This article is Copyright 1990-1996 by Steve Summit.  Content from the
book _C Programming FAQs: Frequently Asked Questions_ is made available
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Certain topics come up again and again on this newsgroup.  They are good
questions, and the answers may not be immediately obvious, but each time
they recur, much net bandwidth and reader time is wasted on repetitive
responses, and on tedious corrections to the incorrect answers which are
inevitably posted.

This article, which is posted monthly, attempts to answer these common
questions definitively and succinctly, so that net discussion can move
on to more constructive topics without continual regression to first

No mere newsgroup article can substitute for thoughtful perusal of a
full-length tutorial or language reference manual.  Anyone interested
enough in C to be following this newsgroup should also be interested
enough to read and study one or more such manuals, preferably several
times.  Some C books and compiler manuals are unfortunately inadequate;
a few even perpetuate some of the myths which this article attempts to
refute.  Several noteworthy books on C are listed in this article's
bibliography; see also question 18.10.  Many of the questions and
answers are cross-referenced to these books, for further study by the
interested and dedicated reader (but beware of ANSI vs. ISO C Standard
section numbers; see question 11.1).

If you have a question about C which is not answered in this article,
first try to answer it by checking a few of the referenced books, or by
asking knowledgeable colleagues, before posing your question to the net
at large.  There are many people on the net who are happy to answer
questions, but the volume of repetitive answers posted to one question,
as well as the growing number of questions as the net attracts more
readers, can become oppressive.  If you have questions or comments
prompted by this article, please reply by mail rather than following up --
this article is meant to decrease net traffic, not increase it.

Besides listing frequently-asked questions, this article also summarizes
frequently-posted answers.  Even if you know all the answers, it's worth
skimming through this list once in a while, so that when you see one of
its questions unwittingly posted, you won't have to waste time

This article was last modified on February 8, 1996, and its travels
may have taken it far from its original home on Usenet.  It may now
be out-of-date, particularly if you are looking at a printed copy or one
retrieved from a tertiary archive site or CD-ROM.  You can always obtain
the most up-to-date copy by anonymous ftp from sites,, or (see questions 18.16 and 20.40), or by
sending the e-mail message "help" to [email protected] .  Since
this list is modified from time to time, its question numbers may not
match those in older or newer copies which are in circulation; be
careful when referring to FAQ list entries by number alone.

This article was produced for free redistribution.  You should not need
to pay anyone for a copy of it.

Other versions of this document are also available.  Posted along with
it are an abridged version and (when there are changes) a list of
differences with respect to the previous version.  A
hypertext version is available on the world-wide web (WWW); see URL
.  Finally, for those who
might prefer a bound, hardcopy version (and even longer answers to even
more questions!), a book-length version has been published by Addison-
Wesley (ISBN 0-201-84519-9).

This article is always being improved.  Your input is welcomed.  Send
your comments to [email protected] .

The questions answered here are divided into several categories:

	 1. Declarations and Initializations
	 2. Structures, Unions, and Enumerations
	 3. Expressions
	 4. Pointers
	 5. Null Pointers
	 6. Arrays and Pointers
	 7. Memory Allocation
	 8. Characters and Strings
	 9. Boolean Expressions and Variables
	10. C Preprocessor
	11. ANSI/ISO Standard C
	12. Stdio
	13. Library Functions
	14. Floating Point
	15. Variable-Length Argument Lists
	16. Strange Problems
	17. Style
	18. Tools and Resources
	19. System Dependencies
	20. Miscellaneous

(The question numbers within each section are not continuous because
they are aligned with the forthcoming book-length version, which
contains even more questions.)

Herewith, some frequently-asked questions and their answers:

Section 1. Declarations and Initializations

1.1:	How do you decide which integer type to use?

A:	If you might need large values (above 32,767 or below -32,767),
	use long.  Otherwise, if space is very important (i.e. if there
	are large arrays or many structures), use short.  Otherwise, use
	int.  If well-defined overflow characteristics are important and
	negative values are not, or if you want to steer clear of sign-
	extension problems when manipulating bits or bytes, use one of
	the corresponding unsigned types.  (Beware when mixing signed
	and unsigned values in expressions, though.)

	Although character types (especially unsigned char) can be used
	as "tiny" integers, doing so is sometimes more trouble than it's
	worth, due to unpredictable sign extension and increased code
	size.  (Using unsigned char can help; see question 12.1 for a
	related problem.)

	A similar space/time tradeoff applies when deciding between
	float and double.  None of the above rules apply if the address
	of a variable is taken and must have a particular type.

	If for some reason you need to declare something with an *exact*
	size (usually the only good reason for doing so is when
	attempting to conform to some externally-imposed storage layout,
	but see question 20.5), be sure to encapsulate the choice behind
	an appropriate typedef.

	References: K&R1 Sec. 2.2 p. 34; K&R2 Sec. 2.2 p. 36, Sec. A4.2
	pp. 195-6, Sec. B11 p. 257; ANSI Sec., Sec.;
	ISO Sec., Sec.; H&S Secs. 5.1,5.2 pp. 110-114.

1.4:	What should the 64-bit type on new, 64-bit machines be?

A:	Some vendors of C products for 64-bit machines support 64-bit
	long ints.  Others fear that too much existing code is written
	to assume that ints and longs are the same size, or that one or
	the other of them is exactly 32 bits, and introduce a new,
	nonstandard, 64-bit long long (or __longlong) type instead.

	Programmers interested in writing portable code should therefore
	insulate their 64-bit type needs behind appropriate typedefs.
	Vendors who feel compelled to introduce a new, longer integral
	type should advertise it as being "at least 64 bits" (which is
	truly new, a type traditional C does not have), and not "exactly
	64 bits."

	References: ANSI Sec. F.5.6; ISO Sec. G.5.6.

1.7:	What's the best way to declare and define global variables?

A:	First, though there can be many "declarations" (and in many
	translation units) of a single "global" (strictly speaking,
	"external") variable or function, there must be exactly one
	"definition".  (The definition is the declaration that actually
	allocates space, and provides an initialization value, if any.)
	The best arrangement is to place each definition in some
	relevant .c file, with an external declaration in a header
	(".h") file, which is #included wherever the declaration is
	needed.  The .c file containing the definition should also
	#include the same header file, so that the compiler can check
	that the definition matches the declarations.

	This rule promotes a high degree of portability: it is
	consistent with the requirements of the ANSI C Standard, and is
	also consistent with most pre-ANSI compilers and linkers.  (Unix
	compilers and linkers typically use a "common model" which
	allows multiple definitions, as long as at most one is
	initialized; this behavior is mentioned as a "common extension"
	by the ANSI Standard, no pun intended.  A few very odd systems
	may require an explicit initializer to distinguish a definition
	from an external declaration.)

	It is possible to use preprocessor tricks to arrange that a line

		DEFINE(int, i);

	need only be entered once in one header file, and turned into a
	definition or a declaration depending on the setting of some
	macro, but it's not clear if this is worth the trouble.

	It's especially important to put global declarations in header
	files if you want the compiler to catch inconsistent
	declarations for you.  In particular, never place a prototype
	for an external function in a .c file: it wouldn't generally be
	checked for consistency with the definition, and an incompatible
	prototype is worse than useless.

	See also questions 10.6 and 18.8.

	References: K&R1 Sec. 4.5 pp. 76-7; K&R2 Sec. 4.4 pp. 80-1; ANSI
	Sec., Sec. 3.7, Sec. 3.7.2, Sec. F.5.11; ISO
	Sec., Sec. 6.7, Sec. 6.7.2, Sec. G.5.11; Rationale
	Sec.; H&S Sec. 4.8 pp. 101-104, Sec. 9.2.3 p. 267; CT&P
	Sec. 4.2 pp. 54-56.

1.11:	What does extern mean in a function declaration?

A:	It can be used as a stylistic hint to indicate that the
	function's definition is probably in another source file, but
	there is no formal difference between

		extern int f();


		int f();

	References: ANSI Sec., Sec. 3.5.1; ISO Sec.,
	Sec. 6.5.1; Rationale Sec.; H&S Secs. 4.3,4.3.1 pp. 75-

1.12:	What's the auto keyword good for?

A:	Nothing; it's archaic.  See also question 20.37.

	References: K&R1 Sec. A8.1 p. 193; ANSI Sec.,
	Sec. 3.5.1; ISO Sec., Sec. 6.5.1; H&S Sec. 4.3 p. 75,
	Sec. 4.3.1 p. 76.

1.14:	I can't seem to define a linked list successfully.  I tried

		typedef struct {
			char *item;
			NODEPTR next;

	but the compiler gave me error messages.  Can't a structure in C
	contain a pointer to itself?

A:	Structures in C can certainly contain pointers to themselves;
	the discussion and example in section 6.5 of K&R make this
	clear.  The problem with the NODEPTR example is that the typedef
	has not been defined at the point where the "next" field is
	declared.  To fix this code, first give the structure a tag
	("struct node").  Then, declare the "next" field as a simple
	"struct node *", or disentangle the typedef declaration from the
	structure definition, or both.  One corrected version would be

		struct node {
			char *item;
			struct node *next;

		typedef struct node *NODEPTR;

	and there are at least three other equivalently correct ways of
	arranging it.

	A similar problem, with a similar solution, can arise when
	attempting to declare a pair of typedef'ed mutually referential

	See also question 2.1.

	References: K&R1 Sec. 6.5 p. 101; K&R2 Sec. 6.5 p. 139; ANSI
	Sec. 3.5.2, Sec., esp. examples; ISO Sec. 6.5.2,
	Sec.; H&S Sec. 5.6.1 pp. 132-3.

1.21:	How do I declare an array of N pointers to functions returning
	pointers to functions returning pointers to characters?

A:	The first part of this question can be answered in at least
	three ways:

	1.  char *(*(*a[N])())();

	2.  Build the declaration up incrementally, using typedefs:

		typedef char *pc;	/* pointer to char */
		typedef pc fpc();	/* function returning pointer to char */
		typedef fpc *pfpc;	/* pointer to above */
		typedef pfpc fpfpc();	/* function returning... */
		typedef fpfpc *pfpfpc;	/* pointer to... */
		pfpfpc a[N];		/* array of... */

	3.  Use the cdecl program, which turns English into C and vice

		cdecl> declare a as array of pointer to function returning
			pointer to function returning pointer to char
		char *(*(*a[])())()

	    cdecl can also explain complicated declarations, help with
	    casts, and indicate which set of parentheses the arguments
	    go in (for complicated function definitions, like the one
	    above).  Versions of cdecl are in volume 14 of
	    comp.sources.unix (see question 18.16) and K&R2.

	Any good book on C should explain how to read these complicated
	C declarations "inside out" to understand them ("declaration
	mimics use").

	The pointer-to-function declarations in the examples above have
	not included parameter type information.  When the parameters
	have complicated types, declarations can *really* get messy.
	(Modern versions of cdecl can help here, too.)

	References: K&R2 Sec. 5.12 p. 122; ANSI Sec. 3.5ff (esp.
	Sec. 3.5.4); ISO Sec. 6.5ff (esp. Sec. 6.5.4); H&S Sec. 4.5 pp.
	85-92, Sec. 5.10.1 pp. 149-50.

1.22:	How can I declare a function that can return a pointer to a
	function of the same type?  I'm building a state machine with
	one function for each state, each of which returns a pointer to
	the function for the next state.  But I can't find a way to
	declare the functions.

A:	You can't quite do it directly.  Either have the function return
	a generic function pointer, with some judicious casts to adjust
	the types as the pointers are passed around; or have it return a
	structure containing only a pointer to a function returning that

1.25:	My compiler is complaining about an invalid redeclaration of a
	function, but I only define it once and call it once.

A:	Functions which are called without a declaration in scope
	(perhaps because the first call precedes the function's
	definition) are assumed to be declared as returning int (and
	without any argument type information), leading to discrepancies
	if the function is later declared or defined otherwise.  Non-int
	functions must be declared before they are called.

	Another possible source of this problem is that the function has
	the same name as another one declared in some header file.

	See also questions 11.3 and 15.1.

	References: K&R1 Sec. 4.2 p. 70; K&R2 Sec. 4.2 p. 72; ANSI
	Sec.; ISO Sec.; H&S Sec. 4.7 p. 101.

1.30:	What can I safely assume about the initial values of variables
	which are not explicitly initialized?  If global variables start
	out as "zero," is that good enough for null pointers and
	floating-point zeroes?

A:	Variables with "static" duration (that is, those declared
	outside of functions, and those declared with the storage class
	static), are guaranteed initialized (just once, at program
	startup) to zero, as if the programmer had typed "= 0".
	Therefore, such variables are initialized to the null pointer
	(of the correct type; see also section 5) if they are pointers,
	and to 0.0 if they are floating-point.

	Variables with "automatic" duration (i.e. local variables
	without the static storage class) start out containing garbage,
	unless they are explicitly initialized.  (Nothing useful can be
	predicted about the garbage.)

	Dynamically-allocated memory obtained with malloc() and
	realloc() is also likely to contain garbage, and must be
	initialized by the calling program, as appropriate.  Memory
	obtained with calloc() is all-bits-0, but this is not
	necessarily useful for pointer or floating-point values (see
	question 7.31, and section 5).

	References: K&R1 Sec. 4.9 pp. 82-4; K&R2 Sec. 4.9 pp. 85-86;
	ANSI Sec. 3.5.7, Sec., Sec.; ISO Sec. 6.5.7,
	Sec., Sec.; H&S Sec. 4.2.8 pp. 72-3, Sec. 4.6
	pp. 92-3, Sec. 4.6.2 pp. 94-5, Sec. 4.6.3 p. 96, Sec. 16.1 p.

1.31:	This code, straight out of a book, isn't compiling:

			char a[] = "Hello, world!";

A:	Perhaps you have a pre-ANSI compiler, which doesn't allow
	initialization of "automatic aggregates" (i.e. non-static local
	arrays, structures, and unions).  As a workaround, you can make
	the array global or static (if you won't need a fresh copy
	during any subsequent calls), or replace it with a pointer (if
	the array won't be written to).  (You can always initialize
	local char * variables to point to string literals, but see
	question 1.32 below.)  If neither of these conditions hold,
	you'll have to initialize the array by hand with strcpy() when
	f() is called.  See also question 11.29.

1.32:	What is the difference between these initializations?

		char a[] = "string literal";
		char *p  = "string literal";

	My program crashes if I try to assign a new value to p[i].

A:	A string literal can be used in two slightly different ways.  As
	an array initializer (as in the declaration of char a[]), it
	specifies the initial values of the characters in that array.
	Anywhere else, it turns into an unnamed, static array of
	characters, which may be stored in read-only memory, which is
	why you can't safely modify it.  In an expression context, the
	array is converted at once to a pointer, as usual (see section
	6), so the second declaration initializes p to point to the
	unnamed array's first element.

	(For compiling old code, some compilers have a switch
	controlling whether strings are writable or not.)

	See also questions 1.31, 6.1, 6.2, and 6.8.

	References: K&R2 Sec. 5.5 p. 104; ANSI Sec. 3.1.4, Sec. 3.5.7;
	ISO Sec. 6.1.4, Sec. 6.5.7; Rationale Sec. 3.1.4; H&S Sec. 2.7.4
	pp. 31-2.

1.34:	I finally figured out the syntax for declaring pointers to
	functions, but now how do I initialize one?

A:	Use something like

		extern int func();
		int (*fp)() = func;

	When the name of a function appears in an expression like this,
	it "decays" into a pointer (that is, it has its address
	implicitly taken), much as an array name does.

	An explicit declaration for the function is normally needed,
	since implicit external function declaration does not happen in
	this case (because the function name in the initialization is
	not part of a function call).

	See also question 4.12.

Section 2. Structures, Unions, and Enumerations

2.1:	What's the difference between these two declarations?

		struct x1 { ... };
		typedef struct { ... } x2;

A:	The first form declares a "structure tag"; the second declares a
	"typedef".  The main difference is that the second declaration
	is of a slightly more abstract type -- its users don't
	necessarily know that it is a structure, and the keyword struct
	is not used when declaring instances of it.

2.2:	Why doesn't

		struct x { ... };
		x thestruct;


A:	C is not C++.  Typedef names are not automatically generated for
	structure tags.  See also question 2.1 above.

2.3:	Can a structure contain a pointer to itself?

A:	Most certainly.  See question 1.14.

2.4:	What's the best way of implementing opaque (abstract) data types
	in C?

A:	One good way is for clients to use structure pointers (perhaps
	additionally hidden behind typedefs) which point to structure
	types which are not publicly defined.

2.6:	I came across some code that declared a structure like this:

		struct name {
			int namelen;
			char namestr[1];

	and then did some tricky allocation to make the namestr array
	act like it had several elements.  Is this legal or portable?

A:	This technique is popular, although Dennis Ritchie has called it
	"unwarranted chumminess with the C implementation."  An official
	interpretation has deemed that it is not strictly conforming
	with the C Standard.  (A thorough treatment of the arguments
	surrounding the legality of the technique is beyond the scope of
	this list.)  It does seem to be portable to all known
	implementations.  (Compilers which check array bounds carefully
	might issue warnings.)

	Another possibility is to declare the variable-size element very
	large, rather than very small; in the case of the above example:

		char namestr[MAXSIZE];

	where MAXSIZE is larger than any name which will be stored.
	However, it looks like this technique is disallowed by a strict
	interpretation of the Standard as well.

	References: Rationale Sec.

2.7:	I heard that structures could be assigned to variables and
	passed to and from functions, but K&R1 says not.

A:	What K&R1 said was that the restrictions on structure operations
	would be lifted in a forthcoming version of the compiler, and in
	fact structure assignment and passing were fully functional in
	Ritchie's compiler even as K&R1 was being published.  Although a
	few early C compilers lacked these operations, all modern
	compilers support them, and they are part of the ANSI C
	standard, so there should be no reluctance to use them.

	(Note that when a structure is assigned, passed, or returned,
	the copying is done monolithically; anything pointed to by any
	pointer fields is *not* copied.)

	References: K&R1 Sec. 6.2 p. 121; K&R2 Sec. 6.2 p. 129; ANSI
	Sec., Sec., Sec. 3.3.16; ISO Sec.,
	Sec., Sec. 6.3.16; H&S Sec. 5.6.2 p. 133.

2.8:	Why can't you compare structures?

A:	There is no single, good way for a compiler to implement
	structure comparison which is consistent with C's low-level
	flavor.  A simple byte-by-byte comparison could founder on
	random bits present in unused "holes" in the structure (such
	padding is used to keep the alignment of later fields correct;
	see question 2.12).  A field-by-field comparison might require
	unacceptable amounts of repetitive code for large structures.

	If you need to compare two structures, you'll have to write your
	own function to do so, field by field.

	References: K&R2 Sec. 6.2 p. 129; ANSI Sec. footnote
	136; Rationale Sec. 3.3.9; H&S Sec. 5.6.2 p. 133.

2.9:	How are structure passing and returning implemented?

A:	When structures are passed as arguments to functions, the entire
	structure is typically pushed on the stack, using as many words
	as are required.  (Programmers often choose to use pointers to
	structures instead, precisely to avoid this overhead.)  Some
	compilers merely pass a pointer to the structure, though they
	may have to make a local copy to preserve pass-by-value

	Structures are often returned from functions in a location
	pointed to by an extra, compiler-supplied "hidden" argument to
	the function.  Some older compilers used a special, static
	location for structure returns, although this made structure-
	valued functions non-reentrant, which ANSI C disallows.

	References: ANSI Sec. 2.2.3; ISO Sec. 5.2.3.

2.10:	How can I pass constant values to functions which accept
	structure arguments?

A:	C has no way of generating anonymous structure values.  You will
	have to use a temporary structure variable or a little structure-
	building function.  (gcc provides structure constants as an
	extension, and the mechanism will probably be added to a future
	revision of the C Standard.)  See also question 4.10.

2.11:	How can I read/write structures from/to data files?

A:	It is relatively straightforward to write a structure out using

		fwrite(&somestruct, sizeof somestruct, 1, fp);

	and a corresponding fread invocation can read it back in.
	(Under pre-ANSI C, a (char *) cast on the first argument is
	required.  What's important is that fwrite() receive a byte
	pointer, not a structure pointer.)  However, data files so
	written will *not* be portable (see questions 2.12 and 20.5).
	Note also that if the structure contains any pointers, only the
	pointer values will be written, and they are most unlikely to be
	valid when read back in.  Finally, note that for widespread
	portability you must use the "b" flag when fopening the files;
	see question 12.38.

	A more portable solution, though it's a bit more work initially,
	is to write a pair of functions for writing and reading a
	structure, field-by-field, in a portable (perhaps even human-
	readable) way.

	References: H&S Sec. 15.13 p. 381.

2.12:	My compiler is leaving holes in structures, which is wasting
	space and preventing "binary" I/O to external data files.  Can I
	turn off the padding, or otherwise control the alignment of
	structure fields?

A:	Your compiler may provide an extension to give you this control
	(perhaps a #pragma; see question 11.20), but there is no
	standard method.

	See also question 20.5.

	References: K&R2 Sec. 6.4 p. 138; H&S Sec. 5.6.4 p. 135.

2.13:	Why does sizeof report a larger size than I expect for a
	structure type, as if there were padding at the end?

A:	Structures may have this padding (as well as internal padding),
	if necessary, to ensure that alignment properties will be
	preserved when an array of contiguous structures is allocated.
	Even when the structure is not part of an array, the end padding
	remains, so that sizeof can always return a consistent size.
	See question 2.12 above.

	References: H&S Sec. 5.6.7 pp. 139-40.

2.14:	How can I determine the byte offset of a field within a

A:	ANSI C defines the offsetof() macro, which should be used if
	available; see .  If you don't have it, one possible
	implementation is

		#define offsetof(type, mem) ((size_t) \
			((char *)&((type *)0)->mem - (char *)(type *)0))

	This implementation is not 100% portable; some compilers may
	legitimately refuse to accept it.

	See question 2.15 below for a usage hint.

	References: ANSI Sec. 4.1.5; ISO Sec. 7.1.6; Rationale
	Sec.; H&S Sec. 11.1 pp. 292-3.

2.15:	How can I access structure fields by name at run time?

A:	Build a table of names and offsets, using the offsetof() macro.
	The offset of field b in struct a is

		offsetb = offsetof(struct a, b)

	If structp is a pointer to an instance of this structure, and
	field b is an int (with offset as computed above), b's value can
	be set indirectly with

		*(int *)((char *)structp + offsetb) = value;

2.18:	This program works correctly, but it dumps core after it
	finishes.  Why?

		struct list {
			char *item;
			struct list *next;

		/* Here is the main program. */

		main(argc, argv)
		{ ... }

A:	A missing semicolon causes main() to be declared as returning a
	structure.  (The connection is hard to see because of the
	intervening comment.)  Since structure-valued functions are
	usually implemented by adding a hidden return pointer (see
	question 2.9), the generated code for main() tries to accept
	three arguments, although only two are passed (in this case, by
	the C start-up code).  See also questions 10.9 and 16.4.

	References: CT&P Sec. 2.3 pp. 21-2.

2.20:	Can I initialize unions?

A:	ANSI Standard C allows an initializer for the first member of a
	union.  There is no standard way of initializing any other
	member (nor, under a pre-ANSI compiler, is there generally any
	way of initializing a union at all).

	References: K&R2 Sec. 6.8 pp. 148-9; ANSI Sec. 3.5.7; ISO
	Sec. 6.5.7; H&S Sec. 4.6.7 p. 100.

2.22:	What is the difference between an enumeration and a set of
	preprocessor #defines?

A:	At the present time, there is little difference.  Although many
	people might have wished otherwise, the C Standard says that
	enumerations may be freely intermixed with other integral types,
	without errors.  (If such intermixing were disallowed without
	explicit casts, judicious use of enumerations could catch
	certain programming errors.)

	Some advantages of enumerations are that the numeric values are
	automatically assigned, that a debugger may be able to display
	the symbolic values when enumeration variables are examined, and
	that they obey block scope.  (A compiler may also generate
	nonfatal warnings when enumerations and integers are
	indiscriminately mixed, since doing so can still be considered
	bad style even though it is not strictly illegal.)  A
	disadvantage is that the programmer has little control over
	those nonfatal warnings; some programmers also resent not having
	control over the sizes of enumeration variables.

	References: K&R2 Sec. 2.3 p. 39, Sec. A4.2 p. 196; ANSI
	Sec., Sec. 3.5.2, Sec., Appendix E; ISO
	Sec., Sec. 6.5.2, Sec., Annex F; H&S Sec. 5.5
	pp. 127-9, Sec. 5.11.2 p. 153.

2.24:	Is there an easy way to print enumeration values symbolically?

A:	No.  You can write a little function to map an enumeration
	constant to a string.  (If all you're worried about is
	debugging, a good debugger should automatically print
	enumeration constants symbolically.)

Section 3. Expressions

3.1:	Why doesn't this code:

		a[i] = i++;


A:	The subexpression i++ causes a side effect -- it modifies i's
	value -- which leads to undefined behavior since i is also
	referenced elsewhere in the same expression.  (Note that
	although the language in K&R suggests that the behavior of this
	expression is unspecified, the C Standard makes the stronger
	statement that it is undefined -- see question 11.33.)

	References: K&R1 Sec. 2.12; K&R2 Sec. 2.12; ANSI Sec. 3.3; ISO
	Sec. 6.3.

3.2:	Under my compiler, the code

		int i = 7;
		printf("%d\n", i++ * i++);

	prints 49.  Regardless of the order of evaluation, shouldn't it
	print 56?

A:	Although the postincrement and postdecrement operators ++ and --
	perform their operations after yielding the former value, the
	implication of "after" is often misunderstood.  It is *not*
	guaranteed that an increment or decrement is performed
	immediately after giving up the previous value and before any
	other part of the expression is evaluated.  It is merely
	guaranteed that the update will be performed sometime before the
	expression is considered "finished" (before the next "sequence
	point," in ANSI C's terminology; see question 3.8).  In the
	example, the compiler chose to multiply the previous value by
	itself and to perform both increments afterwards.

	The behavior of code which contains multiple, ambiguous side
	effects has always been undefined.  (Loosely speaking, by
	"multiple, ambiguous side effects" we mean any combination of
	++, --, =, +=, -=, etc. in a single expression which causes the
	same object either to be modified twice or modified and then
	inspected.  This is a rough definition; see question 3.8 for a
	precise one, and question 11.33 for the meaning of "undefined.")
	Don't even try to find out how your compiler implements such
	things (contrary to the ill-advised exercises in many C
	textbooks); as K&R wisely point out, "if you don't know *how*
	they are done on various machines, that innocence may help to
	protect you."

	References: K&R1 Sec. 2.12 p. 50; K&R2 Sec. 2.12 p. 54; ANSI
	Sec. 3.3; ISO Sec. 6.3; CT&P Sec. 3.7 p. 47; PCS Sec. 9.5 pp.

3.3:	I've experimented with the code


	on several compilers.  Some gave i the value 3, some gave 4, but
	one gave 7.  I know the behavior is undefined, but how could it
	give 7?

A:	[I apologize for the censorship of the question, but the
	expression that used to be there was indecent, and by the
	newly-passed Communications Decency Act of the U.S., I am
	prohibited from transmitting "indecent" material, whatever that
	is.  Suffice it to say that the expression tried to modify the
	same variable twice between sequence points.  --scs]

	Undefined behavior means *anything* can happen.  See questions
	3.9 and 11.33.  (Also, note that neither i++ nor ++i is the same
	as i+1.  If you want to increment i, use i=i+1 or i++ or ++i,
	not some combination.  See also question 3.12.)

3.4:	Can I use explicit parentheses to force the order of evaluation
	I want?  Even if I don't, doesn't precedence dictate it?

A:	Not in general.

	Operator precedence and explicit parentheses impose only a
	partial ordering on the evaluation of an expression.  In the

		f() + g() * h()

	although we know that the multiplication will happen before the
	addition, there is no telling which of the three functions will
	be called first.

	When you need to ensure the order of subexpression evaluation,
	you may need to use explicit temporary variables and separate

	References: K&R1 Sec. 2.12 p. 49, Sec. A.7 p. 185; K&R2
	Sec. 2.12 pp. 52-3, Sec. A.7 p. 200.

3.5:	But what about the && and || operators?
	I see code like "while((c = getchar()) != EOF && c != '\n')" ...

A:	There is a special exception for those operators (as well as the
	?: operator): left-to-right evaluation is guaranteed (as is an
	intermediate sequence point, see question 3.8).  Any book on C
	should make this clear.

	References: K&R1 Sec. 2.6 p. 38, Secs. A7.11-12 pp. 190-1; K&R2
	Sec. 2.6 p. 41, Secs. A7.14-15 pp. 207-8; ANSI Sec. 3.3.13,
	Sec. 3.3.14, Sec. 3.3.15; ISO Sec. 6.3.13, Sec. 6.3.14,
	Sec. 6.3.15; H&S Sec. 7.7 pp. 217-8, Sec. 7.8 pp. 218-20,
	Sec. 7.12.1 p. 229; CT&P Sec. 3.7 pp. 46-7.

3.8:	How can I understand these complex expressions?  What's a
	"sequence point"?

A:	A sequence point is the point (at the end of a full expression,
	or at the ||, &&, ?:, or comma operators, or just before a
	function call) at which the dust has settled and all side
	effects are guaranteed to be complete.  The ANSI/ISO C Standard
	states that

		Between the previous and next sequence point an
		object shall have its stored value modified at
		most once by the evaluation of an expression.
		Furthermore, the prior value shall be accessed
		only to determine the value to be stored.

	The second sentence can be difficult to understand.  It says
	that if an object is written to within a full expression, any
	and all accesses to it within the same expression must be for
	the purposes of computing the value to be written.  This rule
	effectively constrains legal expressions to those in which the
	accesses demonstrably precede the modification.

	See also question 3.9 below.

	References: ANSI Sec., Sec. 3.3, Appendix B; ISO
	Sec., Sec. 6.3, Annex C; Rationale Sec.; H&S
	Sec. 7.12.1 pp. 228-9.

3.9:	So given

		a[i] = i++;

	we don't know which cell of a[] gets written to, but i does get
	incremented by one.

A:	*No.*  Once an expression or program becomes undefined, *all*
	aspects of it become undefined.  See questions 3.2, 3.3, 11.33,
	and 11.35.

3.12:	If I'm not using the value of the expression, should I use i++
	or ++i to increment a variable?

A:	Since the two forms differ only in the value yielded, they are
	entirely equivalent when only their side effect is needed.

	See also question 3.3.

	References: K&R1 Sec. 2.8 p. 43; K&R2 Sec. 2.8 p. 47; ANSI
	Sec., Sec.; ISO Sec., Sec.; H&S
	Sec. 7.4.4 pp. 192-3, Sec. 7.5.8 pp. 199-200.

3.14:	Why doesn't the code

		int a = 1000, b = 1000;
		long int c = a * b;


A:	Under C's integral promotion rules, the multiplication is
	carried out using int arithmetic, and the result may overflow or
	be truncated before being promoted and assigned to the long int
	left-hand side.  Use an explicit cast to force long arithmetic:

		long int c = (long int)a * b;

	Note that (long int)(a * b) would *not* have the desired effect.

	A similar problem can arise when two integers are divided, with
	the result assigned to a floating-point variable.

	References: K&R1 Sec. 2.7 p. 41; K&R2 Sec. 2.7 p. 44; ANSI
	Sec.; ISO Sec.; H&S Sec. 6.3.4 p. 176; CT&P
	Sec. 3.9 pp. 49-50.

3.16:	I have a complicated expression which I have to assign to one of
	two variables, depending on a condition.  Can I use code like

		((condition) ? a : b) = complicated_expression;

A:	No.  The ?: operator, like most operators, yields a value, and
	you can't assign to a value.  (In other words, ?: does not yield
	an "lvalue".)  If you really want to, you can try something like

		*((condition) ? &a : &b) = complicated_expression;

	although this is admittedly not as pretty.

	References: ANSI Sec. 3.3.15 esp. footnote 50; ISO Sec. 6.3.15;
	H&S Sec. 7.1 pp. 179-180.

Section 4. Pointers

4.2:	I'm trying to declare a pointer and allocate some space for it,
	but it's not working.  What's wrong with this code?

		char *p;
		*p = malloc(10);

A:	The pointer you declared is p, not *p.  To make a pointer point
	somewhere, you just use the name of the pointer:

		p = malloc(10);

	It's when you're manipulating the pointed-to memory that you use
	* as an indirection operator:

		*p = 'H';

	See also questions 1.21, 7.1, and 8.3.

	References: CT&P Sec. 3.1 p. 28.

4.3:	Does *p++ increment p, or what it points to?

A:	Unary operators like *, ++, and -- all associate (group) from
	right to left.  Therefore, *p++ increments p (and returns the
	value pointed to by p before the increment).  To increment the
	value pointed to by p, use (*p)++ (or perhaps ++*p, if the order
	of the side effect doesn't matter).

	References: K&R1 Sec. 5.1 p. 91; K&R2 Sec. 5.1 p. 95; ANSI
	Sec. 3.3.2, Sec. 3.3.3; ISO Sec. 6.3.2, Sec. 6.3.3; H&S
	Sec. 7.4.4 pp. 192-3, Sec. 7.5 p. 193, Secs. 7.5.7,7.5.8 pp. 199-

4.5:	I have a char * pointer that happens to point to some ints, and
	I want to step it over them.  Why doesn't

		((int *)p)++;


A:	In C, a cast operator does not mean "pretend these bits have a
	different type, and treat them accordingly"; it is a conversion
	operator, and by definition it yields an rvalue, which cannot be
	assigned to, or incremented with ++.  (It is an anomaly in pcc-
	derived compilers, and an extension in gcc, that expressions
	such as the above are ever accepted.)  Say what you mean: use

		p = (char *)((int *)p + 1);

	or (since p is a char *) simply

		p += sizeof(int);

	Whenever possible, you should choose appropriate pointer types
	in the first place, instead of trying to treat one type as

	References: K&R2 Sec. A7.5 p. 205; ANSI Sec. 3.3.4 (esp.
	footnote 14); ISO Sec. 6.3.4; Rationale Sec.; H&S
	Sec. 7.1 pp. 179-80.

4.8:	I have a function which accepts, and is supposed to initialize,
	a pointer:

		void f(ip)
		int *ip;
			static int dummy = 5;
			ip = &dummy;

	But when I call it like this:

		int *ip;

	the pointer in the caller remains unchanged.

A:	Are you sure the function initialized what you thought it did?
	Remember that arguments in C are passed by value.  The called
	function altered only the passed copy of the pointer.  You'll
	either want to pass the address of the pointer (the function
	will end up accepting a pointer-to-a-pointer), or have the
	function return the pointer.

	See also questions 4.9 and 4.11.

4.9:	Can I use a void ** pointer to pass a generic pointer to a
	function by reference?

A:	Not portably.  There is no generic pointer-to-pointer type in C.
	void * acts as a generic pointer only because conversions are
	applied automatically when other pointer types are assigned to
	and from void *'s; these conversions cannot be performed (the
	correct underlying pointer type is not known) if an attempt is
	made to indirect upon a void ** value which points at something
	other than a void *.

4.10:	I have a function

		extern int f(int *);

	which accepts a pointer to an int.  How can I pass a constant by
	reference?  A call like


	doesn't seem to work.

A:	You can't do this directly.  You will have to declare a
	temporary variable, and then pass its address to the function:

		int five = 5;

	See also questions 2.10, 4.8, and 20.1.

4.11:	Does C even have "pass by reference"?

A:	Not really.  Strictly speaking, C always uses pass by value.
	You can simulate pass by reference yourself, by defining
	functions which accept pointers and then using the & operator
	when calling, and the compiler will essentially simulate it for
	you when you pass an array to a function (by passing a pointer
	instead, see question 6.4 et al.), but C has nothing truly
	equivalent to formal pass by reference or C++ reference
	parameters.  (However, function-like preprocessor macros do
	provide a form of "call by name".)

	See also questions 4.8 and 20.1.

	References: K&R1 Sec. 1.8 pp. 24-5, Sec. 5.2 pp. 91-3; K&R2
	Sec. 1.8 pp. 27-8, Sec. 5.2 pp. 91-3; ANSI Sec., esp.
	footnote 39; ISO Sec.; H&S Sec. 9.5 pp. 273-4.

4.12:	I've seen different methods used for calling functions via
	pointers.  What's the story?

A:	Originally, a pointer to a function had to be "turned into" a
	"real" function, with the * operator (and an extra pair of
	parentheses, to keep the precedence straight), before calling:

		int r, func(), (*fp)() = func;
		r = (*fp)();

	It can also be argued that functions are always called via
	pointers, and that "real" function names always decay implicitly
	into pointers (in expressions, as they do in initializations;
	see question 1.34).  This reasoning, made widespread through pcc
	and adopted in the ANSI standard, means that

		r = fp();

	is legal and works correctly, whether fp is the name of a
	function or a pointer to one.  (The usage has always been
	unambiguous; there is nothing you ever could have done with a
	function pointer followed by an argument list except call the
	function pointed to.)  An explicit * is still allowed (and
	recommended, if portability to older compilers is important).

	See also question 1.34.

	References: K&R1 Sec. 5.12 p. 116; K&R2 Sec. 5.11 p. 120; ANSI
	Sec.; ISO Sec.; Rationale Sec.; H&S
	Sec. 5.8 p. 147, Sec. 7.4.3 p. 190.

Section 5. Null Pointers

5.1:	What is this infamous null pointer, anyway?

A:	The language definition states that for each pointer type, there
	is a special value -- the "null pointer" -- which is
	distinguishable from all other pointer values and which is
	"guaranteed to compare unequal to a pointer to any object or
	function."  That is, the address-of operator & will never yield
	a null pointer, nor will a successful call to malloc().
	(malloc() does return a null pointer when it fails, and this is
	a typical use of null pointers: as a "special" pointer value
	with some other meaning, usually "not allocated" or "not
	pointing anywhere yet.")

	A null pointer is conceptually different from an uninitialized
	pointer.  A null pointer is known not to point to any object or
	function; an uninitialized pointer might point anywhere.  See
	also questions 1.30, 7.1, and 7.31.

	As mentioned above, there is a null pointer for each pointer
	type, and the internal values of null pointers for different
	types may be different.  Although programmers need not know the
	internal values, the compiler must always be informed which type
	of null pointer is required, so that it can make the distinction
	if necessary (see questions 5.2, 5.5, and 5.6 below).

	References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102; ANSI
	Sec.; ISO Sec.; Rationale Sec.; H&S
	Sec. 5.3.2 pp. 121-3.

5.2:	How do I get a null pointer in my programs?

A:	According to the language definition, a constant 0 in a pointer
	context is converted into a null pointer at compile time.  That
	is, in an initialization, assignment, or comparison when one
	side is a variable or expression of pointer type, the compiler
	can tell that a constant 0 on the other side requests a null
	pointer, and generate the correctly-typed null pointer value.
	Therefore, the following fragments are perfectly legal:

		char *p = 0;
		if(p != 0)

	(See also question 5.3.)

	However, an argument being passed to a function is not
	necessarily recognizable as a pointer context, and the compiler
	may not be able to tell that an unadorned 0 "means" a null
	pointer.  To generate a null pointer in a function call context,
	an explicit cast may be required, to force the 0 to be
	recognized as a pointer.  For example, the Unix system call
	execl takes a variable-length, null-pointer-terminated list of
	character pointer arguments, and is correctly called like this:

		execl("/bin/sh", "sh", "-c", "date", (char *)0);

	If the (char *) cast on the last argument were omitted, the
	compiler would not know to pass a null pointer, and would pass
	an integer 0 instead.  (Note that many Unix manuals get this
	example wrong .)

	When function prototypes are in scope, argument passing becomes
	an "assignment context," and most casts may safely be omitted,
	since the prototype tells the compiler that a pointer is
	required, and of which type, enabling it to correctly convert an
	unadorned 0.  Function prototypes cannot provide the types for
	variable arguments in variable-length argument lists however, so
	explicit casts are still required for those arguments.  (See
	also question 15.3.)  It is safest to properly cast all null
	pointer constants in function calls: to guard against varargs
	functions or those without prototypes, to allow interim use of
	non-ANSI compilers, and to demonstrate that you know what you
	are doing.  (Incidentally, it's also a simpler rule to


		Unadorned 0 okay:	Explicit cast required:

		initialization		function call,
					no prototype in scope
					variable argument in
		comparison		varargs function call

		function call,
		prototype in scope,
		fixed argument

	References: K&R1 Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R2
	Sec. A7.10 p. 207, Sec. A7.17 p. 209; ANSI Sec.; ISO
	Sec.; H&S Sec. 4.6.3 p. 95, Sec. 6.2.7 p. 171.

5.3:	Is the abbreviated pointer comparison "if(p)" to test for non-
	null pointers valid?  What if the internal representation for
	null pointers is nonzero?

A:	When C requires the Boolean value of an expression (in the if,
	while, for, and do statements, and with the &&, ||, !, and ?:
	operators), a false value is inferred when the expression
	compares equal to zero, and a true value otherwise.  That is,
	whenever one writes


	where "expr" is any expression at all, the compiler essentially
	acts as if it had been written as

		if((expr) != 0)

	Substituting the trivial pointer expression "p" for "expr," we

		if(p)	is equivalent to		if(p != 0)

	and this is a comparison context, so the compiler can tell that
	the (implicit) 0 is actually a null pointer constant, and use
	the correct null pointer value.  There is no trickery involved
	here; compilers do work this way, and generate identical code
	for both constructs.  The internal representation of a null
	pointer does *not* matter.

	The boolean negation operator, !, can be described as follows:

		!expr	is essentially equivalent to	(expr)?0:1
			or to				((expr) == 0)

	which leads to the conclusion that

		if(!p)	is equivalent to		if(p == 0)

	"Abbreviations" such as if(p), though perfectly legal, are
	considered by some to be bad style (and by others to be good
	style; see question 17.10).

	See also question 9.2.

	References: K&R2 Sec. A7.4.7 p. 204; ANSI Sec.,
	Sec. 3.3.9, Sec. 3.3.13, Sec. 3.3.14, Sec. 3.3.15, Sec.,
	Sec. 3.6.5; ISO Sec., Sec. 6.3.9, Sec. 6.3.13,
	Sec. 6.3.14, Sec. 6.3.15, Sec., Sec. 6.6.5; H&S
	Sec. 5.3.2 p. 122.

5.4:	What is NULL and how is it #defined?

A:	As a matter of style, many programmers prefer not to have
	unadorned 0's scattered through their programs.  Therefore, the
	preprocessor macro NULL is #defined (by  or )
	with the value 0, possibly cast to (void *) (see also question
	5.6).  A programmer who wishes to make explicit the distinction
	between 0 the integer and 0 the null pointer constant can then
	use NULL whenever a null pointer is required.

	Using NULL is a stylistic convention only; the preprocessor
	turns NULL back into 0 which is then recognized by the compiler,
	in pointer contexts, as before.  In particular, a cast may still
	be necessary before NULL (as before 0) in a function call
	argument.  The table under question 5.2 above applies for NULL
	as well as 0 (an unadorned NULL is equivalent to an unadorned

	NULL should *only* be used for pointers; see question 5.9.

	References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102; ANSI
	Sec. 4.1.5, Sec.; ISO Sec. 7.1.6, Sec.;
	Rationale Sec. 4.1.5; H&S Sec. 5.3.2 p. 122, Sec. 11.1 p. 292.

5.5:	How should NULL be defined on a machine which uses a nonzero bit
	pattern as the internal representation of a null pointer?

A:	The same as on any other machine: as 0 (or ((void *)0)).

	Whenever a programmer requests a null pointer, either by writing
	"0" or "NULL," it is the compiler's responsibility to generate
	whatever bit pattern the machine uses for that null pointer.
	Therefore, #defining NULL as 0 on a machine for which internal
	null pointers are nonzero is as valid as on any other: the
	compiler must always be able to generate the machine's correct
	null pointers in response to unadorned 0's seen in pointer
	contexts.  See also questions 5.2, 5.10, and 5.17.

	References: ANSI Sec. 4.1.5; ISO Sec. 7.1.6; Rationale
	Sec. 4.1.5.

5.6:	If NULL were defined as follows:

		#define NULL ((char *)0)

	wouldn't that make function calls which pass an uncast NULL

A:	Not in general.  The problem is that there are machines which
	use different internal representations for pointers to different
	types of data.  The suggested definition would make uncast NULL
	arguments to functions expecting pointers to characters work
	correctly, but pointer arguments of other types would still be
	problematical, and legal constructions such as

		FILE *fp = NULL;

	could fail.

	Nevertheless, ANSI C allows the alternate definition

		#define NULL ((void *)0)

	for NULL.  Besides potentially helping incorrect programs to
	work (but only on machines with homogeneous pointers, thus
	questionably valid assistance), this definition may catch
	programs which use NULL incorrectly (e.g. when the ASCII NUL
	character was really intended; see question 5.9).

	References: Rationale Sec. 4.1.5.

5.9:	If NULL and 0 are equivalent as null pointer constants, which
	should I use?

A:	Many programmers believe that NULL should be used in all pointer
	contexts, as a reminder that the value is to be thought of as a
	pointer.  Others feel that the confusion surrounding NULL and 0
	is only compounded by hiding 0 behind a macro, and prefer to use
	unadorned 0 instead.  There is no one right answer.  (See also
	questions 9.2 and 17.10.)  C programmers must understand that
	NULL and 0 are interchangeable in pointer contexts, and that an
	uncast 0 is perfectly acceptable.  Any usage of NULL (as opposed
	to 0) should be considered a gentle reminder that a pointer is
	involved; programmers should not depend on it (either for their
	own understanding or the compiler's) for distinguishing pointer
	0's from integer 0's.

	NULL should *not* be used when another kind of 0 is required,
	even though it might work, because doing so sends the wrong
	stylistic message.  (Furthermore, ANSI allows the definition of
	NULL to be ((void *)0), which will not work at all in non-
	pointer contexts.)  In particular, do not use NULL when the
	ASCII null character (NUL) is desired.  Provide your own

		#define NUL '\0'

	if you must.

	References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102.

5.10:	But wouldn't it be better to use NULL (rather than 0), in case
	the value of NULL changes, perhaps on a machine with nonzero
	internal null pointers?

A:	No.  (Using NULL may be preferable, but not for this reason.)
	Although symbolic constants are often used in place of numbers
	because the numbers might change, this is *not* the reason that
	NULL is used in place of 0.  Once again, the language guarantees
	that source-code 0's (in pointer contexts) generate null
	pointers.  NULL is used only as a stylistic convention.  See
	questions 5.5 and 9.2.

5.12:	I use the preprocessor macro

		#define Nullptr(type) (type *)0

	to help me build null pointers of the correct type.

A:	This trick, though popular and superficially attractive, does
	not buy much.  It is not needed in assignments and comparisons;
	see question 5.2.  It does not even save keystrokes.  Its use
	may suggest to the reader that the program's author is shaky on
	the subject of null pointers, requiring that the #definition of
	the macro, its invocations, and *all* other pointer usages be
	checked.  See also questions 9.1 and 10.2.

5.13:	This is strange.  NULL is guaranteed to be 0, but the null
	pointer is not?

A:	When the term "null" or "NULL" is casually used, one of several
	things may be meant:

	1.	The conceptual null pointer, the abstract language concept
		defined in question 5.1.  It is implemented with...

	2.	The internal (or run-time) representation of a null
		pointer, which may or may not be all-bits-0 and which may
		be different for different pointer types.  The actual
		values should be of concern only to compiler writers.
		Authors of C programs never see them, since they use...

	3.	The null pointer constant, which is a constant integer 0
		(see question 5.2).  It is often hidden behind...

	4.	The NULL macro, which is #defined to be "0" or
		"((void *)0)" (see question 5.4).  Finally, as red
		herrings, we have...

	5.	The ASCII null character (NUL), which does have all bits
		zero, but has no necessary relation to the null pointer
		except in name; and...

	6.	The "null string," which is another name for the empty
		string ("").  Using the term "null string" can be
		confusing in C, because an empty string involves a null
		('\0') character, but *not* a null pointer, which brings
		us full circle...

	This article uses the phrase "null pointer" (in lower case) for
	sense 1, the character "0" or the phrase "null pointer constant"
	for sense 3, and the capitalized word "NULL" for sense 4.

5.14:	Why is there so much confusion surrounding null pointers?  Why
	do these questions come up so often?

A:	C programmers traditionally like to know more than they need to
	about the underlying machine implementation.  The fact that null
	pointers are represented both in source code, and internally to
	most machines, as zero invites unwarranted assumptions.  The use
	of a preprocessor macro (NULL) may seem to suggest that the
	value could change some day, or on some weird machine.  The
	construct "if(p == 0)" is easily misread as calling for
	conversion of p to an integral type, rather than 0 to a pointer
	type, before the comparison.  Finally, the distinction between
	the several uses of the term "null" (listed in question 5.13
	above) is often overlooked.

	One good way to wade out of the confusion is to imagine that C
	used a keyword (perhaps "nil", like Pascal) as a null pointer
	constant.  The compiler could either turn "nil" into the correct
	type of null pointer when it could determine the type from the
	source code, or complain when it could not.  Now in fact, in C
	the keyword for a null pointer constant is not "nil" but "0",
	which works almost as well, except that an uncast "0" in a non-
	pointer context generates an integer zero instead of an error
	message, and if that uncast 0 was supposed to be a null pointer
	constant, the code may not work.

5.15:	I'm confused.  I just can't understand all this null pointer

A:	Follow these two simple rules:

	1.	When you want a null pointer constant in source code,
		use "0" or "NULL".

	2.	If the usage of "0" or "NULL" is an argument in a
		function call, cast it to the pointer type expected by
		the function being called.

	The rest of the discussion has to do with other people's
	misunderstandings, with the internal representation of null
	pointers (which you shouldn't need to know), and with ANSI C
	refinements.  Understand questions 5.1, 5.2, and 5.4, and
	consider 5.3, 5.9, 5.13, and 5.14, and you'll do fine.

5.16:	Given all the confusion surrounding null pointers, wouldn't it
	be easier simply to require them to be represented internally by

A:	If for no other reason, doing so would be ill-advised because it
	would unnecessarily constrain implementations which would
	otherwise naturally represent null pointers by special, nonzero
	bit patterns, particularly when those values would trigger
	automatic hardware traps for invalid accesses.

	Besides, what would such a requirement really accomplish?
	Proper understanding of null pointers does not require knowledge
	of the internal representation, whether zero or nonzero.
	Assuming that null pointers are internally zero does not make
	any code easier to write (except for a certain ill-advised usage
	of calloc(); see question 7.31).  Known-zero internal pointers
	would not obviate casts in function calls, because the *size* of
	the pointer might still be different from that of an int.  (If
	"nil" were used to request null pointers, as mentioned in
	question 5.14 above, the urge to assume an internal zero
	representation would not even arise.)

5.17:	Seriously, have any actual machines really used nonzero null
	pointers, or different representations for pointers to different

A:	The Prime 50 series used segment 07777, offset 0 for the null
	pointer, at least for PL/I.  Later models used segment 0, offset
	0 for null pointers in C, necessitating new instructions such as
	TCNP (Test C Null Pointer), evidently as a sop to all the extant
	poorly-written C code which made incorrect assumptions.  Older,
	word-addressed Prime machines were also notorious for requiring
	larger byte pointers (char *'s) than word pointers (int *'s).

	The Eclipse MV series from Data General has three
	architecturally supported pointer formats (word, byte, and bit
	pointers), two of which are used by C compilers: byte pointers
	for char * and void *, and word pointers for everything else.

	Some Honeywell-Bull mainframes use the bit pattern 06000 for
	(internal) null pointers.

	The CDC Cyber 180 Series has 48-bit pointers consisting of a
	ring, segment, and offset.  Most users (in ring 11) have null
	pointers of 0xB00000000000.  It was common on old CDC ones-
	complement machines to use an all-one-bits word as a special
	flag for all kinds of data, including invalid addresses.

	The old HP 3000 series uses a different addressing scheme for
	byte addresses than for word addresses; like several of the
	machines above it therefore uses different representations for
	char * and void * pointers than for other pointers.

	The Symbolics Lisp Machine, a tagged architecture, does not even
	have conventional numeric pointers; it uses the pair 
	(basically a nonexistent  handle) as a C null

	Depending on the "memory model" in use, 8086-family processors
	(PC compatibles) may use 16-bit data pointers and 32-bit
	function pointers, or vice versa.

	Some 64-bit Cray machines represent int * in the lower 48 bits
	of a word; char * additionally uses the upper 16 bits to
	indicate a byte address within a word.

	References: K&R1 Sec. A14.4 p. 211.

5.20:	What does a run-time "null pointer assignment" error mean?  How
	do I track it down?

A:	This message, which typically occurs with MS-DOS compilers (see,
	therefore, section 19) means that you've written, via a null
	(perhaps because uninitialized) pointer, to location 0.  (See
	also question 16.8.)

	A debugger may let you set a data breakpoint or watchpoint or
	something on location 0.  Alternatively, you could write a bit
	of code to stash away a copy of 20 or so bytes from location 0,
	and periodically check that the memory at location 0 hasn't

Section 6.  Arrays and Pointers

6.1:	I had the definition char a[6] in one source file, and in
	another I declared extern char *a.  Why didn't it work?

A:	The declaration extern char *a simply does not match the actual
	definition.  The type pointer-to-type-T is not the same as array-
	of-type-T.  Use extern char a[].

	References: ANSI Sec.; ISO Sec.; CT&P Sec. 3.3
	pp. 33-4, Sec. 4.5 pp. 64-5.

6.2:	But I heard that char a[] was identical to char *a.

A:	Not at all.  (What you heard has to do with formal parameters to
	functions; see question 6.4.)  Arrays are not pointers.  The
	array declaration char a[6] requests that space for six
	characters be set aside, to be known by the name "a."  That is,
	there is a location named "a" at which six characters can sit.
	The pointer declaration char *p, on the other hand, requests a
	place which holds a pointer, to be known by the name "p."  This
	pointer can point almost anywhere: to any char, or to any
	contiguous array of chars, or nowhere (see also questions 5.1
	and 1.30).

	As usual, a picture is worth a thousand words.  The declarations

		char a[] = "hello";
		char *p = "world";

	would initialize data structures which could be represented like
		a: | h | e | l | l | o |\0 |
		   +-----+     +---+---+---+---+---+---+
		p: |  *======> | w | o | r | l | d |\0 |
		   +-----+     +---+---+---+---+---+---+

	It is important to realize that a reference like *x*[3]
	generates different code depending on whether *x* is an array or
	a pointer.  Given the declarations above, when the compiler sees
	the expression a[3], it emits code to start at the location "a,"
	move three past it, and fetch the character there.  When it sees
	the expression p[3], it emits code to start at the location "p,"
	fetch the pointer value there, add three to the pointer, and
	finally fetch the character pointed to.  In other words, a[3] is
	three places past (the start of) the object *named* a, while
	p[3] is three places past the object *pointed to* by p.  In the
	example above, both a[3] and p[3] happen to be the character
	'l', but the compiler gets there differently.

	References: K&R2 Sec. 5.5 p. 104; CT&P Sec. 4.5 pp. 64-5.

6.3:	So what is meant by the "equivalence of pointers and arrays" in

A:	Much of the confusion surrounding arrays and pointers in C can
	be traced to a misunderstanding of this statement.  Saying that
	arrays and pointers are "equivalent" means neither that they are
	identical nor even interchangeable.

	"Equivalence" refers to the following key definition:

		An lvalue of type array-of-T which appears in an
		expression decays (with three exceptions) into a
		pointer to its first element; the type of the
		resultant pointer is pointer-to-T.

	(The exceptions are when the array is the operand of a sizeof or
	& operator, or is a string literal initializer for a character

	As a consequence of this definition, the compiler doesn't apply
	the array subscripting operator [] that differently to arrays
	and pointers, after all.  In an expression of the form a[i], the
	array decays into a pointer, following the rule above, and is
	then subscripted just as would be a pointer variable in the
	expression p[i] (although the eventual memory accesses will be
	different, as explained in question 6.2).  If you were to assign
	the array's address to the pointer:

		p = a;

	then p[3] and a[3] would access the same element.

	See also question 6.8.

	References: K&R1 Sec. 5.3 pp. 93-6; K&R2 Sec. 5.3 p. 99; ANSI
	Sec., Sec., Sec. 3.3.6; ISO Sec.,
	Sec., Sec. 6.3.6; H&S Sec. 5.4.1 p. 124.

6.4:	Then why are array and pointer declarations interchangeable as
	function formal parameters?

A:	It's supposed to be a convenience.

	Since arrays decay immediately into pointers, an array is never
	actually passed to a function.  Allowing pointer parameters to
	be declared as arrays is a simply a way of making it look as
	though the array was being passed -- a programmer may wish to
	emphasize that a parameter is traditionally treated as if it
	were an array, or that an array (strictly speaking, the address)
	is traditionally passed.  As a convenience, therefore, any
	parameter declarations which "look like" arrays, e.g.

		char a[];
		{ ... }

	are treated by the compiler as if they were pointers, since that
	is what the function will receive if an array is passed:

		char *a;
		{ ... }

	This conversion holds only within function formal parameter
	declarations, nowhere else.  If the conversion bothers you,
	avoid it; many people have concluded that the confusion it
	causes outweighs the small advantage of having the declaration
	"look like" the call or the uses within the function.

	See also question 6.21.

	References: K&R1 Sec. 5.3 p. 95, Sec. A10.1 p. 205; K&R2
	Sec. 5.3 p. 100, Sec. A8.6.3 p. 218, Sec. A10.1 p. 226; ANSI
	Sec., Sec. 3.7.1, Sec. 3.9.6; ISO Sec.,
	Sec. 6.7.1, Sec. 6.9.6; H&S Sec. 9.3 p. 271; CT&P Sec. 3.3 pp.

6.7:	How can an array be an lvalue, if you can't assign to it?

A:	The ANSI C Standard defines a "modifiable lvalue," which an
	array is not.

	References: ANSI Sec.; ISO Sec.; Rationale
	Sec.; H&S Sec. 7.1 p. 179.

6.8:	Practically speaking, what is the difference between arrays and

A:	Arrays automatically allocate space, but can't be relocated or
	resized.  Pointers must be explicitly assigned to point to
	allocated space (perhaps using malloc), but can be reassigned
	(i.e. pointed at different objects) at will, and have many other
	uses besides serving as the base of blocks of memory.

	Due to the so-called equivalence of arrays and pointers (see
	question 6.3), arrays and pointers often seem interchangeable,
	and in particular a pointer to a block of memory assigned by
	malloc is frequently treated (and can be referenced using [])
	exactly as if it were a true array.  See questions 6.14 and
	6.16.  (Be careful with sizeof, though.)

	See also questions 1.32 and 20.14.

6.9:	Someone explained to me that arrays were really just constant

A:	This is a bit of an oversimplification.  An array name is
	"constant" in that it cannot be assigned to, but an array is
	*not* a pointer, as the discussion and pictures in question 6.2
	should make clear.  See also questions 6.3 and 6.8.

6.11:	I came across some "joke" code containing the "expression"
	5["abcdef"] .  How can this be legal C?

A:	Yes, Virginia, array subscripting is commutative in C.  This
	curious fact follows from the pointer definition of array
	subscripting, namely that a[e] is identical to *((a)+(e)), for
	*any* two expressions a and e, as long as one of them is a
	pointer expression and one is integral.  This unsuspected
	commutativity is often mentioned in C texts as if it were
	something to be proud of, but it finds no useful application
	outside of the Obfuscated C Contest (see question 20.36).

	References: Rationale Sec.; H&S Sec. 5.4.1 p. 124,
	Sec. 7.4.1 pp. 186-7.

6.12:	Since array references decay into pointers, if arr is an array,
	what's the difference between arr and &arr?

A:	The type.

	In Standard C, &arr yields a pointer, of type pointer-to-array-
	of-T, to the entire array.  (In pre-ANSI C, the & in &arr
	generally elicited a warning, and was generally ignored.)  Under
	all C compilers, a simple reference (without an explicit &) to
	an array yields a pointer, of type pointer-to-T, to the array's
	first element.  (See also questions 6.3, 6.13, and 6.18.)

	References: ANSI Sec., Sec.; ISO Sec.,
	Sec.; Rationale Sec.; H&S Sec. 7.5.6 p. 198.

6.13:	How do I declare a pointer to an array?

A:	Usually, you don't want to.  When people speak casually of a
	pointer to an array, they usually mean a pointer to its first

	Instead of a pointer to an array, consider using a pointer to
	one of the array's elements.  Arrays of type T decay into
	pointers to type T (see question 6.3), which is convenient;
	subscripting or incrementing the resultant pointer will access
	the individual members of the array.  True pointers to arrays,
	when subscripted or incremented, step over entire arrays, and
	are generally useful only when operating on arrays of arrays, if
	at all.  (See question 6.18.)

	If you really need to declare a pointer to an entire array, use
	something like "int (*ap)[N];" where N is the size of the array.
	(See also question 1.21.)  If the size of the array is unknown,
	N can in principle be omitted, but the resulting type, "pointer
	to array of unknown size," is useless.

	See also question 6.12 above.

	References: ANSI Sec.; ISO Sec.

6.14:	How can I set an array's size at compile time?
	How can I avoid fixed-sized arrays?

A:	The equivalence between arrays and pointers (see question 6.3)
	allows a pointer to malloc'ed memory to simulate an array
	quite effectively.  After executing

		int *dynarray = (int *)malloc(10 * sizeof(int));

	(and if the call to malloc() succeeds), you can reference
	dynarray[i] (for i from 0 to 9) just as if dynarray were a
	conventional, statically-allocated array (int a[10]).  See also
	question 6.16.

6.15:	How can I declare local arrays of a size matching a passed-in

A:	You can't, in C.  Array dimensions must be compile-time
	constants.  (gcc provides parameterized arrays as an extension.)
	You'll have to use malloc(), and remember to call free() before
	the function returns.  See also questions 6.14, 6.16, 6.19,
	7.22, and maybe 7.32.

	References: ANSI Sec. 3.4, Sec.; ISO Sec. 6.4,

6.16:	How can I dynamically allocate a multidimensional array?

A:	It is usually best to allocate an array of pointers, and then
	initialize each pointer to a dynamically-allocated "row."  Here
	is a two-dimensional example:


		int **array1 = (int **)malloc(nrows * sizeof(int *));
		for(i = 0; i < nrows; i++)
			array1[i] = (int *)malloc(ncolumns * sizeof(int));

	(In real code, of course, all of malloc's return values would
	be checked.)

	You can keep the array's contents contiguous, while making later
	reallocation of individual rows difficult, with a bit of
	explicit pointer arithmetic:

		int **array2 = (int **)malloc(nrows * sizeof(int *));
		array2[0] = (int *)malloc(nrows * ncolumns * sizeof(int));
		for(i = 1; i < nrows; i++)
			array2[i] = array2[0] + i * ncolumns;

	In either case, the elements of the dynamic array can be
	accessed with normal-looking array subscripts: arrayx[i][j] (for
	0 <= i <= NROWS and 0 <= j <= NCOLUMNS).

	If the double indirection implied by the above schemes is for
	some reason unacceptable, you can simulate a two-dimensional
	array with a single, dynamically-allocated one-dimensional

		int *array3 = (int *)malloc(nrows * ncolumns * sizeof(int));

	However, you must now perform subscript calculations manually,
	accessing the i,jth element with array3[i * ncolumns + j].  (A
	macro could hide the explicit calculation, but invoking it would
	require parentheses and commas which wouldn't look exactly like
	multidimensional array syntax, and the macro would need access
	to at least one of the dimensions, as well.  See also question

	Finally, you could use pointers to arrays:

		int (*array4)[NCOLUMNS] =
			(int (*)[NCOLUMNS])malloc(nrows * sizeof(*array4));

	but the syntax starts getting horrific and at most one dimension
	may be specified at run time.

	With all of these techniques, you may of course need to remember
	to free the arrays (which may take several steps; see question
	7.23) when they are no longer needed, and you cannot necessarily
	intermix dynamically-allocated arrays with conventional,
	statically-allocated ones (see question 6.20, and also question

	All of these techniques can also be extended to three or more

6.17:	Here's a neat trick: if I write

		int realarray[10];
		int *array = &realarray[-1];

	I can treat "array" as if it were a 1-based array.

A:	Although this technique is attractive (and was used in old
	editions of the book _Numerical Recipes in C_), it does not
	conform to the C standards.  Pointer arithmetic is defined only
	as long as the pointer points within the same allocated block of
	memory, or to the imaginary "terminating" element one past it;
	otherwise, the behavior is undefined, *even if the pointer is
	not dereferenced*.  The code above could fail if, while
	subtracting the offset, an illegal address were generated
	(perhaps because the address tried to "wrap around" past the
	beginning of some memory segment).

	References: K&R2 Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3, Sec. A7.7
	pp. 205-6; ANSI Sec. 3.3.6; ISO Sec. 6.3.6; Rationale

6.18:	My compiler complained when I passed a two-dimensional array to
	a function expecting a pointer to a pointer.

A:	The rule (see question 6.3) by which arrays decay into pointers
	is not applied recursively.  An array of arrays (i.e. a two-
	dimensional array in C) decays into a pointer to an array, not a
	pointer to a pointer.  Pointers to arrays can be confusing, and
	must be treated carefully; see also question 6.13.  (The
	confusion is heightened by the existence of incorrect compilers,
	including some old versions of pcc and pcc-derived lints, which
	improperly accept assignments of multi-dimensional arrays to
	multi-level pointers.)

	If you are passing a two-dimensional array to a function:

		int array[NROWS][NCOLUMNS];

	the function's declaration must match:

		f(int a[][NCOLUMNS])
		{ ... }


		f(int (*ap)[NCOLUMNS])  /* ap is a pointer to an array */
		{ ... }

	In the first declaration, the compiler performs the usual
	implicit parameter rewriting of "array of array" to "pointer to
	array" (see questions 6.3 and 6.4); in the second form the
	pointer declaration is explicit.  Since the called function does
	not allocate space for the array, it does not need to know the
	overall size, so the number of rows, NROWS, can be omitted.  The
	"shape" of the array is still important, so the column dimension
	NCOLUMNS (and, for three- or more dimensional arrays, the
	intervening ones) must be retained.

	If a function is already declared as accepting a pointer to a
	pointer, it is probably meaningless to pass a two-dimensional
	array directly to it.

	See also questions 6.12 and 6.15.

	References: K&R1 Sec. 5.10 p. 110; K&R2 Sec. 5.9 p. 113; H&S
	Sec. 5.4.3 p. 126.

6.19:	How do I write functions which accept two-dimensional arrays
	when the "width" is not known at compile time?

A:	It's not easy.  One way is to pass in a pointer to the [0][0]
	element, along with the two dimensions, and simulate array
	subscripting "by hand:"

		f2(aryp, nrows, ncolumns)
		int *aryp;
		int nrows, ncolumns;
		{ ... array[i][j] is accessed as aryp[i * ncolumns + j] ... }

	This function could be called with the array from question 6.18

		f2(&array[0][0], NROWS, NCOLUMNS);

	It must be noted, however, that a program which performs
	multidimensional array subscripting "by hand" in this way is not
	in strict conformance with the ANSI C Standard; according to an
	official interpretation, the behavior of accessing
	(&array[0][0])[x] is not defined for x >= NCOLUMNS.

	gcc allows local arrays to be declared having sizes which are
	specified by a function's arguments, but this is a nonstandard

	When you want to be able to use a function on multidimensional
	arrays of various sizes, one solution is to simulate all the
	arrays dynamically, as in question 6.16.

	See also questions 6.18, 6.20, and 6.15.

	References: ANSI Sec. 3.3.6; ISO Sec. 6.3.6.

6.20:	How can I use statically- and dynamically-allocated
	multidimensional arrays interchangeably when passing them to

A:	There is no single perfect method.  Given the declarations

		int array[NROWS][NCOLUMNS];
		int **array1;			/* ragged */
		int **array2;			/* contiguous */
		int *array3;			/* "flattened" */
		int (*array4)[NCOLUMNS];

	with the pointers initialized as in the code fragments in
	question 6.16, and functions declared as

		f1(int a[][NCOLUMNS], int nrows, int ncolumns);
		f2(int *aryp, int nrows, int ncolumns);
		f3(int **pp, int nrows, int ncolumns);

	where f1() accepts a conventional two-dimensional array, f2()
	accepts a "flattened" two-dimensional array, and f3() accepts a
	pointer-to-pointer, simulated array (see also questions 6.18 and
	6.19), the following calls should work as expected:

		f1(array, NROWS, NCOLUMNS);
		f1(array4, nrows, NCOLUMNS);
		f2(&array[0][0], NROWS, NCOLUMNS);
		f2(*array, NROWS, NCOLUMNS);
		f2(*array2, nrows, ncolumns);
		f2(array3, nrows, ncolumns);
		f2(*array4, nrows, NCOLUMNS);
		f3(array1, nrows, ncolumns);
		f3(array2, nrows, ncolumns);

	The following two calls would probably work on most systems, but
	involve questionable casts, and work only if the dynamic
	ncolumns matches the static NCOLUMNS:

		f1((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);
		f1((int (*)[NCOLUMNS])array3, nrows, ncolumns);

	It must again be noted that passing &array[0][0] (or,
	equivalently, *array) to f2() is not strictly conforming; see
	question 6.19.

	If you can understand why all of the above calls work and are
	written as they are, and if you understand why the combinations
	that are not listed would not work, then you have a *very* good
	understanding of arrays and pointers in C.

	Rather than worrying about all of this, one approach to using
	multidimensional arrays of various sizes is to make them *all*
	dynamic, as in question 6.16.  If there are no static
	multidimensional arrays -- if all arrays are allocated like
	array1 or array2 in question 6.16 -- then all functions can be
	written like f3().

6.21:	Why doesn't sizeof properly report the size of an array when the
	array is a parameter to a function?

A:	The compiler pretends that the array parameter was declared as a
	pointer (see question 6.4), and sizeof reports the size of the

	References: H&S Sec. 7.5.2 p. 195.

Section 7. Memory Allocation

7.1:	Why doesn't this fragment work?

		char *answer;
		printf("Type something:\n");
		printf("You typed \"%s\"\n", answer);

A:	The pointer variable answer(), which is handed to gets() as the
	location into which the response should be stored, has not been
	set to point to any valid storage.  That is, we cannot say where
	the pointer answer() points.  (Since local variables are not
	initialized, and typically contain garbage, it is not even
	guaranteed that answer() starts out as a null pointer.  See
	questions 1.30 and 5.1.)

	The simplest way to correct the question-asking program is to
	use a local array, instead of a pointer, and let the compiler
	worry about allocation:


		char answer[100], *p;
		printf("Type something:\n");
		fgets(answer, sizeof answer, stdin);
		if((p = strchr(answer, '\n')) != NULL)
			*p = '\0';
		printf("You typed \"%s\"\n", answer);

	This example also uses fgets() instead of gets(), so that the
	end of the array cannot be overwritten.  (See question 12.23.
	Unfortunately for this example, fgets() does not automatically
	delete the trailing \n, as gets() would.)  It would also be
	possible to use malloc() to allocate the answer buffer.

7.2:	I can't get strcat() to work.  I tried

		char *s1 = "Hello, ";
		char *s2 = "world!";
		char *s3 = strcat(s1, s2);

	but I got strange results.

A:	As in question 7.1 above, the main problem here is that space
	for the concatenated result is not properly allocated.  C does
	not provide an automatically-managed string type.  C compilers
	only allocate memory for objects explicitly mentioned in the
	source code (in the case of "strings," this includes character
	arrays and string literals).  The programmer must arrange for
	sufficient space for the results of run-time operations such as
	string concatenation, typically by declaring arrays, or by
	calling malloc().

	strcat() performs no allocation; the second string is appended
	to the first one, in place.  Therefore, one fix would be to
	declare the first string as an array:

		char s1[20] = "Hello, ";

	Since strcat() returns the value of its first argument (s1, in
	this case), the variable s3 is superfluous.

	The original call to strcat() in the question actually has two
	problems: the string literal pointed to by s1, besides not being
	big enough for any concatenated text, is not necessarily
	writable at all.  See question 1.32.

	References: CT&P Sec. 3.2 p. 32.

7.3:	But the man page for strcat() says that it takes two char *'s as
	arguments.  How am I supposed to know to allocate things?

A:	In general, when using pointers you *always* have to consider
	memory allocation, if only to make sure that the compiler is
	doing it for you.  If a library function's documentation does
	not explicitly mention allocation, it is usually the caller's

	The Synopsis section at the top of a Unix-style man page or in
	the ANSI C standard can be misleading.  The code fragments
	presented there are closer to the function definitions used by
	an implementor than the invocations used by the caller.  In
	particular, many functions which accept pointers (e.g. to
	structures or strings) are usually called with the address of
	some object (a structure, or an array -- see questions 6.3 and
	6.4).  Other common examples are time() (see question 13.12)
	and stat().

7.5:	I have a function that is supposed to return a string, but when
	it returns to its caller, the returned string is garbage.

A:	Make sure that the pointed-to memory is properly allocated.  The
	returned pointer should be to a statically-allocated buffer, or
	to a buffer passed in by the caller, or to memory obtained with
	malloc(), but *not* to a local (automatic) array.  In other
	words, never do something like

		char *itoa(int n)
			char retbuf[20];		/* WRONG */
			sprintf(retbuf, "%d", n);
			return retbuf;			/* WRONG */

	One fix (which is imperfect, especially if the function in
	question is called recursively, or if several of its return
	values are needed simultaneously) would be to declare the return
	buffer as

			static char retbuf[20];

	See also questions 12.21 and 20.1.

	References: ANSI Sec.; ISO Sec.

7.6:	Why am I getting "warning: assignment of pointer from integer
	lacks a cast" for calls to malloc()?

A:	Have you #included , or otherwise arranged for
	malloc() to be declared properly?

	References: H&S Sec. 4.7 p. 101.

7.7:	Why does some code carefully cast the values returned by malloc
	to the pointer type being allocated?

A:	Before ANSI/ISO Standard C introduced the void * generic pointer
	type, these casts were typically required to silence warnings
	(and perhaps induce conversions) when assigning between
	incompatible pointer types.  (Under ANSI/ISO Standard C, these
	casts are no longer necessary.)

	References: H&S Sec. 16.1 pp. 386-7.

7.8:	I see code like

		char *p = malloc(strlen(s) + 1);
		strcpy(p, s);

	Shouldn't that be malloc((strlen(s) + 1) * sizeof(char))?

A:	It's never necessary to multiply by sizeof(char), since
	sizeof(char) is, by definition, exactly 1.  (On the other hand,
	multiplying by sizeof(char) doesn't hurt, and may help by
	introducing a size_t into the expression.)  See also question

	References: ANSI Sec.; ISO Sec.; H&S Sec. 7.5.2
	p. 195.

7.14:	I've heard that some operating systems don't actually allocate
	malloc'ed memory until the program tries to use it.  Is this

A:	It's hard to say.  The Standard doesn't say that systems can act
	this way, but it doesn't explicitly say that they can't, either.

	References: ANSI Sec. 4.10.3; ISO Sec. 7.10.3.

7.16:	I'm allocating a large array for some numeric work, using the

		double *array = malloc(256 * 256 * sizeof(double));

	malloc() isn't returning null, but the program is acting
	strangely, as if it's overwriting memory, or malloc() isn't
	allocating as much as I asked for, or something.

A:	Notice that 256 x 256 is 65,536, which will not fit in a 16-bit
	int, even before you multiply it by sizeof(double).  If you need
	to allocate this much memory, you'll have to be careful.  If
	size_t (the type accepted by malloc()) is a 32-bit type on your
	machine, but int is 16 bits, you might be able to get away with
	writing 256 * (256 * sizeof(double)) (see question 3.14).
	Otherwise, you'll have to break your data structure up into
	smaller chunks, or use a 32-bit machine, or use some nonstandard
	memory allocation routines.  See also question 19.23.

7.17:	I've got 8 meg of memory in my PC.  Why can I only seem to
	malloc() 640K or so?

A:	Under the segmented architecture of PC compatibles, it can be
	difficult to use more than 640K with any degree of transparency.
	See also question 19.23.

7.19:	My program is crashing, apparently somewhere down inside malloc,
	but I can't see anything wrong with it.

A:	It is unfortunately very easy to corrupt malloc's internal data
	structures, and the resulting problems can be stubborn.  The
	most common source of problems is writing more to a malloc'ed
	region than it was allocated to hold; a particularly common bug
	is to malloc(strlen(s)) instead of strlen(s) + 1.  Other
	problems may involve using pointers to freed storage, freeing
	pointers twice, freeing pointers not obtained from malloc, or
	trying to realloc a null pointer (see question 7.30).

	See also questions 7.26, 16.8, and 18.2.

7.20:	You can't use dynamically-allocated memory after you free it,
	can you?

A:	No.  Some early documentation for malloc() stated that the
	contents of freed memory were "left undisturbed," but this ill-
	advised guarantee was never universal and is not required by the
	C Standard.

	Few programmers would use the contents of freed memory
	deliberately, but it is easy to do so accidentally.  Consider
	the following (correct) code for freeing a singly-linked list:

		struct list *listp, *nextp;
		for(listp = base; listp != NULL; listp = nextp) {
			nextp = listp->next;
			free((void *)listp);

	and notice what would happen if the more-obvious loop iteration
	expression listp = listp->next were used, without the temporary
	nextp pointer.

	References: K&R2 Sec. 7.8.5 p. 167; ANSI Sec. 4.10.3; ISO
	Sec. 7.10.3; Rationale Sec.; H&S Sec. 16.2 p. 387; CT&P
	Sec. 7.10 p. 95.

7.21:	Why isn't a pointer null after calling free()?
	How unsafe is it to use (assign, compare) a pointer value after
	it's been freed?

A:	When you call free(), the memory pointed to by the passed
	pointer is freed, but the value of the pointer in the caller
	remains unchanged, because C's pass-by-value semantics mean that
	called functions never permanently change the values of their
	arguments.  (See also question 4.8.)

	A pointer value which has been freed is, strictly speaking,
	invalid, and *any* use of it, even if is not dereferenced can
	theoretically lead to trouble, though as a quality of
	implementation issue, most implementations will probably not go
	out of their way to generate exceptions for innocuous uses of
	invalid pointers.

	References: ANSI Sec. 4.10.3; ISO Sec. 7.10.3; Rationale

7.22:	When I call malloc() to allocate memory for a local pointer, do
	I have to explicitly free() it?

A:	Yes.  Remember that a pointer is different from what it points
	to.  Local variables are deallocated when the function returns,
	but in the case of a pointer variable, this means that the
	pointer is deallocated, *not* what it points to.  Memory
	allocated with malloc() always persists until you explicitly
	free it.  In general, for every call to malloc(), there should
	be a corresponding call to free().

7.23:	I'm allocating structures which contain pointers to other
	dynamically-allocated objects.  When I free a structure, do I
	have to free each subsidiary pointer first?

A:	Yes.  In general, you must arrange that each pointer returned
	from malloc() be individually passed to free(), exactly once (if
	it is freed at all).

	A good rule of thumb is that for each call to malloc() in a
	program, you should be able to point at the call to free() which
	frees the memory allocated by that malloc() call.

	See also question 7.24.

7.24:	Must I free allocated memory before the program exits?

A:	You shouldn't have to.  A real operating system definitively
	reclaims all memory when a program exits.  Nevertheless, some
	personal computers are said not to reliably recover memory, and
	all that can be inferred from the ANSI/ISO C Standard is that
	this is a "quality of implementation issue."

	References: ANSI Sec.; ISO Sec.

7.25:	I have a program which mallocs and later frees a lot of memory,
	but memory usage (as reported by ps) doesn't seem to go back

A:	Most implementations of malloc/free do not return freed memory
	to the operating system (if there is one), but merely make it
	available for future malloc() calls within the same program.

7.26:	How does free() know how many bytes to free?

A:	The malloc/free implementation remembers the size of each block
	allocated and returned, so it is not necessary to remind it of
	the size when freeing.

7.27:	So can I query the malloc package to find out how big an
	allocated block is?

A:	Not portably.

7.30:	Is it legal to pass a null pointer as the first argument to
	realloc()?  Why would you want to?

A:	ANSI C sanctions this usage (and the related realloc(..., 0),
	which frees), although several earlier implementations do not
	support it, so it may not be fully portable.  Passing an
	initially-null pointer to realloc() can make it easier to write
	a self-starting incremental allocation algorithm.

	References: ANSI Sec.; ISO Sec.; H&S Sec. 16.3
	p. 388.

7.31:	What's the difference between calloc() and malloc()?  Is it safe
	to take advantage of calloc's zero-filling?  Does free() work
	on memory allocated with calloc(), or do you need a cfree()?

A:	calloc(m, n) is essentially equivalent to

		p = malloc(m * n);
		memset(p, 0, m * n);

	The zero fill is all-bits-zero, and does *not* therefore
	guarantee useful null pointer values (see section 5 of this
	list) or floating-point zero values.  free() is properly used to
	free the memory allocated by calloc().

	References: ANSI Sec. 4.10.3 to; ISO Sec. 7.10.3 to; H&S Sec. 16.1 p. 386, Sec. 16.2 p. 386; PCS Sec. 11
	pp. 141,142.

7.32:	What is alloca() and why is its use discouraged?

A:	alloca() allocates memory which is automatically freed when the
	function which called alloca() returns.  That is, memory
	allocated with alloca is local to a particular function's "stack
	frame" or context.

	alloca() cannot be written portably, and is difficult to
	implement on machines without a conventional stack.  Its use is
	problematical (and the obvious implementation on a stack-based
	machine fails) when its return value is passed directly to
	another function, as in fgets(alloca(100), 100, stdin).

	For these reasons, alloca() is not Standard and cannot be used
	in programs which must be widely portable, no matter how useful
	it might be.

	See also question 7.22.

	References: Rationale Sec. 4.10.3.

Section 8. Characters and Strings

8.1:	Why doesn't

		strcat(string, '!');


A:	There is a very real difference between characters and strings,
	and strcat() concatenates *strings*.

	Characters in C are represented by small integers corresponding
	to their character set values (see also question 8.6 below).
	Strings are represented by arrays of characters; you usually
	manipulate a pointer to the first character of the array.  It is
	never correct to use one when the other is expected.  To append
	a ! to a string, use

		strcat(string, "!");

	See also questions 1.32, 7.2, and 16.6.

	References: CT&P Sec. 1.5 pp. 9-10.

8.2:	I'm checking a string to see if it matches a particular value.
	Why isn't this code working?

		char *string;
		if(string == "value") {
			/* string matches "value" */

A:	Strings in C are represented as arrays of characters, and C
	never manipulates (assigns, compares, etc.) arrays as a whole.
	The == operator in the code fragment above compares two pointers
	-- the value of the pointer variable string and a pointer to the
	string literal "value" -- to see if they are equal, that is, if
	they point to the same place.  They probably don't, so the
	comparison never succeeds.

	To compare two strings, you generally use the library function

		if(strcmp(string, "value") == 0) {
			/* string matches "value" */

8.3:	If I can say

		char a[] = "Hello, world!";

	why can't I say

		char a[14];
		a = "Hello, world!";

A:	Strings are arrays, and you can't assign arrays directly.  Use
	strcpy() instead:

		strcpy(a, "Hello, world!");

	See also questions 1.32, 4.2, and 7.2.

8.6:	How can I get the numeric (character set) value corresponding to
	a character, or vice versa?

A:	In C, characters are represented by small integers corresponding
	to their values (in the machine's character set), so you don't
	need a conversion routine: if you have the character, you have
	its value.

8.9:	I think something's wrong with my compiler: I just noticed that
	sizeof('a') is 2, not 1 (i.e. not sizeof(char)).

A:	Perhaps surprisingly, character constants in C are of type int,
	so sizeof('a') is sizeof(int) (though it's different in C++).
	See also question 7.8.

	References: ANSI Sec.; ISO Sec.; H&S Sec. 2.7.3
	p. 29.

Section 9. Boolean Expressions

9.1:	What is the right type to use for Boolean values in C?  Why
	isn't it a standard type?  Should I use #defines or enums for
	the true and false values?

A:	C does not provide a standard Boolean type, in part because
	picking one involves a space/time tradeoff which can best be
	decided by the programmer.  (Using an int may be faster, while
	using char may save data space.  Smaller types may make the
	generated code bigger or slower, though, if they require lots of
	conversions to and from int.)

	The choice between #defines and enumeration constants for the
	true/false values is arbitrary and not terribly interesting (see
	also questions 2.22 and 17.10).  Use any of

		#define TRUE  1			#define YES 1
		#define FALSE 0			#define NO  0

		enum bool {false, true};	enum bool {no, yes};

	or use raw 1 and 0, as long as you are consistent within one
	program or project.  (An enumeration may be preferable if your
	debugger shows the names of enumeration constants when examining

	Some people prefer variants like

		#define TRUE (1==1)
		#define FALSE (!TRUE)

	or define "helper" macros such as

		#define Istrue(e) ((e) != 0)

	These don't buy anything (see question 9.2 below; see also
	questions 5.12 and 10.2).

9.2:	Isn't #defining TRUE to be 1 dangerous, since any nonzero value
	is considered "true" in C?  What if a built-in logical or
	relational operator "returns" something other than 1?

A:	It is true (sic) that any nonzero value is considered true in C,
	but this applies only "on input", i.e. where a Boolean value is
	expected.  When a Boolean value is generated by a built-in
	operator, it is guaranteed to be 1 or 0.  Therefore, the test

		if((a == b) == TRUE)

	would work as expected (as long as TRUE is 1), but it is
	obviously silly.  In general, explicit tests against TRUE and
	FALSE are inappropriate, because some library functions (notably
	isupper(), isalpha(), etc.) return, on success, a nonzero value
	which is *not* necessarily 1.  (Besides, if you believe that
	"if((a == b) == TRUE)" is an improvement over "if(a == b)", why
	stop there?  Why not use "if(((a == b) == TRUE) == TRUE)"?)  A
	good rule of thumb is to use TRUE and FALSE (or the like) only
	for assignment to a Boolean variable or function parameter, or
	as the return value from a Boolean function, but never in a

	The preprocessor macros TRUE and FALSE (and, of course, NULL)
	are used for code readability, not because the underlying values
	might ever change.  (See also questions 5.3 and 5.10.)

	On the other hand, Boolean values and definitions can evidently
	be confusing, and some programmers feel that TRUE and FALSE
	macros only compound the confusion.  (See also question 5.9.)

	References: K&R1 Sec. 2.6 p. 39, Sec. 2.7 p. 41; K&R2 Sec. 2.6
	p. 42, Sec. 2.7 p. 44, Sec. A7.4.7 p. 204, Sec. A7.9 p. 206;
	ANSI Sec., Sec. 3.3.8, Sec. 3.3.9, Sec. 3.3.13,
	Sec. 3.3.14, Sec. 3.3.15, Sec., Sec. 3.6.5; ISO
	Sec., Sec. 6.3.8, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14,
	Sec. 6.3.15, Sec., Sec. 6.6.5; H&S Sec. 7.5.4 pp. 196-7,
	Sec. 7.6.4 pp. 207-8, Sec. 7.6.5 pp. 208-9, Sec. 7.7 pp. 217-8,
	Sec. 7.8 pp. 218-9, Sec. 8.5 pp. 238-9, Sec. 8.6 pp. 241-4;
	"What the Tortoise Said to Achilles".

9.3:	Is if(p), where p is a pointer, a valid conditional?

A:	Yes.  See question 5.3.

Section 10. C Preprocessor

10.2:	Here are some cute preprocessor macros:

		#define begin	{
		#define end	}

	What do y'all think?

A:	Bleah.  See also section 17.

10.3:	How can I write a generic macro to swap two values?

A:	There is no good answer to this question.  If the values are
	integers, a well-known trick using exclusive-OR could perhaps be
	used, but it will not work for floating-point values or
	pointers, or if the two values are the same variable (and the
	"obvious" supercompressed implementation for integral types
	a^=b^=a^=b is illegal due to multiple side-effects; see question
	3.2).  If the macro is intended to be used on values of
	arbitrary type (the usual goal), it cannot use a temporary,
	since it does not know what type of temporary it needs (and
	would have a hard time naming it if it did), and standard C does
	not provide a typeof operator.

	The best all-around solution is probably to forget about using a
	macro, unless you're willing to pass in the type as a third

10.4:	What's the best way to write a multi-statement macro?

A:	The usual goal is to write a macro that can be invoked as if it
	were a statement consisting of a single function call.  This
	means that the "caller" will be supplying the final semicolon,
	so the macro body should not.  The macro body cannot therefore
	be a simple brace-enclosed compound statement, because syntax
	errors would result if it were invoked (apparently as a single
	statement, but with a resultant extra semicolon) as the if
	branch of an if/else statement with an explicit else clause.

	The traditional solution, therefore, is to use

		#define MACRO(arg1, arg2) do {	\
			/* declarations */	\
			stmt1;			\
			stmt2;			\
			/* ... */		\
			} while(0)	/* (no trailing ; ) */

	When the caller appends a semicolon, this expansion becomes a
	single statement regardless of context.  (An optimizing compiler
	will remove any "dead" tests or branches on the constant
	condition 0, although lint may complain.)

	If all of the statements in the intended macro are simple
	expressions, with no declarations or loops, another technique is
	to write a single, parenthesized expression using one or more
	comma operators.  (For an example, see the first DEBUG() macro
	in question 10.26.)  This technique also allows a value to be

	References: H&S Sec. 3.3.2 p. 45; CT&P Sec. 6.3 pp. 82-3.

10.6:	I'm splitting up a program into multiple source files for the
	first time, and I'm wondering what to put in .c files and what
	to put in .h files.  (What does ".h" mean, anyway?)

A:	As a general rule, you should put these things in header (.h)

		macro definitions (preprocessor #defines)
		structure, union, and enumeration declarations
		typedef declarations
		external function declarations (see also question 1.11)
		global variable declarations

	It's especially important to put a declaration or definition in
	a header file when it will be shared between several other
	files.  (In particular, never put external function prototypes
	in .c files.  See also question 1.7.)

	On the other hand, when a definition or declaration should
	remain private to one source file, it's fine to leave it there.

	See also questions 1.7 and 10.7.

	References: K&R2 Sec. 4.5 pp. 81-2; H&S Sec. 9.2.3 p. 267; CT&P
	Sec. 4.6 pp. 66-7.

10.7:	Is it acceptable for one header file to #include another?

A:	It's a question of style, and thus receives considerable debate.
	Many people believe that "nested #include files" are to be
	avoided: the prestigious Indian Hill Style Guide (see question
	17.9) disparages them; they can make it harder to find relevant
	definitions; they can lead to multiple-definition errors if a
	file is #included twice; and they make manual Makefile
	maintenance very difficult.  On the other hand, they make it
	possible to use header files in a modular way (a header file can
	#include what it needs itself, rather than requiring each
	#includer to do so); a tool like grep (or a tags file) makes it
	easy to find definitions no matter where they are; a popular
	trick along the lines of:

		...header file contents...

	(where a different bracketing macro name is used for each header
	file) makes a header file "idempotent" so that it can safely be
	#included multiple times; and automated Makefile maintenance
	tools (which are a virtual necessity in large projects anyway;
	see question 18.1) handle dependency generation in the face of
	nested #include files easily.  See also question 17.10.

	References: Rationale Sec. 4.1.2.

10.8:	Where are header ("#include") files searched for?

A:	The exact behavior is implementation-defined (which means that
	it is supposed to be documented; see question 11.33).
	Typically, headers named with <> syntax are searched for in one
	or more standard places.  Header files named with "" syntax are
	first searched for in the "current directory," then (if not
	found) in the same standard places.

	Traditionally (especially under Unix compilers), the current
	directory is taken to be the directory containing the file
	containing the #include directive.  Under other compilers,
	however, the current directory (if any) is the directory in
	which the compiler was initially invoked.  Check your compiler

	References: K&R2 Sec. A12.4 p. 231; ANSI Sec. 3.8.2; ISO
	Sec. 6.8.2; H&S Sec. 3.4 p. 55.

10.9:	I'm getting strange syntax errors on the very first declaration
	in a file, but it looks fine.

A:	Perhaps there's a missing semicolon at the end of the last
	declaration in the last header file you're #including.  See also
	questions 2.18 and 11.29.

10.11:	I seem to be missing the system header file .  Can
	someone send me a copy?

A:	Standard headers exist in part so that definitions appropriate
	to your compiler, operating system, and processor can be
	supplied.  You cannot just pick up a copy of someone else's
	header file and expect it to work, unless that person is using
	exactly the same environment.  Ask your compiler vendor why the
	file was not provided (or to send a replacement copy).

10.12:	How can I construct preprocessor #if expressions which compare

A:	You can't do it directly; preprocessor #if arithmetic uses only
	integers.  You can #define several manifest constants, however,
	and implement  conditionals on those.

	See also question 20.17.

	References: K&R2 Sec. 4.11.3 p. 91; ANSI Sec. 3.8.1; ISO
	Sec. 6.8.1; H&S Sec. 7.11.1 p. 225.

10.13:	Does the sizeof operator work in preprocessor #if directives?

A:	No.  Preprocessing happens during an earlier phase of
	compilation, before type names have been parsed.  Instead of
	sizeof, consider using the predefined constants in ANSI's
	, if applicable, or perhaps a "configure" script.
	(Better yet, try to write code which is inherently insensitive
	to type sizes.)

	References: ANSI Sec., Sec. 3.8.1 footnote 83; ISO
	Sec., Sec. 6.8.1; H&S Sec. 7.11.1 p. 225.

10.14:	Can I use an #ifdef in a #define line, to define something two
	different ways?

A:	No.  You can't "run the preprocessor on itself," so to speak.
	What you can do is use one of two completely separate #define
	lines, depending on the #ifdef setting.

	References: ANSI Sec. 3.8.3, Sec.; ISO Sec. 6.8.3,
	Sec.; H&S Sec. 3.2 pp. 40-1.

10.15:	Is there anything like an #ifdef for typedefs?

A:	Unfortunately, no.  (See also question 10.13.)

	References: ANSI Sec., Sec. 3.8.1 footnote 83; ISO
	Sec., Sec. 6.8.1; H&S Sec. 7.11.1 p. 225.

10.16:	How can I use a preprocessor #if expression to tell if a machine
	is big-endian or little-endian?

A:	You probably can't.  (Preprocessor arithmetic uses only long
	integers, and there is no concept of addressing.  )  Are you
	sure you need to know the machine's endianness explicitly?
	Usually it's better to write code which doesn't care ).  See
	also question 20.9.

	References: ANSI Sec. 3.8.1; ISO Sec. 6.8.1; H&S Sec. 7.11.1
	p. 225.

10.18:	I inherited some code which contains far too many #ifdef's for
	my taste.  How can I preprocess the code to leave only one
	conditional compilation set, without running it through the
	preprocessor and expanding all of the #include's and #define's
	as well?

A:	There are programs floating around called unifdef, rmifdef, and
	scpp ("selective C preprocessor") which do exactly this.  See
	question 18.16.

10.19:	How can I list all of the pre#defined identifiers?

A:	There's no standard way, although it is a common need.  If the
	compiler documentation is unhelpful, the most expedient way is
	probably to extract printable strings from the compiler or
	preprocessor executable with something like the Unix strings
	utility.  Beware that many traditional system-specific
	pre#defined identifiers (e.g. "unix") are non-Standard (because
	they clash with the user's namespace) and are being removed or

10.20:	I have some old code that tries to construct identifiers with a
	macro like

		#define Paste(a, b) a/**/b

	but it doesn't work any more.

A:	It was an undocumented feature of some early preprocessor
	implementations (notably John Reiser's) that comments
	disappeared entirely and could therefore be used for token
	pasting.  ANSI affirms (as did K&R1) that comments are replaced
	with white space.  However, since the need for pasting tokens
	was demonstrated and real, ANSI introduced a well-defined token-
	pasting operator, ##, which can be used like this:

		#define Paste(a, b) a##b

	See also question 11.17.

	References: ANSI Sec.; ISO Sec.; Rationale
	Sec.; H&S Sec. 3.3.9 p. 52.

10.22:	Why is the macro

		#define TRACE(n) printf("TRACE: %d\n", n)

	giving me the warning "macro replacement within a string
	literal"?  It seems to be expanding

		printf("TRACE: %d\count", count);

A:	See question 11.18.

10.23:	How can I use a macro argument inside a string literal in the
	macro expansion?

A:	See question 11.18.

10.25:	I've got this tricky preprocessing I want to do and I can't
	figure out a way to do it.

A:	C's preprocessor is not intended as a general-purpose tool.
	(Note also that it is not guaranteed to be available as a
	separate program.)  Rather than forcing it to do something
	inappropriate, consider writing your own little special-purpose
	preprocessing tool, instead.  You can easily get a utility like
	make(1) to run it for you automatically.

	If you are trying to preprocess something other than C, consider
	using a general-purpose preprocessor.  (One older one available
	on most Unix systems is m4.)

10.26:	How can I write a macro which takes a variable number of

A:	One popular trick is to define and invoke the macro with a
	single, parenthesized "argument" which in the macro expansion
	becomes the entire argument list, parentheses and all, for a
	function such as printf():

		#define DEBUG(args) (printf("DEBUG: "), printf args)

		if(n != 0) DEBUG(("n is %d\n", n));

	The obvious disadvantage is that the caller must always remember
	to use the extra parentheses.

	gcc has an extension which allows a function-like macro to
	accept a variable number of arguments, but it's not standard.
	Other possible solutions are to use different macros (DEBUG1,
	DEBUG2, etc.) depending on the number of arguments, to play
	games with commas:

		#define DEBUG(args) (printf("DEBUG: "), printf(args))
		#define _ ,

		DEBUG("i = %d" _ i)

	It is often better to use a bona-fide function, which can take a
	variable number of arguments in a well-defined way.  See
	questions 15.4 and 15.5.

Section 11.  ANSI/ISO Standard C

11.1:	What is the "ANSI C Standard?"

A:	In 1983, the American National Standards Institute (ANSI)
	commissioned a committee, X3J11, to standardize the C language.
	After a long, arduous process, including several widespread
	public reviews, the committee's work was finally ratified as ANS
	X3.159-1989 on December 14, 1989, and published in the spring of
	1990.	For the most part, ANSI C standardizes existing practice,
	with a few additions from C++ (most notably function prototypes)
	and support for multinational character sets (including the
	controversial trigraph sequences).  The ANSI C standard also
	formalizes the C run-time library support routines.

	More recently, the Standard has been adopted as an international
	standard, ISO/IEC 9899:1990, and this ISO Standard replaces the
	earlier X3.159 even within the United States.  Its sections are
	numbered differently (briefly, ISO sections 5 through 7
	correspond roughly to the old ANSI sections 2 through 4).  As an
	ISO Standard, it is subject to ongoing revision through the
	release of Technical Corrigenda and Normative Addenda.

	In 1994, Technical Corrigendum 1 amended the Standard in about
	40 places, most of them minor corrections or clarifications.
	More recently, Normative Addendum 1 added about 50 pages of new
	material, mostly specifying new library functions for
	internationalization.  The production of Technical Corrigenda is
	an ongoing process, and a second one is expected in late 1995.
	In addition, both ANSI and ISO require periodic review of their
	standards.  This process is beginning in 1995, and will likely
	result in a completely revised standard (nicknamed "C9X," on the
	assumption of completion by 1999).

	The original ANSI Standard included a "Rationale," explaining
	many of its decisions, and discussing a number of subtle points,
	including several of those covered here.  (The Rationale was
	"not part of ANSI Standard X3.159-1989, but... included for
	information only," and is not included with the ISO Standard.)

11.2:	How can I get a copy of the Standard?

A:	Copies are available in the United States from

		American National Standards Institute
		11 W. 42nd St., 13th floor
		New York, NY  10036  USA
		(+1) 212 642 4900


		Global Engineering Documents
		15 Inverness Way E
		Englewood, CO  80112  USA
		(+1) 303 397 2715
		(800) 854 7179  (U.S. & Canada)

	In other countries, contact the appropriate national standards
	body, or ISO in Geneva at:

		ISO Sales
		Case Postale 56
		CH-1211 Geneve 20

	(or see URL or check the comp.std.internat FAQ
	list, Standards.Faq).

	At the time of this writing, the cost is $130.00 from ANSI or
	$410.00 from Global.  Copies of the original X3.159 (including
	the Rationale) may still be available at $205.00 from ANSI or
	$162.50 from Global.  Note that ANSI derives revenues to support
	its operations from the sale of printed standards, so electronic
	copies are *not* available.

	In the U.S., it may be possible to get a copy of the original
	ANSI X3.159 (including the Rationale) as "FIPS PUB 160" from

		National Technical Information Service (NTIS)
		U.S. Department of Commerce
		Springfield, VA  22161
		703 487 4650

	The mistitled _Annotated ANSI C Standard_, with annotations by
	Herbert Schildt, contains most of the text of ISO 9899; it is
	published by Osborne/McGraw-Hill, ISBN 0-07-881952-0, and sells
	in the U.S. for approximately $40.  It has been suggested that
	the price differential between this work and the official
	standard reflects the value of the annotations: they are plagued
	by numerous errors and omissions, and a few pages of the
	Standard itself are missing.  Many people on the net recommend
	ignoring the annotations entirely.  A review of the annotations
	("annotated annotations") by Clive Feather can be found on the
	web at .

	The text of the Rationale (not the full Standard) can be
	obtained by anonymous ftp from (see question 18.16)
	in directory doc/standards/ansi/X3.159-1989, and is also
	available on the web at .  The Rationale has
	also been printed by Silicon Press, ISBN 0-929306-07-4.

11.3:	My ANSI compiler complains about a mismatch when it sees

		extern int func(float);

		int func(x)
		float x;
		{ ...

A:	You have mixed the new-style prototype declaration
	"extern int func(float);" with the old-style definition
	"int func(x) float x;".  It is usually safe to mix the two
	styles (see question 11.4), but not in this case.

	Old C (and ANSI C, in the absence of prototypes, and in variable-
	length argument lists; see question 15.2) "widens" certain
	arguments when they are passed to functions.  floats are
	promoted to double, and characters and short integers are
	promoted to int.  (For old-style function definitions, the
	values are automatically converted back to the corresponding
	narrower types within the body of the called function, if they
	are declared that way there.)

	This problem can be fixed either by using new-style syntax
	consistently in the definition:

		int func(float x) { ... }

	or by changing the new-style prototype declaration to match the
	old-style definition:

		extern int func(double);

	(In this case, it would be clearest to change the old-style
	definition to use double as well, as long as the address of that
	parameter is not taken.)

	It may also be safer to avoid "narrow" (char, short int, and
	float) function arguments and return types altogether.

	See also question 1.25.

	References: K&R1 Sec. A7.1 p. 186; K&R2 Sec. A7.3.2 p. 202; ANSI
	Sec., Sec.; ISO Sec., Sec.;
	Rationale Sec., Sec.; H&S Sec. 9.2 pp. 265-7,
	Sec. 9.4 pp. 272-3.

11.4:	Can you mix old-style and new-style function syntax?

A:	Doing so is perfectly legal, as long as you're careful (see
	especially question 11.3).  Note however that old-style syntax
	is marked as obsolescent, so official support for it may be
	removed some day.

	References: ANSI Sec. 3.7.1, Sec. 3.9.5; ISO Sec. 6.7.1,
	Sec. 6.9.5; H&S Sec. 9.2.2 pp. 265-7, Sec. 9.2.5 pp. 269-70.

11.5:	Why does the declaration

		extern f(struct x *p);

	give me an obscure warning message about "struct x introduced in
	prototype scope"?

A:	In a quirk of C's normal block scoping rules, a structure
	declared (or even mentioned) for the first time within a
	prototype cannot be compatible with other structures declared in
	the same source file (it goes out of scope at the end of the

	To resolve the problem, precede the prototype with the vacuous-
	looking declaration

		struct x;

	which places an (incomplete) declaration of struct x at file
	scope, so that all following declarations involving struct x can
	at least be sure they're referring to the same struct x.

	References: ANSI Sec., Sec., Sec.; ISO
	Sec., Sec., Sec.

11.8:	I don't understand why I can't use const values in initializers
	and array dimensions, as in

		const int n = 5;
		int a[n];

A:	The const qualifier really means "read-only;" an object so
	qualified is a run-time object which cannot (normally) be
	assigned to.  The value of a const-qualified object is therefore
	*not* a constant expression in the full sense of the term.  (C
	is unlike C++ in this regard.)  When you need a true compile-
	time constant, use a preprocessor #define.

	References: ANSI Sec. 3.4; ISO Sec. 6.4; H&S Secs. 7.11.2,7.11.3
	pp. 226-7.

11.9:	What's the difference between "const char *p" and
	"char * const p"?

A:	"char const *p" declares a pointer to a constant character (you
	can't change the character); "char * const p" declares a
	constant pointer to a (variable) character (i.e. you can't
	change the pointer).

	Read these "inside out" to understand them; see also question

	References: ANSI Sec. examples; ISO Sec.;
	Rationale Sec.; H&S Sec. 4.4.4 p. 81.

11.10:	Why can't I pass a char ** to a function which expects a
	const char **?

A:	You can use a pointer-to-T (for any type T) where a pointer-to-
	const-T is expected.  However, the rule (an explicit exception)
	which permits slight mismatches in qualified pointer types is
	not applied recursively, but only at the top level.

	You must use explicit casts (e.g. (const char **) in this case)
	when assigning (or passing) pointers which have qualifier
	mismatches at other than the first level of indirection.

	References: ANSI Sec., Sec., Sec. 3.5.3; ISO
	Sec., Sec., Sec. 6.5.3; H&S Sec. 7.9.1 pp. 221-

11.12:	Can I declare main() as void, to shut off these annoying "main
	returns no value" messages?

A:	No.  main() must be declared as returning an int, and as taking
	either zero or two arguments, of the appropriate types.  If
	you're calling exit() but still getting warnings, you may have
	to insert a redundant return statement (or use some kind of "not
	reached" directive, if available).

	Declaring a function as void does not merely shut off or
	rearrange warnings: it may also result in a different function
	call/return sequence, incompatible with what the caller (in
	main's case, the C run-time startup code) expects.

	(Note that this discussion of main() pertains only to "hosted"
	implementations; none of it applies to "freestanding"
	implementations, which may not even have main().  However,
	freestanding implementations are comparatively rare, and if
	you're using one, you probably know it.  If you've never heard
	of the distinction, you're probably using a hosted
	implementation, and the above rules apply.)

	References: ANSI Sec., Sec. F.5.1; ISO Sec.,
	Sec. G.5.1; H&S Sec. 20.1 p. 416; CT&P Sec. 3.10 pp. 50-51.

11.13:	But what about main's third argument, envp?

A:	It's a non-standard (though common) extension.  If you really
	need to access the environment in ways beyind what the standard
	getenv() function provides, though, the global variable environ
	is probably a better avenue (though it's equally non-standard).

	References: ANSI Sec. F.5.1; ISO Sec. G.5.1; H&S Sec. 20.1 pp.

11.14:	I believe that declaring void main() can't fail, since I'm
	calling exit() instead of returning, and anyway my operating
	system ignores a program's exit/return status.

A:	It doesn't matter whether main() returns or not, or whether
	anyone looks at the status; the problem is that when main() is
	misdeclared, its caller (the runtime startup code) may not even
	be able to *call* it correctly (due to the potential clash of
	calling conventions; see question 11.12).  Your operating system
	may ignore the exit status, and void main() may work for you,
	but it is not portable and not correct.

11.15:	The book I've been using, _C Programing for the Compleat Idiot_,
	always uses void main().

A:	Perhaps its author counts himself among the target audience.
	Many books unaccountably use void main() in examples.  They're

11.16:	Is exit(status) truly equivalent to returning the same status
	from main()?

A:	Yes and no.  The Standard says that they are equivalent.
	However, a few older, nonconforming systems may have problems
	with one or the other form.  Also, a return from main() cannot
	be expected to work if data local to main() might be needed
	during cleanup; see also question 16.4.  (Finally, the two forms
	are obviously not equivalent in a recursive call to main().)

	References: K&R2 Sec. 7.6 pp. 163-4; ANSI Sec.; ISO

11.17:	I'm trying to use the ANSI "stringizing" preprocessing operator
	`#' to insert the value of a symbolic constant into a message,
	but it keeps stringizing the macro's name rather than its value.

A:	You can use something like the following two-step procedure to
	force a macro to be expanded as well as stringized:

		#define Str(x) #x
		#define Xstr(x) Str(x)
		#define OP plus
		char *opname = Xstr(OP);

	This code sets opname to "plus" rather than "OP".

	An equivalent circumlocution is necessary with the token-pasting
	operator ## when the values (rather than the names) of two
	macros are to be concatenated.

	References: ANSI Sec., Sec. example; ISO
	Sec., Sec.

11.18:	What does the message "warning: macro replacement within a
	string literal" mean?

A:	Some pre-ANSI compilers/preprocessors interpreted macro
	definitions like

		#define TRACE(var, fmt) printf("TRACE: var = fmt\n", var)

	such that invocations like

		TRACE(i, %d);

	were expanded as

		printf("TRACE: i = %d\n", i);

	In other words, macro parameters were expanded even inside
	string literals and character constants.

	Macro expansion is *not* defined in this way by K&R or by
	Standard C.  When you do want to turn macro arguments into
	strings, you can use the new # preprocessing operator, along
	with string literal concatenation (another new ANSI feature):

		#define TRACE(var, fmt) \
			printf("TRACE: " #var " = " #fmt "\n", var)

	See also question 11.17 above.

	References: H&S Sec. 3.3.8 p. 51.

11.19:	I'm getting strange syntax errors inside lines I've #ifdeffed

A:	Under ANSI C, the text inside a "turned off" #if, #ifdef, or
	#ifndef must still consist of "valid preprocessing tokens."
	This means that there must be no newlines inside quotes, and no
	unterminated comments or quotes (note particularly that an
	apostrophe within a contracted word looks like the beginning of
	a character constant).  Therefore, natural-language comments and
	pseudocode should always be written between the "official"
	comment delimiters /* and */.  (But see question 20.20, and also

	References: ANSI Sec., Sec. 3.1; ISO Sec.,
	Sec. 6.1; H&S Sec. 3.2 p. 40.

11.20:	What are #pragmas and what are they good for?

A:	The #pragma directive provides a single, well-defined "escape
	hatch" which can be used for all sorts of implementation-
	specific controls and extensions: source listing control,
	structure packing, warning suppression (like lint's old
	/* NOTREACHED */ comments), etc.

	References: ANSI Sec. 3.8.6; ISO Sec. 6.8.6; H&S Sec. 3.7 p. 61.

11.21:	What does "#pragma once" mean?  I found it in some header files.

A:	It is an extension implemented by some preprocessors to help
	make header files idempotent; it is essentially equivalent to
	the #ifndef trick mentioned in question 10.7.

11.22:	Is char a[3] = "abc"; legal?  What does it mean?

A:	It is legal in ANSI C (and perhaps in a few pre-ANSI systems),
	though useful only in rare circumstances.  It declares an array
	of size three, initialized with the three characters 'a', 'b',
	and 'c', *without* the usual terminating '\0' character.  The
	array is therefore not a true C string and cannot be used with
	strcpy, printf %s, etc.

	Most of the time, you should let the compiler count the
	initializers when initializing arrays (in the case of the
	initializer "abc", of course, the computed size will be 4).

	References: ANSI Sec. 3.5.7; ISO Sec. 6.5.7; H&S Sec. 4.6.4 p.

11.24:	Why can't I perform arithmetic on a void * pointer?

A:	The compiler doesn't know the size of the pointed-to objects.
	Before performing arithmetic, convert the pointer either to
	char * or to the pointer type you're trying to manipulate (but
	see also question 4.5).

	References: ANSI Sec., Sec. 3.3.6; ISO Sec.,
	Sec. 6.3.6; H&S Sec. 7.6.2 p. 204.

11.25:	What's the difference between memcpy() and memmove()?

A:	memmove() offers guaranteed behavior if the source and
	destination arguments overlap.  memcpy() makes no such
	guarantee, and may therefore be more efficiently implementable.
	When in doubt, it's safer to use memmove().

	References: K&R2 Sec. B3 p. 250; ANSI Sec.,
	Sec.; ISO Sec., Sec.; Rationale
	Sec. 4.11.2; H&S Sec. 14.3 pp. 341-2; PCS Sec. 11 pp. 165-6.

11.26:	What should malloc(0) do?  Return a null pointer or a pointer to
	0 bytes?

A:	The ANSI/ISO Standard says that it may do either; the behavior
	is implementation-defined (see question 11.33).

	References: ANSI Sec. 4.10.3; ISO Sec. 7.10.3; PCS Sec. 16.1 p.

11.27:	Why does the ANSI Standard not guarantee more than six case-
	insensitive characters of external identifier significance?

A:	The problem is older linkers which are under the control of
	neither the ANSI/ISO Standard nor the C compiler developers on
	the systems which have them.  The limitation is only that
	identifiers be *significant* in the first six characters, not
	that they be restricted to six characters in length.  This
	limitation is annoying, but certainly not unbearable, and is
	marked in the Standard as "obsolescent," i.e. a future revision
	will likely relax it.

	This concession to current, restrictive linkers really had to be
	made, no matter how vehemently some people oppose it.  (The
	Rationale notes that its retention was "most painful.")  If you
	disagree, or have thought of a trick by which a compiler
	burdened with a restrictive linker could present the C
	programmer with the appearance of more significance in external
	identifiers, read the excellently-worded section 3.1.2 in the
	X3.159 Rationale (see question 11.1), which discusses several
	such schemes and explains why they could not be mandated.

	References: ANSI Sec. 3.1.2, Sec. 3.9.1; ISO Sec. 6.1.2,
	Sec. 6.9.1; Rationale Sec. 3.1.2; H&S Sec. 2.5 pp. 22-3.

11.29:	My compiler is rejecting the simplest possible test programs,
	with all kinds of syntax errors.

A:	Perhaps it is a pre-ANSI compiler, unable to accept function
	prototypes and the like.

	See also questions 1.31, 10.9, and 11.30.

11.30:	Why are some ANSI/ISO Standard library routines showing up as
	undefined, even though I've got an ANSI compiler?

A:	It's possible to have a compiler available which accepts ANSI
	syntax, but not to have ANSI-compatible header files or run-time
	libraries installed.  (In fact, this situation is rather common
	when using a non-vendor-supplied compiler such as gcc.)  See
	also questions 11.29, 13.25, and 13.26.

11.31:	Does anyone have a tool for converting old-style C programs to
	ANSI C, or vice versa, or for automatically generating

A:	Two programs, protoize and unprotoize, convert back and forth
	between prototyped and "old style" function definitions and
	declarations.  (These programs do *not* handle full-blown
	translation between "Classic" C and ANSI C.)  These programs are
	part of the FSF's GNU C compiler distribution; see question

	The unproto program (/pub/unix/unproto5.shar.Z on is a filter which sits between the preprocessor
	and the next compiler pass, converting most of ANSI C to
	traditional C on-the-fly.

	The GNU GhostScript package comes with a little program called

	Before converting ANSI C back to old-style, beware that such a
	conversion cannot always be made both safely and automatically.
	ANSI C introduces new features and complexities not found in K&R
	C.  You'll especially need to be careful of prototyped function
	calls; you'll probably need to insert explicit casts.  See also
	questions 11.3 and 11.29.

	Several prototype generators exist, many as modifications to
	lint.  A program called CPROTO was posted to comp.sources.misc
	in March, 1992.  There is another program called "cextract."
	Many vendors supply simple utilities like these with their
	compilers.  See also question 18.16.  (But be careful when
	generating prototypes for old functions with "narrow"
	parameters; see question 11.3.)

	Finally, are you sure you really need to convert lots of old
	code to ANSI C?  The old-style function syntax is still
	acceptable, and a hasty conversion can easily introduce bugs.
	(See question 11.3.)

11.32:	Why won't the Frobozz Magic C Compiler, which claims to be ANSI
	compliant, accept this code?  I know that the code is ANSI,
	because gcc accepts it.

A:	Many compilers support a few non-Standard extensions, gcc more
	so than most.  Are you sure that the code being rejected doesn't
	rely on such an extension?  It is usually a bad idea to perform
	experiments with a particular compiler to determine properties
	of a language; the applicable standard may permit variations, or
	the compiler may be wrong.  See also question 11.35.

11.33:	People seem to make a point of distinguishing between
	implementation-defined, unspecified, and undefined behavior.
	What's the difference?

A:	Briefly: implementation-defined means that an implementation
	must choose some behavior and document it.  Unspecified means
	that an implementation should choose some behavior, but need not
	document it.  Undefined means that absolutely anything might
	happen.  In no case does the Standard impose requirements; in
	the first two cases it occasionally suggests (and may require a
	choice from among) a small set of likely behaviors.

	Note that since the Standard imposes *no* requirements on the
	behavior of a compiler faced with an instance of undefined
	behavior, the compiler can do absolutely anything.  In
	particular, there is no guarantee that the rest of the program
	will perform normally.  It's perilous to think that you can
	tolerate undefined behavior in a program; see question 3.2 for a
	relatively simple example.

	If you're interested in writing portable code, you can ignore
	the distinctions, as you'll want to avoid code that depends on
	any of the three behaviors.

	See also questions 3.9, and 11.34.

	References: ANSI Sec. 1.6; ISO Sec. 3.10, Sec. 3.16, Sec. 3.17;
	Rationale Sec. 1.6.

11.34:	I'm appalled that the ANSI Standard leaves so many issues
	undefined.  Isn't a Standard's whole job to standardize these

A:	It has always been a characteristic of C that certain constructs
	behaved in whatever way a particular compiler or a particular
	piece of hardware chose to implement them.  This deliberate
	imprecision often allows compilers to generate more efficient
	code for common cases, without having to burden all programs
	with extra code to assure well-defined behavior of cases deemed
	to be less reasonable.  Therefore, the Standard is simply
	codifying existing practice.

	A programming language standard can be thought of as a treaty
	between the language user and the compiler implementor.  Parts
	of that treaty consist of features which the compiler
	implementor agrees to provide, and which the user may assume
	will be available.  Other parts, however, consist of rules which
	the user agrees to follow and which the implementor may assume
	will be followed.  As long as both sides uphold their
	guarantees, programs have a fighting chance of working
	correctly.  If *either* side reneges on any of its commitments,
	nothing is guaranteed to work.

	See also question 11.35.

	References: Rationale Sec. 1.1.

11.35:	People keep saying that the behavior of i = i++ is undefined,
	but I just tried it on an ANSI-conforming compiler, and got the
	results I expected.

A:	A compiler may do anything it likes when faced with undefined
	behavior (and, within limits, with implementation-defined and
	unspecified behavior), including doing what you expect.  It's
	unwise to depend on it, though.  See also questions 11.32,
	11.33, and 11.34.

Section 12. Stdio

12.1:	What's wrong with this code?

		char c;
		while((c = getchar()) != EOF) ...

A:	For one thing, the variable to hold getchar's return value
	must be an int.  getchar() can return all possible character
	values, as well as EOF.  By passing getchar's return value
	through a char, either a normal character might be
	misinterpreted as EOF, or the EOF might be altered (particularly
	if type char is unsigned) and so never seen.

	References: K&R1 Sec. 1.5 p. 14; K&R2 Sec. 1.5.1 p. 16; ANSI
	Sec., Sec. 4.9.1, Sec.; ISO Sec.,
	Sec. 7.9.1, Sec.; H&S Sec. 5.1.3 p. 116, Sec. 15.1,
	Sec. 15.6; CT&P Sec. 5.1 p. 70; PCS Sec. 11 p. 157.

12.2:	Why does the code

		while(!feof(infp)) {
			fgets(buf, MAXLINE, infp);
			fputs(buf, outfp);

	copy the last line twice?

A:	In C, EOF is only indicated *after* an input routine has tried
	to read, and has reached end-of-file.  (In other words, C's I/O
	is not like Pascal's.)  Usually, you should just check the
	return value of the input routine (fgets() in this case); often,
	you don't need to use feof() at all.

	References: K&R2 Sec. 7.6 p. 164; ANSI Sec. 4.9.3, Sec.,
	Sec.; ISO Sec. 7.9.3, Sec., Sec.; H&S
	Sec. 15.14 p. 382.

12.4:	My program's prompts and intermediate output don't always show
	up on the screen, especially when I pipe the output through
	another program.

A:	It's best to use an explicit fflush(stdout) whenever output
	should definitely be visible.  Several mechanisms attempt to
	perform the fflush() for you, at the "right time," but they tend
	to apply only when stdout is an interactive terminal.  (See also
	question 12.24.)

	References: ANSI Sec.; ISO Sec.

12.5:	How can I read one character at a time, without waiting for the
	RETURN key?

A:	See question 19.1.

12.6:	How can I print a '%' character in a printf format string?  I
	tried \%, but it didn't work.

A:	Simply double the percent sign: %% .

	\% can't work, because the backslash \ is the *compiler's*
	escape character, while here our problem is that the % is
	printf's escape character.

	See also question 19.17.

	References: K&R1 Sec. 7.3 p. 147; K&R2 Sec. 7.2 p. 154; ANSI
	Sec.; ISO Sec.

12.9:	Someone told me it was wrong to use %lf with printf().  How can
	printf() use %f for type double, if scanf() requires %lf?

A:	It's true that printf's %f specifier works with both float and
	double arguments.  Due to the "default argument promotions"
	(which apply in variable-length argument lists such as
	printf's, whether or not prototypes are in scope), values of
	type float are promoted to double, and printf() therefore sees
	only doubles.  See also questions 12.13 and 15.2.

	References: K&R1 Sec. 7.3 pp. 145-47, Sec. 7.4 pp. 147-50; K&R2
	Sec. 7.2 pp. 153-44, Sec. 7.4 pp. 157-59; ANSI Sec.,
	Sec.; ISO Sec., Sec.; H&S Sec. 15.8 pp.
	357-64, Sec. 15.11 pp. 366-78; CT&P Sec. A.1 pp. 121-33.

12.10:	How can I implement a variable field width with printf?  That
	is, instead of %8d, I want the width to be specified at run

A:	printf("%*d", width, n) will do just what you want.  See also
	question 12.15.

	References: K&R1 Sec. 7.3; K&R2 Sec. 7.2; ANSI Sec.; ISO
	Sec.; H&S Sec. 15.11.6; CT&P Sec. A.1.

12.11:	How can I print numbers with commas separating the thousands?
	What about currency formatted numbers?

A:	The routines in  begin to provide some support for
	these operations, but there is no standard routine for doing
	either task.  (The only thing printf() does in response to a
	custom locale setting is to change its decimal-point character.)

	References: ANSI Sec. 4.4; ISO Sec. 7.4; H&S Sec. 11.6 pp. 301-4.

12.12:	Why doesn't the call scanf("%d", i) work?

A:	The arguments you pass to scanf() must always be pointers.
	To fix the fragment above, change it to scanf("%d", &i) .

12.13:	Why doesn't this code:

		double d;
		scanf("%f", &d);


A:	Unlike printf(), scanf() uses %lf for values of type double, and
	%f for float.  See also question 12.9.

12.15:	How can I specify a variable width in a scanf() format string?

A:	You can't; an asterisk in a scanf() format string means to
	suppress assignment.  You may be able to use ANSI stringizing
	and string concatenation to accomplish about the same thing, or
	to construct a scanf format string on-the-fly.

12.17:	When I read numbers from the keyboard with scanf "%d\n", it
	seems to hang until I type one extra line of input.

A:	Perhaps surprisingly, \n in a scanf format string does *not*
	mean to expect a newline, but rather to read and discard
	characters as long as each is a whitespace character.  See also
	question 12.20.

	References: K&R2 Sec. B1.3 pp. 245-6; ANSI Sec.; ISO
	Sec.; H&S Sec. 15.8 pp. 357-64.

12.18:	I'm reading a number with scanf %d and then a string with
	gets(), but the compiler seems to be skipping the call to

A:	scanf %d won't consume a trailing newline.  If the input number
	is immediately followed by a newline, that newline will
	immediately satisfy the gets().

	As a general rule, you shouldn't try to interlace calls to
	scanf() with calls to gets() (or any other input routines);
	scanf's peculiar treatment of newlines almost always leads to
	trouble.  Either use scanf() to read everything or nothing.

	See also questions 12.20 and 12.23.

	References: ANSI Sec.; ISO Sec.; H&S Sec. 15.8
	pp. 357-64.

12.19:	I figured I could use scanf() more safely if I checked its
	return value to make sure that the user typed the numeric values
	I expect, but sometimes it seems to go into an infinite loop.

A:	When scanf() is attempting to convert numbers, any non-numeric
	characters it encounters terminate the conversion *and are left
	on the input stream*.  Therefore, unless some other steps are
	taken, unexpected non-numeric input "jams" scanf() again and
	again: scanf() never gets past the bad character(s) to encounter
	later, valid data.  If the user types a character like `x' in
	response to a numeric scanf format such as %d or %f, code that
	simply re-prompts and retries the same scanf() call will
	immediately reencounter the same `x'.

	See also question 12.20.

	References: ANSI Sec.; ISO Sec.; H&S Sec. 15.8
	pp. 357-64.

12.20:	Why does everyone say not to use scanf()?  What should I use

A:	scanf() has a number of problems -- see questions 12.17, 12.18,
	and 12.19.  Also, its %s format has the same problem that gets()
	has (see question 12.23) -- it's hard to guarantee that the
	receiving buffer won't overflow.

	More generally, scanf() is designed for relatively structured,
	formatted input (its name is in fact derived from "scan
	formatted").  If you pay attention, it will tell you whether it
	succeeded or failed, but it can tell you only approximately
	where it failed, and not at all how or why.  It's nearly
	impossible to do decent error recovery with scanf(); usually
	it's far easier to read entire lines (with fgets() or the like),
	then interpret them, either using sscanf() or some other
	techniques.  (Routines like strtol(), strtok(), and atoi() are
	often useful; see also question 13.6.)  If you do use sscanf(),
	don't forget to check the return value to make sure that the
	expected number of items were found.

	References: K&R2 Sec. 7.4 p. 159.

12.21:	How can I tell how much destination buffer space I'll need for
	an arbitrary sprintf call?  How can I avoid overflowing the
	destination buffer with sprintf()?

A:	There are not (yet) any good answers to either of these
	excellent questions, and this represents perhaps the biggest
	deficiency in the traditional stdio library.

	When the format string being used with sprintf() is known and
	relatively simple, you can usually predict a buffer size in an
	ad-hoc way.  If the format consists of one or two %s's, you can
	count the fixed characters in the format string yourself (or let
	sizeof count them for you) and add in the result of calling
	strlen() on the string(s) to be inserted.  You can
	conservatively estimate the size that %d will expand to with
	code like:

		char buf[(sizeof(int) * CHAR_BIT + 2) / 3 + 1 + 1];
		sprintf(buf, "%d", n);

	(This code computes the number of characters required for a base-
	8 representation of a number; a base-10 expansion is guaranteed
	to take as much room or less.)

	When the format string is more complicated, or is not even known
	until run time, predicting the buffer size becomes as difficult
	as reimplementing sprintf(), and correspondingly error-prone
	(and inadvisable).  A last-ditch technique which is sometimes
	suggested is to use fprintf() to print the same text to a bit
	bucket or temporary file, and then to look at fprintf's return
	value or the size of the file (but see question 19.12).

	If there's any chance that the buffer might not be big enough,
	you won't want to call sprintf() without some guarantee that the
	buffer will not overflow and overwrite some other part of
	memory.  Several stdio's (including GNU and 4.4bsd) provide the
	obvious snprintf() function, which can be used like this:

		snprintf(buf, bufsize, "You typed \"%s\"", answer);

	and we can hope that a future revision of the ANSI/ISO C
	Standard will include this function.

12.23:	Why does everyone say not to use gets()?

A:	Unlike fgets(), gets() cannot be told the size of the buffer
	it's to read into, so it cannot be prevented from overflowing
	that buffer.  As a general rule, always use fgets().  See
	question 7.1 for a code fragment illustrating the replacement of
	gets() with fgets().

	References: Rationale Sec.; H&S Sec. 15.7 p. 356.

12.24:	Why does errno contain ENOTTY after a call to printf()?

A:	Many implementations of the stdio package adjust their behavior
	slightly if stdout is a terminal.  To make the determination,
	these implementations perform some operation which happens to
	fail (with ENOTTY) if stdout is not a terminal.  Although the
	output operation goes on to complete successfully, errno still
	contains ENOTTY.  (Note that it is only meaningful for a program
	to inspect the contents of errno after an error has been

	References: ANSI Sec. 4.1.3, Sec.; ISO Sec. 7.1.4,
	Sec.; CT&P Sec. 5.4 p. 73; PCS Sec. 14 p. 254.

12.25:	What's the difference between fgetpos/fsetpos and ftell/fseek?
	What are fgetpos() and fsetpos() good for?

A:	fgetpos() and fsetpos() use a special typedef, fpos_t, for
	representing offsets (positions) in a file.  The type behind
	this typedef, if chosen appropriately, can represent arbitrarily
	large offsets, allowing fgetpos() and fsetpos() to be used with
	arbitrarily huge files.  ftell() and fseek(), on the other hand,
	use long int, and are therefore limited to offsets which can be
	represented in a long int.  See also question 1.4.

	References: K&R2 Sec. B1.6 p. 248; ANSI Sec. 4.9.1,
	Secs.,; ISO Sec. 7.9.1, Secs.,;
	H&S Sec. 15.5 p. 252.

12.26:	How can I flush pending input so that a user's typeahead isn't
	read at the next prompt?  Will fflush(stdin) work?

A:	fflush() is defined only for output streams.  Since its
	definition of "flush" is to complete the writing of buffered
	characters (not to discard them), discarding unread input would
	not be an analogous meaning for fflush on input streams.

	There is no standard way to discard unread characters from a
	stdio input stream, nor would such a way be sufficient unread
	characters can also accumulate in other, OS-level input buffers.

	References: ANSI Sec.; ISO Sec.; H&S Sec. 15.2.

12.30:	I'm trying to update a file in place, by using fopen mode "r+",
	reading a certain string, and writing back a modified string,
	but it's not working.

A:	Be sure to call fseek before you write, both to seek back to the
	beginning of the string you're trying to overwrite, and because
	an fseek or fflush is always required between reading and
	writing in the read/write "+" modes.  Also, remember that you
	can only overwrite characters with the same number of
	replacement characters; see also question 19.14.

	References: ANSI Sec.; ISO Sec.

12.33:	How can I redirect stdin or stdout to a file from within a

A:	Use freopen() (but see question 12.34 below).

	References: ANSI Sec.; ISO Sec.; H&S Sec. 15.2.

12.34:	Once I've used freopen(), how can I get the original stdout (or
	stdin) back?

A:	There isn't a good way.  If you need to switch back, the best
	solution is not to have used freopen() in the first place.  Try
	using your own explicit output (or input) stream variable, which
	you can reassign at will, while leaving the original stdout (or
	stdin) undisturbed.

12.38:	How can I read a binary data file properly?  I'm occasionally
	seeing 0x0a and 0x0d values getting garbled, and it seems to hit
	EOF prematurely if the data contains the value 0x1a.

A:	When you're reading a binary data file, you should specify "rb"
	mode when calling fopen(), to make sure that text file
	translations do not occur.  Similarly, when writing binary data
	files, use "wb".

	Note that the text/binary distinction is made when you open the
	file: once a file is open, it doesn't matter which I/O calls you
	use on it.  See also question 20.5.

	References: ANSI Sec.; ISO Sec.; H&S Sec. 15.2.1
	p. 348.

Section 13. Library Functions

13.1:	How can I convert numbers to strings (the opposite of atoi)?  Is
	there an itoa function?

A:	Just use sprintf().  (Don't worry that sprintf() may be
	overkill, potentially wasting run time or code space; it works
	well in practice.)  See the examples in the answer to question
	7.5; see also question 12.21.

	You can obviously use sprintf() to convert long or floating-
	point numbers to strings as well (using %ld or %f).

	References: K&R1 Sec. 3.6 p. 60; K&R2 Sec. 3.6 p. 64.

13.2:	Why does strncpy() not always place a '\0' terminator in the
	destination string?

A:	strncpy() was first designed to handle a now-obsolete data
	structure, the fixed-length, not-necessarily-\0-terminated
	"string."  (A related quirk of strncpy's is that it pads short
	strings with multiple \0's, out to the specified length.)
	strncpy() is admittedly a bit cumbersome to use in other
	contexts, since you must often append a '\0' to the destination
	string by hand.  You can get around the problem by using
	strncat() instead of strncpy(): if the destination string starts
	out empty, strncat() does what you probably wanted strncpy() to
	do.  Another possibility is sprintf(dest, "%.*s", n, source) .

	When arbitrary bytes (as opposed to strings) are being copied,
	memcpy() is usually a more appropriate routine to use than

13.5:	Why do some versions of toupper() act strangely if given an
	upper-case letter?
	Why does some code call islower() before toupper()?

A:	Older versions of toupper() and tolower() did not always work
	correctly on arguments which did not need converting (i.e. on
	digits or punctuation or letters already of the desired case).
	In ANSI/ISO Standard C, these functions are guaranteed to work
	appropriately on all character arguments.

	References: ANSI Sec. 4.3.2; ISO Sec. 7.3.2; H&S Sec. 12.9 pp.
	320-1; PCS p. 182.

13.6:	How can I split up a string into whitespace-separated fields?
	How can I duplicate the process by which main() is handed argc
	and argv?

A:	The only Standard routine available for this kind of
	"tokenizing" is strtok, although it can be tricky to use and it
	may not do everything you want it to.  (For instance, it does
	not handle quoting.)

	References: K&R2 Sec. B3 p. 250; ANSI Sec.; ISO
	Sec.; H&S Sec. 13.7 pp. 333-4; PCS p. 178.

13.7:	I need some code to do regular expression and wildcard matching.

A:	Make sure you recognize the difference between classic regular
	expressions (variants of which are used in such Unix utilities
	as ed and grep), and filename wildcards (variants of which are
	used by most operating systems).

	There are a number of packages available for matching regular
	expressions.  Most packages use a pair of functions, one for
	"compiling" the regular expression, and one for "executing" it
	(i.e. matching strings against it).  Look for header files named
	 or , and functions called regcmp()/regex(),
	regcomp()/regexec(), or re_comp()/re_exec().  (These functions
	may exist in a separate regexp library.)  A popular, freely-
	redistributable regexp package by Henry Spencer is available
	from in pub/regexp.shar.Z or in several other
	archives.  The GNU project has a package called rx.  See also
	question 18.16.

	Filename wildcard matching (sometimes called "globbing") is done
	in a variety of ways on different systems.  On Unix, wildcards
	are automatically expanded by the shell before a process is
	invoked, so programs rarely have to worry about them explicitly.
	Under MS-DOS compilers, there is often a special object file
	which can be linked in to a program to expand wildcards while
	argv is being built.  Several systems (including MS-DOS and VMS)
	provide system services for listing or opening files specified
	by wildcards.  Check your compiler/library documentation.

13.8:	I'm trying to sort an array of strings with qsort(), using
	strcmp() as the comparison function, but it's not working.

A:	By "array of strings" you probably mean "array of pointers to
	char."  The arguments to qsort's comparison function are
	pointers to the objects being sorted, in this case, pointers to
	pointers to char.  strcmp(), however, accepts simple pointers to
	char.  Therefore, strcmp() can't be used directly.  Write an
	intermediate comparison function like this:

		/* compare strings via pointers */
		int pstrcmp(const void *p1, const void *p2)
			return strcmp(*(char * const *)p1, *(char * const *)p2);

	The comparison function's arguments are expressed as "generic
	pointers," const void *.  They are converted back to what they
	"really are" (char **) and dereferenced, yielding char *'s which
	can be passed to strcmp().  (Under a pre-ANSI compiler, declare
	the pointer parameters as char * instead of void *, and drop the

	(Don't be misled by the discussion in K&R2 Sec. 5.11 pp. 119-20,
	which is not discussing the Standard library's qsort).

	References: ANSI Sec.; ISO Sec.; H&S Sec. 20.5
	p. 419.

13.9:	Now I'm trying to sort an array of structures with qsort().  My
	comparison function takes pointers to structures, but the
	compiler complains that the function is of the wrong type for
	qsort().  How can I cast the function pointer to shut off the

A:	The conversions must be in the comparison function, which must
	be declared as accepting "generic pointers" (const void *) as
	discussed in question 13.8 above.  The comparison function might
	look like

		int mystructcmp(const void *p1, const void *p2)
			const struct mystruct *sp1 = p1;
			const struct mystruct *sp2 = p2;
			/* now compare sp1->whatever and sp2-> ... */

	(The conversions from generic pointers to struct mystruct
	pointers happen in the initializations sp1 = p1 and sp2 = p2;
	the compiler performs the conversions implicitly since p1 and p2
	are void pointers.  Explicit casts, and char * pointers, would
	be required under a pre-ANSI compiler.  See also question 7.7.)

	If, on the other hand, you're sorting pointers to structures,
	you'll need indirection, as in question 13.8:
	sp1 = *(struct mystruct **)p1 .

	In general, it is a bad idea to insert casts just to "shut the
	compiler up."  Compiler warnings are usually trying to tell you
	something, and unless you really know what you're doing, you
	ignore or muzzle them at your peril.  See also question 4.9.

	References: ANSI Sec.; ISO Sec.; H&S Sec. 20.5
	p. 419.

13.10:	How can I sort a linked list?

A:	Sometimes it's easier to keep the list in order as you build it
	(or perhaps to use a tree instead).  Algorithms like insertion
	sort and merge sort lend themselves ideally to use with linked
	lists.  If you want to use a standard library function, you can
	allocate a temporary array of pointers, fill it in with pointers
	to all your list nodes, call qsort(), and finally rebuild the
	list pointers based on the sorted array.

	References: Knuth Sec. 5.2.1 pp. 80-102, Sec. 5.2.4 pp. 159-168;
	Sedgewick Sec. 8 pp. 98-100, Sec. 12 pp. 163-175.

13.11:	How can I sort more data than will fit in memory?

A:	You want an "external sort," which you can read about in Knuth,
	Volume 3.  The basic idea is to sort the data in chunks (as much
	as will fit in memory at one time), write each sorted chunk to a
	temporary file, and then merge the files.  Your operating system
	may provide a general-purpose sort utility, and if so, you can
	try invoking it from within your program: see questions 19.27
	and 19.30.

	References: Knuth Sec. 5.4 pp. 247-378; Sedgewick Sec. 13 pp.

13.12:	How can I get the current date or time of day in a C program?

A:	Just use the time, ctime, and/or localtime functions.  (These
	routines have been around for years, and are in the ANSI
	standard.)  Here is a simple example:


			time_t now;
			printf("It's %.24s.\n", ctime(&now));
			return 0;

	References: K&R2 Sec. B10 pp. 255-7; ANSI Sec. 4.12; ISO
	Sec. 7.12; H&S Sec. 18.

13.13:	I know that the library routine localtime() will convert a
	time_t into a broken-down struct tm, and that ctime() will
	convert a time_t to a printable string.  How can I perform the
	inverse operations of converting a struct tm or a string into a

A:	ANSI C specifies a library routine, mktime(), which converts a
	struct tm to a time_t.

	Converting a string to a time_t is harder, because of the wide
	variety of date and time formats which might be encountered.
	Some systems provide a strptime() function, which is basically
	the inverse of strftime().  Other popular routines are partime()
	(widely distributed with the RCS package) and getdate() (and a
	few others, from the C news distribution).  See question 18.16.

	References: K&R2 Sec. B10 p. 256; ANSI Sec.; ISO
	Sec.; H&S Sec. 18.4 pp. 401-2.

13.14:	How can I add N days to a date?  How can I find the difference
	between two dates?

A:	The ANSI/ISO Standard C mktime() and difftime() functions
	provide some support for both problems.  mktime() accepts non-
	normalized dates, so it is straightforward to take a filled-in
	struct tm, add or subtract from the tm_mday field, and call
	mktime() to normalize the year, month, and day fields (and
	incidentally convert to a time_t value).  difftime() computes
	the difference, in seconds, between two time_t values; mktime()
	can be used to compute time_t values for two dates to be

	These solutions are only guaranteed to work correctly for dates
	in the range which can be represented as time_t's.  The tm_mday
	field is an int, so day offsets of more than 32,736 or so may
	cause overflow.  Note also that at daylight saving time
	changeovers, local days are not 24 hours long.

	Another approach to both problems is to use "Julian day"
	numbers.  Implementations of Julian day routines can be found in
	the file JULCAL10.ZIP from the Simtel/Oakland archives (see
	question 18.16) and the "Date conversions" article mentioned in
	the References.

	See also questions 13.13, 20.31, and 20.32.

	References: K&R2 Sec. B10 p. 256; ANSI Secs.,;
	ISO Secs.,; H&S Secs. 18.4,18.5 pp. 401-2;
	David Burki, "Date Conversions".

13.15:	I need a random number generator.

A:	The Standard C library has one: rand().  The implementation on
	your system may not be perfect, but writing a better one isn't
	necessarily easy, either.

	If you do find yourself needing to implement your own random
	number generator, there is plenty of literature out there; see
	the References.  There are also any number of packages on the
	net: look for r250, RANLIB, and FSULTRA (see question 18.16).

	References: K&R2 Sec. 2.7 p. 46, Sec. 7.8.7 p. 168; ANSI
	Sec.; ISO Sec.; H&S Sec. 17.7 p. 393; PCS
	Sec. 11 p. 172; Knuth Vol. 2 Chap. 3 pp. 1-177; Park and Miller,
	"Random Number Generators: Good Ones are hard to Find".

13.16:	How can I get random integers in a certain range?

A:	The obvious way,

		rand() % N		/* POOR */

	(which tries to return numbers from 0 to N-1) is poor, because
	the low-order bits of many random number generators are
	distressingly *non*-random.  (See question 13.18.)  A better
	method is something like

		(int)((double)rand() / ((double)RAND_MAX + 1) * N)

	If you're worried about using floating point, you could use

		rand() / (RAND_MAX / N + 1)

	Both methods obviously require knowing RAND_MAX (which ANSI
	#defines in ), and assume that N is much less than

	(Note, by the way, that RAND_MAX is a *constant* telling you
	what the fixed range of the C library rand() function is.  You
	cannot set RAND_MAX to some other value, and there is no way of
	requesting that rand() return numbers in some other range.)

	If you're starting with a random number generator which returns
	floating-point values between 0 and 1, all you have to do to get
	integers from 0 to N-1 is multiply the output of that generator
	by N.

	References: K&R2 Sec. 7.8.7 p. 168; PCS Sec. 11 p. 172.

13.17:	Each time I run my program, I get the same sequence of numbers
	back from rand().

A:	You can call srand() to seed the pseudo-random number generator
	with a truly random initial value.  Popular seed values are the
	time of day, or the elapsed time before the user presses a key
	(although keypress times are hard to determine portably; see
	question 19.37).  (Note also that it's rarely useful to call
	srand() more than once during a run of a program; in particular,
	don't try calling srand() before each call to rand(), in an
	attempt to get "really random" numbers.)

	References: K&R2 Sec. 7.8.7 p. 168; ANSI Sec.; ISO
	Sec.; H&S Sec. 17.7 p. 393.

13.18:	I need a random true/false value, so I'm just taking rand() % 2,
	but it's alternating 0, 1, 0, 1, 0...

A:	Poor pseudorandom number generators (such as the ones
	unfortunately supplied with some systems) are not very random in
	the low-order bits.  Try using the higher-order bits: see
	question 13.16.

	References: Knuth Sec. pp. 12-14.

13.20:	How can I generate random numbers with a normal or Gaussian

A:	Here is one method, by Box and Muller, and recommended by Knuth:


		double gaussrand()
			static double V1, V2, S;
			static int phase = 0;
			double X;

			if(phase == 0) {
				do {
					double U1 = (double)rand() / RAND_MAX;
					double U2 = (double)rand() / RAND_MAX;

					V1 = 2 * U1 - 1;
					V2 = 2 * U2 - 1;
					S = V1 * V1 + V2 * V2;
					} while(S >= 1 || S == 0);

				X = V1 * sqrt(-2 * log(S) / S);
			} else
				X = V2 * sqrt(-2 * log(S) / S);

			phase = 1 - phase;

			return X;

	See the extended versions of this list (see question 20.40) for
	other ideas.

	References: Knuth Sec. 3.4.1 p. 117; Box and Muller, "A Note on
	the Generation of Random Normal Deviates"; Press et al.,
	_Numerical Recipes in C_ Sec. 7.2 pp. 288-290.

13.24:	I'm trying to port this	     A:	Those routines are variously
	old program.  Why do I		obsolete; you should
	get "undefined external"	instead:
	errors for:

	index?				use strchr.
	rindex?				use strrchr.
	bcopy?				use memmove, after
					interchanging the first and
					second arguments (see also
					question 11.25).
	bcmp?				use memcmp.
	bzero?				use memset, with a second
					argument of 0.

	Contrariwise, if you're using an older system which is missing
	the functions in the second column, you may be able to implement
	them in terms of, or substitute, the functions in the first.

	References: PCS Sec. 11.

13.25:	I keep getting errors due to library functions being undefined,
	but I'm #including all the right header files.

A:	In some cases (especially if the functions are nonstandard) you
	may have to explicitly ask for the correct libraries to be
	searched when you link the program.  See also questions 11.30,
	13.26, and 14.3.

13.26:	I'm still getting errors due to library functions being
	undefined, even though I'm explicitly requesting the right
	libraries while linking.

A:	Many linkers make one pass over the list of object files and
	libraries you specify, and extract from libraries only those
	modules which satisfy references which have so far come up as
	undefined.  Therefore, the order in which libraries are listed
	with respect to object files (and each other) is significant;
	usually, you want to search the libraries last.  (For example,
	under Unix, put any -l options towards the end of the command
	line.)  See also question 13.28.

13.28:	What does it mean when the linker says that _end is undefined?

A:	That message is a quirk of the old Unix linkers.  You only get
	an error about _end being undefined when other things are
	undefined, too -- fix the others, and the error about _end will
	disappear.  (See also questions 13.25 and 13.26.)

Section 14. Floating Point

14.1:	When I set a float variable to, say, 3.1, why is printf()
	printing it as 3.0999999?

A:	Most computers use base 2 for floating-point numbers as well as
	for integers.  In base 2, 1/1010 (that is, 1/10 decimal) is an
	infinitely-repeating fraction: its binary representation is
	0.0001100110011... .  Depending on how carefully your compiler's
	binary/decimal conversion routines (such as those used by
	printf) have been written, you may see discrepancies when
	numbers (especially low-precision floats) not exactly
	representable in base 2 are assigned or read in and then printed
	(i.e. converted from base 10 to base 2 and back again).  See
	also question 14.6.

14.2:	I'm trying to take some square roots, but I'm getting crazy

A:	Make sure that you have #included , and correctly
	declared other functions returning double.  (Another library
	routine to be careful with is atof(), which is declared in
	.)  See also question 14.3 below.

	References: CT&P Sec. 4.5 pp. 65-6.

14.3:	I'm trying to do some simple trig, and I am #including ,
	but I keep getting "undefined: sin" compilation errors.

A:	Make sure you're actually linking with the math library.  For
	instance, under Unix, you usually need to use the -lm option, at
	the *end* of the command line, when compiling/linking.  See also
	questions 13.25 and 13.26.

14.4:	My floating-point calculations are acting strangely and giving
	me different answers on different machines.

A:	First, see question 14.2 above.

	If the problem isn't that simple, recall that digital computers
	usually use floating-point formats which provide a close but by
	no means exact simulation of real number arithmetic.  Underflow,
	cumulative precision loss, and other anomalies are often

	Don't assume that floating-point results will be exact, and
	especially don't assume that floating-point values can be
	compared for equality.  (Don't throw haphazard "fuzz factors"
	in, either; see question 14.5.)

	These problems are no worse for C than they are for any other
	computer language.  Certain aspects of floating-point are
	usually defined as "however the processor does them" (see also
	question 11.34), otherwise a compiler for a machine without the
	"right" model would have to do prohibitively expensive

	This article cannot begin to list the pitfalls associated with,
	and workarounds appropriate for, floating-point work.  A good
	numerical programming text should cover the basics; see also the
	references below.

	References: Kernighan and Plauger, _The Elements of Programming
	Style_ Sec. 6 pp. 115-8; Knuth, Volume 2 chapter 4; David
	Goldberg, "What Every Computer Scientist Should Know about
	Floating-Point Arithmetic".

14.5:	What's a good way to check for "close enough" floating-point

A:	Since the absolute accuracy of floating point values varies, by
	definition, with their magnitude, the best way of comparing two
	floating point values is to use an accuracy threshold which is
	relative to the magnitude of the numbers being compared.  Rather

		double a, b;
		if(a == b)	/* WRONG */

	use something like


		if(fabs(a - b) <= epsilon * a)

	for some suitably-chosen epsilon.

	References: Knuth Sec. 4.2.2 pp. 217-8.

14.6:	How do I round numbers?

A:	The simplest and most straightforward way is with code like

		(int)(x + 0.5)

	This technique won't work properly for negative numbers, though.

14.7:	Why doesn't C have an exponentiation operator?

A:	Because few processors have an exponentiation instruction.  C
	has a pow() function, declared in , although explicit
	multiplication is often better for small positive integral

	References: ANSI Sec.; ISO Sec.; H&S Sec. 17.6
	p. 393.

14.8:	The pre-#defined constant M_PI seems to be missing from my
	machine's copy of .

A:	That constant (which is apparently supposed to be the value of
	pi, accurate to the machine's precision), is not standard.  If
	you need pi, you'll have to #define it yourself.

	References: PCS Sec. 13 p. 237.

14.9:	How do I test for IEEE NaN and other special values?

A:	Many systems with high-quality IEEE floating-point
	implementations provide facilities (e.g. predefined constants,
	and functions like isnan(), either as nonstandard extensions in
	 or perhaps in  or ) to deal with these
	values cleanly, and work is being done to formally standardize
	such facilities.  A crude but usually effective test for NaN is
	exemplified by

		#define isnan(x) ((x) != (x))

	although non-IEEE-aware compilers may optimize the test away.

	Another possibility is to to format the value in question using
	sprintf(): on many systems it generates strings like "NaN" and
	"Inf" which you could compare for in a pinch.

	See also question 19.39.

14.11:	What's a good way to implement complex numbers in C?

A:	It is straightforward to define a simple structure and some
	arithmetic functions to manipulate them.  See also questions
	2.7, 2.10, and 14.12.

14.12:	I'm looking for some code to do:
		Fast Fourier Transforms (FFT's)
		matrix arithmetic (multiplication, inversion, etc.)
		complex arithmetic

A:	Ajay Shah maintains an index of free numerical software; it is
	posted periodically, and available where this FAQ list is
	archived (see question 20.40).  See also question 18.16.

14.13:	I'm having trouble with a Turbo C program which crashes and says
	something like "floating point formats not linked."

A:	The message in the question has to do with an indecent problem
	in Borland's compilers, which for some unfathomable reason has
	still not been fixed.  However, by the newly-passed
	Communications Decency Act of the U.S., I am prohibited from
	transmitting or discussing "indecent" material.  (If the fact
	that users of Borland's compilers are still having this problem
	isn't indecent, I don't know what is.)  If you send me e-mail
	certifying that you are over 18 years of age, I may be able to
	help you.  (You may also be able to see the
	comp.os.msdos.programmer FAQ list for more information.)

Section 15. Variable-Length Argument Lists

15.1:	I heard that you have to #include  before calling
	printf().  Why?

A:	So that a proper prototype for printf() will be in scope.

	A compiler may use a different calling sequence for functions
	which accept variable-length argument lists.  (It might do so if
	calls using variable-length argument lists were less efficient
	than those using fixed-length.)  Therefore, a prototype
	(indicating, using the ellipsis notation "...", that the
	argument list is of variable length) must be in scope whenever a
	varargs function is called, so that the compiler knows to use
	the varargs calling mechanism.

	References: ANSI Sec., Sec. 4.1.6; ISO Sec.,
	Sec. 7.1.7; Rationale Sec., Sec. 4.1.6; H&S Sec. 9.2.4
	pp. 268-9, Sec. 9.6 pp. 275-6.

15.2:	How can %f be used for both float and double arguments in
	printf()?  Aren't they different types?

A:	In the variable-length part of a variable-length argument list,
	the "default argument promotions" apply: types char and
	short int are promoted to int, and float is promoted to double.
	(These are the same promotions that apply to function calls
	without a prototype in scope, also known as "old style" function
	calls; see question 11.3.)  Therefore, printf's %f format always
	sees a double.  (Similarly, %c always sees an int, as does %hd.)
	See also questions 12.9 and 12.13.

	References: ANSI Sec.; ISO Sec.; H&S Sec. 6.3.5
	p. 177, Sec. 9.4 pp. 272-3.

15.3:	I had a frustrating problem which turned out to be caused by the

		printf("%d", n);

	where n was actually a long int.  I thought that ANSI function
	prototypes were supposed to guard against argument type
	mismatches like this.

A:	When a function accepts a variable number of arguments, its
	prototype does not (and cannot) provide any information about
	the number and types of those variable arguments.  Therefore,
	the usual protections do *not* apply in the variable-length part
	of variable-length argument lists: the compiler cannot perform
	implicit conversions or (in general) warn about mismatches.

	See also questions 5.2, 11.3, 12.9, and 15.2.

15.4:	How can I write a function that takes a variable number of

A:	Use the facilities of the  header.

	Here is a function which concatenates an arbitrary number of
	strings into malloc'ed memory:

		#include 		/* for malloc, NULL, size_t */
		#include 		/* for va_ stuff */
		#include 		/* for strcat et al. */

		char *vstrcat(char *first, ...)
			size_t len;
			char *retbuf;
			va_list argp;
			char *p;

			if(first == NULL)
				return NULL;

			len = strlen(first);

			va_start(argp, first);

			while((p = va_arg(argp, char *)) != NULL)
				len += strlen(p);


			retbuf = malloc(len + 1);	/* +1 for trailing \0 */

			if(retbuf == NULL)
				return NULL;		/* error */

			(void)strcpy(retbuf, first);

			va_start(argp, first);		/* restart for second scan */

			while((p = va_arg(argp, char *)) != NULL)
				(void)strcat(retbuf, p);


			return retbuf;

	Usage is something like

		char *str = vstrcat("Hello, ", "world!", (char *)NULL);

	Note the cast on the last argument; see questions 5.2 and 15.3.
	(Also note that the caller must free the returned, malloc'ed

	Under a pre-ANSI compiler, rewrite the function definition
	without a prototype ("char *vstrcat(first) char *first; {"),
	include  rather than , add "extern
	char *malloc();", and use int instead of size_t.  You may also
	have to delete the (void) casts, and use the older varargs
	package instead of stdarg.  See also question 15.7.

	References: K&R2 Sec. 7.3 p. 155, Sec. B7 p. 254; ANSI Sec. 4.8;
	ISO Sec. 7.8; Rationale Sec. 4.8; H&S Sec. 11.4 pp. 296-9; CT&P
	Sec. A.3 pp. 139-141; PCS Sec. 11 pp. 184-5, Sec. 13 p. 242.

15.5:	How can I write a function that takes a format string and a
	variable number of arguments, like printf(), and passes them to
	printf() to do most of the work?

A:	Use vprintf(), vfprintf(), or vsprintf().

	Here is an error() routine which prints an error message,
	preceded by the string "error: " and terminated with a newline:


		void error(char *fmt, ...)
			va_list argp;
			fprintf(stderr, "error: ");
			va_start(argp, fmt);
			vfprintf(stderr, fmt, argp);
			fprintf(stderr, "\n");

	See also question 15.7.

	References: K&R2 Sec. 8.3 p. 174, Sec. B1.2 p. 245; ANSI
	Secs.,,; ISO
	Secs.,,; H&S Sec. 15.12 pp. 379-80; PCS
	Sec. 11 pp. 186-7.

15.6:	How can I write a function analogous to scanf(), that calls
	scanf() to do most of the work?

A:	Unfortunately, vscanf and the like are not standard.  You're on
	your own.

15.7:	I have a pre-ANSI compiler, without .  What can I do?

A:	There's an older header, , which offers about the
	same functionality.

	To rewrite the error() function from question 15.5 to use
	, change the function header to:

		void error(va_alist)
			char *fmt;

	change the va_start line to


	and add the line

		fmt = va_arg(argp, char *);

	between the calls to va_start and vfprintf.  (Note that there is
	no semicolon after va_dcl.)

	References: H&S Sec. 11.4 pp. 296-9; CT&P Sec. A.2 pp. 134-139;
	PCS Sec. 11 pp. 184-5, Sec. 13 p. 250.

15.8:	How can I discover how many arguments a function was actually
	called with?

A:	This information is not available to a portable program.  Some
	old systems provided a nonstandard nargs() function, but its use
	was always questionable, since it typically returned the number
	of words passed, not the number of arguments.  (Structures, long
	ints, and floating point values are usually passed as several

	Any function which takes a variable number of arguments must be
	able to determine *from the arguments themselves* how many of
	them there are.  printf-like functions do this by looking for
	formatting specifiers (%d and the like) in the format string
	(which is why these functions fail badly if the format string
	does not match the argument list).  Another common technique,
	applicable when the arguments are all of the same type, is to
	use a sentinel value (often 0, -1, or an appropriately-cast null
	pointer) at the end of the list (see the execl() and vstrcat()
	examples in questions 5.2 and 15.4).  Finally, if their types
	are predictable, you can pass an explicit count of the number of
	variable arguments (although it's usually a nuisance for the
	caller to generate).

	References: PCS Sec. 11 pp. 167-8.

15.9:	My compiler isn't letting me declare a function

		int f(...)

	i.e. with no fixed arguments.

A:	Standard C requires at least one fixed argument, in part so that
	you can hand it to va_start().

	References: ANSI Sec. 3.5.4, Sec., Sec.; ISO
	Sec. 6.5.4, Sec., Sec.; H&S Sec. 9.2 p. 263.

15.10:	I have a varargs function which accepts a float parameter.  Why

		va_arg(argp, float)


A:	In the variable-length part of variable-length argument lists,
	the old "default argument promotions" apply: arguments of type
	float are always promoted (widened) to type double, and types
	char and short int are promoted to int.  Therefore, it is never
	correct to invoke va_arg(argp, float); instead you should always
	use va_arg(argp, double).  Similarly, use va_arg(argp, int) to
	retrieve arguments which were originally char, short, or int.
	See also questions 11.3 and 15.2.

	References: ANSI Sec.; ISO Sec.; Rationale
	Sec.; H&S Sec. 11.4 p. 297.

15.11:	I can't get va_arg() to pull in an argument of type pointer-to-

A:	The type-rewriting games which the va_arg() macro typically
	plays are stymied by overly-complicated types such as pointer-to-
	function.  If you use a typedef for the function pointer type,
	however, all will be well.  See also question 1.21.

	References: ANSI Sec.; ISO Sec.; Rationale

15.12:	How can I write a function which takes a variable number of
	arguments and passes them to some other function (which takes a
	variable number of arguments)?

A:	In general, you cannot.  Ideally, you should provide a version
	of that other function which accepts a va_list pointer
	(analogous to vfprintf(); see question 15.5 above).  If the
	arguments must be passed directly as actual arguments, or if you
	do not have the option of rewriting the second function to
	accept a va_list (in other words, if the second, called function
	must accept a variable number of arguments, not a va_list), no
	portable solution is possible.  (The problem could perhaps be
	solved by resorting to machine-specific assembly language; see
	also question 15.13 below.)

15.13:	How can I call a function with an argument list built up at run

A:	There is no guaranteed or portable way to do this.  If you're
	curious, ask this list's editor, who has a few wacky ideas you
	could try...

	Instead of an actual argument list, you might consider passing
	an array of generic (void *) pointers.  The called function can
	then step through the array, much like main() might step through
	argv.  (Obviously this works only if you have control over all
	the called functions.)

	(See also question 19.36.)

Section 16. Strange Problems

16.3:	This program crashes before it even runs!  (When single-stepping
	with a debugger, it dies before the first statement in main().)

A:	You probably have one or more very large (kilobyte or more)
	local arrays.  Many systems have fixed-size stacks, and those
	which perform dynamic stack allocation automatically (e.g. Unix)
	can be confused when the stack tries to grow by a huge chunk all
	at once.  It is often better to declare large arrays with static
	duration (unless of course you need a fresh set with each
	recursive call, in which case you could dynamically allocate
	them with malloc(); see also question 1.31).

	(See also questions 11.12, 16.4, 16.5, and 18.4.)

16.4:	I have a program that seems to run correctly, but it crashes as
	it's exiting, *after* the last statement in main().  What could
	be causing this?

A:	Look for a misdeclared main() (see questions 2.18 and 10.9), or
	local buffers passed to setbuf() or setvbuf(), or problems in
	cleanup functions registered by atexit().  See also questions
	7.5 and 11.16.

	References: CT&P Sec. 5.3 pp. 72-3.

16.5:	This program runs perfectly on one machine, but I get weird
	results on another.  Stranger still, adding or removing
	debugging printouts changes the symptoms...

A:	Lots of things could be going wrong; here are a few of the more
	common things to check:

		uninitialized local variables (see also question 7.1)

		integer overflow, especially on 16-bit machines,
		especially of an intermediate result when doing things
		like a * b / c (see also question 3.14)

		undefined evaluation order (see questions 3.1 through 3.4)

		omitted declaration of external functions, especially
		those which return something other than int
		(see questions 1.25 and 14.2)

		dereferenced null pointers (see section 5)

		improper malloc/free use: assuming malloc'ed memory
		contains 0, assuming freed storage persists, freeing
		something twice (see also questions 7.20 and 7.19)

		pointer problems in general (see also question 16.8)

		mismatch between printf() format and arguments, especially
		trying to print long ints using %d (see question 12.9)

		trying to malloc(256 * 256 * sizeof(double)), especially
		on machines with limited memory (see also questions 7.16
		and 19.23)

		array bounds problems, especially of small, temporary
		buffers, perhaps used for constructing strings with
		sprintf() (see also questions 7.1 and 12.21)

		invalid assumptions about the mapping of typedefs,
		especially size_t

		floating point problems (see questions 14.1 and 14.4)

		anything you thought was a clever exploitation of the way
		you believe code is generated for your specific system

	Proper use of function prototypes can catch several of these
	problems; lint would catch several more.  See also questions
	16.3, 16.4, and 18.4.

16.6:	Why does this code:

		char *p = "hello, world!";
		p[0] = 'H';


A:	String literals are not necessarily modifiable, except (in
	effect) when they are used as array initializers.  Try

		char a[] = "hello, world!";

	See also question 1.32.

	References: ANSI Sec. 3.1.4; ISO Sec. 6.1.4; H&S Sec. 2.7.4 pp.

16.8:	What do "Segmentation violation" and "Bus error" mean?

A:	These generally mean that your program tried to access memory it
	shouldn't have, invariably as a result of improper pointer use.
	Likely causes are inadvertent use of null pointers (see also
	questions 5.2 and 5.20) or uninitialized, misaligned, or
	otherwise improperly allocated pointers (see questions 7.1 and
	7.2); corruption of the malloc arena (see question 7.19); and
	mismatched function arguments, especially involving pointers;
	two possibilities are scanf() (see question 12.12) and fprintf()
	(make sure it receives its first FILE * argument).

	See also questions 16.3 and 16.4.

Section 17. Style

17.1:	What's the best style for code layout in C?

A:	K&R, while providing the example most often copied, also supply
	a good excuse for disregarding it:

		The position of braces is less important,
		although people hold passionate beliefs.
		We have chosen one of several popular styles.
		Pick a style that suits you, then use it

	It is more important that the layout chosen be consistent (with
	itself, and with nearby or common code) than that it be
	"perfect."  If your coding environment (i.e. local custom or
	company policy) does not suggest a style, and you don't feel
	like inventing your own, just copy K&R.  (The tradeoffs between
	various indenting and brace placement options can be
	exhaustively and minutely examined, but don't warrant repetition
	here.  See also the Indian Hill Style Guide.)

	The elusive quality of "good style" involves much more than mere
	code layout details; don't spend time on formatting to the
	exclusion of more substantive code quality issues.

	See also question 10.6.

	References: K&R1 Sec. 1.2 p. 10; K&R2 Sec. 1.2 p. 10.

17.3:	Here's a neat trick for checking whether two strings are equal:

		if(!strcmp(s1, s2))

	Is this good style?

A:	It is not particularly good style, although it is a popular
	idiom.  The test succeeds if the two strings are equal, but the
	use of ! ("not") suggests that it tests for inequality.

	A better option is to use a macro:

		#define Streq(s1, s2) (strcmp((s1), (s2)) == 0)

	Opinions on code style, like those on religion, can be debated
	endlessly.  Though good style is a worthy goal, and can usually
	be recognized, it cannot be rigorously codified.  See also
	question 17.10.

17.4:	Why do some people write if(0 == x) instead of if(x == 0)?

A:	It's a trick to guard against the common error of writing

		if(x = 0)

	If you're in the habit of writing the constant before the ==,
	the compiler will complain if you accidentally type

		if(0 = x)

	Evidently it can be easier to remember to reverse the test than
	it is to remember to type the doubled = sign.

	References: H&S Sec. 7.6.5 pp. 209-10.

17.5:	I came across some code that puts a (void) cast before each call
	to printf().  Why?

A:	printf() does return a value, though few programs bother to
	check the return values from each call.  Since some compilers
	(and lint) will warn about discarded return values, an explicit
	cast to (void) is a way of saying "Yes, I've decided to ignore
	the return value from this call, but please continue to warn me
	about other (perhaps inadvertently) ignored return values."
	It's also common to use void casts on calls to strcpy() and
	strcat(), since the return value is never surprising.

	References: K&R2 Sec. A6.7 p. 199; Rationale Sec. 3.3.4; H&S
	Sec. 6.2.9 p. 172, Sec. 7.13 pp. 229-30.

17.8:	What is "Hungarian Notation"?  Is it worthwhile?

A:	Hungarian Notation is a naming convention, invented by Charles
	Simonyi, which encodes things about a variable's type (and
	perhaps its intended use) in its name.  It is well-loved in some
	circles and roundly castigated in others.  Its chief advantage
	is that it makes a variable's type or intended use obvious from
	its name; its chief disadvantage is that type information is not
	necessarily a worthwhile thing to carry around in the name of a

	References: Simonyi and Heller, "The Hungarian Revolution" .

17.9:	Where can I get the "Indian Hill Style Guide" and other coding

A:	Various documents are available for anonymous ftp from:

		Site:			File or directory:	pub/cstyle.tar.Z
					(the updated Indian Hill guide)	doc/programming
					(including Henry Spencer's
					"10 Commandments for C Programmers")		pub/style-guide

	You may also be interested in the books _The Elements of
	Programming Style_, _Plum Hall Programming Guidelines_, and _C
	Style: Standards and Guidelines_; see the Bibliography.  (The
	_Standards and Guidelines_ book is not in fact a style guide,
	but a set of guidelines on selecting and creating style guides.)

	See also question 18.9.

17.10:	Some people say that goto's are evil and that I should never use
	them.  Isn't that a bit extreme?

A:	Programming style, like writing style, is somewhat of an art and
	cannot be codified by inflexible rules, although discussions
	about style often seem to center exclusively around such rules.

	In the case of the goto statement, it has long been observed
	that unfettered use of goto's quickly leads to unmaintainable
	spaghetti code.  However, a simple, unthinking ban on the goto
	statement does not necessarily lead immediately to beautiful
	programming: an unstructured programmer is just as capable of
	constructing a Byzantine tangle without using any goto's
	(perhaps substituting oddly-nested loops and Boolean control
	variables, instead).

	Most observations or "rules" about programming style usually
	work better as guidelines than rules, and work much better if
	programmers understand what the guidelines are trying to
	accomplish.  Blindly avoiding certain constructs or following
	rules without understanding them can lead to just as many
	problems as the rules were supposed to avert.

	Furthermore, many opinions on programming style are just that:
	opinions.  It's usually futile to get dragged into "style wars,"
	because on certain issues (such as those referred to in
	questions 9.2, 5.3, 5.9, and 10.7), opponents can never seem to
	agree, or agree to disagree, or stop arguing.

Section 18. Tools and Resources

18.1:	I need:			     A:	Look for programs (see also
					question 18.16) named:

	a C cross-reference		cflow, cxref, calls, cscope,
	generator			xscope, or ixfw

	a C beautifier/pretty-		cb, indent, GNU indent, or
	printer				vgrind

	a revision control or		RCS or SCCS
	configuration management

	a C source obfuscator		obfus, shroud, or opqcp

	a "make" dependency		makedepend, or try cc -M or
	generator			cpp -M

	tools to compute code		ccount, Metre, lcount, or
	metrics				csize, or see URL
					Engineering/Cmetrics.html ;
					there is also a package sold
					by McCabe and Associates

	a C lines-of-source		this can be done very
	counter				crudely with the standard
					Unix utility wc, and
					considerably better with
					grep -c ";"

	a prototype generator		see question 11.31

	a tool to track down
	malloc problems			see question 18.2

	a "selective" C
	preprocessor			see question 10.18

	language translation		see questions 11.31 and
	tools				 20.26

	C verifiers (lint)		see question 18.7

	a C compiler!			see question 18.3

	(This list of tools is by no means complete; if you know of tools
	not mentioned, you're welcome to contact this list's maintainer.)

	Other lists of tools, and discussion about them, can be found in
	the Usenet newsgroups comp.compilers and .

	See also questions 18.16 and 18.3.

18.2:	How can I track down these pesky malloc problems?

A:	A number of debugging packages exist to help track down malloc
	problems; one popular one is Conor P. Cahill's "dbmalloc,"
	posted to comp.sources.misc in 1992, volume 32.  Others are
	"leak," available in volume 27 of the comp.sources.unix
	archives; JMalloc.c and JMalloc.h in the "Snippets" collection;
	and MEMDEBUG from in pub/sources/memdebug .  See
	also question 18.16.

	A number of commercial debugging tools exist, and can be
	invaluable in tracking down malloc-related and other stubborn

		Bounds-Checker for DOS, from Nu-Mega Technologies,
		P.O. Box 7780, Nashua, NH 03060-7780, USA, 603-889-2386.

		CodeCenter (formerly Saber-C) from Centerline Software
		(formerly Saber), 10 Fawcett Street, Cambridge, MA
		02138-1110, USA, 617-498-3000.

		Insight, from ParaSoft Corporation, 2500 E. Foothill
		Blvd., Pasadena, CA 91107, USA, 818-792-9941,
		[email protected] .

		Purify, from Pure Software, 1309 S. Mary Ave., Sunnyvale,
		CA 94087, USA, 800-224-7873, [email protected] .

		SENTINEL, from AIB Software, 46030 Manekin Plaza, Dulles,
		VA 20166, USA, 703-430-9247, 800-296-3000, [email protected] .

18.3:	What's a free or cheap C compiler I can use?

A:	A popular and high-quality free C compiler is the FSF's GNU C
	compiler, or gcc.  It is available by anonymous ftp from in directory pub/gnu, or at several other FSF
	archive sites.  An MS-DOS port, djgpp, is also available; it can
	be found in the Simtel and Oakland archives and probably many
	others, usually in a directory like pub/msdos/djgpp/ or

	There is a shareware compiler called PCC, available as

	A very inexpensive MS-DOS compiler is Power C from Mix Software,
	1132 Commerce Drive, Richardson, TX 75801, USA, 214-783-6001.

	Another recently-developed compiler is lcc, available for
	anonymous ftp from in pub/lcc.

	Archives associated with comp.compilers contain a great deal of
	information about available compilers, interpreters, grammars,
	etc. (for many languages).  The comp.compilers archives
	(including an FAQ list), maintained by the moderator, John R.
	Levine, are at .  A list of available compilers and
	related resources, maintained by Mark Hopkins, Steven Robenalt,
	and David Muir Sharnoff, is at in pub/compilers-
	list/.  (See also the comp.compilers directory in the
	news.answers archives at and; see
	question 20.40.)

	See also question 18.16.

18.4:	I just typed in this program, and it's acting strangely.  Can
	you see anything wrong with it?

A:	See if you can run lint first (perhaps with the -a, -c, -h, -p
	or other options).  Many C compilers are really only half-
	compilers, electing not to diagnose numerous source code
	difficulties which would not actively preclude code generation.

	See also questions 16.5 and 16.8.

	References: Ian Darwin, _Checking C Programs with lint_ .

18.5:	How can I shut off the "warning: possible pointer alignment
	problem" message which lint gives me for each call to malloc()?

A:	The problem is that traditional versions of lint do not know,
	and cannot be told, that malloc() "returns a pointer to space
	suitably aligned for storage of any type of object."  It is
	possible to provide a pseudoimplementation of malloc(), using a
	#define inside of #ifdef lint, which effectively shuts this
	warning off, but a simpleminded definition will also suppress
	meaningful messages about truly incorrect invocations.  It may
	be easier simply to ignore the message, perhaps in an automated
	way with grep -v.  (But don't get in the habit of ignoring too
	many lint messages, otherwise one day you'll overlook a
	significant one.)

18.7:	Where can I get an ANSI-compatible lint?

A:	Products called PC-Lint and FlexeLint (in "shrouded source
	form," for compilation on 'most any system) are available from

		Gimpel Software
		3207 Hogarth Lane
		Collegeville, PA  19426  USA
		(+1) 610 584 4261
		[email protected]

	The Unix System V release 4 lint is ANSI-compatible, and is
	available separately (bundled with other C tools) from UNIX
	Support Labs or from System V resellers.

	Another ANSI-compatible lint (which can also perform higher-
	level formal verification) is LCLint, available via anonymous
	ftp from in pub/Larch/lclint/.

	In the absence of lint, many modern compilers do attempt to
	diagnose almost as many problems as lint does.

18.8:	Don't ANSI function prototypes render lint obsolete?

A:	Not really.  First of all, prototypes work only if they are
	present and correct; an inadvertently incorrect prototype is
	worse than useless.  Secondly, lint checks consistency across
	multiple source files, and checks data declarations as well as
	functions.  Finally, an independent program like lint will
	probably always be more scrupulous at enforcing compatible,
	portable coding practices than will any particular,
	implementation-specific, feature- and extension-laden compiler.

	If you do want to use function prototypes instead of lint for
	cross-file consistency checking, make sure that you set the
	prototypes up correctly in header files.  See questions 1.7 and

18.9:	Are there any C tutorials or other resources on the net?

A:	There are several of them:

	"Notes for C programmers," by Christopher Sawtell, are
	available from in misc/sawtell_C.shar and in /pc/c-lang/ .

	Tim Love's "C for Programmers" is available by ftp from svr- in the misc directory.  An html version is at .

	The Coronado Enterprises C tutorials are available on Simtel
	mirrors in pub/msdos/c/.

	Rick Rowe has a tutorial which is available from
	as pub/rowe/ or as
	pub/MSDOS_UPLOADS/programming/c_language/ .

	There is evidently a web-based course at .

	Finally, on some Unix machines you can try typing learn c at the
	shell prompt.

	[Disclaimer: I have not reviewed these tutorials; I have heard
	that at least one of them contains a number of errors.  Also,
	this sort of information rapidly becomes out-of-date; these
	addresses may not work by the time you read this and try them.]

	Several of these tutorials, plus a great deal of other
	information about C, are accessible via the web at .

	Vinit Carpenter maintains a list of resources for learning C and
	C++; it is posted to comp.lang.c and comp.lang.c++, and archived
	where this FAQ list is (see question 20.40), or on the web at .

	See also question 18.10 below.

18.10:	What's a good book for learning C?

A:	There are far too many books on C to list here; it's impossible
	to rate them all.  Many people believe that the best one was
	also the first: _The C Programming Language_, by Kernighan and
	Ritchie ("K&R," now in its second edition).  Opinions vary on
	K&R's suitability as an initial programming text: many of us did
	learn C from it, and learned it well; some, however, feel that
	it is a bit too clinical as a first tutorial for those without
	much programming background.

	An excellent reference manual is _C: A Reference Manual_, by
	Samuel P. Harbison and Guy L. Steele, now in its fourth edition.

	Though not suitable for learning C from scratch, this FAQ list
	has been published in book form; see the Bibliography.

	Mitch Wright maintains an annotated bibliography of C and Unix
	books; it is available for anonymous ftp from in
	directory pub/mitch/YABL/.

	This FAQ list's editor maintains a collection of previous
	answers to this question, which is available upon request.  See
	also question 18.9 above.

18.13:	Where can I find the sources of the standard C libraries?

A:	One source (though not public domain) is _The Standard C
	Library_, by P.J. Plauger (see the Bibliography).
	Implementations of all or part of the C library have been
	written and are readily available as part of the netBSD and GNU
	(also Linux) projects.  See also question 18.16.

18.14:	I need code to parse and evaluate expressions.

A:	Two available packages are "defunc," posted to comp.sources.misc
	in December, 1993 (V41 i32,33), to alt.sources in January, 1994,
	and available from in
	pub/packages/development/libraries/defunc-1.3.tar.Z, and
	"parse," at  Other options include the
	S-Lang interpreter, available via anonymous ftp from in pub/slang, and the shareware Cmm ("C-
	minus-minus" or "C minus the hard stuff").  See also question

	There is also some parsing/evaluation code in _Software
	Solutions in C_ (chapter 12, pp. 235-55).

18.15:	Where can I get a BNF or YACC grammar for C?

A:	The definitive grammar is of course the one in the ANSI
	standard; see question 11.2.  Another grammar (along with one
	for C++) by Jim Roskind is in pub/c++grammar1.1.tar.Z at .  A fleshed-out, working instance of the ANSI
	grammar (due to Jeff Lee) is on (see question 18.16)
	in usenet/net.sources/ansi.c.grammar.Z (including a companion
	lexer).  The FSF's GNU C compiler contains a grammar, as does
	the appendix to K&R2.

	The comp.compilers archives contain more information about
	grammars; see question 18.3.

	References: K&R1 Sec. A18 pp. 214-219; K&R2 Sec. A13 pp. 234-
	239; ANSI Sec. A.2; ISO Sec. B.2; H&S pp. 423-435 Appendix B.

18.15a: Does anyone have a C compiler test suite I can use?

A:	Plum Hall (formerly in Cardiff, NJ; now in Hawaii) sells one;
	another package is Ronald Guilmette's RoadTest(tm) Compiler Test
	Suites (ftp to, pub/rfg/roadtest/announce.txt for
	information).  The FSF's GNU C (gcc) distribution includes a c-
	torture-test which checks a number of common problems with
	compilers.  Kahan's paranoia test, found in netlib/paranoia on, strenuously tests a C implementation's floating
	point capabilities.

18.16:	Where and how can I get copies of all these freely distributable

A:	As the number of available programs, the number of publicly
	accessible archive sites, and the number of people trying to
	access them all grow, this question becomes both easier and more
	difficult to answer.

	There are a number of large, public-spirited archive sites out
	there, such as,,,, and, which have huge
	amounts of software and other information all freely available.
	For the FSF's GNU project, the central distribution site is .  These well-known sites tend to be extremely
	busy and hard to reach, but there are also numerous "mirror"
	sites which try to spread the load around.

	On the connected Internet, the traditional way to retrieve files
	from an archive site is with anonymous ftp.  For those without
	ftp access, there are also several ftp-by-mail servers in
	operation.  More and more, the world-wide web (WWW) is being
	used to announce, index, and even transfer large data files.
	There are probably yet newer access methods, too.

	Those are some of the easy parts of the question to answer.  The
	hard part is in the details -- this article cannot begin to
	track or list all of the available archive sites or all of the
	various ways of accessing them.  If you have access to the net
	at all, you probably have access to more up-to-date information
	about active sites and useful access methods than this FAQ list

	The other easy-and-hard aspect of the question, of course, is
	simply *finding* which site has what you're looking for.  There
	is a tremendous amount of work going on in this area, and there
	are probably new indexing services springing up every day.  One
	of the first was "archie": for any program or resource available
	on the net, if you know its name, an archie server can usually
	tell you which anonymous ftp sites have it.  Your system may
	have an archie command, or you can send the mail message "help"
	to [email protected] for information.

	If you have access to Usenet, see the regular postings in the
	comp.sources.unix and comp.sources.misc newsgroups, which
	describe the archiving policies for those groups and how to
	access their archives.  The comp.archives newsgroup contains
	numerous announcements of anonymous ftp availability of various
	items.  Finally, the newsgroup comp.sources.wanted is generally
	a more appropriate place to post queries for source
	availability, but check *its* FAQ list, "How to find sources,"
	before posting there.

	See also question 14.12.

Section 19. System Dependencies

19.1:	How can I read a single character from the keyboard without
	waiting for the RETURN key?  How can I stop characters from
	being echoed on the screen as they're typed?

A:	Alas, there is no standard or portable way to do these things in
	C.  Concepts such as screens and keyboards are not even
	mentioned in the Standard, which deals only with simple I/O
	"streams" of characters.

	At some level, interactive keyboard input is usually collected
	and presented to the requesting program a line at a time.  This
	gives the operating system a chance to support input line
	editing (backspace/delete/rubout, etc.) in a consistent way,
	without requiring that it be built into every program.  Only
	when the user is satisfied and presses the RETURN key (or
	equivalent) is the line made available to the calling program.
	Even if the calling program appears to be reading input a
	character at a time (with getchar() or the like), the first call
	blocks until the user has typed an entire line, at which point
	potentially many characters become available and many character
	requests (e.g. getchar() calls) are satisfied in quick

	When a program wants to read each character immediately as it
	arrives, its course of action will depend on where in the input
	stream the line collection is happening and how it can be
	disabled.  Under some systems (e.g. MS-DOS, VMS in some modes),
	a program can use a different or modified set of OS-level input
	calls to bypass line-at-a-time input processing.  Under other
	systems (e.g. Unix, VMS in other modes), the part of the
	operating system responsible for serial input (often called the
	"terminal driver") must be placed in a mode which turns off line-
	at-a-time processing, after which all calls to the usual input
	routines (e.g. read(), getchar(), etc.) will return characters
	immediately.  Finally, a few systems (particularly older, batch-
	oriented mainframes) perform input processing in peripheral
	processors which cannot be told to do anything other than line-
	at-a-time input.

	Therefore, when you need to do character-at-a-time input (or
	disable keyboard echo, which is an analogous problem), you will
	have to use a technique specific to the system you're using,
	assuming it provides one.  Since comp.lang.c is oriented towards
	topics that C does deal with, you will usually get better
	answers to these questions by referring to a system-specific
	newsgroup such as comp.unix.questions or
	comp.os.msdos.programmer, and to the FAQ lists for these groups.
	Note that the answers are often not unique even across different
	variants of a system; bear in mind when answering system-
	specific questions that the answer that applies to your system
	may not apply to everyone else's.

	However, since these questions are frequently asked here, here
	are brief answers for some common situations.

	Some versions of curses have functions called cbreak(),
	noecho(), and getch() which do what you want.  If you're
	specifically trying to read a short password without echo, you
	might try getpass().  Under Unix, you can use ioctl() to play
	with the terminal driver modes (CBREAK or RAW under "classic"
	versions; ICANON, c_cc[VMIN] and c_cc[VTIME] under System V or
	POSIX systems; ECHO under all versions), or in a pinch, system()
	and the stty command.  (For more information, see  and
	tty(4) under classic versions,  and termio(4) under
	System V, or  and termios(4) under POSIX.)  Under MS-
	DOS, use getch() or getche(), or the corresponding BIOS
	interrupts.  Under VMS, try the Screen Management (SMG$)
	routines, or curses, or issue low-level $QIO's with the
	IO$_READVBLK function code (and perhaps IO$M_NOECHO, and others)
	to ask for one character at a time.  (It's also possible to set
	character-at-a-time or "pass through" modes in the VMS terminal
	driver.)  Under other operating systems, you're on your own.

	(As an aside, note that simply using setbuf() or setvbuf() to
	set stdin to unbuffered will *not* generally serve to allow
	character-at-a-time input.)

	If you're trying to write a portable program, a good approach is
	to define your own suite of three functions to (1) set the
	terminal driver or input system into character-at-a-time mode
	(if necessary), (2) get characters, and (3) return the terminal
	driver to its initial state when the program is finished.
	(Ideally, such a set of functions might be part of the C
	Standard, some day.)  The extended versions of this FAQ list
	(see question 20.40) contain examples of such functions for
	several popular systems.

	See also question 19.2.

	References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1; POSIX
	Sec. 7.

19.2:	How can I find out if there are characters available for reading
	(and if so, how many)?  Alternatively, how can I do a read that
	will not block if there are no characters available?

A:	These, too, are entirely operating-system-specific.  Some
	versions of curses have a nodelay() function.  Depending on your
	system, you may also be able to use "nonblocking I/O", or a
	system call named "select" or "poll", or the FIONREAD ioctl, or
	c_cc[VTIME], or kbhit(), or rdchk(), or the O_NDELAY option to
	open() or fcntl().  See also question 19.1.

19.3:	How can I display a percentage-done indication that updates
	itself in place, or show one of those "twirling baton" progress

A:	These simple things, at least, you can do fairly portably.
	Printing the character '\r' will usually give you a carriage
	return without a line feed, so that you can overwrite the
	current line.  The character '\b' is a backspace, and will
	usually move the cursor one position to the left.

	References: ANSI Sec. 2.2.2; ISO Sec. 5.2.2.

19.4:	How can I clear the screen?
	How can I print things in inverse video?
	How can I move the cursor to a specific x, y position?

A:	Such things depend on the terminal type (or display) you're
	using.  You will have to use a library such as termcap,
	terminfo, or curses, or some system-specific routines, to
	perform these operations.

	For clearing the screen, a halfway portable solution is to print
	a form-feed character ('\f'), which will cause some displays to
	clear.  Even more portable would be to print enough newlines to
	scroll everything away.  As a last resort, you could use
	system() (see question 19.27) to invoke an operating system
	clear-screen command.

	References: PCS Sec. 5.1.4 pp. 54-60, Sec. 5.1.5 pp. 60-62.

19.5:	How do I read the arrow keys?  What about function keys?

A:	Terminfo, some versions of termcap, and some versions of curses
	have support for these non-ASCII keys.  Typically, a special key
	sends a multicharacter sequence (usually beginning with ESC,
	'\033'); parsing these can be tricky.  (curses will do the
	parsing for you, if you call keypad() first.)

	Under MS-DOS, if you receive a character with value 0 (*not*
	'0'!) while reading the keyboard, it's a flag indicating that
	the next character read will be a code indicating a special key.
	See any DOS programming guide for lists of keyboard codes.
	(Very briefly: the up, left, right, and down arrow keys are 72,
	75, 77, and 80, and the function keys are 59 through 68.)

	References: PCS Sec. 5.1.4 pp. 56-7.

19.6:	How do I read the mouse?

A:	Consult your system documentation, or ask on an appropriate
	system-specific newsgroup (but check its FAQ list first).  Mouse
	handling is completely different under the X window system, MS-
	DOS, the Macintosh, and probably every other system.

	References: PCS Sec. 5.5 pp. 78-80.

19.7:	How can I do serial ("comm") port I/O?

A:	It's system-dependent.  Under Unix, you typically open, read,
	and write a device file in /dev, and use the facilities of the
	terminal driver to adjust its characteristics.  (See also
	questions 19.1 and 19.2.)  Under MS-DOS, you can use the
	predefined stream stdaux, or a special file like COM1, or some
	primitive BIOS interrupts, or (if you require decent
	performance) any number of interrupt-driven serial I/O packages.
	Several netters recommend the book _C Programmer's Guide to
	Serial Communications_, by Joe Campbell.

19.8:	How can I direct output to the printer?

A:	Under Unix, either use popen() (see question 19.30) to write to
	the lp or lpr program, or perhaps open a special file like
	/dev/lp.  Under MS-DOS, write to the (nonstandard) predefined
	stdio stream stdprn, or open the special files PRN or LPT1.

	References: PCS Sec. 5.3 pp. 72-74.

19.9:	How do I send escape sequences to control a terminal or other

A:	If you can figure out how to send characters to the device at
	all (see question 19.8 above), it's easy enough to send escape
	sequences.  In ASCII, the ESC code is 033 (27 decimal), so code

		fprintf(ofd, "\033[J");

	sends the sequence ESC [ J .

19.10:	How can I do graphics?

A:	Once upon a time, Unix had a fairly nice little set of device-
	independent plot routines described in plot(3) and plot(5), but
	they've largely fallen into disuse.

	If you're programming for MS-DOS, you'll probably want to use
	libraries conforming to the VESA or BGI standards.

	If you're trying to talk to a particular plotter, making it draw
	is usually a matter of sending it the appropriate escape
	sequences; see also question 19.9.  The vendor may supply a C-
	callable library, or you may be able to find one on the net.

	If you're programming for a particular window system (Macintosh,
	X windows, Microsoft Windows), you will use its facilities; see
	the relevant documentation or newsgroup or FAQ list.

	References: PCS Sec. 5.4 pp. 75-77.

19.11:	How can I check whether a file exists?  I want to warn the user
	if a requested input file is missing.

A:	It's surprisingly difficult to make this determination reliably
	and portably.  Any test you make can be invalidated if the file
	is created or deleted (i.e. by some other process) between the
	time you make the test and the time you try to open the file.

	Three possible test routines are stat(), access(), and fopen().
	(To make an approximate test for file existence with fopen(),
	just open for reading and close immediately.)  Of these, only
	fopen() is widely portable, and access(), where it exists, must
	be used carefully if the program uses the Unix set-UID feature.

	Rather than trying to predict in advance whether an operation
	such as opening a file will succeed, it's often better to try
	it, check the return value, and complain if it fails.
	(Obviously, this approach won't work if you're trying to avoid
	overwriting an existing file, unless you've got something like
	the O_EXCL file opening option available, which does just what
	you want in this case.)

	References: PCS Sec. 12 pp. 189,213; POSIX Sec. 5.3.1,
	Sec. 5.6.2, Sec. 5.6.3.

19.12:	How can I find out the size of a file, prior to reading it in?

A:	If the "size of a file" is the number of characters you'll be
	able to read from it in C, it is difficult or impossible to
	determine this number exactly).

	Under Unix, the stat() call will give you an exact answer.
	Several other systems supply a Unix-like stat() which will give
	an approximate answer.  You can fseek() to the end and then use
	ftell(), but these tend to have the same problems: fstat() is
	not portable, and generally tells you the same thing stat()
	tells you; ftell() is not guaranteed to return a byte count
	except for binary files.  Some systems provide routines called
	filesize() or filelength(), but these are not portable, either.

	Are you sure you have to determine the file's size in advance?
	Since the most accurate way of determining the size of a file as
	a C program will see it is to open the file and read it, perhaps
	you can rearrange the code to learn the size as it reads.

	References: ANSI Sec.; ISO Sec.; H&S
	Sec. 15.5.1; PCS Sec. 12 p. 213; POSIX Sec. 5.6.2.

19.13:	How can a file be shortened in-place without completely clearing
	or rewriting it?

A:	BSD systems provide ftruncate(), several others supply chsize(),
	and a few may provide a (possibly undocumented) fcntl option
	F_FREESP.  Under MS-DOS, you can sometimes use write(fd, "", 0).
	However, there is no portable solution, nor a way to delete
	blocks at the beginning.  See also question 19.14.

19.14:	How can I insert or delete a line (or record) in the middle of a

A:	Short of rewriting the file, you probably can't.  The usual
	solution is simply to rewrite the file.  (Instead of deleting
	records, you might consider simply marking them as deleted, to
	avoid rewriting.)  See also questions 12.30 and 19.13.

19.15:	How can I recover the file name given an open stream or file

A:	This problem is, in general, insoluble.  Under Unix, for
	instance, a scan of the entire disk (perhaps involving special
	permissions) would theoretically be required, and would fail if
	the descriptor were connected to a pipe or referred to a deleted
	file (and could give a misleading answer for a file with
	multiple links).  It is best to remember the names of files
	yourself when you open them (perhaps with a wrapper function
	around fopen()).

19.16:	How can I delete a file?

A:	The Standard C Library function is remove().  (This is therefore
	one of the few questions in this section for which the answer is
	*not* "It's system-dependent.")  On older, pre-ANSI Unix
	systems, remove() may not exist, in which case you can try

	References: K&R2 Sec. B1.1 p. 242; ANSI Sec.; ISO
	Sec.; H&S Sec. 15.15 p. 382; PCS Sec. 12 pp. 208,220-
	221; POSIX Sec. 5.5.1, Sec. 8.2.4.

19.17:	Why can't I open a file by its explicit path?  The call

		fopen("c:\newdir\file.dat", "r")

	is failing.

A:	The file you actually requested -- with the characters \n and \f
	in its name -- probably doesn't exist, and isn't what you
	thought you were trying to open.

	In character constants and string literals, the backslash \ is
	an escape character, giving special meaning to the character
	following it.  In order for literal backslashes in a pathname to
	be passed through to fopen() (or any other routine) correctly,
	they have to be doubled, so that the first backslash in each
	pair quotes the second one:

		fopen("c:\\newdir\\file.dat", "r");

	Alternatively, under MS-DOS, it turns out that forward slashes
	are also accepted as directory separators, so you could use

		fopen("c:/newdir/file.dat", "r");

	(Note, by the way, that header file names mentioned in
	preprocessor #include directives are *not* string literals, so
	you may not have to worry about backslashes there.)

19.18:	I'm getting an error, "Too many open files".  How can I increase
	the allowable number of simultaneously open files?

A:	There are actually at least two resource limitations on the
	number of simultaneously open files: the number of low-level
	"file descriptors" or "file handles" available in the operating
	system, and the number of FILE structures available in the stdio
	library.  Both must be sufficient.  Under MS-DOS systems, you
	can control the number of operating system file handles with a
	line in CONFIG.SYS.  Some compilers come with instructions (and
	perhaps a source file or two) for increasing the number of stdio
	FILE structures.

19.20:	How can I read a directory in a C program?

A:	See if you can use the opendir() and readdir() routines, which
	are part of the POSIX standard and are available on most Unix
	variants.  Implementations also exist for MS-DOS, VMS, and other
	systems.  (MS-DOS also has FINDFIRST and FINDNEXT routines which
	do essentially the same thing.)  readdir() only returns file
	names; if you need more information about the file, try calling
	stat().  To match filenames to some wildcard pattern, see
	question 13.7.

	References: K&R2 Sec. 8.6 pp. 179-184; PCS Sec. 13 pp. 230-1;
	POSIX Sec. 5.1; Schumacher, ed., _Software Solutions in C_
	Sec. 8.

19.22:	How can I find out how much memory is available?

A:	Your operating system may provide a routine which returns this
	information, but it's quite system-dependent.

19.23:	How can I allocate arrays or structures bigger than 64K?

A:	A reasonable computer ought to give you transparent access to
	all available memory.  If you're not so lucky, you'll either
	have to rethink your program's use of memory, or use various
	system-specific techniques.

	64K is (still) a pretty big chunk of memory.  No matter how much
	memory your computer has available, it's asking a lot to be able
	to allocate huge amounts of it contiguously.  (The C Standard
	does not guarantee that a single object can be larger than 32K.)
	Often it's a good idea to use data structures which don't
	require that all memory be contiguous.  For dynamically-
	allocated multidimensional arrays, you can use pointers to
	pointers, as illustrated in question 6.16.  Instead of a large
	array of structures, you can use a linked list, or an array of
	pointers to structures.

	If you're using a PC-compatible (8086-based) system, and running
	up against a 640K limit, consider using "huge" memory model, or
	expanded or extended memory, or malloc variants such as halloc()
	or farmalloc(), or a 32-bit "flat" compiler (e.g. djgpp, see
	question 18.3), or some kind of a DOS extender, or another
	operating system.

	References: ANSI Sec.; ISO Sec.

19.24:	What does the error message "DGROUP data allocation exceeds 64K"
	mean, and what can I do about it?  I thought that using large
	model meant that I could use more than 64K of data!

A:	Even in large memory models, MS-DOS compilers apparently toss
	certain data (strings, some initialized global or static
	variables) into a default data segment, and it's this segment
	that is overflowing.  Either use less global data, or, if you're
	already limiting yourself to reasonable amounts (and if the
	problem is due to something like the number of strings), you may
	be able to coax the compiler into not using the default data
	segment for so much.  Some compilers place only "small" data
	objects in the default data segment, and give you a way (e.g.
	the /Gt option under Microsoft compilers) to configure the
	threshold for "small."

19.25:	How can I access memory (a memory-mapped device, or graphics
	memory) located at a certain address?

A:	Set a pointer, of the appropriate type, to the right number
	(using an explicit cast to assure the compiler that you really
	do intend this nonportable conversion):

		unsigned int *magicloc = (unsigned int *)0x12345678;

	Then, *magicloc refers to the location you want.  (Under MS-DOS,
	you may find a macro like MK_FP() handy for working with
	segments and offsets.)

	References: K&R1 Sec. A14.4 p. 210; K&R2 Sec. A6.6 p. 199; ANSI
	Sec. 3.3.4; ISO Sec. 6.3.4; Rationale Sec. 3.3.4; H&S Sec. 6.2.7
	pp. 171-2.

19.27:	How can I invoke another program (a standalone executable, or an
	operating system command) from within a C program?

A:	Use the library function system(), which does exactly that.
	Note that system's return value is the command's exit status,
	and usually has nothing to do with the output of the command.
	Note also that system() accepts a single string representing the
	command to be invoked; if you need to build up a complex command
	line, you can use sprintf().  See also question 19.30.

	References: K&R1 Sec. 7.9 p. 157; K&R2 Sec. 7.8.4 p. 167,
	Sec. B6 p. 253; ANSI Sec.; ISO Sec.; H&S
	Sec. 19.2 p. 407; PCS Sec. 11 p. 179.

19.30:	How can I invoke another program or command and trap its output?

A:	Unix and some other systems provide a popen() routine, which
	sets up a stdio stream on a pipe connected to the process
	running a command, so that the output can be read (or the input
	supplied).  (Also, remember to call pclose().)

	If you can't use popen(), you may be able to use system(), with
	the output going to a file which you then open and read.

	If you're using Unix and popen() isn't sufficient, you can learn
	about pipe(), dup(), fork(), and exec().

	(One thing that probably would *not* work, by the way, would be
	to use freopen().)

	References: PCS Sec. 11 p. 169.

19.31:	How can my program discover the complete pathname to the
	executable from which it was invoked?

A:	argv[0] may contain all or part of the pathname, or it may
	contain nothing.  You may be able to duplicate the command
	language interpreter's search path logic to locate the
	executable if the name in argv[0] is present but incomplete.
	However, there is no guaranteed solution.

	References: K&R1 Sec. 5.11 p. 111; K&R2 Sec. 5.10 p. 115; ANSI
	Sec.; ISO Sec.; H&S Sec. 20.1 p. 416.

19.32:	How can I automatically locate a program's configuration files
	in the same directory as the executable?

A:	It's hard; see also question 19.31 above.  Even if you can
	figure out a workable way to do it, you might want to consider
	making the program's auxiliary (library) directory configurable,
	perhaps with an environment variable.  (It's especially
	important to allow variable placement of a program's
	configuration files when the program will be used by several
	people, e.g. on a multiuser system.)

19.33:	How can a process change an environment variable in its caller?

A:	It may or may not be possible to do so at all.  Different
	operating systems implement global name/value functionality
	similar to the Unix environment in different ways.  Whether the
	"environment" can be usefully altered by a running program, and
	if so, how, is system-dependent.

	Under Unix, a process can modify its own environment (some
	systems provide setenv() or putenv() functions for the purpose),
	and the modified environment is generally passed on to child
	processes, but it is *not* propagated back to the parent

19.36:	How can I read in an object file and jump to routines in it?

A:	You want a dynamic linker or loader.  It may be possible to
	malloc some space and read in object files, but you have to know
	an awful lot about object file formats, relocation, etc.  Under
	BSD Unix, you could use system() and ld -A to do the linking for
	you.  Many versions of SunOS and System V have the -ldl library
	which allows object files to be dynamically loaded.  Under VMS,
	use LIB$FIND_IMAGE_SYMBOL.  GNU has a package called "dld".  See
	also question 15.13.

19.37:	How can I implement a delay, or time a user's response, with sub-
	second resolution?

A:	Unfortunately, there is no portable way.  V7 Unix, and derived
	systems, provided a fairly useful ftime() routine with
	resolution up to a millisecond, but it has disappeared from
	System V and POSIX.  Other routines you might look for on your
	system include clock(), delay(), gettimeofday(), msleep(),
	nap(), napms(), setitimer(), sleep(), times(), and usleep().
	(A routine called wait(), however, is at least under Unix *not*
	what you want.)  The select() and poll() calls (if available)
	can be pressed into service to implement simple delays.  On MS-
	DOS machines, it is possible to reprogram the system timer and
	timer interrupts.

	Of these, only clock() is part of the ANSI Standard.  The
	difference between two calls to clock() gives elapsed execution
	time, and if CLOCKS_PER_SEC is greater than 1, the difference will
	have subsecond resolution.  However, clock() gives elapsed
	processor time used by the current program, which on a
	multitasking system may differ considerably from real time.

	If you're trying to implement a delay and all you have available
	is a time-reporting function, you can implement a CPU-intensive
	busy-wait, but this is only an option on a single-user, single-
	tasking machine as it is terribly antisocial to any other
	processes.  Under a multi-tasking operating system, be sure to
	use a call which puts your process to sleep for the duration,
	such as sleep() or select(), or pause() in conjunction with
	alarm() or setitimer().

	For really brief delays, it's tempting to use a do-nothing loop

		long int i;
		for(i = 0; i < 1000000; i++)

	but resist this temptation if at all possible!  For one thing,
	your carefully-calculated delay loops will stop working next
	month when a faster processor comes out.  Perhaps worse, a
	clever compiler may notice that the loop does nothing and
	optimize it away completely.

	References: H&S Sec. 18.1 pp. 398-9; PCS Sec. 12 pp. 197-8,215-
	6; POSIX Sec. 4.5.2.

19.38:	How can I trap or ignore keyboard interrupts like control-C?

A:	The basic step is to call signal(), either as

		signal(SIGINT, SIG_IGN);

	to ignore the interrupt signal, or as

		extern void func(int);
		signal(SIGINT, func);

	to cause control to transfer to function func() on receipt of an
	interrupt signal.

	On a multi-tasking system such as Unix, it's best to use a
	slightly more involved technique:

		extern void func(int);
		if(signal(SIGINT, SIG_IGN) != SIG_IGN)
			signal(SIGINT, func);

	The test and extra call ensure that a keyboard interrupt typed
	in the foreground won't inadvertently interrupt a program
	running in the background (and it doesn't hurt to code calls to
	signal() this way on any system).

	On some systems, keyboard interrupt handling is also a function
	of the mode of the terminal-input subsystem; see question 19.1.
	On some systems, checking for keyboard interrupts is only
	performed when the program is reading input, and keyboard
	interrupt handling may therefore depend on which input routines
	are being called (and *whether* any input routines are active at
	all).  On MS-DOS systems, setcbrk() or ctrlbrk() functions may
	also be involved.

	References: ANSI Secs. 4.7,4.7.1; ISO Secs. 7.7,7.7.1; H&S
	Sec. 19.6 pp. 411-3; PCS Sec. 12 pp. 210-2; POSIX
	Secs. 3.3.1,3.3.4.

19.39:	How can I handle floating-point exceptions gracefully?

A:	On many systems, you can define a routine matherr() which will
	be called when there are certain floating-point errors, such as
	errors in the math routines in .  You may also be able
	to use signal() (see question 19.38 above) to catch SIGFPE.  See
	also question 14.9.

	References: Rationale Sec. 4.5.1.

19.40:	How do I...  Use sockets?  Do networking?  Write client/server

A:	All of these questions are outside of the scope of this list and
	have much more to do with the networking facilities which you
	have available than they do with C.  Good books on the subject
	are Douglas Comer's three-volume _Internetworking with TCP/IP_
	and W. R. Stevens's _UNIX Network Programming_.  (There is also
	plenty of information out on the net itself.)

19.40b: How do I use BIOS calls?  How can I write ISR's?  How can I
	create TSR's?

A:	These are very particular to specific systems (PC compatibles
	running MS-DOS, most likely).  You'll get much better
	information in a specific newsgroup such as
	comp.os.msdos.programmer or its FAQ list; another excellent
	resource is Ralf Brown's interrupt list.

19.41:	But I can't use all these nonstandard, system-dependent
	functions, because my program has to be ANSI compatible!

A:	You're out of luck.  Either you misunderstood your requirement,
	or it's an impossible one to meet.  ANSI/ISO Standard C simply
	does not define ways of doing these things.  (POSIX defines a
	few.)  It is possible, and desirable, for *most* of a program to
	be ANSI-compatible, deferring the system-dependent functionality
	to a few routines in a few files which are rewritten for each
	system ported to.

Section 20. Miscellaneous

20.1:	How can I return multiple values from a function?

A:	Either pass pointers to several locations which the function can
	fill in, or have the function return a structure containing the
	desired values, or (in a pinch) consider global variables.  See
	also questions 2.7, 4.8, and 7.5.

20.3:	How do I access command-line arguments?

A:	They are pointed to by the argv array with which main() is

	References: K&R1 Sec. 5.11 pp. 110-114; K&R2 Sec. 5.10 pp. 114-
	118; ANSI Sec.; ISO Sec.; H&S Sec. 20.1 p.
	416; PCS Sec. 5.6 pp. 81-2, Sec. 11 p. 159, pp. 339-40 Appendix
	F; Schumacher, ed., _Software Solutions in C_ Sec. 4 pp. 75-85.

20.5:	How can I write data files which can be read on other machines
	with different word size, byte order, or floating point formats?

A:	The most portable solution is to use text files (usually ASCII),
	written with fprintf() and read with fscanf() or the like.
	(Similar advice also applies to network protocols.)  Be
	skeptical of arguments which imply that text files are too big,
	or that reading and writing them is too slow.  Not only is their
	efficiency frequently acceptable in practice, but the advantages
	of being able to interchange them easily between machines, and
	manipulate them with standard tools, can be overwhelming.

	If you must use a binary format, you can improve portability,
	and perhaps take advantage of prewritten I/O libraries, by
	making use of standardized formats such as Sun's XDR (RFC 1014),
	OSI's ASN.1 (referenced in CCITT X.409 and ISO 8825 "Basic
	Encoding Rules"), CDF, netCDF, or HDF.  See also questions 2.12
	and 12.38.

	References: PCS Sec. 6 pp. 86,88.

20.6:	If I have a char * variable pointing to the name of a function,
	how can I call that function?

A:	The most straightforward thing to do is to maintain a
	correspondence table of names and function pointers:

		int func(), anotherfunc();

		struct { char *name; int (*funcptr)(); } symtab[] = {
			"func",		func,
			"anotherfunc",	anotherfunc,

	Then, search the table for the name, and call via the associated
	function pointer.  See also questions 2.15 and 19.36.

	References: PCS Sec. 11 p. 168.

20.8:	How can I implement sets or arrays of bits?

A:	Use arrays of char or int, with a few macros to access the
	desired bit at the proper index.  Here are some simple macros to
	use with arrays of char:

		#include 		/* for CHAR_BIT */

		#define BITMASK(b) (1 << ((b) % CHAR_BIT))
		#define BITSLOT(b) ((b) / CHAR_BIT)
		#define BITSET(a, b) ((a)[BITSLOT(b)] |= BITMASK(b))
		#define BITTEST(a, b) ((a)[BITSLOT(b)] & BITMASK(b))

	(If you don't have , try using 8 for CHAR_BIT.)

	References: H&S Sec. 7.6.7 pp. 211-216.

20.9:	How can I determine whether a machine's byte order is big-endian
	or little-endian?

A:	One way is to use a pointer:

		int x = 1;
		if(*(char *)&x == 1)
		else	printf("big-endian\n");

	It's also possible to use a union.

	See also question 10.16.

	References: H&S Sec. 6.1.2 pp. 163-4.

20.10:	How can I convert integers to binary or hexadecimal?

A:	Make sure you really know what you're asking.  Integers are
	stored internally in binary, although for most purposes it is
	not incorrect to think of them as being in octal, decimal, or
	hexadecimal, whichever is convenient.  The base in which a
	number is expressed matters only when that number is read in
	from or written out to the outside world.

	In source code, a non-decimal base is indicated by a leading 0
	or 0x (for octal or hexadecimal, respectively).  During I/O, the
	base of a formatted number is controlled in the printf and scanf
	family of functions by the choice of format specifier (%d, %o,
	%x, etc.) and in the strtol() and strtoul() functions by the
	third argument.  During *binary* I/O, however, the base again
	becomes immaterial.

	For more information about "binary" I/O, see question 2.11.  See
	also questions 8.6 and 13.1.

	References: ANSI Secs.,; ISO

20.11:	Can I use base-2 constants (something like 0b101010)?
	Is there a printf() format for binary?

A:	No, on both counts.  You can convert base-2 string
	representations to integers with strtol().

20.12:	What is the most efficient way to count the number of bits which
	are set in a value?

A:	Many "bit-fiddling" problems like this one can be sped up and
	streamlined using lookup tables (but see question 20.13 below).

20.13:	How can I make my code more efficient?

A:	Efficiency, though a favorite comp.lang.c topic, is not
	important nearly as often as people tend to think it is.  Most
	of the code in most programs is not time-critical.  When code is
	not time-critical, it is far more important that it be written
	clearly and portably than that it be written maximally
	efficiently.  (Remember that computers are very, very fast, and
	that even "inefficient" code can run without apparent delay.)

	It is notoriously difficult to predict what the "hot spots" in a
	program will be.  When efficiency is a concern, it is important
	to use profiling software to determine which parts of the
	program deserve attention.  Often, actual computation time is
	swamped by peripheral tasks such as I/O and memory allocation,
	which can be sped up by using buffering and caching techniques.

	Even for code that *is* time-critical, it is not as important to
	"microoptimize" the coding details.  Many of the "efficient
	coding tricks" which are frequently suggested (e.g. substituting
	shift operators for multiplication by powers of two) are
	performed automatically by even simpleminded compilers.
	Heavyhanded optimization attempts can make code so bulky that
	performance is actually degraded, and are rarely portable (i.e.
	they may speed things up on one machine but slow them down on
	another).  In any case, tweaking the coding usually results in
	at best linear performance improvements; the big payoffs are in
	better algorithms.

	For more discussion of efficiency tradeoffs, as well as good
	advice on how to improve efficiency when it is important, see
	chapter 7 of Kernighan and Plauger's _The Elements of
	Programming Style_, and Jon Bentley's _Writing Efficient

20.14:	Are pointers really faster than arrays?  How much do function
	calls slow things down?  Is ++i faster than i = i + 1?

A:	Precise answers to these and many similar questions depend of
	course on the processor and compiler in use.  If you simply must
	know, you'll have to time test programs carefully.  (Often the
	differences are so slight that hundreds of thousands of
	iterations are required even to see them.  Check the compiler's
	assembly language output, if available, to see if two purported
	alternatives aren't compiled identically.)

	It is "usually" faster to march through large arrays with
	pointers rather than array subscripts, but for some processors
	the reverse is true.

	Function calls, though obviously incrementally slower than in-
	line code, contribute so much to modularity and code clarity
	that there is rarely good reason to avoid them.

	Before rearranging expressions such as i = i + 1, remember that
	you are dealing with a compiler, not a keystroke-programmable
	calculator.  Any decent compiler will generate identical code
	for ++i, i += 1, and i = i + 1.  The reasons for using ++i or
	i += 1 over i = i + 1 have to do with style, not efficiency.
	(See also question 3.12.)

20.17:	Is there a way to switch on strings?

A:	Not directly.  Sometimes, it's appropriate to use a separate
	function to map strings to integer codes, and then switch on
	those.  Otherwise, of course, you can fall back on strcmp() and
	a conventional if/else chain.  See also questions 10.12, 20.18,
	and 20.29.

	References: K&R1 Sec. 3.4 p. 55; K&R2 Sec. 3.4 p. 58; ANSI
	Sec.; ISO Sec.; H&S Sec. 8.7 p. 248.

20.18:	Is there a way to have non-constant case labels (i.e. ranges or
	arbitrary expressions)?

A:	No.  The switch statement was originally designed to be quite
	simple for the compiler to translate, therefore case labels are
	limited to single, constant, integral expressions.  You *can*
	attach several case labels to the same statement, which will let
	you cover a small range if you don't mind listing all cases

	If you want to select on arbitrary ranges or non-constant
	expressions, you'll have to use an if/else chain.

	See also questions question 20.17.

	References: K&R1 Sec. 3.4 p. 55; K&R2 Sec. 3.4 p. 58; ANSI
	Sec.; ISO Sec.; Rationale Sec.; H&S
	Sec. 8.7 p. 248.

20.19:	Are the outer parentheses in return statements really optional?

A:	Yes.

	Long ago, in the early days of C, they were required, and just
	enough people learned C then, and wrote code which is still in
	circulation, that the notion that they might still be required
	is widespread.

	(As it happens, parentheses are optional with the sizeof
	operator, too, as long as its operand is a variable or a unary

	References: K&R1 Sec. A18.3 p. 218; ANSI Sec. 3.3.3, Sec. 3.6.6;
	ISO Sec. 6.3.3, Sec. 6.6.6; H&S Sec. 8.9 p. 254.

20.20:	Why don't C comments nest?  How am I supposed to comment out
	code containing comments?  Are comments legal inside quoted

A:	C comments don't nest mostly because PL/I's comments, which C's
	are borrowed from, don't either.  Therefore, it is usually
	better to "comment out" large sections of code, which might
	contain comments, with #ifdef or #if 0 (but see question 11.19).

	The character sequences /* and */ are not special within double-
	quoted strings, and do not therefore introduce comments, because
	a program (particularly one which is generating C code as
	output) might want to print them.

	Note also that // comments, as in C++, are not currently legal
	in C, so it's not a good idea to use them in C programs (even if
	your compiler supports them as an extension).

	References: K&R1 Sec. A2.1 p. 179; K&R2 Sec. A2.2 p. 192; ANSI
	Sec. 3.1.9 (esp. footnote 26), Appendix E; ISO Sec. 6.1.9, Annex
	F; Rationale Sec. 3.1.9; H&S Sec. 2.2 pp. 18-9; PCS Sec. 10 p.

20.24:	Why doesn't C have nested functions?

A:	It's not trivial to implement nested functions such that they
	have the proper access to local variables in the containing
	function(s), so they were deliberately left out of C as a
	simplification.  (gcc does allow them, as an extension.)  For
	many potential uses of nested functions (e.g. qsort comparison
	functions), an adequate if slightly cumbersome solution is to
	use an adjacent function with static declaration, communicating
	if necessary via a few static variables.  (A cleaner solution
	when such functions must communicate is to pass around a pointer
	to a structure containing the necessary context.)

20.25:	How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions
	from C?  (And vice versa?)

A:	The answer is entirely dependent on the machine and the specific
	calling sequences of the various compilers in use, and may not
	be possible at all.  Read your compiler documentation very
	carefully; sometimes there is a "mixed-language programming
	guide," although the techniques for passing arguments and
	ensuring correct run-time startup are often arcane.  More
	information may be found in FORT.gz by Glenn Geers, available
	via anonymous ftp from in the src

	cfortran.h, a C header file, simplifies C/FORTRAN interfacing on
	many popular machines.  It is available via anonymous ftp from (

	In C++, a "C" modifier in an external function declaration
	indicates that the function is to be called using C calling

	References: H&S Sec. 4.9.8 pp. 106-7.

20.26:	Does anyone know of a program for converting Pascal or FORTRAN
	(or LISP, Ada, awk, "Old" C, ...) to C?

A:	Several freely distributable programs are available:

	p2c	A Pascal to C converter written by Dave Gillespie,
		posted to comp.sources.unix in March, 1990 (Volume 21);
		also available by anonymous ftp from, file pub/p2c-1.20.tar.Z .

	ptoc	Another Pascal to C converter, this one written in
		Pascal (comp.sources.unix, Volume 10, also patches in
		Volume 13?).

	f2c	A Fortran to C converter jointly developed by people
		from Bell Labs, Bellcore, and Carnegie Mellon.  To find
		out more about f2c, send the mail message "send index
		from f2c" to [email protected] or research!netlib.
		(It is also available via anonymous ftp on, in directory netlib/f2c.)

	This FAQ list's maintainer also has available a list of a few
	other commercial translation products, and some for more obscure

	See also questions 11.31 and 18.16.

20.27:	Is C++ a superset of C?  Can I use a C++ compiler to compile C

A:	C++ was derived from C, and is largely based on it, but there
	are some legal C constructs which are not legal C++.
	Conversely, ANSI C inherited several features from C++,
	including prototypes and const, so neither language is really a
	subset or superset of the other.  In spite of the differences,
	many C programs will compile correctly in a C++ environment, and
	many recent compilers offer both C and C++ compilation modes.

	References: H&S p. xviii, Sec. 1.1.5 p. 6, Sec. 2.8 pp. 36-7,
	Sec. 4.9 pp. 104-107.

20.28:	I need a sort of an "approximate" strcmp routine, for comparing
	two strings for close, but not necessarily exact, equality.

A:	Some nice information and algorithms having to do with
	approximate string matching, as well as a useful bibliography,
	can be found in Sun Wu and Udi Manber's paper "AGREP -- A Fast
	Approximate Pattern-Matching Tool."

	Another approach involves the "soundex" algorithm, which maps
	similar-sounding words to the same codes.  Soundex was designed
	for discovering similar-sounding names (for telephone directory
	assistance, as it happens), but it can be pressed into service
	for processing arbitrary words.

	References: Knuth Sec. 6 pp. 391-2 Volume 3; Wu and Manber,
	"AGREP -- A Fast Approximate Pattern-Matching Tool" .

20.29:	What is hashing?

A:	Hashing is the process of mapping strings to integers, usually
	in a relatively small range.  A "hash function" maps a string
	(or some other data structure) to a a bounded number (the "hash
	bucket") which can more easily be used as an index in an array,
	or for performing repeated comparisons.  (Obviously, a mapping
	from a potentially huge set of strings to a small set of
	integers will not be unique.  Any algorithm using hashing
	therefore has to deal with the possibility of "collisions.")
	Many hashing functions and related algorithms have been
	developed; a full treatment is beyond the scope of this list.

	References: K&R2 Sec. 6.6; Knuth Sec. 6.4 pp. 506-549 Volume 3;
	Sedgewick Sec. 16 pp. 231-244.

20.31:	How can I find the day of the week given the date?

A:	Use mktime() or localtime() (see questions 13.13 and 13.14, but
	beware of DST adjustments if tm_hour is 0), or Zeller's
	congruence (see the sci.math FAQ list), or this elegant code by
	Tomohiko Sakamoto:

		dayofweek(y, m, d)	/* 0 = Sunday */
		int y, m, d;		/* 1 <= m <= 12, y > 1752 or so */
			static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
			y -= m < 3;
			return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;

	See also questions 13.14 and 20.32.

	References: ANSI Sec.; ISO Sec.

20.32:	Will 2000 be a leap year?  Is (year % 4 == 0) an accurate test
	for leap years?

A:	Yes and no, respectively.  The full expression for the present
	Gregorian calendar is

		year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)

	See a good astronomical almanac or other reference for details.
	(To forestall an eternal debate: references which claim the
	existence of a 4000-year rule are wrong.)

20.34:	Here's a good puzzle: how do you write a program which produces
	its own source code as its output?

A:	It is actually quite difficult to write a self-reproducing
	program that is truly portable, due particularly to quoting and
	character set difficulties.

	Here is a classic example (which is normally presented on one
	line, although it will "fix" itself the first time it's run):


	(This program, like many of the genre, assumes that the double-
	quote character " has the value 34, as it does in ASCII.)

20.35:	What is "Duff's Device"?

A:	It's a devastatingly deviously unrolled byte-copying loop,
	devised by Tom Duff while he was at Lucasfilm.  In its "classic"
	form, it looks like:

		register n = (count + 7) / 8;	/* count > 0 assumed */
		switch (count % 8)
		case 0:    do { *to = *from++;
		case 7:		*to = *from++;
		case 6:         *to = *from++;
		case 5:		*to = *from++;
		case 4:		*to = *from++;
		case 3:		*to = *from++;
		case 2:		*to = *from++;
		case 1:		*to = *from++;
			       } while (--n > 0);

	where count bytes are to be copied from the array pointed to by
	from to the memory location pointed to by to (which is a memory-
	mapped device output register, which is why to isn't
	incremented).  It solves the problem of handling the leftover
	bytes (when count isn't a multiple of 8) by interleaving a
	switch statement with the loop which copies bytes 8 at a time.
	(Believe it or not, it *is* legal to have case labels buried
	within blocks nested in a switch statement like this.  In his
	announcement of the technique to C's developers and the world,
	Duff noted that C's switch syntax, in particular its "fall
	through" behavior, had long been controversial, and that "This
	code forms some sort of argument in that debate, but I'm not
	sure whether it's for or against.")

20.36:	When will the next International Obfuscated C Code Contest
	(IOCCC) be held?  How can I get a copy of the current and
	previous winning entries?

A:	The contest schedule is tied to the dates of the USENIX
	conferences at which the winners are announced.  At the time of
	this writing, it is expected that the yearly contest will open
	in October.  To obtain a current copy of the rules and
	guidelines, send e-mail with the Subject: line "send rules" to:

		{apple,pyramid,sun,uunet}!hoptoad!judges  or
		[email protected]

	(Note that these are *not* the addresses for submitting

	Contest winners should be announced at the winter USENIX
	conference in January, and are posted to the net sometime
	thereafter.  Winning entries from previous years (back to 1984)
	are archived at (see question 18.16) under the
	directory pub/ioccc/.

	As a last resort, previous winners may be obtained by sending e-
	mail to the above address, using the Subject: "send YEAR
	winners", where YEAR is a single four-digit year, a year range,
	or "all".

20.37:	What was the entry keyword mentioned in K&R1?

A:	It was reserved to allow the possibility of having functions
	with multiple, differently-named entry points, a la FORTRAN.  It
	was not, to anyone's knowledge, ever implemented (nor does
	anyone remember what sort of syntax might have been imagined for
	it).  It has been withdrawn, and is not a keyword in ANSI C.
	(See also question 1.12.)

	References: K&R2 p. 259 Appendix C.

20.38:	Where does the name "C" come from, anyway?

A:	C was derived from Ken Thompson's experimental language B, which
	was inspired by Martin Richards's BCPL (Basic Combined
	Programming Language), which was a simplification of CPL
	(Cambridge Programming Language).  For a while, there was
	speculation that C's successor might be named P (the third
	letter in BCPL) instead of D, but of course the most visible
	descendant language today is C++.

20.39:	How do you pronounce "char"?

A:	You can pronounce the C keyword "char" in at least three ways:
	like the English words "char," "care," or "car;" the choice is

20.40:	Where can I get extra copies of this list?  What about back

A:	An up-to-date copy may be obtained from in
	directory u/s/scs/C-faq/.  You can also just pull it off the
	net; it is normally posted to comp.lang.c on the first of each
	month, with an Expires: line which should keep it around all
	month.  A parallel, abridged version is available (and posted),
	as is a list of changes accompanying each significantly updated

	The various versions of this list are also posted to the
	newsgroups comp.answers and news.answers .  Several sites
	archive news.answers postings and other FAQ lists, including
	this one; two sites are (directories
	pub/usenet/news.answers/C-faq/ and pub/usenet/comp.lang.c/) and (directory usenet/news.answers/C-faq/).  An archie
	server (see question 18.16) should help you find others; ask it
	to "find C-faq".  If you don't have ftp access, a mailserver at can mail you FAQ lists: send a message containing
	the single word help to [email protected] .  See the meta-
	FAQ list in news.answers for more information.

	A hypertext (HTML) version of this FAQ list is available on the
	World-Wide Web; the URL is .
	URL's pointing at all FAQ lists (these may also allow topic
	searching) are http://www.cis.ohio- and .

	An extended version of this FAQ list is being published by
	Addison-Wesley as _C Programming FAQs: Frequently Asked
	Questions_ (ISBN 0-201-84519-9).  It should be available in
	November 1995.

	This list is an evolving document of questions which have been
	Frequent since before the Great Renaming, not just a collection
	of this month's interesting questions.  Older copies are
	obsolete and don't contain much, except the occasional typo,
	that the current list doesn't.


Americal National Standards Institute, _American National Standard for
Information Systems -- Programming Language -- C_, ANSI X3.159-1989 (see
question 11.2).  [ANSI]

Americal National Standards Institute, _Rationale for American National
Standard for Information Systems -- Programming Language -- C_ (see
question 11.2).  [Rationale]

Jon Bentley, _Writing Efficient Programs_, Prentice-Hall, 1982, ISBN 0-

G.E.P. Box and Mervin E. Muller, "A Note on the Generation of Random
Normal Deviates," _Annals of Mathematical Statistics_, Vol. 29 #2, June,
1958, pp. 610-611.

David Burki, "Date Conversions," _The C Users Journal_, February 1993,
pp. 29-34.

Ian F. Darwin, _Checking C Programs with lint_, O'Reilly, 1988, ISBN 0-

David Goldberg, "What Every Computer Scientist Should Know about
Floating-Point Arithmetic," _ACM Computing Surveys_, Vol. 23 #1, March,
1991, pp. 5-48.

Samuel P. Harbison and Guy L. Steele, Jr., _C: A Reference Manual_,
Fourth Edition, Prentice-Hall, 1995, ISBN 0-13-326224-3.  [H&S]

Mark R. Horton, _Portable C Software_, Prentice Hall, 1990, ISBN 0-13-
868050-7.  [PCS]

Institute of Electrical and Electronics Engineers, _Portable Operating
System Interface (POSIX) -- Part 1: System Application Program Interface
(API) [C Language_, IEEE Std. 1003.1, ISO/IEC 9945-1.

International Organization for Standardization, ISO 9899:1990 (see
question 11.2).  [ISO]

Brian W. Kernighan and P.J. Plauger, _The Elements of Programming
Style_, Second Edition, McGraw-Hill, 1978, ISBN 0-07-034207-5.

Brian W. Kernighan and Dennis M. Ritchie, _The C Programming Language_,
Prentice-Hall, 1978, ISBN 0-13-110163-3.  [K&R1]

Brian W. Kernighan and Dennis M. Ritchie, _The C Programming Language_,
Second Edition, Prentice Hall, 1988, ISBN 0-13-110362-8, 0-13-110370-9.

Donald E. Knuth, _The Art of Computer Programming_.  Volume 1:
_Fundamental Algorithms_, Second Edition, Addison-Wesley, 1973, ISBN 0-
201-03809-9.  Volume 2: _Seminumerical Algorithms_, Second Edition,
Addison-Wesley, 1981, ISBN 0-201-03822-6.  Volume 3: _Sorting and
Searching_, Addison-Wesley, 1973, ISBN 0-201-03803-X.  [Knuth]

Andrew Koenig, _C Traps and Pitfalls_, Addison-Wesley, 1989, ISBN 0-201-
17928-8.  [CT&P]

Stephen K. Park and Keith W. Miller, "Random Number Generators: Good
Ones are Hard to Find," _Communications of the ACM_, Vol. 31 #10,
October, 1988, pp. 1192-1201 (also technical correspondence August,
1989, pp. 1020-1024, and July, 1993, pp. 108-110).

P.J. Plauger, _The Standard C Library_, Prentice Hall, 1992, ISBN 0-13-

Thomas Plum, _C Programming Guidelines_, Second Edition, Plum Hall,
1989, ISBN 0-911537-07-4.

William H. Press, Saul A. Teukolsky, William T. Vetterling, and Brian P.
Flannery, _Numerical Recipes in C_, Second Edition, Cambridge University
Press, 1992, ISBN 0-521-43108-5.

Dale Schumacher, Ed., _Software Solutions in C_, AP Professional, 1994,
ISBN 0-12-632360-7.

Robert Sedgewick, _Algorithms in C_, Addison-Wesley, 1990, ISBN 0-201-

Charles Simonyi and Martin Heller, "The Hungarian Revolution," _Byte_,
August, 1991, pp.131-138.

David Straker, _C Style: Standards and Guidelines_, Prentice Hall, ISBN

Steve Summit, _C Programming FAQs: Frequently Asked Questions_, Addison-
Wesley, 1995, ISBN 0-201-84519-9.  [The book version of this FAQ list.]

Sun Wu and Udi Manber, "AGREP -- A Fast Approximate Pattern-Matching
Tool," USENIX Conference Proceedings, Winter, 1992, pp. 153-162.

There is another bibliography in the revised Indian Hill style guide
(see question 17.9).  See also question 18.10.


Thanks to Jamshid Afshar, David Anderson, Tanner Andrews, Sudheer Apte,
Joseph Arceneaux, Randall Atkinson, Rick Beem, Peter Bennett, Wayne
Berke, Dan Bernstein, Tanmoy Bhattacharya, John Bickers, Gary Blaine,
Yuan Bo, Dave Boutcher, Michael Bresnahan, Vincent Broman, Stan Brown,
Joe Buehler, Kimberley Burchett, Gordon Burditt, Burkhard Burow, Conor
P. Cahill, D'Arcy J.M. Cain, Christopher Calabrese, Ian Cargill, Vinit
Carpenter, Paul Carter, Mike Chambers, Billy Chambless, Franklin Chen,
Jonathan Chen, Raymond Chen, Richard Cheung, Steve Clamage, Ken Corbin,
Ian Cottam, Russ Cox, Jonathan Coxhead, Lee Crawford, Steve Dahmer,
Andrew Daviel, James Davies, John E. Davis, Ken Delong,
Norm Diamond, Jeff Dunlop, Ray Dunn, Stephen M. Dunn, Michael J. Eager,
Scott Ehrlich, Arno Eigenwillig, Dave Eisen, Bjorn Engsig, David Evans,
Clive D.W. Feather, Dominic Feeley, Simao Ferraz, Chris Flatters, Rod
Flores, Alexander Forst, Steve Fosdick, Jeff Francis, Tom Gambill, Dave
Gillespie, Samuel Goldstein, Tim Goodwin, Alasdair Grant, Ron Guilmette,
Doug Gwyn, Michael Hafner, Tony Hansen, Elliotte Rusty Harold, Joe
Harrington, Des Herriott, Guy Harris, John Hascall, Ger Hobbelt, Jos
Horsmeier, Blair Houghton, James C. Hu, Chin Huang, David Hurt, Einar
Indridason, Vladimir Ivanovic, Jon Jagger, Ke Jin, Kirk Johnson, Larry
Jones, Arjan Kenter, James Kew, Lawrence Kirby, Kin-ichi Kitano, Peter
Klausler, Andrew Koenig, Tom Koenig, Adam Kolawa, Jukka Korpela, Ajoy
Krishnan T, Markus Kuhn, Deepak Kulkarni, Oliver Laumann, John Lauro,
Felix Lee, Mike Lee, Timothy J. Lee, Tony Lee, Marty Leisner, Don Libes,
Brian Liedtke, Philip Lijnzaad, Keith Lindsay, Yen-Wei Liu, Paul Long,
Christopher Lott, Tim Love, Tim McDaniel, Kevin McMahon, Stuart
MacMartin, John R. MacMillan, Andrew Main, Bob Makowski, Evan Manning,
Barry Margolin, George Matas, Brad Mears, Roger Miller, Bill Mitchell,
Mark Moraes, Darren Morby, Bernhard Muenzer, David Murphy, Walter
Murray, Ralf Muschall, Ken Nakata, Todd Nathan, Landon Curt Noll, Tim
Norman, Paul Nulsen, David O'Brien, Richard A. O'Keefe, Adam Kolawa,
James Ojaste, Hans Olsson, Bob Peck, Andrew Phillips, Christopher
Phillips, Francois Pinard, Nick Pitfield, Wayne Pollock, Dan Pop, Lutz
Prechelt, Lynn Pye, Kevin D. Quitt, Pat Rankin, Arjun Ray, Eric S.
Raymond, Peter W. Richards, Eric Roode, Manfred Rosenboom, J. M.
Rosenstock, Rick Rowe, Erkki Ruohtula, John Rushford, Kadda Sahnine,
Tomohiko Sakamoto, Matthew Saltzman, Rich Salz, Chip Salzenberg, Matthew
Sams, Paul Sand, DaviD W. Sanderson, Frank Sandy, Christopher Sawtell,
Jonas Schlein, Paul Schlyter, Doug Schmidt, Rene Schmit, Russell Schulz,
Dean Schulze, Chris Sears, Patricia Shanahan, Raymond Shwake, Peter da
Silva, Joshua Simons, Ross Smith, Henri Socha, Leslie J. Somos, Henry
Spencer, David Spuler, James Stern, Bob Stout, Steve Sullivan, Melanie
Summit, Erik Talvola, Dave Taylor, Clarke Thatcher, Wayne Throop, Chris
Torek, Steve Traugott, Ilya Tsindlekht, Andrew Tucker, Goran Uddeborg,
Rodrigo Vanegas, Jim Van Zandt, Wietse Venema, Tom Verhoeff, Ed
Vielmetti, Larry Virden, Chris Volpe, Mark Warren, Alan Watson, Kurt
Watzka, Larry Weiss, Martin Weitzel, Howard West, Tom White, Freek
Wiedijk, Dik T. Winter, Lars Wirzenius, Dave Wolverton, Mitch Wright,
Conway Yee, Ozan S. Yigit, and Zhuo Zang, who have contributed, directly
or indirectly, to this article.  Special thanks to Karl Heuer, Jutta
Degener, and particularly to Mark Brader, who (to borrow a line from
Steve Johnson) have goaded me beyond my inclination, and occasionally
beyond my endurance, in relentless pursuit of a better FAQ list.

					Steve Summit
					[email protected]

Internet C Sites

Welcome to of the index of resources for numerical computation
in C or C++.  It is a collection of pointers to:

    - free source code available on the net,
    - articles and documents, especially those available over the net.

This file is
or c/numcomp-free-c on netlib (slightly outdated).
Also see the last item on

The book reviews which were here have been deleted in order to
combat bloat.  You can find them in
but be warned that we have a slow leased line and our systems are only
up from 9 to 7 GMT+0530.

Please see the section "interesting sites" below to get some help on
how to retrieve software listed here.

Table of Contents:

    * Explanations of fields
    * The index
    * f2c
    * Other pointers
    * Interesting sites
    * Credits

The index is biased towards fields I work in.
Please send me suggestions, corrections and improvements.

        -Ajay Shah, [email protected]

Explanations of fields

Name        if the archive has a obvious name, then that is shown.
            Otherwise I invent something sensible.
Where       is a pointer into a ftp site, or sufficient information to
            figure that out.  The information at EOF may enlighten
            you if you are still stuck.  Ideally I try to give information
            which is explicit enough for use with (say) ftpmail.
Systems     The default is Unix.  If it runs on other systems this is shown,
            if it does NOT run on Unix this is shown.
Language    The default is ANSI C.  The alternatives are K&R and C++.
Author      I try to give the name(s) and email addresses.  Sometimes the
            email address is a contact person, even if it's not the author.
Version     This tries to identify a most-recent version and gives it's date.
Description A one-line description
Comments    Are a few keywords thrown in to help you egrep.

Many things are incomplete; tell me of anything which hurts your eyes.
Please point me to goodies I've overlooked.  If you have source code
which may be of wide interest, please make it available to the net.

The index

Name        : AIPS++ library (beta)
Where       :
Systems     : Unix
Language    : C++
Author      : AIPS++ consortium, [email protected]
Version     : 3
Description : A class library under development for radio astronomical
              calibration and imaging.
Comments    : Released library has multidimensional array classes, FFT's
              gridding of ungridded data, containers, tables, a documentation
              extractor (from comments), etc.

Name        : ADOL-C
Author      : Andreas Griewank et al. ([email protected])
Systems     : Unix, cfront or g++
Version     : 1.5 (Dec 1993)
Description : Automatic differentiation package in C++
Where       : In pub/ADOLC at ftp sites
Comments    : Contains LaTeX documentation.
              Associated with article in TOMS.
              See book "Automatic differentiation of algorithms" edited
              by George Corliss and Andreas Griewank, SIAM, Dec 1991, where
              the chapter by D. Juedes lists many other automatic
              differentiation packages.

Name        : ajay
Where       : in general on Statlib
Description : cholesky decomposition and drawing from MVN
Author      : Ajay Shah, [email protected]
Version     : 23 Sept 1991

Name        : as274_fc.tar.z (42748 bytes)
Author      : Alan Miller ([email protected])
              Port to C and packaging by Ajay Shah ([email protected])
Systems     : Unix
Version     : 1 May 1993
Description : High accuracy least squares routines with facilities for
              WLS for a subset of variables, changing the order
              of variables, dealing with singularities, calculating an
              estimated covariance matrix of the coefficients.
              Both fortran and C versions are presented, along with
              a regression testing setup using ten test programs.
              See article "Least Squares Routines to Supplement those
              of Gentleman" in Applied Statistics 41(2), 1992 by
              Alan Miller.
Where       : pub/C-numanal on
Comments    : note the .z is the new gzip compression.

Name        : ASA
Where       :
Description : adaptive simulated annealing: performing adaptive global
              optimization on multivariate nonlinear stochastic systems
Language    : either K&R or ANSI C
Authors     : Lester Ingber ([email protected])
Comments    : Is very actively developed.
Version     : 12.1, 10 Feb 1996

Name        : AutoClass C
Authors     : Diane Cook & Joe Potts - U. Texas at Arlington
Description : C implementation of AutoClass: an unsupervised Bayesian 
              classification system that seeks a maximum posterior
              probability classification.
Systems     : SunSparc SunOS 4.1.3
Where       :
              or send e-mail to [email protected]
Language    : ANSI C, GNU gcc version 2.6.3
Version     : 1.0
Comments    : source code provided
Date        : 26 April 95

Name        : awesime
Description : a C++ task library explicitly designed for simulation.
Where       : pub/cs/misc/Awesime on
Author      : Dirk Grunwald ([email protected])
Version     : II

Name        : bignum
Where       : pub/bignum on ;
Description : directory filled with bignum software, and a file
              BIGNUMS.TXT which summaries bignum alternatives.
Author      : BIGNUMS.TXT is by Mark Riordan ([email protected])
              The ftp site is maintained by him.
Version     : April 1993.

Name        : bignum.tar.Z
Where       : in tars/math on (
Systems     : Unix
Description : Arbitrary Precision Integer Arithmetic
Author      : Serpette, Vuillemin, Jean-Claude Herve 
Version     : 23 Sept 1990
Comments    : Excellent. very fast. possible problems with unalloc call. 

Name        : blas.cpp.shar.z
Where       : in pub/C-numanal on
Author      : Damian McGuckin ([email protected])
Description : a BLAS in C++
Version     : beta, 8 May 1993

Name        : c++ (5665 bytes)
Author      : U. Ruede ([email protected])
Description : Summary of 1992 workshop "Scientific Computing in C++"
              (plain text file)
Where       : mgnet/papers/Ruede on
Date        : August 4 1992

Name        : C++SIM
Where       : on
Description : SIMULA and SIMSET style simulation package in C++
              with accompanying documentation.
Authors     : Mark Little ([email protected])
              Daniel McCue ([email protected])
Version     : 1.0 (June 14th 1993)
Comments    : A complete simulation package for creating process based
              discrete event simulation as in SIMULA. The linked-list
              manipulation facilities provided by SIMSET are also
              provided in the package. The system is built in an object-
              oriented manner and the documentation provides information
              on how it can be modified and extended. Active objects in
              C++ can also be provided outside of the simulation package
              by simply inheriting the desired thread characteristic.

Name        : cdhc
Where       :
Systems     : Unix
Language    : C
Author      : Darrell McCauley, [email protected]
Version     : 1.0 (12 Sep 1994)
Description : A library for testing normality & exponentiality
Comments    : Draft docs at\
              grass/tutorials/ Includes
              D'Agostino's D, Anderson-Darling, Cramer-Von Mises W^2,
              Kolmogorov-Smirnov, Chi-Square, Shapiro-Wilk, many others.
              Expands and fixes bugs in general/cdh in statlib.

Name        : cephes
Author      : Stephen L. Moshier, [email protected]
Description : Emphasis on high accuracy special functions, but
              also contains useful code for matrices, eigenvalues,
              integration, ODEs, complex arithmetic, chebyshev approximation.
Where       : the many files in directory cephes on netlib
Version     : 2.2, June 1992

Name        : Cfortran
Where       : []
Systems     : VAX VMS or Ultrix, DECstation, Silicon Graphics, 
              IBM RS/6000, Sun, CRAY, Apollo and HP9000.
Language    : C, FORTRAN 
Author      : Burkhard Burow, [email protected], University of Toronto
Version     : 2.5
Description : A set of macros (cfortran.h = 1000 lines) allowing function
              calls to be made from C to FORTRAN and vice-versa.
Comments    : Good compact way of calling functions without translating.
              Easy to use.

Name        : chernikov
Author      : Ata Etemadi ([email protected])
Where       : Volume 26, Issue 91 of comp.sources.unix
Description : computes the stochastic webs produced by the Chernikov
              equations (see Nature Vol. 326, April 1987) and produces
              a PGM image based on occupancy of cells. The equations
              essentially describe the path of a non-relativistic
              charged particle rotating about a magnetic field line,
              and experiencing a periodic electric field impulse.
Version     : v1.0, 3 April 1993

Name        : clapack
Where       : Start at
     (10187795 bytes)
Description : f2c translation of Lapack, with minor, mostly cosmetic
Author      : Jim Demmel ([email protected],
              Xiaoye Li ([email protected])
              Contact [email protected]
Version     : Built off Lapack 2.0, 30 Sep 1994
Comments    : They have ported the Lapack testing and timing code also.
              Their clapack is known to pass all the tests.

Name        : code++
Where       : in pub/code++ on
Language    : C++
Systems     : UNIX, GNU g++ and cfront
Description : C++ class library for ordinary differential equations
              and related problems. Contains lots of useful classes
              for linear algebra (vectors, matrices, linear solvers,
              pseudoinverses), and other utilities (minimal tool command
              language, etc.).
              The integration classes for ODEs are based on adaptive
              extrapolation methods (explicit Euler discretization
              for non-stiff, and implicit for stiff ODEs). Classes
              for continuous output, stepsize freezing, and variational
              equations are also provided, as well as an experimental
              multiple shooting environment for BVPs.
Author      : Andreas Hohmann, [email protected]

Name        : (12263 bytes)
Where       : in pub/C-numanal on
Systems     : Unix
Language    : C++
Description : An include file to make complex math look like
              regular math.
Author      : Leonard Kamlet, [email protected]
Version     : 8 March 1993
Comments    : The file uses a lot of operator overloading, so that 
              if x=a+ib and y=c+id, the code for multiplying the two
              together looks like z = x*y;  Also, the file includes nrutil
              from Numerical Recipes, and adds the complex versions for
              vectors and matrices.

Name        : CVODE
Where       : netlib/ode/cvode.tar.Z
Author      : Choen, Scott D. and Alan C. Hindmarsh
Version     : 5 October 1994
Language    : Ansi C
Description : ODE solver
Comments    : Integrates ODE's. BDF or Adams-Moulton Formula.
              Implicit equation is solved with Functional or Newtontype
              iteration. Direct or iterative solution of the lin. eq. of 
              the Newton iteration. Dense, diagonal, banded or sparse
              approximation of the Jacobimatrix of the right hand side of
              the ODE. Manual (91 p.) in postscript.

Name        : dcdflib
Authors     : Barry W. Brown, James Lovato, Kathy Russell
Description : Library of Routines for Cumulative Distribution
              Functions, Inverses, and Other Parameters
Systems     : Unix
Where       : in pub/unix/dcdflib.c-1.0-tar.Z
Language    : K&R and ANSI C available.
Version     : 1.0, February 1994

Name        : dcg.shar
Where       : in c on Netlib
Description : preconditioned conjugate gradient method
Author      : Mark K. Seager, [email protected]

Name        : dddd
Where       : in pub/dddd on
Description : dynamical data determinism detector (works with time-series
              data).  exploits Open windows 3.
Systems     : Unix
Version     : 21 Oct 1992
Author      : Dave Watson, [email protected]

Name        : Diffpack
Authors     : Hans Petter Langtangen ([email protected])
              Are Magnus Bruaset     ([email protected])
              + contributions from several other people
Description : A development environment for object-oriented simulators
              based on PDEs. C++ source code and documentation.
Systems     : Tested on
                SGI/IRIX 5.2,      C++ 3.2.1  
                HP/HP-UX 9.05,     C++ 3.50
                SPARC/Solaris 2.3, C++ 4.0
                IBM/AIX 3.2.5,     C++
                Relatively easy to port to other Unix platforms, does not
                work very well with g++.
Where       : and mirror sites, directory diffpack
Language    : C++ (and a few C functions for GUI)
Version     : 1.0
Comments    : See the Diffpack WWW Home Page on
              for presentation and the latest news.
Date        : April 28, 1995

Name        : drpn
Where       :
Systems     : Unix
Description : RPN calculator for digital signal processing
Author      : Dan Kegel, JPL
Version     : 1.1
Comments    : A simple way to do add, multiply, FFT, sum, shift operators
              on a stream of fixed-length records of data.  Handles several
              data types (16 bit int, 32 bit float). Used, for example, to
              process a synthetic aperture radar image.

Name        : dstool
Where       : somewhere on
Systems     : Unix, uses xview3 and open windows 3
Description : Dynamical systems simulation package 
              Plots Lorenz attractors and "other chaotic things" in realtime.
              Includes a expression evaluator.
Author      : 
Version     : 1.1

Name        : dtoa.c
Where       : in fp on Netlib
Description : correctly rounded decimal <--> binary conversion

Name        : eigen.1.01.shar.Z (80545 bytes)
Version     : 1.01, 25 March 1993
Author      : Nadav Har'El ([email protected])
Description : Find the N largest eigenvalues and their eigenvectors of a
              real matrix ( < 700x700).  Includes postscript documentation.
Where       : eigen directory on (

Name        : Euler
Where       : By anonymous ftp from
Files       : 212 kb /pub/unix/math/euler.tar.Z
Language    : ANSI-C
Author      : Rene Grothmann ([email protected])
Version     : 3.18
Description : Runs on UNIX/XWindow systems (OS/2 version available).
              Real and complex numbers and matrices. Lots of built in
              functions. Programming language. 2D/3D plots. ASCII-
              documentation and demo mode.  Matlab like.
Comments    : Tested on IBM Risc, Linux and Sun (with acc compiler)

Name        : fec
Authors     : B. Bagheri (email?)
Description : A collection of finite element libraries in C++
Where       : pub/Math on
Language    : GNU C++
Version     : 1.1
Date        :

Name        : FElt
Where       : pub/felt on
Description : introductory finite element analysis
Systems     : Unix commandline or Unix + X
              HP-SUX, Sun, Linux, DOS.
Version     : 2.0, 28 February 1994
Author      : Jason Gobat, [email protected]
              Darren Atkinson, [email protected]
Comments    : postscript manual and mailing list exists.

Name        : femlib-1.1.tar.gz
Author      : Michael Tiller ([email protected])
Where       : pub/C-numanal on
Systems     : UNIX
Language    : C++
Version     : 1.1, June 17 1993
Description : C++ class libraries for doing Finite Element simulations,
              Garbage Collection, Automatic Differentiation as
              well as a library for Sparse Matrices.
Comments    : This release is still pretty rough but should compile
              with gcc-2.4.3, gnumake-3.6x, libg++-2.3.1 and
              makedepend (from X11 distribution).  

Name        : fft.shar
Where       : in c++ on Netlib
Description : radix 2 FFT

Name        : fft-sstuff.tar.z
Where       : in pub/C-numanal on
Description : summary about FFT code in C, including lots of source
Author      : Peter J. McKinney ([email protected])
              and Ron Mayer ([email protected])
Version     : 19 March 1993
Comments    : Includes DDJ's improved version of Numerical Recipes four1().

Name        : fftsing
Where       :
Description : FFT of extremely long series; Singleton's mixed radix algo
Author      : Javier Soley, [email protected]

Name        : frac
Where       : in c on Netlib
Description : finds rational approximation to floating point value
Author      : Robert Craig, AT&T Bell Labs - Naperville

Name        : fromskip
Where       : send email to Skip Carter (address at EOF)
Language    : C++
Description : numerical derivatives with richardson extrapolation,
              runge-kutta code, monte-carlo integration, fredholm and
              voltera integral equation solvers, etc.

Name        : FSQP, CFSQP
Where       : send email to [email protected]
Systems     : many (including DOS)
Language    : FORTRAN (FSQP), C (CFSQP)                ,
Authors     : Jian L. Zhou ([email protected]) and Andre L. Tits 
              ([email protected]) (FSQP);
              Craig T. Lawrence ([email protected]), Zhou and Tits (CFSQP). 
Version     : FSQP: 3.3b, 9/93; CFSQP: 2.3, 11/8/95
Description : solution of constrained continuous optimization problems,
              possibly minimax (cost function is max of finitely many 
              functions).   CFSQP also includes efficient scheme to
              handle problems with many "sequentially related" objectives
              or constraints (e.g., finely discretized minimax problems
              or semi-infinite problems).
Comments    : modified SQP scheme; successive iterates are all feasible
              (inequality constraints) or "semi-feasible" (equality
              constraints).  70 page manual.
              keywords nonlinear minimisation maximisation
                       nonlinear programming

Name        : fudgit_2.31.tar.Z (451691 bytes)
Author      : Martin-D. Lacasse, [email protected]
Where       : pub/Fudgit on
Description : C-based fitting and data manipulation program (works on
              top of gnuplot).  Gives you a C-like interpreted script
Systems     : Unix only.
Comments    : See entry on gnuplot elsewhere in this document.
Version     : 2.31, 13 April 1993

Name        : gaut
Where       : in general on Statlib
Description : upper-tail probabilities on normal and t densities
Author      : Ajay Shah, [email protected]
Version     : 12 May 1991

Name        : ga's
Where       : pub/galist/source-code/ga-source on
Description : many genetic algorithm optimisation libraries, all in C
Comments    : they are GAucsd 1.4 (Nici Schraudolph, [email protected]),
              GENEsYS 1.0 (Thomas Baeck, [email protected])
              Genesis 5.0 (John J. Grefenstette, [email protected]),
              Goldberg's SGA in C (with a nCube version) by Rob Smith,
              [email protected]
              Also see survey of GA software in file GAsoft.txt at

Name        : GAlib
Where       :
Systems     : UNIX, DOS/Windows, MacOS
Language    : C++
Author      : Matthew Wall   [email protected]
Version     : 2.3.2   (2.4 coming December 1995)
Description : Objects for doing genetic algorithm optimization

Name        : gemmw
Description : a highly portable Level 3 BLAS implementation of Winograd's
              variant of Strassen's matrix multiplication algorithm
Where       : in misc on Netlib
Author      : Craig C. Douglas, [email protected]
Version     : 22 May 1992

Name        : genocop{,2}.tar.Z
Where       : in coe/evol on (
Description : nonlinear maximisation with linear constraints.  You write C
              code for the function to optimise and link into genocop.
              Allowable ranges for each parameter can be defined.  Author
              plans to do nonlinear constraints "soon".
Author      : ??, [email protected]
Version     : 2

Name        : geometry
Description : archive containing many programs on geometry
Where       : pub/contrib/comp_geom on
Comments    : Short summary as of 5 June 1993
              geomview -- interactive geometry viewing for SGI IRIS
              evolver -- models evolution of surfaces driven by forces
              hcad -- drawing hyperbolic polyhedra in 3d poincare disk (for X)
              invriemann -- inverse riemann mapping by circle packing
              riemannmap -- riemann mapping by circle packing
              kali - 2D euclidean symmetry pattern editor for SGI IRIS
              minneview -- general 3d viewing program for SGI IRIS
              polycut -- covering spaces of 3d euclidian space from inside
              crsolver -- conformal mapping, complex analytic functions (NeXT)
              automata -- automatic groups programs
              epsilon -- utility for squashing FP roundoff errors in data files
              hyper -- projective <--> conformal models of hyperbolic space
              omni_interp and
              interpolate -- interpolating between formatted data files
              kaleido -- constructing uniform polyhedra
              qhull -- general dimension convex hull computations program
              snappea -- hyperbolic structures computations
              vcs -- 3d voronoi diagram program
              viewwld -- viewing line drawings in 3d space (for Suns)
              vor2d -- 2d voronoi and delaunay diagrams, with cheyenne graphics

              kaos -- interactive dynamical systems package (Suns)

Name        : gle 
Description : graphics layout editor
              script or menu driven program for composing a graphics
              page.  Graphics primitives + PostScript file inclusion,
              plot generation from equations or tabular data + manipulation.
              Various output formats (X,ps,hpgl..) and utility programs
              (contour, surface, fits..)
Systems     : Unix, PC
Where       :
Version     : 3.3b
Language    : ANSI C
Author      : Chris Pugmire, [email protected]

Name        : gmp-1.3.tar.z
Description : GNU multiple precision library
Where       : in pub/gnu on
Version     : 1.3, May 10 1993
Author      : ?

Name        : gmt
Where       :
Description : great scientific graphics
Author      : ?
Systems     : Unix
Comments    : Fits the Unix philosophy.  Postscript output supported.
Language    : C

Name        : Gnans
Where       : in 
Systems     : Solaris 2.x, SunOS 4.1.x, SGI IRIX 5.x.
Language    : C++
Author      : Bengt Martensson 
Version     : 1.3, 26 August 1994
Description : Analyse deterministic and stochastic dynamic systems
Comments    : A program (and language) for dynamical systems. Includes
              simple scripting language. Graphical user interface. Copyleft.
              There is a mailing list.

Name        : gnufit10.tar.gz
Author      : Carsten Grammes ([email protected])
Description : Gnuplot 3.2 with nonlinear regression features added
Systems     : Most Unix, OS/2 2.x.  Needs popen(3).
Where       : pub/utils in
Version     : 1.0
Comments    : Levenberg-Marquadt nonlinear least squares
Date        : 28 June 1993

Name        : gnuplot3.5.tar.Z
Authors     : coordinated by Alex Woo ([email protected])
Description : plotting package for functions and data
Systems     : all systems, all graphics file formats, all output devices
Where       : in pub/gnuplot on
Version     : 3.5
Comments    : Includes probability functions, 3d plotting with contours
              and hidden line removal, parametric functions.  Has
              manual, online help, commandline editing and a newsgroup
              Can be used as a C library.
Date        : 17 August 1993

Name        : go.c.Z (7288 bytes)
Where       : in pub/C-numanal on
Description : Calculate gaussian quadrature rules.  Translation of 
              Netlib: go/gausq.f using f2c with some hand-cleaning.  You
              need a log gamma function.
Comments    : numerical integration

Name        : hare (Hazard Regression)
Where       : file hare (a shar file) in S directory on statlib
Author      : Charles Kooperberg ([email protected])
Description : estimates the conditional hazard rate based on possibly
              censored data and covariates. Includes parametric and
              non-parametric, additive and non-additive proportional and
              non-proportional hazards model as special cases.  Addition
              and deletion of basis functions make the fit highly adaptive.
Version     : statlib, last update April 21, 1993
Comments    : actually the objective of this file is to give a end-user
              of the S statistical package this functionality.  But the
              actual computation is done in C.
              Described in Univ. of California, Berkeley, Stat tech rep 389.
              Available from the author.

Name        : heft (Hazard Estimation with Flexible Tails)
Where       : file heft (a shar file) in S directory on statlib
Author      : Charles Kooperberg ([email protected])
Description : estimates the unconditional hazard rate using splines. Knot
              addition, deletion and two extra tail terms make the fit
              highly adaptive.
Version     : statlib, last update April 21, 1993
Comments    : actually the objective of this file is to give a end-user
              of the S statistical package this functionality.  But the
              actual computation is done in C.
              Described in Univ. of California, Berkeley, Stat tech rep 388.
              Available from the author.

Name        : hepC++.html
Authors     : Marcus Speh (?)
Description : Information on C++ applications in HEP
Where       : in pub/www/projects on
Language    : access through WWW
Date        : June 21 1993

Name        : HL_Vector.shar
Authors     : [email protected], [email protected]
Description : Aitken-Lagrange interpolation over the table of uniform or
              arbitrary mesh, and the Hook-Jeevse multidimensional minimizer.
Systems     : Unix
Where       : netlib (
Language    : C++ (gcc 2.5.8)
Version     : 1.0
Comments    : Test drivers and test run outputs are included, too. Commented.
              Needs LinAlg.shar
Date        : May 27, 1992

Name        : hooke.c
Authors     : Mark Johnson
Description : Hooke and Jeeves Algorithm
Where       : netlib/opt/hooke.c
Language    : C

Name        : IR-STAT-PAK
Where       :
Systems     : Written for Solaris but it has been compiled under Linux and
            : SunOS.  An AIX version required a few changes but not many.
            : The documentation discusses the O/S specific code.
Language    : ANSI C except that there are three arguments to main().  This
            : is unnecessary and will be removed in the next release.
Author      : J. Blustein 
Version     : 1.02 (released 30 August 1995)
Description : Descriptive and analytic statistics for the TREC IR trials
Comments    : information retrieval recall precision Tukey

Name        : ieeetest.zoo (65783 bytes)
Where       : in pub/C-numanal on
Author      : Stephen L. Moshier, [email protected]
Description : includes a improved version of paranoia, and code for
              testing the precision of the C I/O library on FP I/O.
Version     : 8 March 1993

Name        : IND Tree Package
Where       : available in the US only, contact author
Systems     : Unix
Description : Tree classification routines (supervised learning) including
              reimplementations of parts of CART, C4.5, and Bayesian
              and MDL methods with tree smoothing and "decision graphs".
              The package is made up of a collection of interconnected
              Unix tools.  It comes with a lot of documentation.
Author      : Wray Buntine, [email protected]
Version     : Version 2.1, January 1993

Name        : in-spice
Where       : part of Spice.  SPICE3E1 is free, SPICE3E2 is not-free
Description : files src/lib/ni/ni{integ,comcof}.c are first- (backward
              euler) and second- (trapezoidal) order integrator and a >6
              order GEAR.

Name        : jgraph.Z
Author      : Jim Plank ([email protected])
Description : filter for producing {encapsulated,} postscript
              using input in a script language.  Presentation quality results.
Systems     : Unix
Where       : in pub on, also jgraph.shar in misc on netlib
Language    : C
Version     : 8.3
Comments    : Very useful for post-processing the results of a computational
              program.  E.g. an awk program can turn numbers into jgraph
              code, or a C program can generate jgraph directly.
              The script language gives a very high degree of control over
              the final appearance.  There is a mailing list.
Date        : Nov 30 1992

Name        : kalman.tar.gz (22747 bytes)
Where       : in pub/C-numanal on
Author      : Skip Carter ([email protected])
Description : A class library for Kalman filtering
Language    : C++ (works with g++ 2.4.2 also)
Version     : v1.0, 3 July 1993

Name        : Karma
Where       : graphics/graphics/packages/karma on
Description : DSP package

Name        : Kaskade
Description : Linear elliptic FEM solver written in C. Reads problem
              description from plain text file - can be (mis)used as 
              triangular mesh generator. Graphical output under X11 and MacOS.
              Mailing list.
Authors     : 2-D -- Rainer Roitzsch ([email protected])
              3-D -- Bodo Erdmann ([email protected])
              Konrad-Zuse-Zentrum fuer Informationstechnik (ZIB)
Systems     : compiles on Unix and Macintosh
Where       : (The slightly outdated 
              user manual is in pub/kaskade/AltesZeug/ - in 

Ajay Shah, (213)749-8133, [email protected]
From: [email protected]
Newsgroups: comp.lang.c,comp.lang.c++,sci.math.num-analysis,sci.comp-aided,sci.op-research
Subject: Part 2 of 3: Free C,C++ for numerical computation
Followup-To: sci.math.num-analysis
Date: 6 Mar 1996 05:24:13 -0800
Organization: Centre for Monitoring Indian Economy, Bombay, India
Lines: 802
Sender: [email protected]
Distribution: world
Message-ID: <[email protected]>
Reply-To: [email protected]
Keywords: source code, numerical statistical scientific computation
Xref: comp.lang.c:177098 comp.lang.c++:176366 sci.math.num-analysis:26774 sci.comp-aided:1258 sci.op-research:5408

Name        : Kinetic Compiler and Integrator (kci)
Where       :
Systems     : Unix and MS-DOS
Language    : ANSI-C and the tools lex and yacc
Author      : Kenneth Geisshirt ([email protected]) and Keld Nielsen
Version     : 1.05
Description : Chemical reaction simulator and ODE solver
Comments    : The kci package is able to simulate a set of chemical
              reactions and/or solve ODEs. The package also comes with
              many numerical libraries e.g. matrices (very small lib.),
              ODE solvers, integration of real functions, and find eigen-
              values/vectors of general matrices. There is also a small
              library for symbolically manipulating expressions.

Name        : Lapack++
Authors     : J. Dongarra, R. Pozo, D. Walker
Description : C++ version of some of lapack fortran code.
Where       : ftp from in lapack++/*
Language    : C++
Version     : 0.9 beta
Comments    : C++ version of some of lapack fortran code.
              Developmental version of proposed C++ version of lapack.
              Contains blas.h++ etc, but needs Fortran library to link.
              Has overview paper (9 pages ps), release notes (7 page ps)
Date        :

Name        : LASSPTools
Where       : /pub/LASSPTools at
Systems     : Unix
Description : Data manipulation and entry tools for Unix.
Author      : Various people in the Cornell physics department
Comments    : A diverse set of tools by various people at the Laboratory
              of Atomic and Solid State Physics at Cornell.  Most useful for
              a set of X-windows applications and UNIX filters for
              interactive data manipulation. For instance, there's a
              mouse-operated track-ball that outputs a rotation matrix
              describing the orientation of the ball.

Name        : leda
Description : library of efficient data types and algorithms
Version     : v3.3, Jan 1996
Where       :
        in   pub/LEDA
              email [email protected]
Author      : Christian Uhrig
Comments    : includes code on computational geometry
              There is a mailing list on it; contact [email protected]

Name        : LinAlg.shar
Authors     : [email protected], [email protected]
Description : Basic Linear algebra in C++
Systems     : Unix/Mac
Where       : netlib ( or
Language    : C++
Version     : 3.1
Comments    : Contains declarations of the Matrix, Vector, subMatrix
              over the real domain, and *efficient* and fool-proof 
              implementations of level 1 & 2 BLAS (element-wise operations +
              various multiplications), transpositions and determinant
              evaluation/inversion. There are operations on a single
              row/col/diagonal of a matrix.
              The "new style" of returning matrices (via LazyMatrix) and
              filling them out.
              See LinAlg.h for the complete list of classes and functions,
              and, test drivers as to how the features
              can be used. See README for hints.
              The code made ANSI-C++ compliant and very portable
              (compiles with gcc v2.6.3).
Date        : Feb 7, 1995

Name        : logspline 
Where       : file logspline (a shar file) in S directory on statlib
Author      : Charles Kooperberg ([email protected])
Description : logspline density estimation
              fully automatic nonparametric density estimation
              adaptive smoothing using splines
Version     : statlib, last update April 21, 1993
Comments    : actually the objective of this file is to give a end-user
              of the S statistical package this functionality.  But the
              actual computation is done in C.
              Described in Journal of Computational and Graphical Statistics,
              (1993), vol 1, 301-328.

Name        : lpsolve
Where       : volume02 of comp.sources.reviewed
Description : very good mixed integer linear program solver
Author      : Michel Berkelaar ([email protected])
Version     : 1.4, 18 January 1994
Comments    : Its core is a sparse matrix dual simplex LP solver.  MILP
              problems are solved with a branch-and-bound iteration over LP
              solutions. It uses a lex+yacc parser to read a human-friendly
              algebraic input format.  The author has used the program to
              solve LP problems up to about 30000 variables and 50000
              constraints (on a 22 MFLOPS HP9000/750).

Name        :
Author      : Michael Courtney ([email protected])
Systems     : OS/2 2.x, UNIX
Version     : 1.5, 28 February 1994
Description : Non-linear least squares fitting program that opens
              a pipe to gnuplot and plots data and attempted fit.
              It's easy to define your own functions and recompile.
              Can fit multidimensional data to functions of more
              than one independent variable.  You can choose whether
              to vary parameters.
Language    : ANSI C
Where       : pub/os2/2_x/unix/lsqrft*zip on

Name        : machar
Where       : in misc on Netlib
Description : find out properties of floating point hardware
Author      : William J. Cody, [email protected], and Tim Hopkins
Version     : October 1985

Name        : madpack
Where       : Netlib, in pdes/madpack
Description : MADPACK is a a compact package for solving systems of
              linear equations using multigrid or aggregation
              disaggregation methods.  Imbedded in the algorithms
              are implementations for sparse Gaussian elimination
              and symmetric Gauss-Seidel (unaccelerated or
              accelerated by conjugate gradients or Orthomin(1)).
              This package is particularly useful for solving
              problems which arise from discretizing partial
              differential equations, regardless of whether finite
              differences, finite elements, or finite volumes are
Author      : Craig Douglas, [email protected]
Comments    : see directory mgnet on too

Name        : marsaglia-random
Where       :*
Systems     : highly portable
Language    : C, Pascal, Ada
Authors     : G Marsaglia, M G Harmon & T P Baker, V Broman.
Description : highly machine-independent uniform RNG,
              requires 24-bit fixed point or floating point arithmetic.
              953118087 different seed pairs give pseudo-random sequences
              with period about 2**144.  passes stringent randomness tests.
Comments    : correct operation with 24-bit floats seems to require
              a guard bit.  failing that, try fixed point arithmetic.

Name        : matcalc
Author      : M. Gerberg, E.J. Moore, University of New South Wales, Australia
Version     : 2.1
Systems     : Unix, VMS and DOS installation scripts exist
Description : Matlab-like numerical solver. Good support of singular
              problems. Well structured - easy extension with own C routines
              which can use the matcalc library.
Where       : netlib/matcalc on

Name        : matclass_info
Author      : Keith Briggs ([email protected]).
Where       : Posted on sci.math.num-analysis and comp.lang.c++
              Also see
Description : A comprehensive catalog of C++ matrix classes.
              I am not a C++ junkie (yet); it has a lot of information
              not present here.
Version     : Last posted 6 April 1994.

Name        : Matclass
Description : a C++ class for numerical computation
Author      : Chris Birchenhall ([email protected]}
Systems     : Unix and PC
Where       : ftp from pub/matclass/pc and pub/matclass/unix
Comments    : Offers a general purpose dense, real matrix class
              Has a family of decomposition classes based on
                LU, Cholesky, Householder QR and SVD
              Has a family of OLS regression classes based on 
                above decompositons
              A family of special function classes
              Random number class
              Has a simplified I/O structure
              Very good tex manual.
Date        :
Version     :

Name        : matcom
Authors     : [email protected] (Keren Yaron)
Description : Matlab --> C++ translator
Systems     : SunOS (324k), MSW (1.1M)
Where       :
Language    :
Version     : beta2
Comments    :
Date        : 10 Feb 1996

Name        : matmult.tar.z
Where       : in pub/C-numanal on
Author      : Clark Thomborson
Description : Several C-language codes for n * n matrix multiply, n a
              power of 2, developed as a laboratory exercise in the
              Spring of 1993 for MIT course 6.891, "Source Code
              Optimization for Workstations and Supercomputers."  The
              sources are commented, however the recursive SRM
              (shuffled-row major) algorithm is obscure.  Offered "as
              is" into the public domain by the course instructor.
Version     : 7 May 1993

Name        : matrices.asc
Where       : inside in published/dr-dobbs on
Description : efficiently raise matrices to an integer power
Author      : Victor Duvanenko
Version     : June 1991

Name        : matrix-multiply.shar.z
Where       : in pub/C-numanal on
Description : collection of net postings and email about fast matrix multiply
              Includes C source.
Version     : 1 May 1993, updated 4 June 1993
Comments    : also see matmult.tar.z in this file.

Name        : matrix.tar.Z
Where       : in ftp-raimund/pub/src/Math on
Author      : Paul Schmidt, TI
Description : Small matrix library, including SOR, WLS

Name        :
Where       : in mirrors/msdos/c on
Version     : 0.41, Sept 23 1993
Description : Small matrix toolbox

Name        : Matrix.tar.Z
Where       : in pub
Description : The C++ Matrix class, including a matrix implementation
              of the backward error propagation (backprop) algorithm for
              training multi-layer, feed-forward artificial neural networks
Version     : 10 July 1993
Systems     : Can use either g++ or cfront.
              SunOS, Solaris 2, NeXT, SGI, Linux.
Author      : E. Robert (Bob) Tisdale, [email protected]

Name        : mclaughl.lst
Where       : inside ddj8909.arc in published/dr-dobbs on
Description : source code (500 lines) associated with article on
              Simulated Annealing by Michael P. McLaughlin.
Version     : September 1989

Name        : meschach
Where       : in c/meschach on netlib
              pub/meschach on
Systems     : Unix, PC
Description : a library for matrix computation; matrix,
              vector, permutation, sparse matrix data structures; basic
              linear algebra; min/max, sorting & componentwise operations;
              dense LU, Cholesky, QR, LDL factorisations; dense
              eigenvalues/vectors, singular value decomposition; sparse
              matrix factorisations (LU, Cholesky, BKP); iterative
              methods; error handling; input/output
Author      : David E. Stewart, [email protected]
Version     : 1.2a, 28 February 1994

Name        : meschach
Where       : in c/meschach on netlib
Systems     : Unix, PC
Description : a library for matrix computation; more functionality than
              Linpack; nonstandard matrices
Author      : David E. Stewart, [email protected]
Version     : 1.1, 8 April 1993

Name        : mfloat
Where       : in math on simtel.
Systems     : DOS
Language    : written C++ and 80x86 assembly, useful for C, C++, Pascal
Author      : Kaufmann Friedrich, [email protected]
              Mueller Walter, [email protected]
Version     : 2.0 into beta testing 2 June 1994.
Description : fast high precision FP arithmetic (upto 77 digits)
Comments    : Shareware ($25).

Name        : MG-mglib.html
Authors     : Marcus Speh
Description : Information on development of a C++ library for multigrid
Where       : in pub/www/projects on
Language    : access through WWW
Date        : June 21 1993

Name        : MIDAS
Authors     : European Southern Observatory
Description : Tools for image processing and data reduction, with an
              accent on applications in astronomy.
Systems     : all major Unix, Linux, VMS
Where       : for example
Language    : ANSI C and Fortran f77.
Version     : 94MAYpl2
Comments    : Binaries are freely available, source is free to nonprofit
              research institutions.  A contact person is [email protected]
Date        : 19 October 1994

Name        : minit
Where       : volume 7 of comp.sources.misc
Systems     : Unix
Description : linear programming by dual simplex method
Author      : Badri Lokanathan
Version     : 1.0, July 1989
Comments    : don't miss minit.p1

Name        : mm.c and mmgen.c
Author      : Mark Smotherman ([email protected])
Description : benchmarking matrix multiply
Where       : in pub/programs/mark on
Comments    : includes a lot of code for fast matrix multiply
Date        : 24 June 1993

Name        : morrow.arc and gamaze.asc
Where       : inside in published/dr-dobbs on
Description : genetic algorithm for optimisation, associated with
              article on the subject by Mike Morrow.
Version     : April 1991

Name        : Mrandom (version 1)
Where       : Comp.sources.unix, Volume 25, Issue 23, December 1991
Systems     : 4.3bsd Unix
Language    : C
Author      : Clark Thomborson
Version     : 1, 12/91
Description : bug fix for 4.3bsd Unix random()
Comments    : random number generator, 4.3bsd Unix library routine

Name        : Mrandom (version 2.3)
Where       : anon ftp from, directory pub/cthombor,
              have submitted to comp.sources.unix
Systems     : 4.3bsd Unix
Language    : C
Author      : Clark Thomborson
Version     : 2.3, 8/92
Description : bug fix for 4.3bsd Unix random(), interface to other RNGs
Comments    : random number generator, 4.3bsd Unix library routine

Name        : MXYZPTLK  (mxyzptlk.tar.Z)
Author      : Leo Michelotti ([email protected])
Systems     : Unix, CC++, g++  (has been ported to others)
Version     : 3.1 (Sep, 1994)
Description : Automatic differentiation and
            : differential algebra package in C++
Where       : ftp 
            : (in directory pub/outgoing/michelotti/MXYZPTLK
Comments    : Contains old 1990 PostScript documentation.
            : Some demo programs demonstrate features not documented.
Review      : Complements ADOL-C.   Features complex mode.
              Easier to use, but possibly not as efficient for large problems. 
              (Keith Briggs ([email protected])

Name        : newmat
Where       : volume47, issue 38-47 of comp.sources.misc
              SIMTEL msdos/cpluspls/
Language    : C++
Systems     : Unix (g++, AT&T), MS-DOS (Borland, Watcom, MS)
Description : a very thorough matrix class
Author      : Robert Davies ([email protected])
Version     : v8, 19 Jan 1995

Name        : nlmdl
Where       : in pub/arg/nlmdl at (
              in volume 16 of comp.sources.misc
Language    : C++
Systems     : Unix, MS-DOS (Turbo C++)
Description : a library for estimation of nonlinear models
Author      : A. Ronald Gallant, [email protected]
Comments    : nonlinear maximisation, estimation, includes a real matrix class
Version     : January 1991

Name        : nonlinear
Where       : in pub/inls-ucsd on
Language    : various
Description : archive of programs in nonlinear dynamics, signal processing
Author      : various, contact person is [email protected] (Matt Kennel)

Name        : or plplot4p99i.tar.gz
Authors     : Maurice LeBrun and Geoff Furnish
Description : scientific plotting package and Tk plotting widget
Systems     : Unix, VMS, MSDOS, Amiga.  Wide variety of output drivers.
Where       : in /plplot
Language    : Fortran, C, C++, Tcl
Version     : 4.99i (beta)
Comments    : Wide range of plot types including line (linear, log), 
              contour, 3D, fill, etc.  Approx 1000 characters (including
              Greek and mathematical) in extended font set (Hershey).
              Strong X-windows support, with Tk plotting widget that
              supports zoom, pan, dump to file or printer, page layout,
              etc.  Distributed rendering supported.
Date        : 6 Sep 1994

Name        : Project Northstar
Where       : (
Description : courseware supporting mathematics and engineering classes
Systems     : Unix, known to work on IBM,HP,Sun,DEC,Convex.
Comments    : Not free, but freely available for .edu use.

Name        : QMG
Author      : S. Vavasis, Cornell, [email protected]
Description : Unstructured finite element mesh generation for 3D
              polyhedral objects with complicated geometry
Systems     : Sun/SunOS 4.1, Sun/Solaris, IBM RS6000/AIX, HP9000s800/HP-UX
Where       :
Language    : C++
Version     : 1.0
Date        : 9 May 95

Name        : nrutil
Where       :
Description : Appendix B of Numerical Recipes 2nd ed, a group of
              vector/matrix initialisation function which NR has
              standardised on.
Author      : Numerical Recipes is by William Press et al.
              This package is maintained by James C. Hu, [email protected]
Version     : 1 August 1994
Comments    : Note this is public domain, while none of the other NR
              source is.

Name        : nurbs.tar.Z
Where       : in /pub/misc/unix/nurbs/nurbs.tar.Z on
Author      : W. T. Hewitt
Description : Data structures and procedures for creation and
              manipulation of B-Spline curves and surfaces.

Name        : ObjectProDSP
Authors     : Mountain Math Software
              Contact Paul Budnik, [email protected]
              P. O. Box 2124, Saratoga, CA 95070
              (408) 353-3989
Description : Tool for DSP and object framework for interactive
              science and engineering applications.
Systems     : Linux binaries available, you can build on any Unix+X.
Where       : pub/linux/packages/dsp on
              pub/Linux/devel/opd on
Language    : C++
Version     : Beta 0.1, but likely to be more stable than your usual
              beta 0.1 product.
Comments    : Released under GPL.  Copious documentation.
Date        : 1 October 1994.

Name        : Octave
Where       :
              Binaries for some systems are also available.
Systems     : Compiles and runs on SPARC, RS/6000, DEC/Ultrix,
              i386/Linux, and probably most Unix systems that have a
              working port of g++ and libg++.  A port to OS/2 and DOS
              is mostly working but not quite ready for release yet.
Language    : C/C++/Fortran
Author      : John W. Eaton 
Version     : 1.1, Mon Jan 23 10:16:43 GMT+0530 1995
Description : Matlab-like interactive system for numerical computations
Comments    : Includes C++ classes for matrix manipulation, numerical
              integration, and the solution of systems of nonlinear equations,
              ODEs and DAEs.  Distributed under the GPL.
              230 page texinfo manual.  2d and 3d plotting using gnuplot.

Name        : ols
Where       : in usenet/comp.sources.reviewed/volume01/ols
Systems     : almost anything, but it's most useful under Unix
Description : A small linear regression package dressed as a Unix tool
Author      : Ajay Shah, [email protected]
Version     : v1.00, late 1991

Name        : opbdp
Where       :
Systems     : Unix
Language    : C++ (needs a compiler that supports templates)
Author      : Peter Barth ([email protected])
Version     : 1.0 #0 (29.5.95)
Description : An implicit enumeration algorithm for solving linear 0-1
              optimization problems. A bunch of heuristics for selecting
              a branching literal. Several preprocessing techniques
              (coefficient reduction, fixing, equation detection).
              Preprocessed problem can written to a file readable for
              CPLEX and lp_solve.          
Comments    : Technical report included. If your favorite linear-programming
              based solver fails on your problem you might give opbdp a chance.

Name        : p-wavelets.tar.Z
Where       :
Author      : Eric L. Veum ([email protected])
Language    : ANSI C
Systems     : Unix, with X-windows
Version     : March, 1995
Description : Compactly Supported Wavelets Transform/Inverse Transform
Comments    : Transform and inverse transform for compactly supported
              wavelets with variable scaling factors, of which the 
              special case of 2 are the Daubechies wavelets. Generates
              phase space time-frequency 3-D graphics if desired.

Name        : pdes (sortof)
Where       : pub/pdetools at
Description : extensive collection of C for linear and nonlinear systems,
              derived principally from pdes.

Name        : p4.tar.Z
Where       : pub/p4 on
Description : a library for writing parallel programs for shared-memory
              or message-passing.  It will work on a network of workstations
              or on parallel hardware.
Author      : [email protected]
Version     : July 28, 1992

Name        : Para++
Where       :
Systems     : Unix
Language    : C++,PVM or MPI
Author      : O. Coulaud ([email protected]), E. Dillon
Version     : 0.9
Description : C++ Bindings for Message Passing Libraries.
Comments    : The aim of Para++ is to provide C++ bindings to use a message 
              passing library (currently PVM or MPI), without the user had 
              to worry about PVM or MPI. Para++ is based on the SPMD parallel 
              programming method.

Name        : paranoia
Where       : in dist; check netlib/paranoia too
Systems     : Unix
Description : exercise the edges of your floating point implementation
Comments    : also see `ieeetest' in this file.

Name        : Pari/GP
Where       :
              Also at
              Mailing lists exist. Contact [email protected]
              or [email protected]
Description : PARI/GP is a package which is aimed at efficient
              computations in number theory, but also contains a large
              number of functions unrelated to number theory. It is
              somewhat related to a Computer Algebra System, but is
              not really one since it treats symbolic expressions as
              mathematical entities such as polynomials, series,
              matrices, etc..., and not as expressions per se. However
              it is often much faster than other CAS, and contains a
              huge number of specific functions not found elsewhere,
              essentially for use in number theory.  In particular,
              and especially so in the present release, there is a
              very large package for working in general algebraic
              number fields.
Systems     : Binaries available for SPARC v7, v8, DEC Alpha, HP-PA.
              In the future, Mac (68k and powerPC).
              Intel hardware may have unsupported versions.
Version     : 1.39a, 19 January 1995
Author      : [email protected]

Name        : pca
Where       : in multi on Statlib
Description : principal component analysis

Name        : perlman.Z
Where       : in a on Netlib
Description : normal, chi-squared and F distributions
Author      : Gary Perlman

Name        : piecewise.tar.Z (68025 bytes)
Where       : pub/math on (
Language    : C
Systems     : Unix (DOS if getopt available)
Description : Piecewise finds a piecewise linear approximation to a 
              1D function. The program provides two methods to find 
              the approximating segments, both satisfying an L infinity 
              error norm and both SUB-OPTIMAL. The user specifies the 
              tabulated function values and an error bound and the program 
              returns the endpoints of the line segments that approximate the
              function. The operation is fast (essentially a single pass 
              through the data) and works reasonably well on data with
              low noise. If the noise level is too high an alternative 
              approach using smoothing splines should be used.
Author      : Original algorithms by Ivan Tomek and
              F. Gritzali & G.Papakonstantinou
              Port to C and packaging by Tim Monks ([email protected]) 
Version     : 3 March 1991
Comments    : keywords linear splines

Name        : pierreQP.tar.Z (17680 bytes)
Where       : in pub/C-numanal on
Author      : Pierre Asselin, [email protected]
Description : Extremely good package for calculation of gaussian
              quadrature rules
Comments    : numerical integration

Name        : polyfit.tar.Z
Description : fit polynomials to data
Where       : in ftp-raimund/pub/src/Math on
Author      : Ted Stefanik, [email protected]
Version     : 8 August 1989

Name        : praxis
Where       : in math on Simtel
Description : derivative-free maximisation
Version     : July 1987

Name        : presto
Where       : pub/presto1.0.tar.Z on
Language    : C++
Systems     : Unix-like OS on (moderate) multiprocessor machines
Description : C++ routines for fine-grained parallel programming
              (lightweight threads) on multiprocessors. Tuned for the Sequent
              machines, but highly adaptable and customizable.
Author      : Brian N. Bershad, Edward D. Lazowska, Henry M. Levy 
Version     : Version 1.0 is an optimized version by John E. Faust. (All
              above are from U. Washington, Seattle)
Comments    : Presto was the subject of a number of research papers in
              multiprocessor OS. Version 1.0 looks usable (ie not
              experimental anymore).

Name        : proj-4.?.tar.Z
Authors     : Gerald I. Evenden ([email protected])
Description : Unix tool for cartographic projection and unprojection
Where       : in pub on
Language    : ANSI and POSIX C
Comments    : has beautiful (TeX) manual in postscript form

Name        : psuedo.asc
Where       : inside in published/dr-dobbs on
Description : implements R250 random number generator, from 
              S. Kirkpatrick and E. Stoll, Journal of Computational Physics,
              40, p. 517 (1981).
Author      : W. L. Maier

Name        : Radix-2 FFT
Authors     : [email protected], [email protected]
Description : Radix-2 DFT of a real or complex sequence, or sin/cos/complex
              Fourier integral of an evenly tabulated function.
Systems     : Unix/Mac
Where       : netlib (
Language    : C++ (gcc 2.5.8)
Version     : 1.0
Comments    : The input can be either real or complex with/without
              zero padding, the full complex transform or only
              real/im/abs_value part of it can be obtained.
              Test drivers and test run outputs are included, too. Commented.
              Needs LinAlg.shar
Date        : May 27, 1992 

Name        : random
Where       : bsd-sources/src/lib/libc/gen on
Description : the BSD C library random number generator

Name        : random-c
Where       : in c on Simtel
Description : portable, good random number generator

Name        : range.tar.Z (206015 bytes)
Where       : in pub on
Description : C++ class for interval arithmetic.
              Associated with article in TOMS, Dec 1992 title
              "Precise computation using range arithmetic, via C++"
Author      : Oliver Aberth and Mark J. Schaefer
Version     : October 1994

Name        : ranpm
Where       : in prog/libraries on (
              also in volume5 of comp.sources.misc in "random"
Description : the Park-Miller "minimal standard" random-number generator
Author      : Ajay Shah, [email protected]
Version     : February 1992
Comments    : there are several other independent implementations,
              all are quite alike

Name        : ranlib-c
Where       : pub/unix/ranlib.c-1.1-tar.Z on
Description : large library for random variate generation from many
              univariate and multivariate distributions
Author      : Barry Brown, [email protected]
Version     : v1.1, 24 Mar 1994

Name        : rktec.c.Z (20870 bytes)
Where       : in misc on netlib, or pub/papers/Hosea on
Description : computing truncation error coefficients of Albrecht's error
              expansion for Runge-Kutta formulas.  Version 2.1 adds
              a radial stability region "plotter".
Author      : Mike Hosea ([email protected])
Version     : v2.1, 5 June 1994
Comments    : The niu site also has some techreports.

Name        : rlab
Where       :
                        702 kb  rlab-1.19a.tar.gz
                        384 kb  rlap-2.0.tar.gz
                        74 kb   rblas-1.1.tar.gz
                        30 kb   rfft-1.2.tar.gz
                        31 kb   rnlib-1.1.tar.gz
Systems     : Compiles and runs on Sun4, RS/6000, DEC/Ultrix, SysV/R4 i386,
              Linux, HP-UX, SGI.  Broadly, should work on any Unix.
Language    : C + Fortran
Author      : Ian Searle ([email protected])
Version     : 1.18d, 16-Mar-95
Description : Matrix oriented, interactive programing environment.
              Rlab is _not_ a clone of languages such as those used by
              tools like MATLAB or matrix_X/Xmath. However, as Rlab
              focuses on creating a good experimental environment (or
              laboratory) in which to do matrix math, it can be called
              "MATLAB-like" since its programming language possesses
              similar operators and concepts.  Extensive use has been
              made of the LAPACK, FFTPACK and RANLIB sources available
              from netlib.
Comments    : Includes online help and LaTeX manual.
              There is a mailing list.
              The distribution is under GPL

Name        : robot
Description : a scientific graph plotting and data analysis package.
              Works for Xview v3, and knows to generates postscript.
Where       : in pub/astrod on (
Version     : v0.46, 7 Feb 1993
Author      : Robin Corbet ([email protected])

Name        : rpart (113799 bytes)
Where       : in general on Statlib
Description : Routines for recursive partitioning
Author      : Terry Therneau, [email protected]
Version     : 9 July 1993

Name        : sa.tar.gz (30473 bytes)
Where       : in pub/C-numanal on
Description : library for simulated annealing
Language    : versions for C, C++ and Ada exist.  Works with g++ 2.4.2.
Author      : Skip Carter ([email protected])
Version     : 3 July 1993

Name        : sabre.tar.Z (813499 bytes)
Where       : in pub on
Ajay Shah, (213)749-8133, [email protected]
From: [email protected]
Newsgroups: comp.lang.c,comp.lang.c++,sci.math.num-analysis,sci.comp-aided,sci.op-research
Subject: Part 3 of 3: Free C,C++ for numerical computation
Followup-To: sci.math.num-analysis
Date: 6 Mar 1996 05:24:42 -0800
Organization: Centre for Monitoring Indian Economy, Bombay, India
Lines: 547
Sender: [email protected]
Distribution: world
Message-ID: <[email protected]>
Reply-To: [email protected]
Keywords: source code, numerical statistical scientific computation
Xref: comp.lang.c:177131 comp.lang.c++:176396 sci.math.num-analysis:26783 sci.comp-aided:1260 sci.op-research:5413

Description : (not clear) a linear/nonlinear simulation system
Comments    : the `portable math library' directory is definitely
              very useful (5k lines).  I noticed some interesting
              interpolation, integration, banded LU decomposition,
              nonlinear solver, etc.
Author      : ?
Version     : ?

Name        : Scilab
Authors     : [email protected]
Description : Matrix--based scientific computation
Systems     : Sun, RS6000, HP9000, Mips, Alpha, i386 (Linux)
              Requires X.
Where       : ( in INRIA/Projects/Meta2/Scilab
Language    : ?
Version     : 2.1
Comments    : Resembles Matlab and Xmath.  Has hundreds of builtin
              mathematical functions, sophisticated data structures,
              a high--level interpreter, a macro language, and excellent
              graphics.  C and fortran functions can be added as new
              Comes with toolboxes for control and signal processing,
              and for analysis of graphs and optimisation of utility networks.
Date        : 8 March 1995

Name        : sdeint.tar.z
Where       : in pub/C-numanal on
Systems     : Unix, MS-DOS
Language    : C++
Description : A Runge-Kutta like class for integrating systems of Stochastic
              Differential Equations
Author      : Skip Carter, [email protected]
Version     : v1.9 4 May 1993

Name        : sga-c
Authors     : David Goldberg
Description : C source for simple genetic algorithm
Where       :

Name        : sge.shar
Where       : in c on Netlib
Description : Linpack functions geco, gefa, gesl and a little of BLAS;
              nonstandard matrices
Author      : Mark K. Seager, [email protected]
Version     : April 88

Name        : SGPC
Description : Simple Genetic Programming in C
Author      : Walter Alden Tackett ([email protected])
Where       : in the pub/Users/tackett on
Version     : 28 May 1993
Comments    : genetic algorithms, nonlinear maximisation

Name        : SIMATH 
Author      : SIMATH-Gruppe, Saarbruecken, Germany
Systems     : Unix
Where       : via anonymous ftp: (, 
     ( in pub/simath
Version     : 3.6.1
Description : SIMATH contains a lot of C-functions over algebraic 
              structures as arbitrary long integers, rational 
              numbers, polynomials, Galois fields, matrices, 
              elliptic curves, algebraic number fields, modular
              integers, etc. There is also an interactive calculator
              (simath) which uses the C-libraries of SIMATH.
Comments    : version 3.6.1 contains a handbook written in English. 
              The SIMATH package also includes a user interface,
              which makes it possible to use the on-line
              documentation of the functions and the keyword index.
              It is free, but you have to first register, in order
              to get a "license" file without which it won't compile.

Name        : simlab
Authors     : ?
Description : circuit simulation environment
Systems     : Unix, optimised version for connection machine exists.
Where       :
Language    : C

Name        : simpack-2.1.tar.Z (287965 bytes), simpack-2.1++.tar.Z (82683 bytes)
Author      : Paul A. Fishwick, [email protected]
Description : tools for writing simulations with a EECS bias
Where       : pub/simdigest/tools on, also see
     from cis/tech-reports/tr92
Language    : C and C++ versions exist
Version     : v2.0, June 1992

Name        : smirnov.shar.Z (3599 bytes)
Author      : David Rapoport ([email protected])
Version     : 22 February 1993
Description : Kolmogorov Smirnov two-sample statistic 
Where       : in pub/C-numanal on

Name        : SMMS (Sparse Matrix Manipulation System)
Description : A collection of about 80 commands to do almost
              anything you wish to do with sparse matrices VERY
              EASILY.  It is designed as an instructional and
              prototyping tool, not for "production" work.
Where       : /pub/smms93/* on
Systems     : Any Unix system with X-windows, but tested only on Sun,
              HP and DEC.  Also works under DOS
Language    : Mostly C (any version).  One or two routines in Fortran
Author      : Fernando Alvarado ([email protected])
Version     : Release 2 May 1993
Comments    : Includes online help for every command and LaTeX and 
              PostScript versions a manual.  Expandable by the user.  
              Release 2 handles complex sparse matrices, interval 
              matrices, blocked matrices adn symbolic matrices. 
              Visualization tools.  Interfaces to Harwell routines 
              and Boeing-Harwell sparse matrix data.

Name        : smooth.tar.Z
Description : Unix tool for smoothing
Where       : in ftp-raimund/pub/src/Math on
Author      : Bill Davidsen ([email protected])
Version     : v1.9, 15 Aug 1989

Name        : smoothwb (209947 bytes)
Authors     : Lise Manchester ([email protected])
              David Trueman ([email protected])
Description : Smoothing Workbench
Systems     : Unix + Xview (e.g. SunOS, Linux)
Where       : in general on statlib
Language    : C (2613 lines) and fortran (1458 lines)
Comments    : interactive program for exploring smoothing methods
              Includes postscript documentation.
Date        : 28 June 1993

Name        : SPARSE
Where       : in sparse on Netlib
Description : library for LU factorisation for large sparse matrices 
Author      : Ken Kundert, Alberto Sangiovanni-Vincentelli,
              [email protected]

Name        :
Where       : in mirrors/msdos/c on
Description : Interpolation using splines under tension, dressed up as
              a Unix tool
Author      : James. R. Van Zandt
Version     : v2.9, 21 Nov 1992

Name        : |STAT
Where       : in pub/stat on (
Description : collection of around 30 Unix tools for statistical analysis
Author      : Gary Perlman ([email protected])
Version     : 5.4, 27 May 1993
Systems     : Unix, MS-DOS
Comments    : Has been in use for 13 years.  There is a troff|ps manual
              and man pages.  Explicitly designed to work with Unix
              philosophy.  The file stat.tar.Z.crypt.uu is ENCRYPTED;
              you have to send email asking for the password.
              There is a handbook available.

Name        : submit1
Where       : in jcgs on Statlib
Description : damped convex minorant algorithm
Author      : David Eberly, [email protected]
Version     : May 1992

Name        : surf
Authors     : Weimin Zhao, [email protected]
Description : Xlib program to debug, monitor, control largescale
              numerical simulations (in either fortran or C).  Does
              realtime 3d display.
Systems     : Aix, HP-UX, Linux.
Where       :, pub/Linux/Incoming
Version     : 1.0, 12 May 1994

Name        : SVDPACKC.tar.Z
Where       : in pub/berry on
Systems     : Sun, IBM RS/6000, HP9000, DECstation, Macintosh II/fx, Cray Y-MP
Language    : C
Description : an ANSI-C library for the singular value decomposition
            : of large sparse matrices.  Lanczos- and subspace iteraton-
            : based methods are used to iteratively compute several
            : of the largest (or smallest) singular values and corres-
            : ponding singular vectors.  Sample  UNIX C-SHELL scripts
            : are provided for automatic compiling and testing of the
            : library routines.   Cray Y-MP compatible routines provided.
Author      : Michael W. Berry ([email protected])
Version     : 1.0, June 1993

Name        : svd.c.Z (8704 bytes)
Where       : in pub/C-numanal on
Description : SVD based on pascal from J. C. Nash book
Author      : Bryant Marks ([email protected])
              Brian Collett ([email protected])
Version     : 14 April 1993

Name        : taranto-1.0.shar.Z
Where       : in prog/libraries on (
Description : portable, accurate FP to decimal conversion.

Name        : totinfo
Where       : in volume7 of comp.sources.misc
Description : info statistic and chi-square for 2-D contingency tables
Date        : August 1989

Name        : Tela
Authors     : 
Description : Tensor Language
Systems     : Unix (SGI, Linux, Aix, Sun)
Where       :
Language    :
Version     : 1.21, 24 Feb 1995
Comments    : Includes a C translation of FFTPACK, 20-page user manual,
              FAQ, graphics examples, etc.
              Email addresses are [email protected] and [email protected]

Name        : Tensor.tar.Z
Authors     : E. Robert Tisdale, [email protected]
Description : Experimental Tensor Class
Systems     : Solaris
Where       :
Language    : Gnu C++ 2.6.2
Date        : Thu Jan 12 18:10:53 GMT+0530 1995

Name        : tsp
Where       :
Systems     : Any C environment
Description : Simple heuristic Travelling Salesman Problem solver
Author      : Dan Kegel - from "Discrete Optimization Algorithms," Maciej Syslo
Version     : 1.1

Name        : tsp_solve
Where       : e-mail request to [email protected]
Systems     : Borland, sco and Sun with gcc
Language    : C++
Authors     : Chad Hurwitz ([email protected])
              Robert.J.Craig ([email protected]) and anyone else who'd
              like to test their own TSP tour finder's performance
Version     : 1.0beta
Description : Finds Optimal and Heuristic Solutions to many types of
              Traveling Salesman Problems (TSP).
Comments    : tsp_solve finds optimal solutions to geometric TSPs with 100
              cities in about an hour (don't go to lenscrafters for this one.)
              It will soon have an asymmetric TSP optimal solution finder that
              will perform at approximately the same level.

Name        : UDouble
Where       :
Systems     : Source code: should be portable (developed with gnu gcc)
Language    : C++
Author      : Evan M. Manning, [email protected]
Version     : 1.00
Description : A class library for tracking propagation of uncertainties
              through systems of equations.
Comments    : Described in part in an article in the March issue of
              the _C/C++_Users_Journal_.

Name        : using-lapack.Z (8478 bytes)
Where       : pub/C-numanal on
Description : Notes on using Lapack through f2c.
Author      : S. Sullivan ([email protected])
Version     : 14 April 1993

Name        : vis5d
Where       : (
Systems     : SGI, Stardent, IBM PC
Language    : C, Fortran
Authors     : Brian Paul ([email protected]) and Bill Hibbard
Version     : 3.0 (soon to be 3.1)
Description : visualizing/animating data made by numerical weather
              models and similar sources
Comments    : vis5d interactively provides 3-D isosurfaces, vector-field
              slices, horizontal and vertical contour and colored slices,
              and ribbon "particle" trajectories (integral curves)

Name        : vregion
Authors     : Seth Teller, [email protected]
Description : Computes the voronoi diagram, delaunay triangulation,
              and convex hull of a two-dimensional point set.  It's based
              on Steve Fortune's algorithm, and partially on his 
Systems     : Unix
Where       : comp.sources.misc, volume 41, issue 30
Language    : C
Date        : 14 December 1993

Name        : vspline
Where       : in gcv on Netlib
Description : non-parametric estimate of a smooth vector-valued
              function from noisy data
Author      : Jeff Fessler
Comments    : splines

Name        : wavethresh (wavelet.shar)
Where       : in directory S on Statlib, and anonymous ftp from 
    , in directory pub/masgpn
Language    : C (and S functions)
Author      : Guy Nason ([email protected])
Version     : 2.1 (March 26 1993)
Description : wavelet transform & thresholding software in C for linking 
              into S.
Comments    : Performs 1- and 2-D discrete wavelet transforms using 
              Daubechie's wavelets. Also performs thresholding according to 
              Donoho and Johnstone.

Name        : weisfeld-simplex.shar (7457 bytes)
Where       : pub/C-numanal on
Description : small implementation of simplex method for linear programming.
Author      : Matt Weisfeld (not on Internet)
Version     : Feb 1993
Comments    : associated with article in Feb 1993 CUJ.
              For production use (where you want a black-box solver),
              the `lpsolve' package (above) is better.  If you want to
              open up a simplex implementation and modify it, then this is
              quite good, using the article as documentation.

Name        : xgobi
Where       : in general on Statlib
Systems     : Unix, needs X Windows
Description : a data analysis package emphasising graphical data exploration
Author      : Debby Swayne, [email protected]
              Dianne Cook, [email protected]
              Andreas Buja, [email protected]
Date        : 23 March 1993
Comments    : EDA

Name        : XLispStat
Where       : pub/xlispstat on
Systems     : Unix, Macintosh, MSW
Description : a statistical package
Author      : Luke Tierney, luke%[email protected]
Version     :
Comments    : object-oriented, EDA, graphics, lisp

Name        : xtrap.c.Z (4463 bytes)
Author      : Bryan M. Gorman, [email protected]
Version     : 28 July 1992
Description : extrapolation program.  Supports 6 algorithms: VBS
              approximants, Aitken delta-squared, Wynn epsilon algo,
              Wynn rho algo, Brezenski theta algo, Levin u-transform.
              Is dressed up as a Unix tool.
Where       : pub/C-numanal on

Name        : xvgr/xmgr (open look or motif versions)
Where       : /CCALMR/pub/acegr on
Systems     : Unix, with either open look or motif
Description : graphics for EDA
Author      : Paul J. Turner, [email protected]
Version     : 2.10, 2 May 1993
              3.01 (Motif only), 17 August 1994.
Comments    : Linux and SunOS 4.1.3 binaries are in bin directory

Name:           Yorick
Author:         David Munro - Lawrence Livermore National Laboratory
                [email protected]
Version:        1.1
Date:           18 May 1995
Description:    Yorick is a very fast interpreted language designed for
                scientific computing and numerical analysis.  The syntax
                is similar to C, but without declarative statements.
                Operations between arrays yield array results, and Yorick
                provides a very rich selection of multi-dimensional array
                indexing operations.  Yorick also features a binary I/O
                package which automatically translates floating point and
                integer representations on the machine where it is running
                to and from the format of any other machine.  Thus, you can
                easily share binary files between, for example, Cray YMPs
                and DEC alphas, or "teach" Yorick to read existing binary
                databases.  Yorick also offers an interactive graphics
                package based on X windows.  X-Y plots, quadrilateral
                meshes, filled meshes, cell arrays, and contours are
                supported.  Finally, you can embed compiled routines in
                custom versions of Yorick to solve problems for which the
                interpreter is too slow.  The primary use of Yorick to date
                has been as a pre- and post-processor for large physical
                simulation programs.
                A binary distribution for Linux is available at sunsite.
                Freely Redistributable
Language:       ANSI C (some support for Fortran customization)
Keywords:       interpreter, language, interactive graphics, data analysis,
Where: /pub/Yorick
                1.4 MB yorick-1.1.tar.gz
                or /languages/yorick
Systems:        Sun SPARC (SunOS or Solaris), HP PA-RISC (HPUX), IBM RS/6000
                (AIX), DEC alpha, SGI (IRIX), Cray YMP (UNICOS), Ix86 (Linux)
                Requires ANSI C compiler.  Interactive graphics requires
                X window system.  Tested on Sun (SunOS and Solaris), HP
                PA-RISC, IBM RS/6000, DEC alpha, SGI, Cray YMP, and Linux;
                should not be difficult to build on other UNIX machines.


In case you had not already noticed it: a public domain, industrial
strength, fortran-to-C translator named f2c exists.  It has one great
strength and one great weakness: "It is a true compiler".  Thus the
code generated always "works", at the price of frequently looking like

A lot of useful fortran libraries can readily be turned into working C
using f2c, and the resulting C can often be made almost human after
some hand-editing.  The weakest link of f2c is code which involves

A pointer to f2c is at EOF.  f2c is also inside Netlib, so you are
probably better off figuring out how to use Netlib.

Other pointers

There is a lot of interesting C source in these fields which I know nothing
        - signal processing
        - pattern recognition, neural networks
The comp.dsp FAQ has some pointers to source code.
Please send me complete entries to include in the above index.

A lot of 3rd party source code which hooks into the S statistical package
uses computational engines written in C.  With a little work you can extract
useful source from this.  Look in the S directory on Statlib for more
pointers.  If you find something which is remarkably useful and easy
to extract, please tell me about it.

The same phenomenon operates to some extent for the XLispStat package.
Look around on the site.

Interesting sites

If you don't have ftp access, send email to [email protected]
saying "help".  You will get instructions on how to do ftp via email.

Juhana Kouhia ([email protected]) has setup a very nice service:
Everything in this index (except for what is on {net,stat}lib) is
mirrored in pub/sci/math/numcomp-free-c on
Note: this site is in finland.  If you are in the US, please try
to find a site closer to you.

source-code newsgroups:  (e.g. usenet/comp.sources.reviewed archives the
        comp.sources.reviewed newsgroup).
        netlib/f2c on
        pub/gnu on
Netlib:       email, ftp             email, xnetlib               email, xnetlib for Europe
           (e.g. send email to [email protected] to access by email) is a mail server useful for Europe. ( in Australia
Statlib: (as statlib) (
others: has a small collection in pub/math, including
        fft stuff not listed above. is quite interesting


The following people helped me put this index together:

Bardo Muller                    [email protected] 
David E. Stewart                [email protected] 
Skip Carter                     [email protected]
John Gregory                    [email protected]
John Eaton                      [email protected]
P. G. Hamer                     [email protected]
Alan Magnuson                   [email protected]
David Rapoport                  [email protected]
Peter Fraenkel                  [email protected]
Martin-D. Lacasse               [email protected]
Matthew Koebbe                  [email protected]
Nicolas Ratier                  [email protected]
Henri Cohen                     [email protected]
Bill Hutchison                  [email protected]
Ronald F. Guilmette             [email protected]
Jay Han                         [email protected]
Van Snyder                      [email protected]
Alan Cabrera                    [email protected]
Vincent Broman                  [email protected]
Piercarlo Grandi                [email protected]
Abed Hammoud                    [email protected]
Richard A. O'Keefe              [email protected]
Fumiaki Kamiya                  [email protected]
Keith Briggs                    [email protected]
Brian Glendenning               [email protected]
Bill Gropp                      [email protected]
Emmett McLean                   [email protected]
Wenfu Ku                        [email protected]
Adrian Ireland                  [email protected]
Alexander Frink                 [email protected]
Douglas N Arnold                [email protected]
Jens Ehlers                     [email protected]
Bruce Haggerty                  [email protected]
Peter Espen                     [email protected]
M J Olesen                      [email protected] 
Vincent Broman                  [email protected]
Luiz Henrique de Figueiredo     [email protected]
Jose E. Korneluk                [email protected]

Of course, we owe infinite gratitude to the authors themselves, for
making their work available in the public domain.
Ajay Shah, (213)749-8133, [email protected]