From: scs@adam.mit.edu (Steve Summit)
Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List)
Followup-To: poster
Supersedes: <1992Aug2.041414.15818@athena.mit.edu>
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Date: Tue, 1 Sep 1992 04:02:13 GMT
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Archive-name: C-faq/faq

[Last modified May 30, 1992 by scs.]

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
principles.

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 vendors' 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.  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
4.1).

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
answering.

This article is always being improved.  Your input is welcomed.  Send
your comments to scs@adam.mit.edu, scs%adam.mit.edu@mit.edu, and/or
mit-eddie!adam.mit.edu!scs; this article's From: line may be unusable.

The questions answered here are divided into several categories:

	 1. Null Pointers
	 2. Arrays and Pointers
	 3. Expressions
	 4. ANSI C
	 5. C Preprocessor
	 6. Variable-Length Argument Lists
	 7. Lint
	 8. Memory Allocation
	 9. Structures
	10. Declarations
	11. Boolean Expressions and Variables
	12. System Dependencies
	13. Stdio
	14. Library Subroutines
	15. Style
	16. Miscellaneous (Fortran to C converters, YACC grammars, etc.)

Herewith, some frequently-asked questions and their answers:


Section 1. Null Pointers

1.1:	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 not
	the address of any object.  That is, the address-of operator &
	will never yield a null pointer, nor will a successful call to
	malloc.  (malloc returns 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; an
	uninitialized pointer might point anywhere.  See also questions
	8.1, 8.9, and 16.1.

	As mentioned in the definition 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 it can make
	the distinction if necessary (see below).

	References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S
	Sec. 5.3 p. 91; ANSI Sec. 3.2.2.3 p. 38.

1.2:	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)

	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.  For instance, the Unix system call "execl" takes a
	variable-length, null-pointer-terminated list of character
	pointer arguments.  To generate a null pointer in a function
	call context, an explicit cast is typically required:

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

	If the (char *) cast 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
	unadorned 0's.  Function prototypes cannot provide the types for
	variable arguments in variable-length argument lists, however,
	so explicit casts are still required for those arguments.  It is
	safest always to cast null pointer function arguments, 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 remember.)

	Summary:

		Unadorned 0 okay:	Explicit cast required:

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

		function call,
		prototype in scope,
		fixed argument

	References: K&R I Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R II
	Sec. A7.10 p. 207, Sec. A7.17 p. 209; H&S Sec. 4.6.3 p. 72; ANSI
	Sec. 3.2.2.3 .

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

A:	As a matter of style, many people prefer not to have unadorned
	0's scattered throughout their programs.  For this reason, the
	preprocessor macro NULL is #defined (by <stdio.h> or
	<stddef.h>), with value 0 (or (void *)0, about which more
	later).  A programmer who wishes to make explicit the
	distinction between 0 the integer and 0 the null pointer can
	then use NULL whenever a null pointer is required.  This is a
	stylistic convention only; the preprocessor turns NULL back to 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 1.2 above applies for NULL as well as 0.)

	NULL should _only_ be used for pointers; see question 1.8.

	References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S
	Sec. 13.1 p. 283; ANSI Sec. 4.1.5 p. 99, Sec. 3.2.2.3 p. 38,
	Rationale Sec. 4.1.5 p. 74.

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

A:	Programmers should never need to know the internal
	representation(s) of null pointers, because they are normally
	taken care of by the compiler.  If a machine uses a nonzero bit
	pattern for null pointers, it is the compiler's responsibility
	to generate it when the programmer requests, by writing "0" or
	"NULL," a null pointer.  Therefore, #defining NULL as 0 on a
	machine for which internal null pointers are nonzero is as valid
	as on any other, because the compiler must (and can) still
	generate the machine's correct null pointers in response to
	unadorned 0's seen in pointer contexts.

1.5:	If NULL were defined as follows:

		#define NULL (char *)0

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

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 to work
	correctly, but pointer arguments to other types would still be
	problematical, and legal constructions such as

		FILE *fp = NULL;

	could fail.

	Nevertheless, ANSI C allows the alternate

		#define NULL ((void *)0)

	definition for NULL.  Besides 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 1.8).

1.6:	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 in some circles, does not buy much.
	It is not needed in assignments and comparisons; see question
	1.2.  It does not even save keystrokes.  Its use suggests to the
	reader that the author is shaky on the subject of null pointers,
	and requires the reader to check the #definition of the macro,
	its invocations, and _all_ other pointer usages much more
	carefully.

1.7:	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 produced when the expression
	compares equal to zero, and a true value otherwise.  That is,
	whenever one writes

		if(expr)

	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
	have

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

	and this is a comparison context, so the compiler can tell that
	the (implicit) 0 is a null pointer, and use the correct value.
	There is no trickery involved here; compilers do work this way,
	and generate identical code for both statements.  The internal
	representation of a pointer does _not_ matter.

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

		!expr	is essentially equivalent to	expr?0:1

	It is left as an exercise for the reader to show 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.

	See also question 11.2.

	References: K&R II Sec. A7.4.7 p. 204; H&S Sec. 5.3 p. 91; ANSI
	Secs. 3.3.3.3, 3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, and
	3.6.5 .

1.8:	If "NULL" and "0" are equivalent, 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
	#definition, and prefer to use unadorned "0" instead.  There is
	no one right answer.  C programmers must understand that "NULL"
	and "0" are interchangeable and that an uncast "0" is perfectly
	acceptable in initialization, assignment, and comparison
	contexts.  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.  (ANSI allows the #definition of NULL to be
	(void *)0, which will not work in non-pointer contexts.)  In
	particular, do not use NULL when the ASCII null character (NUL)
	is desired.  Provide your own definition

		#define NUL '\0'

	if you must.

	Reference: K&R II Sec. 5.4 p. 102.

1.9:	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
	null pointers?

A:	No.  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.

1.10:	I'm confused.  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 1.1.  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 source code syntax for null pointers, which is the
		single character "0".  It is often hidden behind...

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

	5.	The ASCII null character (NUL), which does have all bits
		zero, but has no relation to the null pointer except in
		name.  Since this character terminates strings in C, an
		otherwise empty string may be called...

	6.	The "null string," consisting only of the terminating
		'\0'.  This term is best avoided, because it could be
		construed as having something to do with null pointers,
		which brings us full circle...

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

1.11:	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) suggests that the value might
	change later, 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 above) is often overlooked.

	One good way to wade out of the confusion is to imagine that C
	had a keyword (perhaps "nil", like Pascal) with which null
	pointers were requested.  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 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, the code may not work.

1.12:	I'm still confused.  I just can't understand all this null
	pointer stuff.

A:	Follow these two simple rules:

	1.	When you want to refer to a null pointer 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, or with the internal representation of null
	pointers (which you shouldn't need to know), or with ANSI C
	refinements.  Understand questions 1.1, 1.2, and 1.3, and
	consider 1.8 and 1.11, and you'll do fine.

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

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 this 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 8.9).  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 rather than "0," as mentioned in question
	1.11, the urge to assume an internal zero representation would
	not even arise.)

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

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).

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

	The Symbolics Lisp Machine, a tagged architecture, does not even
	have conventional numeric pointers; it uses the pair <NIL, 0>
	(basically a nonexistent <object, offset> handle) as a C null
	pointer.

	Depending on the "memory model" in use, 80*86 processors (PC's)
	may use 16 bit data pointers and 32 bit function pointers, or
	vice versa.


Section 2. Arrays and Pointers

2.1:	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: CT&P Sec. 3.3 pp. 33-4, Sec. 4.5 pp. 64-5.

2.2:	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 2.4.)  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.  The pointer is to be known by the
	name "p," and can point to any char (or contiguous array of
	chars) anywhere.

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

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

	would result in data structures which could be represented like
	this:

		   +---+---+---+---+---+---+
		a: | h | e | l | l | o |\0 |
		   +---+---+---+---+---+---+

		   +-----+     +---+---+---+---+---+---+
		p: |  *======> | w | o | r | l | d |\0 |
		   +-----+     +---+---+---+---+---+---+

	It is important to remember 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 the example above,
	both a[3] and p[3] happen to be the character 'l', but the
	compiler gets there differently.  (See also question 16.11.)

2.3:	So what is meant by the "equivalence of pointers and arrays" in
	C?

A:	Much of the confusion surrounding pointers in C can be traced to
	a misunderstanding of this statement.  Saying that arrays and
	pointers are "equivalent" does not by any means imply that they
	are 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 literal string initializer for a
	character array.)

	As a consequence of this definition, there is not really any
	difference in the behavior of the "array subscripting" operator
	[] as it applies to arrays and pointers.  In an expression of
	the form a[i], the array reference "a" decays into a pointer,
	following the rule above, and is then subscripted exactly as
	would be a pointer variable in the expression p[i].  In either
	case, the expression x[i] (where x is an array or a pointer) is,
	by definition, exactly equivalent to *((x)+(i)).

	References: K&R I Sec. 5.3 pp. 93-6; K&R II Sec. 5.3 p. 99; H&S
	Sec. 5.4.1 p. 93; ANSI Sec. 3.2.2.1, Sec. 3.3.2.1, Sec. 3.3.6 .

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

A:	Since arrays decay immediately into pointers, an array is never
	actually passed to a function.  Therefore, any parameter
	declarations which "look like" arrays, e.g.

		f(a)
		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:

		f(a)
		char *a;

	This conversion holds only within function formal parameter
	declarations, nowhere else.  If this 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 and/or the uses within the function.

	References: K&R I Sec. 5.3 p. 95, Sec. A10.1 p. 205; K&R II
	Sec. 5.3 p. 100, Sec. A8.6.3 p. 218, Sec. A10.1 p. 226; H&S
	Sec. 5.4.3 p. 96; ANSI Sec. 3.5.4.3, Sec. 3.7.1, CT&P Sec. 3.3
	pp. 33-4.

2.5:	Why doesn't sizeof() properly report the size of an array that's
	a parameter to a subroutine?

A:	sizeof() reports the size of the pointer parameter which the
	subroutine really receives (see question 2.4).

2.6:	Someone explained to me that arrays were really just constant
	pointers.

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 2.2
	should make clear.

2.7:	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 exactly equivalent to
	*((a)+(e)), for _any_ expression e and primary expression a, 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
	16.7).

	References: ANSI Rationale Sec. 3.3.2.1 p. 41.

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

A:	The rule 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.  (The confusion is heightened by the
	existence of incorrect compilers, including some versions of pcc
	and pcc-derived lint's, 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[YSIZE][XSIZE];
		f(array);

	the function's declaration should match:

		f(int a[][XSIZE]) {...}
	or
		f(int (*ap)[XSIZE]) {...}     /* 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;" 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," YSIZE, can be omitted.  The "shape" of the array is
	still important, so the "column" dimension XSIZE (and, for 3- or
	more dimensional arrays, the intervening ones) must be included.

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

	References: K&R I Sec. 5.10 p. 110; K&R II Sec. 5.9 p. 113.

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

A:	Usually, you don't want to.  Consider using a pointer to one of
	the array's elements instead.  Arrays of type T decay into
	pointers to type T (see question 2.3), which is convenient;
	subscripting or incrementing the resultant pointer accesses the
	individual members of the array.  True pointers to arrays, when
	subscripted or incremented, step over entire arrays, and are
	generally only useful when operating on arrays of arrays, if at
	all.  (See question 2.8 above.)  When people speak casually of a
	pointer to an array, they usually mean a pointer to its first
	element.

	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 10.3.)  If the size of the array is unknown,
	N can be omitted, but the resulting type, "pointer to array of
	unknown size," is useless.

2.10:	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."  The
	resulting "ragged" array can save space, although it is not
	necessarily contiguous in memory as a real array would be.  Here
	is a two-dimensional example:

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

	(In "real" code, of course, malloc would be declared correctly,
	and each return value 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 **array = (int **)malloc(nrows * sizeof(int *));
		array[0] = (int *)malloc(nrows * ncolumns * sizeof(int));
		for(i = 1; i < nrows; i++)
			array[i] = array[0] + i * ncolumns;

	In either case, the elements of the dynamic array can be
	accessed with normal-looking array subscripts: array[i][j].

	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
	array:

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

	However, you must now perform subscript calculations manually,
	accessing the i,jth element with array[i * ncolumns + j].  (A
	macro can hide the explicit calculation, but invoking it then
	requires parentheses and commas which don't look exactly like
	multidimensional array subscripts.)

2.11:	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:	This technique, though attractive, is not strictly conforming to
	the C standards (in spite of its appearance in Numerical Recipes
	in C).  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&R II Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3,
	Sec. A7.7 pp. 205-6; ANSI Rationale Sec. 3.2.2.3 p. 38.

2.12:	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)++;

	work?

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);

	References: ANSI Sec. 3.3.4, Rationale Sec. 3.3.2.4 p. 43.


Section 3. Expressions

3.1:	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 the operations after yielding the former value, the
	implication of "after" is often misunderstood.  It is _not_
	guaranteed that the operation 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).  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.  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&R I Sec. 2.12 p. 50; K&R II Sec. 2.12 p. 54; ANSI
	Sec. 3.3 p. 39; CT&P Sec. 3.7 p. 47; PCS Sec. 9.5 pp. 120-1.
	(Ignore H&S Sec. 7.12 pp. 190-1, which is obsolete.)

3.2:	But what about the &&, ||, and comma operators?
	I see code like "if((c = getchar()) == EOF || c == '\n')" ...

A:	There is a special exception for those operators, (as well as
	?: ); each of them does imply a sequence point (i.e. left-to-
	right evaluation is guaranteed).  Any book on C should make this
	clear.

	References: K&R I Sec. 2.6 p. 38, Secs. A7.11-12 pp. 190-1;
	K&R II Sec. 2.6 p. 41, Secs. A7.14-15 pp. 207-8; ANSI
	Secs. 3.3.13 p. 52, 3.3.14 p. 52, 3.3.15 p. 53, 3.3.17 p. 55,
	CT&P Sec. 3.7 pp. 46-7.

3.3:	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.  Some
	people will tell you that in the old days one form was preferred
	over the other because it utilized a PDP-11 autoincrement
	addressing mode, but those people are confused.

3.4:	Why doesn't the code

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

	work?

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

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


Section 4. ANSI C

4.1:	What is the "ANSI C Standard?"

A:	In 1983, the American National Standards Institute 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 an
	American National Standard, 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 much-lambasted trigraph
	sequences).  The ANSI C standard also formalizes the C run-time
	library support routines.

	The published Standard includes a "Rationale," which explains
	many of its decisions, and discusses a number of subtle points,
	including several of those covered here.  (The Rationale is "not
	part of ANSI Standard X3.159-1989, but is included for
	information only.")

	The Standard has been adopted as an international standard,
	ISO/IEC 9899:1990, although the sections are numbered
	differently (briefly, ANSI sections 2 through 4 correspond
	roughly to ISO sections 5 through 7), and the Rationale is
	currently not included.

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

A:	Copies are available from

		American National Standards Institute
		1430 Broadway
		New York, NY  10018  USA
		(+1) 212 642 4900

	or

		Global Engineering Documents
		2805 McGaw Avenue
		Irvine, CA  92714  USA
		(+1) 714 261 1455
		(800) 854 7179  (U.S. & Canada)

	The cost from ANSI is $50.00, plus $6.00 shipping.  Quantity
	discounts are available.  (Note that ANSI derives revenues to
	support its operations from the sale of printed standards, so
	electronic copies are _not_ available.)

	The Rationale, by itself, has been printed by Silicon Press,
	ISBN 0-929306-07-4.

4.3:	Does anyone have a tool for converting old-style C programs to
	ANSI C, or for automatically generating prototypes?

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
	exist as patches to the FSF GNU C compiler, gcc.  Look for the
	file protoize-1.39.0.5.Z in pub/gnu at prep.ai.mit.edu
	(18.71.0.38), or at several other FSF archive sites.

	The unproto program (/pub/unix/unproto4.shar.Z on
	ftp.win.tue.nl) is a filter which sits between the preprocessor
	and the next compiler pass, converting most of ANSI C to
	traditional C on-the-fly.

	Several prototype generators exist, many as modifications to
	lint.  Version 3 of CPROTO was posted to comp.sources.misc in
	March, 1992.  (See also question 16.6.)

4.4:	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 must use something like the following two-step procedure to
	force the 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 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. 3.8.3.2, Sec. 3.8.3.5 example p. 93.

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

A:	"char const *p" is a pointer to a constant character (you can't
	change the character); "char * const p" is a constant pointer to
	a (variable) character (i.e. you can't change the pointer).
	(Read these "inside out" to understand them.  See question
	10.3.)

	References: ANSI Sec. 3.5.4.1 .

4.6:	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;".  Old C (and ANSI C, in the absence of
	prototypes) silently promotes floats to doubles when passing
	them as arguments, and arranges that doubles being passed are
	coerced back to floats if the formal parameters are declared
	that way.

	The 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.

	References: ANSI Sec. 3.3.2.2 .

4.7:	I'm getting strange syntax errors inside code which I've
	#ifdeffed out.

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 unterminated comments or quotes
	(note particularly that an apostrophe within a contracted word
	could look like the beginning of a character constant), and no
	newlines inside quotes.  Therefore, natural-language comments
	and pseudocode should always be written between the "official"
	comment delimiters /* and */.  (But see also question 16.8.)

	References: ANSI Sec. 2.1.1.2 p. 6, Sec. 3.1 p. 19 line 37.

4.8:	Can I declare main as void, to shut off these annoying "main
	returns no value" messages?  (I'm calling exit(), so main
	doesn't return.)

A:	No.  main must be declared as returning an int, and as taking
	either zero or two arguments (of the appropriate type).  If
	you're calling exit() but still getting warnings, you'll have to
	insert a redundant return statement (or use some kind of
	"notreached" directive, if available).

	References: ANSI Sec. 2.1.2.2.1 pp. 7-8.

4.9:	Why does the ANSI Standard not guarantee more than six monocase
	characters of external identifier significance?

A:	The problem is older linkers which are neither under the control
	of the ANSI 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 4.1), which discusses several
	such schemes and explains why they could not be mandated.

	References: ANSI Sec. 3.1.2 p. 21, Sec. 3.9.1 p. 96, Rationale
	Sec. 3.1.2 pp. 19-21.

4.10:	What is 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: ANSI Secs. 4.11.2.1, 4.11.2.2, Rationale
	Sec. 4.11.2 .

4.11:	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 the old lint
	/* NOTREACHED */ comments), etc.

	References: ANSI Sec. 3.8.6 .


Section 5. C Preprocessor

5.1:	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, (and it will not work if the two values are the same
	variable, and the "obvious" supercompressed implementation for
	integral types a^=b^=a^=b is, strictly speaking, illegal due to
	multiple side-effects, and...).  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 standard C does not provide a typeof operator.

	The best all-around solution is probably to forget about using a
	macro, unless you don't mind passing in the type as a third
	argument.

5.2:	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:	That comments disappeared entirely and could therefore be used
	for token pasting was an undocumented feature of some early
	preprocessor implementations, notably Reiser's.  ANSI affirms
	(as did K&R) 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 4.4.)

	Reference: ANSI Sec. 3.8.3.3 p. 91, Rationale pp. 66-7.

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

A:	The usual goal is to write a macro that can be invoked as if it
	were a single function-call statement.  This means that the
	"caller" will be supplying the final semicolon, so the macro
	body should not.  The macro body cannot be a simple brace-
	delineated 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 is to use

		#define Func() 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.  (See the example under question 5.8 below.
	This technique also allows a value to be "returned.")

	Reference: CT&P Sec. 6.3 pp. 82-3.

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

A:	There has been considerable debate surrounding this question.
	Many people believe that "nested #include files" are to be
	avoided: the prestigious Indian Hill Style Guide disparages
	them; they can make it harder to find relevant definitions; they
	can lead to multiple-declaration 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 #includes what it needs itself,
	rather than requiring each #includer to do so, a requirement
	that can lead to intractable headaches); a tool like grep (or a
	tags file) makes it easy to find definitions no matter where
	they are; a popular trick:

		#ifndef HEADER_FILE_NAME
		#define HEADER_FILE_NAME
		...header file contents...
		#endif

	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)
	handle dependency generation in the face of nested #include
	files easily.

5.5:	Does sizeof() work in preprocessor #if directives?

A:	No.  Preprocessing happens during an earlier pass of
	compilation, before type names have been parsed.  Consider using
	the predefined constants in <limits.h>, or a "configure" script,
	instead.

	References: ANSI Sec. 2.1.1.2 pp. 6-7, Sec. 3.8.1 p. 87
	footnote 83.

5.6:	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
	ints, 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.

5.7:	I've got this tricky processing I want to do at compile time and
	I can't figure out a way to get cpp to do it.

A:	cpp is not intended as a general-purpose preprocessor.  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.

5.8:	How can I write a cpp macro which takes a variable number of
	arguments?

A:	One popular trick is to define the macro with a single argument,
	and call it with a double set of parentheses, which appear to
	the preprocessor to indicate a single argument:

		#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.  (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 6.1 and 6.2
	below.)


Section 6. Variable-Length Argument Lists

6.1:	How can I write a function that takes a variable number of
	arguments?

A:	Use the <stdarg.h> header (or, if you must, the older
	<varargs.h>).

	Here is a function which concatenates an arbitrary number of
	strings into malloc'ed memory:

		#include <stddef.h>		/* for NULL, size_t */
		#include <stdarg.h>		/* for va_ stuff */
		#include <string.h>		/* for strcat et al */
		#include <stdlib.h>		/* for malloc */

		char *vstrcat(char *first, ...)
		{
			size_t len = 0;
			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);

			va_end(argp);

			retbuf = malloc(len + 1);	/* +1 for trailing \0 */

			if(retbuf == NULL)
				return NULL;		/* error */

			(void)strcpy(retbuf, first);

			va_start(argp, first);

			while((p = va_arg(argp, char *)) != NULL)
				(void)strcat(retbuf, p);

			va_end(argp);

			return retbuf;
		}

	Usage is something like

		char *str = vstrcat("Hello, ", "world!", (char *)NULL);

	Note the cast on the last argument.  (Also note that the caller
	must free the returned, malloc'ed storage.)

	Under a pre-ANSI compiler, rewrite the function definition
	without a prototype ("char *vstrcat(first) char *first; {"),
	include <stdio.h> rather than <stddef.h>, replace "#include
	<stdlib.h>" with "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 the next
	question for hints.

	References: K&R II Sec. 7.3 p. 155, Sec. B7 p. 254; H&S
	Sec. 13.4 pp. 286-9; ANSI Secs. 4.8 through 4.8.1.3 .

6.2:	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:

		#include <stdio.h>
		#include <stdarg.h>

		void
		error(char *fmt, ...)
		{
			va_list argp;
			fprintf(stderr, "error: ");
			va_start(argp, fmt);
			vfprintf(stderr, fmt, argp);
			va_end(argp);
			fprintf(stderr, "\n");
		}

	To use the older <varargs.h> package, instead of <stdarg.h>,
	change the function header to:

		void error(va_alist)
		va_dcl
		{
			char *fmt;

	change the va_start line to

		va_start(argp);

	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: K&R II Sec. 8.3 p. 174, Sec. B1.2 p. 245; H&S
	Sec. 17.12 p. 337; ANSI Secs. 4.9.6.7, 4.9.6.8, 4.9.6.9 .

6.3:	How can I discover how many arguments a function was actually
	called with?

A:	This information is not available to a portable program.  Some
	systems provide a nonstandard nargs() function, but its use is
	questionable, since it typically returns the number of words
	passed, not the number of arguments.  (Floating point values and
	structures are usually passed as several words.)

	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
	(useful 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 under questions 1.2 and 6.1 above).

6.4:	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.  You must provide a version of that
	other function which accepts a va_list pointer, as does vfprintf
	in the example above.  If the arguments must be passed directly
	as actual arguments (not indirectly through a va_list pointer)
	to another function which is itself variadic (for which you do
	not have the option of creating an alternate, va_list-accepting
	version) no portable solution is possible.  (The problem can be
	solved by resorting to machine-specific assembly language.)


Section 7. Lint

7.1:	I just typed in this program, and it's acting strangely.  Can
	you see anything wrong with it?

A:	Try running lint first.  Many C compilers are really only half-
	compilers, electing not to diagnose numerous source code
	difficulties which would not actively preclude code generation.

7.2:	How can I shut off the "warning: possible pointer alignment
	problem" message 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.

7.3:	Where can I get an ANSI-compatible lint?

A:	A product called FlexeLint is available (in "shrouded source
	form," for compilation on 'most any system) from

		Gimpel Software
		3207 Hogarth Lane
		Collegeville, PA  19426  USA
		(+1) 215 584 4261

	The System V release 4 lint is ANSI-compatible, and is available
	separately (bundled with other C tools) from Unix Support Labs
	(a subsidiary of AT&T), or from System V resellers.


Section 8. Memory Allocation

8.1:	Why doesn't this fragment work?

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

A:	The pointer variable "answer," which is handed to the gets
	function 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 question 16.1.)

	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:

		#include <string.h>

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

	Note that this example also uses fgets instead of gets (always a
	good idea), so that the size of the array can be specified, so
	that fgets will not overwrite the end of the array if the user
	types an overly-long line.  (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, and/or to parameterize its size
	(#define ANSWERSIZE 100).

8.2:	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:	Again, the problem 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 (explicitly) 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 with sufficient space:

		char s1[20] = "Hello, ";

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

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

8.3:	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, at least to make sure that the compiler is
	doing it for you.  If a library routine's documentation does not
	explicitly mention allocation, it is usually the caller's
	problem.

	The Synopsis section at the top of a Unix-style man page can be
	misleading.  The code fragments presented there are closer to
	the function definition used by the call's implementor than the
	invocation used by the caller.  In particular, many routines
	which accept pointers (e.g. to structs or strings), are usually
	called with the address of some object (a struct, or an array --
	see questions 2.3 and 2.4.)  Another common example is stat().

8.4:	I have a function that returns a string, and it seems to work
	fine under the debugger, but when it returns to its caller, the
	returned string is garbage.

A:	Make sure that the memory to which the function returns a
	pointer is correctly allocated.  The returned pointer should be
	to a statically-allocated buffer, or to a buffer passed in by
	the caller, but _not_ to a local array.

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

A:	No.  Some early man pages for malloc stated that the contents of
	freed memory was "left undisturbed;" this ill-advised guarantee
	was never universal and is not required by ANSI.

	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((char *)listp);
		}

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

	References: ANSI Rationale Sec. 4.10.3.2 p. 102; CT&P Sec. 7.10
	p. 95.

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

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

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

A:	Not portably.

8.8:	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), but several earlier implementations do not support
	it, so it is not widely portable.  Passing an initially-null
	pointer to realloc can make it easier to write a self-starting
	incremental allocation algorithm.

	References: ANSI Sec. 4.10.3.4 .

8.9:	What is the difference between calloc and malloc?  Is it safe to
	use calloc's zero-fill guarantee for pointer and floating-point
	values?  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 zero values for pointers (see section 1 of this list) or
	floating-point values.  free can (and should) be used to free
	the memory allocated by calloc.

	References: ANSI Secs. 4.10.3 to 4.10.3.2 .

8.10:	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 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 cannot be used in programs which must
	be widely portable, no matter how useful it might be.


Section 9. Structures

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

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

	References: K&R I Sec. 6.2 p. 121; K&R II Sec. 6.2 p. 129; H&S
	Sec. 5.6.2 p. 103; ANSI Secs. 3.1.2.5, 3.2.2.1, 3.3.16 .

9.2:	How does struct passing and returning work?

A:	When structures are passed as arguments to functions, the entire
	struct 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.)

	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 struct-valued
	functions nonreentrant, which ANSI C disallows.

	Reference: ANSI Sec. 2.2.3 p. 13.

9.3:	The following 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 the compiler to believe that main
	returns a structure.  (The connection is hard to see because of
	the intervening comment.)  Since struct-valued functions are
	usually implemented by adding a hidden return pointer, 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 question 16.15.

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

9.4:	Why can't you compare structs?

A:	There is no reasonable way for a compiler to implement struct
	comparison which is consistent with C's low-level flavor.  A
	byte-by-byte comparison could be invalidated by random bits
	present in unused "holes" in the structure (such padding is used
	to keep the alignment of later fields correct).  A field-by-
	field comparison would require unacceptable amounts of
	repetitive, in-line code for large structures.

	If you want to compare two structures, you must write your own
	function to do so.  C++ would let you arrange for the ==
	operator to map to your function.

	References: K&R II Sec. 6.2 p. 129; H&S Sec. 5.6.2 p. 103; ANSI
	Rationale Sec. 3.3.9 p. 47.

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

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

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

A:	This technique is popular, although Dennis Ritchie has called it
	"unwarranted chumminess with the compiler."  The ANSI C standard
	allows it only implicitly.  It seems to be portable to all known
	implementations.  (Compilers which check array bounds carefully
	might issue warnings.)

	References: ANSI Rationale Sec. 3.5.4.2 pp. 54-5.

9.6:	How can I determine the byte offset of a field within a
	structure?

A:	ANSI C defines the offsetof macro, which should be used if
	available; see <stddef.h>.  If you don't have it, a suggested
	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 the next question for a usage hint.

	References: ANSI Sec. 4.1.5 , Rationale Sec. 3.5.4.2 p. 55.

9.7:	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 b
	is an int field with offset as computed above, b's value can be
	set indirectly with

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


Section 10. Declarations

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

A:	If you might need large values (above 32767 or below -32767),
	use long.  Otherwise, if space is very important (there are
	large arrays or many structures), use short.  Otherwise, use
	int.  If well-defined overflow characteristics are important
	and/or negative values are not, use the corresponding unsigned
	types.  (But beware mixtures of signed and unsigned.)  Similar
	arguments apply when deciding between float and double.

	Although char or unsigned char can be used as a "tiny" int type,
	doing so is often more trouble than it's worth, due to
	unpredictable sign extension and increased code size.

	These rules obviously don't apply if the address of a variable
	is taken and must have a particular type.

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

		typedef struct
			{
			char *item;
			NODEPTR next;
			} *NODEPTR;

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

A:	Structs 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 this example is that the NODEPTR typedef is not
	complete at the point where the "next" field is declared.  To
	fix it, first give the structure a tag ("struct node").  Then,
	declare the "next" field as "struct node *next;", and/or move
	the typedef declaration wholly before or wholly after the struct
	declaration.  One fixed 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 recursive
	structures.

	References: K&R I Sec. 6.5 p. 101; K&R II Sec. 6.5 p. 139; H&S
	Sec. 5.6.1 p. 102; ANSI Sec. 3.5.2.3 .

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

A:	This question can be answered in at least three ways (all
	declare the hypothetical array with 5 elements):

	1.  char *(*(*a[5])())();

	2.  Build the declaration up in stages, 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[5];		/* array of... */

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

		cdecl> declare a as array 5 of pointer to function returning
			 pointer to function returning pointer to char
		char *(*(*a[5])())()

	    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
	    above).  Versions of cdecl are in volume 14 of
	    comp.sources.unix (see question 16.6) and K&R II.

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

	References: K&R II Sec. 5.12 p. 122; H&S Sec. 5.10.1 p. 116.

10.4:	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.)
	It is best to place the definition in some central (to the
	program, or to the module) .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 header file containing the external
	declaration, so that the compiler can check that the
	declarations match.

	This rule promotes a high degree of portability, and is
	consistent with the requirements of the ANSI C Standard.  Note
	that Unix compilers and linkers typically use a "common model"
	which allows multiple (uninitialized) definitions.  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 the
	declaration need only be typed once, in the header file, and
	"turned into" a definition, during exactly one #inclusion, via a
	special #define.

	References: K&R I Sec. 4.5 pp. 76-7; K&R II Sec. 4.4 pp. 80-1;
	ANSI Sec. 3.1.2.2 (esp. Rationale), Secs. 3.7, 3.7.2,
	Sec. F.5.11 .

10.5:	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 but is not
	being called (i.e. is not followed by a "("), it "decays" into a
	pointer (i.e. it has its address implicitly taken), much as an
	array name does.

	An explicit extern declaration for the function is normally
	needed, since implicit external function declaration does not
	happen in this case (again, because the function name is not
	followed by a "(").

10.6:	I've seen different methods used for calling through pointers to
	functions.  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, f(), (*fp)() = f;
		r = (*fp)();

	It can also be argued that functions are always called through
	pointers, but that "real" functions decay implicitly into
	pointers (in expressions, as they do in initializations) and so
	cause no trouble.  This reasoning, made widespread through pcc
	and adopted in the ANSI standard, means that

		r = fp();

	is legal and works correctly, whether fp is 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 through it.)  An
	explicit * is harmless, and still allowed (and recommended, if
	portability to older compilers is important).

	References: ANSI Sec. 3.3.2.2 p. 41, Rationale p. 41.


Section 11. Boolean Expressions and Variables

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

A:	C does not provide a standard boolean type, because picking one
	involves a space/time tradeoff which is best decided by the
	programmer.  (Using an int for a boolean may be faster, while
	using char may save data space.)

	The choice between #defines and enums is arbitrary and not
	terribly interesting.  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 enum may be preferable if your debugger
	expands enum values when examining variables.)

	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 below).

11.2:	Isn't #defining TRUE to be 1 dangerous, since any nonzero value
	is considered "true" in C?  What if a built-in boolean 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)

	will work as expected (as long as TRUE is 1), but it is
	obviously silly.  In general, explicit tests against TRUE and
	FALSE are undesirable, 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 as the return value
	from a Boolean function, never in a comparison.

	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 question 1.7.)

	References: K&R I Sec. 2.7 p. 41; K&R II Sec. 2.6 p. 42,
	Sec. A7.4.7 p. 204, Sec. A7.9 p. 206; ANSI Secs. 3.3.3.3, 3.3.8,
	3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, 3.6.5; Achilles and the
	Tortoise.

11.3:	What is the difference between an enum and a series of
	preprocessor #defines?

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

	The primary advantages of enums are that the numeric values are
	automatically assigned, and that a debugger may be able to
	display the symbolic values when enum variables are examined.
	(A compiler may also generate nonfatal warnings when enums and
	ints 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 the
	size (or over those nonfatal warnings).

	References: K&R II Sec. 2.3 p. 39, Sec. A4.2 p. 196; H&S
	Sec. 5.5 p. 100; ANSI Secs. 3.1.2.5, 3.5.2, 3.5.2.2 .


Section 12. System Dependencies

12.1:	How can I read a single character from the keyboard without
	waiting for a newline?

A:	Contrary to popular belief and many people's wishes, this is not
	a C-related question.  (Nor are closely-related questions
	concerning the echo of keyboard input.)  The delivery of
	characters from a "keyboard" to a C program is a function of the
	operating system in use, and cannot be standardized by the C
	language.  Some versions of curses have a cbreak() function
	which does what you want.  Under UNIX, 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).  Under MS-DOS, use getch().  Under VMS, try the
	Screen Management (SMG$) routines.  Under other operating
	systems, you're on your own.  Beware that some operating systems
	make this sort of thing impossible, because character collection
	into input lines is done by peripheral processors not under
	direct control of the CPU running your program.

	Operating system specific questions are not appropriate for
	comp.lang.c .  Many common questions are answered in
	frequently-asked questions postings in such groups as
	comp.unix.questions and comp.os.msdos.programmer .  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.

	References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1.

12.2:	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 the FIONREAD ioctl, or kbhit(),
	or rdchk(), or the O_NDELAY option to open() or fcntl().

12.3:	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 than
	it is under MS-DOS than it is on the Macintosh.

12.4:	How can my program discover the complete pathname to the
	executable file 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 or portable solution.

12.5:	How can a process change an environment variable in its caller?

A:	In general, it cannot.  Different operating systems implement
	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() and/or putenv() functions to do this),
	and the modified environment is usually passed on to any child
	processes, but it is _not_ propagated back to the parent
	process.

12.6:	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 in general impossible to
	determine this number in advance.  Under Unix, the stat()
	function will give you an exact answer, and 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 this
	usage is nonportable (it gives you an accurate answer only under
	Unix, and a guaranteed, but potentially approximate answer only
	for ANSI C "binary" files).

12.7:	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, but there is no truly portable solution.

12.8:	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.


Section 13. Stdio

13.1:	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 an operation which fails (with
	ENOTTY) if stdout is not a terminal.  Although the output
	operation goes on to complete successfully, errno still contains
	ENOTTY.

	Reference: CT&P Sec. 5.4 p. 73.

13.2:	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 is 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 a terminal.  (See question 13.1.)

13.3:	When I read from the keyboard with scanf(), it seems to hang
	until I type one extra line of input.

A:	scanf() was designed for free-format input, which is seldom what
	you want when reading from the keyboard.  In particular, "\n" in
	a format string means, not to expect a newline, but to read and
	discard characters as long as each is a whitespace character.

	It is usually better to use fgets() to read a whole line, and
	then use sscanf() or other string functions to parse the line
	buffer.

13.4:	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
	buffer, nor would such a way be sufficient; unread characters
	can also accumulate in other, OS-level input buffers.

13.5:	How can I recover the file name given an open file descriptor?

A:	This problem is, in general, insoluble.  Under Unix, for
	instance, a scan of the entire disk, (perhaps requiring special
	permissions) would theoretically be required, and would fail if
	the file descriptor was 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 open files yourself
	(perhaps with a wrapper function around fopen).


Section 14. Library Subroutines

14.1:	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, of course, accepts simple pointers
	to char.)

	The comparison routine's arguments are expressed as "generic
	pointers," void * or char *.  They must be converted back to
	what they "really are" (char **) and dereferenced, yielding
	char *'s which can be usefully compared.  Write a comparison
	function like this:

		int pstrcmp(p1, p2)	/* compare strings through pointers */
		char *p1, *p2;		/* void * for ANSI C */
		{
			return strcmp(*(char **)p1, *(char **)p2);
		}

14.2:	Now I'm trying to sort an array of structures with qsort.  My
	comparison routine 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
	warning?

A:	The conversions must be in the comparison function, which must
	be declared as accepting "generic pointers" (void * or char *)
	as discussed above.

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

A:	Just use sprintf.  (You'll have to allocate space for the result
	somewhere anyway; see questions 8.1 and 8.2.  Don't worry that
	sprintf may be overkill, potentially wasting run time or code
	space; it works well in practice.)

	References: K&R I Sec. 3.6 p. 60; K&R II Sec. 3.6 p. 64.

14.4:	How can I get the 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:

		#include <stdio.h>
		#include <time.h>

		main()
		{
			time_t now;
			time(&now);
			printf("It's %.24s.\n", ctime(&now));
			exit(0);
		}

	References: ANSI Sec. 4.12 .

14.5:	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 time_t?

A:	ANSI C specifies a library routine, mktime, which converts a
	struct tm to a time_t.  Several public-domain versions of this
	routine are available in case your compiler does not support it
	yet.

	Converting a string to a time_t is harder, because of the wide
	variety of date and time formats which should be parsed.
	Public-domain routines have been written for performing this
	function (see, for example, the file partime.c, widely
	distributed with the RCS package), but they are less likely to
	become standardized.

	References: K&R II Sec. B10 p. 256; H&S Sec. 20.4 p. 361; ANSI
	Sec. 4.12.2.3 .

14.6:	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.

	References: ANSI Sec. 4.10.2.1 p. 154.

14.7:	Each time I run my program, I get the same sequence of random
	numbers.

A:	You can call srand() to seed the pseudo-random number generator
	with a more random value.  Popular random initial seeds are the
	time of day, or the elapsed time before the user presses a key.

	References: ANSI Sec. 4.10.2.2 p. 154.

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

14.9-13: I'm trying to port this old	A:  These routines are variously
	program.  Why do I get		    obsolete; you should
	"undefined external" errors	    instead:
	for:

14.9:	index?				A:  use strchr.
14.10:	rindex?				A:  use strrchr.
14.11:	bcopy?				A:  use memmove, after
					    interchanging the first and
					    second arguments (see also
					    question 4.10).
14.12:	bcmp?				A:  use memcmp.
14.13:	bzero?				A:  use memset, with a second
					    argument of 0.

14.14:	How can I execute a command with system() and read its output
	into a program?

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).

14.15:	How can I read a directory in a C program?

A:	See if you can use the opendir() and readdir() routines, which
	are available on most Unix systems.  Implementations also exist
	for MS-DOS, VMS, and other systems.  (MS-DOS also has FINDFIRST
	and FINDNEXT routines which do essentially the same thing.)


Section 15. Style

15.1:	Here's a neat trick:

		if(!strcmp(s1, s2))

	Is this good style?

A:	Perhaps; perhaps not.  The test succeeds if the two strings are
	equal, but its form suggests that it tests for inequality.

	Another solution 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 codified.

15.2:	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 avoiding 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
		consistently.

	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.

	Reference: K&R Sec. 1.2 p. 10.

15.3:	Where can I get the "Indian Hill Style Guide" and other coding
	standards?

A:	Various documents are available for anonymous ftp from:

	    Site:			File or directory:

	    cs.washington.edu		~ftp/pub/cstyle.tar.Z
	    (128.95.1.4)		(the updated Indian Hill guide)

	    cs.toronto.edu		doc/programming

	    giza.cis.ohio-state.edu	pub/style-guide


Section 16. Miscellaneous

16.1:	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 to zero, as if the
	programmer had typed "= 0".  Therefore, such variables are
	initialized to the null pointer (of the correct type) 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
	contains all-bits-0, but this is not necessarily useful for
	pointer or floating-point values (see section 1 and question
	8.9).

16.2:	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 best 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 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, CCITT's X.409, or ISO 8825 "Basic Encoding Rules."

16.3:	I seem to be missing the system header file <sgtty.h>.  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).

16.4:	How can I call Fortran (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.

	cfortran.h, a C header file, simplifies C/Fortran interfacing on
	many popular machines.  It is available via anonymous ftp from
	zebra.desy.de (131.169.2.244).

16.5:	Does anyone know of a program for converting Pascal (Fortran,
	lisp, "Old" C, ...) to C?

A:	Several public-domain programs are available:

	p2c	written by Dave Gillespie, and posted to
		comp.sources.unix in March, 1990 (Volume 21); also
		available by anonymous ftp from csvax.cs.caltech.edu,
		file pub/p2c-1.20.tar.Z .

	ptoc	another comp.sources.unix contribution, this one written
		in Pascal (comp.sources.unix, Volume 10, also patches in
		Volume 13?).

	f2c	jointly developed by people from Bell Labs, Bellcore,
		and Carnegie Mellon.  To find about f2c, send the mail
		message "send index from f2c" to netlib@research.att.com
		or research!netlib.  (It is also available via anonymous
		ftp on research.att.com, in directory dist/f2c.)

	A PL/M to C converter was posted to alt.sources in April, 1991.

	The following companies sell various translation tools and
	services:

		Cobalt Blue
		2940 Union Ave., Suite C
		San Jose, CA  95124  USA
		(+1) 408 723 0474

		Promula Development Corp.
		3620 N. High St., Suite 301
		Columbus, OH  43214  USA
		(+1) 614 263 5454

		Micro-Processor Services Inc.
		92 Stone Hurst Lane
		Dix Hills, NY  11746  USA
		(+1) 519 499 4461

	See also question 4.3.

16.6:	Where can I get copies of all these public-domain programs?

A:	If you have access to Usenet, see the regular postings in the
	comp.sources.unix and comp.sources.misc newsgroups, which
	describe, in some detail, the archiving policies and how to
	retrieve copies.  The usual approach is to use anonymous ftp
	and/or uucp from a central, public-spirited site, such as uunet
	(ftp.uu.net, 192.48.96.9).  However, this article cannot track
	or list all of the available archive sites and how to access
	them.  The comp.archives newsgroup contains numerous
	announcements of anonymous ftp availability of various items.
	The "archie" mailserver can tell you which anonymous ftp sites
	have which packages; send the mail message "help" to
	archie@quiche.cs.mcgill.ca for information.  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.

16.7:	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 typically runs from early March through mid-May.  To
	obtain a current copy of the rules and other information, send
	e-mail with the Subject: line "send rules" to:

		{apple,pyramid,sun,uunet}!hoptoad!judges  or  judges@toad.com

	Contest winners are first announced at the Summer Usenix
	Conference in mid-June, and posted to the net in July.  Previous
	winners are available on uunet (see question 16.6) under the
	directory ~/pub/ioccc.

16.8:	Why don't C comments nest?  Are they legal inside quoted
	strings?

A:	Nested comments would cause more harm than good, mostly because
	of the possibility of accidentally leaving comments unclosed by
	including the characters "/*" within them.  For this reason, it
	is usually better to "comment out" large sections of code, which
	might contain comments, with #ifdef or #if 0 (but see question
	4.7).

	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.

	Reference: ANSI Rationale Sec. 3.1.9 p. 33.

16.9:	What is the most efficient way to count the number of bits which
	are set in a value?

A:	This and many other similar bit-twiddling problems can often be
	sped up and streamlined using lookup tables (but see the next
	question).

16.10:	How can I make this 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 cacheing techniques.

	For the small fraction of code that is time-critical, it is
	vital to pick a good algorithm; it is less 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 degraded.

	For more discussion of efficiency tradeoffs, as well as good
	advice on how to increase efficiency when it is important, see
	chapter 7 of Kernighan and Plauger's The Elements of Programming
	Style, and Jon Bentley's Writing Efficient Programs.

16.11:	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 C compiler, not a keystroke-programmable
	calculator.  A good 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 3.3.)

16.12:	My floating-point calculations are acting strangely and giving
	me different answers on different machines.

A:	First, make sure that you have #included <math.h>, and correctly
	declared other functions returning double.

	If the problem isn't that simple, recall that most digital
	computers use floating-point formats which provide a close but
	by no means exact simulation of real number arithmetic.  Among
	other things, the associative and distributive laws do not hold
	completely (i.e. order of operation may be important, and
	repeated addition is not necessarily equivalent to
	multiplication).  Underflow or cumulative precision loss is
	often a problem.

	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.)

	These problems are no worse for C than they are for any other
	computer language.  Floating-point semantics are usually defined
	as "however the processor does them;" otherwise a compiler for a
	machine without the "right" model would have to do prohibitively
	expensive emulations.

	This article cannot begin to list the pitfalls associated with,
	and workarounds appropriate for, floating-point work.  A good
	programming text should cover the basics.

	References: EoPS Sec. 6 pp. 115-8.

16.13:	Why doesn't C have an exponentiation operator?

A:	You can #include <math.h> and use the pow() function, although
	explicit multiplication is often better for small integral
	exponents.

	References: ANSI Sec. 4.5.5.1 .

16.14:	I'm having trouble with a Turbo C program which crashes and says
	something like "floating point formats not linked."

A:	Some compilers for small machines, including Turbo C (and
	Ritchie's original PDP-11 compiler), leave out floating point
	support if it looks like it will not be needed.  In particular,
	the non-floating-point versions of printf and scanf save space
	by not including code to handle %e, %f, and %g.  It happens that
	Turbo C's heuristics for determining whether the program uses
	floating point are occasionally insufficient, and the programmer
	must sometimes insert a dummy explicit floating-point call to
	force loading of floating-point support.

	In general, questions about a particular compiler are
	inappropriate for comp.lang.c .  Problems with PC compilers, for
	instance, will find a more receptive audience in a PC newsgroup
	(e.g. comp.os.msdos.programmer).

16.15:	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).

	(See also question 9.3.)

16.16:	What does "Segmentation violation" mean?

A:	It means your program tried to access memory it shouldn't have,
	invariably as a result of improper pointer use, often involving
	malloc.

16.17:	Does anyone have a C compiler test suite I can use?

A:	Plum Hall (1 Spruce Ave., Cardiff, NJ 08232, USA), among others,
	sells one.

16.18:	Where can I get a YACC grammar for C?

A:	The definitive grammar is of course the one in the ANSI
	standard.  Several copies are floating around; keep your eyes
	open.  There is one on uunet (see question 16.6) 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&R II.

	References: ANSI Sec. A.2 .

16.19:	How do you pronounce "char"?  What's that funny name for the "#"
	character?

A:	You can pronounce the C keyword "char" in at least three ways:
	like the English words "char," "care," or "car;" the choice is
	arbitrary.  Bell Labs once proposed the (now obsolete) term
	"octothorpe" for the "#" character.

	Trivia questions like these aren't any more pertinent for
	comp.lang.c than they are for most of the other groups they
	frequently come up in.  You can find lots of information in the
	net.announce.newusers frequently-asked questions postings, the
	"jargon file" (also published as _The [New] Hacker's
	Dictionary_), and the old Usenet ASCII pronunciation list.

16.20:	Where can I get extra copies of this list?  What about back
	issues?

A:	For now, just pull it off the net; it is normally posted to
	comp.lang.c on the first of each month, with an Expiration: line
	which should keep it around all month.  It can also be found in
	the newsgroup news.answers .  Several sites archive news.answers
	postings and other FAQ lists, including this one; the archie
	server (see question 16.6) should help you find them.  See the
	meta-FAQ list in news.answers for more information.

	This list is an evolving document, 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.


Bibliography

ANSI	American National Standard for Information Systems --
	Programming Language -- C, ANSI X3.159-1989 (see question 4.2).

JLB	Jon Louis Bentley, Writing Efficient Programs, Prentice-Hall,
	1982, ISBN 0-13-970244-X.

H&S	Samuel P. Harbison and Guy L. Steele, C: A Reference Manual,
	Second Edition, Prentice-Hall, 1987, ISBN 0-13-109802-0.
	(A third edition has recently been released.)

PCS	Mark R. Horton, Portable C Software, Prentice Hall, 1990,
	ISBN 0-13-868050-7.

EoPS	Brian W. Kernighan and P.J. Plauger, The Elements of Programming
	Style, Second Edition, McGraw-Hill, 1978, ISBN 0-07-034207-5.

K&R I	Brian W. Kernighan and Dennis M. Ritchie, The C Programming
	Language, Prentice Hall, 1978, ISBN 0-13-110163-3.

K&R II	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.

CT&P	Andrew Koenig, C Traps and Pitfalls, Addison-Wesley, 1989,
	ISBN 0-201-17928-8.

	P.J. Plauger, The Standard C Library, Prentice Hall, 1992,
	ISBN 0-13-131509-9.

	H. Rabinowitz and Chaim Schaap, Portable C, Prentice-Hall, 1990.

TNHD	Eric Raymond, Ed., The New Hacker's Dictionary, MIT Press, 1991,
	ISBN 0-262-68069-6.

There is a more extensive bibliography in the revised Indian Hill style
guide (see question 15.3).


Acknowledgements

Thanks to Jamshid Afshar, Sudheer Apte, Dan Bernstein, Stan Brown, Joe
Buehler, Burkhard Burow, Raymond Chen, Christopher Calabrese, James
Davies, Norm Diamond, Ray Dunn, Stephen M. Dunn, Bjorn Engsig, Dave
Gillespie, Samuel Goldstein, Ron Guilmette, Doug Gwyn, Tony Hansen, Joe
Harrington, Guy Harris, Jos Horsmeier, Blair Houghton, Kirk Johnson,
Peter Klausler, Andrew Koenig, John Lauro, Don Libes, Christopher Lott,
Tim McDaniel, Evan Manning, Mark Moraes, Darren Morby, Hans Olsson,
Francois Pinard, randall@virginia, Pat Rankin, Rich Salz, Chip
Salzenberg, Paul Sand, Doug Schmidt, Patricia Shanahan, Peter da Silva,
Joshua Simons, Henry Spencer, Erik Talvola, Clarke Thatcher, Chris
Torek, Goran Uddeborg, Wietse Venema, Ed Vielmetti, Larry Virden, Freek
Wiedijk, and Dave Wolverton, who have contributed, directly or
indirectly, to this article.  Special thanks to Karl Heuer, 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
					scs@adam.mit.edu
					scs%adam.mit.edu@mit.edu
					mit-eddie!adam.mit.edu!scs

This article is Copyright 1988, 1990-1992 by Steve Summit.
It may be freely redistributed so long as the author's name, and this
notice, are retained.
The C code in this article (vstrcat(), error(), etc.) is public domain
and may be used without restriction.
Newsgroups: comp.lang.c