From: scs@adam.mit.edu (Steve Summit)
Newsgroups: comp.lang.c,news.answers
Subject: comp.lang.c Answers (Abridged) to Frequently Asked Questions (FAQ)
Date: 15 Apr 92 22:55:45 GMT
Expires: Sun, 3 May 1992 00:00:00 GMT
Reply-To: scs@adam.mit.edu
Followup-To: poster
Organization: none, at the moment
Supersedes: <1992Apr1.052837.25269@athena.mit.edu>
X-Archive-Name: C-faq/abridged
X-Last-Modified: February 29, 1992

Archive-name: C-faq/abridged

[Last modified February 29, 1992 by scs.]

This article contains minimal answers to the comp.lang.c frequently-
asked questions list.  Please see the long version (posted on the
first of each month) for more detailed explanations and references.


Section 1. Null Pointers

1.1:	What is this infamous null pointer, anyway?

A:	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.

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

A:	A constant 0 in a pointer context is converted into a null
	pointer at compile time.  A "pointer context" is an
	initialization, assignment, or comparison with one side a
	variable or expression of pointer type, and (in ANSI standard C)
	a function argument which has a prototype in scope declaring a
	certain parameter as being of pointer type.  In other contexts
	(function arguments without prototypes, or in the variable part
	of variadic function calls) a constant 0 with an appropriate
	explicit cast is required.

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

A:	NULL is simply a preprocessor macro, #defined as 0 (or
	(void *)0), which is used (as a stylistic convention, in
	preference to unadorned 0's) to generate null pointers,

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:	The same as any other machine: as 0 (or (void *)0).  (The
	compiler makes the translation, upon seeing a 0, not the
	preprocessor.)

1.5:	If NULL were defined as "(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.  A cast is still required to tell the compiler
	which kind of null pointer is required, since it may be
	different from (char *)0.

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 valid, does not buy much.

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:	The construction "if(p)" works, regardless of the internal
	representation of null pointers, because the compiler
	essentially rewrites it as "if(p != 0)" and goes on to convert 0
	into the correct null pointer.

1.8:	If "NULL" and "0" are equivalent, which should I use?

A:	Either; the distinction is entirely stylistic.

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.  NULL is, and will always be, 0.

1.10:	I'm confused.  NULL is guaranteed to be 0, but the null pointer
	is not?

A:	A "null pointer" is a language concept whose particular internal
	value does not matter.  A null pointer is requested in source
	code with the character "0".  "NULL" is a preprocessor macro,
	which is always #defined as 0 (or (void *)0).

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

A:	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.

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

A:	A simple rule is, "Always use `0' or `NULL' for null pointers,
	and always cast them when they are used as arguments in function
	calls."

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:	What would such a requirement really accomplish?

1.14:	Seriously, have any actual machines really used nonzero null
	pointers?

A:	Machines manufactured by Prime and by Honeywell-Bull, as well as
	Symbolics Lisp Machines, have done so.


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.   Use extern char a[].

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

A:	Not at all.  Arrays are not pointers.  A reference like x[3]
	generates different code depending on whether x is an array or a
	pointer.

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

A:	An lvalue of type array-of-T which appears in an expression
	decays into a pointer to its first element; the type of the
	resultant pointer is pointer-to-T.

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

A:	Since functions can never receive arrays as parameters, any
	parameter declarations which "look like" arrays are treated by
	the compiler as if they were pointers.

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

A:	An array name is "constant" in that it cannot be assigned to,
	but an array is _not_ a pointer.

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

A:	Yes, array subscripting is commutative in C.  The array
	subscripting operation a[e] is defined as being equivalent to
	*((a)+(e)).

2.7:	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.

2.8:	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.

2.9:	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."  See
	the full list for code samples.

2.10:	Can I simulate a non-0-based array with a pointer?

A:	Not if the pointer points outside of the block of memory it is
	intended to access.

2.11:	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 is a conversion operator, and by
	definition it yields an rvalue, which cannot be assigned to, or
	incremented with ++.


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:	The operations implied by the postincrement and postdecrement
	operators ++ and -- are performed at some time after the
	operand's former values are yielded and before the end of the
	expression, but not necessarily immediately after, or before
	other parts of the expression are evaluated.

3.2:	But what about the &&, ||, and comma operators?

A:	There is a special exception for those operators, (as well as
	?: ); left-to-right evaluation is guaranteed.

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.


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, 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.  The Standard has
	also been adopted as ISO/IEC 9899:1990.

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

A:	Copies are available from the American National Standards
	Institute in New York, or from Global Engineering Documents in
	Irvine, CA.  See the unabridged list for addresses.

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

A:	See the full list for details.

4.4:	How do I keep the ANSI "stringizing" preprocessing operator from
	stringizing the macro's name rather than its value?

A:	You must use a two-step #definition to force the macro to be
	expanded as well as stringized.

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

A:	The former is a pointer to a constant character; the latter is a
	constant pointer to a character.

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;".  The problem can be fixed by using
	either new-style (prototype) or old-style syntax consistently.

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

A:	Under ANSI C, #ifdeffed-out text must still consist of "valid
	preprocessing tokens."  This means that there must be no
	unterminated comments or quotes (i.e. no single apostrophes),
	and no newlines inside quotes.

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

A:	No.

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 cannot be forced (by mere
	words in a Standard) to upgrade.

4.10:	What is the difference between memcpy and memmove?

A:	memmove offers guaranteed behavior if the source and destination
	arguments overlap.

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 extensions.


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.  The best all-around
	solution is probably to forget about using a macro.

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:	Try the ANSI token-pasting operator ##.

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

A:	#define Func() do {stmt1; stmt2; ... } while(0)  /* (no trailing ;) */

5.4:	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));


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.

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.

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

A:	Any function which takes a variable number of arguments must be
	able to determine from the arguments themselves how many of them
	there are.

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.


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.

7.2:	How can I shut off the "warning: possible pointer alignment
	problem" message lint gives me for each call to malloc?

A:	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:	See the unabridged list for two commercial products.


Section 8. Memory Allocation

8.1:	Why doesn't the code "char *answer; gets(answer);" work?

A:	The pointer variable "answer" has not been set to point to any
	valid storage.  The simplest way to correct this fragment is to
	use a local array, instead of a pointer.

8.2:	I can't get strcat to work.  I tried "char *s1 = "Hello, ",
	*s2 = "world!", *s3 = strcat(s1, s2);" but I got strange
	results.

A:	Again, the problem is that space for the concatenated result is
	not properly allocated.

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.

8.4:	I have a function that returns a string, 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 (i.e. not locally) allocated.

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

A:	No.  Some early man pages implied otherwise, but the claim is no
	longer valid.

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.

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()?

A:	ANSI C sanctions this usage, but several earlier implementations
	do not support it.

8.9:	Is it safe to use calloc's zero-fill guarantee for pointer and
	floating-point values?

A:	No.

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.  alloca cannot be written
	portably, is difficult to implement on machines without a stack,
	and fails under certain conditions if implemented simply.


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:	These operations are supported by all modern compilers.

9.2:	How does struct passing and returning work?

A:	If you really need to know, see the unabridged list.

9.3:	I have a program which works correctly, but dumps core after it
	finishes.  Why?

A:	Check to see if a structure type declaration just before main is
	missing its trailing semicolon, causing the compiler to believe
	that main returns a structure.  See also question 16.14.

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.

9.5:	I came across some code that declared a structure with the last
	member an array of one element, and then did some tricky
	allocation to make the array act like it had several elements.
	Is this legal and/or portable?

A:	The ANSI C standard allows it, but only implicitly.

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.

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.


Section 10. Declarations

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

A:	If you might need large values, use long.  Otherwise, if space
	is very important, use short.  Otherwise, use int.

10.2:	I can't seem to define a linked list node which contains 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.
	Problems arise if an attempt is made to define (and use) a
	typedef in the midst of such a declaration; avoid this.

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

A:	char *(*(*a[5])())();
	Using a chain of typedefs, or the cdecl program, makes these
	declarations easier.

10.4:	So where can I get cdecl?

A:	Several public-domain versions are available.  See the full list
	for details.

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

A:	It is best to place the definition in some central .c file, with
	an external declaration in a header file.

10.6:	How do I initialize a pointer to a function?

A:	Use something like "extern int func(); int (*fp)() = func;" .

10.7:	I've seen different methods used for calling through pointers to
	functions.

A:	The extra parentheses and explicit * are now officially
	optional, although some older implementations require them.


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.  The choice between #defines and enums is arbitrary
	and not terribly interesting.

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.  (This is _not_ true
	for some library routines such as isalpha.)

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

A:	At the present time, there is little difference.  The ANSI
	standard states that enumerations are compatible with integral
	types.


Section 12. Operating 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.  How to do so is a function of the
	operating system in use.

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.

12.3:	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.  You may be
	able to duplicate the command language interpreter's search path
	logic to locate the executable.

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

A:	In general, it cannot.

12.5:	How can a file be shortened in-place without completely clearing
	or rewriting it?

A:	There are various ways to do this, but there is no truly
	portable solution.


Section 13. Stdio

13.1:	Why does errno contain ENOTTY after a call to printf?

A:	Don't worry about it.  It is only meaningful for a program to
	inspect the contents of errno after an error has occurred.

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.

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.

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

A:	This problem is, in general, insoluble.  It is best to remember
	the names of open files yourself.


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:	You'll have to write a "helper" comparison function which takes
	two generic pointer arguments, converts them to char **, and
	dereferences them, yielding char *'s which can be usefully
	compared.

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:	Any casts must be in the comparison function, which must be
	declared as accepting "generic pointers" (void * or char *).

14.3:	Can someone tell me how to write itoa?

A:	Just use sprintf.

14.4:	How can I get the time of day in a C program?

A:	Just use the time, ctime, and/or localtime functions.

14.5:	How can I convert a struct tm or a string into a time_t?

A:	The ANSI mktime routine converts a struct tm to a time_t.  No
	standard routine exists to parse strings.

14.6:	I need a random number generator.

A:	The standard C library has one: rand().

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.

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:	Try using the higher-order bits.

14.9-13: I'm trying to port this old program.  Why do I get "undefined
	external" errors for some library routines?

A:	Some semistandard routines have been renamed or replaced over
	the years; see the full list for details.


Section 15. Style

15.1:	Is the code "if(!strcmp(s1, s2))" good style?

A:	No.

15.2:	What's the best style for code layout in C?

A:	There is no one "best style," but see the full list for a few
	suggestions.

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

A:	See the unabridged list.


Section 16. Miscellaneous

16.1:	What can I safely assume about the initial values of variables
	which are not explicitly initialized?

A:	Variables with "static" duration start out as 0, as if the
	programmer had initialized them.  Variables with "automatic"
	duration, and dynamically-allocated memory, start out containing
	garbage (with the exception of calloc).

16.2:	How can I write data files which can be read on other machines
	with different data formats?

A:	The best solution is to use text files.

16.3:	I seem to be missing the system header file <sgtty.h>.  Can
	someone send me a copy?

A:	You cannot just pick up a copy of someone else's header file and
	expect it to work, since the definitions within header files are
	frequently system-dependent.  Contact your vendor.

16.4:	How can I call Fortran (BASIC, Pascal, ADA, lisp) functions from C?

A:	The answer is entirely dependent on the machine and the specific
	calling sequences of the various compilers in use.

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

A:	Several public-domain programs are available, namely ptoc, p2c,
	and f2c.  See the full list for details.

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

A:	See the regular postings in the comp.sources.unix and
	comp.sources.misc newsgroups for information.

16.7:	When will the next Obfuscated C Contest be held?  How can I get
	a copy of the previous winning entries?

A:	See the full list, or send email to judges@toad.com .

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

A:	Nested comments would cause more harm than good.  The character
	sequences /* and */ are not special within double-quoted
	strings.

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.

16.10:	How can I make this code more efficient?

A:	Efficiency is not important nearly as often as people tend to
	think it is.  Most of the time, by simply paying attention to
	good algorithm choices, perfectly acceptable results can be
	achieved.

16.11:	Are pointers really faster than arrays?  How much do function
	calls slow things down?

A:	Precise answers to these and many similar questions depend of
	course on the processor and compiler in use.

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, see the full list for a brief explanation, or any
	good programming book for a better one.

16.13:	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, attempt to
	leave out floating point support if it looks like it will not be
	needed.  The programmer must occasionally insert a dummy
	explicit floating-point call to force loading of floating-point
	support.

16.14:	This program crashes before it even runs!

A:	Look for very large, local arrays.
	(See also question 9.3.)

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

A:	Plum Hall, among others, sells one.

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

A:	See the ANSI Standard, or the unabridged list.

16.17:	How do you pronounce "char"?

A:	Like the English words "char," "care," or "car" (your choice).

16.18:	Where can I get extra copies of this list?

A:	For now, just pull it off the net; the unabridged version is
	normally posted 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 should help you find them.


					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.