Subject: comp.lang.c Answers (Abridged) to Frequently Asked Questions (FAQ)

From: [email-masked] (Steve Summit)
Date: Sat, 1 Oct 1994 10:02:03 GMT

Archive-name: C-faq/abridged
Comp-lang-c-archive-name: C-FAQ-list.abridged

[Last modified April 16, 1994 by scs.]

This article contains minimal answers to the comp.lang.c frequently-
asked questions list.  Please see the long version (see question 17.33
for availability) 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 or function.

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:	Such a requirement would accomplish little.

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

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

1.15:	What does a run-time "null pointer assignment" error mean?

A:	It means that you've written through a null pointer.

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.  So for an array a and
	pointer p, you can say "p = a;" and then p[3] and a[3] will
	access the same element.

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

A:	An array is not a "modifiable lvalue."

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

A:	The sizeof operator reports the size of the pointer parameter
	which the function actually receives.

2.7:	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.

2.8:	What is the real difference between arrays and pointers?

A:	Arrays automatically allocate space which is fixed in size and
	location; pointers are dynamic.

2.9:	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 identical to
	*((a)+(e)).

2.10:	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.

2.11:	How do I write functions which accept 2-dimensional arrays when
	the "width" is not known at compile time?

A:	It's not particularly easy.

2.12:	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.

2.13:	What's the difference between array and &array?

A:	Under ANSI/ISO Standard C, &array yields a pointer to the entire
	array.  An unadorned reference to an array yields a pointer to
	the array's first element.

2.14:	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.

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

A:	There is no single perfect method, but see the full list for
	some ideas.

2.16:	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.

2.17:	I passed a pointer to a function which initialized it, but the
	pointer in the caller was unchanged.

A:	The called function probably altered only the passed copy of the
	pointer.

2.18:	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 ++.

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

A:	Not portably.

Section 3. Memory Allocation

3.1:	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.

3.2:	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.

3.3:	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.

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

A:	Make sure that the memory to which the function returns a
	pointer is correctly (i.e. not locally) allocated.

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

A:	Before ANSI/ISO C, these casts were required to silence certain
	warnings.

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

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

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

A:	The malloc/free package remembers the size of each block it
	allocates and returns.

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

A:	Not portably.

3.9:	When I free a dynamically-allocated structure containing
	pointers, do I have to free each subsidiary pointer first?

A:	Yes.

3.10:	Why doesn't my program's memory usage go down when I free
	memory?

A:	Most implementations of malloc/free do not return freed memory
	to the operating system.

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

A:	You shouldn't have to.

3.12:	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.

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

A:	No.

3.14:	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 4. Expressions

4.1:	Why doesn't the code "a[i] = i++;" work?

A:	The variable i is both referenced and modified in the same
	expression.

4.2:	Under my compiler, the code "int i = 7;
	printf("%d\n", i++ * i++);" prints 49.  Regardless of the order
	of evaluation, shouldn't it print 56?

A:	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.

4.3:	How could the code "int i = 2; i = i++;" ever give 4?

A:	Undefined behavior means _anything_ can happen.

4.4:	I just tried some allegedly-undefined code on an ANSI-conforming
	compiler, and got the results I expected.

A:	A compiler may do anything it likes when faced with undefined
	behavior, including doing what you expect.

4.5:	Don't precedence and parentheses dictate order of evaluation?

A:	Operator precedence and explicit parentheses impose only a
	partial ordering on the evaluation of an expression, which does
	not generally include the order of side effects.

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

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

4.7:	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.

4.8:	Why doesn't the code "int a = 1000, b = 1000;
	long int c = a * b;" work?

A:	You must manually cast one of the operands to (long).

Section 5. ANSI C

5.1:	What is the "ANSI C Standard?"

A:	In 1983, the American National Standards Institute (ANSI)
	commissioned a committee to standardize the C language.  Their
	work was ratified as ANS X3.159-1989, and has since been adopted
	as ISO/IEC 9899:1990.

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

A:	ANSI X3.159 has been officially superseded by ISO 9899.  Copies
	are available from ANSI in New York, or from Global Engineering
	Documents in Irvine, CA.  See the unabridged list for addresses.

5.3:	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.

5.4:	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.

5.5:	Why can't I use const values in initializers and array
	dimensions?

A:	The value of a const-qualified object is _not_ a constant
	expression in the full sense of the term.

5.6:	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.

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

A:	The rule which permits slight mismatches in qualified pointer
	assignments is not applied recursively.

5.8:	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;".  "Narrow" types are treated differently
	according to which syntax is used.  This problem can be fixed by
	avoiding narrow types, or by using either new-style (prototype)
	or old-style syntax consistently.

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

A:	Doing so is currently perfectly legal.

5.10:	Why does the declaration "extern f(struct x {int s;} *p);" give
	me a warning message?

A:	A struct declared only within a prototype cannot be compatible
	with other structs declared in the same source file.

5.11:	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.

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

A:	No.

5.13:	Is exit(status) truly equivalent to returning status from main?

A:	Formally, yes.

5.14:	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.

5.15:	What is the difference between memcpy and memmove?

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

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

A:	Perhaps it is a pre-ANSI compiler.

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

A:	Perhaps you don't have ANSI-compatible headers and libraries.

5.18:	Why won't frobozz-cc, which claims to be ANSI compliant, accept
	this code?

A:	Are you sure that the code being rejected doesn't rely on some
	non-Standard extension?

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

A:	The compiler doesn't know the size of the pointed-to objects.

5.20:	Is char a[3] = "abc"; legal?

A:	Yes, in ANSI C.

5.21:	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.

5.22:	What does "#pragma once" mean?

A:	It is an extension implemented by some preprocessors to help
	make header files idempotent.

5.23:	What's the difference between implementation-defined,
	unspecified, and undefined behavior?

A:	If you're writing portable code, ignore the distinctions.
	Otherwise, see the full list.

Section 6. C Preprocessor

6.1:	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.

6.2:	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 ##.

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

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

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

A:	It's a question of style, and thus receives considerable debate.

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

A:	No.

6.6:	How can I use a preprocessor #if expression to detect
	endianness?

A:	You probably can't.

6.7:	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:	Consider writing your own little special-purpose preprocessing
	tool, instead.

6.8:	How can I preprocess some code to remove selected conditional
	compilations, without preprocessing everything?

A:	Look for a program called unifdef, rmifdef, or scpp.

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

A:	If the compiler documentation is unhelpful, try extracting
	printable strings from the compiler or preprocessor executable.

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

A:	Here is one popular trick.  Note that the parentheses around
	printf's argument list are in the macro call, not the
	definition.

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

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

Section 7. Variable-Length Argument Lists

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

A:	Use the <stdarg.h> (or older <varargs.h>) header.

7.2:	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.

7.3:	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.

7.4:	I can't get the va_arg macro to pull in an argument of type
	pointer-to-function.

A:	Use a typedef.

7.5:	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.

7.6:	How can I call a function with an argument list built up at run
	time?

A:	You can't.

Section 8. Boolean Expressions and Variables

8.1:	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.

8.2:	What if a built-in boolean or relational operator "returns"
	something other than 1?

A:	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.)

Section 9. Structs, Enums, and Unions

9.1:	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.

9.2:	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.

9.3:	How does struct passing and returning work?

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

9.4:	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 17.21.

9.5:	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.

9.6:	How can I read/write structs from/to data files?

A:	It is relatively straightforward to use fread and fwrite.

9.7:	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:	An ANSI Interpretation Ruling has deemed it to be not strictly
	conforming.

9.8:	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.

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

A:	Build a table of names and offsets, using the offsetof() macro.

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

A:	The alignment of arrays of structures must be preserved.

9.11:	How can I turn off structure padding?

A:	There is no standard method.

9.12:	Can I initialize unions?

A:	ANSI Standard C allows an initializer for the first member.

9.13:	Can I pass constant values to routines which accept struct
	arguments?

A:	No.  C has no way of generating anonymous struct values.

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

A:	There are arguments in favor of long int and long long int,
	among other options.

10.3:	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.

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

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

10.5:	How can I declare a function that returns a pointer to a
	function of its own type?

A:	You can't do it directly.  Use a cast, or wrap a struct around
	the pointer and return that.

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

A:	Non-int functions must be declared before they are called.

10.7:	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.

10.8:	What does extern mean in a function declaration?

A:	Nothing, really.

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

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

10.10:	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.

10.11:	What's the auto keyword good for?

A:	Nothing.

Section 11. Stdio

11.1:	What's wrong with the code "char c; while((c = getchar()) != EOF)..." ?

A:	The variable to hold getchar's return value must be an int.

11.2:	How can I print a '%' character with printf?

A:	%% .

11.3:	Why doesn't the code scanf("%d", i); work?

A:	scanf needs pointers to the variables it is to fill in.

11.4:	Why doesn't the code double d; scanf("%f", &d); work?

A:	Unlike printf, scanf uses %lf for double, and %f for float.

11.5:	Why won't the code "while(!feof(infp)) {
	fgets(buf, MAXLINE, infp); fputs(buf, outfp); }" work?

A:	EOF is only indicated _after_ an input routine has reached end-
	of-file.

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

A:	It cannot be prevented from overflowing the input buffer.

11.7:	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.

11.8:	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.

11.9:	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.

11.10:	I'm trying to update a file in place, by using fopen mode "r+",
	but it's not working.

A:	Be sure to call fseek between reading and writing.

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

A:	See question 16.1.

11.12:	Will fflush(stdin) flush unread characters from the standard
	input stream?

A:	No.

11.13:	How can I redirect stdin or stdout from within a program?

A:	Use freopen.

11.14:	Once I've used freopen, how can I get the original stdout back?

A:	It's not easy.  Try avoiding freopen.

11.15:	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 files yourself when you open them.

Section 12. Library Subroutines

12.1:	Why does strncpy not always write a '\0'?

A:	For mildly-interesting historical reasons.

12.2:	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.

12.3:	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" (const void * or
	char *).

12.4:	How can I convert numbers to strings?

A:	Just use sprintf.

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

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

12.6:	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.

12.7:	How can I perform calendar manipulations?

A:	The ANSI/ISO Standard C mktime and difftime functions provide
	some support for both problems.

12.8:	I need a random number generator.

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

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

A:	One method is something like

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

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

A:	You can call srand() to seed the pseudo-random number generator
	with a more random initial value.

12.11:	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.

12.12:	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.

12.13:	I get errors due to library routines being undefined even though
	I #include the right header files.

A:	You may have to explicitly ask for the correct libraries to be
	searched.

12.14:	I'm still getting errors due to library routines being
	undefined, even though I'm requesting the right libraries.

A:	Library search order is significant; usually, you must search
	the libraries last.

12.15:	I need some code to do regular expression matching.

A:	regexp libraries abound; see the full list for details.

12.16:	How can I split up a string into whitespace-separated arguments?

A:	Try strtok.

Section 13. Lint

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

A:	Try running lint first.

13.2:	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.

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

A:	See the unabridged list for two commercial products.

Section 14. Style

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

A:	Not particularly.

14.2:	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.

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

A:	See the unabridged list.

Section 15. Floating Point

15.1:	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.

15.2:	I keep getting "undefined: _sin" compilation errors.

A:	Make sure you're linking with the correct math library.

15.3:	Where is C's exponentiation operator?

A:	Try using the pow() function.

15.4:	How do I round numbers?

A:	The simplest way is with code like (int)(x + 0.5) .

15.5:	How do I test for IEEE NaN and other special values?

A:	There is not yet a portable way, but see the full list for
	ideas.

15.6:	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 an extra,
	explicit call to a floating-point library routine to force
	loading of floating-point support.

Section 16. System Dependencies

16.1:	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.

16.2:	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.

16.3:	How can I clear the screen?

A:	Such things depend on the output device you're using.

16.4:	How do I read the mouse?

A:	What system are you using?

16.5:	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.

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

A:	In general, it cannot.

16.7:	How can I check whether a file exists?

A:	You can try the access() or stat() routines.  Otherwise, the
	only guaranteed and portable way is to try opening the file.

16.8:	How can I find out the size of a file, prior to reading it in?

A:	You might be able to get an estimate using stat() or
	fseek/ftell.

16.9:	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.

16.10:	How can I implement a delay, or time a user's response, with
	sub-second resolution?

A:	Unfortunately, there is no portable way.

16.11:	How can I read in an object file and jump to routines in it?

A:	You want a dynamic linker and/or loader.

16.12:	How can I invoke an operating system command from within a
	program?

A:	Use system().

16.13:	How can I invoke an operating system command and trap its
	output?

A:	Unix and some other systems provide a popen() routine.

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

A:	See if you can use the opendir() and readdir() routines.

16.15:	How can I do serial ("comm") port I/O?

A:	It's system-dependent.

Section 17. Miscellaneous

17.1:	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).

17.2:	What's wrong with

		f() { char a[] = "Hello, world!"; }

A:	Perhaps you have a pre-ANSI compiler.

17.3:	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.

17.4:	How can I insert or delete a line in the middle of a file?

A:	Short of rewriting the file, you probably can't.

17.5:	How can I return several values from a function?

A:	Either pass pointers to locations which the function can fill
	in, or have the function return a structure containing the
	desired values.

17.6:	How can I call a function, given its name as a string?

A:	The most straightforward thing to do is maintain a
	correspondence table of names and function pointers.

17.7:	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.

17.8:	How can I call FORTRAN (C++, 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.

17.9:	Does anyone know of a program for converting Pascal or FORTRAN
	to C?

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

17.10:	Can I use a C++ compiler to compile C code?

A:	Not necessarily; C++ is not a strict superset of C.

17.11:	I'm looking for C development tools (cross-reference generators,
	code beautifiers, etc.).

A:	See the full list for a few names.

17.12:	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.

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

A:	See the full list, or send e-mail to [email-masked] .

17.14:	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.

17.15:	How can I get the ASCII value corresponding to a character?

A:	In C, if you have the character, you have its value.

17.16:	How can I implement sets and/or arrays of bits?

A:	Use arrays of char or int, with a few macros to access the right
	bit at the right index.

17.17:	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.

17.18:	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.

17.19:	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.

17.20:	Why does the code "char *p = "Hello, world!";
	p[0] = tolower(p[0]);" crash?

A:	String literals are not modifiable, except (in effect) when they
	are used as array initializers.

17.21:	This program crashes before it even runs!

A:	Look for very large, local arrays.
	(See also question 9.4.)

17.22:	What does "Segmentation violation" mean?

A:	It generally means that your program tried to access memory it
	shouldn't have.

17.23:	My program is crashing, apparently somewhere down inside malloc.

A:	Make sure you aren't using more memory than you malloc'ed,
	especially for strings (which need strlen() + 1 bytes).

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

A:	See the full list for several sources.

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

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

17.26:	I need code to parse and evaluate expressions.

A:	Two available packages are mentioned in the full list.

17.27:	I need to compare two strings for close, but not necessarily
	exact, equality.

A:	The traditional routine for doing this sort of thing involves
	the "soundex" algorithm.

17.28:	How can I find the day of the week given the date?

A:	Use Zeller's congruence.

17.29:	Will 2000 be a leap year?

A:	Yes.

17.30:	How do you pronounce "char"?

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

17.31:	What's a good book for learning C?

A:	There are far too many to list here; the full list contains a
	few pointers.

17.32:	Are there any C tutorials on the net?

A:	There are at least two of them.

17.33:	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 Expires:
	line which should keep it around all month.  It can also be
	found in the newsgroups comp.answers and news.answers .  Several
	sites archive news.answers postings and other FAQ lists,
	including this one; two sites are rtfm.mit.edu (directory
	pub/usenet), and ftp.uu.net (directory usenet).  The archie
	server should help you find others.

					Steve Summit
					[email-masked]

This article is Copyright 1988, 1990-1994 by Steve Summit.
It may be freely redistributed so long as the author's name, and this
notice, are retained.