From: scs@eskimo.com (Steve Summit)
Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List)
This article is always being improved. Your input is welcomed. Send your comments to scs@eskimo.com .

The questions answered here are divided into several categories:

         1. Null Pointers
         2. Arrays and Pointers
         3. Memory Allocation
         4. Expressions
         5. ANSI C
         6. C Preprocessor
         7. Variable-Length Argument Lists
         8. Boolean Expressions and Variables
         9. Structs, Enums, and Unions
        10. Declarations
        11. Stdio
        12. Library Subroutines
        13. Lint
        14. Style
        15. Floating Point
        16. System Dependencies
        17. 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 or function. 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
        3.1, 3.13, and 17.1.

        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, to force
        the 0 to be in a pointer context:

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).

References: ANSI Rationale Sec. 4.1.5 p. 74.

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. See also question 8.1.

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 17.5.

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

A:      Before ANSI/ISO Standard C introduced the void * generic pointer
        type, these casts were typically required to silence warnings
        about assignment between incompatible pointer types. (Under
        ANSI/ISO Standard C, these casts are not required.)

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

A:      No. Some early documentation for malloc stated that the
        contents of freed memory 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.

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, so it is not necessary to remind it of
        the size when freeing.

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

A: Not portably.

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

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

3.10:   I have a program which mallocs but then frees a lot of memory,
        but memory usage (as reported by ps) doesn't seem to go back
        down.

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

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

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

References: ANSI Sec. 4.10.3.2 .

3.12: 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 .

3.13:   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 .

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. 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.

References: ANSI Rationale Sec. 4.10.3 p. 102.

Section 4. Expressions

4.1: Why doesn't this code:

a[i] = i++;

work?

A:      The subexpression i++ causes a side effect -- it modifies i's
        value -- which leads to undefined behavior if i is also
        referenced elsewhere in the same expression. (Note that
        although the language in K&R suggests that the behavior of this
        expression is unspecified, the ANSI/ISO C Standard makes the
        stronger statement that it is undefined -- see question 5.23.)

References: ANSI Sec. 3.3 p. 39.

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:      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 (see question 5.23). 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.)

4.3: I've experimented with the code

int i = 2;
i = i++;

        on several compilers. Some gave i the value 2, some gave 3, but
        one gave 4. I know the behavior is undefined, but how could it
        give 4?

A: Undefined behavior means _anything_ can happen. See question
5.23.

4.4:    People keep saying the behavior is undefined, but I just tried
        it on an ANSI-conforming compiler, and got the results I
        expected.

A:      A compiler may do anything it likes when faced with undefined
        behavior (and, within limits, with implementation-defined and
        unspecified behavior), including doing what you expect. It's
        unwise to depend on it, though. See also question 5.18.

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

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

f() + g() * h()

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

4.6: 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
        the ?: operator); 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.

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:      Under C's integral promotion rules, the multiplication is
        carried out using int arithmetic, and the result may overflow
        and/or be 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;

Note that the code (long int)(a * b) would _not_ have the
desired effect.

Section 5. ANSI C

5.1: What is the "ANSI C Standard?"

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

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 in the United States from

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

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

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

                ISO Sales
                Case Postale 56
                CH-1211 Geneve 20
                Switzerland

        The cost is $130.00 from ANSI or $162.50 from Global. Copies of
        the original X3.159 (including the Rationale) are still
        available at $205.00 from ANSI or $200.50 from Global. Note
        that ANSI derives revenues to support its operations from the
        sale of printed standards, so electronic copies are _not_
        available.

        The mistitled _Annotated ANSI C Standard_, with annotations by
        Herbert Schildt, contains the full text of ISO 9899; it is
        published by Osborne/McGraw-Hill, ISBN 0-07-881952-0, and sells
        in the U.S. for approximately $40. (It has been suggested that
        the price differential between this work and the official
        standard reflects the value of the annotations.)

        The text of the Rationale (not the full Standard) is now
        available for anonymous ftp from ftp.uu.net (see question 17.12)
        in directory doc/standards/ansi/X3.159-1989 . The Rationale has
        also been printed by Silicon Press, ISBN 0-929306-07-4.

5.3:    Does anyone have a tool for converting old-style C programs to
        ANSI C, or vice versa, 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
        were once patches to the FSF GNU C compiler, gcc, but are now
        part of the main gcc distribution; look in pub/gnu at
        prep.ai.mit.edu (18.71.0.38), or at several other FSF archive
        sites.

        The unproto program (/pub/unix/unproto5.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.

The GNU GhostScript package comes with a little program called
ansi2knr.

        Several prototype generators exist, many as modifications to
        lint. Version 3 of CPROTO was posted to comp.sources.misc in
        March, 1992. There is another program called "cextract." See
        also question 17.12.

        Finally, are you sure you really need to convert lots of old
        code to ANSI C? The old-style function syntax is still
        acceptable.

5.4:    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.

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

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

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

References: ANSI Sec. 3.4 .

5.6: 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
        10.4.)

References: ANSI Sec. 3.5.4.1 .

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

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

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

References: ANSI Sec. 3.1.2.6 p. 26, Sec. 3.3.16.1 p. 54,
Sec. 3.5.3 p. 65.

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;". It is usually safe to mix the two
        styles (see question 5.9), but not in this case. Old C (and
        ANSI C, in the absence of prototypes, and in variable-length
        argument lists) "widens" certain arguments when they are passed
        to functions. floats are promoted to double, and characters and
        short integers are promoted to ints. (For old-style function
        definitions, the values are automatically converted back to the
        corresponding narrower types within the body of the called
        function, if they are declared that way there.)

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

int func(float x) { ... }

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

extern int func(double);

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

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

References: ANSI Sec. 3.3.2.2 .

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

A:      Doing so is perfectly legal, as long as you're careful (see
        especially question 5.8). Note however that old-style syntax is
        marked as obsolescent, and support for it may be removed some
        day.

References: ANSI Secs. 3.7.1, 3.9.5 .

5.10: Why does the declaration

extern f(struct x {int s;} *p);

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

A:      In a quirk of C's normal block scoping rules, a struct declared
        only within a prototype cannot be compatible with other structs
        declared in the same source file, nor can the struct tag be used
        later as you'd expect (it goes out of scope at the end of the
        prototype).

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

struct x;

        , which will reserve a place at file scope for struct x's
        definition, which will be completed by the struct declaration
        within the prototype.

References: ANSI Sec. 3.1.2.1 p. 21, Sec. 3.1.2.6 p. 26,
Sec. 3.5.2.3 p. 63.

5.11: 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 17.14, and
        6.7.)

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

5.12:   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).

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

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

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

A:      Formally, yes, although discrepancies arise under a few older,
        nonconforming systems, or if data local to main() might be needed
        during cleanup (due perhaps to a setbuf or atexit call), or if
        main() is called recursively.

References: ANSI Sec. 2.1.2.2.3 p. 8.

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 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 5.1), 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.

5.15: 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 .

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, unable to accept function
prototypes and the like. See also questions 5.17 and 17.2.

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

A:      It's not unusual to have a compiler available which accepts ANSI
        syntax, but not to have ANSI-compatible header files or run-time
        libraries installed. See also questions 5.16 and 17.2.

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

A:      Most compilers support a few non-Standard extensions, gcc more
        so than most. Are you sure that the code being rejected doesn't
        rely on such an extension? It is usually a bad idea to perform
        experiments with a particular compiler to determine properties
        of a language; the applicable standard may permit variations, or
        the compiler may be wrong. See also question 4.4.

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.
        Before performing arithmetic, cast the pointer either to char *
        or to the type you're trying to manipulate (but see question
        2.18).

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

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

References: ANSI Sec. 3.5.7 pp. 72-3.

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 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 .

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

A:      It is an extension implemented by some preprocessors to help
        make header files idempotent; it is essentially equivalent to
        the #ifndef trick mentioned in question 6.4.

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

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

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

References: ANSI Sec. 1.6, especially the Rationale.

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. If the values are
        integers, a well-known trick using exclusive-OR could perhaps be
        used, but it will not work for floating-point values or
        pointers, or if the two values are the same variable (and the
        "obvious" supercompressed implementation for integral types
        a^=b^=a^=b is in fact illegal due to multiple side-effects; see
        questions 4.1 and 4.2). If the macro is intended to be used on
        values of arbitrary type (the usual goal), it cannot use a
        temporary, since it does not know what type of temporary it
        needs, and standard C does not provide a typeof operator.

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

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:      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 5.4.)

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

6.3: 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 6.10 below.
        This technique also allows a value to be "returned.")

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

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

A:      It's a question of style, and thus receives considerable debate.
        Many people believe that "nested #include files" are to be
        avoided: the prestigious Indian Hill Style Guide (see question
        14.3) 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 HEADERUSED
                #define HEADERUSED
                ...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. See also section 14.

6.5: Does the sizeof operator 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 ANSI's <limits.h>, if applicable, or
        a "configure" script, instead. (Better yet, try to write code
        which is inherently insensitive to type sizes.)

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

6.6: 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.

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:      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.

If you are trying to preprocess something other than C, consider
using a general-purpose preprocessor (such as m4).

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

A: There are programs floating around called unifdef, rmifdef, and
scpp which do exactly this. (See question 17.12.)

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

A:      There's no standard way, although it is a frequent need. If the
        compiler documentation is unhelpful, the most expedient way is
        probably to extract printable strings from the compiler or
        preprocessor executable with something like the Unix strings(1)
        utility. Beware that many traditional system-selective
        pre#defined identifiers (e.g. "unix") are non-Standard (because
        they clash with the user's namespace) and are being removed or
        renamed.

6.10: 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. Other solutions are to use
        different macros (DEBUG1, DEBUG2, etc.) depending on the number
        of arguments, or to play games with commas:

                #define DEBUG(args) (printf("DEBUG: "), printf(args))
                #define _ ,
                DEBUG("i = %d" _ i)

        It is often better to use a bona-fide function, which can take a
        variable number of arguments in a well-defined way. See
        questions 7.1 and 7.2.

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

                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 <stdlib.h>, add "extern
        char *malloc();", and use int instead of size_t. You may also
        have to delete the (void) casts, and use the older varargs
        package instead of stdarg. See the next question for hints.

        Remember that in variable-length argument lists, function
        prototypes do not supply parameter type information; therefore,
        default argument promotions apply (see question 5.8), and null
        pointer arguments must be typed explicitly (see question 1.2).

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 .

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.

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 .

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

A:      This information is not available to a portable program. Some
        old systems provided a nonstandard nargs() function, but its use
        was always questionable, since it typically returned the number
        of words passed, not the number of arguments. (Structures and
        floating point values 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 7.1 above).

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

A:      The type-rewriting games which the va_arg macro typically plays
        are stymied by overly-complicated types such as pointer-to-
        function. If you use a typedef for the function pointer type,
        however, all will be well.

References: ANSI Sec. 4.8.1.2 p. 124.

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. 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.)

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

A:      There is no guaranteed or portable way to do this. If you're
        curious, ask this list's editor, who has a few wacky ideas you
        could try... (See also question 16.11.)

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. (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 (see also question 9.1). 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 question 8.2 below; see also
question 1.6).

8.2:    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 function parameter, or
        as the return value from a Boolean function, but never in a
        comparison.

        The preprocessor macros TRUE and FALSE are used for code
        readability, not because the underlying values might ever
        change. (See also questions 1.7 and 1.9.)

        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.

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. 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.)

        Some advantages of enums are that the numeric values are
        automatically assigned, that a debugger may be able to display
        the symbolic values when enum variables are examined, and that
        they obey block scope. (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 .

9.2: 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 .

9.3: 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.

References: ANSI Sec. 2.2.3 p. 13.

9.4: 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 17.21.

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

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. 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; see questions
        9.10 and 9.11). 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.

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

A: It is relatively straightforward to write a struct out using
fwrite:

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

        and a corresponding fread invocation can read it back in.
        However, data files so written will _not_ be very portable (see
        questions 9.11 and 17.3). Note also that on many systems you
        must use the "b" flag when fopening the files.

9.7: 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 C implementation." An ANSI
        Interpretation Ruling has deemed it (more precisely, access
        beyond the declared size of the name field) to be not strictly
        conforming, although a thorough treatment of the arguments
        surrounding the legality of the technique is beyond the scope of
        this list. It seems, however, to be portable to all known
        implementations. (Compilers which check array bounds carefully
        might issue warnings.)

        To be on the safe side, it may be preferable to declare the
        variable-size element very large, rather than very small; in the
        case of the above example:

                ...
                char name[MAXSIZE];
                ...

        where MAXSIZE is larger than any name which will be stored.
        (The trick so modified is said to be in conformance with the
        Standard.)

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

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; 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.

9.9: 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;

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:      Structures may have this padding (as well as internal padding;
        see also question 9.5), so that alignment properties will be
        preserved when an array of contiguous structures is allocated.

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

A:      Your compiler may provide an extension to give you this control
        (perhaps a #pragma), but there is no standard method. See also
        question 17.3.

9.12: Can I initialize unions?

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

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

A: C has no way of generating anonymous struct values. You will
have to use a temporary struct variable.

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 of mixing signed and unsigned in
        expressions.) 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.

        If for some reason you need to declare something with an _exact_
        size (usually the only good reason for doing so is when
        attempting to conform to some externally-imposed storage layout,
        but see question 17.3), be sure to encapsulate the choice behind
        an appropriate typedef.

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

A:      Some vendors of C products for 64-bit machines support 64-bit
        long ints. Others fear that too much existing code depends on
        sizeof(int) == sizeof(long) == 32 bits, and introduce a new 64-
        bit long long (or __longlong) type instead.

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

10.3: 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 corrected version would be

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

typedef struct node *NODEPTR;

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

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

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

A: This question can be answered in at least three ways:

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

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[N];            /* array of... */

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

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

            cdecl can also explain complicated declarations, help with
            casts, and indicate which set of parentheses the arguments
            go in (for complicated function definitions, like the
            above). Versions of cdecl are in volume 14 of
            comp.sources.unix (see question 17.12) 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.

10.5:   I'm building a state machine with a bunch of functions, one for
        each state. I want to implement state transitions by having
        each function return a pointer to the next state function. I
        find a limitation in C's declaration mechanism: there's no way
        to declare these functions as returning a pointer to a function
        returning a pointer to a function returning a pointer to a
        function...

A:      You can't do it directly. Either have the function return a
        generic function pointer type, and apply a cast before calling
        through it; or have it return a structure containing only a
        pointer to a function returning that structure.

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

A:      Functions which are called without a declaration in scope (or
        before they are declared) are assumed to be declared as
        returning int, leading to discrepancies if the function is later
        declared otherwise. Non-int functions must be declared before
        they are called.

References: K&R I Sec. 4.2 pp. 70; K&R II Sec. 4.2 p. 72; ANSI
Sec. 3.3.2.2 .

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

A:      First, though there can be many _declarations_ (and in many
        translation units) of a single "global" (strictly speaking,
        "external") variable (or function), there must be exactly one
        _definition_. (The definition is the declaration that actually
        allocates space, and provides an initialization value, if any.)
        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; H&S Sec. 4.8 pp. 79-80; CT&P Sec. 4.2 pp. 54-56.

10.8: What does extern mean in a function declaration?

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

                extern int f();
        and
                int f();

References: ANSI Sec. 3.1.2.2 .

10.9: 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 "(").

10.10: 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, func(), (*fp)() = func;
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.

10.11: What's the auto keyword good for?

A: Nothing; it's obsolete.

Section 11. Stdio

11.1: What's wrong with this code:

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

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

References: CT&P Sec. 5.1 p. 70.

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

A: Simply double the percent sign: %% .

References: K&R I Sec. 7.3 p. 147; K&R II Sec. 7.2 p. 154; ANSI
Sec. 4.9.6.1 .

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

A: scanf needs pointers to the variables it is to fill in; you must
call scanf("%d", &i);

11.4: Why doesn't this code:

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

work?

A:      scanf uses %lf for values of type double, and %f for float.
        (Note the discrepancy with printf, which uses %f for both double
        and float, due to C's default argument promotion rules.)

11.5: Why won't the code

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

work?

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

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

A:      It cannot be told the size of the buffer it's to read into, so
        it cannot be prevented from overflowing that buffer. See
        question 3.1 for a code fragment illustrating the replacement of
        gets() with fgets().

11.7: 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.

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

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. 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 11.7.)

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. In particular, "\n" in
        a format string does _not_ mean to expect a newline, but rather
        to read and discard characters as long as each is a whitespace
        character.

        A related problem is that unexpected non-numeric input can cause
        scanf to "jam." Because of these problems, it is usually better
        to use fgets to read a whole line, and then use sscanf or other
        string functions to pick apart the line buffer. If you do use
        sscanf, don't forget to check the return value to make sure that
        the expected number of items were found.

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

A:      Be sure to call fseek before you write, both to seek back to the
        beginning of the string you're trying to overwrite, and because
        an fseek or fflush is always required between reading and
        writing in the read/write "+" modes. Also, remember that you
        can only overwrite characters with the same number of
        replacement characters; see also question 17.4.

References: ANSI Sec. 4.9.5.3 p. 131.

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

A: See question 16.1.

11.12: 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.

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

A: Use freopen.

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

A:      If you need to switch back and forth, the best all-around
        solution is not to use freopen in the first place. Try using
        your own explicit output (or input) stream variable, which you
        can reassign at will, while leaving the original stdout (or
        stdin) undisturbed.

11.15: 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 files yourself when
        you open them (perhaps with a wrapper function around fopen).

Section 12. Library Subroutines

12.1: Why does strncpy not always place a '\0' termination in the
destination string?

A:      strncpy was first designed to handle a now-obsolete data
        structure, the fixed-length, not-necessarily-\0-terminated
        "string." strncpy is admittedly a bit cumbersome to use in
        other contexts, since you must often append a '\0' to the
        destination string by hand.

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:      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," const 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;          /* const void * for ANSI C */
                {
                        return strcmp(*(char **)p1, *(char **)p2);
                }

Beware of the discussion in K&R II Sec. 5.11 pp. 119-20, which
is not discussing Standard library qsort.

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 *) as discussed in question 12.2 above. The code might
        look like

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

        (If, on the other hand, you're sorting pointers to structures,
        you'll need indirection, as in question 12.2:
        sp1 = *(struct mystruct **)p1 .)

12.4: 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 3.1 and 3.2. 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.

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

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

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

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

References: ANSI Sec. 4.12 .

12.6:   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. Some
        systems provide a strptime function; another popular routine is
        partime (widely distributed with the RCS package), but these 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 .

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

A:      The ANSI/ISO Standard C mktime and difftime functions provide
        some support for both problems. mktime() accepts non-normalized
        dates, so it is straightforward to take a filled-in struct tm,
        add or subtract from the tm_mday field, and call mktime() to
        normalize the year, month, and day fields (and convert to a
        time_t value). difftime() computes the difference, in seconds,
        between two time_t values; mktime() can be used to compute
        time_t values for two dates to be subtracted. (Note, however,
        that these solutions only work for dates in the range which can
        be represented as time_t's, and that not all days are 86400
        seconds long.) See also questions 12.6 and 17.28.

References: K&R II Sec. B10 p. 256; H&S Secs. 20.4, 20.5
pp. 361-362; ANSI Secs. 4.12.2.2, 4.12.2.3 .

12.8: 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; Knuth Vol. 2 Chap. 3
pp. 1-177.

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

A: The obvious way,

rand() % N

        (where N is of course the range) is poor, because the low-order
        bits of many random number generators are distressingly non-
        random. (See question 12.11.) A better method is something
        like

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

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

rand() / (RAND_MAX / N + 1)

        Both methods obviously require knowing RAND_MAX (which ANSI
        defines in <stdlib.h>), and assume that N is much less than
        RAND_MAX.

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. Popular seed values are the
        time of day, or the elapsed time before the user presses a key
        (although keypress times are hard to determine portably; see
        question 16.10).

References: ANSI Sec. 4.10.2.2 p. 154.

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:      Poor pseudorandom number generators (such as the ones
        unfortunately supplied with some systems) are not very random in
        the low-order bits. Try using the higher-order bits. See
        question 12.9.

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

        index?                              use strchr.
        rindex?                             use strrchr.
        bcopy?                              use memmove, after
                                            interchanging the first and
                                            second arguments (see also
                                            question 5.15).
        bcmp?                               use memcmp.
        bzero?                              use memset, with a second
                                            argument of 0.

12.13: I keep getting errors due to library routines being undefined,
but I'm #including all the right header files.

A:      In some cases (especially if the routines are nonstandard) you
        may have to explicitly ask for the correct libraries to be
        searched when you link the program. See also question 15.2.

12.14: I'm still getting errors due to library routines being
undefined, even though I'm using -l to request the libraries
while linking.

A:      Many linkers make one pass over the list of object files and
        libraries you specify, and extract from libraries only those
        modules which satisfy references which have so far come up as
        undefined. Therefore, the order in which libraries are listed
        with respect to object files (and each other) is significant;
        usually, you want to search the libraries last. (For example,
        under Unix, put any -l switches towards the end of the command
        line.)

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

A:      Look for the regexp library (supplied with many Unix systems),
        or get Henry Spencer's regexp package from cs.toronto.edu in
        pub/regexp.shar.Z (see also question 17.12).

12.16: How can I split up a command line into whitespace-separated
arguments, like main's argc and argv?

A:      Most systems have a routine called strtok, although it can be
        tricky to use and it may not do everything you want it to (e.g.,
        quoting).

References: ANSI Sec. 4.11.5.8; K&R II Sec. B3 p. 250; H&S
Sec. 15.7; PCS p. 178.

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 (perhaps with the -a, -c, -h, -p and/or
        other options). Many C compilers are really only half-
        compilers, electing not to diagnose numerous source code
        difficulties which would not actively preclude code generation.

13.2: 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.

13.3: 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
        or from System V resellers.

In the absence of lint, many modern compilers attempt to
diagnose almost as many problems as a good lint does.

Section 14. Style

14.1: Here's a neat trick:

if(!strcmp(s1, s2))

Is this good style?

A:      It is not particularly good style, although it is a popular
        idiom. The test succeeds if the two strings are equal, but 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.

14.2: 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.

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

14.3: 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

ftp.cs.umd.edu pub/style-guide

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, recall that most digital
        computers use floating-point formats which provide a close but
        by no means exact simulation of real number arithmetic.
        Underflow, cumulative precision loss, and other anomalies are
        often troublesome.

        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.

15.2: I'm trying to do some simple trig, and I am #including <math.h>,
but I keep getting "undefined: _sin" compilation errors.

A:      Make sure you're linking with the correct math library. For
        instance, under Unix, you usually need to use the -lm option,
        and at the _end_ of the command line, when compiling/linking.
        See also question 12.14.

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

A:      Because few processors have an exponentiation instruction.
        Instead, you can #include <math.h> and use the pow() function,
        although explicit multiplication is often better for small
        positive integral exponents.

References: ANSI Sec. 4.5.5.1 .

15.4: How do I round numbers?

A: The simplest and most straightforward way is with code like

(int)(x + 0.5)

This won't work properly for negative numbers, though.

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

A:      Many systems with high-quality IEEE floating-point
        implementations provide facilities (e.g. an isnan() macro) to
        deal with these values cleanly, and the Numerical C Extensions
        Group (NCEG) is working to formally standardize such facilities.
        A crude but usually effective test for NaN is exemplified by

#define isnan(x) ((x) != (x))

although non-IEEE-aware compilers may optimize the test away.

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 (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 insufficient, and the programmer must
        sometimes 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. (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 has not been standardized by the C
        language. Some versions of curses have a cbreak() function
        which does what you want. If you're specifically trying to read
        a short password without echo, you might try getpass(). 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,
        or curses, or issue low-level $QIO's with the IO$_READVBLK (and
        perhaps IO$M_NOECHO) function codes to ask for one character at
        a time. 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.

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. 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().

16.3: How can I clear the screen? How can I print things in inverse
video?

A:      Such things depend on the terminal type (or display) you're
        using. You will have to use a library such as termcap or
        curses, or some system-specific routines, to perform these
        functions.

16.4: How do I read the mouse?

A:      Consult your system documentation, or ask on an appropriate
        system-specific newsgroup (but check its FAQ list first). Mouse
        handling is completely different under the X window system, MS-
        DOS, Macintosh, and probably every other system.

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, 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.

16.6: 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.

16.7: How can I check whether a file exists? I want to query the user
before overwriting existing files.

A:      On Unix-like systems, you can try the access() routine, although
        it's got a few problems. (It isn't atomic with respect to the
        following action, and can have anomalies if used in setuid
        programs.) Another option (perhaps preferable) is to call
        stat() on the file. Otherwise, the only guaranteed and portable
        way to test for file existence is to try opening the file (which
        doesn't help if you're trying to avoid overwriting an existing
        file, unless you've got something like the BSD Unix O_EXCL open
        option available).

16.8: 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 call
        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 otherwise a quasi-accurate answer only for ANSI C "binary"
        files). Some systems provide routines called filesize or
        filelength.

        Are you sure you have to determine the file's size in advance?
        Since the most accurate way of determining the size of a file as
        a C program will see it is to open the file and read it, perhaps
        you can rearrange the code to learn the size as it reads.

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

A:      BSD systems provide ftruncate(), several others supply chsize(),
        and a few may provide a (possibly undocumented) fcntl option
        F_FREESP. Under MS-DOS, you can sometimes use write(fd, "", 0).
        However, there is no 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. V7 Unix, and derived
        systems, provided a fairly useful ftime() routine with
        resolution up to a millisecond, but it has disappeared from
        System V and Posix. Other routines you might look for on your
        system include nap(), setitimer(), msleep(), usleep(), clock(),
        and gettimeofday(). The select() and poll() calls (if
        available) can be pressed into service to implement simple
        delays. On MS-DOS machines, it is possible to reprogram the
        system timer and timer interrupts.

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. It is possible to
        malloc some space and read in object files, but you have to know
        an awful lot about object file formats, relocation, etc. Under
        BSD Unix, you could use system() and ld -A to do the linking for
        you. Many (most?) versions of SunOS and System V have the -ldl
        library which allows object files to be dynamically loaded.
        There is also a GNU package called "dld". See also question
        7.6.

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

A: Use system().

References: K&R II Sec. B6 p. 253; ANSI Sec. 4.10.4.5; H&S
Sec. 21.2; PCS Sec. 11 p. 179;

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

A:      Unix and some other systems provide a popen() routine, which
        sets up a stdio stream on a pipe connected to the process
        running a command, so that the output can be read (or the input
        supplied). Alternately, invoke the command simply (see question
        16.12) in such a way that it writes its output to a file, then
        open and read that file.

References: PCS Sec. 11 p. 169 .

16.14: 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.)

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

A:      It's system-dependent. Under Unix, you typically open, read,
        and write a device in /dev, and use the facilities of the
        terminal driver to adjust its characteristics. Under MS-DOS,
        you can either use some primitive BIOS interrupts, or (if you
        require decent performance) one of any number of interrupt-
        driven serial I/O packages.

Section 17. Miscellaneous

17.1:   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 (just once, at program
        startup) to zero, as if the programmer had typed "= 0".
        Therefore, such variables are initialized to the null pointer
        (of the correct type; see also Section 1) 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 question 3.13, and section
        1).

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

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

A:      Perhaps you have a pre-ANSI compiler, which doesn't allow
        initialization of "automatic aggregates" (i.e. non-static local
        arrays and structures). As a workaround, you can make the array
        global or static, and initialize it with strcpy when f is
        called. (You can always initialize local char * variables with
        string literals, but see question 17.20). See also questions
        5.16 and 5.17.

17.3: 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."
        See also question 9.11.

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

A: Short of rewriting the file, you probably can't. See also
question 16.9.

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, or (in a pinch) consider global variables. See
        also questions 2.17, 3.4, and 9.2.

17.6: If I have a char * variable pointing to the name of a function
as a string, how can I call that function?

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

int function1(), function2();

                struct {char *name; int (*funcptr)(); } symtab[] =
                        {
                        "function1",    function1,
                        "function2",    function2,
                        };

Then, just search the table for the name, and call through the
associated function pointer. See also questions 9.9 and 16.11.

17.7: 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).

17.8: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions
from C? (And vice versa?)

A:      The answer is entirely dependent on the machine and the specific
        calling sequences of the various compilers in use, and may not
        be possible at all. Read your compiler documentation very
        carefully; sometimes there is a "mixed-language programming
        guide," although the techniques for passing arguments and
        ensuring correct run-time startup are often arcane. More
        information may be found in FORT.gz by Glenn Geers, available
        via anonymous ftp from suphys.physics.su.oz.au in the src
        directory.

        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).

        In C++, a "C" modifier in an external function declaration
        indicates that the function is to be called using C calling
        conventions.

17.9: Does anyone know of a program for converting Pascal or FORTRAN
(or LISP, Ada, awk, "Old" C, ...) to C?

A: Several public-domain programs are available:

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

        ptoc    Another Pascal to C converter, this one written in
                Pascal (comp.sources.unix, Volume 10, also patches in
                Volume 13?).

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

        This FAQ list's maintainer also has available a list of other
        commercial translation products, and some for more obscure
        languages.