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Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List)
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[Last modified May 30, 1992 by scs.]

Certain topics come up again and again on this newsgroup.  They are good
questions, and the answers may not be immediately obvious, but each time
they recur, much net bandwidth and reader time is wasted on repetitive
responses, and on tedious corrections to the incorrect answers which are
inevitably posted.

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

No mere newsgroup article can substitute for thoughtful perusal of a
full-length tutorial or language reference manual.  Anyone interested
enough in C to be following this newsgroup should also be interested
enough to read and study one or more such manuals, preferably several
times.  Some vendors' compiler manuals are unfortunately inadequate; a
few even perpetuate some of the myths which this article attempts to
refute.  Several noteworthy books on C are listed in this article's
bibliography.  Many of the questions and answers are cross-referenced to
these books, for further study by the interested and dedicated reader
(but beware of ANSI vs. ISO C Standard section numbers; see question
4.1).

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

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

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

The questions answered here are divided into several categories:

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

Herewith, some frequently-asked questions and their answers:


Section 1. Null Pointers

1.1:    What is this infamous null pointer, anyway?

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

    A null pointer is conceptually different from an uninitialized
    pointer.  A null pointer is known not to point to any object; an
    uninitialized pointer might point anywhere.  See also questions
    8.1, 8.9, and 16.1.

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

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

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

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

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

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

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

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

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

    Summary:

        Unadorned 0 okay:    Explicit cast required:

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

        function call,
        prototype in scope,
        fixed argument

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

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

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

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

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

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

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

1.5:    If NULL were defined as follows:

        #define NULL (char *)0

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

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

        FILE *fp = NULL;

    could fail.

    Nevertheless, ANSI C allows the alternate

        #define NULL ((void *)0)

    definition for NULL.  Besides helping incorrect programs to work
    (but only on machines with homogeneous pointers, thus
    questionably valid assistance) this definition may catch
    programs which use NULL incorrectly (e.g. when the ASCII  NUL
    character was really intended; see question 1.8).

1.6:    I use the preprocessor macro

        #define Nullptr(type) (type *)0

    to help me build null pointers of the correct type.

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

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

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

        if(expr)

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

        if(expr != 0)

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

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

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

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

        !expr    is essentially equivalent to    expr?0:1

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

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

    "Abbreviations" such as if(p), though perfectly legal, are
    considered by some to be bad style.

    See also question 11.2.

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

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

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

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

        #define NUL '\0'

    if you must.

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

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

A:    No.  Although symbolic constants are often used in place of
    numbers because the numbers might change, this is _not_ the
    reason that NULL is used in place of 0.  Once again, the
    language guarantees that source-code 0's (in pointer contexts)
    generate null pointers.  NULL is used only as a stylistic
    convention.

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

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

    1.    The conceptual null pointer, the abstract language
        concept defined in question 1.1.  It is implemented
        with...

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

    3.    The source code syntax for null pointers, which is the
        single character "0".  It is often hidden behind...

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

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

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

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

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

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

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

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

A:    Follow these two simple rules:

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

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

    The rest of the discussion has to do with other people's
    misunderstandings, or with the internal representation of null
    pointers (which you shouldn't need to know), or with ANSI C
    refinements.  Understand questions 1.1, 1.2, and 1.3, and
    consider 1.8 and 1.11, and you'll do fine.

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

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

    Besides, what would this requirement really accomplish?  Proper
    understanding of null pointers does not require knowledge of the
    internal representation, whether zero or nonzero.  Assuming that
    null pointers are internally zero does not make any code easier
    to write (except for a certain ill-advised usage of calloc; see
    question 8.9).  Known-zero internal pointers would not obviate
    casts in function calls, because the _size_ of the pointer might
    still be different from that of an int.  (If "nil" were used to
    request null pointers rather than "0," as mentioned in question
    1.11, the urge to assume an internal zero representation would
    not even arise.)

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

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

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

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

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


Section 2. Arrays and Pointers

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

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

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

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

A:    Not at all.  (What you heard has to do with formal parameters to
    functions; see question 2.4.)  Arrays are not pointers.  The
    array declaration "char a[6];" requests that space for six
    characters be set aside, to be known by the name "a."  That is,
    there is a location named "a" at which six characters can sit.
    The pointer declaration "char *p;" on the other hand, requests a
    place which holds a pointer.  The pointer is to be known by the
    name "p," and can point to any char (or contiguous array of
    chars) anywhere.

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

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

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

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

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

    It is important to remember that a reference like x[3] generates
    different code depending on whether x is an array or a pointer.
    Given the declarations above, when the compiler sees the
    expression a[3], it emits code to start at the location "a,"
    move three past it, and fetch the character there.  When it sees
    the expression p[3], it emits code to start at the location "p,"
    fetch the pointer value there, add three to the pointer, and
    finally fetch the character pointed to.  In the example above,
    both a[3] and p[3] happen to be the character 'l', but the
    compiler gets there differently.  (See also question 16.11.)

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

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

    "Equivalence" refers to the following key definition:

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

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

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

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

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

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

        f(a)
        char a[];

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

        f(a)
        char *a;

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

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

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

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

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

A:    This is a bit of an oversimplification.  An array name is
    "constant" in that it cannot be assigned to, but an array is
    _not_ a pointer, as the discussion and pictures in question 2.2
    should make clear.

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

A:    Yes, Virginia, array subscripting is commutative in C.  This
    curious fact follows from the pointer definition of array
    subscripting, namely that a[e] is exactly equivalent to
    *((a)+(e)), for _any_ expression e and primary expression a, as
    long as one of them is a pointer expression and one is integral.
    This unsuspected commutativity is often mentioned in C texts as
    if it were something to be proud of, but it finds no useful
    application outside of the Obfuscated C Contest (see question
    16.7).

    References: ANSI Rationale Sec. 3.3.2.1 p. 41.

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

A:    The rule by which arrays decay into pointers is not applied
    recursively.  An array of arrays (i.e. a two-dimensional array
    in C) decays into a pointer to an array, not a pointer to a
    pointer.  Pointers to arrays can be confusing, and must be
    treated carefully.  (The confusion is heightened by the
    existence of incorrect compilers, including some versions of pcc
    and pcc-derived lint's, which improperly accept assignments of
    multi-dimensional arrays to multi-level pointers.)  If you are
    passing a two-dimensional array to a function:

        int array[YSIZE][XSIZE];
        f(array);

    the function's declaration should match:

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

    In the first declaration, the compiler performs the usual
    implicit parameter rewriting of "array of array" to "pointer to
    array;" in the second form the pointer declaration is explicit.
    Since the called function does not allocate space for the array,
    it does not need to know the overall size, so the number of
    "rows," YSIZE, can be omitted.  The "shape" of the array is
    still important, so the "column" dimension XSIZE (and, for 3- or
    more dimensional arrays, the intervening ones) must be included.

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

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

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

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

    If you really need to declare a pointer to an entire array, use
    something like "int (*ap)[N];" where N is the size of the array.
    (See also question 10.3.)  If the size of the array is unknown,
    N can be omitted, but the resulting type, "pointer to array of
    unknown size," is useless.

2.10:    How can I dynamically allocate a multidimensional array?

A:    It is usually best to allocate an array of pointers, and then
    initialize each pointer to a dynamically-allocated "row."  The
    resulting "ragged" array can save space, although it is not
    necessarily contiguous in memory as a real array would be.  Here
    is a two-dimensional example:

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

    (In "real" code, of course, malloc would be declared correctly,
    and each return value checked.)

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

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

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

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

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

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

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

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

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

A:    This technique, though attractive, is not strictly conforming to
    the C standards (in spite of its appearance in Numerical Recipes
    in C).  Pointer arithmetic is defined only as long as the
    pointer points within the same allocated block of memory, or to
    the imaginary "terminating" element one past it; otherwise, the
    behavior is undefined, _even if the pointer is not
    dereferenced_.  The code above could fail if, while subtracting
    the offset, an illegal address were generated (perhaps because
    the address tried to "wrap around" past the beginning of some
    memory segment).

    References: K&R II Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3,
    Sec. A7.7 pp. 205-6; ANSI Rationale Sec. 3.2.2.3 p. 38.

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

        ((int *)p)++;

    work?

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

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

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


Section 3. Expressions

3.1:    Under my compiler, the code

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

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

A:    Although the postincrement and postdecrement operators ++ and --
    perform the operations after yielding the former value, the
    implication of "after" is often misunderstood.  It is _not_
    guaranteed that the operation is performed immediately after
    giving up the previous value and before any other part of the
    expression is evaluated.  It is merely guaranteed that the
    update will be performed sometime before the expression is
    considered "finished" (before the next "sequence point," in ANSI
    C's terminology).  In the example, the compiler chose to
    multiply the previous value by itself and to perform both
    increments afterwards.

    The behavior of code which contains multiple, ambiguous side
    effects has always been undefined.  Don't even try to find out
    how your compiler implements such things (contrary to the ill-
    advised exercises in many C textbooks); as K&R wisely point out,
    "if you don't know _how_ they are done on various machines, that
    innocence may help to protect you."

    References: K&R I Sec. 2.12 p. 50; K&R II Sec. 2.12 p. 54; ANSI
    Sec. 3.3 p. 39; CT&P Sec. 3.7 p. 47; PCS Sec. 9.5 pp. 120-1.
    (Ignore H&S Sec. 7.12 pp. 190-1, which is obsolete.)

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

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

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

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

A:    Since the two forms differ only in the value yielded, they are
    entirely equivalent when only their side effect is needed.  Some
    people will tell you that in the old days one form was preferred
    over the other because it utilized a PDP-11 autoincrement
    addressing mode, but those people are confused.

3.4:    Why doesn't the code

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

    work?

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

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


Section 4. ANSI C

4.1:    What is the "ANSI C Standard?"

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

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

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

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

A:    Copies are available from

        American National Standards Institute
        1430 Broadway
        New York, NY  10018  USA
        ([phone]

    or

        Global Engineering Documents
        2805 McGaw Avenue
        Irvine, CA  92714  USA
        ([phone]
        [phone]  (U.S. & Canada)

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

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

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

A:    Two programs, protoize and unprotoize, convert back and forth
    between prototyped and "old style" function definitions and
    declarations.  (These programs do _not_ handle full-blown
    translation between "Classic" C and ANSI C.)  These programs
    exist as patches to the FSF GNU C compiler, gcc.  Look for the
    file protoize-1.39.0.5.Z in pub/gnu at prep.ai.mit.edu
    (18.71.0.38), or at several other FSF archive sites.

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

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

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

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

        #define str(x) #x
        #define xstr(x) str(x)
        #define OP plus
        char *opname = xstr(OP);

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

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

    References: ANSI Sec. 3.8.3.2, Sec. 3.8.3.5 example p. 93.

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

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

    References: ANSI Sec. 3.5.4.1 .

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

        extern int func(float);

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

A:    You have mixed the new-style prototype declaration
    "extern int func(float);" with the old-style definition
    "int func(x) float x;".  Old C (and ANSI C, in the absence of
    prototypes) silently promotes floats to doubles when passing
    them as arguments, and arranges that doubles being passed are
    coerced back to floats if the formal parameters are declared
    that way.

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

        int func(float x) { ... }

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

        extern int func(double);

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

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

    References: ANSI Sec. 3.3.2.2 .

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

A:    Under ANSI C, the text inside a "turned off" #if, #ifdef, or
    #ifndef must still consist of "valid preprocessing tokens."
    This means that there must be no unterminated comments or quotes
    (note particularly that an apostrophe within a contracted word
    could look like the beginning of a character constant), and no
    newlines inside quotes.  Therefore, natural-language comments
    and pseudocode should always be written between the "official"
    comment delimiters /* and */.  (But see also question 16.8.)

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

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

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

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

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

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

    This concession to current, restrictive linkers really had to be
    made, no matter how vehemently some people oppose it.  (The
    Rationale notes that its retention was "most painful.")  If you
    disagree, or have thought of a trick by which a compiler
    burdened with a restrictive linker could present the C
    programmer with the appearance of more significance in external
    identifiers, read the excellently-worded section 3.1.2 in the
    X3.159 Rationale (see question 4.1), which discusses several
    such schemes and explains why they could not be mandated.

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

4.10:    What is the difference between memcpy and memmove?

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

    References: ANSI Secs. 4.11.2.1, 4.11.2.2, Rationale
    Sec. 4.11.2 .

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

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

    References: ANSI Sec. 3.8.6 .


Section 5. C Preprocessor

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

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

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

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

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

    but it doesn't work any more.

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

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

    (See also question 4.4.)

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

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

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

    The traditional solution is to use

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

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

    If all of the statements in the intended macro are simple
    expressions, with no declarations or loops, another technique is
    to write a single, parenthesized expression using one or more
    comma operators.  (See the example under question 5.8 below.
    This technique also allows a value to be "returned.")

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

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

A:    There has been considerable debate surrounding this question.
    Many people believe that "nested #include files" are to be
    avoided: the prestigious Indian Hill Style Guide disparages
    them; they can make it harder to find relevant definitions; they
    can lead to multiple-declaration errors if a file is #included
    twice; and they make manual Makefile maintenance very difficult.
    On the other hand, they make it possible to use header files in
    a modular way (a header file #includes what it needs itself,
    rather than requiring each #includer to do so, a requirement
    that can lead to intractable headaches); a tool like grep (or a
    tags file) makes it easy to find definitions no matter where
    they are; a popular trick:

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

    makes a header file "idempotent" so that it can safely be
    #included multiple times; and automated Makefile maintenance
    tools (which are a virtual necessity in large projects anyway)
    handle dependency generation in the face of nested #include
    files easily.

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

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

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

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

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

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

A:    cpp is not intended as a general-purpose preprocessor.  Rather
    than forcing it to do something inappropriate, consider writing
    your own little special-purpose preprocessing tool, instead.
    You can easily get a utility like make(1) to run it for you
    automatically.

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

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

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

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

    The obvious disadvantage is that the caller must always remember
    to use the extra parentheses.  (It is often better to use a
    bona-fide function, which can take a variable number of
    arguments in a well-defined way.  See questions 6.1 and 6.2
    below.)


Section 6. Variable-Length Argument Lists

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

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

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

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

        char *vstrcat(char *first, ...)
        {
            size_t len = 0;
            char *retbuf;
            va_list argp;
            char *p;

            if(first == NULL)
                return NULL;

            len = strlen(first);

            va_start(argp, first);

            while((p = va_arg(argp, char *)) != NULL)
                len += strlen(p);

            va_end(argp);

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

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

            (void)strcpy(retbuf, first);

            va_start(argp, first);

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

            va_end(argp);

            return retbuf;
        }

    Usage is something like

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

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

    Under a pre-ANSI compiler, rewrite the function definition
    without a prototype ("char *vstrcat(first) char *first; {"),
    include <stdio.h> rather than <stddef.h>, replace "#include
    <stdlib.h>" with "extern char *malloc();", and use int instead
    of size_t.  You may also have to delete the (void) casts, and
    use the older varargs package instead of stdarg.  See the next
    question for hints.

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

6.2:    How can I write a function that takes a format string and a
    variable number of arguments, like printf, and passes them to
    printf to do most of the work?

A:    Use vprintf, vfprintf, or vsprintf.

    Here is an "error" routine which prints an error message,
    preceded by the string "error: " and terminated with a newline:

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

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

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

        void error(va_alist)
        va_dcl
        {
            char *fmt;

    change the va_start line to

        va_start(argp);

    and add the line

        fmt = va_arg(argp, char *);

    between the calls to va_start and vfprintf.  (Note that there is
    no semicolon after va_dcl.)

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

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

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

    Any function which takes a variable number of arguments must be
    able to determine from the arguments themselves how many of them
    there are.  printf-like functions do this by looking for
    formatting specifiers (%d and the like) in the format string
    (which is why these functions fail badly if the format string
    does not match the argument list).  Another common technique
    (useful when the arguments are all of the same type) is to use a
    sentinel value (often 0, -1, or an appropriately-cast null
    pointer) at the end of the list (see the execl and vstrcat
    examples under questions 1.2 and 6.1 above).

6.4:    How can I write a function which takes a variable number of
    arguments and passes them to some other function (which takes a
    variable number of arguments)?

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


Section 7. Lint

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

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

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

A:    The problem is that traditional versions of lint do not know,
    and cannot be told, that malloc "returns a pointer to space
    suitably aligned for storage of any type of object."  It is
    possible to provide a pseudoimplementation of malloc, using a
    #define inside of #ifdef lint, which effectively shuts this
    warning off, but a simpleminded #definition will also suppress
    meaningful messages about truly incorrect invocations.  It may
    be easier simply to ignore the message, perhaps in an automated
    way with grep -v.

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

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

        Gimpel Software
        3207 Hogarth Lane
        Collegeville, PA  19426  USA
        ([phone]

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


Section 8. Memory Allocation

8.1:    Why doesn't this fragment work?

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

A:    The pointer variable "answer," which is handed to the gets
    function as the location into which the response should be
    stored, has not been set to point to any valid storage.  That
    is, we cannot say where the pointer "answer" points.  (Since
    local variables are not initialized, and typically contain
    garbage, it is not even guaranteed that "answer" starts out as a
    null pointer.  See question 16.1.)

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

        #include <string.h>

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

    Note that this example also uses fgets instead of gets (always a
    good idea), so that the size of the array can be specified, so
    that fgets will not overwrite the end of the array if the user
    types an overly-long line.  (Unfortunately for this example,
    fgets does not automatically delete the trailing \n, as gets
    would.)  It would also be possible to use malloc to allocate the
    answer buffer, and/or to parameterize its size
    (#define ANSWERSIZE 100).

8.2:    I can't get strcat to work.  I tried

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

    but I got strange results.

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

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

        char s1[20] = "Hello, ";

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

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

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

A:    In general, when using pointers you _always_ have to consider
    memory allocation, at least to make sure that the compiler is
    doing it for you.  If a library routine's documentation does not
    explicitly mention allocation, it is usually the caller's
    problem.

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

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

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

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

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

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

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

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

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

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

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

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

A:    Not portably.

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

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

    References: ANSI Sec. 4.10.3.4 .

8.9:    What is the difference between calloc and malloc?  Is it safe to
    use calloc's zero-fill guarantee for pointer and floating-point
    values?  Does free work on memory allocated with calloc, or do
    you need a cfree?

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

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

    The zero fill is all-bits-zero, and does not therefore guarantee
    useful zero values for pointers (see section 1 of this list) or
    floating-point values.  free can (and should) be used to free
    the memory allocated by calloc.

    References: ANSI Secs. 4.10.3 to 4.10.3.2 .

8.10:    What is alloca and why is its use discouraged?

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

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

    For these reasons, alloca cannot be used in programs which must
    be widely portable, no matter how useful it might be.


Section 9. Structures

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

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

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

9.2:    How does struct passing and returning work?

A:    When structures are passed as arguments to functions, the entire
    struct is typically pushed on the stack, using as many words as
    are required.  (Programmers often choose to use pointers to
    structures instead, precisely to avoid this overhead.)

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

    Reference: ANSI Sec. 2.2.3 p. 13.

9.3:    The following program works correctly, but it dumps core after
    it finishes.  Why?

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

        /* Here is the main program. */

        main(argc, argv)
        ...

A:    A missing semicolon causes the compiler to believe that main
    returns a structure.  (The connection is hard to see because of
    the intervening comment.)  Since struct-valued functions are
    usually implemented by adding a hidden return pointer, the
    generated code for main() tries to accept three arguments,
    although only two are passed (in this case, by the C start-up
    code).  See also question 16.15.

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

9.4:    Why can't you compare structs?

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

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

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

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

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

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

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

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

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

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

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

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

    See the next question for a usage hint.

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

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

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

        offsetb = offsetof(struct a, b)

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

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


Section 10. Declarations

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

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

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

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

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

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

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

A:    Structs in C can certainly contain pointers to themselves; the
    discussion and example in section 6.5 of K&R make this clear.
    The problem with this example is that the NODEPTR typedef is not
    complete at the point where the "next" field is declared.  To
    fix it, first give the structure a tag ("struct node").  Then,
    declare the "next" field as "struct node *next;", and/or move
    the typedef declaration wholly before or wholly after the struct
    declaration.  One fixed version would be

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

        typedef struct node *NODEPTR;

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

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

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

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

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

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

    2.  Build the declaration up in stages, using typedefs:

        typedef char *pc;    /* pointer to char */
        typedef pc fpc();    /* function returning pointer to char */
        typedef fpc *pfpc;    /* pointer to above */
        typedef pfpc fpfpc();    /* function returning... */
        typedef fpfpc *pfpfpc;    /* pointer to... */
        pfpfpc a[5];        /* array of... */

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

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

        cdecl can also explain complicated declarations, help with
        casts, and indicate which set of parentheses the arguments
        go in (for complicated function definitions, like the
        above).  Versions of cdecl are in volume 14 of
        comp.sources.unix (see question 16.6) and K&R II.

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

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

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

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

    This rule promotes a high degree of portability, and is
    consistent with the requirements of the ANSI C Standard.  Note
    that Unix compilers and linkers typically use a "common model"
    which allows multiple (uninitialized) definitions.  A few very
    odd systems may require an explicit initializer to distinguish a
    definition from an external declaration.

    It is possible to use preprocessor tricks to arrange that the
    declaration need only be typed once, in the header file, and
    "turned into" a definition, during exactly one #inclusion, via a
    special #define.

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

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

A:    Use something like

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

    When the name of a function appears in an expression but is not
    being called (i.e. is not followed by a "("), it "decays" into a
    pointer (i.e. it has its address implicitly taken), much as an
    array name does.

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

10.6:    I've seen different methods used for calling through pointers to
    functions.  What's the story?

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

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

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

        r = fp();

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

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


Section 11. Boolean Expressions and Variables

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

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

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

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

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

    or use raw 1 and 0, as long as you are consistent within one
    program or project.  (An enum may be preferable if your debugger
    expands enum values when examining variables.)

    Some people prefer variants like

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

    or define "helper" macros such as

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

    These don't buy anything (see below).

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

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

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

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

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

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

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

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

    The primary advantages of enums are that the numeric values are
    automatically assigned, and that a debugger may be able to
    display the symbolic values when enum variables are examined.
    (A compiler may also generate nonfatal warnings when enums and
    ints are indiscriminately mixed, since doing so can still be
    considered bad style even though it is not strictly illegal).  A
    disadvantage is that the programmer has little control over the
    size (or over those nonfatal warnings).

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


Section 12. System Dependencies

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

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

    Operating system specific questions are not appropriate for
    comp.lang.c .  Many common questions are answered in
    frequently-asked questions postings in such groups as
    comp.unix.questions and comp.os.msdos.programmer .  Note that
    the answers are often not unique even across different variants
    of a system; bear in mind when answering system-specific
    questions that the answer that applies to your system may not
    apply to everyone else's.

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

12.2:    How can I find out if there are characters available for reading
    (and if so, how many)?  Alternatively, how can I do a read that
    will not block if there are no characters available?

A:    These, too, are entirely operating-system-specific.  Some
    versions of curses have a nodelay() function.  Depending on your
    system, you may also be able to use "nonblocking I/O", or a
    system call named "select", or the FIONREAD ioctl, or kbhit(),
    or rdchk(), or the O_NDELAY option to open() or fcntl().

12.3:    How do I read the mouse?

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

12.4:    How can my program discover the complete pathname to the
    executable file from which it was invoked?

A:    argv[0] may contain all or part of the pathname, or it may
    contain nothing.  You may be able to duplicate the command
    language interpreter's search path logic to locate the
    executable if the name in argv[0] is present but incomplete.
    However, there is no guaranteed or portable solution.

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

A:    In general, it cannot.  Different operating systems implement
    name/value functionality similar to the Unix environment in
    different ways.  Whether the "environment" can be usefully
    altered by a running program, and if so, how, is system-
    dependent.

    Under Unix, a process can modify its own environment (some
    systems provide setenv() and/or putenv() functions to do this),
    and the modified environment is usually passed on to any child
    processes, but it is _not_ propagated back to the parent
    process.

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

A:    If the "size of a file" is the number of characters you'll be
    able to read from it in C, it is in general impossible to
    determine this number in advance.  Under Unix, the stat()
    function will give you an exact answer, and several other
    systems supply a Unix-like stat() which will give an approximate
    answer.  You can fseek to the end and then use ftell, but this
    usage is nonportable (it gives you an accurate answer only under
    Unix, and a guaranteed, but potentially approximate answer only
    for ANSI C "binary" files).

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

A:    BSD systems provide ftruncate(), several others supply chsize(),
    and a few may provide a (possibly undocumented) fcntl option
    F_FREESP, but there is no truly portable solution.

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

A:    Unfortunately, there is no portable way.  V7 Unix, and derived
    systems, provided a fairly useful ftime() routine with
    resolution up to a millisecond, but it has disappeared from
    System V and Posix.


Section 13. Stdio

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

A:    Many implementations of the stdio package adjust their behavior
    slightly if stdout is a terminal.  To make the determination,
    these implementations perform an operation which fails (with
    ENOTTY) if stdout is not a terminal.  Although the output
    operation goes on to complete successfully, errno still contains
    ENOTTY.

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

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

A:    It is best to use an explicit fflush(stdout) whenever output
    should definitely be visible.  Several mechanisms attempt to
    perform the fflush for you, at the "right time," but they tend
    to apply only when stdout is a terminal.  (See question 13.1.)

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

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

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

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

A:    fflush is defined only for output streams.  Since its definition
    of "flush" is to complete the writing of buffered characters
    (not to discard them), discarding unread input would not be an
    analogous meaning for fflush on input streams.  There is no
    standard way to discard unread characters from a stdio input
    buffer, nor would such a way be sufficient; unread characters
    can also accumulate in other, OS-level input buffers.

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

A:    This problem is, in general, insoluble.  Under Unix, for
    instance, a scan of the entire disk, (perhaps requiring special
    permissions) would theoretically be required, and would fail if
    the file descriptor was a pipe or referred to a deleted file
    (and could give a misleading answer for a file with multiple
    links).  It is best to remember the names of open files yourself
    (perhaps with a wrapper function around fopen).


Section 14. Library Subroutines

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

A:    By "array of strings" you probably mean "array of pointers to
    char."  The arguments to qsort's comparison function are
    pointers to the objects being sorted, in this case, pointers to
    pointers to char.  (strcmp, of course, accepts simple pointers
    to char.)

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

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

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

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

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

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

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

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

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

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

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

    References: ANSI Sec. 4.12 .

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

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

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

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

14.6:    I need a random number generator.

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

    References: ANSI Sec. 4.10.2.1 p. 154.

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

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

    References: ANSI Sec. 4.10.2.2 p. 154.

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

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

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

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

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

A:    Unix and some other systems provide a popen() routine, which
    sets up a stdio stream on a pipe connected to the process
    running a command, so that the output can be read (or the input
    supplied).

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

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


Section 15. Style

15.1:    Here's a neat trick:

        if(!strcmp(s1, s2))

    Is this good style?

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

    Another solution is to use a macro:

        #define Streq(s1, s2) (strcmp((s1), (s2)) == 0)

    Opinions on code style, like those on religion, can be debated
    endlessly.  Though good style is a worthy goal, and can usually
    be recognized, it cannot be codified.

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

A:    K&R, while providing the example most often copied, also supply
    a good excuse for avoiding it:

        The position of braces is less important,
        although people hold passionate beliefs.
        We have chosen one of several popular styles.
        Pick a style that suits you, then use it
        consistently.

    It is more important that the layout chosen be consistent (with
    itself, and with nearby or common code) than that it be
    "perfect."  If your coding environment (i.e. local custom or
    company policy) does not suggest a style, and you don't feel
    like inventing your own, just copy K&R.  (The tradeoffs between
    various indenting and brace placement options can be
    exhaustively and minutely examined, but don't warrant repetition
    here.  See also the Indian Hill Style Guide.)

    The elusive quality of "good style" involves much more than mere
    code layout details; don't spend time on formatting to the
    exclusion of more substantive code quality issues.

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

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

A:    Various documents are available for anonymous ftp from:

        Site:            File or directory:

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

        cs.toronto.edu        doc/programming

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


Section 16. Miscellaneous

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

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

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

    Dynamically-allocated memory obtained with malloc and realloc is
    also likely to contain garbage, and must be initialized by the
    calling program, as appropriate.  Memory obtained with calloc
    contains all-bits-0, but this is not necessarily useful for
    pointer or floating-point values (see section 1 and question
    8.9).

16.2:    How can I write data files which can be read on other machines
    with different word size, byte order, or floating point formats?

A:    The best solution is to use text files (usually ASCII), written
    with fprintf and read with fscanf or the like.  (Similar advice
    also applies to network protocols.)  Be skeptical of arguments
    which imply that text files are too big, or that reading and
    writing them is too slow.  Not only is their efficiency
    frequently acceptable in practice, but the advantages of being
    able to manipulate them with standard tools can be overwhelming.

    If you must use a binary format, you can improve portability,
    and perhaps take advantage of prewritten I/O libraries, by
    making use of standardized formats such as Sun's XDR (RFC 1014),
    OSI's ASN.1, CCITT's X.409, or ISO 8825 "Basic Encoding Rules."

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

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

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

A:    The answer is entirely dependent on the machine and the specific
    calling sequences of the various compilers in use, and may not
    be possible at all.  Read your compiler documentation very
    carefully; sometimes there is a "mixed-language programming
    guide," although the techniques for passing arguments and
    ensuring correct run-time startup are often arcane.

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

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

A:    Several public-domain programs are available:

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

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

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

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

    The following companies sell various translation tools and
    services:

        Cobalt Blue
        2940 Union Ave., Suite C
        San Jose, CA  95124  USA
        ([phone]

        Promula Development Corp.
        3620 N. High St., Suite 301
        Columbus, OH  43214  USA
        ([phone]

        Micro-Processor Services Inc.
        92 Stone Hurst Lane
        Dix Hills, NY  11746  USA
        ([phone]

    See also question 4.3.

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

A:    If you have access to Usenet, see the regular postings in the
    comp.sources.unix and comp.sources.misc newsgroups, which
    describe, in some detail, the archiving policies and how to
    retrieve copies.  The usual approach is to use anonymous ftp
    and/or uucp from a central, public-spirited site, such as uunet
    (ftp.uu.net, 192.48.96.9).  However, this article cannot track
    or list all of the available archive sites and how to access
    them.  The comp.archives newsgroup contains numerous
    announcements of anonymous ftp availability of various items.
    The "archie" mailserver can tell you which anonymous ftp sites
    have which packages; send the mail message "help" to
    [e-mail] for information.  Finally, the
    newsgroup comp.sources.wanted is generally a more appropriate
    place to post queries for source availability, but check _its_
    FAQ list, "How to find sources," before posting there.

16.7:    When will the next International Obfuscated C Code Contest
    (IOCCC) be held?  How can I get a copy of the current and
    previous winning entries?

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

        {apple,pyramid,sun,uunet}!hoptoad!judges  or  [e-mail]

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

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

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

    The character sequences /* and */ are not special within
    double-quoted strings, and do not therefore introduce comments,
    because a program (particularly one which is generating C code
    as output) might want to print them.

    Reference: ANSI Rationale Sec. 3.1.9 p. 33.

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

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

16.10:    How can I make this code more efficient?

A:    Efficiency, though a favorite comp.lang.c topic, is not
    important nearly as often as people tend to think it is.  Most
    of the code in most programs is not time-critical.  When code is
    not time-critical, it is far more important that it be written
    clearly and portably than that it be written maximally
    efficiently.  (Remember that computers are very, very fast, and
    that even "inefficient" code can run without apparent delay.)

    It is notoriously difficult to predict what the "hot spots" in a
    program will be.  When efficiency is a concern, it is important
    to use profiling software to determine which parts of the
    program deserve attention.  Often, actual computation time is
    swamped by peripheral tasks such as I/O and memory allocation,
    which can be sped up by using buffering and cacheing techniques.

    For the small fraction of code that is time-critical, it is
    vital to pick a good algorithm; it is less important to
    "microoptimize" the coding details.  Many of the "efficient
    coding tricks" which are frequently suggested (e.g. substituting
    shift operators for multiplication by powers of two) are
    performed automatically by even simpleminded compilers.
    Heavyhanded "optimization" attempts can make code so bulky that
    performance is degraded.

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

16.11:    Are pointers really faster than arrays?  How much do function
    calls slow things down?  Is ++i faster than i = i + 1?

A:    Precise answers to these and many similar questions depend of
    course on the processor and compiler in use.  If you simply must
    know, you'll have to time test programs carefully.  (Often the
    differences are so slight that hundreds of thousands of
    iterations are required even to see them.  Check the compiler's
    assembly language output, if available, to see if two purported
    alternatives aren't compiled identically.)

    It is "usually" faster to march through large arrays with
    pointers rather than array subscripts, but for some processors
    the reverse is true.

    Function calls, though obviously incrementally slower than in-
    line code, contribute so much to modularity and code clarity
    that there is rarely good reason to avoid them.

    Before rearranging expressions such as i = i + 1, remember that
    you are dealing with a C compiler, not a keystroke-programmable
    calculator.  A good compiler will generate identical code for
    ++i, i += 1, and i = i + 1.  The reasons for using ++i or i += 1
    over i = i + 1 have to do with style, not efficiency.  (See also
    question 3.3.)

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

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

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

    Don't assume that floating-point results will be exact, and
    especially don't assume that floating-point values can be
    compared for equality.  (Don't throw haphazard "fuzz factors"
    in, either.)

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

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

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

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

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

    References: ANSI Sec. 4.5.5.1 .

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

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

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

16.15:    This program crashes before it even runs!  (When single-stepping
    with a debugger, it dies before the first statement in main.)

A:    You probably have one or more very large (kilobyte or more)
    local arrays.  Many systems have fixed-size stacks, and those
    which perform dynamic stack allocation automatically (e.g. Unix)
    can be confused when the stack tries to grow by a huge chunk all
    at once.

    It is often better to declare large arrays with static duration
    (unless of course you need a fresh set with each recursive
    call).

    (See also question 9.3.)

16.16:    What does "Segmentation violation" mean?

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

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

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

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

A:    The definitive grammar is of course the one in the ANSI
    standard.  Several copies are floating around; keep your eyes
    open.  There is one on uunet (see question 16.6) in
    usenet/net.sources/ansi.c.grammar.Z (including a companion
    lexer).  The FSF's GNU C compiler contains a grammar, as does
    the appendix to K&R II.

    References: ANSI Sec. A.2 .

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

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

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

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

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

    This list is an evolving document, not just a collection of this
    month's interesting questions.  Older copies are obsolete and
    don't contain much, except the occasional typo, that the current
    list doesn't.


Bibliography

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

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

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

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

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

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

K&R II    Brian W. Kernighan and Dennis M. Ritchie, The C Programming
    Language, Second Edition, Prentice Hall, 1988,
    ISBN 0-13-110362-8, 0-13-110370-9.

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

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

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

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

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


Acknowledgements

Thanks to Jamshid Afshar, Sudheer Apte, Dan Bernstein, Stan Brown, Joe
Buehler, Burkhard Burow, Raymond Chen, Christopher Calabrese, James
Davies, Norm Diamond, Ray Dunn, Stephen M. Dunn, Bjorn Engsig, Dave
Gillespie, Samuel Goldstein, Ron Guilmette, Doug Gwyn, Tony Hansen, Joe
Harrington, Guy Harris, Jos Horsmeier, Blair Houghton, Kirk Johnson,
Peter Klausler, Andrew Koenig, John Lauro, Don Libes, Christopher Lott,
Tim McDaniel, Evan Manning, Mark Moraes, Darren Morby, Hans Olsson,
Francois Pinard, randall@virginia, Pat Rankin, Rich Salz, Chip
Salzenberg, Paul Sand, Doug Schmidt, Patricia Shanahan, Peter da Silva,
Joshua Simons, Henry Spencer, Erik Talvola, Clarke Thatcher, Chris
Torek, Goran Uddeborg, Wietse Venema, Ed Vielmetti, Larry Virden, Freek
Wiedijk, and Dave Wolverton, who have contributed, directly or
indirectly, to this article.  Special thanks to Karl Heuer, and
particularly to Mark Brader, who (to borrow a line from Steve Johnson)
have goaded me beyond my inclination, and occasionally beyond my
endurance, in relentless pursuit of a better FAQ list.

                    Steve Summit
                    [e-mail]
                    [e-mail]
                    mit-eddie!adam.mit.edu!scs

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