Meeting Pearls 1/Meeting Pearls Vol 1 (1994).iso/installed_progs/text/faqs/c-faq.faq

Internet Message Format | 1994-05-02 | 143.1 KB

Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List)
Newsgroups: comp.lang.c,comp.answers,news.answers
From: [e-mail] (Steve Summit)
Date: 1 May 94 10:02:39 GMT

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

[Last modified April 16, 1994 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 C books and 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
5.1).

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] .

The questions answered here are divided into several categories:

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

Herewith, some frequently-asked questions and their answers:


Section 1. Null Pointers

1.1:    What is this infamous null pointer, anyway?

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

    A null pointer is conceptually different from an uninitialized
    pointer.  A null pointer is known not to point to any object; an
    uninitialized pointer might point anywhere.  See also questions
    3.1, 3.13, and 17.1.

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

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

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

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

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

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

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

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

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

    Summary:

        Unadorned 0 okay:    Explicit cast required:

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

        function call,
        prototype in scope,
        fixed argument

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

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

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

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

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

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

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

1.5:    If NULL were defined as follows:

        #define NULL ((char *)0)

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

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

        FILE *fp = NULL;

    could fail.

    Nevertheless, ANSI C allows the alternate

        #define NULL ((void *)0)

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

    References: ANSI Rationale Sec. 4.1.5 p. 74.

1.6:    I use the preprocessor macro

        #define Nullptr(type) (type *)0

    to help me build null pointers of the correct type.

A:    This trick, though popular in some circles, does not buy much.
    It is not needed in assignments and comparisons; see question
    1.2.  It does not even save keystrokes.  Its use suggests to the
    reader that the author is shaky on the subject of null pointers,
    and requires the reader to check the #definition of the macro,
    its invocations, and _all_ other pointer usages much more
    carefully.  See also question 8.1.

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

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

        if(expr)

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

        if(expr != 0)

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

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

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

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

        !expr    is essentially equivalent to    expr?0:1

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

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

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

    See also question 8.2.

    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.

    References: 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 necessary relation to the null pointer
        except in name; and...

    6.    The "null string," which is another name for an empty
        string ("").  The term "null string" can be confusing in
        C (and should perhaps be avoided), because it involves a
        null ('\0') character, but not a null pointer, 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 3.13).  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).

    The Eclipse MV series from Data General has three
    architecturally supported pointer formats (word, byte, and bit
    pointers), two of which are used by C compilers: byte pointers
    for char * and void *, and word pointers for everything else.

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

    The CDC Cyber 180 Series has 48-bit pointers consisting of a
    ring, segment, and offset.  Most users (in ring 11) have null
    pointers of 0xB00000000000.

    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.

    The old HP 3000 series computers use a different addressing
    scheme for byte addresses than for word addresses; void and char
    pointers therefore have a different representation than an int
    (structure, etc.) pointer to the same address would have.

1.15:    What does a run-time "null pointer assignment" error mean?  How
    do I track it down?

A:    This message, which occurs only under MS-DOS (see, therefore,
    section 16) means that you've written, via an unintialized
    and/or null pointer, to location zero.

    A debugger will usually let you set a data breakpoint on
    location 0.  Alternately, you could write a bit of code to copy
    20 or so bytes from location 0 into another buffer, and
    periodically check that it hasn't changed.


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 realize 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 questions 17.19 and
    17.20.)

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" neither means that they are identical
    nor even interchangeable.

    "Equivalence" refers to the following key definition:

        An lvalue [see question 2.5] 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 no apparent
    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 just as would
    be a pointer variable in the expression p[i] (although the
    eventual memory accesses will be different, as explained in
    question 2.2).  In either case, the expression x[i] (where x is
    an array or a pointer) is, by definition, identical 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.  As a convenience, 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:    How can an array be an lvalue, if you can't assign to it?

A:    The ANSI C Standard defines a "modifiable lvalue," which an
    array is not.

    References: ANSI Sec. 3.2.2.1 p. 37.

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

A:    The sizeof operator reports the size of the pointer parameter
    which the function actually receives (see question 2.4).

2.7:    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.

2.8:    Practically speaking, what is the difference between arrays and
    pointers?

A:    Arrays automatically allocate space, but can't be relocated or
    resized.  Pointers must be explicitly assigned to point to
    allocated space (perhaps using malloc), but can be reassigned
    (i.e. pointed at different objects) at will, and have many other
    uses besides serving as the base of blocks of memory.

    Due to the so-called equivalence of arrays and pointers (see
    question 2.3), arrays and pointers often seem interchangeable,
    and in particular a pointer to a block of memory assigned by
    malloc is frequently treated (and can be referenced using []
    exactly) as if it were a true array.  (See question 2.14; see
    also question 17.20.)

2.9:    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 identical 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
    17.13).

    References: ANSI Rationale Sec. 3.3.2.1 p. 41.

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

A:    The rule by which arrays decay into pointers is not applied
    recursively.  An array of arrays (i.e. a two-dimensional array
    in C) decays into a pointer to an array, not a pointer to a
    pointer.  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[NROWS][NCOLUMNS];
        f(array);

    the function's declaration should match:

        f(int a[][NCOLUMNS]) {...}
    or
        f(int (*ap)[NCOLUMNS]) {...}   /* 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," NROWS, can be omitted.  The "shape" of the array is
    still important, so the "column" dimension NCOLUMNS (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.

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

A:    It's not easy.  One way is to pass in a pointer to the [0][0]
    element, along with the two dimensions, and simulate array
    subscripting "by hand:"

        f2(aryp, nrows, ncolumns)
        int *aryp;
        int nrows, ncolumns;
        { ... ary[i][j] is really aryp[i * ncolumns + j] ... }

    This function could be called with the array from question 2.10
    as

        f2(&array[0][0], NROWS, NCOLUMNS);

    It must be noted, however, that a program which performs
    multidimensional array subscripting "by hand" in this way is not
    in strict conformance with the ANSI C Standard; the behavior of
    accessing (&array[0][0])[x] is not defined for x > NCOLUMNS.

    gcc allows local arrays to be declared having sizes which are
    specified by a function's arguments, but this is a nonstandard
    extension.

    See also question 2.15.

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

A:    Usually, you don't want to.  When people speak casually of a
    pointer to an array, they usually mean a pointer to its first
    element.

    Instead of a pointer to an array, consider using a pointer to
    one of the array's elements.  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 2.10 above.)

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

2.13:    Since array references decay to pointers, given

        int array[NROWS][NCOLUMNS];

    what's the difference between array and &array?

A:    Under ANSI/ISO Standard C, &array yields a pointer, of type
    pointer-to-array-of-T, to the entire array (see also question
    2.12).  Under pre-ANSI C, the & in &array generally elicited a
    warning, and was generally ignored.  Under all C compilers, an
    unadorned reference to an array yields a pointer, of type
    pointer-to-T, to the array's first element.  (See also question
    2.3.)

2.14:    How can I dynamically allocate a multidimensional array?

A:    It is usually best to allocate an array of pointers, and then
    initialize each pointer to a dynamically-allocated "row."  Here
    is a two-dimensional example:

        int **array1 = (int **)malloc(nrows * sizeof(int *));
        for(i = 0; i < nrows; i++)
            array1[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 **array2 = (int **)malloc(nrows * sizeof(int *));
        array2[0] = (int *)malloc(nrows * ncolumns * sizeof(int));
        for(i = 1; i < nrows; i++)
            array2[i] = array2[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 *array3 = (int *)malloc(nrows * ncolumns * sizeof(int));

    However, you must now perform subscript calculations manually,
    accessing the i,jth element with array3[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.)

    Finally, you can use pointers-to-arrays:

        int (*array4)[NCOLUMNS] =
            (int (*)[NCOLUMNS])malloc(nrows * sizeof(*array4));

    , but the syntax gets horrific and all but one dimension must be
    known at compile time.

    With all of these techniques, you may of course need to remember
    to free the arrays (which may take several steps; see question
    3.9) when they are no longer needed, and you cannot necessarily
    intermix the dynamically-allocated arrays with conventional,
    statically-allocated ones (see question 2.15 below, and also
    question 2.10).

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

A:    There is no single perfect method.  Given the declarations

        int array[NROWS][NCOLUMNS];
        int **array1;
        int **array2;
        int *array3;
        int (*array4)[NCOLUMNS];

    as initialized in the code fragments in questions 2.10 and 2.14,
    and functions declared as

        f1(int a[][NCOLUMNS], int m, int n);
        f2(int *aryp, int nrows, int ncolumns);
        f3(int **pp, int m, int n);

    (see questions 2.10 and 2.11), the following calls should work
    as expected:

        f1(array, NROWS, NCOLUMNS);
        f1(array4, nrows, NCOLUMNS);
        f2(&array[0][0], NROWS, NCOLUMNS);
        f2(*array2, nrows, ncolumns);
        f2(array3, nrows, ncolumns);
        f2(*array4, nrows, NCOLUMNS);
        f3(array1, nrows, ncolumns);
        f3(array2, nrows, ncolumns);

    The following two calls would probably work, but involve
    questionable casts, and work only if the dynamic ncolumns
    matches the static NCOLUMNS:

        f1((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);
        f1((int (*)[NCOLUMNS])array3, nrows, ncolumns);

    It must again be noted that passing &array[0][0] to f2() is not
    strictly conforming; see question 2.11.

    If you can understand why all of the above calls work and are
    written as they are, and if you understand why the combinations
    that are not listed would not work, then you have a _very_ good
    understanding of arrays and pointers (and several other areas)
    in C.

2.16:    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:    Although this technique is attractive (and was used in old
    editions of the book Numerical Recipes in C), it does not
    conform to the C standards.  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: ANSI Sec. 3.3.6 p. 48, Rationale Sec. 3.2.2.3 p. 38;
    K&R II Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3, Sec. A7.7 pp. 205-6.

2.17:    I passed a pointer to a function which initialized it:

        ...
        int *ip;
        f(ip);
        ...

        void f(ip)
        int *ip;
        {
            static int dummy = 5;
            ip = &dummy;
        }

    , but the pointer in the caller was unchanged.

A:    Did the function try to initialize the pointer itself, or just
    what it pointed to?  Remember that arguments in C are passed by
    value.  The called function altered only the passed copy of the
    pointer.  You'll either want to pass the address of the pointer
    (the function will end up accepting a pointer-to-a-pointer), or
    have the function return the pointer.

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

        ((int *)p)++;

    work?

A:    In C, a cast operator 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);

    , or simply

        p += sizeof(int);

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

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

A:    Not portably.  There is no generic pointer-to-pointer type in C.
    void * acts as a generic pointer only because conversions are
    applied automatically when other pointer types are assigned to
    and from void *'s; these conversions cannot be performed (the
    correct underlying pointer type is not known) if an attempt is
    made to indirect upon a void ** value which points at something
    other than a void *.


Section 3. Memory Allocation

3.1:    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 17.1.)

    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, sizeof(answer), 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; see question 11.6), allowing the size of
    the array to be specified, so that the end of the array will not
    be overwritten 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.

3.2:    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.  (See also question 17.20.)

    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.

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

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

A:    In general, when using pointers you _always_ have to consider
    memory allocation, at least to make sure that the compiler is
    doing it for you.  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().

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

A:    Make sure that the memory to which the function returns a
    pointer is correctly allocated.  The returned pointer should be
    to a statically-allocated buffer, or to a buffer passed in by
    the caller, or to memory obtained with malloc(), but _not_ to a
    local (auto) array.  In other words, never do something like

        char *f()
        {
            char buf[10];
            /* ... */
            return buf;
        }

    One fix (which is imperfect, especially if f() is called
    recursively, or if several of its return values are needed
    simultaneously) would to to declare the buffer as

            static char buf[10];

    See also question 17.5.

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

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

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

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

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

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

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

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

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

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

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

A:    Not portably.

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

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

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

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

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

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

    References: ANSI Sec. 4.10.3.2 .

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

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

    References: ANSI Sec. 4.10.3.4 .

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

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

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

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

    References: ANSI Secs. 4.10.3 to 4.10.3.2 .

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

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

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

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

    References: ANSI Rationale Sec. 4.10.3 p. 102.


Section 4. Expressions

4.1:    Why doesn't this code:

        a[i] = i++;

    work?

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

    References: ANSI Sec. 3.3 p. 39.

4.2:    Under my compiler, the code

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

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

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

    The behavior of code which contains multiple, ambiguous side
    effects has always been undefined (see question 5.23).  Don't
    even try to find out how your compiler implements such things
    (contrary to the ill-advised exercises in many C textbooks); as
    K&R wisely point out, "if you don't know _how_ they are done on
    various machines, that innocence may help to protect you."

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

4.3:    I've experimented with the code

        int i = 2;
        i = i++;

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

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

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

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

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

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

        f() + g() * h()

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

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

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

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

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

A:    Since the two forms differ only in the value yielded, they are
    entirely equivalent when only their side effect is needed.

4.8:    Why doesn't the code

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

    work?

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

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

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


Section 5. ANSI C

5.1:    What is the "ANSI C Standard?"

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

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

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

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

A:    ANSI X3.159 has been officially superseded by ISO 9899.  Copies
    are available in the United States from

        American National Standards Institute
        11 W. 42nd St., 13th floor
        New York, NY  10036  USA
        ([phone]

    or

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

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

        ISO Sales
        Case Postale 56
        CH-1211 Geneve 20
        Switzerland

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

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

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

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

A:    Two programs, protoize and unprotoize, convert back and forth
    between prototyped and "old style" function definitions and
    declarations.  (These programs do _not_ handle full-blown
    translation between "Classic" C and ANSI C.)  These programs
    were once patches to the FSF GNU C compiler, gcc, but are now
    part of the main gcc distribution; look in pub/gnu at
    prep.ai.mit.edu (18.71.0.38), or at several other FSF archive
    sites.

    The unproto program (/pub/unix/unproto5.shar.Z on
    ftp.win.tue.nl) is a filter which sits between the preprocessor
    and the next compiler pass, converting most of ANSI C to
    traditional C on-the-fly.

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

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

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

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

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

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

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

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

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

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

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

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

    References: ANSI Sec. 3.4 .

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

A:    "char const *p" is a pointer to a constant character (you can't
    change the character); "char * const p" is a constant pointer to
    a (variable) character (i.e. you can't change the pointer).
    (Read these "inside out" to understand them.  See question
    10.4.)

    References: ANSI Sec. 3.5.4.1 .

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

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

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

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

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

        extern int func(float);

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

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

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

        int func(float x) { ... }

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

        extern int func(double);

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

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

    References: ANSI Sec. 3.3.2.2 .

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

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

    References: ANSI Secs. 3.7.1, 3.9.5 .

5.10:    Why does the declaration

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

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

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

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

        struct x;

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

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

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

A:    Under ANSI C, the text inside a "turned off" #if, #ifdef, or
    #ifndef must still consist of "valid preprocessing tokens."
    This means that there must be no unterminated comments or quotes
    (note particularly that an apostrophe within a contracted word
    could look like the beginning of a character constant), and no
    newlines inside quotes.  Therefore, natural-language comments
    and pseudocode should always be written between the "official"
    comment delimiters /* and */.  (But see also question 17.14, and
    6.7.)

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

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

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

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

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

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

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

    References: ANSI Sec. 2.1.2.2.3 p. 8.

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

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

    This concession to current, restrictive linkers really had to be
    made, no matter how vehemently some people oppose it.  (The
    Rationale notes that its retention was "most painful.")  If you
    disagree, or have thought of a trick by which a compiler
    burdened with a restrictive linker could present the C
    programmer with the appearance of more significance in external
    identifiers, read the excellently-worded section 3.1.2 in the
    X3.159 Rationale (see question 5.1), which discusses several
    such schemes and explains why they could not be mandated.

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

5.15:    What is the difference between memcpy and memmove?

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

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

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

A:    Perhaps it is a pre-ANSI compiler, unable to accept function
    prototypes and the like.  See also questions 5.17 and 17.2.

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

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

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

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

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

A:    The compiler doesn't know the size of the pointed-to objects.
    Before performing arithmetic, cast the pointer either to char *
    or to the type you're trying to manipulate (but see question
    2.18).

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

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

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

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

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

    References: ANSI Sec. 3.8.6 .

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

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

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

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

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

    References: ANSI Sec. 1.6, especially the Rationale.


Section 6. C Preprocessor

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

A:    There is no good answer to this question.  If the values are
    integers, a well-known trick using exclusive-OR could perhaps be
    used, but it will not work for floating-point values or
    pointers, or if the two values are the same variable (and the
    "obvious" supercompressed implementation for integral types
    a^=b^=a^=b is in fact illegal due to multiple side-effects; see
    questions 4.1 and 4.2).  If the macro is intended to be used on
    values of arbitrary type (the usual goal), it cannot use a
    temporary, since it does not know what type of temporary it
    needs, and standard C does not provide a typeof operator.

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

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

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

    but it doesn't work any more.

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

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

    (See also question 5.4.)

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

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

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

    The traditional solution is to use

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

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

    If all of the statements in the intended macro are simple
    expressions, with no declarations or loops, another technique is
    to write a single, parenthesized expression using one or more
    comma operators.  (See the example under question 6.10 below.
    This technique also allows a value to be "returned.")

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

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

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

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

    makes a header file "idempotent" so that it can safely be
    #included multiple times; and automated Makefile maintenance
    tools (which are a virtual necessity in large projects anyway)
    handle dependency generation in the face of nested #include
    files easily.  See also section 14.

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

A:    No.  Preprocessing happens during an earlier pass of
    compilation, before type names have been parsed.  Consider using
    the predefined constants in ANSI's <limits.h>, if applicable, or
    a "configure" script, instead.  (Better yet, try to write code
    which is inherently insensitive to type sizes.)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    The obvious disadvantage is that the caller must always remember
    to use the extra parentheses.  Other solutions are to use
    different macros (DEBUG1, DEBUG2, etc.) depending on the number
    of arguments, or to play games with commas:

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

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


Section 7. Variable-Length Argument Lists

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

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

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

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

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

            if(first == NULL)
                return NULL;

            len = strlen(first);

            va_start(argp, first);

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

            va_end(argp);

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

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

            (void)strcpy(retbuf, first);

            va_start(argp, first);

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

            va_end(argp);

            return retbuf;
        }

    Usage is something like

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

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

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

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

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

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

A:    Use vprintf, vfprintf, or vsprintf.

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

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

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

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

        void error(va_alist)
        va_dcl
        {
            char *fmt;

    change the va_start line to

        va_start(argp);

    and add the line

        fmt = va_arg(argp, char *);

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

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

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

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

    Any function which takes a variable number of arguments must be
    able to determine from the arguments themselves how many of them
    there are.  printf-like functions do this by looking for
    formatting specifiers (%d and the like) in the format string
    (which is why these functions fail badly if the format string
    does not match the argument list).  Another common technique
    (useful when the arguments are all of the same type) is to use a
    sentinel value (often 0, -1, or an appropriately-cast null
    pointer) at the end of the list (see the execl and vstrcat
    examples under questions 1.2 and 7.1 above).

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

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

    References: ANSI Sec. 4.8.1.2 p. 124.

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

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

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

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


Section 8. Boolean Expressions and Variables

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

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

    The choice between #defines and enums is arbitrary and not
    terribly interesting (see also question 9.1).  Use any of

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

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

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

    Some people prefer variants like

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

    or define "helper" macros such as

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

    These don't buy anything (see question 8.2 below; see also
    question 1.6).

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

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

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

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

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

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


Section 9. Structs, Enums, and Unions

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

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

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

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

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

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

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

9.3:    How does struct passing and returning work?

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

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

    References: ANSI Sec. 2.2.3 p. 13.

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

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

        /* Here is the main program. */

        main(argc, argv)
        ...

A:    A missing semicolon causes the compiler to believe that main
    returns a structure.  (The connection is hard to see because of
    the intervening comment.)  Since struct-valued functions are
    usually implemented by adding a hidden return pointer, the
    generated code for main() tries to accept three arguments,
    although only two are passed (in this case, by the C start-up
    code).  See also question 17.21.

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

9.5:    Why can't you compare structs?

A:    There is no reasonable way for a compiler to implement struct
    comparison which is consistent with C's low-level flavor.  A
    byte-by-byte comparison could be invalidated by random bits
    present in unused "holes" in the structure (such padding is used
    to keep the alignment of later fields correct; see questions
    9.10 and 9.11).  A field-by-field comparison would require
    unacceptable amounts of repetitive, in-line code for large
    structures.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    See the next question for a usage hint.

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

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

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

        offsetb = offsetof(struct a, b)

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

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

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

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

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

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

9.12:    Can I initialize unions?

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

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

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


Section 10. Declarations

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

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

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

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

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

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

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

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

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

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

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

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

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

        typedef struct node *NODEPTR;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    References: K&R I Sec. 4.5 pp. 76-7; K&R II Sec. 4.4 pp. 80-1;
    ANSI Sec. 3.1.2.2 (esp. Rationale), Secs. 3.7, 3.7.2,
    Sec. F.5.11; H&S Sec. 4.8 pp. 79-80; CT&P Sec. 4.2 pp. 54-56.

10.8:    What does extern mean in a function declaration?

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

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

    References: ANSI Sec. 3.1.2.2 .

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

A:    Use something like

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

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

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

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

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

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

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

        r = fp();

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

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

10.11:    What's the auto keyword good for?

A:    Nothing; it's obsolete.


Section 11. Stdio

11.1:    What's wrong with this code:

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

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

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

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

A:    Simply double the percent sign: %% .

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

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

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

11.4:    Why doesn't this code:

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

    work?

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

11.5:    Why won't the code

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

    work?

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

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

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

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

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

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

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

A:    It is best to use an explicit fflush(stdout) whenever output
    should definitely be visible.  Several mechanisms attempt to
    perform the fflush for you, at the "right time," but they tend
    to apply only when stdout is a terminal.  (See question 11.7.)

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

A:    scanf was designed for free-format input, which is seldom what
    you want when reading from the keyboard.  In particular, "\n" in
    a format string does _not_ mean to expect a newline, but rather
    to read and discard characters as long as each is a whitespace
    character.

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

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

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

    References: ANSI Sec. 4.9.5.3 p. 131.

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

A:    See question 16.1.

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

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

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

A:    Use freopen.

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

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

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

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


Section 12. Library Subroutines

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

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

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

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

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

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

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

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

A:    The conversions must be in the comparison function, which must
    be declared as accepting "generic pointers" (const void * or
    char *) as discussed in question 12.2 above.  The code might
    look like

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

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

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

A:    Just use sprintf.  (You'll have to allocate space for the result
    somewhere anyway; see questions 3.1 and 3.2.  Don't worry that
    sprintf may be overkill, potentially wasting run time or code
    space; it works well in practice.)

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

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

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

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

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

    References: ANSI Sec. 4.12 .

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

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

    Converting a string to a time_t is harder, because of the wide
    variety of date and time formats which should be parsed.  Some
    systems provide a strptime function; another popular routine is
    partime (widely distributed with the RCS package), but these are
    less likely to become standardized.

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

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

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

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

12.8:    I need a random number generator.

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

    References: ANSI Sec. 4.10.2.1 p. 154; Knuth Vol. 2 Chap. 3
    pp. 1-177.

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

A:    The obvious way,

        rand() % N

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

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

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

        rand() / (RAND_MAX / N + 1)

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

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

A:    You can call srand() to seed the pseudo-random number generator
    with a more random initial value.  Popular seed values are the
    time of day, or the elapsed time before the user presses a key
    (although keypress times are hard to determine portably; see
    question 16.10).

    References: ANSI Sec. 4.10.2.2 p. 154.

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

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

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

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

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

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

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

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

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

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

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

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

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


Section 13. Lint

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

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

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

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

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

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

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

    The System V release 4 lint is ANSI-compatible, and is available
    separately (bundled with other C tools) from UNIX Support Labs
    or from System V resellers.

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

Section 14. Style

14.1:    Here's a neat trick:

        if(!strcmp(s1, s2))

    Is this good style?

A:    It is not particularly good style, although it is a popular
    idiom.  The test succeeds if the two strings are equal, but its
    form suggests that it tests for inequality.

    Another solution is to use a macro:

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

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

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

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

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

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

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

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

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

A:    Various documents are available for anonymous ftp from:

        Site:            File or directory:

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

        cs.toronto.edu        doc/programming

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


Section 15. Floating Point

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

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

    If the problem isn't that simple, recall that most digital
    computers use floating-point formats which provide a close but
    by no means exact simulation of real number arithmetic.
    Underflow, cumulative precision loss, and other anomalies are
    often troublesome.

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

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

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

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

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

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

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

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

    References: ANSI Sec. 4.5.5.1 .

15.4:    How do I round numbers?

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

        (int)(x + 0.5)

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

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

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

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

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

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

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


Section 16. System Dependencies

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

A:    Contrary to popular belief and many people's wishes, this is not
    a C-related question.  (Nor are closely-related questions
    concerning the echo of keyboard input.)  The delivery of
    characters from a "keyboard" to a C program is a function of the
    operating system in use, and has not been standardized by the C
    language.  Some versions of curses have a cbreak() function
    which does what you want.  If you're specifically trying to read
    a short password without echo, you might try getpass().  Under
    Unix, use ioctl to play with the terminal driver modes (CBREAK
    or RAW under "classic" versions; ICANON, c_cc[VMIN] and
    c_cc[VTIME] under System V or Posix systems).  Under MS-DOS, use
    getch().  Under VMS, try the Screen Management (SMG$) routines,
    or curses, or issue low-level $QIO's with the IO$_READVBLK (and
    perhaps IO$M_NOECHO) function codes to ask for one character at
    a time.  Under other operating systems, you're on your own.
    Beware that some operating systems make this sort of thing
    impossible, because character collection into input lines is
    done by peripheral processors not under direct control of the
    CPU running your program.

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

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

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

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

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

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

16.4:    How do I read the mouse?

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

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

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

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

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

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

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

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

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

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

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

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

A:    BSD systems provide ftruncate(), several others supply chsize(),
    and a few may provide a (possibly undocumented) fcntl option
    F_FREESP.  Under MS-DOS, you can sometimes use write(fd, "", 0).
    However, there is no truly portable solution.

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

A:    Unfortunately, there is no portable way.  V7 Unix, and derived
    systems, provided a fairly useful ftime() routine with
    resolution up to a millisecond, but it has disappeared from
    System V and Posix.  Other routines you might look for on your
    system include nap(), setitimer(), msleep(), usleep(), clock(),
    and gettimeofday().  The select() and poll() calls (if
    available) can be pressed into service to implement simple
    delays.  On MS-DOS machines, it is possible to reprogram the
    system timer and timer interrupts.

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

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

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

A:    Use system().

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

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

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

    References: PCS Sec. 11 p. 169 .

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

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

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

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


Section 17. Miscellaneous

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

A:    Variables with "static" duration (that is, those declared
    outside of functions, and those declared with the storage class
    static), are guaranteed initialized (just once, at program
    startup) to zero, as if the programmer had typed "= 0".
    Therefore, such variables are initialized to the null pointer
    (of the correct type; see also Section 1) if they are pointers,
    and to 0.0 if they are floating-point.

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

    Dynamically-allocated memory obtained with malloc and realloc is
    also likely to contain garbage, and must be initialized by the
    calling program, as appropriate.  Memory obtained with calloc
    contains all-bits-0, but this is not necessarily useful for
    pointer or floating-point values (see question 3.13, and section
    1).

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

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

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

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

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

    If you must use a binary format, you can improve portability,
    and perhaps take advantage of prewritten I/O libraries, by
    making use of standardized formats such as Sun's XDR (RFC 1014),
    OSI's ASN.1, CCITT's X.409, or ISO 8825 "Basic Encoding Rules."
    See also question 9.11.

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

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

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

A:    Either pass pointers to locations which the function can fill
    in, or have the function return a structure containing the
    desired values, or (in a pinch) consider global variables.  See
    also questions 2.17, 3.4, and 9.2.

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

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

        int function1(), function2();

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

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

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

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

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

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

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

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

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

A:    Several public-domain programs are available:

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

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

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

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

    See also question 5.3.

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

A:    C++ was derived from C, and is largely based on it, but there
    are some legal C constructs which are not legal C++.  (Many C
    programs will nevertheless compile correctly in a C++
    environment.)

17.11:    I need:                A:  Look for programs (see also
                        question 17.12) named:

    a C cross-reference            cflow, calls, cscope
    generator

    a C beautifier/pretty-            cb, indent
    printer

17.12:    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.

    Ajay Shah maintains an index of free numerical software; it is
    posted periodically, and available where this FAQ list is
    archived (see question 17.33).  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.

17.13:    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 guidelines, send e-mail
    with the Subject: line "send rules" to:

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

    (Note that these are _not_ the addresses for submitting
    entries.)

    Contest winners are first announced at the Summer Usenix
    Conference in mid-June, and posted to the net sometime in July-
    August.  Winning entries from previous years (to 1984) are
    archived at uunet (see question 17.12) under the directory
    ~/pub/ioccc.

    As a last resort, previous winners may be obtained by sending
    e-mail to the above address, using the Subject: "send YEAR
    winners", where YEAR is a single four-digit year, a year range,
    or "all".

17.14:    Why don't C comments nest?  How am I supposed to comment out
    code containing comments?  Are comments 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
    5.11).

    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.

    References: ANSI Appendix E p. 198, Rationale Sec. 3.1.9 p. 33.

17.15:    How can I get the ASCII value corresponding to a character, or
    vice versa?

A:    In C, characters are represented by small integers corresponding
    to their values (in the machine's character set) so you don't
    need a conversion routine: if you have the character, you have
    its value.

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

A:    Use arrays of char or int, with a few macros to access the right
    bit at the right index (try using 8 for CHAR_BIT if you don't
    have <limits.h>):

        #include <limits.h>        /* for CHAR_BIT */

        #define BITMASK(bit) (1 << ((bit) % CHAR_BIT))
        #define BITSLOT(bit) ((bit) / CHAR_BIT)
        #define BITSET(ary, bit) ((ary)[BITSLOT(bit)] |= BITMASK(bit))
        #define BITTEST(ary, bit) ((ary)[BITSLOT(bit)] & BITMASK(bit))

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

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

17.18:    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 caching 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.

17.19:    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.  Any decent 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 4.7.)

17.20:    Why does this code:

        char *p = "Hello, world!";
        p[0] = tolower(p[0]);

    crash?

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

        char a[] = "Hello, world!";

    (For compiling old code, some compilers have a switch
    controlling whether strings are writable or not.)  See also
    questions 2.1, 2.2, 2.8, and 17.2.

    References: ANSI Sec. 3.1.4 .

17.21:    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 9.4.)

17.22:    What do "Segmentation violation" and "Bus error" mean?

A:    These generally mean that your program tried to access memory it
    shouldn't have, invariably as a result of improper pointer use,
    often involving uninitialized or improperly allocated pointers
    (see questions 3.1 and 3.2), or malloc (see question 17.23), or
    perhaps scanf (see question 11.3).

17.23:    My program is crashing, apparently somewhere down inside malloc,
    but I can't see anything wrong with it.

A:    It is unfortunately very easy to corrupt malloc's internal data
    structures, and the resulting problems can be hard to track
    down.  The most common source of problems is writing more to a
    malloc'ed region than it was allocated to hold; a particularly
    common bug is to malloc(strlen(s)) instead of strlen(s) + 1.
    Other problems involve freeing pointers not obtained from
    malloc, or trying to realloc a null pointer (see question 3.12).

    A number of debugging packages exist to help track down malloc
    problems; one popular one is Conor P. Cahill's "dbmalloc,"
    posted to comp.sources.misc in September of 1992.  Others are
    "leak," available in volume 27 of the comp.sources.unix
    archives; JMalloc.c and JMalloc.h in Fidonet's C_ECHO Snippets
    (or ask archie; see question 17.12); and MEMDEBUG from
    ftp.crpht.lu in pub/sources/memdebug .  See also question 17.12.

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

A:    Plum Hall (formerly in Cardiff, NJ; now in Hawaii) sells one.
    The FSF's GNU C (gcc) distribution includes a c-torture-
    test.tar.Z which checks a number of common problems with
    compilers.  Kahan's paranoia test, found in netlib/paranoia on
    netlib.att.com, strenuously tests a C implementation's floating
    point capabilities.

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

A:    The definitive grammar is of course the one in the ANSI
    standard.  Another grammar, by Jim Roskind, is in pub/*grammar*
    at ics.uci.edu .  A fleshed-out, working instance of the ANSI
    grammar (due to Jeff Lee) is on uunet (see question 17.12) 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 .

17.26:    I need code to parse and evaluate expressions.

A:    Two available packages are "defunc," posted to comp.source.misc
    in December, 1993 (V41 i32,33), to alt.sources in January, 1994,
    and available from sunsite.unc.edu in
    pub/packages/development/libraries/defunc-1.3.tar.Z; and
    "parse," at lamont.ldgo.columbia.edu.

17.27:    I need a sort of an "approximate" strcmp routine, for comparing
    two strings for close, but not necessarily exact, equality.

A:    The traditional routine for doing this sort of thing involves
    the "soundex" algorithm, which maps similar-sounding words to
    the same numeric codes.  Soundex is described in the Searching
    and Sorting volume of Donald Knuth's classic _The Art of
    Computer Programming_.

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

A:    Use mktime (see questions 12.6 and 12.7), or Zeller's
    congruence, or see the sci.math FAQ list, or try this code
    posted by Tomohiko Sakamoto:

        dayofweek(y, m, d)    /* 0 = Sunday */
        int y, m, d;        /* 1 <= m <= 12,  y > 1752 or so */
        {
            static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
            y -= m < 3;
            return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
        }

17.29:    Will 2000 be a leap year?  Is (year % 4 == 0) an accurate test
    for leap years?

A:    Yes and no, respectively.  The full expression for the Gregorian
    calendar is

        year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)

    See a good astronomical almanac or other reference for details.

17.30:    How do you pronounce "char"?

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.

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

A:    Mitch Wright maintains an annotated bibliography of C and Unix
    books; it is available for anonymous ftp from ftp.rahul.net in
    directory pub/mitch/YABL.

    This FAQ list's editor maintains a collection of previous
    answers to this question, which is available upon request.

17.32:    Are there any C tutorials on the net?

A:    There are at least two of them:

    "Notes for C programmers," by Christopher Sawtell,
    available from:
    svr-ftp.eng.cam.ac.uk:misc/sawtell_C.shar
    garbo.uwasa.fi:/pc/c-lang/c-lesson.zip
    paris7.jussieu.fr:/contributions/docs

    Tim Love's "C for Programmers,"
    available from svr-ftp.eng.cam.ac.uk in the misc directory.

17.33:    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 Expires: line
    which should keep it around all month.  An abridged version is
    also available (and posted), as is a list of changes
    accompanying each significantly updated version.  These lists
    can also be found in the newsgroups comp.answers and
    news.answers .  Several sites archive news.answers postings and
    other FAQ lists, including this one; two sites are rtfm.mit.edu
    (directories pub/usenet/news.answers/C-faq/ and
    pub/usenet/comp.lang.c/ ) and ftp.uu.net (directory
    usenet/news.answers/C-faq/ ).  The archie server should help you
    find others; query it for "prog C-faq".  See the meta-FAQ list
    in news.answers for more information; see also question 17.12.

    This list is an evolving document of questions which have been
    Frequent since before the Great Renaming, 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 5.2).

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.

Knuth    Donald E. Knuth, The Art of Computer Programming, (3 vols.),
    Addison-Wesley, 1981.

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.

    Harry Rabinowitz and Chaim Schaap, Portable C, Prentice-Hall,
    1990, ISBN 0-13-685967-4.

There is a more extensive bibliography in the revised Indian Hill style
guide (see question 14.3).  See also question 17.31.


Acknowledgements

Thanks to Jamshid Afshar, Sudheer Apte, Randall Atkinson, Dan Bernstein,
Vincent Broman, Stan Brown, Joe Buehler, Gordon Burditt, Burkhard Burow,
Conor P. Cahill, D'Arcy J.M. Cain, Christopher Calabrese, Ian Cargill,
Paul Carter, Billy Chambless, Raymond Chen, Jonathan Coxhead, Lee
Crawford, Steve Dahmer, Andrew Daviel, James Davies, Jutta Degener, Norm
Diamond, Jeff Dunlop, Ray Dunn, Stephen M. Dunn, Michael J. Eager, Dave
Eisen, Bjorn Engsig, Chris Flatters, Rod Flores, Alexander Forst, Jeff
Francis, Dave Gillespie, Samuel Goldstein, Alasdair Grant, Ron
Guilmette, Doug Gwyn, Tony Hansen, Joe Harrington, Guy Harris, Elliotte
Rusty Harold, Jos Horsmeier, Blair Houghton, Ke Jin, Kirk Johnson, Larry
Jones, Kin-ichi Kitano, Peter Klausler, Andrew Koenig, Tom Koenig, Ajoy
Krishnan T, Markus Kuhn, John Lauro, Felix Lee, Mike Lee, Timothy J.
Lee, Tony Lee, Don Libes, Christopher Lott, Tim Love, Tim McDaniel,
Stuart MacMartin, John R. MacMillan, Bob Makowski, Evan Manning, Barry
Margolin, George Matas, Brad Mears, Bill Mitchell, Mark Moraes, Darren
Morby, Ken Nakata, Landon Curt Noll, David O'Brien, Richard A. O'Keefe,
Hans Olsson, Philip ([e-mail]), Andrew Phillips,
Christopher Phillips, Francois Pinard, Dan Pop, Kevin D. Quitt, Pat
Rankin, J. M. Rosenstock, Erkki Ruohtula, Tomohiko Sakamoto, Rich Salz,
Chip Salzenberg, Paul Sand, DaviD W. Sanderson, Christopher Sawtell,
Paul Schlyter, Doug Schmidt, Rene Schmit, Russell Schulz, Patricia
Shanahan, Peter da Silva, Joshua Simons, Henry Spencer, David Spuler,
Melanie Summit, Erik Talvola, Clarke Thatcher, Wayne Throop, Chris
Torek, Andrew Tucker, Goran Uddeborg, Rodrigo Vanegas, Jim Van Zandt,
Wietse Venema, Ed Vielmetti, Larry Virden, Chris Volpe, Mark Warren,
Larry Weiss, Freek Wiedijk, Lars Wirzenius, Dave Wolverton, Mitch
Wright, Conway Yee, and Zhuo Zang, 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]

This article is Copyright 1988, 1990-1994 by Steve Summit.
It may be freely redistributed so long as the author's name, and this
notice, are retained.
The C code in this article (vstrcat(), error(), etc.) is public domain
and may be used without restriction.