Internet Surfer 2.0/Internet Surfer 2.0 (Wayzata Technology) (1996).iso/pc/text/mac/faqs.223

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Frequently Asked Questions (FAQS);faqs.223



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.)

7.5:    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 16.10.)


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 as the return value
    from a Boolean function, never in a comparison.

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

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


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.)

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

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

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.

    Reference: 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 17.15.

    Reference: 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).  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:    I came across some code that declared a structure like this:

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

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

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

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

9.7:    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.

9.8:    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;

9.9:    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.

9.10:    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 17.2.

9.11:    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).


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 17.2), be sure to encapsulate the choice behind
    an appropriate typedef.

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

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

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

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

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

        typedef struct node *NODEPTR;

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

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

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

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

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

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

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

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

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

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

        cdecl can also explain complicated declarations, help with
        casts, and indicate which set of parentheses the arguments
        go in (for complicated function definitions, like the
        above).  Versions of cdecl are in volume 14 of
        comp.sources.unix (see question 17.8) and K&R II.

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

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

10.4:    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.

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

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

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

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

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

10.6:    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 "(").

10.7:    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.


Section 11. Stdio

11.1:    Why doesn't this code:

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

    work?

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 and so never seen.

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

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

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

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

11.3:    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 11.2.)

11.4:    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.

    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.

11.5:    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.

11.6:    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.

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

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

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

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

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

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

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

A:    Just use sprintf.  (You'll have to allocate space for the result
    somewhere anyway; see questions 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.

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

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

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

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

    References: ANSI Sec. 4.12 .

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

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

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

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

12.6:    I need a random number generator.

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

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

12.7:    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 random initial seeds
    are the time of day, or the elapsed time before the user presses
    a key.

    References: ANSI Sec. 4.10.2.2 p. 154.

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

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

12.9-13: I'm trying to port this old    A:  These routines are variously
    program.  Why do I get            obsolete; you should
    "undefined external" errors        instead:
    for:

12.9:    index?                A:  use strchr.
12.10:    rindex?                A:  use strrchr.
12.11:    bcopy?                A:  use memmove, after
                        interchanging the first and
                        second arguments (see also
                        question 5.10).
12.12:    bcmp?                A:  use memcmp.
12.13:    bzero?                A:  use memset, with a second
                        argument of 0.

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

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

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

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