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From: [e-mail] (Steve Summit)
Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List)
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[Last modified April 3, 1996 by scs.]

This article is Copyright 1990-1996 by Steve Summit.  Content from the
book _C Programming FAQs: Frequently Asked Questions_ is made available
here by permission of the author and the publisher as a service to the
community.  It is intended to complement the use of the published text
and is protected by international copyright laws.  The content is made
available here and may be accessed freely for personal use but may not
be republished without permission.

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; see also question 18.10.  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 11.1).

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 was last modified on April 3, 1996, and its travels
may have taken it far from its original home on Usenet.  It may now
be out-of-date, particularly if you are looking at a printed copy or one
retrieved from a tertiary archive site or CD-ROM.  You can always obtain
the most up-to-date copy by anonymous ftp from sites ftp.eskimo.com,
rtfm.mit.edu, or ftp.uu.net (see questions 18.16 and 20.40), or by
sending the e-mail message "help" to [e-mail] .  Since
this list is modified from time to time, its question numbers may not
match those in older or newer copies which are in circulation; be
careful when referring to FAQ list entries by number alone.

This article was produced for free redistribution.  You should not need
to pay anyone for a copy of it.

Other versions of this document are also available.  Posted along with
it are an abridged version and (when there are changes) a list of
differences with respect to the previous version.  A
hypertext version is available on the world-wide web (WWW); see URL
http://www.eskimo.com/~scs/C-faq/top.html .  Finally, for those who
might prefer a bound, hardcopy version (and even longer answers to even
more questions!), a book-length version has been published by Addison-
Wesley (ISBN 0-201-84519-9).

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. Declarations and Initializations
     2. Structures, Unions, and Enumerations
     3. Expressions
     4. Pointers
     5. Null Pointers
     6. Arrays and Pointers
     7. Memory Allocation
     8. Characters and Strings
     9. Boolean Expressions and Variables
    10. C Preprocessor
    11. ANSI/ISO Standard C
    12. Stdio
    13. Library Functions
    14. Floating Point
    15. Variable-Length Argument Lists
    16. Strange Problems
    17. Style
    18. Tools and Resources
    19. System Dependencies
    20. Miscellaneous

(The question numbers within each section are not continuous because
they are aligned with the forthcoming book-length version, which
contains even more questions.)

Herewith, some frequently-asked questions and their answers:


Section 1. Declarations and Initializations

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

A:    If you might need large values (above 32,767 or below -32,767),
    use long.  Otherwise, if space is very important (i.e. if there
    are large arrays or many structures), use short.  Otherwise, use
    int.  If well-defined overflow characteristics are important and
    negative values are not, or if you want to steer clear of sign-
    extension problems when manipulating bits or bytes, use one of
    the corresponding unsigned types.  (Beware when mixing signed
    and unsigned values in expressions, though.)

    Although character types (especially unsigned char) can be used
    as "tiny" integers, doing so is sometimes more trouble than it's
    worth, due to unpredictable sign extension and increased code
    size.  (Using unsigned char can help; see question 12.1 for a
    related problem.)

    A similar space/time tradeoff applies when deciding between
    float and double.  None of the above rules 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 20.5), be sure to encapsulate the choice behind
    an appropriate typedef.

    References: K&R1 Sec. 2.2 p. 34; K&R2 Sec. 2.2 p. 36, Sec. A4.2
    pp. 195-6, Sec. B11 p. 257; ANSI Sec. 2.2.4.2.1, Sec. 3.1.2.5;
    ISO Sec. 5.2.4.2.1, Sec. 6.1.2.5; H&S Secs. 5.1,5.2 pp. 110-114.

1.4:    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 is written
    to assume that ints and longs are the same size, or that one or
    the other of them is exactly 32 bits, and introduce a new,
    nonstandard, 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 does not have), and not "exactly
    64 bits."

    References: ANSI Sec. F.5.6; ISO Sec. G.5.6.

1.7:    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.)
    The best arrangement is to place each definition in some
    relevant .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 same header file, so that the compiler can check
    that the definition matches the declarations.

    This rule promotes a high degree of portability: it is
    consistent with the requirements of the ANSI C Standard, and is
    also consistent with most pre-ANSI compilers and linkers.  (Unix
    compilers and linkers typically use a "common model" which
    allows multiple definitions, as long as at most one is
    initialized; this behavior is mentioned as a "common extension"
    by the ANSI Standard, no pun intended.  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 a line
    like

        DEFINE(int, i);

    need only be entered once in one header file, and turned into a
    definition or a declaration depending on the setting of some
    macro, but it's not clear if this is worth the trouble.

    It's especially important to put global declarations in header
    files if you want the compiler to catch inconsistent
    declarations for you.  In particular, never place a prototype
    for an external function in a .c file: it wouldn't generally be
    checked for consistency with the definition, and an incompatible
    prototype is worse than useless.

    See also questions 10.6 and 18.8.

    References: K&R1 Sec. 4.5 pp. 76-7; K&R2 Sec. 4.4 pp. 80-1; ANSI
    Sec. 3.1.2.2, Sec. 3.7, Sec. 3.7.2, Sec. F.5.11; ISO
    Sec. 6.1.2.2, Sec. 6.7, Sec. 6.7.2, Sec. G.5.11; Rationale
    Sec. 3.1.2.2; H&S Sec. 4.8 pp. 101-104, Sec. 9.2.3 p. 267; CT&P
    Sec. 4.2 pp. 54-56.

1.11:    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, Sec. 3.5.1; ISO Sec. 6.1.2.2,
    Sec. 6.5.1; Rationale Sec. 3.1.2.2; H&S Secs. 4.3,4.3.1 pp. 75-
    6.

1.12:    What's the auto keyword good for?

A:    Nothing; it's archaic.  See also question 20.37.

    References: K&R1 Sec. A8.1 p. 193; ANSI Sec. 3.1.2.4,
    Sec. 3.5.1; ISO Sec. 6.1.2.4, Sec. 6.5.1; H&S Sec. 4.3 p. 75,
    Sec. 4.3.1 p. 76.

1.14:    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 structure in C
    contain a pointer to itself?

A:    Structures 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 the NODEPTR example is that the typedef
    has not been defined at the point where the "next" field is
    declared.  To fix this code, first give the structure a tag
    ("struct node").  Then, declare the "next" field as a simple
    "struct node *", or disentangle the typedef declaration from the
    structure definition, or both.  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.

    See also question 2.1.

    References: K&R1 Sec. 6.5 p. 101; K&R2 Sec. 6.5 p. 139; ANSI
    Sec. 3.5.2, Sec. 3.5.2.3, esp. examples; ISO Sec. 6.5.2,
    Sec. 6.5.2.3; H&S Sec. 5.6.1 pp. 132-3.

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

A:    The first part of this question can be answered in at least
    three ways:

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

    2.  Build the declaration up incrementally, 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 one
        above).  Versions of cdecl are in volume 14 of
        comp.sources.unix (see question 18.16) and K&R2.

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

    The pointer-to-function declarations in the examples above have
    not included parameter type information.  When the parameters
    have complicated types, declarations can *really* get messy.
    (Modern versions of cdecl can help here, too.)

    References: K&R2 Sec. 5.12 p. 122; ANSI Sec. 3.5ff (esp.
    Sec. 3.5.4); ISO Sec. 6.5ff (esp. Sec. 6.5.4); H&S Sec. 4.5 pp.
    85-92, Sec. 5.10.1 pp. 149-50.

1.22:    How can I declare a function that can return a pointer to a
    function of the same type?  I'm building a state machine with
    one function for each state, each of which returns a pointer to
    the function for the next state.  But I can't find a way to
    declare the functions.

A:    You can't quite do it directly.  Either have the function return
    a generic function pointer, with some judicious casts to adjust
    the types as the pointers are passed around; or have it return a
    structure containing only a pointer to a function returning that
    structure.

1.25:    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
    (perhaps because the first call precedes the function's
    definition) are assumed to be declared as returning int (and
    without any argument type information), leading to discrepancies
    if the function is later declared or defined otherwise.  Non-int
    functions must be declared before they are called.

    Another possible source of this problem is that the function has
    the same name as another one declared in some header file.

    See also questions 11.3 and 15.1.

    References: K&R1 Sec. 4.2 p. 70; K&R2 Sec. 4.2 p. 72; ANSI
    Sec. 3.3.2.2; ISO Sec. 6.3.2.2; H&S Sec. 4.7 p. 101.

1.30:    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 5) 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() is all-bits-0, but this is not
    necessarily useful for pointer or floating-point values (see
    question 7.31, and section 5).

    References: K&R1 Sec. 4.9 pp. 82-4; K&R2 Sec. 4.9 pp. 85-86;
    ANSI Sec. 3.5.7, Sec. 4.10.3.1, Sec. 4.10.5.3; ISO Sec. 6.5.7,
    Sec. 7.10.3.1, Sec. 7.10.5.3; H&S Sec. 4.2.8 pp. 72-3, Sec. 4.6
    pp. 92-3, Sec. 4.6.2 pp. 94-5, Sec. 4.6.3 p. 96, Sec. 16.1 p.
    386.

1.31:    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, structures, and unions).  As a workaround, you can make
    the array global or static (if you won't need a fresh copy
    during any subsequent calls), or replace it with a pointer (if
    the array won't be written to).  (You can always initialize
    local char * variables to point to string literals, but see
    question 1.32 below.)  If neither of these conditions hold,
    you'll have to initialize the array by hand with strcpy() when
    f() is called.  See also question 11.29.

1.32:    What is the difference between these initializations?

        char a[] = "string literal";
        char *p  = "string literal";

    My program crashes if I try to assign a new value to p[i].

A:    A string literal can be used in two slightly different ways.  As
    an array initializer (as in the declaration of char a[]), it
    specifies the initial values of the characters in that array.
    Anywhere else, it turns into an unnamed, static array of
    characters, which may be stored in read-only memory, which is
    why you can't safely modify it.  In an expression context, the
    array is converted at once to a pointer, as usual (see section
    6), so the second declaration initializes p to point to the
    unnamed array's first element.

    (For compiling old code, some compilers have a switch
    controlling whether strings are writable or not.)

    See also questions 1.31, 6.1, 6.2, and 6.8.

    References: K&R2 Sec. 5.5 p. 104; ANSI Sec. 3.1.4, Sec. 3.5.7;
    ISO Sec. 6.1.4, Sec. 6.5.7; Rationale Sec. 3.1.4; H&S Sec. 2.7.4
    pp. 31-2.

1.34:    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 like this,
    it "decays" into a pointer (that is, it has its address
    implicitly taken), much as an array name does.

    An explicit declaration for the function is normally needed,
    since implicit external function declaration does not happen in
    this case (because the function name in the initialization is
    not part of a function call).

    See also question 4.12.


Section 2. Structures, Unions, and Enumerations

2.1:    What's the difference between these two declarations?

        struct x1 { ... };
        typedef struct { ... } x2;

A:    The first form declares a "structure tag"; the second declares a
    "typedef".  The main difference is that the second declaration
    is of a slightly more abstract type -- its users don't
    necessarily know that it is a structure, and the keyword struct
    is not used when declaring instances of it.

2.2:    Why doesn't

        struct x { ... };
        x thestruct;

    work?

A:    C is not C++.  Typedef names are not automatically generated for
    structure tags.  See also question 2.1 above.

2.3:    Can a structure contain a pointer to itself?

A:    Most certainly.  See question 1.14.

2.4:    What's the best way of implementing opaque (abstract) data types
    in C?

A:    One good way is for clients to use structure pointers (perhaps
    additionally hidden behind typedefs) which point to structure
    types which are not publicly defined.

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

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

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

A:    This technique is popular, although Dennis Ritchie has called it
    "unwarranted chumminess with the C implementation."  An official
    interpretation has deemed that it is not strictly conforming
    with the C Standard.  (A thorough treatment of the arguments
    surrounding the legality of the technique is beyond the scope of
    this list.)  It does seem to be portable to all known
    implementations.  (Compilers which check array bounds carefully
    might issue warnings.)

    Another possibility is to declare the variable-size element very
    large, rather than very small; in the case of the above example:

        ...
        char namestr[MAXSIZE];
        ...

    where MAXSIZE is larger than any name which will be stored.
    However, it looks like this technique is disallowed by a strict
    interpretation of the Standard as well.

    References: Rationale Sec. 3.5.4.2.

2.7:    I heard that structures could be assigned to variables and
    passed to and from functions, but K&R1 says not.

A:    What K&R1 said was that the restrictions on structure operations
    would be lifted in a forthcoming version of the compiler, and in
    fact structure assignment and passing were fully functional in
    Ritchie's compiler even as K&R1 was being published.  Although a
    few early C compilers lacked these operations, all modern
    compilers support them, and they are part of the ANSI C
    standard, so there should be no reluctance to use them.

    (Note that when a structure is assigned, passed, or returned,
    the copying is done monolithically; anything pointed to by any
    pointer fields is *not* copied.)

    References: K&R1 Sec. 6.2 p. 121; K&R2 Sec. 6.2 p. 129; ANSI
    Sec. 3.1.2.5, Sec. 3.2.2.1, Sec. 3.3.16; ISO Sec. 6.1.2.5,
    Sec. 6.2.2.1, Sec. 6.3.16; H&S Sec. 5.6.2 p. 133.

2.8:    Why can't you compare structures?

A:    There is no single, good way for a compiler to implement
    structure comparison which is consistent with C's low-level
    flavor.  A simple byte-by-byte comparison could founder on
    random bits present in unused "holes" in the structure (such
    padding is used to keep the alignment of later fields correct;
    see question 2.12).  A field-by-field comparison might require
    unacceptable amounts of repetitive code for large structures.

    If you need to compare two structures, you'll have to write your
    own function to do so, field by field.

    References: K&R2 Sec. 6.2 p. 129; ANSI Sec. 4.11.4.1 footnote
    136; Rationale Sec. 3.3.9; H&S Sec. 5.6.2 p. 133.

2.9:    How are structure passing and returning implemented?

A:    When structures are passed as arguments to functions, the entire
    structure 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.)  Some
    compilers merely pass a pointer to the structure, though they
    may have to make a local copy to preserve pass-by-value
    semantics.

    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 structure-
    valued functions non-reentrant, which ANSI C disallows.

    References: ANSI Sec. 2.2.3; ISO Sec. 5.2.3.

2.10:    How can I pass constant values to functions which accept
    structure arguments?

A:    C has no way of generating anonymous structure values.  You will
    have to use a temporary structure variable or a little structure-
    building function.  (gcc provides structure constants as an
    extension, and the mechanism will probably be added to a future
    revision of the C Standard.)  See also question 4.10.

2.11:    How can I read/write structures from/to data files?

A:    It is relatively straightforward to write a structure out using
    fwrite():

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

    and a corresponding fread invocation can read it back in.
    (Under pre-ANSI C, a (char *) cast on the first argument is
    required.  What's important is that fwrite() receive a byte
    pointer, not a structure pointer.)  However, data files so
    written will *not* be portable (see questions 2.12 and 20.5).
    Note also that if the structure contains any pointers, only the
    pointer values will be written, and they are most unlikely to be
    valid when read back in.  Finally, note that for widespread
    portability you must use the "b" flag when fopening the files;
    see question 12.38.

    A more portable solution, though it's a bit more work initially,
    is to write a pair of functions for writing and reading a
    structure, field-by-field, in a portable (perhaps even human-
    readable) way.

    References: H&S Sec. 15.13 p. 381.

2.12:    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
    structure fields?

A:    Your compiler may provide an extension to give you this control
    (perhaps a #pragma; see question 11.20), but there is no
    standard method.

    See also question 20.5.

    References: K&R2 Sec. 6.4 p. 138; H&S Sec. 5.6.4 p. 135.

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

A:    Structures may have this padding (as well as internal padding),
    if necessary, to ensure that alignment properties will be
    preserved when an array of contiguous structures is allocated.
    Even when the structure is not part of an array, the end padding
    remains, so that sizeof can always return a consistent size.
    See question 2.12 above.

    References: H&S Sec. 5.6.7 pp. 139-40.

2.14:    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, one possible
    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 question 2.15 below for a usage hint.

    References: ANSI Sec. 4.1.5; ISO Sec. 7.1.6; Rationale
    Sec. 3.5.4.2; H&S Sec. 11.1 pp. 292-3.

2.15:    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
    field b is an int (with offset as computed above), b's value can
    be set indirectly with

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

2.18:    This 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 main() to be declared as returning a
    structure.  (The connection is hard to see because of the
    intervening comment.)  Since structure-valued functions are
    usually implemented by adding a hidden return pointer (see
    question 2.9), 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 questions 10.9 and 16.4.

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

2.20:    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 any other
    member (nor, under a pre-ANSI compiler, is there generally any
    way of initializing a union at all).

    References: K&R2 Sec. 6.8 pp. 148-9; ANSI Sec. 3.5.7; ISO
    Sec. 6.5.7; H&S Sec. 4.6.7 p. 100.

2.22:    What is the difference between an enumeration and a set of
    preprocessor #defines?

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

    Some advantages of enumerations are that the numeric values are
    automatically assigned, that a debugger may be able to display
    the symbolic values when enumeration variables are examined, and
    that they obey block scope.  (A compiler may also generate
    nonfatal warnings when enumerations and integers 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
    those nonfatal warnings; some programmers also resent not having
    control over the sizes of enumeration variables.

    References: K&R2 Sec. 2.3 p. 39, Sec. A4.2 p. 196; ANSI
    Sec. 3.1.2.5, Sec. 3.5.2, Sec. 3.5.2.2, Appendix E; ISO
    Sec. 6.1.2.5, Sec. 6.5.2, Sec. 6.5.2.2, Annex F; H&S Sec. 5.5
    pp. 127-9, Sec. 5.11.2 p. 153.

2.24:    Is there an easy way to print enumeration values symbolically?

A:    No.  You can write a little function to map an enumeration
    constant to a string.  (If all you're worried about is
    debugging, a good debugger should automatically print
    enumeration constants symbolically.)


Section 3. Expressions

3.1:    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 since 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 C Standard makes the stronger
    statement that it is undefined -- see question 11.33.)

    References: K&R1 Sec. 2.12; K&R2 Sec. 2.12; ANSI Sec. 3.3; ISO
    Sec. 6.3.

3.2:    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 their operations after yielding the former value, the
    implication of "after" is often misunderstood.  It is *not*
    guaranteed that an increment or decrement 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; see question 3.8).  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.  (Loosely speaking, by
    "multiple, ambiguous side effects" we mean any combination of
    ++, --, =, +=, -=, etc. in a single expression which causes the
    same object either to be modified twice or modified and then
    inspected.  This is a rough definition; see question 3.8 for a
    precise one, and question 11.33 for the meaning of "undefined.")
    Don't even try to find out how your compiler implements such
    things (contrary to the ill-advised exercises in many C
    textbooks); as K&R wisely point out, "if you don't know *how*
    they are done on various machines, that innocence may help to
    protect you."

    References: K&R1 Sec. 2.12 p. 50; K&R2 Sec. 2.12 p. 54; ANSI
    Sec. 3.3; ISO Sec. 6.3; CT&P Sec. 3.7 p. 47; PCS Sec. 9.5 pp.
    120-1.

3.3:    I've experimented with the code

        [CENSORED]

    on several compilers.  Some gave i the value 3, some gave 4, but
    one gave 7.  I know the behavior is undefined, but how could it
    give 7?

A:    [I apologize for the censorship of the question, but the
    expression that used to be there was indecent, and by the
    newly-passed Communications Decency Act of the U.S., I am
    prohibited from transmitting "indecent" material, whatever that
    is.  Suffice it to say that the expression tried to modify the
    same variable twice between sequence points.  --scs]

    Undefined behavior means *anything* can happen.  See questions
    3.9 and 11.33.  (Also, note that neither i++ nor ++i is the same
    as i+1.  If you want to increment i, use i=i+1 or i++ or ++i,
    not some combination.  See also question 3.12.)

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

A:    Not in general.

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

    When you need to ensure the order of subexpression evaluation,
    you may need to use explicit temporary variables and separate
    statements.

    References: K&R1 Sec. 2.12 p. 49, Sec. A.7 p. 185; K&R2
    Sec. 2.12 pp. 52-3, Sec. A.7 p. 200.

3.5:    But what about the && and || operators?
    I see code like "while((c = getchar()) != EOF && c != '\n')" ...

A:    There is a special exception for those operators (as well as the
    ?: operator): left-to-right evaluation is guaranteed (as is an
    intermediate sequence point, see question 3.8).  Any book on C
    should make this clear.

    References: K&R1 Sec. 2.6 p. 38, Secs. A7.11-12 pp. 190-1; K&R2
    Sec. 2.6 p. 41, Secs. A7.14-15 pp. 207-8; ANSI Sec. 3.3.13,
    Sec. 3.3.14, Sec. 3.3.15; ISO Sec. 6.3.13, Sec. 6.3.14,
    Sec. 6.3.15; H&S Sec. 7.7 pp. 217-8, Sec. 7.8 pp. 218-20,
    Sec. 7.12.1 p. 229; CT&P Sec. 3.7 pp. 46-7.

3.8:    How can I understand these complex expressions?  What's a
    "sequence point"?

A:    A sequence point is the point (at the end of a full expression,
    or at the ||, &&, ?:, or comma operators, or just before a
    function call) at which the dust has settled and all side
    effects are guaranteed to be complete.  The ANSI/ISO C Standard
    states that

        Between the previous and next sequence point an
        object shall have its stored value modified at
        most once by the evaluation of an expression.
        Furthermore, the prior value shall be accessed
        only to determine the value to be stored.

    The second sentence can be difficult to understand.  It says
    that if an object is written to within a full expression, any
    and all accesses to it within the same expression must be for
    the purposes of computing the value to be written.  This rule
    effectively constrains legal expressions to those in which the
    accesses demonstrably precede the modification.

    See also question 3.9 below.

    References: ANSI Sec. 2.1.2.3, Sec. 3.3, Appendix B; ISO
    Sec. 5.1.2.3, Sec. 6.3, Annex C; Rationale Sec. 2.1.2.3; H&S
    Sec. 7.12.1 pp. 228-9.

3.9:    So given

        a[i] = i++;

    we don't know which cell of a[] gets written to, but i does get
    incremented by one.

A:    *No.*  Once an expression or program becomes undefined, *all*
    aspects of it become undefined.  See questions 3.2, 3.3, 11.33,
    and 11.35.

3.12:    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.

    See also question 3.3.

    References: K&R1 Sec. 2.8 p. 43; K&R2 Sec. 2.8 p. 47; ANSI
    Sec. 3.3.2.4, Sec. 3.3.3.1; ISO Sec. 6.3.2.4, Sec. 6.3.3.1; H&S
    Sec. 7.4.4 pp. 192-3, Sec. 7.5.8 pp. 199-200.


3.14:    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 or
    be truncated before being promoted and 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 (long int)(a * b) would *not* have the desired effect.

    A similar problem can arise when two integers are divided, with
    the result assigned to a floating-point variable.

    References: K&R1 Sec. 2.7 p. 41; K&R2 Sec. 2.7 p. 44; ANSI
    Sec. 3.2.1.5; ISO Sec. 6.2.1.5; H&S Sec. 6.3.4 p. 176; CT&P
    Sec. 3.9 pp. 49-50.

3.16:    I have a complicated expression which I have to assign to one of
    two variables, depending on a condition.  Can I use code like
    this?

        ((condition) ? a : b) = complicated_expression;

A:    No.  The ?: operator, like most operators, yields a value, and
    you can't assign to a value.  (In other words, ?: does not yield
    an "lvalue".)  If you really want to, you can try something like

        *((condition) ? &a : &b) = complicated_expression;

    although this is admittedly not as pretty.

    References: ANSI Sec. 3.3.15 esp. footnote 50; ISO Sec. 6.3.15;
    H&S Sec. 7.1 pp. 179-180.


Section 4. Pointers

4.2:    I'm trying to declare a pointer and allocate some space for it,
    but it's not working.  What's wrong with this code?

        char *p;
        *p = malloc(10);

A:    The pointer you declared is p, not *p.  To make a pointer point
    somewhere, you just use the name of the pointer:

        p = malloc(10);

    It's when you're manipulating the pointed-to memory that you use
    * as an indirection operator:

        *p = 'H';

    See also questions 1.21, 7.1, and 8.3.

    References: CT&P Sec. 3.1 p. 28.

4.3:    Does *p++ increment p, or what it points to?

A:    Unary operators like *, ++, and -- all associate (group) from
    right to left.  Therefore, *p++ increments p (and returns the
    value pointed to by p before the increment).  To increment the
    value pointed to by p, use (*p)++ (or perhaps ++*p, if the order
    of the side effect doesn't matter).

    References: K&R1 Sec. 5.1 p. 91; K&R2 Sec. 5.1 p. 95; ANSI
    Sec. 3.3.2, Sec. 3.3.3; ISO Sec. 6.3.2, Sec. 6.3.3; H&S
    Sec. 7.4.4 pp. 192-3, Sec. 7.5 p. 193, Secs. 7.5.7,7.5.8 pp. 199-
    200.

4.5:    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 (since p is a char *) simply

        p += sizeof(int);

    Whenever possible, you should choose appropriate pointer types
    in the first place, instead of trying to treat one type as
    another.

    References: K&R2 Sec. A7.5 p. 205; ANSI Sec. 3.3.4 (esp.
    footnote 14); ISO Sec. 6.3.4; Rationale Sec. 3.3.2.4; H&S
    Sec. 7.1 pp. 179-80.

4.8:    I have a function which accepts, and is supposed to initialize,
    a pointer:

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

    But when I call it like this:

        int *ip;
        f(ip);

    the pointer in the caller remains unchanged.

A:    Are you sure the function initialized what you thought it did?
    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.

    See also questions 4.9 and 4.11.


4.9:    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 *.

4.10:    I have a function

        extern int f(int *);

    which accepts a pointer to an int.  How can I pass a constant by
    reference?  A call like

        f(&5);

    doesn't seem to work.

A:    You can't do this directly.  You will have to declare a
    temporary variable, and then pass its address to the function:

        int five = 5;
        f(&five);

    See also questions 2.10, 4.8, and 20.1.

4.11:    Does C even have "pass by reference"?

A:    Not really.  Strictly speaking, C always uses pass by value.
    You can simulate pass by reference yourself, by defining
    functions which accept pointers and then using the & operator
    when calling, and the compiler will essentially simulate it for
    you when you pass an array to a function (by passing a pointer
    instead, see question 6.4 et al.), but C has nothing truly
    equivalent to formal pass by reference or C++ reference
    parameters.  (However, function-like preprocessor macros do
    provide a form of "call by name".)

    See also questions 4.8 and 20.1.

    References: K&R1 Sec. 1.8 pp. 24-5, Sec. 5.2 pp. 91-3; K&R2
    Sec. 1.8 pp. 27-8, Sec. 5.2 pp. 91-3; ANSI Sec. 3.3.2.2, esp.
    footnote 39; ISO Sec. 6.3.2.2; H&S Sec. 9.5 pp. 273-4.

4.12:    I've seen different methods used for calling functions via
    pointers.  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 via
    pointers, and that "real" function names always decay implicitly
    into pointers (in expressions, as they do in initializations;
    see question 1.34).  This reasoning, made widespread through pcc
    and adopted in the ANSI standard, means that


        r = fp();

    is legal and works correctly, whether fp is the name of 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 the
    function pointed to.)  An explicit * is still allowed (and
    recommended, if portability to older compilers is important).

    See also question 1.34.

    References: K&R1 Sec. 5.12 p. 116; K&R2 Sec. 5.11 p. 120; ANSI
    Sec. 3.3.2.2; ISO Sec. 6.3.2.2; Rationale Sec. 3.3.2.2; H&S
    Sec. 5.8 p. 147, Sec. 7.4.3 p. 190.


Section 5. Null Pointers

5.1:    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
    "guaranteed to compare unequal to a pointer to any object or
    function."  That is, the address-of operator & will never yield
    a null pointer, nor will a successful call to malloc().
    (malloc() does return 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 or
    function; an uninitialized pointer might point anywhere.  See
    also questions 1.30, 7.1, and 7.31.

    As mentioned 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 that it can make the distinction
    if necessary (see questions 5.2, 5.5, and 5.6 below).

    References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102; ANSI
    Sec. 3.2.2.3; ISO Sec. 6.2.2.3; Rationale Sec. 3.2.2.3; H&S
    Sec. 5.3.2 pp. 121-3.

5.2:    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)

    (See also question 5.3.)

    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.  To generate a null pointer in a function call context,
    an explicit cast may be required, to force the 0 to be
    recognized as a pointer.  For example, the Unix system call
    execl takes a variable-length, null-pointer-terminated list of
    character pointer arguments, and is correctly called like this:

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

    If the (char *) cast on the last argument 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 an
    unadorned 0.  Function prototypes cannot provide the types for
    variable arguments in variable-length argument lists however, so
    explicit casts are still required for those arguments.  (See
    also question 15.3.)  It is safest to properly cast all null
    pointer constants in function calls: 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&R1 Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R2
    Sec. A7.10 p. 207, Sec. A7.17 p. 209; ANSI Sec. 3.2.2.3; ISO
    Sec. 6.2.2.3; H&S Sec. 4.6.3 p. 95, Sec. 6.2.7 p. 171.

5.3:    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 inferred 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 actually a null pointer constant, and use
    the correct null pointer value.  There is no trickery involved
    here; compilers do work this way, and generate identical code
    for both constructs.  The internal representation of a null
    pointer does *not* matter.

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

        !expr    is essentially equivalent to    (expr)?0:1
            or to                ((expr) == 0)

    which leads to the conclusion 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 (and by others to be good
    style; see question 17.10).

    See also question 9.2.

    References: K&R2 Sec. A7.4.7 p. 204; ANSI Sec. 3.3.3.3,
    Sec. 3.3.9, Sec. 3.3.13, Sec. 3.3.14, Sec. 3.3.15, Sec. 3.6.4.1,
    Sec. 3.6.5; ISO Sec. 6.3.3.3, Sec. 6.3.9, Sec. 6.3.13,
    Sec. 6.3.14, Sec. 6.3.15, Sec. 6.6.4.1, Sec. 6.6.5; H&S
    Sec. 5.3.2 p. 122.

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

A:    As a matter of style, many programmers prefer not to have
    unadorned 0's scattered through their programs.  Therefore, the
    preprocessor macro NULL is #defined (by <stdio.h> or <stddef.h>)
    with the value 0, possibly cast to (void *) (see also question
    5.6).  A programmer who wishes to make explicit the distinction
    between 0 the integer and 0 the null pointer constant can then
    use NULL whenever a null pointer is required.

    Using NULL is a stylistic convention only; the preprocessor
    turns NULL back into 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 5.2 above applies for NULL
    as well as 0 (an unadorned NULL is equivalent to an unadorned
    0).

    NULL should *only* be used for pointers; see question 5.9.

    References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102; ANSI
    Sec. 4.1.5, Sec. 3.2.2.3; ISO Sec. 7.1.6, Sec. 6.2.2.3;
    Rationale Sec. 4.1.5; H&S Sec. 5.3.2 p. 122, Sec. 11.1 p. 292.


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

A:    The same as on any other machine: as 0 (or ((void *)0)).

    Whenever a programmer requests a null pointer, either by writing
    "0" or "NULL," it is the compiler's responsibility to generate
    whatever bit pattern the machine uses for that null pointer.
    Therefore, #defining NULL as 0 on a machine for which internal
    null pointers are nonzero is as valid as on any other: the
    compiler must always be able to generate the machine's correct
    null pointers in response to unadorned 0's seen in pointer
    contexts.  See also questions 5.2, 5.10, and 5.17.

    References: ANSI Sec. 4.1.5; ISO Sec. 7.1.6; Rationale
    Sec. 4.1.5.

5.6:    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 work
    correctly, but pointer arguments of other types would still be
    problematical, and legal constructions such as

        FILE *fp = NULL;

    could fail.

    Nevertheless, ANSI C allows the alternate definition

        #define NULL ((void *)0)

    for NULL.  Besides potentially 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 5.9).

    References: Rationale Sec. 4.1.5.

5.9:    If NULL and 0 are equivalent as null pointer constants, 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 macro, and prefer to use
    unadorned 0 instead.  There is no one right answer.  (See also
    questions 9.2 and 17.10.)  C programmers must understand that
    NULL and 0 are interchangeable in pointer contexts, and that an
    uncast 0 is perfectly acceptable.  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.  (Furthermore, ANSI allows the definition of
    NULL to be ((void *)0), which will not work at all 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&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102.

5.10:    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
    internal null pointers?

A:    No.  (Using NULL may be preferable, but not for this reason.)
    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.  See
    questions 5.5 and 9.2.

5.12:    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 and superficially attractive, does
    not buy much.  It is not needed in assignments and comparisons;
    see question 5.2.  It does not even save keystrokes.  Its use
    may suggest to the reader that the program's author is shaky on
    the subject of null pointers, requiring that the #definition of
    the macro, its invocations, and *all* other pointer usages be
    checked.  See also questions 9.1 and 10.2.

5.13:    This is strange.  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 5.1.  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 null pointer constant, which is a constant integer 0
        (see question 5.2).  It is often hidden behind...

    4.    The NULL macro, which is #defined to be "0" or
        "((void *)0)" (see question 5.4).  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 the empty
        string ("").  Using the term "null string" can be
        confusing in C, because an empty string involves a null
        ('\0') character, but *not* a null pointer, which brings
        us full circle...

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

5.14:    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) may seem to suggest that the
    value could change some day, 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 in question 5.13
    above) is often overlooked.

    One good way to wade out of the confusion is to imagine that C
    used a keyword (perhaps "nil", like Pascal) as a null pointer
    constant.  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 constant 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
    constant, the code may not work.

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

A:    Follow these two simple rules:

    1.    When you want a null pointer constant 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, with the internal representation of null
    pointers (which you shouldn't need to know), and with ANSI C
    refinements.  Understand questions 5.1, 5.2, and 5.4, and
    consider 5.3, 5.9, 5.13, and 5.14, and you'll do fine.

5.16:    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 such a 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 7.31).  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, as mentioned in
    question 5.14 above, the urge to assume an internal zero
    representation would not even arise.)

5.17:    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.  It was common on old CDC ones-
    complement machines to use an all-one-bits word as a special
    flag for all kinds of data, including invalid addresses.

    The old HP 3000 series uses a different addressing scheme for
    byte addresses than for word addresses; like several of the
    machines above it therefore uses different representations for
    char * and void * pointers than for other pointers.

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

    Depending on the "memory model" in use, 8086-family processors
    (PC compatibles) may use 16-bit data pointers and 32-bit
    function pointers, or vice versa.

    Some 64-bit Cray machines represent int * in the lower 48 bits
    of a word; char * additionally uses the upper 16 bits to
    indicate a byte address within a word.

    References: K&R1 Sec. A14.4 p. 211.

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

A:    This message, which typically occurs with MS-DOS compilers (see,
    therefore, section 19) means that you've written, via a null
    (perhaps because uninitialized) pointer, to location 0.  (See
    also question 16.8.)

    A debugger may let you set a data breakpoint or watchpoint or
    something on location 0.  Alternatively, you could write a bit
    of code to stash away a copy of 20 or so bytes from location 0,
    and periodically check that the memory at location 0 hasn't
    changed.


Section 6.  Arrays and Pointers

6.1:    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: ANSI Sec. 3.5.4.2; ISO Sec. 6.5.4.2; CT&P Sec. 3.3
    pp. 33-4, Sec. 4.5 pp. 64-5.

6.2:    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 6.4.)  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, to be known by the name "p."  This
    pointer can point almost anywhere: to any char, or to any
    contiguous array of chars, or nowhere (see also questions 5.1
    and 1.30).

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

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

    would initialize 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 other words, a[3] is
    three places past (the start of) the object *named* a, while
    p[3] is three places past the object *pointed to* by p.  In the
    example above, both a[3] and p[3] happen to be the character
    'l', but the compiler gets there differently.

    References: K&R2 Sec. 5.5 p. 104; CT&P Sec. 4.5 pp. 64-5.

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

A:    Much of the confusion surrounding arrays and pointers in C can
    be traced to a misunderstanding of this statement.  Saying that
    arrays and pointers are "equivalent" means neither that they are
    identical nor even interchangeable.

    "Equivalence" refers to the following key definition:

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

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

    As a consequence of this definition, the compiler doesn't apply
    the array subscripting operator [] that differently to arrays
    and pointers, after all.  In an expression of the form a[i], the
    array 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 6.2).  If you were to assign
    the array's address to the pointer:

        p = a;

    then p[3] and a[3] would access the same element.

    See also question 6.8.

    References: K&R1 Sec. 5.3 pp. 93-6; K&R2 Sec. 5.3 p. 99; ANSI
    Sec. 3.2.2.1, Sec. 3.3.2.1, Sec. 3.3.6; ISO Sec. 6.2.2.1,
    Sec. 6.3.2.1, Sec. 6.3.6; H&S Sec. 5.4.1 p. 124.

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

A:    It's supposed to be a convenience.

    Since arrays decay immediately into pointers, an array is never
    actually passed to a function.  Allowing pointer parameters to
    be declared as arrays is a simply a way of making it look as
    though the array was being passed -- a programmer may wish to
    emphasize that a parameter is traditionally treated as if it
    were an array, or that an array (strictly speaking, the address)
    is traditionally passed.  As a convenience, therefore, any
    parameter declarations which "look like" arrays, e.g.

        f(a)
        char a[];
        { ... }

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

        f(a)
        char *a;
        { ... }

    This conversion holds only within function formal parameter
    declarations, nowhere else.  If the 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 or the uses within the function.

    See also question 6.21.

    References: K&R1 Sec. 5.3 p. 95, Sec. A10.1 p. 205; K&R2
    Sec. 5.3 p. 100, Sec. A8.6.3 p. 218, Sec. A10.1 p. 226; ANSI
    Sec. 3.5.4.3, Sec. 3.7.1, Sec. 3.9.6; ISO Sec. 6.5.4.3,
    Sec. 6.7.1, Sec. 6.9.6; H&S Sec. 9.3 p. 271; CT&P Sec. 3.3 pp.
    33-4.

6.7:    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; ISO Sec. 6.2.2.1; Rationale
    Sec. 3.2.2.1; H&S Sec. 7.1 p. 179.

6.8:    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 6.3), 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 questions 6.14 and
    6.16.  (Be careful with sizeof, though.)

    See also questions 1.32 and 20.14.

6.9:    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 6.2
    should make clear.  See also questions 6.3 and 6.8.

6.11:    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* two expressions a and e, 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 20.36).

    References: Rationale Sec. 3.3.2.1; H&S Sec. 5.4.1 p. 124,
    Sec. 7.4.1 pp. 186-7.

6.12:    Since array references decay into pointers, if arr is an array,
    what's the difference between arr and &arr?

A:    The type.

    In Standard C, &arr yields a pointer, of type pointer-to-array-
    of-T, to the entire array.  (In pre-ANSI C, the & in &arr
    generally elicited a warning, and was generally ignored.)  Under
    all C compilers, a simple reference (without an explicit &) to
    an array yields a pointer, of type pointer-to-T, to the array's
    first element.  (See also questions 6.3, 6.13, and 6.18.)

    References: ANSI Sec. 3.2.2.1, Sec. 3.3.3.2; ISO Sec. 6.2.2.1,
    Sec. 6.3.3.2; Rationale Sec. 3.3.3.2; H&S Sec. 7.5.6 p. 198.

6.13:    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 6.3), which is convenient;
    subscripting or incrementing the resultant pointer will access
    the individual members of the array.  True pointers to arrays,
    when subscripted or incremented, step over entire arrays, and
    are generally useful only when operating on arrays of arrays, if
    at all.  (See question 6.18.)

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

    See also question 6.12 above.

    References: ANSI Sec. 3.2.2.1; ISO Sec. 6.2.2.1.

6.14:    How can I set an array's size at compile time?
    How can I avoid fixed-sized arrays?

A:    The equivalence between arrays and pointers (see question 6.3)
    allows a pointer to malloc'ed memory to simulate an array
    quite effectively.  After executing

        #include <stdlib.h>
        int *dynarray = (int *)malloc(10 * sizeof(int));

    (and if the call to malloc() succeeds), you can reference
    dynarray[i] (for i from 0 to 9) just as if dynarray were a
    conventional, statically-allocated array (int a[10]).  See also
    question 6.16.

6.15:    How can I declare local arrays of a size matching a passed-in
    array?

A:    You can't, in C.  Array dimensions must be compile-time
    constants.  (gcc provides parameterized arrays as an extension.)
    You'll have to use malloc(), and remember to call free() before
    the function returns.  See also questions 6.14, 6.16, 6.19,
    7.22, and maybe 7.32.

    References: ANSI Sec. 3.4, Sec. 3.5.4.2; ISO Sec. 6.4,
    Sec. 6.5.4.2.

6.16:    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:

        #include <stdlib.h>

        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, all of malloc's return values would
    be 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: arrayx[i][j] (for
    0 <= i <= NROWS and 0 <= j <= NCOLUMNS).

    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 could hide the explicit calculation, but invoking it would
    require parentheses and commas which wouldn't look exactly like
    multidimensional array syntax, and the macro would need access
    to at least one of the dimensions, as well.  See also question
    6.19.)

    Finally, you could use pointers to arrays:

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

    but the syntax starts getting horrific and at most one dimension
    may be specified at run time.

    With all of these techniques, you may of course need to remember
    to free the arrays (which may take several steps; see question
    7.23) when they are no longer needed, and you cannot necessarily
    intermix dynamically-allocated arrays with conventional,
    statically-allocated ones (see question 6.20, and also question
    6.18).

    All of these techniques can also be extended to three or more
    dimensions.

6.17:    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: K&R2 Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3, Sec. A7.7
    pp. 205-6; ANSI Sec. 3.3.6; ISO Sec. 6.3.6; Rationale
    Sec. 3.2.2.3.

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

A:    The rule (see question 6.3) 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; see also question 6.13.  (The
    confusion is heightened by the existence of incorrect compilers,
    including some old versions of pcc and pcc-derived lints, 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 must 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" (see questions 6.3 and 6.4); 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 three- or more dimensional arrays, the
    intervening ones) must be retained.

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

    See also questions 6.12 and 6.15.

    References: K&R1 Sec. 5.10 p. 110; K&R2 Sec. 5.9 p. 113; H&S
    Sec. 5.4.3 p. 126.

6.19:    How do I write functions which accept two-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;
        { ... array[i][j] is accessed as aryp[i * ncolumns + j] ... }

    This function could be called with the array from question 6.18
    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; according to an
    official interpretation, 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.

    When you want to be able to use a function on multidimensional
    arrays of various sizes, one solution is to simulate all the
    arrays dynamically, as in question 6.16.

    See also questions 6.18, 6.20, and 6.15.

    References: ANSI Sec. 3.3.6; ISO Sec. 6.3.6.

6.20:    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;            /* ragged */
        int **array2;            /* contiguous */
        int *array3;            /* "flattened" */
        int (*array4)[NCOLUMNS];

    with the pointers initialized as in the code fragments in
    question 6.16, and functions declared as

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

    where f1() accepts a conventional two-dimensional array, f2()
    accepts a "flattened" two-dimensional array, and f3() accepts a
    pointer-to-pointer, simulated array (see also questions 6.18 and
    6.19), the following calls should work as expected:

        f1(array, NROWS, NCOLUMNS);
        f1(array4, nrows, NCOLUMNS);
        f2(&array[0][0], NROWS, NCOLUMNS);
        f2(*array, 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 on most systems, 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] (or,
    equivalently, *array) to f2() is not strictly conforming; see
    question 6.19.

    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 in C.

    Rather than worrying about all of this, one approach to using
    multidimensional arrays of various sizes is to make them *all*
    dynamic, as in question 6.16.  If there are no static
    multidimensional arrays -- if all arrays are allocated like
    array1 or array2 in question 6.16 -- then all functions can be
    written like f3().

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

A:    The compiler pretends that the array parameter was declared as a
    pointer (see question 6.4), and sizeof reports the size of the
    pointer.

    References: H&S Sec. 7.5.2 p. 195.


Section 7. Memory Allocation

7.1:    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 gets() 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
    questions 1.30 and 5.1.)

    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 <stdio.h>
        #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);

    This example also uses fgets() instead of gets(), so that the
    end of the array cannot be overwritten.  (See question 12.23.
    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.

7.2:    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:    As in question 7.1 above, the main problem here 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 for
    sufficient space for the results of run-time operations such as
    string concatenation, typically by declaring arrays, or by
    calling malloc().

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

        char s1[20] = "Hello, ";

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

    The original call to strcat() in the question actually has two
    problems: the string literal pointed to by s1, besides not being
    big enough for any concatenated text, is not necessarily
    writable at all.  See question 1.32.

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

7.3:    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, if only to make sure that the compiler is
    doing it for you.  If a library function'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 or in
    the ANSI C standard can be misleading.  The code fragments
    presented there are closer to the function definitions used by
    an implementor than the invocations used by the caller.  In
    particular, many functions which accept pointers (e.g. to
    structures or strings) are usually called with the address of
    some object (a structure, or an array -- see questions 6.3 and
    6.4).  Other common examples are time() (see question 13.12)
    and stat().

7.5:    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 pointed-to memory is properly 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 (automatic) array.  In other
    words, never do something like

        char *itoa(int n)
        {
            char retbuf[20];        /* WRONG */
            sprintf(retbuf, "%d", n);
            return retbuf;            /* WRONG */
        }

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

            static char retbuf[20];

    See also questions 12.21 and 20.1.

    References: ANSI Sec. 3.1.2.4; ISO Sec. 6.1.2.4.

7.6:    Why am I getting "warning: assignment of pointer from integer
    lacks a cast" for calls to malloc()?

A:    Have you #included <stdlib.h>, or otherwise arranged for
    malloc() to be declared properly?

    References: H&S Sec. 4.7 p. 101.

7.7:    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
    (and perhaps induce conversions) when assigning between
    incompatible pointer types.  (Under ANSI/ISO Standard C, these
    casts are no longer necessary.)

    References: H&S Sec. 16.1 pp. 386-7.

7.8:    I see code like

        char *p = malloc(strlen(s) + 1);
        strcpy(p, s);

    Shouldn't that be malloc((strlen(s) + 1) * sizeof(char))?

A:    It's never necessary to multiply by sizeof(char), since
    sizeof(char) is, by definition, exactly 1.  (On the other hand,
    multiplying by sizeof(char) doesn't hurt, and may help by
    introducing a size_t into the expression.)  See also question
    8.9.

    References: ANSI Sec. 3.3.3.4; ISO Sec. 6.3.3.4; H&S Sec. 7.5.2
    p. 195.

7.14:    I've heard that some operating systems don't actually allocate
    malloc'ed memory until the program tries to use it.  Is this
    legal?

A:    It's hard to say.  The Standard doesn't say that systems can act
    this way, but it doesn't explicitly say that they can't, either.

    References: ANSI Sec. 4.10.3; ISO Sec. 7.10.3.

7.16:    I'm allocating a large array for some numeric work, using the
    line

        double *array = malloc(256 * 256 * sizeof(double));

    malloc() isn't returning null, but the program is acting
    strangely, as if it's overwriting memory, or malloc() isn't
    allocating as much as I asked for, or something.

A:    Notice that 256 x 256 is 65,536, which will not fit in a 16-bit
    int, even before you multiply it by sizeof(double).  If you need
    to allocate this much memory, you'll have to be careful.  If
    size_t (the type accepted by malloc()) is a 32-bit type on your
    machine, but int is 16 bits, you might be able to get away with
    writing 256 * (256 * sizeof(double)) (see question 3.14).
    Otherwise, you'll have to break your data structure up into
    smaller chunks, or use a 32-bit machine, or use some nonstandard
    memory allocation routines.  See also question 19.23.

7.17:    I've got 8 meg of memory in my PC.  Why can I only seem to
    malloc() 640K or so?

A:    Under the segmented architecture of PC compatibles, it can be
    difficult to use more than 640K with any degree of transparency.
    See also question 19.23.

7.19:    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 stubborn.  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 may involve using pointers to freed storage, freeing
    pointers twice, freeing pointers not obtained from malloc, or
    trying to realloc a null pointer (see question 7.30).

    See also questions 7.26, 16.8, and 18.2.

7.20:    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 were "left undisturbed," but this ill-
    advised guarantee was never universal and is not required by the
    C Standard.

    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((void *)listp);
        }

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

    References: K&R2 Sec. 7.8.5 p. 167; ANSI Sec. 4.10.3; ISO
    Sec. 7.10.3; Rationale Sec. 4.10.3.2; H&S Sec. 16.2 p. 387; CT&P
    Sec. 7.10 p. 95.

7.21:    Why isn't a pointer null after calling free()?
    How unsafe is it to use (assign, compare) a pointer value after
    it's been freed?

A:    When you call free(), the memory pointed to by the passed
    pointer is freed, but the value of the pointer in the caller
    remains unchanged, because C's pass-by-value semantics mean that
    called functions never permanently change the values of their
    arguments.  (See also question 4.8.)

    A pointer value which has been freed is, strictly speaking,
    invalid, and *any* use of it, even if is not dereferenced can
    theoretically lead to trouble, though as a quality of
    implementation issue, most implementations will probably not go
    out of their way to generate exceptions for innocuous uses of
    invalid pointers.

    References: ANSI Sec. 4.10.3; ISO Sec. 7.10.3; Rationale
    Sec. 3.2.2.3.

7.22:    When I call malloc() to allocate memory for a local pointer, do
    I have to explicitly free() it?

A:    Yes.  Remember that a pointer is different from what it points
    to.  Local variables are deallocated when the function returns,
    but in the case of a pointer variable, this means that the
    pointer is deallocated, *not* what it points to.  Memory
    allocated with malloc() always persists until you explicitly
    free it.  In general, for every call to malloc(), there should
    be a corresponding call to free().

7.23:    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).

    A good rule of thumb is that for each call to malloc() in a
    program, you should be able to point at the call to free() which
    frees the memory allocated by that malloc() call.

    See also question 7.24.

7.24:    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
    this is a "quality of implementation issue."

    References: ANSI Sec. 4.10.3.2; ISO Sec. 7.10.3.2.

7.25:    I have a program which mallocs and later 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 program.

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

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

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

A:    Not portably.

7.30:    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), although several earlier implementations do not
    support it, so it may not be fully 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; ISO Sec. 7.10.3.4; H&S Sec. 16.3
    p. 388.

7.31:    What's the difference between calloc() and malloc()?  Is it safe
    to take advantage of calloc's zero-filling?  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 null pointer values (see section 5 of this
    list) or floating-point zero values.  free() is properly used to
    free the memory allocated by calloc().

    References: ANSI Sec. 4.10.3 to 4.10.3.2; ISO Sec. 7.10.3 to
    7.10.3.2; H&S Sec. 16.1 p. 386, Sec. 16.2 p. 386; PCS Sec. 11
    pp. 141,142.

7.32:    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 conventional 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() is not Standard and cannot be used
    in programs which must be widely portable, no matter how useful
    it might be.

    See also question 7.22.

    References: Rationale Sec. 4.10.3.


Section 8. Characters and Strings

8.1:    Why doesn't

        strcat(string, '!');

    work?

A:    There is a very real difference between characters and strings,
    and strcat() concatenates *strings*.

    Characters in C are represented by small integers corresponding
    to their character set values (see also question 8.6 below).
    Strings are represented by arrays of characters; you usually
    manipulate a pointer to the first character of the array.  It is
    never correct to use one when the other is expected.  To append
    a ! to a string, use

        strcat(string, "!");

    See also questions 1.32, 7.2, and 16.6.

    References: CT&P Sec. 1.5 pp. 9-10.

8.2:    I'm checking a string to see if it matches a particular value.
    Why isn't this code working?

        char *string;
        ...
        if(string == "value") {
            /* string matches "value" */
            ...
        }

A:    Strings in C are represented as arrays of characters, and C
    never manipulates (assigns, compares, etc.) arrays as a whole.
    The == operator in the code fragment above compares two pointers
    -- the value of the pointer variable string and a pointer to the
    string literal "value" -- to see if they are equal, that is, if
    they point to the same place.  They probably don't, so the
    comparison never succeeds.

    To compare two strings, you generally use the library function
    strcmp():

        if(strcmp(string, "value") == 0) {
            /* string matches "value" */
            ...
        }

8.3:    If I can say

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

    why can't I say

        char a[14];
        a = "Hello, world!";

A:    Strings are arrays, and you can't assign arrays directly.  Use
    strcpy() instead:

        strcpy(a, "Hello, world!");

    See also questions 1.32, 4.2, and 7.2.

8.6:    How can I get the numeric (character set) 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.

8.9:    I think something's wrong with my compiler: I just noticed that
    sizeof('a') is 2, not 1 (i.e. not sizeof(char)).

A:    Perhaps surprisingly, character constants in C are of type int,
    so sizeof('a') is sizeof(int) (though it's different in C++).
    See also question 7.8.

    References: ANSI Sec. 3.1.3.4; ISO Sec. 6.1.3.4; H&S Sec. 2.7.3
    p. 29.


Section 9. Boolean Expressions

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

A:    C does not provide a standard Boolean type, in part because
    picking one involves a space/time tradeoff which can best be
    decided by the programmer.  (Using an int may be faster, while
    using char may save data space.  Smaller types may make the
    generated code bigger or slower, though, if they require lots of
    conversions to and from int.)

    The choice between #defines and enumeration constants for the
    true/false values is arbitrary and not terribly interesting (see
    also questions 2.22 and 17.10).  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 enumeration may be preferable if your
    debugger shows the names of enumeration constants 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 9.2 below; see also
    questions 5.12 and 10.2).

9.2:    Isn't #defining TRUE to be 1 dangerous, since any nonzero value
    is considered "true" in C?  What if a built-in logical 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)

    would work as expected (as long as TRUE is 1), but it is
    obviously silly.  In general, explicit tests against TRUE and
    FALSE are inappropriate, 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 (and, of course, NULL)
    are used for code readability, not because the underlying values
    might ever change.  (See also questions 5.3 and 5.10.)

    On the other hand, Boolean values and definitions can evidently
    be confusing, and some programmers feel that TRUE and FALSE
    macros only compound the confusion.  (See also question 5.9.)

    References: K&R1 Sec. 2.6 p. 39, Sec. 2.7 p. 41; K&R2 Sec. 2.6
    p. 42, Sec. 2.7 p. 44, Sec. A7.4.7 p. 204, Sec. A7.9 p. 206;
    ANSI Sec. 3.3.3.3, Sec. 3.3.8, Sec. 3.3.9, Sec. 3.3.13,
    Sec. 3.3.14, Sec. 3.3.15, Sec. 3.6.4.1, Sec. 3.6.5; ISO
    Sec. 6.3.3.3, Sec. 6.3.8, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14,
    Sec. 6.3.15, Sec. 6.6.4.1, Sec. 6.6.5; H&S Sec. 7.5.4 pp. 196-7,
    Sec. 7.6.4 pp. 207-8, Sec. 7.6.5 pp. 208-9, Sec. 7.7 pp. 217-8,
    Sec. 7.8 pp. 218-9, Sec. 8.5 pp. 238-9, Sec. 8.6 pp. 241-4;
    "What the Tortoise Said to Achilles".

9.3:    Is if(p), where p is a pointer, a valid conditional?

A:    Yes.  See question 5.3.


Section 10. C Preprocessor

10.2:    Here are some cute preprocessor macros:

        #define begin    {
        #define end    }

    What do y'all think?

A:    Bleah.  See also section 17.

10.3:    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 illegal due to multiple side-effects; see question
    3.2).  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
    would have a hard time naming it if it did), 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.

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

A:    The usual goal is to write a macro that can be invoked as if it
    were a statement consisting of a single function call.  This
    means that the "caller" will be supplying the final semicolon,
    so the macro body should not.  The macro body cannot therefore
    be a simple brace-enclosed 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, therefore, is to use

        #define MACRO(arg1, arg2) 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.  (For an example, see the first DEBUG() macro
    in question 10.26.)  This technique also allows a value to be
    "returned."

    References: H&S Sec. 3.3.2 p. 45; CT&P Sec. 6.3 pp. 82-3.

10.6:    I'm splitting up a program into multiple source files for the
    first time, and I'm wondering what to put in .c files and what
    to put in .h files.  (What does ".h" mean, anyway?)

A:    As a general rule, you should put these things in header (.h)
    files:

        macro definitions (preprocessor #defines)
        structure, union, and enumeration declarations
        typedef declarations
        external function declarations (see also question 1.11)
        global variable declarations

    It's especially important to put a declaration or definition in
    a header file when it will be shared between several other
    files.  (In particular, never put external function prototypes
    in .c files.  See also question 1.7.)

    On the other hand, when a definition or declaration should
    remain private to one source file, it's fine to leave it there.

    See also questions 1.7 and 10.7.

    References: K&R2 Sec. 4.5 pp. 81-2; H&S Sec. 9.2.3 p. 267; CT&P
    Sec. 4.6 pp. 66-7.

10.7:    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
    17.9) disparages them; they can make it harder to find relevant
    definitions; they can lead to multiple-definition 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 can
    #include what it needs itself, rather than requiring each
    #includer to do so); a tool like grep (or a tags file) makes it
    easy to find definitions no matter where they are; a popular
    trick along the lines of:

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

    (where a different bracketing macro name is used for each header
    file) 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;
    see question 18.1) handle dependency generation in the face of
    nested #include files easily.  See also question 17.10.

    References: Rationale Sec. 4.1.2.

10.8:    Where are header ("#include") files searched for?

A:    The exact behavior is implementation-defined (which means that
    it is supposed to be documented; see question 11.33).
    Typically, headers named with <> syntax are searched for in one
    or more standard places.  Header files named with "" syntax are
    first searched for in the "current directory," then (if not
    found) in the same standard places.

    Traditionally (especially under Unix compilers), the current
    directory is taken to be the directory containing the file
    containing the #include directive.  Under other compilers,
    however, the current directory (if any) is the directory in
    which the compiler was initially invoked.  Check your compiler
    documentation.

    References: K&R2 Sec. A12.4 p. 231; ANSI Sec. 3.8.2; ISO
    Sec. 6.8.2; H&S Sec. 3.4 p. 55.

10.9:    I'm getting strange syntax errors on the very first declaration
    in a file, but it looks fine.

A:    Perhaps there's a missing semicolon at the end of the last
    declaration in the last header file you're #including.  See also
    questions 2.18 and 11.29.

10.11:    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).

10.12:    How can I construct preprocessor #if expressions which compare
    strings?

A:    You can't do it directly; preprocessor #if arithmetic uses only
    integers.  You can #define several manifest constants, however,
    and implement  conditionals on those.

    See also question 20.17.

    References: K&R2 Sec. 4.11.3 p. 91; ANSI Sec. 3.8.1; ISO
    Sec. 6.8.1; H&S Sec. 7.11.1 p. 225.

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

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

    References: ANSI Sec. 2.1.1.2, Sec. 3.8.1 footnote 83; ISO
    Sec. 5.1.1.2, Sec. 6.8.1; H&S Sec. 7.11.1 p. 225.

10.14:    Can I use an #ifdef in a #define line, to define something two
    different ways?

A:    No.  You can't "run the preprocessor on itself," so to speak.
    What you can do is use one of two completely separate #define
    lines, depending on the #ifdef setting.

    References: ANSI Sec. 3.8.3, Sec. 3.8.3.4; ISO Sec. 6.8.3,
    Sec. 6.8.3.4; H&S Sec. 3.2 pp. 40-1.

10.15:    Is there anything like an #ifdef for typedefs?

A:    Unfortunately, no.  (See also question 10.13.)

    References: ANSI Sec. 2.1.1.2, Sec. 3.8.1 footnote 83; ISO
    Sec. 5.1.1.2, Sec. 6.8.1; H&S Sec. 7.11.1 p. 225.

10.16:    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
    integers, 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 ).  See
    also question 20.9.

    References: ANSI Sec. 3.8.1; ISO Sec. 6.8.1; H&S Sec. 7.11.1
    p. 225.

10.18:    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 the
    preprocessor and expanding all of the #include's and #define's
    as well?

A:    There are programs floating around called unifdef, rmifdef, and
    scpp ("selective C preprocessor") which do exactly this.  See
    question 18.16.

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

A:    There's no standard way, although it is a common 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
    utility.  Beware that many traditional system-specific
    pre#defined identifiers (e.g. "unix") are non-Standard (because
    they clash with the user's namespace) and are being removed or
    renamed.

10.20:    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:    It was an undocumented feature of some early preprocessor
    implementations (notably John Reiser's) that comments
    disappeared entirely and could therefore be used for token
    pasting.  ANSI affirms (as did K&R1) 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 11.17.

    References: ANSI Sec. 3.8.3.3; ISO Sec. 6.8.3.3; Rationale
    Sec. 3.8.3.3; H&S Sec. 3.3.9 p. 52.

10.22:    Why is the macro

        #define TRACE(n) printf("TRACE: %d\n", n)

    giving me the warning "macro replacement within a string
    literal"?  It seems to be expanding

        TRACE(count);
    as
        printf("TRACE: %d\count", count);

A:    See question 11.18.

10.23:    How can I use a macro argument inside a string literal in the
    macro expansion?

A:    See question 11.18.

10.25:    I've got this tricky preprocessing I want to do and I can't
    figure out a way to do it.

A:    C's preprocessor is not intended as a general-purpose tool.
    (Note also that it is not guaranteed to be available as a
    separate program.)  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.  (One older one available
    on most Unix systems is m4.)

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

A:    One popular trick is to define and invoke the macro with a
    single, parenthesized "argument" which in the macro expansion
    becomes the entire argument list, parentheses and all, for a
    function such as printf():

        #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.

    gcc has an extension which allows a function-like macro to
    accept a variable number of arguments, but it's not standard.
    Other possible solutions are to use different macros (DEBUG1,
    DEBUG2, etc.) depending on the number of arguments, 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 15.4 and 15.5.


Section 11.  ANSI/ISO Standard C

11.1:    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
    controversial trigraph sequences).  The ANSI C standard also
    formalizes the C run-time library support routines.

    More recently, the Standard has been adopted as an international
    standard, ISO/IEC 9899:1990, and this ISO Standard replaces the
    earlier X3.159 even within the United States.  Its sections are
    numbered differently (briefly, ISO sections 5 through 7
    correspond roughly to the old ANSI sections 2 through 4).  As an
    ISO Standard, it is subject to ongoing revision through the
    release of Technical Corrigenda and Normative Addenda.

    In 1994, Technical Corrigendum 1 amended the Standard in about
    40 places, most of them minor corrections or clarifications.
    More recently, Normative Addendum 1 added about 50 pages of new
    material, mostly specifying new library functions for
    internationalization.  The production of Technical Corrigenda is
    an ongoing process, and a second one is expected in late 1995.
    In addition, both ANSI and ISO require periodic review of their
    standards.  This process is beginning in 1995, and will likely
    result in a completely revised standard (nicknamed "C9X," on the
    assumption of completion by 1999).

    The original ANSI Standard included a "Rationale," explaining
    many of its decisions, and discussing a number of subtle points,
    including several of those covered here.  (The Rationale was
    "not part of ANSI Standard X3.159-1989, but... included for
    information only," and is not included with the ISO Standard.)

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

A:    Copies are available in the United States from

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

    and

        Global Engineering Documents
        15 Inverness Way E
        Englewood, CO  80112  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

    (or see URL http://www.iso.ch or check the comp.std.internat FAQ
    list, Standards.Faq).

    At the time of this writing, the cost is $130.00 from ANSI or
    $410.00 from Global.  Copies of the original X3.159 (including
    the Rationale) may still be available at $205.00 from ANSI or
    $162.50 from Global.  Note that ANSI derives revenues to support
    its operations from the sale of printed standards, so electronic
    copies are *not* available.

    In the U.S., it may be possible to get a copy of the original
    ANSI X3.159 (including the Rationale) as "FIPS PUB 160" from

        National Technical Information Service (NTIS)
        U.S. Department of Commerce
        Springfield, VA  22161
        703 487 4650

    The mistitled _Annotated ANSI C Standard_, with annotations by
    Herbert Schildt, contains most of the 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: they are plagued
    by numerous errors and omissions, and a few pages of the
    Standard itself are missing.  Many people on the net recommend
    ignoring the annotations entirely.  A review of the annotations
    ("annotated annotations") by Clive Feather can be found on the
    web at http://www.lysator.liu.se/c/schildt.html .

    The text of the Rationale (not the full Standard) can be
    obtained by anonymous ftp from ftp.uu.net (see question 18.16)
    in directory doc/standards/ansi/X3.159-1989, and is also
    available on the web at
    http://www.lysator.liu.se/c/rat/title.html .  The Rationale has
    also been printed by Silicon Press, ISBN 0-929306-07-4.

11.3:    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 11.4), but not in this case.

    Old C (and ANSI C, in the absence of prototypes, and in variable-
    length argument lists; see question 15.2) "widens" certain
    arguments when they are passed to functions.  floats are
    promoted to double, and characters and short integers are
    promoted to int.  (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 altogether.

    See also question 1.25.

    References: K&R1 Sec. A7.1 p. 186; K&R2 Sec. A7.3.2 p. 202; ANSI
    Sec. 3.3.2.2, Sec. 3.5.4.3; ISO Sec. 6.3.2.2, Sec. 6.5.4.3;
    Rationale Sec. 3.3.2.2, Sec. 3.5.4.3; H&S Sec. 9.2 pp. 265-7,
    Sec. 9.4 pp. 272-3.

11.4:    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 11.3).  Note however that old-style syntax
    is marked as obsolescent, so official support for it may be
    removed some day.

    References: ANSI Sec. 3.7.1, Sec. 3.9.5; ISO Sec. 6.7.1,
    Sec. 6.9.5; H&S Sec. 9.2.2 pp. 265-7, Sec. 9.2.5 pp. 269-70.

11.5:    Why does the declaration

        extern f(struct x *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 structure
    declared (or even mentioned) for the first time within a
    prototype cannot be compatible with other structures declared in
    the same source file (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 places an (incomplete) declaration of struct x at file
    scope, so that all following declarations involving struct x can
    at least be sure they're referring to the same struct x.

    References: ANSI Sec. 3.1.2.1, Sec. 3.1.2.6, Sec. 3.5.2.3; ISO
    Sec. 6.1.2.1, Sec. 6.1.2.6, Sec. 6.5.2.3.

11.8:    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 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; ISO Sec. 6.4; H&S Secs. 7.11.2,7.11.3
    pp. 226-7.

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

A:    "char const *p" declares a pointer to a constant character (you
    can't change the character); "char * const p" declares a
    constant pointer to a (variable) character (i.e. you can't
    change the pointer).

    Read these "inside out" to understand them; see also question
    1.21.

    References: ANSI Sec. 3.5.4.1 examples; ISO Sec. 6.5.4.1;
    Rationale Sec. 3.5.4.1; H&S Sec. 4.4.4 p. 81.

11.10:    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.  However, 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, Sec. 3.3.16.1, Sec. 3.5.3; ISO
    Sec. 6.1.2.6, Sec. 6.3.16.1, Sec. 6.5.3; H&S Sec. 7.9.1 pp. 221-
    2.

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

A:    No.  main() must be declared as returning an int, and as taking
    either zero or two arguments, of the appropriate types.  If
    you're calling exit() but still getting warnings, you may have
    to insert a redundant return statement (or use some kind of "not
    reached" directive, if available).

    Declaring a function as void does not merely shut off or
    rearrange 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.

    (Note that this discussion of main() pertains only to "hosted"
    implementations; none of it applies to "freestanding"
    implementations, which may not even have main().  However,
    freestanding implementations are comparatively rare, and if
    you're using one, you probably know it.  If you've never heard
    of the distinction, you're probably using a hosted
    implementation, and the above rules apply.)

    References: ANSI Sec. 2.1.2.2.1, Sec. F.5.1; ISO Sec. 5.1.2.2.1,
    Sec. G.5.1; H&S Sec. 20.1 p. 416; CT&P Sec. 3.10 pp. 50-51.

11.13:    But what about main's third argument, envp?

A:    It's a non-standard (though common) extension.  If you really
    need to access the environment in ways beyind what the standard
    getenv() function provides, though, the global variable environ
    is probably a better avenue (though it's equally non-standard).

    References: ANSI Sec. F.5.1; ISO Sec. G.5.1; H&S Sec. 20.1 pp.
    416-7.

11.14:    I believe that declaring void main() can't fail, since I'm
    calling exit() instead of returning, and anyway my operating
    system ignores a program's exit/return status.

A:    It doesn't matter whether main() returns or not, or whether
    anyone looks at the status; the problem is that when main() is
    misdeclared, its caller (the runtime startup code) may not even
    be able to *call* it correctly (due to the potential clash of
    calling conventions; see question 11.12).  Your operating system
    may ignore the exit status, and void main() may work for you,
    but it is not portable and not correct.

11.15:    The book I've been using, _C Programing for the Compleat Idiot_,
    always uses void main().

A:    Perhaps its author counts himself among the target audience.
    Many books unaccountably use void main() in examples.  They're
    wrong.

11.16:    Is exit(status) truly equivalent to returning the same status
    from main()?

A:    Yes and no.  The Standard says that they are equivalent.
    However, a few older, nonconforming systems may have problems
    with one or the other form.  Also, a return from main() cannot
    be expected to work if data local to main() might be needed
    during cleanup; see also question 16.4.  (Finally, the two forms
    are obviously not equivalent in a recursive call to main().)

    References: K&R2 Sec. 7.6 pp. 163-4; ANSI Sec. 2.1.2.2.3; ISO
    Sec. 5.1.2.2.3.

11.17:    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 can use something like the following two-step procedure to
    force a 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 code 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; ISO
    Sec. 6.8.3.2, Sec. 6.8.3.5.

11.18:    What does the message "warning: macro replacement within a
    string literal" mean?

A:    Some pre-ANSI compilers/preprocessors interpreted macro
    definitions like

        #define TRACE(var, fmt) printf("TRACE: var = fmt\n", var)

    such that invocations like

        TRACE(i, %d);

    were expanded as

        printf("TRACE: i = %d\n", i);

    In other words, macro parameters were expanded even inside
    string literals and character constants.

    Macro expansion is *not* defined in this way by K&R or by
    Standard C.  When you do want to turn macro arguments into
    strings, you can use the new # preprocessing operator, along
    with string literal concatenation (another new ANSI feature):

        #define TRACE(var, fmt) \
            printf("TRACE: " #var " = " #fmt "\n", var)

    See also question 11.17 above.

    References: H&S Sec. 3.3.8 p. 51.

11.19:    I'm getting strange syntax errors inside lines 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 newlines inside quotes, and no
    unterminated comments or quotes (note particularly that an
    apostrophe within a contracted word looks like the beginning of
    a character constant).  Therefore, natural-language comments and
    pseudocode should always be written between the "official"
    comment delimiters /* and */.  (But see question 20.20, and also
    10.25.)

    References: ANSI Sec. 2.1.1.2, Sec. 3.1; ISO Sec. 5.1.1.2,
    Sec. 6.1; H&S Sec. 3.2 p. 40.

11.20:    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 lint's old
    /* NOTREACHED */ comments), etc.

    References: ANSI Sec. 3.8.6; ISO Sec. 6.8.6; H&S Sec. 3.7 p. 61.

11.21:    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 10.7.

11.22:    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 useful only in rare circumstances.  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.

    Most of the time, you should let the compiler count the
    initializers when initializing arrays (in the case of the
    initializer "abc", of course, the computed size will be 4).

    References: ANSI Sec. 3.5.7; ISO Sec. 6.5.7; H&S Sec. 4.6.4 p.
    98.

11.24:    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, convert the pointer either to
    char * or to the pointer type you're trying to manipulate (but
    see also question 4.5).

    References: ANSI Sec. 3.1.2.5, Sec. 3.3.6; ISO Sec. 6.1.2.5,
    Sec. 6.3.6; H&S Sec. 7.6.2 p. 204.

11.25:    What's 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: K&R2 Sec. B3 p. 250; ANSI Sec. 4.11.2.1,
    Sec. 4.11.2.2; ISO Sec. 7.11.2.1, Sec. 7.11.2.2; Rationale
    Sec. 4.11.2; H&S Sec. 14.3 pp. 341-2; PCS Sec. 11 pp. 165-6.

11.26:    What should malloc(0) do?  Return a null pointer or a pointer to
    0 bytes?

A:    The ANSI/ISO Standard says that it may do either; the behavior
    is implementation-defined (see question 11.33).

    References: ANSI Sec. 4.10.3; ISO Sec. 7.10.3; PCS Sec. 16.1 p.
    386.

11.27:    Why does the ANSI Standard not guarantee more than six case-
    insensitive characters of external identifier significance?

A:    The problem is older linkers which are under the control of
    neither the ANSI/ISO 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 11.1), which discusses several
    such schemes and explains why they could not be mandated.

    References: ANSI Sec. 3.1.2, Sec. 3.9.1; ISO Sec. 6.1.2,
    Sec. 6.9.1; Rationale Sec. 3.1.2; H&S Sec. 2.5 pp. 22-3.

11.29:    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 1.31, 10.9, and 11.30.

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

A:    It's possible to have a compiler available which accepts ANSI
    syntax, but not to have ANSI-compatible header files or run-time
    libraries installed.  (In fact, this situation is rather common
    when using a non-vendor-supplied compiler such as gcc.)  See
    also questions 11.29, 13.25, and 13.26.

11.31:    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 are
    part of the FSF's GNU C compiler distribution; see question
    18.3.

    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.

    Before converting ANSI C back to old-style, beware that such a
    conversion cannot always be made both safely and automatically.
    ANSI C introduces new features and complexities not found in K&R
    C.  You'll especially need to be careful of prototyped function
    calls; you'll probably need to insert explicit casts.  See also
    questions 11.3 and 11.29.

    Several prototype generators exist, many as modifications to
    lint.  A program called CPROTO was posted to comp.sources.misc
    in March, 1992.  There is another program called "cextract."
    Many vendors supply simple utilities like these with their
    compilers.  See also question 18.16.  (But be careful when
    generating prototypes for old functions with "narrow"
    parameters; see question 11.3.)

    Finally, are you sure you really need to convert lots of old
    code to ANSI C?  The old-style function syntax is still
    acceptable, and a hasty conversion can easily introduce bugs.
    (See question 11.3.)

11.32:    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:    Many 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 11.35.

11.33:    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.

    Note that since the Standard imposes *no* requirements on the
    behavior of a compiler faced with an instance of undefined
    behavior, the compiler can do absolutely anything.  In
    particular, there is no guarantee that the rest of the program
    will perform normally.  It's perilous to think that you can
    tolerate undefined behavior in a program; see question 3.2 for a
    relatively simple example.

    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.

    See also questions 3.9, and 11.34.

    References: ANSI Sec. 1.6; ISO Sec. 3.10, Sec. 3.16, Sec. 3.17;
    Rationale Sec. 1.6.

11.34:    I'm appalled that the ANSI Standard leaves so many issues
    undefined.  Isn't a Standard's whole job to standardize these
    things?

A:    It has always been a characteristic of C that certain constructs
    behaved in whatever way a particular compiler or a particular
    piece of hardware chose to implement them.  This deliberate
    imprecision often allows compilers to generate more efficient
    code for common cases, without having to burden all programs
    with extra code to assure well-defined behavior of cases deemed
    to be less reasonable.  Therefore, the Standard is simply
    codifying existing practice.

    A programming language standard can be thought of as a treaty
    between the language user and the compiler implementor.  Parts
    of that treaty consist of features which the compiler
    implementor agrees to provide, and which the user may assume
    will be available.  Other parts, however, consist of rules which
    the user agrees to follow and which the implementor may assume
    will be followed.  As long as both sides uphold their
    guarantees, programs have a fighting chance of working
    correctly.  If *either* side reneges on any of its commitments,
    nothing is guaranteed to work.

    See also question 11.35.

    References: Rationale Sec. 1.1.

11.35:    People keep saying that the behavior of i = i++ 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 questions 11.32,
    11.33, and 11.34.


Section 12. Stdio

12.1:    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: K&R1 Sec. 1.5 p. 14; K&R2 Sec. 1.5.1 p. 16; ANSI
    Sec. 3.1.2.5, Sec. 4.9.1, Sec. 4.9.7.5; ISO Sec. 6.1.2.5,
    Sec. 7.9.1, Sec. 7.9.7.5; H&S Sec. 5.1.3 p. 116, Sec. 15.1,
    Sec. 15.6; CT&P Sec. 5.1 p. 70; PCS Sec. 11 p. 157.

12.2:    Why does the code

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

    copy the last line twice?

A:    In C, EOF is only indicated *after* an input routine has tried
    to read, and has reached end-of-file.  (In other words, C's I/O
    is not like Pascal's.)  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.

    References: K&R2 Sec. 7.6 p. 164; ANSI Sec. 4.9.3, Sec. 4.9.7.1,
    Sec. 4.9.10.2; ISO Sec. 7.9.3, Sec. 7.9.7.1, Sec. 7.9.10.2; H&S
    Sec. 15.14 p. 382.

12.4:    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's 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 an interactive terminal.  (See also
    question 12.24.)

    References: ANSI Sec. 4.9.5.2; ISO Sec. 7.9.5.2.

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

A:    See question 19.1.

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

A:    Simply double the percent sign: %% .

    \% can't work, because the backslash \ is the *compiler's*
    escape character, while here our problem is that the % is
    printf's escape character.

    See also question 19.17.

    References: K&R1 Sec. 7.3 p. 147; K&R2 Sec. 7.2 p. 154; ANSI
    Sec. 4.9.6.1; ISO Sec. 7.9.6.1.

12.9:    Someone told me it was wrong to use %lf with printf().  How can
    printf() use %f for type double, if scanf() requires %lf?

A:    It's true that printf's %f specifier works with both float and
    double arguments.  Due to the "default argument promotions"
    (which apply in variable-length argument lists such as
    printf's, whether or not prototypes are in scope), values of
    type float are promoted to double, and printf() therefore sees
    only doubles.  See also questions 12.13 and 15.2.

    References: K&R1 Sec. 7.3 pp. 145-47, Sec. 7.4 pp. 147-50; K&R2
    Sec. 7.2 pp. 153-44, Sec. 7.4 pp. 157-59; ANSI Sec. 4.9.6.1,
    Sec. 4.9.6.2; ISO Sec. 7.9.6.1, Sec. 7.9.6.2; H&S Sec. 15.8 pp.
    357-64, Sec. 15.11 pp. 366-78; CT&P Sec. A.1 pp. 121-33.

12.10:    How can I implement a variable field width with printf?  That
    is, instead of %8d, I want the width to be specified at run
    time.

A:    printf("%*d", width, n) will do just what you want.  See also
    question 12.15.

    References: K&R1 Sec. 7.3; K&R2 Sec. 7.2; ANSI Sec. 4.9.6.1; ISO
    Sec. 7.9.6.1; H&S Sec. 15.11.6; CT&P Sec. A.1.

12.11:    How can I print numbers with commas separating the thousands?
    What about currency formatted numbers?

A:    The routines in <locale.h> begin to provide some support for
    these operations, but there is no standard routine for doing
    either task.  (The only thing printf() does in response to a
    custom locale setting is to change its decimal-point character.)

    References: ANSI Sec. 4.4; ISO Sec. 7.4; H&S Sec. 11.6 pp. 301-4.

12.12:    Why doesn't the call scanf("%d", i) work?

A:    The arguments you pass to scanf() must always be pointers.
    To fix the fragment above, change it to scanf("%d", &i) .

12.13:    Why doesn't this code:

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

    work?

A:    Unlike printf(), scanf() uses %lf for values of type double, and
    %f for float.  See also question 12.9.

12.15:    How can I specify a variable width in a scanf() format string?

A:    You can't; an asterisk in a scanf() format string means to
    suppress assignment.  You may be able to use ANSI stringizing
    and string concatenation to accomplish about the same thing, or
    to construct a scanf format string on-the-fly.

12.17:    When I read numbers from the keyboard with scanf "%d\n", it
    seems to hang until I type one extra line of input.

A:    Perhaps surprisingly, \n in a scanf format string does *not*
    mean to expect a newline, but rather to read and discard
    characters as long as each is a whitespace character.  See also
    question 12.20.

    References: K&R2 Sec. B1.3 pp. 245-6; ANSI Sec. 4.9.6.2; ISO
    Sec. 7.9.6.2; H&S Sec. 15.8 pp. 357-64.

12.18:    I'm reading a number with scanf %d and then a string with
    gets(), but the compiler seems to be skipping the call to
    gets()!

A:    scanf %d won't consume a trailing newline.  If the input number
    is immediately followed by a newline, that newline will
    immediately satisfy the gets().

    As a general rule, you shouldn't try to interlace calls to
    scanf() with calls to gets() (or any other input routines);
    scanf's peculiar treatment of newlines almost always leads to
    trouble.  Either use scanf() to read everything or nothing.

    See also questions 12.20 and 12.23.

    References: ANSI Sec. 4.9.6.2; ISO Sec. 7.9.6.2; H&S Sec. 15.8
    pp. 357-64.

12.19:    I figured I could use scanf() more safely if I checked its
    return value to make sure that the user typed the numeric values
    I expect, but sometimes it seems to go into an infinite loop.

A:    When scanf() is attempting to convert numbers, any non-numeric
    characters it encounters terminate the conversion *and are left
    on the input stream*.  Therefore, unless some other steps are
    taken, unexpected non-numeric input "jams" scanf() again and
    again: scanf() never gets past the bad character(s) to encounter
    later, valid data.  If the user types a character like `x' in
    response to a numeric scanf format such as %d or %f, code that
    simply re-prompts and retries the same scanf() call will
    immediately reencounter the same `x'.

    See also question 12.20.

    References: ANSI Sec. 4.9.6.2; ISO Sec. 7.9.6.2; H&S Sec. 15.8
    pp. 357-64.

12.20:    Why does everyone say not to use scanf()?  What should I use
    instead?

A:    scanf() has a number of problems -- see questions 12.17, 12.18,
    and 12.19.  Also, its %s format has the same problem that gets()
    has (see question 12.23) -- it's hard to guarantee that the
    receiving buffer won't overflow.

    More generally, scanf() is designed for relatively structured,
    formatted input (its name is in fact derived from "scan
    formatted").  If you pay attention, it will tell you whether it
    succeeded or failed, but it can tell you only approximately
    where it failed, and not at all how or why.  It's nearly
    impossible to do decent error recovery with scanf(); usually
    it's far easier to read entire lines (with fgets() or the like),
    then interpret them, either using sscanf() or some other
    techniques.  (Routines like strtol(), strtok(), and atoi() are
    often useful; see also question 13.6.)  If you do use sscanf(),
    don't forget to check the return value to make sure that the
    expected number of items were found.

    References: K&R2 Sec. 7.4 p. 159.

12.21:    How can I tell how much destination buffer space I'll need for
    an arbitrary sprintf call?  How can I avoid overflowing the
    destination buffer with sprintf()?

A:    There are not (yet) any good answers to either of these
    excellent questions, and this represents perhaps the biggest
    deficiency in the traditional stdio library.

    When the format string being used with sprintf() is known and
    relatively simple, you can usually predict a buffer size in an
    ad-hoc way.  If the format consists of one or two %s's, you can
    count the fixed characters in the format string yourself (or let
    sizeof count them for you) and add in the result of calling
    strlen() on the string(s) to be inserted.  You can
    conservatively estimate the size that %d will expand to with
    code like:

        #include <limits.h>
        char buf[(sizeof(int) * CHAR_BIT + 2) / 3 + 1 + 1];
        sprintf(buf, "%d", n);

    (This code computes the number of characters required for a base-
    8 representation of a number; a base-10 expansion is guaranteed
    to take as much room or less.)

    When the format string is more complicated, or is not even known
    until run time, predicting the buffer size becomes as difficult
    as reimplementing sprintf(), and correspondingly error-prone
    (and inadvisable).  A last-ditch technique which is sometimes
    suggested is to use fprintf() to print the same text to a bit
    bucket or temporary file, and then to look at fprintf's return
    value or the size of the file (but see question 19.12).

    If there's any chance that the buffer might not be big enough,
    you won't want to call sprintf() without some guarantee that the
    buffer will not overflow and overwrite some other part of
    memory.  Several stdio's (including GNU and 4.4bsd) provide the
    obvious snprintf() function, which can be used like this:

        snprintf(buf, bufsize, "You typed \"%s\"", answer);

    and we can hope that a future revision of the ANSI/ISO C
    Standard will include this function.

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

A:    Unlike fgets(), gets() cannot be told the size of the buffer
    it's to read into, so it cannot be prevented from overflowing
    that buffer.  As a general rule, always use fgets().  See
    question 7.1 for a code fragment illustrating the replacement of
    gets() with fgets().

    References: Rationale Sec. 4.9.7.2; H&S Sec. 15.7 p. 356.

12.24:    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 some operation which happens to
    fail (with ENOTTY) if stdout is not a terminal.  Although the
    output operation goes on to complete successfully, errno still
    contains ENOTTY.  (Note that it is only meaningful for a program
    to inspect the contents of errno after an error has been
    reported.)

    References: ANSI Sec. 4.1.3, Sec. 4.9.10.3; ISO Sec. 7.1.4,
    Sec. 7.9.10.3; CT&P Sec. 5.4 p. 73; PCS Sec. 14 p. 254.

12.25:    What's the difference between fgetpos/fsetpos and ftell/fseek?
    What are fgetpos() and fsetpos() good for?

A:    fgetpos() and fsetpos() use a special typedef, fpos_t, for
    representing offsets (positions) in a file.  The type behind
    this typedef, if chosen appropriately, can represent arbitrarily
    large offsets, allowing fgetpos() and fsetpos() to be used with
    arbitrarily huge files.  ftell() and fseek(), on the other hand,
    use long int, and are therefore limited to offsets which can be
    represented in a long int.  See also question 1.4.

    References: K&R2 Sec. B1.6 p. 248; ANSI Sec. 4.9.1,
    Secs. 4.9.9.1,4.9.9.3; ISO Sec. 7.9.1, Secs. 7.9.9.1,7.9.9.3;
    H&S Sec. 15.5 p. 252.

12.26:    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 stream, nor would such a way be sufficient unread
    characters can also accumulate in other, OS-level input buffers.

    References: ANSI Sec. 4.9.5.2; ISO Sec. 7.9.5.2; H&S Sec. 15.2.

12.30:    I'm trying to update a file in place, by using fopen mode "r+",
    reading a certain string, and 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 19.14.

    References: ANSI Sec. 4.9.5.3; ISO Sec. 7.9.5.3.

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

A:    Use freopen() (but see question 12.34 below).

    References: ANSI Sec. 4.9.5.4; ISO Sec. 7.9.5.4; H&S Sec. 15.2.

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

A:    There isn't a good way.  If you need to switch back, the best
    solution is not to have used 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.

12.38:    How can I read a binary data file properly?  I'm occasionally
    seeing 0x0a and 0x0d values getting garbled, and it seems to hit
    EOF prematurely if the data contains the value 0x1a.

A:    When you're reading a binary data file, you should specify "rb"
    mode when calling fopen(), to make sure that text file
    translations do not occur.  Similarly, when writing binary data
    files, use "wb".

    Note that the text/binary distinction is made when you open the
    file: once a file is open, it doesn't matter which I/O calls you
    use on it.  See also question 20.5.

    References: ANSI Sec. 4.9.5.3; ISO Sec. 7.9.5.3; H&S Sec. 15.2.1
    p. 348.


Section 13. Library Functions

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

A:    Just use sprintf().  (Don't worry that sprintf() may be
    overkill, potentially wasting run time or code space; it works
    well in practice.)  See the examples in the answer to question
    7.5; see also question 12.21.

    You can obviously use sprintf() to convert long or floating-
    point numbers to strings as well (using %ld or %f).

    References: K&R1 Sec. 3.6 p. 60; K&R2 Sec. 3.6 p. 64.

13.2:    Why does strncpy() not always place a '\0' terminator in the
    destination string?

A:    strncpy() was first designed to handle a now-obsolete data
    structure, the fixed-length, not-necessarily-\0-terminated
    "string."  (A related quirk of strncpy's is that it pads short
    strings with multiple \0's, out to the specified length.)
    strncpy() is admittedly a bit cumbersome to use in other
    contexts, since you must often append a '\0' to the destination
    string by hand.  You can get around the problem by using
    strncat() instead of strncpy(): if the destination string starts
    out empty, strncat() does what you probably wanted strncpy() to
    do.  Another possibility is sprintf(dest, "%.*s", n, source) .

    When arbitrary bytes (as opposed to strings) are being copied,
    memcpy() is usually a more appropriate routine to use than
    strncpy().

13.5:    Why do some versions of toupper() act strangely if given an
    upper-case letter?
    Why does some code call islower() before toupper()?

A:    Older versions of toupper() and tolower() did not always work
    correctly on arguments which did not need converting (i.e. on
    digits or punctuation or letters already of the desired case).
    In ANSI/ISO Standard C, these functions are guaranteed to work
    appropriately on all character arguments.

    References: ANSI Sec. 4.3.2; ISO Sec. 7.3.2; H&S Sec. 12.9 pp.
    320-1; PCS p. 182.

13.6:    How can I split up a string into whitespace-separated fields?
    How can I duplicate the process by which main() is handed argc
    and argv?

A:    The only Standard routine available for this kind of
    "tokenizing" is strtok, although it can be tricky to use and it
    may not do everything you want it to.  (For instance, it does
    not handle quoting.)

    References: K&R2 Sec. B3 p. 250; ANSI Sec. 4.11.5.8; ISO
    Sec. 7.11.5.8; H&S Sec. 13.7 pp. 333-4; PCS p. 178.

13.7:    I need some code to do regular expression and wildcard matching.

A:    Make sure you recognize the difference between classic regular
    expressions (variants of which are used in such Unix utilities
    as ed and grep), and filename wildcards (variants of which are
    used by most operating systems).

    There are a number of packages available for matching regular
    expressions.  Most packages use a pair of functions, one for
    "compiling" the regular expression, and one for "executing" it
    (i.e. matching strings against it).  Look for header files named
    <regex.h> or <regexp.h>, and functions called regcmp()/regex(),
    regcomp()/regexec(), or re_comp()/re_exec().  (These functions
    may exist in a separate regexp library.)  A popular, freely-
    redistributable regexp package by Henry Spencer is available
    from ftp.cs.toronto.edu in pub/regexp.shar.Z or in several other
    archives.  The GNU project has a package called rx.  See also
    question 18.16.

    Filename wildcard matching (sometimes called "globbing") is done
    in a variety of ways on different systems.  On Unix, wildcards
    are automatically expanded by the shell before a process is
    invoked, so programs rarely have to worry about them explicitly.
    Under MS-DOS compilers, there is often a special object file
    which can be linked in to a program to expand wildcards while
    argv is being built.  Several systems (including MS-DOS and VMS)
    provide system services for listing or opening files specified
    by wildcards.  Check your compiler/library documentation.

13.8:    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(), however, accepts simple pointers to
    char.  Therefore, strcmp() can't be used directly.  Write an
    intermediate comparison function like this:

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

    The comparison function's arguments are expressed as "generic
    pointers," const void *.  They are converted back to what they
    "really are" (char **) and dereferenced, yielding char *'s which
    can be passed to strcmp().  (Under a pre-ANSI compiler, declare
    the pointer parameters as char * instead of void *, and drop the
    consts.)

    (Don't be misled by the discussion in K&R2 Sec. 5.11 pp. 119-20,
    which is not discussing the Standard library's qsort).

    References: ANSI Sec. 4.10.5.2; ISO Sec. 7.10.5.2; H&S Sec. 20.5
    p. 419.

13.9:    Now I'm trying to sort an array of structures with qsort().  My
    comparison function 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 *) as
    discussed in question 13.8 above.  The comparison function might
    look like

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

    (The conversions from generic pointers to struct mystruct
    pointers happen in the initializations sp1 = p1 and sp2 = p2;
    the compiler performs the conversions implicitly since p1 and p2
    are void pointers.  Explicit casts, and char * pointers, would
    be required under a pre-ANSI compiler.  See also question 7.7.)

    If, on the other hand, you're sorting pointers to structures,
    you'll need indirection, as in question 13.8:
    sp1 = *(struct mystruct **)p1 .

    In general, it is a bad idea to insert casts just to "shut the
    compiler up."  Compiler warnings are usually trying to tell you
    something, and unless you really know what you're doing, you
    ignore or muzzle them at your peril.  See also question 4.9.

    References: ANSI Sec. 4.10.5.2; ISO Sec. 7.10.5.2; H&S Sec. 20.5
    p. 419.

13.10:    How can I sort a linked list?

A:    Sometimes it's easier to keep the list in order as you build it
    (or perhaps to use a tree instead).  Algorithms like insertion
    sort and merge sort lend themselves ideally to use with linked
    lists.  If you want to use a standard library function, you can
    allocate a temporary array of pointers, fill it in with pointers
    to all your list nodes, call qsort(), and finally rebuild the
    list pointers based on the sorted array.

    References: Knuth Sec. 5.2.1 pp. 80-102, Sec. 5.2.4 pp. 159-168;
    Sedgewick Sec. 8 pp. 98-100, Sec. 12 pp. 163-175.

13.11:    How can I sort more data than will fit in memory?

A:    You want an "external sort," which you can read about in Knuth,
    Volume 3.  The basic idea is to sort the data in chunks (as much
    as will fit in memory at one time), write each sorted chunk to a
    temporary file, and then merge the files.  Your operating system
    may provide a general-purpose sort utility, and if so, you can
    try invoking it from within your program: see questions 19.27
    and 19.30.

    References: Knuth Sec. 5.4 pp. 247-378; Sedgewick Sec. 13 pp.
    177-187.

13.12:    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(&now);
            printf("It's %.24s.\n", ctime(&now));
            return 0;
        }

    References: K&R2 Sec. B10 pp. 255-7; ANSI Sec. 4.12; ISO
    Sec. 7.12; H&S Sec. 18.

13.13:    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.

    Converting a string to a time_t is harder, because of the wide
    variety of date and time formats which might be encountered.
    Some systems provide a strptime() function, which is basically
    the inverse of strftime().  Other popular routines are partime()
    (widely distributed with the RCS package) and getdate() (and a
    few others, from the C news distribution).  See question 18.16.

    References: K&R2 Sec. B10 p. 256; ANSI Sec. 4.12.2.3; ISO
    Sec. 7.12.2.3; H&S Sec. 18.4 pp. 401-2.

13.14:    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
    incidentally 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.

    These solutions are only guaranteed to work correctly for dates
    in the range which can be represented as time_t's.  The tm_mday
    field is an int, so day offsets of more than 32,736 or so may
    cause overflow.  Note also that at daylight saving time
    changeovers, local days are not 24 hours long.

    Another approach to both problems is to use "Julian day"
    numbers.  Implementations of Julian day routines can be found in
    the file JULCAL10.ZIP from the Simtel/Oakland archives (see
    question 18.16) and the "Date conversions" article mentioned in
    the References.

    See also questions 13.13, 20.31, and 20.32.

    References: K&R2 Sec. B10 p. 256; ANSI Secs. 4.12.2.2,4.12.2.3;
    ISO Secs. 7.12.2.2,7.12.2.3; H&S Secs. 18.4,18.5 pp. 401-2;
    David Burki, "Date Conversions".

13.15:    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.

    If you do find yourself needing to implement your own random
    number generator, there is plenty of literature out there; see
    the References.  There are also any number of packages on the
    net: look for r250, RANLIB, and FSULTRA (see question 18.16).

    References: K&R2 Sec. 2.7 p. 46, Sec. 7.8.7 p. 168; ANSI
    Sec. 4.10.2.1; ISO Sec. 7.10.2.1; H&S Sec. 17.7 p. 393; PCS
    Sec. 11 p. 172; Knuth Vol. 2 Chap. 3 pp. 1-177; Park and Miller,
    "Random Number Generators: Good Ones are hard to Find".

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

A:    The obvious way,

        rand() % N        /* POOR */

    (which tries to return numbers from 0 to N-1) is poor, because
    the low-order bits of many random number generators are
    distressingly *non*-random.  (See question 13.18.)  A better
    method is something like

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

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

        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.

    (Note, by the way, that RAND_MAX is a *constant* telling you
    what the fixed range of the C library rand() function is.  You
    cannot set RAND_MAX to some other value, and there is no way of
    requesting that rand() return numbers in some other range.)

    If you're starting with a random number generator which returns
    floating-point values between 0 and 1, all you have to do to get
    integers from 0 to N-1 is multiply the output of that generator
    by N.

    References: K&R2 Sec. 7.8.7 p. 168; PCS Sec. 11 p. 172.

13.17:    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 truly 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 19.37).  (Note also that it's rarely useful to call
    srand() more than once during a run of a program; in particular,
    don't try calling srand() before each call to rand(), in an
    attempt to get "really random" numbers.)

    References: K&R2 Sec. 7.8.7 p. 168; ANSI Sec. 4.10.2.2; ISO
    Sec. 7.10.2.2; H&S Sec. 17.7 p. 393.

13.18:    I need a random true/false value, so I'm just taking rand() % 2,
    but it's 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 13.16.

    References: Knuth Sec. 3.2.1.1 pp. 12-14.

13.20:    How can I generate random numbers with a normal or Gaussian
    distribution?

A:    Here is one method, by Box and Muller, and recommended by Knuth:

        #include <stdlib.h>
        #include <math.h>

        double gaussrand()
        {
            static double V1, V2, S;
            static int phase = 0;
            double X;

            if(phase == 0) {
                do {
                    double U1 = (double)rand() / RAND_MAX;
                    double U2 = (double)rand() / RAND_MAX;

                    V1 = 2 * U1 - 1;
                    V2 = 2 * U2 - 1;
                    S = V1 * V1 + V2 * V2;
                    } while(S >= 1 || S == 0);

                X = V1 * sqrt(-2 * log(S) / S);
            } else
                X = V2 * sqrt(-2 * log(S) / S);

            phase = 1 - phase;

            return X;
        }

    See the extended versions of this list (see question 20.40) for
    other ideas.

    References: Knuth Sec. 3.4.1 p. 117; Box and Muller, "A Note on
    the Generation of Random Normal Deviates"; Press et al.,
    _Numerical Recipes in C_ Sec. 7.2 pp. 288-290.

13.24:    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 11.25).
    bcmp?                use memcmp.
    bzero?                use memset, with a second
                    argument of 0.

    Contrariwise, if you're using an older system which is missing
    the functions in the second column, you may be able to implement
    them in terms of, or substitute, the functions in the first.

    References: PCS Sec. 11.

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

A:    In some cases (especially if the functions are nonstandard) you
    may have to explicitly ask for the correct libraries to be
    searched when you link the program.  See also questions 11.30,
    13.26, and 14.3.

13.26:    I'm still getting errors due to library functions being
    undefined, even though I'm explicitly requesting the right
    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 options towards the end of the command
    line.)  See also question 13.28.

13.28:    What does it mean when the linker says that _end is undefined?

A:    That message is a quirk of the old Unix linkers.  You only get
    an error about _end being undefined when other things are
    undefined, too -- fix the others, and the error about _end will
    disappear.  (See also questions 13.25 and 13.26.)


Section 14. Floating Point

14.1:    When I set a float variable to, say, 3.1, why is printf()
    printing it as 3.0999999?

A:    Most computers use base 2 for floating-point numbers as well as
    for integers.  In base 2, 1/1010 (that is, 1/10 decimal) is an
    infinitely-repeating fraction: its binary representation is
    0.0001100110011... .  Depending on how carefully your compiler's
    binary/decimal conversion routines (such as those used by
    printf) have been written, you may see discrepancies when
    numbers (especially low-precision floats) not exactly
    representable in base 2 are assigned or read in and then printed
    (i.e. converted from base 10 to base 2 and back again).  See
    also question 14.6.

14.2:    I'm trying to take some square roots, but I'm getting crazy
    numbers.

A:    Make sure that you have #included <math.h>, and correctly
    declared other functions returning double.  (Another library
    routine to be careful with is atof(), which is declared in
    <stdlib.h>.)  See also question 14.3 below.

    References: CT&P Sec. 4.5 pp. 65-6.

14.3:    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 actually linking with the math library.  For
    instance, under Unix, you usually need to use the -lm option, at
    the *end* of the command line, when compiling/linking.  See also
    questions 13.25 and 13.26.

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

A:    First, see question 14.2 above.

    If the problem isn't that simple, recall that digital computers
    usually 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; see question 14.5.)

    These problems are no worse for C than they are for any other
    computer language.  Certain aspects of floating-point are
    usually defined as "however the processor does them" (see also
    question 11.34), 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
    numerical programming text should cover the basics; see also the
    references below.

    References: Kernighan and Plauger, _The Elements of Programming
    Style_ Sec. 6 pp. 115-8; Knuth, Volume 2 chapter 4; David
    Goldberg, "What Every Computer Scientist Should Know about
    Floating-Point Arithmetic".

14.5:    What's a good way to check for "close enough" floating-point
    equality?

A:    Since the absolute accuracy of floating point values varies, by
    definition, with their magnitude, the best way of comparing two
    floating point values is to use an accuracy threshold which is
    relative to the magnitude of the numbers being compared.  Rather
    than

        double a, b;
        ...
        if(a == b)    /* WRONG */

    use something like

        #include <math.h>

        if(fabs(a - b) <= epsilon * a)

    for some suitably-chosen epsilon.

    References: Knuth Sec. 4.2.2 pp. 217-8.

14.6:    How do I round numbers?

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

        (int)(x + 0.5)

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

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

A:    Because few processors have an exponentiation instruction.  C
    has a pow() function, declared in <math.h>, although explicit
    multiplication is often better for small positive integral
    exponents.

    References: ANSI Sec. 4.5.5.1; ISO Sec. 7.5.5.1; H&S Sec. 17.6
    p. 393.

14.8:    The pre-#defined constant M_PI seems to be missing from my
    machine's copy of <math.h>.

A:    That constant (which is apparently supposed to be the value of
    pi, accurate to the machine's precision), is not standard.  If
    you need pi, you'll have to #define it yourself.

    References: PCS Sec. 13 p. 237.

14.9:    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. predefined constants,
    and functions like isnan(), either as nonstandard extensions in
    <math.h> or perhaps in <ieee.h> or <nan.h>) to deal with these
    values cleanly, and work is being done 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.

    Another possibility is to to format the value in question using
    sprintf(): on many systems it generates strings like "NaN" and
    "Inf" which you could compare for in a pinch.

    See also question 19.39.

14.11:    What's a good way to implement complex numbers in C?

A:    It is straightforward to define a simple structure and some
    arithmetic functions to manipulate them.  See also questions
    2.7, 2.10, and 14.12.

14.12:    I'm looking for some code to do:
        Fast Fourier Transforms (FFT's)
        matrix arithmetic (multiplication, inversion, etc.)
        complex arithmetic

A:    Ajay Shah maintains an index of free numerical software; it is
    posted periodically, and available where this FAQ list is
    archived (see question 20.40).  See also question 18.16.

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

A:    The message in the question has to do with an indecent problem
    in Borland's compilers, which for some unfathomable reason has
    still not been fixed.  However, by the newly-passed
    Communications Decency Act of the U.S., I am prohibited from
    transmitting or discussing "indecent" material.  (If the fact
    that users of Borland's compilers are still having this problem
    isn't indecent, I don't know what is.)  If you send me e-mail
    certifying that you are over 18 years of age, I may be able to
    help you.  (You may also be able to see the
    comp.os.msdos.programmer FAQ list for more information.)


Section 15. Variable-Length Argument Lists

15.1:    I heard that you have to #include <stdio.h> before calling
    printf().  Why?

A:    So that a proper prototype for printf() will be in scope.

    A compiler may use a different calling sequence for functions
    which accept variable-length argument lists.  (It might do so if
    calls using variable-length argument lists were less efficient
    than those using fixed-length.)  Therefore, a prototype
    (indicating, using the ellipsis notation "...", that the
    argument list is of variable length) must be in scope whenever a
    varargs function is called, so that the compiler knows to use
    the varargs calling mechanism.

    References: ANSI Sec. 3.3.2.2, Sec. 4.1.6; ISO Sec. 6.3.2.2,
    Sec. 7.1.7; Rationale Sec. 3.3.2.2, Sec. 4.1.6; H&S Sec. 9.2.4
    pp. 268-9, Sec. 9.6 pp. 275-6.

15.2:    How can %f be used for both float and double arguments in
    printf()?  Aren't they different types?

A:    In the variable-length part of a variable-length argument list,
    the "default argument promotions" apply: types char and
    short int are promoted to int, and float is promoted to double.
    (These are the same promotions that apply to function calls
    without a prototype in scope, also known as "old style" function
    calls; see question 11.3.)  Therefore, printf's %f format always
    sees a double.  (Similarly, %c always sees an int, as does %hd.)
    See also questions 12.9 and 12.13.

    References: ANSI Sec. 3.3.2.2; ISO Sec. 6.3.2.2; H&S Sec. 6.3.5
    p. 177, Sec. 9.4 pp. 272-3.

15.3:    I had a frustrating problem which turned out to be caused by the
    line

        printf("%d", n);

    where n was actually a long int.  I thought that ANSI function
    prototypes were supposed to guard against argument type
    mismatches like this.

A:    When a function accepts a variable number of arguments, its
    prototype does not (and cannot) provide any information about
    the number and types of those variable arguments.  Therefore,
    the usual protections do *not* apply in the variable-length part
    of variable-length argument lists: the compiler cannot perform
    implicit conversions or (in general) warn about mismatches.

    See also questions 5.2, 11.3, 12.9, and 15.2.

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

A:    Use the facilities of the <stdarg.h> header.

    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;
            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);        /* restart for second scan */

            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; see questions 5.2 and 15.3.
    (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 also question 15.7.

    References: K&R2 Sec. 7.3 p. 155, Sec. B7 p. 254; ANSI Sec. 4.8;
    ISO Sec. 7.8; Rationale Sec. 4.8; H&S Sec. 11.4 pp. 296-9; CT&P
    Sec. A.3 pp. 139-141; PCS Sec. 11 pp. 184-5, Sec. 13 p. 242.

15.5:    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");
        }

    See also question 15.7.

    References: K&R2 Sec. 8.3 p. 174, Sec. B1.2 p. 245; ANSI
    Secs. 4.9.6.7,4.9.6.8,4.9.6.9; ISO
    Secs. 7.9.6.7,7.9.6.8,7.9.6.9; H&S Sec. 15.12 pp. 379-80; PCS
    Sec. 11 pp. 186-7.

15.6:    How can I write a function analogous to scanf(), that calls
    scanf() to do most of the work?

A:    Unfortunately, vscanf and the like are not standard.  You're on
    your own.

15.7:    I have a pre-ANSI compiler, without <stdarg.h>.  What can I do?

A:    There's an older header, <varargs.h>, which offers about the
    same functionality.

    To rewrite the error() function from question 15.5 to use
    <varargs.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: H&S Sec. 11.4 pp. 296-9; CT&P Sec. A.2 pp. 134-139;
    PCS Sec. 11 pp. 184-5, Sec. 13 p. 250.

15.8:    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, long
    ints, 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,
    applicable 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 in questions 5.2 and 15.4).  Finally, if their types
    are predictable, you can pass an explicit count of the number of
    variable arguments (although it's usually a nuisance for the
    caller to generate).

    References: PCS Sec. 11 pp. 167-8.

15.9:    My compiler isn't letting me declare a function

        int f(...)
        {
        }

    i.e. with no fixed arguments.

A:    Standard C requires at least one fixed argument, in part so that
    you can hand it to va_start().

    References: ANSI Sec. 3.5.4, Sec. 3.5.4.3, Sec. 4.8.1.1; ISO
    Sec. 6.5.4, Sec. 6.5.4.3, Sec. 7.8.1.1; H&S Sec. 9.2 p. 263.

15.10:    I have a varargs function which accepts a float parameter.  Why
    isn't

        va_arg(argp, float)

    working?

A:    In the variable-length part of variable-length argument lists,
    the old "default argument promotions" apply: arguments of type
    float are always promoted (widened) to type double, and types
    char and short int are promoted to int.  Therefore, it is never
    correct to invoke va_arg(argp, float); instead you should always
    use va_arg(argp, double).  Similarly, use va_arg(argp, int) to
    retrieve arguments which were originally char, short, or int.
    See also questions 11.3 and 15.2.

    References: ANSI Sec. 3.3.2.2; ISO Sec. 6.3.2.2; Rationale
    Sec. 4.8.1.2; H&S Sec. 11.4 p. 297.

15.11:    I can't get va_arg() 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.  See also question 1.21.

    References: ANSI Sec. 4.8.1.2; ISO Sec. 7.8.1.2; Rationale
    Sec. 4.8.1.2.

15.12:    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.  Ideally, you should provide a version
    of that other function which accepts a va_list pointer
    (analogous to vfprintf(); see question 15.5 above).  If the
    arguments must be passed directly as actual arguments, or if you
    do not have the option of rewriting the second function to
    accept a va_list (in other words, if the second, called function
    must accept a variable number of arguments, not a va_list), no
    portable solution is possible.  (The problem could perhaps be
    solved by resorting to machine-specific assembly language; see
    also question 15.13 below.)

15.13:    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...

    Instead of an actual argument list, you might consider passing
    an array of generic (void *) pointers.  The called function can
    then step through the array, much like main() might step through
    argv.  (Obviously this works only if you have control over all
    the called functions.)

    (See also question 19.36.)


Section 16. Strange Problems

16.3:    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, in which case you could dynamically allocate
    them with malloc(); see also question 1.31).

    (See also questions 11.12, 16.4, 16.5, and 18.4.)

16.4:    I have a program that seems to run correctly, but it crashes as
    it's exiting, *after* the last statement in main().  What could
    be causing this?

A:    Look for a misdeclared main() (see questions 2.18 and 10.9), or
    local buffers passed to setbuf() or setvbuf(), or problems in
    cleanup functions registered by atexit().  See also questions
    7.5 and 11.16.

    References: CT&P Sec. 5.3 pp. 72-3.

16.5:    This program runs perfectly on one machine, but I get weird
    results on another.  Stranger still, adding or removing
    debugging printouts changes the symptoms...

A:    Lots of things could be going wrong; here are a few of the more
    common things to check:

        uninitialized local variables (see also question 7.1)

        integer overflow, especially on 16-bit machines,
        especially of an intermediate result when doing things
        like a * b / c (see also question 3.14)

        undefined evaluation order (see questions 3.1 through 3.4)

        omitted declaration of external functions, especially
        those which return something other than int
        (see questions 1.25 and 14.2)

        dereferenced null pointers (see section 5)

        improper malloc/free use: assuming malloc'ed memory
        contains 0, assuming freed storage persists, freeing
        something twice (see also questions 7.20 and 7.19)

        pointer problems in general (see also question 16.8)

        mismatch between printf() format and arguments, especially
        trying to print long ints using %d (see question 12.9)

        trying to malloc(256 * 256 * sizeof(double)), especially
        on machines with limited memory (see also questions 7.16
        and 19.23)

        array bounds problems, especially of small, temporary
        buffers, perhaps used for constructing strings with
        sprintf() (see also questions 7.1 and 12.21)

        invalid assumptions about the mapping of typedefs,
        especially size_t

        floating point problems (see questions 14.1 and 14.4)

        anything you thought was a clever exploitation of the way
        you believe code is generated for your specific system

    Proper use of function prototypes can catch several of these
    problems; lint would catch several more.  See also questions
    16.3, 16.4, and 18.4.

16.6:    Why does this code:

        char *p = "hello, world!";
        p[0] = 'H';

    crash?

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

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

    See also question 1.32.

    References: ANSI Sec. 3.1.4; ISO Sec. 6.1.4; H&S Sec. 2.7.4 pp.
    31-2.

16.8:    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.
    Likely causes are inadvertent use of null pointers (see also
    questions 5.2 and 5.20) or uninitialized, misaligned, or
    otherwise improperly allocated pointers (see questions 7.1 and
    7.2); corruption of the malloc arena (see question 7.19); and
    mismatched function arguments, especially involving pointers;
    two possibilities are scanf() (see question 12.12) and fprintf()
    (make sure it receives its first FILE * argument).

    See also questions 16.3 and 16.4.


Section 17. Style

17.1:    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 disregarding 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.

    See also question 10.6.

    References: K&R1 Sec. 1.2 p. 10; K&R2 Sec. 1.2 p. 10.

17.3:    Here's a neat trick for checking whether two strings are equal:

        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 the
    use of ! ("not") suggests that it tests for inequality.

    A better option 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 rigorously codified.  See also
    question 17.10.

17.4:    Why do some people write if(0 == x) instead of if(x == 0)?

A:    It's a trick to guard against the common error of writing

        if(x = 0)

    If you're in the habit of writing the constant before the ==,
    the compiler will complain if you accidentally type

        if(0 = x)

    Evidently it can be easier to remember to reverse the test than
    it is to remember to type the doubled = sign.

    References: H&S Sec. 7.6.5 pp. 209-10.

17.5:    I came across some code that puts a (void) cast before each call
    to printf().  Why?

A:    printf() does return a value, though few programs bother to
    check the return values from each call.  Since some compilers
    (and lint) will warn about discarded return values, an explicit
    cast to (void) is a way of saying "Yes, I've decided to ignore
    the return value from this call, but please continue to warn me
    about other (perhaps inadvertently) ignored return values."
    It's also common to use void casts on calls to strcpy() and
    strcat(), since the return value is never surprising.

    References: K&R2 Sec. A6.7 p. 199; Rationale Sec. 3.3.4; H&S
    Sec. 6.2.9 p. 172, Sec. 7.13 pp. 229-30.

17.8:    What is "Hungarian Notation"?  Is it worthwhile?

A:    Hungarian Notation is a naming convention, invented by Charles
    Simonyi, which encodes things about a variable's type (and
    perhaps its intended use) in its name.  It is well-loved in some
    circles and roundly castigated in others.  Its chief advantage
    is that it makes a variable's type or intended use obvious from
    its name; its chief disadvantage is that type information is not
    necessarily a worthwhile thing to carry around in the name of a
    variable.

    References: Simonyi and Heller, "The Hungarian Revolution" .

17.9:    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    pub/cstyle.tar.Z
                    (the updated Indian Hill guide)

        ftp.cs.toronto.edu    doc/programming
                    (including Henry Spencer's
                    "10 Commandments for C Programmers")

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

    You may also be interested in the books _The Elements of
    Programming Style_, _Plum Hall Programming Guidelines_, and _C
    Style: Standards and Guidelines_; see the Bibliography.  (The
    _Standards and Guidelines_ book is not in fact a style guide,
    but a set of guidelines on selecting and creating style guides.)

    See also question 18.9.

17.10:    Some people say that goto's are evil and that I should never use
    them.  Isn't that a bit extreme?

A:    Programming style, like writing style, is somewhat of an art and
    cannot be codified by inflexible rules, although discussions
    about style often seem to center exclusively around such rules.

    In the case of the goto statement, it has long been observed
    that unfettered use of goto's quickly leads to unmaintainable
    spaghetti code.  However, a simple, unthinking ban on the goto
    statement does not necessarily lead immediately to beautiful
    programming: an unstructured programmer is just as capable of
    constructing a Byzantine tangle without using any goto's
    (perhaps substituting oddly-nested loops and Boolean control
    variables, instead).

    Most observations or "rules" about programming style usually
    work better as guidelines than rules, and work much better if
    programmers understand what the guidelines are trying to
    accomplish.  Blindly avoiding certain constructs or following
    rules without understanding them can lead to just as many
    problems as the rules were supposed to avert.

    Furthermore, many opinions on programming style are just that:
    opinions.  It's usually futile to get dragged into "style wars,"
    because on certain issues (such as those referred to in
    questions 9.2, 5.3, 5.9, and 10.7), opponents can never seem to
    agree, or agree to disagree, or stop arguing.


Section 18. Tools and Resources

18.1:    I need:                 A:    Look for programs (see also
                    question 18.16) named:

    a C cross-reference        cflow, cxref, calls, cscope,
    generator            xscope, or ixfw

    a C beautifier/pretty-        cb, indent, GNU indent, or
    printer                vgrind

    a revision control or        RCS or SCCS
    configuration management
    tool

    a C source obfuscator        obfus, shroud, or opqcp
    (shrouder)

    a "make" dependency        makedepend, or try cc -M or
    generator            cpp -M

    tools to compute code        ccount, Metre, lcount, or
    metrics                csize, or see URL
                    http://www.qucis.queensu.ca:1999/Software-
                    Engineering/Cmetrics.html ;
                    there is also a package sold
                    by McCabe and Associates

    a C lines-of-source        this can be done very
    counter                crudely with the standard
                    Unix utility wc, and
                    considerably better with
                    grep -c ";"

    a prototype generator        see question 11.31

    a tool to track down
    malloc problems            see question 18.2

    a "selective" C
    preprocessor            see question 10.18

    language translation        see questions 11.31 and
    tools                 20.26

    C verifiers (lint)        see question 18.7

    a C compiler!            see question 18.3

    (This list of tools is by no means complete; if you know of tools
    not mentioned, you're welcome to contact this list's maintainer.)

    Other lists of tools, and discussion about them, can be found in
    the Usenet newsgroups comp.compilers and comp.software-eng .

    See also questions 18.16 and 18.3.

18.2:    How can I track down these pesky malloc problems?

A:    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 1992, volume 32.  Others are
    "leak," available in volume 27 of the comp.sources.unix
    archives; JMalloc.c and JMalloc.h in the "Snippets" collection;
    and MEMDEBUG from ftp.crpht.lu in pub/sources/memdebug .  See
    also question 18.16.

    A number of commercial debugging tools exist, and can be
    invaluable in tracking down malloc-related and other stubborn
    problems:

        Bounds-Checker for DOS, from Nu-Mega Technologies,
        P.O. Box 7780, Nashua, NH 03060-7780, USA, 603-889-2386.

        CodeCenter (formerly Saber-C) from Centerline Software
        (formerly Saber), 10 Fawcett Street, Cambridge, MA
        02138-1110, USA, 617-498-3000.

        Insight, from ParaSoft Corporation, 2500 E. Foothill
        Blvd., Pasadena, CA 91107, USA, 818-792-9941,
        [e-mail] .

        Purify, from Pure Software, 1309 S. Mary Ave., Sunnyvale,
        CA 94087, USA, 800-224-7873, [e-mail] .

        Sentinel, from AIB Software, 1145 Herndon Pkwy #200, Herndon,
        VA 22070, USA, 703-787-7700, 800-296-3000, [e-mail] .

18.3:    What's a free or cheap C compiler I can use?

A:    A popular and high-quality free C compiler is the FSF's GNU C
    compiler, or gcc.  It is available by anonymous ftp from
    prep.ai.mit.edu in directory pub/gnu, or at several other FSF
    archive sites.  An MS-DOS port, djgpp, is also available; it can
    be found in the Simtel and Oakland archives and probably many
    others, usually in a directory like pub/msdos/djgpp/ or
    simtel/msdos/djgpp/.

    There is a shareware compiler called PCC, available as
    PCC12C.ZIP .

    A very inexpensive MS-DOS compiler is Power C from Mix Software,
    1132 Commerce Drive, Richardson, TX 75801, USA, 214-783-6001.

    Another recently-developed compiler is lcc, available for
    anonymous ftp from ftp.cs.princeton.edu in pub/lcc.

    Archives associated with comp.compilers contain a great deal of
    information about available compilers, interpreters, grammars,
    etc. (for many languages).  The comp.compilers archives
    (including an FAQ list), maintained by the moderator, John R.
    Levine, are at iecc.com .  A list of available compilers and
    related resources, maintained by Mark Hopkins, Steven Robenalt,
    and David Muir Sharnoff, is at ftp.idiom.com in pub/compilers-
    list/.  (See also the comp.compilers directory in the
    news.answers archives at rtfm.mit.edu and ftp.uu.net; see
    question 20.40.)

    See also question 18.16.

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

A:    See if you can run lint first (perhaps with the -a, -c, -h, -p
    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.

    See also questions 16.5 and 16.8.

    References: Ian Darwin, _Checking C Programs with lint_ .

18.5:    How can I shut off the "warning: possible pointer alignment
    problem" message which 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.  (But don't get in the habit of ignoring too
    many lint messages, otherwise one day you'll overlook a
    significant one.)

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

A:    Products called PC-Lint and FlexeLint (in "shrouded source
    form," for compilation on 'most any system) are available from

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

    The Unix 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.

    Another ANSI-compatible lint (which can also perform higher-
    level formal verification) is LCLint, available via anonymous
    ftp from larch.lcs.mit.edu in pub/Larch/lclint/.

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

18.8:    Don't ANSI function prototypes render lint obsolete?

A:    Not really.  First of all, prototypes work only if they are
    present and correct; an inadvertently incorrect prototype is
    worse than useless.  Secondly, lint checks consistency across
    multiple source files, and checks data declarations as well as
    functions.  Finally, an independent program like lint will
    probably always be more scrupulous at enforcing compatible,
    portable coding practices than will any particular,
    implementation-specific, feature- and extension-laden compiler.

    If you do want to use function prototypes instead of lint for
    cross-file consistency checking, make sure that you set the
    prototypes up correctly in header files.  See questions 1.7 and
    10.6.

18.9:    Are there any C tutorials or other resources on the net?

A:    There are several of them:

    "Notes for C programmers," by Christopher Sawtell, are
    available from svr-ftp.eng.cam.ac.uk in misc/sawtell_C.shar and
    garbo.uwasa.fi in /pc/c-lang/c-lesson.zip .

    Tim Love's "C for Programmers" is available by ftp from svr-
    ftp.eng.cam.ac.uk in the misc directory.  An html version is at
    http://club.eng.cam.ac.uk/help/tpl/languages/C/teaching_C/teaching_C.html .

    The Coronado Enterprises C tutorials are available on Simtel
    mirrors in pub/msdos/c/.

    Rick Rowe has a tutorial which is available from ftp.netcom.com
    as pub/rowe/tutorde.zip or ftp.wustl.edu as
    pub/MSDOS_UPLOADS/programming/c_language/ctutorde.zip .

    There is evidently a web-based course at
    http://www.strath.ac.uk/CC/Courses/CCourse/CCourse.html .

    Finally, on some Unix machines you can try typing learn c at the
    shell prompt.

    [Disclaimer: I have not reviewed these tutorials; I have heard
    that at least one of them contains a number of errors.  Also,
    this sort of information rapidly becomes out-of-date; these
    addresses may not work by the time you read this and try them.]

    Several of these tutorials, plus a great deal of other
    information about C, are accessible via the web at
    http://www.lysator.liu.se/c/index.html .

    Vinit Carpenter maintains a list of resources for learning C and
    C++; it is posted to comp.lang.c and comp.lang.c++, and archived
    where this FAQ list is (see question 20.40), or on the web at
    http://vinny.csd.mu.edu/ .

    See also question 18.10 below.

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

A:    There are far too many books on C to list here; it's impossible
    to rate them all.  Many people believe that the best one was
    also the first: _The C Programming Language_, by Kernighan and
    Ritchie ("K&R," now in its second edition).  Opinions vary on
    K&R's suitability as an initial programming text: many of us did
    learn C from it, and learned it well; some, however, feel that
    it is a bit too clinical as a first tutorial for those without
    much programming background.

    An excellent reference manual is _C: A Reference Manual_, by
    Samuel P. Harbison and Guy L. Steele, now in its fourth edition.

    Though not suitable for learning C from scratch, this FAQ list
    has been published in book form; see the Bibliography.

    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.  See
    also question 18.9 above.

18.13:    Where can I find the sources of the standard C libraries?

A:    One source (though not public domain) is _The Standard C
    Library_, by P.J. Plauger (see the Bibliography).
    Implementations of all or part of the C library have been
    written and are readily available as part of the netBSD and GNU
    (also Linux) projects.  See also question 18.16.

18.14:    I need code to parse and evaluate expressions.

A:    Two available packages are "defunc," posted to comp.sources.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.  Other options include the
    S-Lang interpreter, available via anonymous ftp from
    amy.tch.harvard.edu in pub/slang, and the shareware Cmm ("C-
    minus-minus" or "C minus the hard stuff").  See also question
    18.16.

    There is also some parsing/evaluation code in _Software
    Solutions in C_ (chapter 12, pp. 235-55).

18.15:    Where can I get a BNF or YACC grammar for C?

A:    The definitive grammar is of course the one in the ANSI
    standard; see question 11.2.  Another grammar (along with one
    for C++) by Jim Roskind is in pub/c++grammar1.1.tar.Z at
    ics.uci.edu .  A fleshed-out, working instance of the ANSI
    grammar (due to Jeff Lee) is on ftp.uu.net (see question 18.16)
    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&R2.

    The comp.compilers archives contain more information about
    grammars; see question 18.3.

    References: K&R1 Sec. A18 pp. 214-219; K&R2 Sec. A13 pp. 234-
    239; ANSI Sec. A.2; ISO Sec. B.2; H&S pp. 423-435 Appendix B.

18.15a: Does anyone have a C compiler test suite I can use?

A:    Plum Hall (formerly in Cardiff, NJ; now in Hawaii) sells one;
    other packages are Ronald Guilmette's RoadTest(tm) Compiler Test
    Suites (ftp to netcom.com, pub/rfg/roadtest/announce.txt for
    information) and Nullstone's Automated Compiler Performance
    Analysis Tool (see http://www.nullstone.com).  The FSF's GNU C
    (gcc) distribution includes a c-torture-test 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.

18.16:    Where and how can I get copies of all these freely distributable
    programs?

A:    As the number of available programs, the number of publicly
    accessible archive sites, and the number of people trying to
    access them all grow, this question becomes both easier and more
    difficult to answer.

    There are a number of large, public-spirited archive sites out
    there, such as ftp.uu.net, archive.umich.edu, oak.oakland.edu,
    sumex-aim.stanford.edu, and wuarchive.wustl.edu, which have huge
    amounts of software and other information all freely available.
    For the FSF's GNU project, the central distribution site is
    prep.ai.mit.edu .  These well-known sites tend to be extremely
    busy and hard to reach, but there are also numerous "mirror"
    sites which try to spread the load around.

    On the connected Internet, the traditional way to retrieve files
    from an archive site is with anonymous ftp.  For those without
    ftp access, there are also several ftp-by-mail servers in
    operation.  More and more, the world-wide web (WWW) is being
    used to announce, index, and even transfer large data files.
    There are probably yet newer access methods, too.

    Those are some of the easy parts of the question to answer.  The
    hard part is in the details -- this article cannot begin to
    track or list all of the available archive sites or all of the
    various ways of accessing them.  If you have access to the net
    at all, you probably have access to more up-to-date information
    about active sites and useful access methods than this FAQ list
    does.

    The other easy-and-hard aspect of the question, of course, is
    simply *finding* which site has what you're looking for.  There
    is a tremendous amount of work going on in this area, and there
    are probably new indexing services springing up every day.  One
    of the first was "archie": for any program or resource available
    on the net, if you know its name, an archie server can usually
    tell you which anonymous ftp sites have it.  Your system may
    have an archie command, or you can send the mail message "help"
    to [e-mail] for information.

    If you have access to Usenet, see the regular postings in the
    comp.sources.unix and comp.sources.misc newsgroups, which
    describe the archiving policies for those groups and how to
    access their archives.  The comp.archives newsgroup contains
    numerous announcements of anonymous ftp availability of various
    items.  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.

    See also question 14.12.

Section 19. System Dependencies

19.1:    How can I read a single character from the keyboard without
    waiting for the RETURN key?  How can I stop characters from
    being echoed on the screen as they're typed?

A:    Alas, there is no standard or portable way to do these things in
    C.  Concepts such as screens and keyboards are not even
    mentioned in the Standard, which deals only with simple I/O
    "streams" of characters.

    At some level, interactive keyboard input is usually collected
    and presented to the requesting program a line at a time.  This
    gives the operating system a chance to support input line
    editing (backspace/delete/rubout, etc.) in a consistent way,
    without requiring that it be built into every program.  Only
    when the user is satisfied and presses the RETURN key (or
    equivalent) is the line made available to the calling program.
    Even if the calling program appears to be reading input a
    character at a time (with getchar() or the like), the first call
    blocks until the user has typed an entire line, at which point
    potentially many characters become available and many character
    requests (e.g. getchar() calls) are satisfied in quick
    succession.

    When a program wants to read each character immediately as it
    arrives, its course of action will depend on where in the input
    stream the line collection is happening and how it can be
    disabled.  Under some systems (e.g. MS-DOS, VMS in some modes),
    a program can use a different or modified set of OS-level input
    calls to bypass line-at-a-time input processing.  Under other
    systems (e.g. Unix, VMS in other modes), the part of the
    operating system responsible for serial input (often called the
    "terminal driver") must be placed in a mode which turns off line-
    at-a-time processing, after which all calls to the usual input
    routines (e.g. read(), getchar(), etc.) will return characters
    immediately.  Finally, a few systems (particularly older, batch-
    oriented mainframes) perform input processing in peripheral
    processors which cannot be told to do anything other than line-
    at-a-time input.

    Therefore, when you need to do character-at-a-time input (or
    disable keyboard echo, which is an analogous problem), you will
    have to use a technique specific to the system you're using,
    assuming it provides one.  Since comp.lang.c is oriented towards
    topics that C does deal with, you will usually get better
    answers to these questions by referring to a system-specific
    newsgroup such as comp.unix.questions or
    comp.os.msdos.programmer, and to the FAQ lists for these groups.
    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.

    However, since these questions are frequently asked here, here
    are brief answers for some common situations.

    Some versions of curses have functions called cbreak(),
    noecho(), and getch() which do what you want.  If you're
    specifically trying to read a short password without echo, you
    might try getpass().  Under Unix, you can 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; ECHO under all versions), or in a pinch, system()
    and the stty command.  (For more information, see <sgtty.h> and
    tty(4) under classic versions, <termio.h> and termio(4) under
    System V, or <termios.h> and termios(4) under POSIX.)  Under MS-
    DOS, use getch() or getche(), or the corresponding BIOS
    interrupts.  Under VMS, try the Screen Management (SMG$)
    routines, or curses, or issue low-level $QIO's with the
    IO$_READVBLK function code (and perhaps IO$M_NOECHO, and others)
    to ask for one character at a time.  (It's also possible to set
    character-at-a-time or "pass through" modes in the VMS terminal
    driver.)  Under other operating systems, you're on your own.

    (As an aside, note that simply using setbuf() or setvbuf() to
    set stdin to unbuffered will *not* generally serve to allow
    character-at-a-time input.)

    If you're trying to write a portable program, a good approach is
    to define your own suite of three functions to (1) set the
    terminal driver or input system into character-at-a-time mode
    (if necessary), (2) get characters, and (3) return the terminal
    driver to its initial state when the program is finished.
    (Ideally, such a set of functions might be part of the C
    Standard, some day.)  The extended versions of this FAQ list
    (see question 20.40) contain examples of such functions for
    several popular systems.

    See also question 19.2.

    References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1; POSIX
    Sec. 7.

19.2:    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 "poll", or the FIONREAD ioctl, or
    c_cc[VTIME], or kbhit(), or rdchk(), or the O_NDELAY option to
    open() or fcntl().  See also question 19.1.

19.3:    How can I display a percentage-done indication that updates
    itself in place, or show one of those "twirling baton" progress
    indicators?

A:    These simple things, at least, you can do fairly portably.
    Printing the character '\r' will usually give you a carriage
    return without a line feed, so that you can overwrite the
    current line.  The character '\b' is a backspace, and will
    usually move the cursor one position to the left.

    References: ANSI Sec. 2.2.2; ISO Sec. 5.2.2.

19.4:    How can I clear the screen?
    How can I print things in inverse video?
    How can I move the cursor to a specific x, y position?

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

    For clearing the screen, a halfway portable solution is to print
    a form-feed character ('\f'), which will cause some displays to
    clear.  Even more portable would be to print enough newlines to
    scroll everything away.  As a last resort, you could use
    system() (see question 19.27) to invoke an operating system
    clear-screen command.

    References: PCS Sec. 5.1.4 pp. 54-60, Sec. 5.1.5 pp. 60-62.

19.5:    How do I read the arrow keys?  What about function keys?

A:    Terminfo, some versions of termcap, and some versions of curses
    have support for these non-ASCII keys.  Typically, a special key
    sends a multicharacter sequence (usually beginning with ESC,
    '\033'); parsing these can be tricky.  (curses will do the
    parsing for you, if you call keypad() first.)

    Under MS-DOS, if you receive a character with value 0 (*not*
    '0'!) while reading the keyboard, it's a flag indicating that
    the next character read will be a code indicating a special key.
    See any DOS programming guide for lists of keyboard codes.
    (Very briefly: the up, left, right, and down arrow keys are 72,
    75, 77, and 80, and the function keys are 59 through 68.)

    References: PCS Sec. 5.1.4 pp. 56-7.

19.6:    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, the Macintosh, and probably every other system.

    References: PCS Sec. 5.5 pp. 78-80.

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

A:    It's system-dependent.  Under Unix, you typically open, read,
    and write a device file in /dev, and use the facilities of the
    terminal driver to adjust its characteristics.  (See also
    questions 19.1 and 19.2.)  Under MS-DOS, you can use the
    predefined stream stdaux, or a special file like COM1, or some
    primitive BIOS interrupts, or (if you require decent
    performance) any number of interrupt-driven serial I/O packages.
    Several netters recommend the book _C Programmer's Guide to
    Serial Communications_, by Joe Campbell.

19.8:    How can I direct output to the printer?

A:    Under Unix, either use popen() (see question 19.30) to write to
    the lp or lpr program, or perhaps open a special file like
    /dev/lp.  Under MS-DOS, write to the (nonstandard) predefined
    stdio stream stdprn, or open the special files PRN or LPT1.

    References: PCS Sec. 5.3 pp. 72-74.

19.9:    How do I send escape sequences to control a terminal or other
    device?

A:    If you can figure out how to send characters to the device at
    all (see question 19.8 above), it's easy enough to send escape
    sequences.  In ASCII, the ESC code is 033 (27 decimal), so code
    like

        fprintf(ofd, "\033[J");

    sends the sequence ESC [ J .

19.10:    How can I do graphics?

A:    Once upon a time, Unix had a fairly nice little set of device-
    independent plot routines described in plot(3) and plot(5), but
    they've largely fallen into disuse.

    If you're programming for MS-DOS, you'll probably want to use
    libraries conforming to the VESA or BGI standards.

    If you're trying to talk to a particular plotter, making it draw
    is usually a matter of sending it the appropriate escape
    sequences; see also question 19.9.  The vendor may supply a C-
    callable library, or you may be able to find one on the net.


    If you're programming for a particular window system (Macintosh,
    X windows, Microsoft Windows), you will use its facilities; see
    the relevant documentation or newsgroup or FAQ list.

    References: PCS Sec. 5.4 pp. 75-77.

19.11:    How can I check whether a file exists?  I want to warn the user
    if a requested input file is missing.

A:    It's surprisingly difficult to make this determination reliably
    and portably.  Any test you make can be invalidated if the file
    is created or deleted (i.e. by some other process) between the
    time you make the test and the time you try to open the file.

    Three possible test routines are stat(), access(), and fopen().
    (To make an approximate test for file existence with fopen(),
    just open for reading and close immediately.)  Of these, only
    fopen() is widely portable, and access(), where it exists, must
    be used carefully if the program uses the Unix set-UID feature.

    Rather than trying to predict in advance whether an operation
    such as opening a file will succeed, it's often better to try
    it, check the return value, and complain if it fails.
    (Obviously, this approach won't work if you're trying to avoid
    overwriting an existing file, unless you've got something like
    the O_EXCL file opening option available, which does just what
    you want in this case.)

    References: PCS Sec. 12 pp. 189,213; POSIX Sec. 5.3.1,
    Sec. 5.6.2, Sec. 5.6.3.

19.12:    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 difficult or impossible to
    determine this number exactly).

    Under Unix, the stat() call will give you an exact answer.
    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 these tend to have the same problems: fstat() is
    not portable, and generally tells you the same thing stat()
    tells you; ftell() is not guaranteed to return a byte count
    except for binary files.  Some systems provide routines called
    filesize() or filelength(), but these are not portable, either.

    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.

    References: ANSI Sec. 4.9.9.4; ISO Sec. 7.9.9.4; H&S
    Sec. 15.5.1; PCS Sec. 12 p. 213; POSIX Sec. 5.6.2.

19.13:    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 portable solution, nor a way to delete
    blocks at the beginning.  See also question 19.14.

19.14:    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.  The usual
    solution is simply to rewrite the file.  (Instead of deleting
    records, you might consider simply marking them as deleted, to
    avoid rewriting.)  See also questions 12.30 and 19.13.

19.15:    How can I recover the file name given an open stream or file
    descriptor?

A:    This problem is, in general, insoluble.  Under Unix, for
    instance, a scan of the entire disk (perhaps involving special
    permissions) would theoretically be required, and would fail if
    the descriptor were connected to 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()).

19.16:    How can I delete a file?

A:    The Standard C Library function is remove().  (This is therefore
    one of the few questions in this section for which the answer is
    *not* "It's system-dependent.")  On older, pre-ANSI Unix
    systems, remove() may not exist, in which case you can try
    unlink().

    References: K&R2 Sec. B1.1 p. 242; ANSI Sec. 4.9.4.1; ISO
    Sec. 7.9.4.1; H&S Sec. 15.15 p. 382; PCS Sec. 12 pp. 208,220-
    221; POSIX Sec. 5.5.1, Sec. 8.2.4.

19.17:    Why can't I open a file by its explicit path?  The call

        fopen("c:\newdir\file.dat", "r")

    is failing.

A:    The file you actually requested -- with the characters \n and \f
    in its name -- probably doesn't exist, and isn't what you
    thought you were trying to open.

    In character constants and string literals, the backslash \ is
    an escape character, giving special meaning to the character
    following it.  In order for literal backslashes in a pathname to
    be passed through to fopen() (or any other routine) correctly,
    they have to be doubled, so that the first backslash in each
    pair quotes the second one:

        fopen("c:\\newdir\\file.dat", "r");

    Alternatively, under MS-DOS, it turns out that forward slashes
    are also accepted as directory separators, so you could use

        fopen("c:/newdir/file.dat", "r");

    (Note, by the way, that header file names mentioned in
    preprocessor #include directives are *not* string literals, so
    you may not have to worry about backslashes there.)

19.18:    I'm getting an error, "Too many open files".  How can I increase
    the allowable number of simultaneously open files?

A:    There are actually at least two resource limitations on the
    number of simultaneously open files: the number of low-level
    "file descriptors" or "file handles" available in the operating
    system, and the number of FILE structures available in the stdio
    library.  Both must be sufficient.  Under MS-DOS systems, you
    can control the number of operating system file handles with a
    line in CONFIG.SYS.  Some compilers come with instructions (and
    perhaps a source file or two) for increasing the number of stdio
    FILE structures.

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

A:    See if you can use the opendir() and readdir() routines, which
    are part of the POSIX standard and are available on most Unix
    variants.  Implementations also exist for MS-DOS, VMS, and other
    systems.  (MS-DOS also has FINDFIRST and FINDNEXT routines which
    do essentially the same thing.)  readdir() only returns file
    names; if you need more information about the file, try calling
    stat().  To match filenames to some wildcard pattern, see
    question 13.7.

    References: K&R2 Sec. 8.6 pp. 179-184; PCS Sec. 13 pp. 230-1;
    POSIX Sec. 5.1; Schumacher, ed., _Software Solutions in C_
    Sec. 8.

19.22:    How can I find out how much memory is available?

A:    Your operating system may provide a routine which returns this
    information, but it's quite system-dependent.

19.23:    How can I allocate arrays or structures bigger than 64K?

A:    A reasonable computer ought to give you transparent access to
    all available memory.  If you're not so lucky, you'll either
    have to rethink your program's use of memory, or use various
    system-specific techniques.

    64K is (still) a pretty big chunk of memory.  No matter how much
    memory your computer has available, it's asking a lot to be able
    to allocate huge amounts of it contiguously.  (The C Standard
    does not guarantee that a single object can be larger than 32K.)
    Often it's a good idea to use data structures which don't
    require that all memory be contiguous.  For dynamically-
    allocated multidimensional arrays, you can use pointers to
    pointers, as illustrated in question 6.16.  Instead of a large
    array of structures, you can use a linked list, or an array of
    pointers to structures.

    If you're using a PC-compatible (8086-based) system, and running
    up against a 640K limit, consider using "huge" memory model, or
    expanded or extended memory, or malloc variants such as halloc()
    or farmalloc(), or a 32-bit "flat" compiler (e.g. djgpp, see
    question 18.3), or some kind of a DOS extender, or another
    operating system.

    References: ANSI Sec. 2.2.4.1; ISO Sec. 5.2.4.1.

19.24:    What does the error message "DGROUP data allocation exceeds 64K"
    mean, and what can I do about it?  I thought that using large
    model meant that I could use more than 64K of data!

A:    Even in large memory models, MS-DOS compilers apparently toss
    certain data (strings, some initialized global or static
    variables) into a default data segment, and it's this segment
    that is overflowing.  Either use less global data, or, if you're
    already limiting yourself to reasonable amounts (and if the
    problem is due to something like the number of strings), you may
    be able to coax the compiler into not using the default data
    segment for so much.  Some compilers place only "small" data
    objects in the default data segment, and give you a way (e.g.
    the /Gt option under Microsoft compilers) to configure the
    threshold for "small."

19.25:    How can I access memory (a memory-mapped device, or graphics
    memory) located at a certain address?

A:    Set a pointer, of the appropriate type, to the right number
    (using an explicit cast to assure the compiler that you really
    do intend this nonportable conversion):

        unsigned int *magicloc = (unsigned int *)0x12345678;

    Then, *magicloc refers to the location you want.  (Under MS-DOS,
    you may find a macro like MK_FP() handy for working with
    segments and offsets.)

    References: K&R1 Sec. A14.4 p. 210; K&R2 Sec. A6.6 p. 199; ANSI
    Sec. 3.3.4; ISO Sec. 6.3.4; Rationale Sec. 3.3.4; H&S Sec. 6.2.7
    pp. 171-2.

19.27:    How can I invoke another program (a standalone executable, or an
    operating system command) from within a C program?

A:    Use the library function system(), which does exactly that.
    Note that system's return value is the command's exit status,
    and usually has nothing to do with the output of the command.
    Note also that system() accepts a single string representing the
    command to be invoked; if you need to build up a complex command
    line, you can use sprintf().  See also question 19.30.

    References: K&R1 Sec. 7.9 p. 157; K&R2 Sec. 7.8.4 p. 167,
    Sec. B6 p. 253; ANSI Sec. 4.10.4.5; ISO Sec. 7.10.4.5; H&S
    Sec. 19.2 p. 407; PCS Sec. 11 p. 179.

19.30:    How can I invoke another program or 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).  (Also, remember to call pclose().)

    If you can't use popen(), you may be able to use system(), with
    the output going to a file which you then open and read.

    If you're using Unix and popen() isn't sufficient, you can learn
    about pipe(), dup(), fork(), and exec().

    (One thing that probably would *not* work, by the way, would be
    to use freopen().)

    References: PCS Sec. 11 p. 169.

19.31:    How can my program discover the complete pathname to the
    executable 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 solution.

    References: K&R1 Sec. 5.11 p. 111; K&R2 Sec. 5.10 p. 115; ANSI
    Sec. 2.1.2.2.1; ISO Sec. 5.1.2.2.1; H&S Sec. 20.1 p. 416.

19.32:    How can I automatically locate a program's configuration files
    in the same directory as the executable?

A:    It's hard; see also question 19.31 above.  Even if you can
    figure out a workable way to do it, you might want to consider
    making the program's auxiliary (library) directory configurable,
    perhaps with an environment variable.  (It's especially
    important to allow variable placement of a program's
    configuration files when the program will be used by several
    people, e.g. on a multiuser system.)

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

A:    It may or may not be possible to do so at all.  Different
    operating systems implement global 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() or putenv() functions for the purpose),
    and the modified environment is generally passed on to child
    processes, but it is *not* propagated back to the parent
    process.

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

A:    You want a dynamic linker or loader.  It may be 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 versions of SunOS and System V have the -ldl library
    which allows object files to be dynamically loaded.  Under VMS,
    use LIB$FIND_IMAGE_SYMBOL.  GNU has a package called "dld".  See
    also question 15.13.

19.37:    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 clock(), delay(), gettimeofday(), msleep(),
    nap(), napms(), setitimer(), sleep(), times(), and usleep().
    (A routine called wait(), however, is at least under Unix *not*
    what you want.)  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.

    Of these, only clock() is part of the ANSI Standard.  The
    difference between two calls to clock() gives elapsed execution
    time, and if CLOCKS_PER_SEC is greater than 1, the difference will
    have subsecond resolution.  However, clock() gives elapsed
    processor time used by the current program, which on a
    multitasking system may differ considerably from real time.

    If you're trying to implement a delay and all you have available
    is a time-reporting function, you can implement a CPU-intensive
    busy-wait, but this is only an option on a single-user, single-
    tasking machine as it is terribly antisocial to any other
    processes.  Under a multi-tasking operating system, be sure to
    use a call which puts your process to sleep for the duration,
    such as sleep() or select(), or pause() in conjunction with
    alarm() or setitimer().

    For really brief delays, it's tempting to use a do-nothing loop
    like

        long int i;
        for(i = 0; i < 1000000; i++)
            ;

    but resist this temptation if at all possible!  For one thing,
    your carefully-calculated delay loops will stop working next
    month when a faster processor comes out.  Perhaps worse, a
    clever compiler may notice that the loop does nothing and
    optimize it away completely.

    References: H&S Sec. 18.1 pp. 398-9; PCS Sec. 12 pp. 197-8,215-
    6; POSIX Sec. 4.5.2.

19.38:    How can I trap or ignore keyboard interrupts like control-C?

A:    The basic step is to call signal(), either as

        #include <signal.h>
        signal(SIGINT, SIG_IGN);

    to ignore the interrupt signal, or as

        extern void func(int);
        signal(SIGINT, func);

    to cause control to transfer to function func() on receipt of an
    interrupt signal.

    On a multi-tasking system such as Unix, it's best to use a
    slightly more involved technique:

        extern void func(int);
        if(signal(SIGINT, SIG_IGN) != SIG_IGN)
            signal(SIGINT, func);

    The test and extra call ensure that a keyboard interrupt typed
    in the foreground won't inadvertently interrupt a program
    running in the background (and it doesn't hurt to code calls to
    signal() this way on any system).

    On some systems, keyboard interrupt handling is also a function
    of the mode of the terminal-input subsystem; see question 19.1.
    On some systems, checking for keyboard interrupts is only
    performed when the program is reading input, and keyboard
    interrupt handling may therefore depend on which input routines
    are being called (and *whether* any input routines are active at
    all).  On MS-DOS systems, setcbrk() or ctrlbrk() functions may
    also be involved.

    References: ANSI Secs. 4.7,4.7.1; ISO Secs. 7.7,7.7.1; H&S
    Sec. 19.6 pp. 411-3; PCS Sec. 12 pp. 210-2; POSIX
    Secs. 3.3.1,3.3.4.

19.39:    How can I handle floating-point exceptions gracefully?

A:    On many systems, you can define a routine matherr() which will
    be called when there are certain floating-point errors, such as
    errors in the math routines in <math.h>.  You may also be able
    to use signal() (see question 19.38 above) to catch SIGFPE.  See
    also question 14.9.

    References: Rationale Sec. 4.5.1.

19.40:    How do I...  Use sockets?  Do networking?  Write client/server
    applications?

A:    All of these questions are outside of the scope of this list and
    have much more to do with the networking facilities which you
    have available than they do with C.  Good books on the subject
    are Douglas Comer's three-volume _Internetworking with TCP/IP_
    and W. R. Stevens's _UNIX Network Programming_.  (There is also
    plenty of information out on the net itself.)

19.40b: How do I use BIOS calls?  How can I write ISR's?  How can I
    create TSR's?

A:    These are very particular to specific systems (PC compatibles
    running MS-DOS, most likely).  You'll get much better
    information in a specific newsgroup such as
    comp.os.msdos.programmer or its FAQ list; another excellent
    resource is Ralf Brown's interrupt list.

19.41:    But I can't use all these nonstandard, system-dependent
    functions, because my program has to be ANSI compatible!

A:    You're out of luck.  Either you misunderstood your requirement,
    or it's an impossible one to meet.  ANSI/ISO Standard C simply
    does not define ways of doing these things.  (POSIX defines a
    few.)  It is possible, and desirable, for *most* of a program to
    be ANSI-compatible, deferring the system-dependent functionality
    to a few routines in a few files which are rewritten for each
    system ported to.


Section 20. Miscellaneous

20.1:    How can I return multiple values from a function?

A:    Either pass pointers to several 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 2.7, 4.8, and 7.5.

20.3:    How do I access command-line arguments?

A:    They are pointed to by the argv array with which main() is
    called.

    References: K&R1 Sec. 5.11 pp. 110-114; K&R2 Sec. 5.10 pp. 114-
    118; ANSI Sec. 2.1.2.2.1; ISO Sec. 5.1.2.2.1; H&S Sec. 20.1 p.
    416; PCS Sec. 5.6 pp. 81-2, Sec. 11 p. 159, pp. 339-40 Appendix
    F; Schumacher, ed., _Software Solutions in C_ Sec. 4 pp. 75-85.

20.5:    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 most portable 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 interchange them easily between machines, and
    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 (referenced in CCITT X.409 and ISO 8825 "Basic
    Encoding Rules"), CDF, netCDF, or HDF.  See also questions 2.12
    and 12.38.

    References: PCS Sec. 6 pp. 86,88.

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

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

        int func(), anotherfunc();

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

    Then, search the table for the name, and call via the associated
    function pointer.  See also questions 2.15 and 19.36.

    References: PCS Sec. 11 p. 168.

20.8:    How can I implement sets or arrays of bits?

A:    Use arrays of char or int, with a few macros to access the
    desired bit at the proper index.  Here are some simple macros to
    use with arrays of char:

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

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

    (If you don't have <limits.h>, try using 8 for CHAR_BIT.)

    References: H&S Sec. 7.6.7 pp. 211-216.

20.9:    How can I determine whether a machine's byte order is big-endian
    or little-endian?

A:    One way is to use a pointer:

        int x = 1;
        if(*(char *)&x == 1)
            printf("little-endian\n");
        else    printf("big-endian\n");

    It's also possible to use a union.

    See also question 10.16.

    References: H&S Sec. 6.1.2 pp. 163-4.

20.10:    How can I convert integers to binary or hexadecimal?

A:    Make sure you really know what you're asking.  Integers are
    stored internally in binary, although for most purposes it is
    not incorrect to think of them as being in octal, decimal, or
    hexadecimal, whichever is convenient.  The base in which a
    number is expressed matters only when that number is read in
    from or written out to the outside world.

    In source code, a non-decimal base is indicated by a leading 0
    or 0x (for octal or hexadecimal, respectively).  During I/O, the
    base of a formatted number is controlled in the printf and scanf
    family of functions by the choice of format specifier (%d, %o,
    %x, etc.) and in the strtol() and strtoul() functions by the
    third argument.  During *binary* I/O, however, the base again
    becomes immaterial.

    For more information about "binary" I/O, see question 2.11.  See
    also questions 8.6 and 13.1.

    References: ANSI Secs. 4.10.1.5,4.10.1.6; ISO
    Secs. 7.10.1.5,7.10.1.6.

20.11:    Can I use base-2 constants (something like 0b101010)?
    Is there a printf() format for binary?

A:    No, on both counts.  You can convert base-2 string
    representations to integers with strtol().

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

A:    Many "bit-fiddling" problems like this one can be sped up and
    streamlined using lookup tables (but see question 20.13 below).

20.13:    How can I make my 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.

    Even for code that *is* time-critical, it is not as 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 actually degraded, and are rarely portable (i.e.
    they may speed things up on one machine but slow them down on
    another).  In any case, tweaking the coding usually results in
    at best linear performance improvements; the big payoffs are in
    better algorithms.

    For more discussion of efficiency tradeoffs, as well as good
    advice on how to improve 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_.

20.14:    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 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 3.12.)

20.17:    Is there a way to switch on strings?

A:    Not directly.  Sometimes, it's appropriate to use a separate
    function to map strings to integer codes, and then switch on
    those.  Otherwise, of course, you can fall back on strcmp() and
    a conventional if/else chain.  See also questions 10.12, 20.18,
    and 20.29.

    References: K&R1 Sec. 3.4 p. 55; K&R2 Sec. 3.4 p. 58; ANSI
    Sec. 3.6.4.2; ISO Sec. 6.6.4.2; H&S Sec. 8.7 p. 248.

20.18:    Is there a way to have non-constant case labels (i.e. ranges or
    arbitrary expressions)?

A:    No.  The switch statement was originally designed to be quite
    simple for the compiler to translate, therefore case labels are
    limited to single, constant, integral expressions.  You *can*
    attach several case labels to the same statement, which will let
    you cover a small range if you don't mind listing all cases
    explicitly.

    If you want to select on arbitrary ranges or non-constant
    expressions, you'll have to use an if/else chain.

    See also questions question 20.17.

    References: K&R1 Sec. 3.4 p. 55; K&R2 Sec. 3.4 p. 58; ANSI
    Sec. 3.6.4.2; ISO Sec. 6.6.4.2; Rationale Sec. 3.6.4.2; H&S
    Sec. 8.7 p. 248.

20.19:    Are the outer parentheses in return statements really optional?

A:    Yes.

    Long ago, in the early days of C, they were required, and just
    enough people learned C then, and wrote code which is still in
    circulation, that the notion that they might still be required
    is widespread.

    (As it happens, parentheses are optional with the sizeof
    operator, too, as long as its operand is a variable or a unary
    expression.)

    References: K&R1 Sec. A18.3 p. 218; ANSI Sec. 3.3.3, Sec. 3.6.6;
    ISO Sec. 6.3.3, Sec. 6.6.6; H&S Sec. 8.9 p. 254.

20.20:    Why don't C comments nest?  How am I supposed to comment out
    code containing comments?  Are comments legal inside quoted
    strings?

A:    C comments don't nest mostly because PL/I's comments, which C's
    are borrowed from, don't either.  Therefore, it is usually
    better to "comment out" large sections of code, which might
    contain comments, with #ifdef or #if 0 (but see question 11.19).

    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.

    Note also that // comments, as in C++, are not currently legal
    in C, so it's not a good idea to use them in C programs (even if
    your compiler supports them as an extension).

    References: K&R1 Sec. A2.1 p. 179; K&R2 Sec. A2.2 p. 192; ANSI
    Sec. 3.1.9 (esp. footnote 26), Appendix E; ISO Sec. 6.1.9, Annex
    F; Rationale Sec. 3.1.9; H&S Sec. 2.2 pp. 18-9; PCS Sec. 10 p.
    130.

20.24:    Why doesn't C have nested functions?

A:    It's not trivial to implement nested functions such that they
    have the proper access to local variables in the containing
    function(s), so they were deliberately left out of C as a
    simplification.  (gcc does allow them, as an extension.)  For
    many potential uses of nested functions (e.g. qsort comparison
    functions), an adequate if slightly cumbersome solution is to
    use an adjacent function with static declaration, communicating
    if necessary via a few static variables.  (A cleaner solution
    when such functions must communicate is to pass around a pointer
    to a structure containing the necessary context.)

20.25:    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.

    References: H&S Sec. 4.9.8 pp. 106-7.

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

A:    Several freely distributable 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 a few
    other commercial translation products, and some for more obscure
    languages.

    See also questions 11.31 and 18.16.

20.27:    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++.
    Conversely, ANSI C inherited several features from C++,
    including prototypes and const, so neither language is really a
    subset or superset of the other.  In spite of the differences,
    many C programs will compile correctly in a C++ environment, and
    many recent compilers offer both C and C++ compilation modes.

    References: H&S p. xviii, Sec. 1.1.5 p. 6, Sec. 2.8 pp. 36-7,
    Sec. 4.9 pp. 104-107.

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

A:    Some nice information and algorithms having to do with
    approximate string matching, as well as a useful bibliography,
    can be found in Sun Wu and Udi Manber's paper "AGREP -- A Fast
    Approximate Pattern-Matching Tool."

    Another approach involves the "soundex" algorithm, which maps
    similar-sounding words to the same codes.  Soundex was designed
    for discovering similar-sounding names (for telephone directory
    assistance, as it happens), but it can be pressed into service
    for processing arbitrary words.

    References: Knuth Sec. 6 pp. 391-2 Volume 3; Wu and Manber,
    "AGREP -- A Fast Approximate Pattern-Matching Tool" .

20.29:    What is hashing?

A:    Hashing is the process of mapping strings to integers, usually
    in a relatively small range.  A "hash function" maps a string
    (or some other data structure) to a a bounded number (the "hash
    bucket") which can more easily be used as an index in an array,
    or for performing repeated comparisons.  (Obviously, a mapping
    from a potentially huge set of strings to a small set of
    integers will not be unique.  Any algorithm using hashing
    therefore has to deal with the possibility of "collisions.")
    Many hashing functions and related algorithms have been
    developed; a full treatment is beyond the scope of this list.

    References: K&R2 Sec. 6.6; Knuth Sec. 6.4 pp. 506-549 Volume 3;
    Sedgewick Sec. 16 pp. 231-244.

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

A:    Use mktime() or localtime() (see questions 13.13 and 13.14, but
    beware of DST adjustments if tm_hour is 0), or Zeller's
    congruence (see the sci.math FAQ list), or this elegant code 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;
        }

    See also questions 13.14 and 20.32.

    References: ANSI Sec. 4.12.2.3; ISO Sec. 7.12.2.3.

20.32:    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 present
    Gregorian calendar is

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

    See a good astronomical almanac or other reference for details.
    (To forestall an eternal debate: references which claim the
    existence of a 4000-year rule are wrong.)

20.34:    Here's a good puzzle: how do you write a program which produces
    its own source code as its output?

A:    It is actually quite difficult to write a self-reproducing
    program that is truly portable, due particularly to quoting and
    character set difficulties.

    Here is a classic example (which is normally presented on one
    line, although it will "fix" itself the first time it's run):

        char*s="char*s=%c%s%c;main(){printf(s,34,s,34);}";
        main(){printf(s,34,s,34);}

    (This program, like many of the genre, assumes that the double-
    quote character " has the value 34, as it does in ASCII.)

20.35:    What is "Duff's Device"?

A:    It's a devastatingly deviously unrolled byte-copying loop,
    devised by Tom Duff while he was at Lucasfilm.  In its "classic"
    form, it looks like:

        register n = (count + 7) / 8;    /* count > 0 assumed */
        switch (count % 8)
        {
        case 0:    do { *to = *from++;
        case 7:        *to = *from++;
        case 6:         *to = *from++;
        case 5:        *to = *from++;
        case 4:        *to = *from++;
        case 3:        *to = *from++;
        case 2:        *to = *from++;
        case 1:        *to = *from++;
                   } while (--n > 0);
        }

    where count bytes are to be copied from the array pointed to by
    from to the memory location pointed to by to (which is a memory-
    mapped device output register, which is why to isn't
    incremented).  It solves the problem of handling the leftover
    bytes (when count isn't a multiple of 8) by interleaving a
    switch statement with the loop which copies bytes 8 at a time.
    (Believe it or not, it *is* legal to have case labels buried
    within blocks nested in a switch statement like this.  In his
    announcement of the technique to C's developers and the world,
    Duff noted that C's switch syntax, in particular its "fall
    through" behavior, had long been controversial, and that "This
    code forms some sort of argument in that debate, but I'm not
    sure whether it's for or against.")

20.36:    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 schedule is tied to the dates of the USENIX
    conferences at which the winners are announced.  At the time of
    this writing, it is expected that the yearly contest will open
    in October.  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 should be announced at the winter USENIX
    conference in January, and are posted to the net sometime
    thereafter.  Winning entries from previous years (back to 1984)
    are archived at ftp.uu.net (see question 18.16) 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".

20.37:    What was the entry keyword mentioned in K&R1?

A:    It was reserved to allow the possibility of having functions
    with multiple, differently-named entry points, a la FORTRAN.  It
    was not, to anyone's knowledge, ever implemented (nor does
    anyone remember what sort of syntax might have been imagined for
    it).  It has been withdrawn, and is not a keyword in ANSI C.
    (See also question 1.12.)

    References: K&R2 p. 259 Appendix C.

20.38:    Where does the name "C" come from, anyway?

A:    C was derived from Ken Thompson's experimental language B, which
    was inspired by Martin Richards's BCPL (Basic Combined
    Programming Language), which was a simplification of CPL
    (Cambridge Programming Language).  For a while, there was
    speculation that C's successor might be named P (the third
    letter in BCPL) instead of D, but of course the most visible
    descendant language today is C++.

20.39:    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.

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

A:    An up-to-date copy may be obtained from ftp.eskimo.com in
    directory u/s/scs/C-faq/.  You can also 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.  A parallel, abridged version is available (and posted),
    as is a list of changes accompanying each significantly updated
    version.

    The various versions of this list are also posted to 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/).  An archie
    server (see question 18.16) should help you find others; ask it
    to "find C-faq".  If you don't have ftp access, a mailserver at
    rtfm.mit.edu can mail you FAQ lists: send a message containing
    the single word help to [e-mail] .  See the meta-
    FAQ list in news.answers for more information.

    A hypertext (HTML) version of this FAQ list is available on the
    World-Wide Web; the URL is http://www.eskimo.com/~scs/C-faq/top.html .
    URL's pointing at all FAQ lists (these may also allow topic
    searching) are http://www.cis.ohio-
    state.edu/hypertext/faq/usenet/FAQ-List.html and
    http://www.luth.se/wais/ .

    An extended version of this FAQ list is being published by
    Addison-Wesley as _C Programming FAQs: Frequently Asked
    Questions_ (ISBN 0-201-84519-9).  It should be available in
    November 1995.

    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

Americal National Standards Institute, _American National Standard for
Information Systems -- Programming Language -- C_, ANSI X3.159-1989 (see
question 11.2).  [ANSI]

Americal National Standards Institute, _Rationale for American National
Standard for Information Systems -- Programming Language -- C_ (see
question 11.2).  [Rationale]

Jon Bentley, _Writing Efficient Programs_, Prentice-Hall, 1982, ISBN 0-
13-970244-X.

G.E.P. Box and Mervin E. Muller, "A Note on the Generation of Random
Normal Deviates," _Annals of Mathematical Statistics_, Vol. 29 #2, June,
1958, pp. 610-611.

David Burki, "Date Conversions," _The C Users Journal_, February 1993,
pp. 29-34.

Ian F. Darwin, _Checking C Programs with lint_, O'Reilly, 1988, ISBN 0-
937175-30-7.

David Goldberg, "What Every Computer Scientist Should Know about
Floating-Point Arithmetic," _ACM Computing Surveys_, Vol. 23 #1, March,
1991, pp. 5-48.

Samuel P. Harbison and Guy L. Steele, Jr., _C: A Reference Manual_,
Fourth Edition, Prentice-Hall, 1995, ISBN 0-13-326224-3.  [H&S]

Mark R. Horton, _Portable C Software_, Prentice Hall, 1990, ISBN 0-13-
868050-7.  [PCS]

Institute of Electrical and Electronics Engineers, _Portable Operating
System Interface (POSIX) -- Part 1: System Application Program Interface
(API) [C Language_, IEEE Std. 1003.1, ISO/IEC 9945-1.

International Organization for Standardization, ISO 9899:1990 (see
question 11.2).  [ISO]

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

Brian W. Kernighan and Dennis M. Ritchie, _The C Programming Language_,
Prentice-Hall, 1978, ISBN 0-13-110163-3.  [K&R1]

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.
[K&R2]

Donald E. Knuth, _The Art of Computer Programming_.  Volume 1:
_Fundamental Algorithms_, Second Edition, Addison-Wesley, 1973, ISBN 0-
201-03809-9.  Volume 2: _Seminumerical Algorithms_, Second Edition,
Addison-Wesley, 1981, ISBN 0-201-03822-6.  Volume 3: _Sorting and
Searching_, Addison-Wesley, 1973, ISBN 0-201-03803-X.  [Knuth]

Andrew Koenig, _C Traps and Pitfalls_, Addison-Wesley, 1989, ISBN 0-201-
17928-8.  [CT&P]

Stephen K. Park and Keith W. Miller, "Random Number Generators: Good
Ones are Hard to Find," _Communications of the ACM_, Vol. 31 #10,
October, 1988, pp. 1192-1201 (also technical correspondence August,
1989, pp. 1020-1024, and July, 1993, pp. 108-110).

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

Thomas Plum, _C Programming Guidelines_, Second Edition, Plum Hall,
1989, ISBN 0-911537-07-4.

William H. Press, Saul A. Teukolsky, William T. Vetterling, and Brian P.
Flannery, _Numerical Recipes in C_, Second Edition, Cambridge University
Press, 1992, ISBN 0-521-43108-5.

Dale Schumacher, Ed., _Software Solutions in C_, AP Professional, 1994,
ISBN 0-12-632360-7.

Robert Sedgewick, _Algorithms in C_, Addison-Wesley, 1990, ISBN 0-201-
51425-7.

Charles Simonyi and Martin Heller, "The Hungarian Revolution," _Byte_,
August, 1991, pp.131-138.

David Straker, _C Style: Standards and Guidelines_, Prentice Hall, ISBN
0-13-116898-3.

Steve Summit, _C Programming FAQs: Frequently Asked Questions_, Addison-
Wesley, 1995, ISBN 0-201-84519-9.  [The book version of this FAQ list.]

Sun Wu and Udi Manber, "AGREP -- A Fast Approximate Pattern-Matching
Tool," USENIX Conference Proceedings, Winter, 1992, pp. 153-162.

There is another bibliography in the revised Indian Hill style guide
(see question 17.9).  See also question 18.10.


Acknowledgements

Thanks to Jamshid Afshar, David Anderson, Tanner Andrews, Sudheer Apte,
Joseph Arceneaux, Randall Atkinson, Rick Beem, Peter Bennett, Wayne
Berke, Dan Bernstein, Tanmoy Bhattacharya, John Bickers, Gary Blaine,
Yuan Bo, Dave Boutcher, Michael Bresnahan, Vincent Broman, Stan Brown,
Joe Buehler, Kimberley Burchett, Gordon Burditt, Burkhard Burow, Conor
P. Cahill, D'Arcy J.M. Cain, Christopher Calabrese, Ian Cargill, Vinit
Carpenter, Paul Carter, Mike Chambers, Billy Chambless, Franklin Chen,
Jonathan Chen, Raymond Chen, Richard Cheung, Steve Clamage, Ken Corbin,
Ian Cottam, Russ Cox, Jonathan Coxhead, Lee Crawford, Steve Dahmer,
Andrew Daviel, James Davies, John E. Davis, Ken Delong,
Norm Diamond, Jeff Dunlop, Ray Dunn, Stephen M. Dunn, Michael J. Eager,
Scott Ehrlich, Arno Eigenwillig, Dave Eisen, Bjorn Engsig, David Evans,
Clive D.W. Feather, Dominic Feeley, Simao Ferraz, Chris Flatters, Rod
Flores, Alexander Forst, Steve Fosdick, Jeff Francis, Tom Gambill, Dave
Gillespie, Samuel Goldstein, Tim Goodwin, Alasdair Grant, Ron Guilmette,
Doug Gwyn, Michael Hafner, Tony Hansen, Elliotte Rusty Harold, Joe
Harrington, Des Herriott, Guy Harris, John Hascall, Ger Hobbelt, Jos
Horsmeier, Blair Houghton, James C. Hu, Chin Huang, David Hurt, Einar
Indridason, Vladimir Ivanovic, Jon Jagger, Ke Jin, Kirk Johnson, Larry
Jones, Arjan Kenter, James Kew, Lawrence Kirby, Kin-ichi Kitano, Peter
Klausler, Andrew Koenig, Tom Koenig, Adam Kolawa, Jukka Korpela, Ajoy
Krishnan T, Markus Kuhn, Deepak Kulkarni, Oliver Laumann, John Lauro,
Felix Lee, Mike Lee, Timothy J. Lee, Tony Lee, Marty Leisner, Don Libes,
Brian Liedtke, Philip Lijnzaad, Keith Lindsay, Yen-Wei Liu, Paul Long,
Christopher Lott, Tim Love, Tim McDaniel, Kevin McMahon, Stuart
MacMartin, John R. MacMillan, Andrew Main, Bob Makowski, Evan Manning,
Barry Margolin, George Matas, Brad Mears, Roger Miller, Bill Mitchell,
Mark Moraes, Darren Morby, Bernhard Muenzer, David Murphy, Walter
Murray, Ralf Muschall, Ken Nakata, Todd Nathan, Landon Curt Noll, Tim
Norman, Paul Nulsen, David O'Brien, Richard A. O'Keefe, Adam Kolawa,
James Ojaste, Hans Olsson, Bob Peck, Andrew Phillips, Christopher
Phillips, Francois Pinard, Nick Pitfield, Wayne Pollock, Dan Pop, Lutz
Prechelt, Lynn Pye, Kevin D. Quitt, Pat Rankin, Arjun Ray, Eric S.
Raymond, Peter W. Richards, Eric Roode, Manfred Rosenboom, J. M.
Rosenstock, Rick Rowe, Erkki Ruohtula, John Rushford, Kadda Sahnine,
Tomohiko Sakamoto, Matthew Saltzman, Rich Salz, Chip Salzenberg, Matthew
Sams, Paul Sand, DaviD W. Sanderson, Frank Sandy, Christopher Sawtell,
Jonas Schlein, Paul Schlyter, Doug Schmidt, Rene Schmit, Russell Schulz,
Dean Schulze, Chris Sears, Patricia Shanahan, Raymond Shwake, Peter da
Silva, Joshua Simons, Ross Smith, Henri Socha, Leslie J. Somos, Henry
Spencer, David Spuler, James Stern, Bob Stout, Steve Sullivan, Melanie
Summit, Erik Talvola, Dave Taylor, Clarke Thatcher, Wayne Throop, Chris
Torek, Steve Traugott, Ilya Tsindlekht, Andrew Tucker, Goran Uddeborg,
Rodrigo Vanegas, Jim Van Zandt, Wietse Venema, Tom Verhoeff, Ed
Vielmetti, Larry Virden, Chris Volpe, Mark Warren, Alan Watson, Kurt
Watzka, Larry Weiss, Martin Weitzel, Howard West, Tom White, Freek
Wiedijk, Dik T. Winter, Lars Wirzenius, Dave Wolverton, Mitch Wright,
Conway Yee, Ozan S. Yigit, and Zhuo Zang, who have contributed, directly
or indirectly, to this article.  Special thanks to Karl Heuer, Jutta
Degener, 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 1990-1996 by Steve Summit.
Content from the book _C Programming FAQs: Frequently Asked Questions_
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