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From: scs@adam.mit.edu (Steve Summit)
Newsgroups: comp.lang.c
Subject: Answers to Frequently Asked Questions on comp.lang.c (Abridged)
Summary: Short answers for those who don't feel like reading the longer explanations
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Date: 1 Jun 90 11:07:32 GMT
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This article contains minimal answers to the comp.lang.c frequently
asked questions list.  Please see the long version for more detailed
explanations.

Null Pointers

1.  What is this infamous null pointer, anyway?

A:  For each pointer type, there is a special value -- the "null
    pointer" -- which is distinguishable from all other pointer values
    and which is not the address of any object.

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

2.  How do I "get" a null pointer in my programs?

A:  A constant 0 in a pointer context is converted into a null pointer
    at compile time.  A "pointer context" is an assignment or comparison
    with one side a variable of pointer type, and (in ANSI standard C) a
    function argument which has a prototype in scope declaring a certain
    argument as being of pointer type.  In other contexts (function
    arguments without prototypes, or in the variable part of variadic
    functions) a constant 0 with an appropriate explicit cast is
    required.

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

3.  But aren't pointers the same as ints?

A:  Not since the early days.

    References: K&R I Sec. 5.6 pp. 102-3.

4.  What is NULL and how is it #defined?

A:  NULL is simply a preprocessor macro, #defined as 0 (or (void *)0)
    which is used (as a stylistic convention, in favor of unadorned 0's)
    to generate null pointers,

    References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S
    Sec. 13.1 p. 283; ANSI X3.159-1989 Sec. 4.1.5 p. 99, Sec. 3.2.2.3
    p. 38, Rationale Sec. 4.1.5 p. 74.

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

A:  The same as any other machine: as 0 (or (void *)0).  (The compiler
    makes the translation, upon seeing a 0, not the preprocessor.)

6.  If NULL were defined as "(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 kinds of pointers for different types of data.  A cast is
    still required to tell the compiler which kind of null pointer is
    required, since it may be different from (char *)0.

7.  My vendor provides header files that #define NULL as 0L.  Why?

A:  This definition, like (void *)0, helps incorrect programs work on
    some common architectures.

8.  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:  The construction "if(p)" works, regardless of the internal
    representation of null pointers, because the compiler essentially
    rewrites it as "if(p != 0)" and goes on to convert 0 into the
    correct null pointer.

    References: K&R II Sec. A7.4.7 p. 204; H&S Sec. 5.3 p. 91.

9.  If "NULL" and "0" are equivalent, which should I use?

A:  Either; the distinction is entirely stylistic.

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

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 null
    pointers?

A:  No.  NULL is, and will always be, 0.

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

A:  A "null pointer" (written in lower case in this article) is a
    language concept whose particular internal value does not matter.  A
    "null pointer" is requested in source code with the character '0'.
    "NULL" (always in capital letters) is a preprocessor macro, which is
    always #defined as 0 (or (void *)0).

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

A:  The fact that null pointers are represented both in source code, and
    internally to most machines, as zero invites unwarranted
    assumptions.  The fact that a preprocessor macro (NULL) is often
    used suggests that this is done because the value might change
    later, or on some weird machine.

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

A:  A simple rule is, "Always use `0' or `NULL' for null pointers, and
    always cast them when they are used in function calls."

Arrays and Pointers

14. I had the declaration char a[5] 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.   Use extern char a[].

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

A:  This identity (that a pointer declaration is interchangeable with an
    array, usually unsized) holds _only_ for formal arguments to
    functions.

    References: K&R I Sec. 5.3 p. 95, Sec. A10.1 p. 205; K&R II Sec. 5.3
    p. 100, Sec. A8.6.3 p. 218, Sec. A10.1 p. 226; H&S Sec. 5.4.3 p. 96.

16. So what is meant by the "equivalence of pointers and arrays" in C?

A:  Saying that arrays and pointers are "equivalent" does not by any
    means imply that they are interchangeable.  "Equivalence" refers to
    the fact that arrays "turn into" pointers within expressions, and
    that pointers and arrays can both be dereferenced using array-like
    subscript notation.

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

Order of Evaluation

17. Under my compiler, the program "int i = 7;
    printf("%d\n", i++ * i++); " prints 49.  Regardless of the order of
    evaluation, shouldn't it print 56?

A:  The operations implied by the postincrement and postdecrement
    operators ++ and -- are performed at some time after the operand's
    former values are yielded and before the end of the expression, but
    not necessarily immediately after, or before other parts of the
    expression are evaluated.

    References: K&R I Sec. 2.12 p. 50; K&R II Sec. 2.12 p. 54.

ANSI C

18. What is the "ANSI C Standard?"

A:  In 1983, the American National Standards Institute commissioned a
    committee, X3J11, to standardize the C language.  After a long and
    arduous process, this C standard was finally ratified as an American
    National Standard, X3.159-1989, on December 14, 1989, and published
    in the spring of 1990.

19. How can I get a copy of the ANSI C standard?

A:  Copies are available from the American National Standards Institute
    in New York, or from Global Engineering Documents in Irvine, CA.
    See the unabridged list for full addresses.

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

A:  There are several such programs, many in the public domain.

21. My ANSI compiler complains about a mismatch when it sees

         extern int blort(float);
         int blort(x)
         float x;
         {...

A:  You have mixed the new-style declaration "extern int blort(float);"
    with the old-style definition "int blort(x) float x;".  The problem
    can be fixed either by using new-style syntax consistently, or by
    changing the new-style declaration to match the old-style
    definition.

C Preprocessor

22. How can I write a macro to swap two values?

A:  There is no good answer to this question.  The best all-around
    solution is probably to forget about using a macro.

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

A:  Under ANSI C, #ifdeffed-out text must still consist of "valid
    preprocessing tokens."  This means that there must be no
    unterminated comments or quotes and no newlines inside quotes.

24. How can I write a cpp macro which takes a variable number of
    arguments?

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

        #define DEBUG(args) {printf("DEBUG: ");printf args;}

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

Variable-Length Argument Lists

25. How can I write a function that takes a variable number of
    arguments?

A:  Use varargs or stdarg.

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

26. 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 v*printf.

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

27. How can I discover how many arguments a function was actually called
    with?

A:  This information is not available to a portable program.  Any
    function which takes a variable number of arguments must be able to
    determine from the arguments themselves how many of them there are.

28. 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.

Memory Allocation

29. Why doesn't the code "char *answer; gets(answer);" work?

A:  The pointer variable "answer" has not been set to point to any valid
    storage.  The simplest way to correct this fragment is to use a
    local array, instead of a pointer.

Structures

30. I heard that structures could be assigned to variables and passed to
    and from functions, but K&R I says no.

A:  These operations are supported by all modern compilers.

    References: K&R I Sec. 6.2 p. 121; K&R II Sec. 6.2 p. 129; H&S Sec.
    5.6.2 p. 103.

31. How does struct passing and returning work?

A:  If you really need to know, see the unabridged list.

32. I have a program which works correctly, but it dumps core after it
    finishes.  Why?

A:  Check to see if a structure declaration just before main is missing
    its trailing semicolon, causing the compiler to think that main
    returns a struct.

33. Why can't you compare structs?

A:  There is no reasonable way for a compiler to implement struct
    comparison which is consistent with C's low-level flavor.

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

34. 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.

35. How can I access structure fields by name at run time?

A:  Build a table of names and offsets, using the offsetof() macro.

Declarations

36. I can't seem to define a linked list node which contains a pointer
    to itself.

    Structs in C can certainly contain pointers to themselves; the
    discussion and example in section 6.5 of K&R make this clear.
    Problems arise when an attempt is made to define (and use) a typedef
    in the midst of such a declaration; avoid this.

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

37. How can I define a pair of mutually referential structures?

A:  The obvious technique works as long as any typedef synonyms are
    defined outside of the struct declarations.

    References: H&S Sec. 5.6.1 p. 102.

38. How do I declare a pointer to a function returning a pointer to a
    double?

A:  double *(*p)();
    Using a chain of typedefs, or the cdecl program, makes these
    declarations easier.

    References: H&S Sec. 5.10.1 p. 116.

39. So where can I get cdecl?

A:  Several public-domain versions are available.

Boolean Expressions and Variables

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

A:  C does not provide a standard boolean type because picking one
    involves a space/time tradeoff which is best decided by the
    programmer.  The choice between #defines and enums is arbitrary and
    not terribly interesting.

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

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

    References: K&R I Sec. 2.7 p. 41; K&R II Sec. 2.6 p. 42, Sec. A7.4.7
    p. 204, Sec. A7.9 p. 206.

42. What is the difference between an enum and a series of preprocessor
    #defines?

A:  At the present time, there is little difference.

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

Operating System Dependencies

43. How can I read a single character from the keyboard without waiting
    for a newline?

A:  Contrary to popular belief and many people's wishes, this is not a
    C-related question.  How to do so is a function of the operating
    system in use.

44. 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.

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

A:  argv[0] may contain all or part of the pathname.  You may be able to
    duplicate the command language interpreter's search path logic to
    locate the executable.

46. How can a process change an environment variable in its caller?

A:  In general, it cannot.

Miscellaneous

47. Why does errno contain ENOTTY after a call to printf?

A:  Don't worry about it.  It is only meaningful for a program to
    inspect the contents of errno after an error has occurred.

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

A:  It is best to use an explicit fflush(stdout) at any point within
    your program at which output should definitely be visible.

49. 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:  The ANSI mktime routine converts a struct tm to a time_t.  No
    standard routine exists to convert strings.

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

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

A:  You cannot just pick up a copy of someone else's header file and
    expect it to work, since the definitions within the file are
    frequently system-dependent.  Contact your vendor.

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

A:  Several public-domain programs are available, namely ptoc, p2c, and
    f2c.  A number of companies, not listed here yet, sell various
    language translation tools, both to and from C.

52. Why don't C comments nest?  Are they legal inside quoted strings?

A:  C comments do not nest.  The character sequences /* and */ are not
    special within double-quoted strings.

53. I'm having trouble with a Turbo C program which crashes and says
    something like "floating point not loaded."

A:  Some compilers for small machines, including Turbo C and Ritchie's
    original pdp11 compiler, attempt to leave out floating point support
    if it looks like it will not be needed.  The programmer must
    occasionally insert one dummy explicit floating-point operation to
    force loading of floating-point support.

54. Does anyone have a C compiler test suite I can use?

A:  Plum Hall, among others, sells one.

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

A:  The classic routine for doing this sort of thing is the "soundex"
    algorithm.

                                             Steve Summit
                                             scs@adam.mit.edu
                                             scs%adam.mit.edu@mit.edu

This article is Copyright 1988, 1990 by Steve Summit.
It may be freely redistributed so long as the author's name, and this
notice, are retained.