Answers to Frequently Asked Questions (FAQ) on comp.lang.c (Abridged)

[Steve Summit]

[Last modified December 16, 1990 by scs.]

This article contains minimal answers to the comp.lang.c frequently-
asked questions list.  Please see the long version (posted on the first
of each month) for more detailed explanations and references.

Section 1. 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.

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 initialization,
    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 parameter as being of pointer type.  In
    other contexts (function arguments without prototypes, or in the
    variable part of variadic function calls) a constant 0 with an
    appropriate explicit cast is required.

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

A:  Not since the early days.

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,

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 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 internal representations for pointers to 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.  I use the preprocessor macro "#define Nullptr(type) (type *)0 " to
    help me build null pointers of the correct type.

A:  This trick does not buy much.

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.

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

A:  Either; the distinction is entirely stylistic.

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 once used a compiler that wouldn't work unless NULL was used.

A:  That compiler was broken.

12. 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).

13. 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 use of a preprocessor macro (NULL) suggests that
    the value might change later, or on some weird machine.

14. 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 as arguments in function calls."

Section 2. Arrays and Pointers

15. 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[].

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

A:  This identity (that a pointer declaration is interchangeable with an
    array declaration, usually unsized) holds _only_ for formal
    parameters to functions.  Otherwise, the two forms are not
    interchangeable.

17. 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 decay into pointers within expressions, and
    that pointers and arrays can both be dereferenced using array-like
    subscript notation.

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

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

19. How do I declare a pointer to an array?

A:  Usually, you don't want to.  Consider using a pointer to one of the
    array's elements instead.

20. 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." See the
    full list for code samples.

Section 3. Order of Evaluation

21. 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:  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.

22. But what about the &&, ||, and comma operators?

A:  There is a special exception for those operators, (as well as ?: );
    left-to-right evaluation is guaranteed.

Section 4. ANSI C

23. What is the "ANSI C Standard?"

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

24. 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 addresses.

25. 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.

26. 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;".  The problem can be fixed by using either new-style
    (prototype) or old-style syntax consistently.

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

A:  The problem is older linkers which cannot be forced (by mere words
    in a Standard) to upgrade.

Section 5. C Preprocessor

28. 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.

29. 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:  Use the ANSI token-pasting operator ##.

30. 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 (i.e. no single apostrophes), and no
    newlines inside quotes.

31. What's the best way to write a multi-statement cpp macro?

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

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

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

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

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

Section 6. Variable-Length Argument Lists

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

A:  Use varargs or stdarg.

34. 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.

35. How can I write a function analogous to scanf?

A:  Unfortunately, vscanf and the like are not standard.

36. 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.

37. 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.

Section 7. Memory Allocation

38. 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.

39. I can't get strcat to work.  I tried "char *s1 = "Hello, ",
    *s2 = "world!", *s3 = strcat(s1, s2);" but I got strange results.

A:  Again, the problem is that space for the concatenated result is not
    properly allocated.

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

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

41. You can't use dynamically-allocated memory after you free it, can
    you?

A:  No.  Some early man pages implied otherwise, but the claim is no
    longer valid.

42. What is alloca and why is its use discouraged?

A:  alloca allocates memory which is automatically freed when the
    function from which alloca was called returns.  alloca cannot be
    written portably, is difficult to implement on machines without a
    stack, and fails under certain conditions if implemented simply.

Section 8. Structures

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

A:  These operations are supported by all modern compilers.

44. How does struct passing and returning work?

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

45. I have a program which works correctly, but 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 believe that main
    returns a struct.

46. 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.

47. I came across some code that declared a structure with the last
    member an array of one element, and then did some tricky allocation
    to make the array act like it had several elements.  Is this legal
    and/or portable?

A:  It is surprisingly difficult to determine whether the ANSI C
    standard allows or disallows it.

48. 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.

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

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

Section 9. Declarations

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

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

51. 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.

52. How do I declare an array of pointers to functions returning
    pointers to functions returning pointers to characters?

A:  char *(*(*a[5])())();
    Using a chain of typedefs, or the cdecl program, makes these
    declarations easier.

53. So where can I get cdecl?

A:  Several public-domain versions are available.  See the full list for
    details.

54. How do I initialize a pointer to a function?

A:  Use something like "extern int func(); int (*fp)() = func; " .

55. I've seen different methods used for calling through pointers to
    functions.

A:  The extra parentheses and explicit * are now officially optional,
    although "Classic C" required them.

Section 10. Boolean Expressions and Variables

56. 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.

57. 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.  (This is _not_ true for some library
    routines such as isalpha.)

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

A:  At the present time, there is little difference.  The ANSI standard
    states that enumerations are compatible with integral types.

Section 11. Operating System Dependencies

59. 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.

60. 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.

61. 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.

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

A:  In general, it cannot.

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

A:  BSD systems provide ftruncate(), and some PC compilers supply
    chsize(), but there is no portable solution.

Section 12. Stdio

64. 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.

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

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

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

A:  scanf() was designed for free-format input, which is seldom what you
    want when reading from the keyboard.

67. So what should I use instead?

A:  Use fgets() to read a whole line, and then use sscanf() or other
    string functions to parse the line buffer.

Section 13. Miscellaneous

68. Can someone tell me how to write itoa?

A:  Just use sprintf.

69. How can I convert 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 parse strings.

70. How can I write data files which can be read on other machines with
    different data formats?

A:  The best solution is to use a text file.

71. 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 header files are
    frequently system-dependent.  Contact your vendor.

72. 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.  See the full list for details.

73. Where can I get copies of all these public-domain programs?

A:  See the regular postings in the comp.sources.unix and
    comp.sources.misc newsgroups for information.

74. How can I call Fortran (BASIC, Pascal, ADA, LISP) functions from C?

A:  The answer is entirely dependent on the machine and the specific
    calling sequences of the various compilers in use.

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

A:  Nested comments would cause more harm than good.  The character
    sequences /* and */ are not special within double-quoted strings.

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

A:  See the full list for a brief explanation, or any good programming
    book for a better one.

77. 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, 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.

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

A:  Plum Hall, among others, sells one.

79. Where can I get a YACC grammar for C?

A:  See the unabridged list.

80. What's the best style for code layout in C?

A:  There is no one "best style," but see the full list for a few
    suggestions.

81. Where can I get the "Indian Hill Style Guide" and other coding
    standards?

A:  See the unabridged list.

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

A:  Like the English words "char," "care," or "car" (your choice);
    "octothorpe."

83. Where can I get extra copies of this list?

A:  For now, just pull it off the net; the unabridged version is
    normally posted on the first of each month, with an Expiration: line
    which should keep it around all month.

                                             Steve Summit
                                             scs at adam.mit.edu
                                             scs%adam.mit.edu at mit.edu
                                             mit-eddie!adam!scs

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.