Answers to Frequently Asked Questions on comp.lang.c (Abridged)
Steve Summit
scs at adam.mit.edu
Fri May 18 22:04:47 AEST 1990
This article contains minimal answers to the comp.lang.c frequently
asked questions list. Please see the long version for more detailed
explanations.
1. What is this infamous null pointer, anyway?
A: For each pointer type, there is a special value -- "null" -- 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 in favor of an unadorned 0 as a stylistic convention.
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.
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. 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.
8. 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.
9. But wouldn't it be better to use NULL (rather than 0) in case the
value of NULL changes, perhaps on a machine with nonzero null
pointers?
A: No. NULL is, and will always be, 0.
10. 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).
11. 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 so because the value might change,
or on some weird machine.
12. 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[].
13. 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.
14. 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.
15. 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.
16. 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, in the spring of 1990.
17. 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.
18. 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.
19. 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));
20. 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.
21. How can I write a function that takes a format string and a variable
number of arguments, like printf, and passes them to printf so it
can 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.
22. 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.
23. 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.
24. 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.
25. 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.
26. How does struct passing and returning work?
A: If you really need to know, see the unabridged list.
27. 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.
28. 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.
29. 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.
30. How can I define a pair of mutually referential structures?
A: The obvious technique works as long as any typedef synonyms are done
later.
References: H&S Sec. 5.6.1 p. 102.
31. 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.
32. So where can I get cdecl?
A: Several public-domain versions are available.
33. 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.
34. 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.
35. What is the difference between an enum and a series of preprocessor
#defines?
A: At the present time, there is essentially no difference.
References: K&R II Sec. 2.3 p. 39, Sec. A4.2 p. 196; H&S Sec. 5.5
p. 100.
36. 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.
37. 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.
38. 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.
39. 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.
40. Does anyone know of a program for converting Pascal (Fortran, lisp,
...) to C?
A: Several public-domain programs are available, namely p2c and f2c. A
number of companies, not listed here yet, sell various language
translation tools, both to and from C.
41. 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.
42. Does anyone have a C compiler test suite I can use?
A: Plum Hall, among others, sells one.
43. 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 at adam.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.
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