Answers to Frequently Asked Questions (FAQ) on comp.lang.c
Steve Summit
scs at adam.mit.edu
Mon Oct 1 14:02:20 AEST 1990
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 will be reposted periodically, 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.
This article does not, and cannot, provide an exhaustive discussion of
all of the subtle points and counterarguments which could be mentioned
with respect to these topics. Cross-references to standard C
publications have been provided, for further study by the interested and
dedicated reader. A few of the more perplexing and pervasive topics may
be further explored in some in-depth minitreatises posted in conjunction
with this article.
No mere newsgroup article can substitute for thoughtful perusal of a
full-length 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 vendors'
compiler manuals are unfortunately inadequate; a few even perpetuate
some of the myths which this article attempts to debunk. Two invaluable
references, which are an excellent addition to any serious programmer's
library, are:
The C Programming Language, by Brian W. Kernighan and Dennis M.
Ritchie.
C: A Reference Manual, by Samuel P. Harbison and Guy L. Steele, Jr.
Both exist in several editions. Andrew Koenig's book _C Traps and
Pitfalls_ also covers many of the difficulties frequently discussed
here.
If you have a question about C which is not answered in this article,
please try to answer it by referring to these or other books, or to
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 numbers 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.
This article is always being improved. Your input is welcomed. Send
your comments to scs at adam.mit.edu and/or scs%adam.mit.edu at mit.edu; this
article's From: line may be unuseable.
Herewith, some frequently-asked questions and their answers:
Null Pointers
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 not the address of any
object. That is, the address-of operator & will never "return" a
null pointer, nor will a successful call to malloc. (malloc returns
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 different from an uninitialized pointer. A null
pointer is known not to point to any object; an uninitialized
pointer might point anywhere (that is, at some random object, or at
a garbage or unallocated address). See also question 34.
As mentioned in the definition 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
null pointer is required, so it can make the distinction if
necessary (see below).
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; ANSI X3.159-1989 Sec. 3.2.2.3 .
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)
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. For instance,
the Unix system call "execl" takes a variable-length, null pointer-
terminated list of character pointer arguments. To generate a null
pointer in a function call context, an explicit cast is typically
required:
execl("/bin/sh", "sh", "-c", "ls", (char *)0);
If the (char *) cast 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 casts may safely be omitted, since the
prototype tells the compiler that a pointer is required, and of
which type, enabling it to correctly cast unadorned 0's. Function
prototypes cannot provide the types for variable arguments in
variable-length argument lists, however, so explicit casts are still
required for those arguments. It is safest always to cast null
pointer function arguments, 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.
Summary:
unadorned 0 okay: explicit cast required:
initialization function call,
no prototype in scope
assignments
variable argument to
comparisons varargs function
function call,
prototype in scope,
fixed argument
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; ANSI X3.159-
1989 Sec. 3.2.2.3 .
3. But aren't pointers the same as ints?
A: Not since the early days. Attempting to push pointers into
integers, or build pointers out of integers, has always been
machine-dependent and unportable, and doing so is strongly
discouraged. (Any object pointer may be cast to the "universal"
pointer type void *, or char * under a pre-ANSI compiler, when
heterogeneous pointers must be passed around.)
References: K&R I Sec. 5.6 pp. 102-3; ANSI X3.159-1989 Sec. 3.3.4 .
4. What is NULL and how is it #defined?
A: As a stylistic convention, many people prefer not to have unadorned
0's scattered throughout their programs. For this reason, the
preprocessor macro NULL is #defined (by stdio.h or stddef.h), with
value 0 (or (void *)0, about which more later). A programmer who
wishes to make explicit the distinction between 0 the integer and 0
the null pointer can then use NULL whenever a null pointer is
required. This is a stylistic convention only; the preprocessor
turns NULL back to 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 2 above applies for NULL as well as 0.)
NULL should _only_ be used for pointers. It should not be used when
another kind of 0 is required, even though it might work, because
doing so sends the wrong stylistic message. (ANSI allows the
#definition of NULL to be (void *)0, which will not work 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&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 a null pointer?
A: Programmers should never need to know the internal representation(s)
of null pointers, because they are normally taken care of by the
compiler. If a machine uses a nonzero bit pattern for null
pointers, it is the compiler's responsibility to generate it when
the programmer requests, by writing "0" or "NULL," a null pointer.
Therefore #defining NULL as 0 on a machine for which internal null
pointers are nonzero is as valid as on any other, because the
compiler must (and can) still generate the machine's correct null
pointers in response to unadorned 0's seen in pointer contexts.
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 to work correctly, but
pointer arguments to other types would still be problematical, and
legal constructions such as
FILE *fp = NULL;
could fail.
Nevertheless, ANSI C allows the alternate
#define NULL (void *)0
definition for NULL. Besides helping incorrect programs to work
(but only on machines with all pointers the same, thus questionably
valid assistance) this definition may catch programs which use NULL
incorrectly (e.g. when the ASCII nul character was really intended).
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: 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 produced 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 a null pointer, and use the correct value. There is
no trickery involved here; compilers do work this way, and generate
identical code for both statements. The internal representation of
a pointer does not matter.
The boolean negation operator, !, can be described as follows:
!expr is essentially equivalent to expr?0:1
It is left as an exercise for the reader to show that
if(!p) is equivalent to if(p == 0)
See also question 48.
References: K&R II Sec. A7.4.7 p. 204; H&S Sec. 5.3 p. 91; ANSI
X3.159-1989 Secs. 3.3.3.3, 3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1,
and 3.6.5 .
8. If "NULL" and "0" are equivalent, 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 #definition, and prefer to
use unadorned "0" instead. There is no one right answer.
C programmers must understand that "NULL" and "0" are
interchangeable and that an uncast "0" is perfectly acceptable in
initialization, assignment, and comparison contexts. 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. Again, NULL should not
be used for other than pointers.
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. Although preprocessor macros 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. The language guarantees that source-code 0's
(in pointer contexts) generate null pointers. NULL is used only as
a stylistic convention.
10. But I once used a compiler that wouldn't work unless NULL was used.
A: This compiler was broken. In general, making decisions about a
language based on the behavior of one particular compiler is likely
to be counterproductive.
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.
(On some machines the internal value is 0; on others it is not.) 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).
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 1. It is implemented with...
2. The internal (or run-time) representation of a null pointer,
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 source code syntax for null pointers, which is the single
character "0". It is often hidden behind...
4. The NULL macro, which is #defined to be "0" or "(void *)0".
Finally, as a red herring, we have
5. The ASCII null character (NUL), which does have all bits zero,
but has no relation to the null pointer except in name.
This article always uses the phrase "null pointer" for sense 1, the
character "0" for sense 3, and the capitalized word "NULL" for
sense 4.
12. 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 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. 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. Finally, the distinction
between the several uses of the term "null" (listed above) is often
overlooked.
One good way to wade out of the confusion is to imagine that C had a
keyword (perhaps "nil", like Pascal) with which null pointers were
requested. The compiler could either turn "nil" into the correct
type of null pointer, when it could determine the type from the
source code (as it does with 0's in reality), or complain when it
could not. Now, in fact, in C the keyword for a null pointer is not
"nil" but "0", which works almost as well, except that an uncast "0"
in a non-pointer context generates an integer zero. If the null
pointer keyword were "nil" the compiler could emit an error message
for an ambiguous usage, but since it is "0" the compiler may end up
emitting incorrect code.
13. I'm still confused. I just can't understand all this null pointer
stuff.
A: Follow these two simple rules:
1. When you want to refer to a null pointer in source code, use
"0" or "NULL".
2. If the usage of "0" or "NULL" is 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, or with the internal representation of null
pointers, which you shouldn't need to know.
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. The type "pointer-to-type-T" is not the same as
"array-of-type-T." 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 declaration, usually unsized) holds _only_ for formal
parameters to functions. This identity is related to the fact that
arrays "turn into" pointers in expressions. That is, when an array
name is mentioned in an expression, it is converted immediately into
a pointer to the array's first element. Therefore, an array is
never passed to a function; rather a pointer to its first element is
passed instead. Allowing pointer parameters to be declared as
arrays is a simply a way of making it look as though the array was
actually being passed. Some programmers prefer, as a matter of
style, to use this syntax to indicate that the pointer parameter is
expected to point to the start of an array rather than to a single
value.
Since functions can never receive arrays as parameters, any
parameter declarations which "look like" arrays, e.g.
f(a)
char a[];
are treated as if they were pointers, since that is what the
function will receive if an array is passed:
f(a)
char *a;
To repeat, however, this conversion holds only within function
formal parameter declarations, nowhere else. If this conversion
confuses you, don't use it; many people have concluded that the
confusion it causes outweighs the small advantage of having the
declaration "look like" the call and/or the uses within the
function.
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;
ANSI X3.159-1989 Sec. 3.5.4.3, Sec. 3.7.1 .
16. So what is meant by the "equivalence of pointers and arrays" in C?
A: Perhaps no aspect of C is more confusing than pointers, and the
confusion is compounded by statements like the one above. Saying
that arrays and pointers are "equivalent" does not by any means
imply that they are interchangeable. (The fact that, as formal
parameters to functions, array-style and pointer-style declarations
are in fact interchangeable does nothing to reduce the confusion.)
"Equivalence" refers to the fact (mentioned above) that arrays decay
into pointers within expressions, and that pointers and arrays can
both be dereferenced using array-like subscript notation. That is,
if we have
char a[10];
char *p;
int i;
we can refer to a[i] and p[i]. (That pointers can be subscripted
like arrays is hardly surprising, since arrays have decayed into
pointers by the time they are subscripted.)
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; ANSI X3.159-1989 Sec. 3.3.2.1, Sec. 3.3.6 .
17. 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.
Pointers to arrays are confusing, and it is best to avoid them.
(The confusion is heightened by incorrect compilers, including some
versions of pcc and pcc-derived lint's, which incorrectly accept
assignments of multi-dimensional arrays to multi-level pointers.)
If you are passing a two-dimensional array to a function:
int array[YSIZE][XSIZE];
f(array);
the function's declaration should match:
f(int a[][XSIZE]) {...}
or
f(int (*a)[XSIZE]) {...}
In the first declaration, the compiler performs the usual implicit
rewriting of "array of array" to "pointer to array;" in the second
form the pointer declaration is explicit. The called function does
not care how big the array is, but it must know its shape, so the
"column" dimension XSIZE must be included. In both cases the number
of "rows" is irrelevant, and omitted.
If a function is already declared as accepting a pointer to a
pointer, an intermediate pointer would need to be used when
attempting to call it with a two-dimensional array:
int *ip = &a[0][0];
g(&ip);
...
g(int **ipp) {...}
Note that this usage is liable to be misleading (if not incorrect),
since the array has been "flattened" (its shape has been lost).
18. How do I declare a pointer to an array?
A: Usually, you don't want one. Think about using a pointer to one of
the array's elements instead. Arrays of type T decay into pointers
to type T, which is convenient; subscripting or incrementing the
resultant pointer accesses the individual members of the array.
True pointers to arrays, when subscripted or incremented, step over
entire arrays, and are generally only useful when operating on
multidimensional arrays. (See the question above.)
19. 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." The
resulting "ragged" array often saves space, although it may not be
contiguous in memory as a real array would be.
int **array = (int **)malloc(nrows * sizeof(int *));
for(i = 0; i < nrows; i++)
array[i] = (int *)malloc(ncolumns * sizeof(int));
(In "real" code, of course, malloc's return value should be
checked.)
You can keep the array's contents contiguous, while losing the
ability to have rows of varying and different lengths, with a bit of
explicit pointer arithmetic:
int **array = (int **)malloc(nrows * sizeof(int *));
array[0] = (int *)malloc(nrows * ncolumns * sizeof(int));
for(i = 1; i < nrows; i++)
array[i] = array[0] + i * ncolumns;
In either case, the elements of the dynamic array can be accessed
with normal-looking array subscripts: array[i][j].
If the double indirection implied by the above scheme is for some
reason unacceptable, you can simulate a two-dimensional array with a
single, dynamically-allocated one-dimensional array:
int *array = (int *)malloc(nrows * ncolumns * sizeof(int));
However, you must now perform subscript calculations manually,
accessing array[i, j] with array[i * ncolumns + j]. (A macro can
hide the explicit calculation, but invoking it then requires
parentheses and commas which don't look exactly like
multidimensional array subscripts.)
Order of Evaluation
20. 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 the operations after yielding the former value, many people
misunderstand the implication of "after." It is _not_ guaranteed
that the operation 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).
In the example, the compiler chose to multiply the previous value by
itself and to perform both increments afterwards.
The order of other embedded side effects is similarly undefined.
For example, the expression i + (i = 2) may or may not have the
value 4. ANSI allows compilers to reject code which contains such
ambiguous or undefined side effects.
References: K&R I Sec. 2.12 p. 50; K&R II Sec. 2.12 p. 54; ANSI
X3.159-1989 Sec. 3.3 .
21. But what about the &&, ||, ?:, and comma operators?
I see code like "if((c = getchar()) == EOF || c == '\n')" ...
A: There is a special exception for those operators; each of them does
imply a sequence point (i.e. left-to-right evaluation is
guaranteed).
References: ANSI X3.159-1989 Secs. 3.3.2.2, 3.3.13, 3.3.14, 3.3.15 .
ANSI C
22. 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. 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 much-lambasted trigraph sequences for transfer of
source code between machines with deficient or multinational
character sets). The ANSI C standard also formalizes the C run-time
library support routines, an unprecedented effort.
23. How can I get a copy of the ANSI C standard?
A: Copies are available from
American National Standards Institute
1430 Broadway
New York, NY 10018
(212) 642-4900
or
Global Engineering Documents
2805 McGaw Avenue
Irvine, CA 92714
(714) 261-1455
The cost is approximately $50.00, plus $6.00 shipping. Quantity
discounts are available.
24. 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. Check
your nearest comp.sources archive. (See also questions 61 and 62.)
25. 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 declaration "extern int func(float);"
with the old-style definition "int func(x) float x;". Old C (and
ANSI C, in the absence of prototypes) silently promotes floats to
doubles when passing them as arguments, and makes a corresponding
silent change to formal parameter declarations, so the old-style
definition actually says that func takes a double.
The 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).
References: ANSI X3.159-1989 Sec. 3.3.2.2 .
C Preprocessor
26. How can I write a 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.
If the macro is intended to be used on values of arbitrary type (the
usual goal), it cannot use a temporary, since it doesn't know what
type of temporary it needs, and standard C does not provide a typeof
operator. (GNU C does.)
The best all-around solution is probably to forget about using a
macro. If you're worried about the use of an ugly temporary, and
know that your machine provides an exchange instruction, convince
your compiler vendor to recognize the standard three-assignment swap
idiom in the optimization phase. Alternatively, use a language
which supports multiple, parallel assignment (a,b := b,a).
27. I'm getting strange syntax errors inside code which 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 unterminated comments or quotes (note particularly
that an apostrophe within a contracted word looks like the beginning
of a character constant) and no newlines inside quotes. Therefore,
natural-language comments should always be written between the
"official" comment delimiters /* and */.
28. 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));
The obvious disadvantage to this trick is that the caller must
always remember to use the extra parentheses. (It is often best to
use a bona-fide function, which can take a variable number of
arguments in a well-defined way, rather than a macro. See questions
29 and 30 below.)
Variable-Length Argument Lists
29. How can I write a function that takes a variable number of
arguments?
A: Use varargs or stdarg.
Here is a function which concatenates an arbitrary number of strings
into malloc'ed memory, using stdarg:
#include <stddef.h> /* for NULL */
#include <stdarg.h> /* for va_ stuff */
#include <string.h> /* for strcat et al */
#include <stdlib.h> /* for malloc */
extern char *malloc(); /* redundant */
/* VARARGS1 */
char *
vstrcat(char *first, ...)
{
int len = 0;
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;
(void)strcpy(retbuf, first);
va_start(argp, first);
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. (Also note that the caller must
free the returned, malloc'ed storage.)
Using the older varargs package, rather than stdarg, requires a few
changes which are not discussed here, in the interests of brevity.
See the next question for hints.
References: K&R II Sec. 7.3 p. 155, Sec. B7 p. 254; H&S Sec. 13.4
pp. 286-9; ANSI X3.159-1989 Secs. 4.8 through 4.8.1.3 .
30. 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");
}
To use varargs, instead of stdarg, 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: K&R II Sec. 8.3 p. 174, Sec. B1.2 p. 245; H&S Sec. 17.12
p. 337; ANSI X3.159-1989 Secs. 4.9.6.7, 4.9.6.8, 4.9.6.9 .
31. How can I write a function analogous to scanf?
A: Unfortunately, vscanf and the like are not standard. You're on your
own.
32. How can I discover how many arguments a function was actually called
with?
A: This information is not available to a portable program. Some
systems have a nonstandard nargs() function available, but its use
is questionable, since it typically returns the number of words
pushed, not the number of arguments. (Floating point values and
structures 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 (useful 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 vstrcat and execl examples under questions 29 and 2 above).
33. 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. You must provide a version of that other
function which accepts a va_list pointer, as does vfprintf in the
example above. If the arguments must be passed directly as actual
arguments (not indirectly through a va_list pointer) to another
function which is itself variadic (for which you do not have the
option of creating an alternate, va_list-accepting version) no
portable solution is possible. (The problem can often be solved by
resorting to machine-specific assembly language.)
Memory Allocation
34. Why doesn't this program work?
main()
{
char *answer;
printf("Type something:\n");
gets(answer);
printf("You typed \"%s\"\n", answer);
}
A: The pointer variable "answer," which is handed to the gets function
as the location into which the response should be stored, has not
been set to point to any valid storage. It is an uninitialized
variable, just as is the variable i in this example:
main()
{
int i;
printf("i = %d\n", i);
}
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.)
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>
main()
{
char answer[100];
printf("Type something:\n");
fgets(answer, 100, stdin);
printf("You typed \"%s\"\n", answer);
}
Note that this example also uses fgets instead of gets (always a
good idea), so that the size of the array can be specified, so that
fgets will not overwrite the end of the array if the user types an
overly-long line. (Unfortunately, gets and fgets differ in their
treatment of the trailing \n.) It would also be possible to use
malloc to allocate the answer buffer, and/or to parameterize its
size (#define ANSWERSIZE 100).
35. You can't use dynamically-allocated memory after you free it, can
you?
A: No. Some early man pages for malloc stated that the contents of
freed memory was "left undisturbed;" this ill-advised guarantee is
not universal and is not required by ANSI.
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((char *)listp);
}
and notice what would happen if the more-obvious loop iteration
expression listp = listp->next were used, without the temporary
nextp pointer.
36. 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. 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 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), stdin, 100).
For these reasons, alloca cannot be used in programs which must be
widely portable, no matter how useful it might be.
Structures
37. I heard that structures could be assigned to variables and passed to
and from functions, but K&R I says not.
A: What K&R I said was that the restrictions on struct operations would
be lifted in a forthcoming version of the compiler, and in fact
struct assignment and passing were fully functional in Ritchie's
compiler even as K&R I was being published. Although a few early C
compilers lacked struct assignment, all modern compilers support it,
and it is part of the ANSI C standard, so there should be no
reluctance to use it.
References: K&R I Sec. 6.2 p. 121; K&R II Sec. 6.2 p. 129; H&S Sec.
5.6.2 p. 103; ANSI X3.159-1989 Secs. 3.1.2.5, 3.2.2.1, 3.3.16 .
38. How does struct passing and returning work?
A: When structures are passed as arguments to functions, the entire
struct is pushed on the stack, which may involve significant
overhead for large structures. It may be preferable in such cases
to pass a pointer to the structure instead.
Structures are returned from functions either in a special, static
place (which may make struct-valued functions nonreentrant) or in a
location pointed to by an extra, "hidden" argument to the function.
39. The following program works correctly, but it dumps core after it
finishes. Why?
struct list
{
char *item;
struct list *next;
}
/* Here is the main program. */
main(argc, argv)
...
A: A missing semicolon causes the compiler to believe that main returns
a struct list. (The connection is hard to see because of the
intervening comment.) When struct-valued functions are implemented
by adding a hidden return pointer, the generated code tries to store
a struct with respect to a pointer which was not actually passed (in
this case, by the C start-up code). Attempting to store a structure
into memory pointed to by the argc or argv value on the stack (where
the compiler expected to find the hidden return pointer) causes the
core dump.
40. Why can't you compare structs?
A: There is no reasonable way for a compiler to implement struct
comparison which is consistent with C's low-level flavor. A byte-
by-byte comparison could be invalidated by random bits present in
unused "holes" in the structure (such padding is used to keep the
alignment of later fields correct). A field-by-field comparison
would require unacceptable amounts of repetitive, in-line code for
large structures. Either method would not necessarily "do the right
thing" with pointer fields: oftentimes, equality should be judged by
equality of the things pointed to rather than strict equality of the
pointers themselves.
If you want to compare two structures, you must write your own
function to do so. C++ (among other languages) would let you
arrange for the == operator to map to your function.
References: K&R II Sec. 6.2 p. 129; H&S Sec. 5.6.2 p. 103.
41. 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. If you don't have it, a suggested implementation is
#define offsetof(type, mem) ((size_t) \
((char *)&((type *) 0)->mem - (char *)((type *) 0)))
This implementation is not 100% portable; some compilers may
legitimately refuse to accept it.
See the next question for a usage hint.
References: ANSI X3.159-1989 Sec. 4.1.5 .
42. 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
offsetof(struct a, b)
If structp is a pointer to an instance of this structure, and b is
an int field with offset as computed above, b's value can be set
indirectly with
*(int *)((char *)structp + offset) = value;
Declarations
43. I can't seem to define a linked list node which contains a pointer
to itself. I tried
typedef struct
{
char *item;
NODEPTR next;
} NODE, *NODEPTR;
but the compiler gave me error messages. Can't a struct in C
contain a pointer to itself?
A: Structs in C can certainly contain pointers to themselves; the
discussion and example in section 6.5 of K&R make this clear. The
problem is that the example above attempts to hide the struct
pointer behind a typedef, which is not complete at the time it is
used. First, rewrite it without a typedef:
struct node
{
char *item;
struct node *next;
};
Then, if you feel you must use typedefs, define them after the fact:
typedef struct node NODE, *NODEPTR;
Alternatively, define the typedefs first (using the line just above)
and follow it with the full definition of struct node, which can
then use the NODEPTR typedef for the "next" field.
References: K&R I Sec. 6.5 p. 101; K&R II Sec. 6.5 p. 139; H&S Sec.
5.6.1 p. 102; ANSI X3.159-1989 Sec. 3.5.2.3 .
44. How can I define a pair of mutually referential structures? I tried
typedef struct
{
int structafield;
STRUCTB *bpointer;
} STRUCTA;
typedef struct
{
int structbfield;
STRUCTA *apointer;
} STRUCTB;
but the compiler doesn't know about STRUCTB when it is used in
struct a.
A: Again, the problem is not the pointers but the typedefs. First,
define the two structures without using typedefs:
struct a
{
int structafield;
struct b *bpointer;
};
struct b
{
int structbfield;
struct a *apointer;
};
The compiler can accept the field declaration struct b *bpointer
within struct a, even though it has not yet heard of struct b.
Occasionally it is necessary to precede this couplet with the empty
declaration
struct b;
to mask the declaration (if in an inner scope) from a different
struct b in an outer scope.
Again, the typedefs could also be defined before, and then used
within, the definitions for struct a and struct b. Problems arise
only when an attempt is made to define and use a typedef within the
same declaration.
References: H&S Sec. 5.6.1 p. 102; ANSI X3.159-1989 Sec. 3.5.2.3 .
45. How do I declare a pointer to a function returning a pointer to a
double?
A: There are at least three answers to this question:
1. double *(*p)();
2. Build it up in stages, using typedefs:
typedef double *pd; /* pointer to double */
typedef pd fpd(); /* func returning ptr to double */
typedef fpd *pfpd; /* ptr to func ret ptr to double */
pfpd p;
3. Use the cdecl program, which turns English into C and vice
versa:
$ cdecl
cdecl> declare p as pointer to function returning pointer to double
double *(*p)();
cdecl>
cdecl can also explain complicated declarations, help with
casts, and indicate which set of parentheses the arguments go
in (for complicated function definitions).
References: H&S Sec. 5.10.1 p. 116.
46. So where can I get cdecl?
A: Several public-domain versions are available. One is in volume 14
of comp.sources.unix . (Commercial versions may also be available,
at least one of which was shamelessly lifted from the public domain
copy submitted by Graham Ross, one of cdecl's originators.)
Boolean Expressions and Variables
47. What is the right type to use for boolean values in C? Why isn't it
a standard type? Should #defines or enums be used for the true and
false values?
A: C does not provide a standard boolean type, because picking one
involves a space/time tradeoff which is best decided by the
programmer. (Using an int for a boolean may be faster, while using
char will probably save data space.)
The choice between #defines and enums is arbitrary and not terribly
interesting. Use any of
#define TRUE 1 #define YES 1
#define FALSE 0 #define NO 0
enum bool {false, true}; enum bool {no, yes};
as long as you are consistent within one program or project. (The
enum may be preferable if your debugger expands enum values when
examining variables.)
Some people prefer variants like
#define TRUE (1==1)
#define FALSE (!TRUE)
These don't buy anything (see below).
48. Isn't #defining TRUE to be 1 dangerous, since any nonzero value is
considered "true" in C? What if a built-in boolean or relational
operator "returns" something other than 1?
A: It is true (sic) that any nonzero value is considered true in C, but
this applies only "on input", i.e. where a boolean value is
expected. When a boolean value is generated by a built-in operator,
it is guaranteed to be 1 or 0. Therefore, the test
if((a == b) == TRUE)
will succeed (if a, in fact, equals b and TRUE is one), but this
code is obviously silly. In general, explicit tests against TRUE
and FALSE are undesirable, because some library functions (notably
isupper, isalpha, etc.) return, on success, a nonzero value which is
_not_ necessarily 1. A good rule of thumb is to use TRUE and FALSE
(or the like) only for assignment to a Boolean variable or as the
return value from a Boolean function, never in a comparison.
Preprocessor macros like TRUE and FALSE (and, in fact, NULL) are
used for code readability, not because the underlying values might
ever change. That "true" is 1 and "false" (and source-code null
pointers) 0 is guaranteed by the language. (See also question 7.)
References: K&R I Sec. 2.7 p. 41; K&R II Sec. 2.6 p. 42, Sec. A7.4.7
p. 204, Sec. A7.9 p. 206; ANSI X3.159-1989 Secs. 3.3.3.3, 3.3.8,
3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, 3.6.5 .
49. What is the difference between an enum and a series of preprocessor
#defines?
A: At the present time, there is little difference. Although many
people might have wished otherwise, the ANSI standard says that
enums may be freely intermixed with integral types, without errors.
(If such intermixing were disallowed without explicit casts,
judicious use of enums could catch certain programming errors.)
The advantages of enums are that the numeric values are
automatically assigned, that a debugger may be able to display the
symbolic values when enum variables are examined, and that a
compiler may generate nonfatal warnings when enums and ints are
indiscriminately mixed (such mixing can still be considered bad
style even though it is not strictly illegal) or when enum cases are
left out of switch statements.
References: K&R II Sec. 2.3 p. 39, Sec. A4.2 p. 196; H&S Sec. 5.5
p. 100; ANSI X3.159-1989 Secs. 3.1.2.5, 3.5.2, 3.5.2.2 .
Operating System Dependencies
50. 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. The delivery of characters from a "keyboard" to
a C program is a function of the operating system, and cannot be
standardized by the C language. If you are using curses, use its
cbreak() function. Under UNIX, 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). Under
MS-DOS, use getch(). Under other operating systems, you're on your
own. Beware that some operating systems make this sort of thing
impossible, because character collection into input lines is done by
peripheral processors not under direct control of the CPU running
your program.
Operating system specific questions are not appropriate for
comp.lang.c . Several common questions are answered in frequently-
asked questions postings in the comp.unix.questions and
comp.sys.ibm.pc newsgroups.
51. 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. Depending on
your system, you may be able to use "nonblocking I/O", or a system
call named "select", or the FIONREAD ioctl, or O_NDELAY, or a
kbhit() routine.
52. How can my program discover the complete pathname to the executable
file from which it was invoked?
A: Depending on the operating system, argv[0] may contain all or part
of the pathname. (It may also 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 incomplete.
However, there is no guaranteed or portable solution.
53. How can a process change an environment variable in its caller?
A: In general, it cannot. If the calling process is prepared to listen
explicitly for some indication that its environment should be
changed, a special-case scheme can be set up. (Under Unix, a child
process cannot directly affect its parent at all. Other operating
systems have different process environments which could
intrinsically support such communication.)
Stdio
54. Why does errno contain ENOTTY after a call to printf?
A: Many implementations of the stdio package adjust their behavior
slightly depending on whether stdout is a terminal or not. To make
this determination, these implementations perform an operation which
fails (with ENOTTY) if stdout is not a terminal. Although the
output operation goes on to complete successfully, errno still
contains ENOTTY. This behavior can be mildly confusing, but it is
not strictly incorrect, because it is only meaningful for a program
to inspect the contents of errno after an error has occurred (that
is, after a library function that sets errno on error has returned
an error code).
55. 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. Several
mechanisms attempt to perform the fflush for you, at the "right
time," but they do not always work, particularly when stdout is a
pipe rather than a terminal.
56. When I read from the keyboard with scanf(), it seems to hang until I
type one extra line of input.
A: scanf() was designed for free-format input, which is seldom what you
want when reading from the keyboard. In particular, "\n" in a
format string does not mean "expect a newline", it means "discard
all whitespace". But the only way to discard all whitespace is to
continue reading the stream until a non-whitespace character is seen
(which is then left in the buffer for the next input), so the effect
is that it keeps going until it sees a nonblank line.
57. So what should I use instead?
A: You could use a "%c" format, which will read one character that you
can then manually compare against a newline; or "%*c" and no
variable if you're willing to trust the user to hit a newline; or
"%*[^\n]%*c" to discard everything up to and including the newline.
Or you could use fgets() to read a whole line, and then use sscanf()
or other string functions to parse the line buffer.
Miscellaneous
58. Can someone tell me how to write itoa (the inverse of atoi)?
A: Just use sprintf.
59. I know that the library routine localtime will convert a time_t into
a broken-down struct tm, and that ctime will convert a time_t to a
printable string. How can I perform the inverse operations of
converting a struct tm or a string into a time_t?
A: ANSI C specifies a library routine, mktime, which converts a
struct tm to a time_t. Several public-domain versions of this
routine are available if your compiler does not support it yet.
Converting a string to a time_t is harder, because of the wide
variety of date and time formats which should be parsed. Public-
domain routines have been written for performing this function, as
well, but they are less likely to become standardized.
References: K&R II Sec. B10 p. 256; H&S Sec. 20.4 p. 361; ANSI
X3.159-1989 Sec. 4.12.2.3 .
60. 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 uses exactly the same environment.
Ask your vendor why the file was not provided (or to send another
copy, if you've merely lost it).
61. Does anyone know of a program for converting Pascal (Fortran, lisp,
"Old" C, ...) to C?
A: Several public-domain programs are available:
p2c written by Dave Gillespie, and posted to
comp.sources.unix in March, 1990 (Volume 21).
ptoc another comp.sources.unix contribution, this one
written in Pascal (comp.sources.unix, Volume 10,
also patches in Volume 13?).
f2c jointly developed by people from Bell Labs,
Bellcore, and Carnegie Mellon. To find about f2c,
send the message "send index from f2c" to
netlib at research.att.com or research!netlib.
FOR_C Available from:
Cobalt Blue
2940 Union Ave., Suite C
San Jose, CA 95124
(408) 723-0474
Promula.Fortran Available from
Promula Development Corp.
3620 N. High St., Suite 301
Columbus, OH 43214
(614) 263-5454
The comp.sources.unix archives also contain converters between
"K&R" C and ANSI C.
62. Where can I get copies of all these public-domain programs?
A: If you have access to Usenet, see the regular postings in the
comp.sources.unix and comp.sources.misc newsgroups, which describe,
in some detail, the archiving policies and how to retrieve copies.
Otherwise, you can try anonymous ftp and/or uucp from a central,
public-spirited site, such as uunet.uu.net, but this article cannot
track or list all of the available sites and how to access them.
63. How can I call Fortran (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 correctly are often arcane.
64. Why don't C comments nest? Are they legal inside quoted strings?
A: Nested comments would cause more harm than good, mostly because of
the possibility of accidentally leaving comments unclosed by
including the characters "/*" within them. For this reason, it is
usually better to "comment out" large sections of code, which might
contain comments, with #ifdef or #if 0.
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.
65. 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. In particular, the non-
floating-point versions of printf and scanf save space by not
including code to handle %e, %f, and %g. Occasionally the
heuristics for "is the program using floating point?" are
insufficient, and the programmer must insert one dummy explicit
floating-point operation to force loading of floating-point support.
Unfortunately, an apparently common sort of program (thus the
frequency of the question) uses scanf to read, and/or printf to
print, floating-point values upon which no arithmetic is done, which
elicits the problem under Turbo C.
In general, questions about a particular compiler are inappropriate
for comp.lang.c . Problems with PC compilers, for instance, will
find a more receptive audience in a PC newsgroup.
66. Does anyone have a C compiler test suite I can use?
A: Plum Hall, (1 Spruce Ave., Cardiff, NJ 08232, USA), among others,
sells one.
67. Where can I get a YACC grammar for C?
A: Several grammars are floating around; keep your eyes open. There is
one on uunet.uu.net (192.48.96.2) in net.sources/ansi.c.grammar.Z .
FSF's GNU C compiler contains a grammar, as does the appendix to
K&R II. Several have recently been posted to the net.
68. Where can I get the "Indian Hill Style Guide" and other coding
standards?
A: Various standards are available for anonymous ftp from:
site file/directory
cs.washington.edu ~ftp/pub/cstyle.tar.Z
(128.95.1.4)
cs.toronto.edu doc/programming
giza.cis.ohio-state.edu pub/style-guide
prep.ai.mit.edu pub/gnu/standards.text
69. Where can I get extra copies of this list?
A: For now, just pull it off the net; it is normally posted on about
the first of the month, with an Expiration: line which should keep
it around all month. Eventually, it may be available for anonymous
ftp, or via a mailserver.
Thanks to Mark Brader, Joe Buehler, Christopher Calabrese, Stephen M.
Dunn, Tony Hansen, Guy Harris, Karl Heuer, Blair Houghton, Kirk Johnson,
Andrew Koenig, John Lauro, Christopher Lott, Rich Salz, Joshua Simons,
and Erik Talvola, who have contributed, directly or indirectly, to this
article.
Steve Summit
scs at adam.mit.edu
scs%adam.mit.edu at 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.
The C code in this article (vstrcat, error, etc.) is public domain and
may be used without restriction.
More information about the Comp.lang.c
mailing list