Answers to Frequently Asked Questions (FAQ) on comp.lang.c
Steve Summit
scs at adam.mit.edu
Wed May 1 14:03:14 AEST 1991
[Last modified April 29, 1991 by scs.]
Certain topics come up again and again on this newsgroup. They are good
questions, and the answers may not be immediately obvious, but each time
they recur, much net bandwidth and reader time is wasted on repetitive
responses, and on tedious corrections to the incorrect answers which are
inevitably posted.
This article, which is posted monthly, attempts to answer these common
questions definitively and succinctly, so that net discussion can move
on to more constructive topics without continual regression to first
principles.
No mere newsgroup article can substitute for thoughtful perusal of a
full-length 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 refute. Several
noteworthy books on C are listed in this article's bibliography. Many
of the questions and answers are cross-referenced to these books, for
further study by the interested and dedicated reader.
If you have a question about C which is not answered in this article,
please try to answer it by checking a few of the referenced books, or by
asking knowledgeable colleagues, before posing your question to the net
at large. There are many people on the net who are happy to answer
questions, but the volume of repetitive answers posted to one question,
as well as the growing number of questions as the net attracts more
readers, can become oppressive. If you have questions or comments
prompted by this article, please reply by mail rather than following up
-- this article is meant to decrease net traffic, not increase it.
Besides listing frequently-asked questions, this article also summarizes
frequently-posted answers. Even if you know all the answers, it's worth
skimming through this list once in a while, so that when you see one of
its questions unwittingly posted, you won't have to waste time
answering.
This article is always being improved. Your input is welcomed. Send
your comments to scs at adam.mit.edu, scs%adam.mit.edu at mit.edu, and/or
mit-eddie!adam!scs; this article's From: line may be unusable.
The questions answered here are divided into several categories:
1. Null Pointers
2. Arrays and Pointers
3. Order of Evaluation
4. ANSI C
5. C Preprocessor
6. Variable-Length Argument Lists
7. Lint
8. Memory Allocation
9. Structures
10. Declarations
11. Boolean Expressions and Variables
12. Operating System Dependencies
13. Stdio
14. Style
15. Miscellaneous (Fortran to C converters, YACC grammars, etc.)
Herewith, some frequently-asked questions and their answers:
Section 1. 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 yield 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 conceptually 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
questions 46, 52, and 82.
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 type of 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 Sec. 3.2.2.3 p. 38.
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 most casts may safely be omitted, since
the prototype tells the compiler that a pointer is required, and of
which type, enabling it to correctly 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
assignment
variable argument in
comparison varargs function call
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
Sec. 3.2.2.3 .
3. What is NULL and how is it #defined?
A: As a matter of style, 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; see question 8.
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 Sec. 4.1.5 p. 99, Sec. 3.2.2.3 p. 38,
Rationale Sec. 4.1.5 p. 74.
4. 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.
5. 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 homogeneous pointers, thus questionably
valid assistance) this definition may catch programs which use NULL
incorrectly (e.g. when the ASCII NUL character was really
intended).
6. I use the preprocessor macro
#define Nullptr(type) (type *)0
to help me build null pointers of the correct type.
A: This trick, though popular in some circles, does not buy much. It
is not needed in assignments and comparisons; see question 2. It
does not even save keystrokes. Its use suggests to the reader that
the author is shaky on the subject of null pointers, and requires
the reader to check the #definition of the macro, its invocations,
and _all_ other pointer usages much more carefully.
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)
"Abbreviations" such as if(p), though perfectly legal, are
considered by some to be bad style.
See also question 68.
References: K&R II Sec. A7.4.7 p. 204; H&S Sec. 5.3 p. 91; ANSI
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.
NULL should _not_ be used when another kind of 0 is required, even
though it might work, because doing so sends the wrong stylistic
message. (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.
Reference: 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 symbolic constants are often used in place of numbers
because the numbers might change, this is _not_ the reason that
NULL is used in place of 0. Once again, the language guarantees
that source-code 0's (in pointer contexts) generate null pointers.
NULL is used only as a stylistic convention.
10. I'm confused. NULL is guaranteed to be 0, but the null pointer is
not?
A: When the term "null" or "NULL" is casually used, one of several
things may be meant:
1. The conceptual null pointer, the abstract language concept
defined in question 1. It is implemented with...
2. The internal (or run-time) representation of a null pointer,
which may or may not be all-bits-0 and which may be different
for different pointer types. The actual values should be of
concern only to compiler writers. Authors of C programs never
see them, since they use...
3. The 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" (in lower case)
for sense 1, the character "0" for sense 3, and the capitalized
word "NULL" for sense 4.
11. Why is there so much confusion surrounding null pointers? Why do
these questions come up so often?
A: C programmers traditionally like to know more than they need to
about the underlying machine implementation. The fact that null
pointers are represented both in source code, and internally to
most machines, as zero invites unwarranted assumptions. The use of
a preprocessor macro (NULL) suggests that 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
instead of an error message, and if that uncast 0 was supposed to
be a null pointer, the code may not work.
12. 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 an argument in a function
call, cast it to the pointer type expected by the function
being called.
The rest of the discussion has to do with other people's
misunderstandings, or with the internal representation of null
pointers, which you shouldn't need to know. Understand questions
1, 2, and 3, and consider 8 and 11, and you'll do fine.
13. Given all the confusion surrounding null pointers, wouldn't it be
easier simply to require them to be represented internally by
zeroes?
A: If for no other reason, doing so would be ill-advised because it
would unnecessarily constrain implementations which would otherwise
naturally represent null pointers by special, nonzero bit patterns,
particularly when those values would trigger automatic hardware
traps for invalid accesses.
Besides, what would this requirement really accomplish? Proper
understanding of null pointers does not require knowledge of the
internal representation, whether zero or nonzero. Assuming that
null pointers are internally zero does not make any code easier to
write (except for a certain ill-advised usage of calloc; see
question 52). Known-zero internal pointers would not obviate casts
in function calls, because the _size_ of the pointer might still be
different from that of an int. (If "nil" were used to request null
pointers rather than "0," as mentioned in question 11, the urge to
assume an internal zero representation would not even arise.)
14. Seriously, have any actual machines really used nonzero null
pointers?
A: "Certain Prime computers use a value different from all-
bits-0 to encode the null pointer. Also, some large
Honeywell-Bull machines use the bit pattern 06000 to encode
the null pointer."
-- Portable C, by H. Rabinowitz and Chaim Schaap,
Prentice-Hall, 1990, page 147.
The "certain Prime computers" were the segmented 50 series, which
used segment 07777, offset 0 for the null pointer, at least for
PL/I. Later models used segment 0, offset 0 for null pointers in
C, necessitating new instructions such as TCNP (Test C Null
Pointer), evidently as a sop to all the extant poorly-written C
code which made incorrect assumptions.
The Symbolics Lisp Machine, a tagged architecture, does not even
have conventional numeric pointers; it uses the pair <NIL, 0>
(basically a nonexistent <object, offset> handle) as a C null
pointer.
Section 2. Arrays and Pointers
15. I had the definition char x[6] in one source file, and in another I
declared extern char *x. Why didn't it work?
A: The declaration extern char *x 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 x[].
References: CT&P Sec. 3.3 pp. 33-4, Sec. 4.5 pp. 64-5.
16. But I heard that char x[] was identical to char *x.
A: Not at all. (What you heard has to do with formal parameters to
functions; see question 19.) Arrays are not pointers. The
declaration "char a[6];" requests that space for six characters be
set aside, to be known by the name "a." That is, there is a
location named "a" at which six characters can sit. The
declaration "char *p;" on the other hand, requests a place which
holds a pointer. The pointer is to be known by the name "p," and
can point to any char (or contiguous array of chars) anywhere.
As usual, a picture is worth a thousand words. The statements
char a[] = "hello";
char *p = "world";
would result in data structures which could be represented like
this:
+---+---+---+---+---+---+
a: | h | e | l | l | o |\0 |
+---+---+---+---+---+---+
+-----+ +---+---+---+---+---+---+
p: | *======> | w | o | r | l | d |\0 |
+-----+ +---+---+---+---+---+---+
17. You mean that a reference like x[3] generates different code
depending on whether x is an array or a pointer?
A: Precisely. Referring back to the sample declarations in the
previous question, when the compiler sees the expression a[3], it
emits code to start at the location "a," move three past it, and
fetch the character there. When it sees the expression p[3], it
emits code to start at the location "p," fetch the pointer value
there, add three to the pointer, and finally fetch the character
pointed to. In the example above, both a[3] and p[3] happen to be
the character 'l', but the compiler gets there differently. (See
also question 93.)
18. So what is meant by the "equivalence of pointers and arrays" in C?
A: Much of the confusion surrounding pointers in C can be traced to a
misunderstanding of this statement. Saying that arrays and
pointers are "equivalent" does not by any means imply that they are
interchangeable.
"Equivalence" refers to the following key definition:
An lvalue of type array-of-T which appears in an expression
decays (with three exceptions) into a pointer to its first
element; the type of the resultant pointer is pointer-to-T.
(The exceptions are when the array is the operand of the sizeof()
operator or of the & operator, or is a literal string initializer
for a character array.)
As a consequence of this definition, there is not really any
difference in the behavior of the "array subscripting" operator []
as it applies to arrays and pointers. In an expression of the form
a[i], the array reference "a" decays into a pointer, following the
rule above, and is then subscripted exactly as would be a pointer
variable in the expression p[i]. In either case, the expression
x[i] (where x is an array or a pointer) is, by definition, exactly
equivalent to *((x)+(i)).
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 Sec. 3.3.2.1, Sec. 3.3.6 .
19. Then why are array and pointer declarations interchangeable as
function formal parameters?
A: Since arrays decay immediately into pointers, an array is never
actually passed to a function. Therefore, any parameter
declarations which "look like" arrays, e.g.
f(a)
char a[];
are treated by the compiler as if they were pointers, since that is
what the function will receive if an array is passed:
f(a)
char *a;
This conversion holds only within function formal parameter
declarations, nowhere else. If this conversion bothers you, avoid
it; many people have concluded that the confusion it causes
outweighs the small advantage of having the declaration "look like"
the call 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 Sec. 3.5.4.3, Sec. 3.7.1, CT&P Sec. 3.3
pp. 33-4.
20. Someone explained to me that arrays were really just constant
pointers.
A: That person did you a disservice. An array name is "constant" in
that it cannot be assigned to, but an array is _not_ a pointer, as
the discussion and pictures in question 16 should make clear.
21. I came across some "joke" code containing the "expression"
5["abcdef"] . How can this be legal C?
A: Yes, Virginia, array subscripting is commutative in C. This
curious fact follows from the pointer definition of array
subscripting, namely that a[e] is exactly equivalent to *((a)+(e)),
for _any_ expression e and primary expression a, as long as one of
them is a pointer expression and one is integral. This unsuspected
commutativity is often mentioned in C texts as if it were something
to be proud of, but it finds no useful application outside of the
Obfuscated C Contest (see question 90).
22. 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 can be confusing, and must be treated carefully.
(The confusion is heightened by the existence of incorrect
compilers, including some versions of pcc and pcc-derived lint's,
which improperly accept assignments of multi-dimensional arrays to
multi-level pointers.) If you are passing a two-dimensional array
to a function:
int array[YSIZE][XSIZE];
f(array);
the function's declaration should match:
f(int a[][XSIZE]) {...}
or
f(int (*ap)[XSIZE]) {...} /* ap is a pointer to an array */
In the first declaration, the compiler performs the usual implicit
parameter rewriting of "array of array" to "pointer to array;" in
the second form the pointer declaration is explicit. Since the
called function does not allocate space for the array, it does not
need to know the overall size, so the number of "rows," YSIZE, can
be omitted. The "shape" of the array is still important, so the
"column" dimension XSIZE (and, for 3- or more dimensional arrays,
the intervening ones) must be included.
If a function is already declared as accepting a pointer to a
pointer, it is probably incorrect to pass a two-dimensional array
directly to it.
23. 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. 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, if at all. (See question 22 above.) When
people speak casually of a pointer to an array, they usually mean a
pointer to its first element.
If you really need to declare a pointer to an entire array, use
something like "int (*ap)[N];" where N is the size of the array.
(See also question 63.) If the size of the array is unknown, N can
be omitted, but the resulting type, "pointer to array of unknown
size," is useless.
24. 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 can save space, although it is not
necessarily contiguous in memory as a real array would be. Here is
a two-dimensional example:
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 should be declared correctly,
and each return value checked.)
You can keep the array's contents contiguous, while making later
reallocation of individual rows difficult, with a bit of explicit
pointer arithmetic:
int **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 schemes 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 the i,jth element 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.)
Section 3. Order of Evaluation
25. 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 behavior of code which contains ambiguous or undefined side
effects (including ambiguous embedded assignments) has always been
undefined. (Note, too, that a compiler's choice, especially under
ANSI rules, for "undefined behavior" may be to refuse to compile
the code.) Don't even try to find out how your compiler implements
such things (contrary to the ill-advised exercises in many C
textbooks); as K&R wisely point out, "if you don't know _how_ they
are done on various machines, that innocence may help to protect
you."
References: K&R I Sec. 2.12 p. 50; K&R II Sec. 2.12 p. 54; ANSI
Sec. 3.3 p. 39; CT&P Sec. 3.7 p. 47; PCS Sec. 9.5 pp. 120-1.
(Ignore H&S Sec. 7.12 pp. 190-1, which is obsolete.)
26. 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, (as well as ?: );
each of them does imply a sequence point (i.e. left-to-right
evaluation is guaranteed). Any book on C should make this clear.
References: K&R I Sec. 2.6 p. 38, Secs. A7.11-12 pp. 190-1; K&R II
Sec. 2.6 p. 41, Secs. A7.14-15 pp. 207-8; ANSI Secs. 3.3.13 p. 52,
3.3.14 p. 52, 3.3.15 p. 53, 3.3.17 p. 55, CT&P Sec. 3.7 pp. 46-7.
Section 4. ANSI C
27. 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, including several widespread public reviews, 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. 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). The ANSI C standard also
formalizes the C run-time library support routines.
The published Standard includes a "Rationale," which explains many
of its decisions, and discusses a number of subtle points,
including several of those covered here. (The Rationale is "not
part of ANSI Standard X3.159-1989, but is included for information
only.")
The Standard has been adopted as an international standard, ISO/IEC
9899:1990, although the Rationale is currently not included.
28. How can I get a copy of the Standard?
A: Copies are available from
American National Standards Institute
1430 Broadway
New York, NY 10018 USA
(+1) 212 642 4900
or
Global Engineering Documents
2805 McGaw Avenue
Irvine, CA 92714 USA
(+1) 714 261 1455
(800) 854 7179 (U.S. & Canada)
The cost from ANSI is $50.00, plus $6.00 shipping. Quantity
discounts are available. (Note that ANSI derives revenues to
support its operations from the sale of printed standards, so
electronic copies are _not_ available.)
The Rationale, by itself, has been printed by Silicon Press, ISBN
0-929306-07-4.
29. Does anyone have a tool for converting old-style C programs to ANSI
C, or for automatically generating prototypes?
A: Two programs, protoize and unprotoize, convert back and forth
between prototyped and "old style" function definitions and
declarations. (These programs do _not_ handle full-blown
translation between "Classic" C and ANSI C.) These programs exist
as patches to the FSF GNU C compiler, gcc. Look for the file
protoize-1.39.0 in pub/gnu at prep.ai.mit.edu (18.71.0.38), or at
several other FSF archive sites.
Several prototype generators exist, many as modifications to lint.
(See also question 89.)
30. What's the difference between "char const *p" and "char * const p"?
A: "char const *p" is a pointer to a constant character (you can't
change the character); "char * const p" is a constant pointer to a
(variable) character (i.e. you can't change the pointer). (Read
these "inside out" to understand them. See question 63.)
31. 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;". Old C (and ANSI C, in the absence of
prototypes) silently promotes floats to doubles when passing them
as arguments, and arranges that doubles being passed are coerced
back to floats if the formal parameters are declared that way.
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, as long as the address of that
parameter is not taken.)
Reference: ANSI Sec. 3.3.2.2 .
32. 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 could look
like the beginning of a character constant), and no newlines inside
quotes. Therefore, natural-language comments and pseudocode should
always be written between the "official" comment delimiters /* and
*/. (But see also question 91.)
References: ANSI Sec. 2.1.1.2 p. 6, Sec. 3.1 p. 19 line 37.
33. Why does the ANSI Standard not guarantee more than six monocase
characters of external identifier significance?
A: The problem is older linkers which are neither under the control of
the ANSI standard nor the C compiler developers on the systems
which have them. The limitation is only that identifiers be
_significant_ in the first six characters, not that they be
restricted to six characters in length. This limitation is
annoying, but certainly not unbearable, and is marked in the
Standard as "obsolescent," i.e. a future revision will likely relax
it.
This concession to current, restrictive linkers really had to be
made, no matter how vehemently some people oppose it. (The
Rationale notes that its retention was "most painful.") If you
disagree, or have thought of a trick by which a compiler burdened
with a restrictive linker could present the C programmer with the
appearance of more significance in external identifiers, read the
excellently-worded section 3.1.2 in the X3.159 Rationale (see
question 27), which discusses several such schemes and explains why
they could not be mandated.
References: ANSI Sec. 3.1.2 p. 21, Sec. 3.9.1 p. 96, Rationale
Sec. 3.1.2 pp. 19-21.
34. What was noalias and what ever happened to it?
A: noalias was another type qualifier, in the same syntactic class as
const and volatile, which was intended to assert that the object
pointed to was not also pointed to ("aliased") by other pointers.
The primary application, which is an important one, would have been
for the formal parameters of subroutines designed to perform
computations on large arrays. A compiler cannot usually take
advantage of vectorization or other parallelization hardware (on
supercomputers which have it) unless it can ensure that the source
and destination arrays do not overlap.
The noalias keyword was not backed up by any "prior art," and it
was introduced late in the review and approval process. It was
phenomenally difficult to define precisely and explain coherently,
and sparked widespread, acrimonious debate. It had far-ranging
implications, particularly on several standard library interfaces,
for which easy fixes were not readily apparent.
Because of the criticism and the difficulty of defining noalias
well, the Committee wisely declined to adopt it, in spite of its
superficial attractions. (When writing a standard, features cannot
be introduced halfway; their full integration, and all
implications, must be understood.) The need for an explicit
mechanism to support parallel implementation of non-overlapping
operations remains unfilled (although the C Numerical Extensions
Working Group is examining the problem).
References: ANSI Sec. 3.9.6 .
35. What are #pragmas and what are they good for?
A: The #pragma directive provides a single, well-defined "escape
hatch" which can be used for all sorts of implementation-specific
controls and extensions: source listing control, structure packing,
warning suppression (like the old lint /* NOTREACHED */ comments),
etc.
References: ANSI Sec. 3.8.6 .
Section 5. C Preprocessor
36. How can I write a generic macro to swap two values?
A: There is no good answer to this question. If the values are
integers, a well-known trick using exclusive-OR could perhaps be
used, but it will not work for floating-point values or pointers,
(and it will not work if the two values are the same variable, and
the "obvious" supercompressed implementation for integral types
a^=b^=a^=b is, strictly speaking, illegal due to multiple side-
effects, and...). If the macro is intended to be used on values of
arbitrary type (the usual goal), it cannot use a temporary, since
it does not know what type of temporary it needs, and standard C
does not provide a typeof operator.
The best all-around solution is probably to forget about using a
macro, unless you don't mind passing in the type as a third
argument.
37. 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: That comments disappeared entirely and could therefore be used for
token pasting was an undocumented feature of some early
preprocessor implementations, notably Reiser's. ANSI affirms (as
did K&R) that comments are replaced with white space. However,
since the need for pasting tokens was demonstrated and real, ANSI
introduced a well-defined token-pasting operator, ##, which can be
used like this:
#define Paste(a, b) a##b
Reference: ANSI Sec. 3.8.3.3 p. 91, Rationale pp. 66-7.
38. What's the best way to write a multi-statement cpp macro?
A: The usual goal is to write a macro that can be invoked as if it
were a single function-call statement. This means that the
"caller" will be supplying the final semicolon, so the macro body
should not. The macro body cannot be a simple brace-delineated
compound statement, because syntax errors would result if it were
invoked (apparently as a single statement, but with a resultant
extra semicolon) as the if branch of an if/else statement with an
explicit else clause.
The traditional solution is to use
#define Func() do { \
/* declarations */ \
stmt1; \
stmt2; \
/* ... */ \
} while(0) /* (no trailing ; ) */
When the "caller" appends a semicolon, this expansion becomes a
single statement regardless of context. (An optimizing compiler
will remove any "dead" tests or branches on the constant condition
0, although lint may complain.)
If all of the statements in the intended macro are simple
expressions, with no declarations or loops, another technique is to
write a single, parenthesized expression using one or more comma
operators. (This technique also allows a value to be "returned.")
Reference: CT&P Sec. 6.3 pp. 82-3.
39. 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));
The obvious disadvantage 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 40 and 41 below.)
Section 6. Variable-Length Argument Lists
40. How can I write a function that takes a variable number of
arguments?
A: Use the <stdarg.h> header (or, if you must, the older <varargs.h>).
Here is a function which concatenates an arbitrary number of
strings into malloc'ed memory:
#include <stddef.h> /* for NULL, size_t */
#include <stdarg.h> /* for va_ stuff */
#include <string.h> /* for strcat et al */
#include <stdlib.h> /* for malloc */
char *vstrcat(char *first, ...)
{
size_t 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; /* error */
(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.)
Under a pre-ANSI compiler, rewrite the function definition without
a prototype ("char *vstrcat(first) char *first; {"), include
<stdio.h> rather than <stddef.h>, replace "#include <stdlib.h>"
with "extern char *malloc();", and use int instead of size_t. You
may also have to delete the (void) casts, and use the older varargs
package instead of stdarg. See 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 Secs. 4.8 through 4.8.1.3 .
41. 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 the older <varargs.h> package, instead of <stdarg.h>, change
the function header to:
void error(va_alist)
va_dcl
{
char *fmt;
change the va_start line to
va_start(argp);
and add the line
fmt = va_arg(argp, char *);
between the calls to va_start and vfprintf. (Note that there is no
semicolon after va_dcl.)
References: K&R II Sec. 8.3 p. 174, Sec. B1.2 p. 245; H&S
Sec. 17.12 p. 337; ANSI Secs. 4.9.6.7, 4.9.6.8, 4.9.6.9 .
42. 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 provide a nonstandard nargs() function, 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 execl and vstrcat examples under questions 2 and
40 above).
Section 7. Lint
43. I just typed in this program, and it's acting strangely. Can you
see anything wrong with it?
A: Try running lint first. Many C compilers are really only half-
compilers, electing not to diagnose numerous source code
difficulties which would not actively preclude code generation.
44. How can I shut off the "warning: possible pointer alignment
problem" message lint gives me for each call to malloc?
A: The problem is that traditional versions of lint do not know, and
cannot be told, that malloc "returns a pointer to space suitably
aligned for storage of any type of object." It is possible to
provide a pseudoimplementation of malloc, using a #define inside of
#ifdef lint, which effectively shuts this warning off, but a
simpleminded #definition will also suppress meaningful messages
about truly incorrect invocations. It may be easier simply to
ignore the message, perhaps in an automated way with grep -v.
45. Where can I get an ANSI-compatible lint?
A: A product called FlexeLint is available (in "shrouded source form,"
for compilation on 'most any system) from
Gimpel Software
3207 Hogarth Lane
Collegeville, PA 19426 USA
(+1) 215 584 4261
The System V release 4 lint is ANSI-compatible, and is available
separately (bundled with other C tools) from Unix Support Labs (a
subsidiary of AT&T), or from System V resellers.
Section 8. Memory Allocation
46. Why doesn't this fragment work?
char *answer;
printf("Type something:\n");
gets(answer);
printf("You typed \"%s\"\n", answer);
A: The pointer variable "answer," which is handed to the gets function
as the location into which the response should be stored, has not
been set to point to any valid storage. That is, we cannot say
where the pointer "answer" points. (Since local variables are not
initialized, and typically contain garbage, it is not even
guaranteed that "answer" starts out as a null pointer. See
question 82.)
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 <string.h>
char answer[100], *p;
printf("Type something:\n");
fgets(answer, 100, stdin);
if((p = strchr(answer, '\n')) != NULL)
*p = '\0';
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 for this example, fgets does not
automatically delete the trailing \n, as gets would.) It would
also be possible to use malloc to allocate the answer buffer,
and/or to parameterize its size (#define ANSWERSIZE 100).
47. I can't get strcat to work. I tried
char *s1 = "Hello, ";
char *s2 = "world!";
char *s3 = strcat(s1, s2);
but I got strange results.
A: Again, the problem is that space for the concatenated result is not
properly allocated. C does not provide an automatically-managed
string type. C compilers only allocate memory for objects
explicitly mentioned in the source code (in the case of "strings,"
this includes character arrays and string literals). The
programmer must arrange (explicitly) for sufficient space for the
results of run-time operations such as string concatenation,
typically by declaring arrays, or by calling malloc.
strcat performs no allocation; the second string is appended to the
first one, in place. Therefore, one fix would be to declare the
first string as an array with sufficient space:
char s1[20] = "Hello, ";
Since strcat returns the value of its first argument (s1, in this
case), the s3 variable is superfluous.
Reference: CT&P Sec. 3.2 p. 32.
48. 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. If a library routine's documentation does not
explicitly mention allocation, it is usually the caller's problem.
The Synopsis section at the top of a Unix-style man page can be
misleading. The code fragments presented there are closer to the
function definition used by the call's implementor than the
invocation used by the caller. In particular, many routines which
accept pointers (e.g. to structs or strings), are usually called
with the address of some object (a struct, or an array -- see
questions 18 and 19.) Another common example is stat().
49. 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 was
never 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.
References: ANSI Rationale Sec. 4.10.3.2 p. 102; CT&P Sec. 7.10
p. 95.
50. How does free() know how many bytes to free?
A: The malloc/free package remembers the size of each block it
allocates and returns, so it is not necessary to remind it of the
size when freeing.
51. Is it legal to pass a null pointer as the first argument to
realloc()? Why would you want to?
A: ANSI C sanctions this usage (and the related realloc(..., 0), which
frees), but several earlier implementations do not support it, so
it is not widely portable. Passing an initially-null pointer to
realloc can make it easier to write a self-starting incremental
allocation algorithm.
References: ANSI Sec. 4.10.3.4 .
52. What is the difference between calloc and malloc? Is it safe to
use calloc's zero-fill guarantee for pointer and floating-point
values? Does free work on memory allocated with calloc, or do you
need a cfree?
A: calloc(m, n) is essentially equivalent to
p = malloc(m * n);
memset(p, 0, m * n);
The zero fill is all-bits-zero, and does not therefore guarantee
useful zero values for pointers (see questions 1-14) or floating-
point values. free can (and should) be used to free the memory
allocated by calloc.
References: ANSI Secs. 4.10.3 to 4.10.3.2 .
53. What is alloca and why is its use discouraged?
A: alloca allocates memory which is automatically freed when the
function which called alloca returns. That is, memory allocated
with alloca is local to a particular function's "stack frame" or
context.
alloca cannot be written portably, and is difficult to implement on
machines without a stack. Its use is problematical (and the
obvious implementation on a stack-based machine fails) when its
return value is passed directly to another function, as in
fgets(alloca(100), 100, stdin).
For these reasons, alloca cannot be used in programs which must be
widely portable, no matter how useful it might be.
Section 9. Structures
54. 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 Secs. 3.1.2.5, 3.2.2.1, 3.3.16 .
55. How does struct passing and returning work?
A: When structures are passed as arguments to functions, the entire
struct is typically pushed on the stack, using as many words as are
required. (Pointers to structures are often chosen precisely to
avoid this overhead.)
Structures are typically returned from functions in a location
pointed to by an extra, compiler-supplied "hidden" argument to the
function. Older compilers often used a special, static location
for structure returns, although this made struct-valued functions
nonreentrant, which ANSI C disallows.
Reference: ANSI Sec. 2.2.3 p. 13.
56. 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.) Since struct-valued functions are
usually implemented by adding a hidden return pointer, the
generated code for main() actually expects three arguments,
although only two were passed (in this case, by the C start-up
code). See also question 96.
Reference: CT&P Sec. 2.3 pp. 21-2.
57. 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.
If you want to compare two structures, you must write your own
function to do so. C++ 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; ANSI
Rationale Sec. 3.3.9 p. 47.
58. I came across some code that declared a structure like this:
struct name
{
int namelen;
char name[1];
};
and then did some tricky allocation to make the name array act like
it had several elements. Is this legal and/or portable?
A: This technique is popular, although Dennis Ritchie has called it
"unwarranted chumminess with the compiler." The ANSI C standard
allows it only implicitly. It seems to be portable to all known
implementations. (Compilers which check array bounds carefully
might issue warnings.)
59. How can I determine the byte offset of a field within a structure?
A: ANSI C defines the offsetof macro, which should be used if
available; see <stddef.h>. If you don't have it, 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.
Reference: ANSI Sec. 4.1.5 .
60. How can I access structure fields by name at run time?
A: Build a table of names and offsets, using the offsetof() macro.
The offset of field b in struct a is
offsetb = offsetof(struct a, b)
If structp is a pointer to an instance of this structure, and b is
an int field with offset as computed above, b's value can be set
indirectly with
*(int *)((char *)structp + offsetb) = value;
Section 10. Declarations
61. How do you decide which integer type to use?
A: If you might need large values (above 32767 or below -32767), use
long. If space is very important (there are large arrays or many
structures), use short. Otherwise, use int. If well-defined
overflow characteristics are important and/or negative values are
not, use the corresponding unsigned types. (But beware mixtures of
signed and unsigned.)
Similar arguments apply when deciding between float and double.
Exceptions apply if the address of a variable is taken and must
have a particular type.
Although char or unsigned char can be used as a "tiny" int type,
doing so is often more trouble than it's worth.
62. I can't seem to define a linked list successfully. I tried
typedef struct
{
char *item;
NODEPTR next;
} *NODEPTR;
but the compiler gave me error messages. Can't a 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 with this example is that the NODEPTR typedef is not
complete when the "next" field is declared. You will have to give
the structure a tag ("struct node"), and declare the "next" field
as "struct node next;".
A similar problem, with a similar solution, can arise when
attempting to declare a pair of typedef'ed mutually recursive
structures.
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 Sec. 3.5.2.3 .
63. How do I declare an array of pointers to functions returning
pointers to functions returning pointers to characters?
A: This question can be answered in at least three ways (all assume
the hypothetical array is to have 5 elements):
1. char *(*(*a[5])())();
2. Build the declaration up in stages, using typedefs:
typedef char *pc; /* pointer to char */
typedef pc fpc(); /* function returning pointer to char */
typedef fpc *pfpc; /* pointer to above */
typedef pfpc fpfpc(); /* function returning... */
typedef fpfpc *pfpfpc; /* pointer to... */
pfpfpc a[5]; /* array of... */
3. Use the cdecl program, which turns English into C and vice
versa:
cdecl> declare a as array 5 of pointer to function returning
pointer to function returning pointer to char
char *(*(*a[5])())()
cdecl can also explain complicated declarations, help with
casts, and indicate which set of parentheses the arguments go
in (for complicated function definitions, like the above).
Any good book on C should explain how to read these complicated C
declarations "inside out" to understand them ("declaration mimics
use").
Reference: H&S Sec. 5.10.1 p. 116.
64. So where can I get cdecl?
A: Several public-domain versions are available. One is in volume 14
of comp.sources.unix . (See question 89.)
Reference: K&R II Sec. 5.12 .
65. I finally figured out the syntax for declaring pointers to
functions, but now how do I initialize one?
A: Use something like
extern int func();
int (*fp)() = func;
When the name of a function appears in an expression but is not
being called (i.e. is not followed by a "("), it "decays" into a
pointer (i.e. it has its address implicitly taken), much as an
array name does.
An explicit extern declaration for the function is normally needed,
since implicit external function declaration does not happen in
this case (again, because the function name is not followed by a
"(").
66. I've seen different methods used for calling through pointers to
functions. What's the story?
A: Originally, a pointer to a function had to be "turned into" a
"real" function, with the * operator (and an extra pair of
parentheses, to keep the precedence straight), before calling:
int r, f(), (*fp)() = f;
r = (*fp)();
Another analysis holds that functions are always called through
pointers, but that "real" functions decay implicitly into pointers
(in expressions, as they do in initializations) and so cause no
trouble. This reasoning, which was adopted in the ANSI standard,
means that
r = fp();
is legal and works correctly, whether fp is a function or a pointer
to one. (The usage has always been unambiguous; there is nothing
you ever could have done with a function pointer followed by an
argument list except call through it.) An explicit * is harmless,
and still allowed (and recommended, if portability to older
compilers is important).
References: ANSI Sec. 3.3.2.2 p. 41, Rationale p. 41.
Section 11. Boolean Expressions and Variables
67. 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 may 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};
or use raw 1 and 0, as long as you are consistent within one
program or project. (An 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)
or define "helper" macros such as
#define Istrue(e) ((e) != 0)
These don't buy anything (see below).
68. 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 work as expected (as long as TRUE is 1), but it 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. (Besides, if you believe that
"if((a == b) == TRUE)" is an improvement over "if(a == b)", why
stop there? Why not use "if(((a == b) == TRUE) == TRUE)"?) A good
rule of thumb is to use TRUE and FALSE (or the like) only for
assignment to a Boolean variable, or as the return value from a
Boolean function, never in a comparison.
The preprocessor macros TRUE and FALSE (and, of course, NULL) are
used for code readability, not because the underlying values might
ever change. That "true" is 1 and "false" 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 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; Achilles and the
Tortoise.
69. 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
enumerations 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 primary advantages of enums are that the numeric values are
automatically assigned, and that a debugger may be able to display
the symbolic values when enum variables are examined. (A compiler
may also generate nonfatal warnings when enums and ints are
indiscriminately mixed, since doing so can still be considered bad
style even though it is not strictly illegal). A disadvantage is
that the programmer has little control over the size (or over those
nonfatal warnings).
References: K&R II Sec. 2.3 p. 39, Sec. A4.2 p. 196; H&S Sec. 5.5
p. 100; ANSI Secs. 3.1.2.5, 3.5.2, 3.5.2.2 .
Section 12. Operating System Dependencies
70. 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 in use, and
cannot be standardized by the C language. Some versions of curses
have a cbreak() function which does what you want. 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 . Many common questions are answered in frequently-
asked questions postings in such groups as comp.unix.questions and
comp.sys.ibm.pc.misc . Note that the answers are often not unique
even across different variants of a system. Bear in mind when
answering system-specific questions that the answer that applies to
your system may not apply to everyone else's.
References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1.
71. How can I find out if there are characters available for reading
(and if so, how many)? Alternatively, how can I do a read that
will not block if there are no characters available?
A: These, too, are entirely operating-system-specific. Some versions
of curses have a nodelay() function. Depending on your system, you
may also be able to use "nonblocking I/O", or a system call named
"select", or the FIONREAD ioctl, or kbhit(), or rdchk(), or the
O_NDELAY option to open() or fcntl().
72. 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, or it may contain
nothing. You may be able to duplicate the command language
interpreter's search path logic to locate the executable if the
name in argv[0] is present but incomplete. However, there is no
guaranteed or portable solution.
73. How can a process change an environment variable in its caller?
A: In general, it cannot. Different operating systems implement
name/value functionality similar to the Unix environment in
different ways. Whether the "environment" can be usefully altered
by a running program, and if so, how, is system-dependent.
Under Unix, a process can modify its own environment (some systems
provide setenv() and/or putenv() functions to do this), and the
modified environment is usually passed on to any child processes,
but it is _not_ propagated back to the parent process.
74. How can a file be shortened in-place without completely clearing or
rewriting it?
A: BSD systems provide ftruncate(), and several others supply
chsize(), but there is no truly portable solution.
Section 13. Stdio
75. Why does errno contain ENOTTY after a call to printf?
A: Many implementations of the stdio package adjust their behavior
slightly if stdout is a terminal. To make the determination, these
implementations perform 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.
Reference: CT&P Sec. 5.4 p. 73.
76. 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) whenever output should
definitely be visible. Several mechanisms attempt to perform the
fflush for you, at the "right time," but they tend to apply only
when stdout is a terminal. (See question 75.)
77. 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".
It is usually better to fgets() to read a whole line, and then use
sscanf() or other string functions to parse the line buffer.
78. How can I recover the file name given an open file descriptor?
A: This problem is, in general, insoluble. Under Unix, for instance,
a scan of the entire disk, (perhaps requiring special permissions)
would theoretically be required, and would fail if the file
descriptor was a pipe or referred to a deleted file (and could give
a misleading answer for a file with multiple links). It is best to
remember the names of open files yourself (perhaps with a wrapper
function around fopen).
Section 14. Style
79. Here's a neat trick:
if(!strcmp(s1, s2))
Is this good style?
A: No. This is a classic example of C minimalism carried to an
obnoxious degree. The test succeeds if the two strings are equal,
but its form strongly suggests that it tests for inequality.
A much better solution is to use a macro:
#define Streq(s1, s2) (strcmp(s1, s2) == 0)
80. What's the best style for code layout in C?
A: K&R, while providing the example most often copied, also supply a
good excuse for avoiding it:
The position of braces is less important, although
people hold passionate beliefs. We have chosen one
of several popular styles. Pick a style that suits
you, then use it consistently.
It is more important that the layout chosen be consistent (with
itself, and with nearby or common code) than that it be "perfect."
If your coding environment (i.e. local custom or company policy)
does not suggest a style, and you don't feel like inventing your
own, just copy K&R. (The tradeoffs between various indenting and
brace placement options can be exhaustively and minutely examined,
but don't warrant repetition here. See also the Indian Hill Style
Guide.)
Reference: K&R Sec. 1.2 p. 10.
81. Where can I get the "Indian Hill Style Guide" and other coding
standards?
A: Various documents are available for anonymous ftp from:
Site: File or directory:
cs.washington.edu ~ftp/pub/cstyle.tar.Z
(128.95.1.4) (the updated Indian Hill guide)
cs.toronto.edu doc/programming
giza.cis.ohio-state.edu pub/style-guide
Section 15. Miscellaneous
82. What can I safely assume about the initial values of variables
which are not explicitly initialized? If global variables start
out as "zero," is that good enough for null pointers and floating-
point zeroes?
A: Variables with "static" duration (that is, those declared outside
of functions, and those declared with the storage class static),
are guaranteed initialized to zero, as if the programmer had typed
"= 0". Therefore, such variables are initialized to the null
pointer (of the correct type) if they are pointers, and to 0.0 if
they are floating-point.
Variables with "automatic" duration (i.e. local variables without
the static storage class) start out containing garbage, unless they
are explicitly initialized. Nothing useful can be predicted about
the garbage.
Dynamically-allocated memory obtained with malloc and realloc is
also likely to contain garbage, and must be initialized by the
calling program, as appropriate. Memory obtained with calloc
contains all-bits-0, but this is not necessarily useful for pointer
or floating-point values (see question 52).
83. Can someone tell me how to write itoa (the inverse of atoi)?
A: Just use sprintf. (You'll have to allocate space for the result
somewhere anyway; see questions 46 and 47. Don't worry that
sprintf may be overkill, potentially wasting run time or code
space; it works well in practice.)
References: K&R I Sec. 3.6 p. 60; K&R II Sec. 3.6 p. 64.
84. 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 in case 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
(see, for example, the file partime.c, widely distributed with the
RCS package), but they are less likely to become standardized.
References: K&R II Sec. B10 p. 256; H&S Sec. 20.4 p. 361; ANSI
Sec. 4.12.2.3 .
85. How can I write data files which can be read on other machines with
different word size, byte order, or floating point formats?
A: The best solution is to use text files (usually ASCII), written
with fprintf and read with fscanf or the like. (Similar advice
also applies to network protocols.) Be skeptical of arguments
which imply that text files are too big, or that reading and
writing them is too slow. Not only is their efficiency frequently
acceptable in practice, but the advantages of being able to
manipulate them with standard tools can be overwhelming.
If you must use a binary format, you can improve portability, and
perhaps take advantage of prewritten I/O libraries, by making use
of standardized formats such as Sun's XDR, OSI's ASN.1, or CCITT's
X.409 .
86. I seem to be missing the system header file <sgtty.h>. Can someone
send me a copy?
A: Standard headers exist in part so that definitions appropriate to
your compiler, operating system, and processor can be supplied.
You cannot just pick up a copy of someone else's header file and
expect it to work, unless that person is using exactly the same
environment. Ask your compiler vendor why the file was not
provided (or to send a replacement copy).
87. 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 and ensuring correct run-time
startup are often arcane.
88. 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 mail message
"send index from f2c" to netlib at research.att.com or
research!netlib. (It is also available via anonymous ftp on
research.att.com, in directory dist/f2c.)
A PL/M to C converter was posted to alt.sources in April, 1991.
The following companies sell various translation tools and
services:
Cobalt Blue
2940 Union Ave., Suite C
San Jose, CA 95124 USA
(+1) 408 723 0474
Promula Development Corp.
3620 N. High St., Suite 301
Columbus, OH 43214 USA
(+1) 614 263 5454
Micro-Processor Services Inc
92 Stone Hurst Lane
Dix Hills, NY 11746 USA
(+1) 519 499 4461
See also question 29.
89. 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.
The usual approach is to use anonymous ftp and/or uucp from a
central, public-spirited site, such as uunet.uu.net (192.48.96.2).
However, this article cannot track or list all of the available
archive sites and how to access them. The comp.archives newsgroup
contains numerous announcements of anonymous ftp availability of
various items. The "archie" mailserver can tell you which
anonymous ftp sites have which packages; send the mail message
"help" to archie at quiche.cs.mcgill.ca for information.
90. When will the next International Obfuscated C Contest (IOCCC) be
held? How can I get a copy of the current and previous winning
entries?
A: The contest typically runs from early March through mid-May. To
obtain a current copy of the rules, send email to:
{pacbell,uunet,utzoo}!hoptoad!judges or judges at toad.com
Contest winners are first announced at the Summer Usenix Conference
in mid-June, and posted to the net in July. Previous winners are
available on uunet (see question 89) under the directory
~/pub/ioccc.
91. 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 (but see question 32).
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.
Reference: ANSI Rationale Sec. 3.1.9 p. 33.
92. How can I make this code more efficient?
A: Efficiency, though a favorite comp.lang.c topic, is not important
nearly as often as people tend to think it is. Most of the code in
most programs is not time-critical. When code is not time-
critical, it is far more important that it be written clearly and
portably than that it be written maximally efficiently. (Remember
that computers are very, very fast, and that even "inefficient"
code can run without apparent delay.)
It is notoriously difficult to predict what the "hot spots" in a
program will be. When efficiency is a concern, it is important to
use profiling software to determine which parts of the program
deserve attention. Often, actual computation time is swamped by
peripheral tasks such as I/O and memory allocation, which can be
sped up by using buffering and cacheing techniques.
For the small fraction of code that is time-critical, it is vital
to pick a good algorithm; it is less important to "microoptimize"
the coding details. Many of the "efficient coding tricks" which
are frequently suggested (e.g. substituting shift operators for
multiplication by powers of two) are performed automatically by
even simpleminded compilers. Heavyhanded "optimization" attempts
can make code so bulky that performance is degraded.
For more discussion of efficiency tradeoffs, as well as good advice
on how to increase efficiency when it is important, see chapter 7
of Kernighan and Plaugher's The Elements of Programming Style, and
Jon Bentley's Writing Efficient Programs.
93. Are pointers really faster than arrays? Do function calls really
slow things down? Is ++i faster than i = i + 1?
A: Precise answers to these and many similar questions depend of
course on the processor and compiler in use. If you simply must
know, you'll have to time test programs carefully. (Often the
differences are so slight that hundreds of thousands of iterations
are required even to see them. Check the compiler's assembly
language output, if available, to see if two purported alternatives
aren't compiled identically.)
It is "usually" faster to march through large arrays with pointers
rather than array subscripts, but for some processors the reverse
is true.
Function calls, though obviously incrementally slower than in-line
code, contribute so much to modularity and code clarity that there
is rarely good reason to avoid them.
Before rearranging expressions such as i = i + 1, remember that you
are dealing with a C compiler, not a keystroke-programmable
calculator. A good compiler will generate identical code for ++i,
i += 1, and i = i + 1. The reasons for using ++i or i += 1 over
i = i + 1 have to do with style, not efficiency.
94. My floating-point calculations are acting strangely and giving me
different answers on different machines.
A: Most digital computers use floating-point formats which provide a
close but by no means exact simulation of real number arithmetic.
Among other things, the associative and distributive laws do not
hold completely (i.e. order of operation may be important, repeated
addition is not necessarily equivalent to multiplication).
Underflow or cumulative precision loss is often a problem.
Don't assume that floating-point results will be exact, and
especially don't assume that floating-point values can be compared
for equality. (Don't throw haphazard "fuzz factors" in, either.)
These problems are no worse for C than they are for any other
computer language. Floating-point semantics are usually defined as
"however the processor does them;" otherwise a compiler for a
machine without the "right" model would have to do prohibitively
expensive emulations.
This article cannot begin to list the pitfalls associated with, and
workarounds appropriate for, floating-point work. A good
programming text should cover the basics. Do make sure that you
have #included <math.h>, and correctly declared other functions
returning double.
References: K&P Sec. 6 pp. 115-8.
95. 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 PDP-11 compiler), 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. It happens that Turbo C's
heuristics for determining whether the program uses floating point
are occasionally insufficient, and the programmer must sometimes
insert a dummy explicit floating-point call to force loading of
floating-point support.
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 (e.g.
comp.os.msdos.programmer).
96. This program crashes before it even runs! (When single-stepping
with a debugger, it dies before the first statement in main.)
A: You probably have one or more very large (kilobyte or more) local
arrays. Many systems have fixed-size stacks, and those which
perform dynamic stack allocation automatically (e.g. Unix) can be
confused when the stack tries to grow by a huge chunk all at once.
It is often better to declare large arrays with static duration
(unless of course you need a fresh set with each recursive call).
(See also question 56.)
97. 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.
98. Where can I get a YACC grammar for C?
A: The definitive grammar is of course the one in the ANSI standard.
Several copies 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 .
The FSF's GNU C compiler contains a grammar, as does the appendix
to K&R II.
References: ANSI Sec. A.2 .
99. How do you pronounce "char"? What's that funny name for the "#"
character?
A: You can pronounce the C keyword "char" like the English words
"char," "care," or "car;" the choice is arbitrary. Bell Labs once
proposed the (now obsolete) term "octothorpe" for the "#"
character.
Trivia questions like these aren't any more pertinent for
comp.lang.c than they are for any of the other groups they
frequently come up in. You can find lots of information in the
net.announce.newusers frequently-asked questions postings, the
"jargon file" (also published as _The Hacker's Dictionary_), and
the Usenet ASCII pronunciation list.
100. Where can I get extra copies of this list? What about back issues?
A: For now, just pull it off the net; it is normally posted to
comp.lang.c on the first of each month, with an Expiration: line
which should keep it around all month. Eventually, it may be
available for anonymous ftp, or via a mailserver.
This list is an evolving document, not just a collection of this
month's interesting questions. Older copies are obsolete and don't
contain much, except the occasional typo, that the current list
doesn't.
Bibliography
ANSI American National Standard for Information Systems --
Programming Language -- C, ANSI X3.159-1989 (see question 28).
Jon Louis Bentley, Writing Efficient Programs, Prentice-Hall,
1982, ISBN 0-13-970244-X.
H&S Samuel P. Harbison and Guy L. Steele, C: A Reference Manual,
Second Edition, Prentice-Hall, 1987, ISBN 0-13-109802-0. (A
third edition has recently been released.)
PCS Mark R. Horton, Portable C Software, Prentice Hall, 1990, ISBN
0-13-868050-7.
K&P Brian W. Kernighan and P.J. Plaugher, The Elements of
Programming Style, Second Edition, McGraw-Hill, 1978, ISBN 0-
07-034207-5.
K&R I Brian W. Kernighan and Dennis M. Ritchie, The C Programming
Language, Prentice Hall, 1978, ISBN 0-13-110163-3.
K&R II Brian W. Kernighan and Dennis M. Ritchie, The C Programming
Language, Second Edition, Prentice Hall, 1988, ISBN 0-13-
110362-8, 0-13-110370-9.
CT&P Andrew Koenig, C Traps and Pitfalls, Addison-Wesley, 1989, ISBN
0-201-17928-8.
There is a more extensive bibliography in the revised Indian Hill style
guide (see question 81).
Acknowledgements
Thanks to Sudheer Apte, Dan Bernstein, Joe Buehler, Raymond Chen,
Christopher Calabrese, James Davies, Norm Diamond, Ray Dunn, Stephen M.
Dunn, Bjorn Engsig, Ron Guilmette, Doug Gwyn, Tony Hansen, Joe
Harrington, Guy Harris, Blair Houghton, Kirk Johnson, Andrew Koenig,
John Lauro, Christopher Lott, Tim McDaniel, Evan Manning, Mark Moraes,
Francois Pinard, randall at virginia, Pat Rankin, Rich Salz, Chip
Salzenberg, Paul Sand, Doug Schmidt, Patricia Shanahan, Peter da Silva,
Joshua Simons, Henry Spencer, Erik Talvola, Clarke Thatcher, Chris
Torek, Ed Vielmetti, Larry Virden, Freek Wiedijk, and Dave Wolverton,
who have contributed, directly or indirectly, to this article. Special
thanks to Karl Heuer, and particularly to Mark Brader, who (to borrow a
line from Steve Johnson) have goaded me beyond my inclination, and
frequently beyond my endurance, in relentless pursuit of a better FAQ
list.
Steve Summit
scs at adam.mit.edu
scs%adam.mit.edu at mit.edu
mit-eddie!adam!scs
This article is Copyright 1988, 1990, 1991 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.
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