Archive-name: C-faq/faq
Comp-lang-c-archive-name: C-FAQ-list
[Last modified March 1, 1994 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 tutorial or 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 C books and 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
(but beware of ANSI vs. ISO C Standard section numbers; see question
5.1).
If you have a question about C which is not answered in this article,
first 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@eskimo.com .
The questions answered here are divided into several categories:
1. Null Pointers
2. Arrays and Pointers
3. Memory Allocation
4. Expressions
5. ANSI C
6. C Preprocessor
7. Variable-Length Argument Lists
8. Boolean Expressions and Variables
9. Structs, Enums, and Unions
10. Declarations
11. Stdio
12. Library Subroutines
13. Lint
14. Style
15. Floating Point
16. System Dependencies
17. Miscellaneous (Fortran to C converters, YACC grammars, etc.)
Herewith, some frequently-asked questions and their answers:
Section 1. Null Pointers
1.1: 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 or function. 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. See also questions
3.1, 3.13, and 17.1.
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.
1.2: 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, to force
the 0 to be in a pointer context:
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 convert
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. (Incidentally, it's also a simpler
rule to remember.)
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 .
1.3: 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 1.2 above applies for NULL as well as 0.)
NULL should _only_ be used for pointers; see question 1.8.
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.
1.4: 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.
1.5: 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; see question 1.8).
References: ANSI Rationale Sec. 4.1.5 p. 74.
1.6: 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
1.2. 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. See also question 8.1.
1.7: 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 8.2.
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 .
1.8: 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.
References: K&R II Sec. 5.4 p. 102.
1.9: 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.
1.10: 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 1.1. 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 red herrings, we have...
5. The ASCII null character (NUL), which does have all bits
zero, but has no necessary relation to the null pointer
except in name; and...
6. The "null string," which is another name for an empty
string (""). The term "null string" can be confusing in
C (and should perhaps be avoided), because it involves a
null ('\0') character, but not a null pointer, which
brings us full circle...
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.
1.11: 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. 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. 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, 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.
1.12: 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), or with ANSI C
refinements. Understand questions 1.1, 1.2, and 1.3, and
consider 1.8 and 1.11, and you'll do fine.
1.13: 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 3.13). 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
1.11, the urge to assume an internal zero representation would
not even arise.)
1.14: Seriously, have any actual machines really used nonzero null
pointers, or different representations for pointers to different
types?
A: The Prime 50 series 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. Older,
word-addressed Prime machines were also notorious for requiring
larger byte pointers (char *'s) than word pointers (int *'s).
The Eclipse MV series from Data General has three
architecturally supported pointer formats (word, byte, and bit
pointers), two of which are used by C compilers: byte pointers
for char * and void *, and word pointers for everything else.
Some Honeywell-Bull mainframes use the bit pattern 06000 for
(internal) null pointers.
The CDC Cyber 180 Series has 48-bit pointers consisting of a
ring, segment, and offset. Most users (in ring 11) have null
pointers of 0xB00000000000.
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.
Depending on the "memory model" in use, 80*86 processors (PC's)
may use 16 bit data pointers and 32 bit function pointers, or
vice versa.
The old HP 3000 series computers use a different addressing
scheme for byte addresses than for word addresses; void and char
pointers therefore have a different representation than an int
(structure, etc.) pointer to the same address would have.
1.15: What does a run-time "null pointer assignment" error mean? How
do I track it down?
A: This message, which occurs only under MS-DOS (see, therefore,
section 16) means that you've written, via a null pointer, to
location zero.
A debugger will usually let you set a data breakpoint on
location 0. Alternately, you could write a bit of code to copy
20 or so bytes from location 0 into another buffer, and
periodically check that it hasn't changed.
Section 2. Arrays and Pointers
2.1: I had the definition char a[6] 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[].
References: CT&P Sec. 3.3 pp. 33-4, Sec. 4.5 pp. 64-5.
2.2: But I heard that char a[] was identical to char *a.
A: Not at all. (What you heard has to do with formal parameters to
functions; see question 2.4.) Arrays are not pointers. The
array 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 pointer 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 |
+-----+ +---+---+---+---+---+---+
It is important to realize that a reference like x[3] generates
different code depending on whether x is an array or a pointer.
Given the declarations above, 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 questions 17.19 and
17.20.)
2.3: 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" neither means that they are identical
nor even interchangeable.
"Equivalence" refers to the following key definition:
An lvalue [see question 2.5] 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 a sizeof or
& operator, or is a literal string initializer for a character
array.)
As a consequence of this definition, there is no apparent
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 just as would
be a pointer variable in the expression p[i] (although the
eventual memory accesses will be different, as explained in
question 2.2). In either case, the expression x[i] (where x is
an array or a pointer) is, by definition, identical 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. 3.2.2.1, Sec. 3.3.2.1, Sec. 3.3.6 .
2.4: 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. As a convenience, 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.
2.5: How can an array be an lvalue, if you can't assign to it?
A: The ANSI C Standard defines a "modifiable lvalue," which an
array is not.
References: ANSI Sec. 3.2.2.1 p. 37.
2.6: Why doesn't sizeof properly report the size of an array which is
a parameter to a function?
A: The sizeof operator reports the size of the pointer parameter
which the function actually receives (see question 2.4).
2.7: Someone explained to me that arrays were really just constant
pointers.
A: This is a bit of an oversimplification. 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 2.2
should make clear.
2.8: Practically speaking, what is the difference between arrays and
pointers?
A: Arrays automatically allocate space, but can't be relocated or
resized. Pointers must be explicitly assigned to point to
allocated space (perhaps using malloc), but can be reassigned
(i.e. pointed at different objects) at will, and have many other
uses besides serving as the base of blocks of memory.
Due to the so-called equivalence of arrays and pointers (see
question 2.3), arrays and pointers often seem interchangeable,
and in particular a pointer to a block of memory assigned by
malloc is frequently treated (and can be referenced using []
exactly) as if it were a true array. (See question 2.14; see
also question 17.20.)
2.9: 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 identical 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
17.13).
References: ANSI Rationale Sec. 3.3.2.1 p. 41.
2.10: 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[NROWS][NCOLUMNS];
f(array);
the function's declaration should match:
f(int a[][NCOLUMNS]) {...}
or
f(int (*ap)[NCOLUMNS]) {...} /* 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," NROWS, can be omitted. The "shape" of the array is
still important, so the "column" dimension NCOLUMNS (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.
References: K&R I Sec. 5.10 p. 110; K&R II Sec. 5.9 p. 113.
2.11: How do I write functions which accept 2-dimensional arrays when
the "width" is not known at compile time?
A: It's not easy. One way is to pass in a pointer to the [0][0]
element, along with the two dimensions, and simulate array
subscripting "by hand:"
f2(aryp, nrows, ncolumns)
int *aryp;
int nrows, ncolumns;
{ ... ary[i][j] is really aryp[i * ncolumns + j] ... }
This function could be called with the array from question 2.10
as
f2(&array[0][0], NROWS, NCOLUMNS);
It must be noted, however, that a program which performs
multidimensional array subscripting "by hand" in this way is not
in strict conformance with the ANSI C Standard; the behavior of
accessing (&array[0][0])[x] is not defined for x > NCOLUMNS.
gcc allows local arrays to be declared having sizes which are
specified by a function's arguments, but this is a nonstandard
extension.
See also question 2.15.
2.12: How do I declare a pointer to an array?
A: Usually, you don't want to. When people speak casually of a
pointer to an array, they usually mean a pointer to its first
element.
Instead of a pointer to an array, consider using a pointer to
one of the array's elements. Arrays of type T decay into
pointers to type T (see question 2.3), 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 arrays of arrays, if at
all. (See question 2.10 above.)
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 10.4.) If the size of the array is unknown,
N can be omitted, but the resulting type, "pointer to array of
unknown size," is useless.
2.13: Since array references decay to pointers, given
int array[NROWS][NCOLUMNS];
what's the difference between array and &array?
A: Under ANSI/ISO Standard C, &array yields a pointer, of type
pointer-to-array-of-T, to the entire array (see also question
2.12). Under pre-ANSI C, the & in &array generally elicited a
warning, and was generally ignored. Under all C compilers, an
unadorned reference to an array yields a pointer, of type
pointer-to-T, to the array's first element. (See also question
2.3.)
2.14: 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." Here
is a two-dimensional example:
int **array1 = (int **)malloc(nrows * sizeof(int *));
for(i = 0; i < nrows; i++)
array1[i] = (int *)malloc(ncolumns * sizeof(int));
(In "real" code, of course, malloc would 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 **array2 = (int **)malloc(nrows * sizeof(int *));
array2[0] = (int *)malloc(nrows * ncolumns * sizeof(int));
for(i = 1; i < nrows; i++)
array2[i] = array2[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 *array3 = (int *)malloc(nrows * ncolumns * sizeof(int));
However, you must now perform subscript calculations manually,
accessing the i,jth element with array3[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.)
Finally, you can use pointers-to-arrays:
int (*array4)[NCOLUMNS] =
(int (*)[NCOLUMNS])malloc(nrows * sizeof(*array4));
, but the syntax gets horrific and all but one dimension must be
known at compile time.
With all of these techniques, you may of course need to remember
to free the arrays (which may take several steps; see question
3.9) when they are no longer needed, and you cannot necessarily
intermix the dynamically-allocated arrays with conventional,
statically-allocated ones (see question 2.15 below, and also
question 2.10).
2.15: How can I use statically- and dynamically-allocated
multidimensional arrays interchangeably when passing them to
functions?
A: There is no single perfect method. Given a function f1()
similar to the f() of question 2.10, the array as declared in
question 2.10, f2() as declared in question 2.11, array1,
array2, array3, and array4 as declared in question 2.14, and a
function f3() declared as:
f3(pp, m, n)
int **pp;
int m, n;
; the following calls should work as expected:
f1(array, NROWS, NCOLUMNS);
f1(array4, nrows, NCOLUMNS);
f2(&array[0][0], NROWS, NCOLUMNS);
f2(*array2, nrows, ncolumns);
f2(array3, nrows, ncolumns);
f2(*array4, nrows, NCOLUMNS);
f3(array1, nrows, ncolumns);
f3(array2, nrows, ncolumns);
The following two calls would probably work, but involve
questionable casts, and work only if the dynamic ncolumns
matches the static NCOLUMNS:
f1((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);
f1((int (*)[NCOLUMNS])array3, nrows, ncolumns);
It must again be noted that passing &array[0][0] to f2() is not
strictly conforming; see question 2.11.
If you can understand why all of the above calls work and are
written as they are, and if you understand why the combinations
that are not listed would not work, then you have a _very_ good
understanding of arrays and pointers (and several other areas)
in C.
2.16: Here's a neat trick: if I write
int realarray[10];
int *array = &realarray[-1];
I can treat "array" as if it were a 1-based array.
A: Although this technique is attractive (and was used in old
editions of the book Numerical Recipes in C), it does not
conform to the C standards. Pointer arithmetic is defined only
as long as the pointer points within the same allocated block of
memory, or to the imaginary "terminating" element one past it;
otherwise, the behavior is undefined, _even if the pointer is
not dereferenced_. The code above could fail if, while
subtracting the offset, an illegal address were generated
(perhaps because the address tried to "wrap around" past the
beginning of some memory segment).
References: ANSI Sec. 3.3.6 p. 48, Rationale Sec. 3.2.2.3 p. 38;
K&R II Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3, Sec. A7.7 pp. 205-6.
2.17: I passed a pointer to a function which initialized it:
...
int *ip;
f(ip);
...
void f(ip)
int *ip;
{
static int dummy = 5;
ip = &dummy;
}
, but the pointer in the caller was unchanged.
A: Did the function try to initialize the pointer itself, or just
what it pointed to? Remember that arguments in C are passed by
value. The called function altered only the passed copy of the
pointer. You'll want to pass the address of the pointer (the
function will end up accepting a pointer-to-a-pointer).
2.18: I have a char * pointer that happens to point to some ints, and
I want to step it over them. Why doesn't
((int *)p)++;
work?
A: In C, a cast operator does not mean "pretend these bits have a
different type, and treat them accordingly;" it is a conversion
operator, and by definition it yields an rvalue, which cannot be
assigned to, or incremented with ++. (It is an anomaly in pcc-
derived compilers, and an extension in gcc, that expressions
such as the above are ever accepted.) Say what you mean: use
p = (char *)((int *)p + 1);
, or simply
p += sizeof(int);
References: ANSI Sec. 3.3.4, Rationale Sec. 3.3.2.4 p. 43.
2.19: Can I use a void ** pointer to pass a generic pointer to a
function by reference?
A: Not portably. There is no generic pointer-to-pointer type in C.
void * acts as a generic pointer only because conversions are
applied automatically when other pointer types are assigned to
and from void *'s; these conversions cannot be performed (the
correct underlying pointer type is not known) if an attempt is
made to indirect upon a void ** value which points at something
other than a void *.
Section 3. Memory Allocation
3.1: 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 17.1.)
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, sizeof(answer), 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; see question 11.5), allowing the size of the array to
be specified, so that the end of the array will not be
overwritten 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.
3.2: 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.
References: CT&P Sec. 3.2 p. 32.
3.3: 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 2.3 and 2.4.) Another common example is stat().
3.4: I have a function that is supposed to return a string, but when
it returns to its caller, the returned string is garbage.
A: Make sure that the memory to which the function returns a
pointer is correctly allocated. The returned pointer should be
to a statically-allocated buffer, or to a buffer passed in by
the caller, but _not_ to a local (auto) array. In other words,
never do something like
char *f()
{
char buf[10];
/* ... */
return buf;
}
One fix would to to declare the buffer as
static char buf[10];
See also question 17.5.
3.5: Why does some code carefully cast the values returned by malloc
to the pointer type being allocated?
A: Before ANSI/ISO Standard C introduced the void * generic pointer
type, these casts were typically required to silence warnings
about assignment between incompatible pointer types.
3.6: You can't use dynamically-allocated memory after you free it,
can you?
A: No. Some early documentation 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.
3.7: 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.
3.8: So can I query the malloc package to find out how big an
allocated block is?
A: Not portably.
3.9: I'm allocating structures which contain pointers to other
dynamically-allocated objects. When I free a structure, do I
have to free each subsidiary pointer first?
A: Yes. In general, you must arrange that each pointer returned
from malloc be individually passed to free, exactly once (if it
is freed at all).
3.10: I have a program which mallocs but then frees a lot of memory,
but memory usage (as reported by ps) doesn't seem to go back
down.
A: Most implementations of malloc/free do not return freed memory
to the operating system (if there is one), but merely make it
available for future malloc calls.
3.11: Must I free allocated memory before the program exits?
A: You shouldn't have to. A real operating system definitively
reclaims all memory when a program exits. Nevertheless, some
personal computers are said not to reliably recover memory, and
all that can be inferred from the ANSI/ISO C Standard is that it
is a "quality of implementation issue."
References: ANSI Sec. 4.10.3.2 .
3.12: 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 .
3.13: 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 section 1 of this list) 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 .
3.14: 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.
References: ANSI Rationale Sec. 4.10.3 p. 102.
Section 4. Expressions
4.1: Why doesn't this code:
a[i] = i++;
work?
A: The subexpression i++ causes a side effect -- it modifies i's
value -- which leads to undefined behavior if i is also
referenced elsewhere in the same expression. (Note that
although the language in K&R suggests that the behavior of this
expression is unspecified, the ANSI/ISO C Standard makes the
stronger statement that it is undefined -- see question 5.23.)
References: ANSI Sec. 3.3 p. 39.
4.2: 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, the
implication of "after" is often misunderstood. 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 multiple, ambiguous side
effects has always been undefined (see question 5.23). 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.)
4.3: I've experimented with the code
int i = 2;
i = i++;
on several compilers. Some gave i the value 2, some gave 3, but
one gave 4. I know the behavior is undefined, but how could it
give 4?
A: Undefined behavior means _anything_ can happen. See question
5.23.
4.4: People keep saying the behavior is undefined, but I just tried
it on an ANSI-conforming compiler, and got the results I
expected.
A: A compiler may do anything it likes when faced with undefined
behavior (and, within limits, with implementation-defined and
unspecified behavior), including doing what you expect. It's
unwise to depend on it, though. See also question 5.18.
4.5: Can I use explicit parentheses to force the order of evaluation
I want? Even if I don't, doesn't precedence dictate it?
A: Operator precedence and explicit parentheses impose only a
partial ordering on the evaluation of an expression. Consider
the expression
f() + g() * h()
-- although we know that the multiplication will happen before
the addition, there is no telling which of the three functions
will be called first.
4.6: 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.
4.7: If I'm not using the value of the expression, should I use i++
or ++i to increment a variable?
A: Since the two forms differ only in the value yielded, they are
entirely equivalent when only their side effect is needed.
4.8: Why doesn't the code
int a = 1000, b = 1000;
long int c = a * b;
work?
A: Under C's integral promotion rules, the multiplication is
carried out using int arithmetic, and the result may overflow
and/or be truncated before being assigned to the long int left-
hand-side. Use an explicit cast to force long arithmetic:
long int c = (long int)a * b;
Section 5. ANSI C
5.1: What is the "ANSI C Standard?"
A: In 1983, the American National Standards Institute (ANSI)
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 ANS
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 sections are numbered
differently (briefly, ANSI sections 2 through 4 correspond
roughly to ISO sections 5 through 7), and the Rationale is
currently not included.
5.2: How can I get a copy of the Standard?
A: ANSI X3.159 has been officially superseded by ISO 9899. Copies
are available in the United States from
American National Standards Institute
11 W. 42nd St., 13th floor
New York, NY 10036 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)
In other countries, contact the appropriate national standards
body, or ISO in Geneva at:
ISO Sales
Case Postale 56
CH-1211 Geneve 20
Switzerland
The cost is $130.00 from ANSI or $162.50 from Global. Copies of
the original X3.159 (including the Rationale) are still
available at $205.00 from ANSI or $200.50 from Global. Note
that ANSI derives revenues to support its operations from the
sale of printed standards, so electronic copies are _not_
available.
The mistitled _Annotated ANSI C Standard_, with annotations by
Herbert Schildt, contains the full text of ISO 9899; it is
published by Osborne/McGraw-Hill, ISBN 0-07-881952-0, and sells
in the U.S. for approximately $40. (It has been suggested that
the price differential between this work and the official
standard reflects the value of the annotations.)
The text of the Rationale (not the full Standard) is now
available for anonymous ftp from ftp.uu.net (see question 17.12)
in directory doc/standards/ansi/X3.159-1989 . The Rationale has
also been printed by Silicon Press, ISBN 0-929306-07-4.
5.3: Does anyone have a tool for converting old-style C programs to
ANSI C, or vice versa, 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
were once patches to the FSF GNU C compiler, gcc, but are now
part of the main gcc distribution; look in pub/gnu at
prep.ai.mit.edu (18.71.0.38), or at several other FSF archive
sites.
The unproto program (/pub/unix/unproto5.shar.Z on
ftp.win.tue.nl) is a filter which sits between the preprocessor
and the next compiler pass, converting most of ANSI C to
traditional C on-the-fly.
The GNU GhostScript package comes with a little program called
ansi2knr.
Several prototype generators exist, many as modifications to
lint. Version 3 of CPROTO was posted to comp.sources.misc in
March, 1992. There is another program called "cextract." See
also question 17.12.
Finally, are you sure you really need to convert lots of old
code to ANSI C? The old-style function syntax is still
acceptable.
5.4: I'm trying to use the ANSI "stringizing" preprocessing operator
# to insert the value of a symbolic constant into a message, but
it keeps stringizing the macro's name rather than its value.
A: You must use something like the following two-step procedure to
force the macro to be expanded as well as stringized:
#define str(x) #x
#define xstr(x) str(x)
#define OP plus
char *opname = xstr(OP);
This sets opname to "plus" rather than "OP".
An equivalent circumlocution is necessary with the token-pasting
operator ## when the values (rather than the names) of two
macros are to be concatenated.
References: ANSI Sec. 3.8.3.2, Sec. 3.8.3.5 example p. 93.
5.5: I don't understand why I can't use const values in initializers
and array dimensions, as in
const int n = 5;
int a[n];
A: The const qualifier really means "read-only;" an object so
qualified is a normal run-time object which cannot (normally) be
assigned to. The value of a const-qualified object is therefore
_not_ a constant expression in the full sense of the term. (C
is unlike C++ in this regard.) When you need a true compile-
time constant, use a preprocessor #define.
References: ANSI Sec. 3.4 .
5.6: 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
10.4.)
References: ANSI Sec. 3.5.4.1 .
5.7: Why can't I pass a char ** to a function which expects a
const char **?
A: You can use a pointer-to-T (for any type T) where a pointer-to-
const-T is expected, but the rule (an explicit exception) which
permits slight mismatches in qualified pointer types is not
applied recursively, but only at the top level.
You must use explicit casts (e.g. (const char **) in this case)
when assigning (or passing) pointers which have qualifier
mismatches at other than the first level of indirection.
References: ANSI Sec. 3.1.2.6 p. 26, Sec. 3.3.16.1 p. 54,
Sec. 3.5.3 p. 65.
5.8: 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;". It is usually safe to mix the two
styles (see question 5.9), but not in this case. Old C (and
ANSI C, in the absence of prototypes, and in variable-length
argument lists) "widens" certain arguments when they are passed
to functions. floats are promoted to double, and characters and
short integers are promoted to ints. (For old-style function
definitions, the values are automatically converted back to the
corresponding narrower types within the body of the called
function, if they are declared that way there.)
This 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.)
It may also be safer to avoid "narrow" (char, short int, and
float) function arguments and return types.
References: ANSI Sec. 3.3.2.2 .
5.9: Can you mix old-style and new-style function syntax?
A: Doing so is perfectly legal, as long as you're careful (see
especially question 5.8). Note however that old-style syntax is
marked as obsolescent, and support for it may be removed some
day.
References: ANSI Secs. 3.7.1, 3.9.5 .
5.10: Why does the declaration
extern f(struct x {int s;} *p);
give me an obscure warning message about "struct x introduced in
prototype scope"?
A: In a quirk of C's normal block scoping rules, a struct declared
only within a prototype cannot be compatible with other structs
declared in the same source file, nor can the struct tag be used
later as you'd expect (it goes out of scope at the end of the
prototype).
To resolve the problem, precede the prototype with the vacuous-
looking declaration
struct x;
, which will reserve a place at file scope for struct x's
definition, which will be completed by the struct declaration
within the prototype.
References: ANSI Sec. 3.1.2.1 p. 21, Sec. 3.1.2.6 p. 26,
Sec. 3.5.2.3 p. 63.
5.11: 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 17.14, and
6.7.)
References: ANSI Sec. 2.1.1.2 p. 6, Sec. 3.1 p. 19 line 37.
5.12: Can I declare main as void, to shut off these annoying "main
returns no value" messages? (I'm calling exit(), so main
doesn't return.)
A: No. main must be declared as returning an int, and as taking
either zero or two arguments (of the appropriate type). If
you're calling exit() but still getting warnings, you'll have to
insert a redundant return statement (or use some kind of
"notreached" directive, if available).
Declaring a function as void does not merely silence warnings;
it may also result in a different function call/return sequence,
incompatible with what the caller (in main's case, the C run-
time startup code) expects.
References: ANSI Sec. 2.1.2.2.1 pp. 7-8.
5.13: Is exit(status) truly equivalent to returning status from main?
A: Essentially, except under a few older, nonconforming systems,
and unless data local to main might be needed during cleanup
(due perhaps to a setbuf or atexit call).
References: ANSI Sec. 2.1.2.2.3 p. 8.
5.14: 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 5.1), 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.
5.15: What is the difference between memcpy and memmove?
A: memmove offers guaranteed behavior if the source and destination
arguments overlap. memcpy makes no such guarantee, and may
therefore be more efficiently implementable. When in doubt,
it's safer to use memmove.
References: ANSI Secs. 4.11.2.1, 4.11.2.2, Rationale
Sec. 4.11.2 .
5.16: My compiler is rejecting the simplest possible test programs,
with all kinds of syntax errors.
A: Perhaps it is a pre-ANSI compiler, unable to accept function
prototypes and the like. See also questions 5.17 and 17.2.
5.17: Why are some ANSI/ISO Standard library routines showing up as
undefined, even though I've got an ANSI compiler?
A: It's not unusual to have a compiler available which accepts ANSI
syntax, but not to have ANSI-compatible header files or run-time
libraries installed. See also questions 5.16 and 17.2.
5.18: Why won't the Frobozz Magic C Compiler, which claims to be ANSI
compliant, accept this code? I know that the code is ANSI,
because gcc accepts it.
A: Most compilers support a few non-Standard extensions, gcc more
so than most. Are you sure that the code being rejected doesn't
rely on such an extension? It is usually a bad idea to perform
experiments with a particular compiler to determine properties
of a language; the applicable standard may permit variations, or
the compiler may be wrong. See also question 4.4.
5.19: Why can't I perform arithmetic on a void * pointer?
A: The compiler doesn't know the size of the pointed-to objects.
Before performing arithmetic, cast the pointer either to char *
or to the type you're trying to manipulate.
5.20: Is char a[3] = "abc"; legal? What does it mean?
A: It is legal in ANSI C (and perhaps in a few pre-ANSI systems),
though questionably useful. It declares an array of size three,
initialized with the three characters 'a', 'b', and 'c', without
the usual terminating '\0' character; the array is therefore not
a true C string and cannot be used with strcpy, printf %s, etc.
References: ANSI Sec. 3.5.7 pp. 72-3.
5.21: 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 .
5.22: What does #pragma once mean? I found it in some header files.
A: It is an extension implemented by some preprocessors to help
make header files idempotent; it is essentially equivalent to
the #ifndef trick mentioned in question 6.4.
5.23: People seem to make a point of distinguishing between
implementation-defined, unspecified, and undefined behavior.
What's the difference?
A: Briefly: implementation-defined means that an implementation
must choose some behavior and document it. Unspecified means
that an implementation should choose some behavior, but need not
document it. Undefined means that absolutely anything might
happen. In no case does the Standard impose requirements; in
the first two cases it occasionally suggests (and may require a
choice from among) a small set of likely behaviors.
If you're interested in writing portable code, you can ignore
the distinctions, as you'll want to avoid code that depends on
any of the three behaviors.
References: ANSI Sec. 1.6, especially the Rationale.
Section 6. C Preprocessor
6.1: 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, or if the two values are the same variable (and the
"obvious" supercompressed implementation for integral types
a^=b^=a^=b is in fact illegal due to multiple side-effects; see
questions 4.1 and 4.2). 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're willing to pass in the type as a third
argument.
6.2: 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
(See also question 5.4.)
References: ANSI Sec. 3.8.3.3 p. 91, Rationale pp. 66-7.
6.3: 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. (See the example under question 6.10 below.
This technique also allows a value to be "returned.")
References: CT&P Sec. 6.3 pp. 82-3.
6.4: Is it acceptable for one header file to #include another?
A: It's a question of style, and thus receives considerable debate.
Many people believe that "nested #include files" are to be
avoided: the prestigious Indian Hill Style Guide (see question
14.3) disparages them; they can make it harder to find relevant
definitions; they can lead to multiple-declaration errors if a
file is #included twice; and they make manual Makefile
maintenance very difficult. On the other hand, they make it
possible to use header files in a modular way (a header file
#includes what it needs itself, rather than requiring each
#includer to do so, a requirement that can lead to intractable
headaches); a tool like grep (or a tags file) makes it easy to
find definitions no matter where they are; a popular trick:
#ifndef HEADERUSED
#define HEADERUSED
...header file contents...
#endif
makes a header file "idempotent" so that it can safely be
#included multiple times; and automated Makefile maintenance
tools (which are a virtual necessity in large projects anyway)
handle dependency generation in the face of nested #include
files easily. See also section 14.
6.5: Does the sizeof operator work in preprocessor #if directives?
A: No. Preprocessing happens during an earlier pass of
compilation, before type names have been parsed. Consider using
the predefined constants in ANSI's <limits.h>, if applicable, or
a "configure" script, instead. (Better yet, try to write code
which is inherently insensitive to type sizes.)
References: ANSI Sec. 2.1.1.2 pp. 6-7, Sec. 3.8.1 p. 87
footnote 83.
6.6: How can I use a preprocessor #if expression to tell if a machine
is big-endian or little-endian?
A: You probably can't. (Preprocessor arithmetic uses only long
ints, and there is no concept of addressing.) Are you sure you
need to know the machine's endianness explicitly? Usually it's
better to write code which doesn't care.
6.7: I've got this tricky processing I want to do at compile time and
I can't figure out a way to get cpp to do it.
A: cpp is not intended as a general-purpose preprocessor. Rather
than forcing it to do something inappropriate, consider writing
your own little special-purpose preprocessing tool, instead.
You can easily get a utility like make(1) to run it for you
automatically.
If you are trying to preprocess something other than C, consider
using a general-purpose preprocessor (such as m4).
6.8: I inherited some code which contains far too many #ifdef's for
my taste. How can I preprocess the code to leave only one
conditional compilation set, without running it through cpp and
expanding all of the #include's and #define's as well?
A: There are programs floating around called unifdef, rmifdef, and
scpp which do exactly this. (See question 17.12.)
6.9: How can I list all of the pre#defined identifiers?
A: There's no standard way, although it is a frequent need. The
most expedient way is probably to extract printable strings from
the compiler or preprocessor executable with something like the
Unix strings(1) utility.
6.10: 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. Other solutions are to use
different macros (DEBUG1, DEBUG2, etc.) depending on the number
of arguments, or to play games with commas:
#define DEBUG(args) (printf("DEBUG: "), printf(args))
#define _ ,
DEBUG("i = %d" _ i)
It is often better to use a bona-fide function, which can take a
variable number of arguments in a well-defined way. See
questions 7.1 and 7.2.
Section 7. Variable-Length Argument Lists
7.1: 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 <stdlib.h> /* for malloc, NULL, size_t */
#include <stdarg.h> /* for va_ stuff */
#include <string.h> /* for strcat et al */
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 <stdlib.h>, add "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.
Remember that in variable-length argument lists, function
prototypes do not supply parameter type information; therefore,
default argument promotions apply (see question 5.8), and null
pointer arguments must be typed explicitly (see question 1.2).
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 .
7.2: 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 .
7.3: How can I discover how many arguments a function was actually
called with?
A: This information is not available to a portable program. Some
old systems provided a nonstandard nargs() function, but its use
was always questionable, since it typically returned the number
of words passed, not the number of arguments. (Structures and
floating point values 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 1.2 and 7.1 above).
7.4: I can't get the va_arg macro to pull in an argument of type
pointer-to-function.
A: The type-rewriting games which the va_arg macro typically plays
are stymied by overly-complicated types such as pointer-to-
function. If you use a typedef for the function pointer type,
however, all will be well.
References: ANSI Sec. 4.8.1.2 p. 124.
7.5: 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 be
solved by resorting to machine-specific assembly language.)
7.6: How can I call a function with an argument list built up at run
time?
A: There is no guaranteed or portable way to do this. If you're
curious, ask this list's editor, who has a few wacky ideas you
could try... (See also question 16.11.)
Section 8. Boolean Expressions and Variables
8.1: 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 (see also question 9.1). 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 question 8.2 below; see also
question 1.6).
8.2: 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 are used for code
readability, not because the underlying values might ever
change. (See also questions 1.7 and 1.9.)
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.
Section 9. Structs, Enums, and Unions
9.1: 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.)
Some 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
they obey block scope. (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 .
9.2: 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 .
9.3: 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. (Programmers often choose to use pointers to
structures instead, precisely to avoid this overhead.)
Structures are often returned from functions in a location
pointed to by an extra, compiler-supplied "hidden" argument to
the function. Some older compilers used a special, static
location for structure returns, although this made struct-valued
functions nonreentrant, which ANSI C disallows.
References: ANSI Sec. 2.2.3 p. 13.
9.4: 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 structure. (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() tries to accept three arguments,
although only two are passed (in this case, by the C start-up
code). See also question 17.21.
References: CT&P Sec. 2.3 pp. 21-2.
9.5: 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; see questions
9.10 and 9.11). 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.
9.6: How can I read/write structs from/to data files?
A: It is relatively straightforward to write a struct out using
fwrite:
fwrite((char *)&somestruct, sizeof(somestruct), 1, fp);
and a corresponding fread invocation can read it back in.
However, data files so written will _not_ be very portable (see
questions 9.11 and 17.3). Note also that on many systems you
must use the "b" flag when fopening the files.
9.7: 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 C implementation." An ANSI
Interpretation Ruling has deemed it (more precisely, access
beyond the declared size of the name field) to be not strictly
conforming, although a thorough treatment of the arguments
surrounding the legality of the technique is beyond the scope of
this list. It seems, however, to be portable to all known
implementations. (Compilers which check array bounds carefully
might issue warnings.)
To be on the safe side, it may be preferable to declare the
variable-size element very large, rather than very small; in the
case of the above example:
...
char name[MAXSIZE];
...
where MAXSIZE is larger than any name which will be stored.
(The trick so modified is said to be in conformance with the
Standard.)
References: ANSI Rationale Sec. 3.5.4.2 pp. 54-5.
9.8: 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.
References: ANSI Sec. 4.1.5, Rationale Sec. 3.5.4.2 p. 55.
9.9: 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;
9.10: Why does sizeof report a larger size than I expect for a
structure type, as if there was padding at the end?
A: Structures may have this padding (as well as internal padding;
see also question 9.5), so that alignment properties will be
preserved when an array of contiguous structures is allocated.
9.11: My compiler is leaving holes in structures, which is wasting
space and preventing "binary" I/O to external data files. Can I
turn off the padding, or otherwise control the alignment of
structs?
A: Your compiler may provide an extension to give you this control
(perhaps a #pragma), but there is no standard method. See also
question 17.3.
9.12: Can I initialize unions?
A: ANSI Standard C allows an initializer for the first member of a
union. There is no standard way of initializing the other
members (nor, under a pre-ANSI compiler, is there generally any
way of initializing any of them).
9.13: How can I pass constant values to routines which accept struct
arguments?
A: C has no way of generating anonymous struct values. You will
have to use a temporary struct variable.
Section 10. Declarations
10.1: How do you decide which integer type to use?
A: If you might need large values (above 32767 or below -32767),
use long. Otherwise, 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 of mixing signed and unsigned in
expressions.) Similar arguments apply when deciding between
float and double.
Although char or unsigned char can be used as a "tiny" int type,
doing so is often more trouble than it's worth, due to
unpredictable sign extension and increased code size.
These rules obviously don't apply if the address of a variable
is taken and must have a particular type.
If for some reason you need to declare something with an _exact_
size (usually the only good reason for doing so is when
attempting to conform to some externally-imposed storage layout,
but see question 17.3), be sure to encapsulate the choice behind
an appropriate typedef.
10.2: What should the 64-bit type on new, 64-bit machines be?
A: Some vendors of C products for 64-bit machines support 64-bit
long ints. Others fear that too much existing code depends on
sizeof(int) == sizeof(long) == 32 bits, and introduce a new 64-
bit long long (or __longlong) type instead.
Programmers interested in writing portable code should therefore
insulate their 64-bit type needs behind appropriate typedefs.
Vendors who feel compelled to introduce a new, longer integral
type should advertise it as being "at least 64 bits" (which is
truly new; a type traditional C doesn't have), and not "exactly
64 bits."
10.3: 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 at the point where the "next" field is declared. To
fix it, first give the structure a tag ("struct node"). Then,
declare the "next" field as "struct node *next;", and/or move
the typedef declaration wholly before or wholly after the struct
declaration. One corrected version would be
struct node
{
char *item;
struct node *next;
};
typedef struct node *NODEPTR;
, and there are at least three other equivalently correct ways
of arranging it.
A similar problem, with a similar solution, can arise when
attempting to declare a pair of typedef'ed mutually referential
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 .
10.4: How do I declare an array of N pointers to functions returning
pointers to functions returning pointers to characters?
A: This question can be answered in at least three ways:
1. char *(*(*a[N])())();
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[N]; /* array of... */
3. Use the cdecl program, which turns English into C and vice
versa:
cdecl> declare a as array of pointer to function returning
pointer to function returning pointer to char
char *(*(*a[])())()
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). Versions of cdecl are in volume 14 of
comp.sources.unix (see question 17.12) and K&R II.
Any good book on C should explain how to read these complicated
C declarations "inside out" to understand them ("declaration
mimics use").
References: K&R II Sec. 5.12 p. 122; H&S Sec. 5.10.1 p. 116.
10.5: I'm building a state machine with a bunch of functions, one for
each state. I want to implement state transitions by having
each function return a pointer to the next state function. I
find a limitation in C's declaration mechanism: there's no way
to declare these functions as returning a pointer to a function
returning a pointer to a function returning a pointer to a
function...
A: You can't do it directly. Either have the function return a
generic function pointer type, and apply a cast before calling
through it; or have it return a structure containing only a
pointer to a function returning that structure.
10.6: My compiler is complaining about an invalid redeclaration of a
function, but I only define it once and call it once.
A: If the first call precedes the definition, the compiler will
assume a function returns an int. Non-int functions must be
declared before they are called.
References: K&R I Sec. 4.2 pp. 70; K&R II Sec. 4.2 p. 72; ANSI
Sec. 3.3.2.2 .
10.7: What's the best way to declare and define global variables?
A: First, though there can be many _declarations_ (and in many
translation units) of a single "global" (strictly speaking,
"external") variable (or function), there must be exactly one
_definition_. (The definition is the declaration that actually
allocates space, and provides an initialization value, if any.)
It is best to place the definition in some central (to the
program, or to the module) .c file, with an external declaration
in a header (".h") file, which is #included wherever the
declaration is needed. The .c file containing the definition
should also #include the header file containing the external
declaration, so that the compiler can check that the
declarations match.
This rule promotes a high degree of portability, and is
consistent with the requirements of the ANSI C Standard. Note
that Unix compilers and linkers typically use a "common model"
which allows multiple (uninitialized) definitions. A few very
odd systems may require an explicit initializer to distinguish a
definition from an external declaration.
It is possible to use preprocessor tricks to arrange that the
declaration need only be typed once, in the header file, and
"turned into" a definition, during exactly one #inclusion, via a
special #define.
References: K&R I Sec. 4.5 pp. 76-7; K&R II Sec. 4.4 pp. 80-1;
ANSI Sec. 3.1.2.2 (esp. Rationale), Secs. 3.7, 3.7.2,
Sec. F.5.11; H&S Sec. 4.8 pp. 79-80; CT&P Sec. 4.2 pp. 54-56.
10.8: What does extern mean in a function declaration?
A: It can be used as a stylistic hint to indicate that the
function's definition is probably in another source file, but
there is no formal difference between
extern int f();
and
int f();
References: ANSI Sec. 3.1.2.2 .
10.9: 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 "(").
10.10: 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, func(), (*fp)() = func;
r = (*fp)();
It can also be argued 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, made widespread through pcc
and 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.
10.11: What's the auto keyword good for?
A: Nothing; it's obsolete.
Section 11. Stdio
11.1: Why doesn't this code:
char c;
while((c = getchar()) != EOF)...
work?
A: For one thing, the variable to hold getchar's return value must
be an int. getchar can return all possible character values, as
well as EOF. By passing getchar's return value through a char,
either a normal character might be misinterpreted as EOF, or the
EOF might be altered and so never seen.
References: CT&P Sec. 5.1 p. 70.
11.2: Why doesn't the code scanf("%d", i); work?
A: scanf needs pointers to the variables it is to fill in; you must
call scanf("%d", &i);
11.3: Why doesn't this code:
double d;
scanf("%f", &d);
work?
A: scanf uses %lf for values of type double, and %f for float.
(Note the discrepancy with printf, which uses %f for both double
and float, due to C's default argument promotion rules.)
11.4: Why won't the code
while(!feof(infp)) {
fgets(buf, MAXLINE, infp);
fputs(buf, outfp);
}
work?
A: C's I/O is not like Pascal's. EOF is only indicated _after_ an
input routine has tried to read, and has reached end-of-file.
Usually, you should just check the return value of the input
routine (fgets in this case); often, you don't need to use
feof() at all.
11.5: Why does everyone say not to use gets()?
A: It cannot be told the size of the buffer it's to read into, so
it cannot be prevented from overflowing that buffer. See
question 3.1 for a code fragment illustrating the replacement of
gets() with fgets().
11.6: 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.
References: CT&P Sec. 5.4 p. 73.
11.7: 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 11.6.)
11.8: 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 to expect a newline, but rather
to read and discard characters as long as each is a whitespace
character.
A related problem is that unexpected non-numeric input can cause
scanf to "jam." Because of these problems, it is usually better
to use fgets to read a whole line, and then use sscanf or other
string functions to pick apart the line buffer. If you do use
sscanf, don't forget to check the return value to make sure that
the expected number of items were found.
11.9: I'm trying to update a file in place, by using fopen mode "r+",
then reading a certain string, and finally writing back a
modified string, but it's not working.
A: Be sure to call fseek before you write, both to seek back to the
beginning of the string you're trying to overwrite, and because
an fseek or fflush is always required between reading and
writing in the read/write "+" modes.
References: ANSI Sec. 4.9.5.3 p. 131.
11.10: How can I read one character at a time, without waiting for the
RETURN key?
A: See question 16.1.
11.11: How can I flush pending input so that a user's typeahead isn't
read at the next prompt? Will fflush(stdin) work?
A: fflush is defined only for output streams. Since its definition
of "flush" is to complete the writing of buffered characters
(not to discard them), discarding unread input would not be an
analogous meaning for fflush on input streams. There is no
standard way to discard unread characters from a stdio input
buffer, nor would such a way be sufficient; unread characters
can also accumulate in other, OS-level input buffers.
11.12: How can I redirect stdin or stdout to a file from within a
program?
A: Use freopen.
11.13: Once I've used freopen, how can I get the original stdout (or
stdin) back?
A: If you need to switch back and forth, the best all-around
solution is not to use freopen in the first place. Try using
your own explicit output (or input) stream variable, which you
can reassign at will, while leaving the original stdout (or
stdin) undisturbed.
11.14: 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 files yourself when
you open them (perhaps with a wrapper function around fopen).
Section 12. Library Subroutines
12.1: Why does strncpy not always place a '\0' termination in the
destination string?
A: strncpy was first designed to handle a now-obsolete data
structure, the fixed-length, not-necessarily-\0-terminated
"string." strncpy is admittedly a bit cumbersome to use in
other contexts, since you must often append a '\0' to the
destination string by hand.
12.2: I'm trying to sort an array of strings with qsort, using strcmp
as the comparison function, but it's not working.
A: By "array of strings" you probably mean "array of pointers to
char." The arguments to qsort's comparison function are
pointers to the objects being sorted, in this case, pointers to
pointers to char. (strcmp, of course, accepts simple pointers
to char.)
The comparison routine's arguments are expressed as "generic
pointers," const void * or char *. They must be converted back
to what they "really are" (char **) and dereferenced, yielding
char *'s which can be usefully compared. Write a comparison
function like this:
int pstrcmp(p1, p2) /* compare strings through pointers */
char *p1, *p2; /* const void * for ANSI C */
{
return strcmp(*(char **)p1, *(char **)p2);
}
Beware of the discussion in K&R II Sec. 5.11 pp. 119-20, which
is not discussing Standard library qsort.
12.3: Now I'm trying to sort an array of structures with qsort. My
comparison routine takes pointers to structures, but the
compiler complains that the function is of the wrong type for
qsort. How can I cast the function pointer to shut off the
warning?
A: The conversions must be in the comparison function, which must
be declared as accepting "generic pointers" (const void * or
char *) as discussed in question 12.2 above. The code might
look like
int mystructcmp(p1, p2)
char *p1, *p2; /* const void * for ANSI C */
{
struct mystruct *sp1 = (struct mystruct *)p1;
struct mystruct *sp2 = (struct mystruct *)p2;
/* now compare sp1->whatever and *sp2-> ... */
}
(If, on the other hand, you're sorting pointers to structures,
you'll need indirection, as in question 12.2:
sp1 = *(struct mystruct **)p1 .)
12.4: How can I convert numbers to strings (the opposite of atoi)? Is
there an itoa function?
A: Just use sprintf. (You'll have to allocate space for the result
somewhere anyway; see questions 3.1 and 3.2. 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.
12.5: How can I get the current date or time of day in a C program?
A: Just use the time, ctime, and/or localtime functions. (These
routines have been around for years, and are in the ANSI
standard.) Here is a simple example:
#include <stdio.h>
#include <time.h>
main()
{
time_t now = time((time_t *)NULL);
printf("It's %.24s.\n", ctime(&now));
return 0;
}
References: ANSI Sec. 4.12 .
12.6: 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. Some
systems provide a strptime function; another popular routine is
partime (widely distributed with the RCS package), but these 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 .
12.7: How can I add n days to a date? How can I find the difference
between two dates?
A: The ANSI/ISO Standard C mktime and difftime functions provide
support for both problems. mktime accepts non-normalized dates,
so it is straightforward to take a filled in struct tm, add or
subtract from the tm_mday field, and call mktime to normalize
the year, month, and day fields (and convert to a time_t value).
difftime computes the difference, in seconds, between two time_t
values; mktime can be used to compute time_t values for two
dates to be subtracted. (Note, however, that these solutions
only work for dates which can be represented as time_t's.) See
also questions 12.6 and 17.28.
References: K&R II Sec. B10 p. 256; H&S Secs. 20.4, 20.5
pp. 361-362; ANSI Secs. 4.12.2.2, 4.12.2.3 .
12.8: I need a random number generator.
A: The standard C library has one: rand(). The implementation on
your system may not be perfect, but writing a better one isn't
necessarily easy, either.
References: ANSI Sec. 4.10.2.1 p. 154; Knuth Vol. 2 Chap. 3
pp. 1-177.
12.9: How can I get random integers in a certain range?
A: The obvious way,
rand() % N
(where N is of course the range) is poor, because the low-order
bits of many random number generators are distressingly non-
random. (See question 12.11.) A better method is something
like
(int)((double)rand() / ((double)RAND_MAX + 1) * N)
If you're worried about using floating point, you could try
rand() / (RAND_MAX / N + 1)
Both methods obviously require knowing RAND_MAX (which ANSI
defines in <stdlib.h>), and assume that N is much less than
RAND_MAX.
12.10: Each time I run my program, I get the same sequence of numbers
back from rand().
A: You can call srand() to seed the pseudo-random number generator
with a more random initial value. Popular seed values are the
time of day, or the elapsed time before the user presses a key
(although keypress times are hard to determine portably; see
question 16.10).
References: ANSI Sec. 4.10.2.2 p. 154.
12.11: I need a random true/false value, so I'm taking rand() % 2, but
it's just alternating 0, 1, 0, 1, 0...
A: Poor pseudorandom number generators (such as the ones
unfortunately supplied with some systems) are not very random in
the low-order bits. Try using the higher-order bits. See
question 12.9.
12.12: I'm trying to port this A: These routines are variously
old program. Why do I obsolete; you should
get "undefined external" instead:
errors for:
index? use strchr.
rindex? use strrchr.
bcopy? use memmove, after
interchanging the first and
second arguments (see also
question 5.15).
bcmp? use memcmp.
bzero? use memset, with a second
argument of 0.
12.13: I keep getting errors due to library routines being undefined,
but I'm #including all the right header files.
A: In some cases (especially if the routines are nonstandard) you
may have to explicitly ask for the correct libraries to be
searched when you link the program. See also question 15.2.
12.14: I'm still getting errors due to library routines being
undefined, even though I'm using -l to request the libraries
while linking.
A: Many linkers make one pass over the list of object files and
libraries you specify, and extract from libraries only those
modules which satisfy references which have so far come up as
undefined. Therefore, the order in which libraries are listed
with respect to object files (and each other) is significant;
usually, you want to search the libraries last (i.e., under
Unix, put any -l switches towards the end of the command line).
12.15: I need some code to do regular expression matching.
A: Look for the regexp library (supplied with many Unix systems),
or get Henry Spencer's regexp package from cs.toronto.edu in
pub/regexp.shar.Z (see also question 17.12).
12.16: How can I split up a command line into argc and argv, like the
shell does?
A: Most systems have a routine called strtok.
References: ANSI Sec. 4.11.5.8; K&R II Sec. B3 p. 250; H&S
Sec. 15.7; PCS p. 178.
Section 13. Lint
13.1: I just typed in this program, and it's acting strangely. Can
you see anything wrong with it?
A: Try running lint first (perhaps with the -a, -c, -h, -p and/or
other options). Many C compilers are really only half-
compilers, electing not to diagnose numerous source code
difficulties which would not actively preclude code generation.
13.2: 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.
13.3: 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 14. Style
14.1: Here's a neat trick:
if(!strcmp(s1, s2))
Is this good style?
A: It is not particularly good style, although it is a popular
idiom. The test succeeds if the two strings are equal, but its
form suggests that it tests for inequality.
Another solution is to use a macro:
#define Streq(s1, s2) (strcmp((s1), (s2)) == 0)
Opinions on code style, like those on religion, can be debated
endlessly. Though good style is a worthy goal, and can usually
be recognized, it cannot be codified.
14.2: 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.)
The elusive quality of "good style" involves much more than mere
code layout details; don't spend time on formatting to the
exclusion of more substantive code quality issues.
References: K&R Sec. 1.2 p. 10.
14.3: 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. Floating Point
15.1: My floating-point calculations are acting strangely and giving
me different answers on different machines.
A: First, make sure that you have #included <math.h>, and correctly
declared other functions returning double.
If the problem isn't that simple, recall that most digital
computers use floating-point formats which provide a close but
by no means exact simulation of real number arithmetic.
Underflow, cumulative precision loss, and other anomalies are
often troublesome.
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.
References: EoPS Sec. 6 pp. 115-8.
15.2: I'm trying to do some simple trig, and I am #including <math.h>,
but I keep getting "undefined: _sin" compilation errors.
A: Make sure you're linking with the correct math library. For
instance, under Unix, you usually need to use the -lm option,
and at the _end_ of the command line, when compiling/linking.
See also question 12.14.
15.3: Why doesn't C have an exponentiation operator?
A: Because few processors have an exponentiation instruction.
Instead, you can #include <math.h> and use the pow() function,
although explicit multiplication is often better for small
positive integral exponents.
References: ANSI Sec. 4.5.5.1 .
15.4: How do I round numbers?
A: The simplest and most straightforward way is with code like
(int)(x + 0.5)
This won't work properly for negative numbers, though.
15.5: How do I test for IEEE NaN and other special values?
A: Many systems with high-quality IEEE floating-point
implementations provide facilities (e.g. an isnan() macro) to
deal with these values cleanly, and the Numerical C Extensions
Group (NCEG) is working to formally standardize such facilities.
A crude but usually effective test for NaN is exemplified by
#define isnan(x) ((x) != (x))
although non-IEEE-aware compilers may optimize the test away.
15.6: I'm having trouble with a Turbo C program which crashes and says
something like "floating point formats not linked."
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 insufficient, and the programmer must
sometimes insert an extra, explicit call to a floating-point
library routine to force loading of floating-point support.
Section 16. System Dependencies
16.1: 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. (Nor are closely-related questions
concerning the echo of keyboard input.) The delivery of
characters from a "keyboard" to a C program is a function of the
operating system in use, and has not been standardized by the C
language. Some versions of curses have a cbreak() function
which does what you want. If you're specifically trying to read
a short password without echo, you might try getpass(). 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 VMS, try the Screen Management (SMG$) routines,
or curses, or issue low-level $QIO's with the IO$_READVBLK (and
perhaps IO$M_NOECHO) function codes to ask for one character at
a time. 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.os.msdos.programmer . 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.
16.2: 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().
16.3: How can I clear the screen? How can I print things in inverse
video?
A: Such things depend on the terminal type (or display) you're
using. You will have to use a library such as termcap or
curses, or some system-specific routines, to perform these
functions.
16.4: How do I read the mouse?
A: Consult your system documentation, or ask on an appropriate
system-specific newsgroup (but check its FAQ list first). Mouse
handling is completely different under the X window system, MS-
DOS, Macintosh, and probably every other system.
16.5: 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.
16.6: 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.
16.7: How can I check whether a file exists? I want to query the user
if a requested output file already exists.
A: You can try the access() routine, although it's got a few
problems. (It isn't atomic with respect to the following
action, and it has anomalies if the program calling it is
running as root.)
16.8: How can I find out the size of a file, prior to reading it in?
A: If the "size of a file" is the number of characters you'll be
able to read from it in C, it is in general impossible to
determine this number in advance. Under Unix, the stat call
will give you an exact answer, and several other systems supply
a Unix-like stat which will give an approximate answer. You can
fseek to the end and then use ftell, but this usage is
nonportable (it gives you an accurate answer only under Unix,
and otherwise a quasi-accurate answer only for ANSI C "binary"
files). Some systems provide routines called filesize or
filelength.
Are you sure you have to determine the file's size in advance?
Since the most accurate way of determining the size of a file as
a C program will see it is to open the file and read it, perhaps
you can rearrange the code to learn the size as it reads.
16.9: How can a file be shortened in-place without completely clearing
or rewriting it?
A: BSD systems provide ftruncate(), several others supply chsize(),
and a few may provide a (possibly undocumented) fcntl option
F_FREESP. Under MS-DOS, you can sometimes use write(fd, "", 0).
However, there is no truly portable solution.
16.10: How can I implement a delay, or time a user's response, with
sub-second resolution?
A: Unfortunately, there is no portable way. V7 Unix, and derived
systems, provided a fairly useful ftime() routine with
resolution up to a millisecond, but it has disappeared from
System V and Posix. Other routines you might look for on your
system include nap(), setitimer(), msleep(), usleep(), clock(),
and gettimeofday(). The select() and poll() calls (if
available) can be pressed into service to implement simple
delays. On MS-DOS machines, it is possible to reprogram the
system timer and timer interrupts.
16.11: How can I read in an object file and jump to routines in it?
A: You want a dynamic linker and/or loader. It is possible to
malloc some space and read in object files, but you have to know
an awful lot about object file formats, relocation, etc. Under
BSD Unix, you could use system() and ld -A to do the linking for
you. Many (most?) versions of SunOS and System V have the -ldl
library which allows object files to be dynamically loaded.
There is also a GNU package called "dld". See also question
7.6.
16.12: How can I invoke an operating system command from within a
program?
A: Use system().
References: K&R II Sec. B6 p. 253; ANSI Sec. 4.10.4.5; H&S
Sec. 21.2; PCS Sec. 11 p. 179;
16.13: How can I invoke an operating system command and trap its
output?
A: Unix and some other systems provide a popen() routine, which
sets up a stdio stream on a pipe connected to the process
running a command, so that the output can be read (or the input
supplied).
References: PCS Sec. 11 p. 169 .
16.14: How can I read a directory in a C program?
A: See if you can use the opendir() and readdir() routines, which
are available on most Unix systems. Implementations also exist
for MS-DOS, VMS, and other systems. (MS-DOS also has FINDFIRST
and FINDNEXT routines which do essentially the same thing.)
16.15: How can I do serial ("comm") port I/O?
A: It's system-dependent. Under Unix, you typically open, read,
and write a device in /dev, and use the facilities of the
terminal driver to adjust its characteristics. Under MS-DOS,
you can either use some primitive BIOS interrupts, or (if you
require decent performance) one of any number of interrupt-
driven serial I/O packages.
Section 17. Miscellaneous
17.1: 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 (just once, at program
startup) to zero, as if the programmer had typed "= 0".
Therefore, such variables are initialized to the null pointer
(of the correct type; see also Section 1) 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 3.13, and section
1).
17.2: This code, straight out of a book, isn't compiling:
f()
{
char a[] = "Hello, world!";
}
A: Perhaps you have a pre-ANSI compiler, which doesn't allow
initialization of "automatic aggregates" (i.e. non-static local
arrays and structures). As a workaround, you can make the array
global or static. (You can always initialize local char *
variables with string literals, but see question 17.20). See
also questions 5.16 and 5.17.
17.3: 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 (RFC 1014),
OSI's ASN.1, CCITT's X.409, or ISO 8825 "Basic Encoding Rules."
See also question 9.11.
17.4: How can I delete a line (or record) from the middle of a file?
A: Short of rewriting the file, you probably can't. See also
question 16.9.
17.5: How can I return several values from a function?
A: Either pass pointers to locations which the function can fill
in, or have the function return a structure containing the
desired values, or (in a pinch) consider global variables. See
also questions 2.17, 3.4, and 9.2.
17.6: If I have a char * variable pointing to the name of a function
as a string, how can I call that function?
A: The most straightforward thing to do is maintain a
correspondence table of names and function pointers:
int function1(), function2();
struct {char *name; int (*funcptr)(); } symtab[] =
{
"function1", function1,
"function2", function2,
};
Then, just search the table for the name, and call through the
associated function pointer. See also questions 9.9 and 16.11.
17.7: 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).
17.8: How can I call FORTRAN (C++, 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. More
information may be found in FORT.gz by Glenn Geers, available
via anonymous ftp from suphys.physics.su.oz.au in the src
directory.
cfortran.h, a C header file, simplifies C/FORTRAN interfacing on
many popular machines. It is available via anonymous ftp from
zebra.desy.de (131.169.2.244).
In C++, a "C" modifier in an external function declaration
indicates that the function is to be called using C calling
conventions.
17.9: Does anyone know of a program for converting Pascal or FORTRAN
(or LISP, Ada, awk, "Old" C, ...) to C?
A: Several public-domain programs are available:
p2c A Pascal to C converter written by Dave Gillespie,
posted to comp.sources.unix in March, 1990 (Volume 21);
also available by anonymous ftp from
csvax.cs.caltech.edu, file pub/p2c-1.20.tar.Z .
ptoc Another Pascal to C converter, this one written in
Pascal (comp.sources.unix, Volume 10, also patches in
Volume 13?).
f2c A Fortran to C converter 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@research.att.com or research!netlib. (It is
also available via anonymous ftp on research.att.com, in
directory dist/f2c.)
This FAQ list's maintainer also has available a list of other
commercial translation products, and some for more obscure
languages.
See also question 5.3.
17.10: Is C++ a superset of C? Can I use a C++ compiler to compile C
code?
A: C++ was derived from C, and is largely based on it, but there
are some legal C constructs which are not legal C++. (Many C
programs will nevertheless compile correctly in a C++
environment.)
17.11: I need: A: Look for programs (see also
question 17.12) named:
a C cross-reference cflow, calls, cscope
generator
a C beautifier/pretty- cb, indent
printer
17.12: 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
(ftp.uu.net, 192.48.96.9). However, this article cannot track
or list all of the available archive sites and how to access
them.
Ajay Shah maintains an index of free numerical software; it is
posted periodically, and available where this FAQ list is
archived (see question 17.33). 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@quiche.cs.mcgill.ca for information. Finally,
the newsgroup comp.sources.wanted is generally a more
appropriate place to post queries for source availability, but
check _its_ FAQ list, "How to find sources," before posting
there.
17.13: When will the next International Obfuscated C Code 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 and guidelines, send e-mail
with the Subject: line "send rules" to:
{apple,pyramid,sun,uunet}!hoptoad!judges or
judges@toad.com
(Note that these are _not_ the addresses for submitting
entries.)
Contest winners are first announced at the Summer Usenix
Conference in mid-June, and posted to the net sometime in July-
August. Winning entries from previous years (to 1984) are
archived at uunet (see question 17.12) under the directory
~/pub/ioccc.
As a last resort, previous winners may be obtained by sending
e-mail to the above address, using the Subject: "send YEAR
winners", where YEAR is a single four-digit year, a year range,
or "all".
17.14: Why don't C comments nest? How am I supposed to comment out
code containing comments? Are comments 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
5.11).
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.
References: ANSI Appendix E p. 198, Rationale Sec. 3.1.9 p. 33.
17.15: How can I get the ASCII value corresponding to a character, or
vice versa?
A: In C, characters are represented by small integers corresponding
to their values (in the machine's character set) so you don't
need a conversion routine: if you have the character, you have
its value.
17.16: How can I implement sets and/or arrays of bits?
A: Use arrays of char or int, with a few macros to access the right
bit at the right index (try using 8 for CHAR_BIT if you don't
have <limits.h>):
#include <limits.h> /* for CHAR_BIT */
#define BITMASK(bit) (1 << ((bit) % CHAR_BIT))
#define BITSLOT(bit) ((bit) / CHAR_BIT)
#define BITSET(ary, bit) ((ary)[BITSLOT(bit)] |= BITMASK(bit))
#define BITTEST(ary, bit) ((ary)[BITSLOT(bit)] & BITMASK(bit))
17.17: What is the most efficient way to count the number of bits which
are set in a value?
A: This and many other similar bit-twiddling problems can often be
sped up and streamlined using lookup tables (but see the next
question).
17.18: 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 caching 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 Plauger's The Elements of Programming
Style, and Jon Bentley's Writing Efficient Programs.
17.19: Are pointers really faster than arrays? How much do function
calls 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. Any decent 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.
(See also question 4.7.)
17.20: Why does this code:
char *p = "Hello, world!";
p[0] = tolower(p[0]);
crash?
A: String literals are not necessarily modifiable, except (in
effect) when they are used as array initializers. Try
char a[] = "Hello, world!";
(For compiling old code, some compilers have a switch
controlling whether strings are writable or not.) See also
questions 2.1, 2.2, 2.8, and 17.2.
References: ANSI Sec. 3.1.4 .
17.21: 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 9.4.)
17.22: What do "Segmentation violation" and "Bus error" mean?
A: These generally mean that your program tried to access memory it
shouldn't have, invariably as a result of improper pointer use,
often involving uninitialized or improperly allocated pointers
(see questions 3.1 and 3.2), or malloc (see question 17.23), or
perhaps scanf (see question 11.2).
17.23: My program is crashing, apparently somewhere down inside malloc,
but I can't see anything wrong with it.
A: It is unfortunately very easy to corrupt malloc's internal data
structures, and the resulting problems can be hard to track
down. The most common source of problems is writing more to a
malloc'ed region than it was allocated to hold; a particularly
common bug is to malloc(strlen(s)) instead of strlen(s) + 1.
Other problems involve freeing pointers not obtained from
malloc, or trying to realloc a null pointer (see question 3.12).
A number of debugging packages exist to help track down malloc
problems; one popular one is Conor P. Cahill's "dbmalloc,"
posted to comp.sources.misc in September of 1992. Others are
"leak," available in volume 27 of the comp.sources.unix
archives; JMalloc.c and JMalloc.h in Fidonet's C_ECHO Snippets
(or ask archie; see question 17.12); and MEMDEBUG from
dorado.crpht.lu in pub/sources/memdebug . See also question
17.12.
17.24: Does anyone have a C compiler test suite I can use?
A: Plum Hall (formerly in Cardiff, NJ; now in Hawaii) sells one.
The FSF's GNU C (gcc) distribution includes a c-torture-
test.tar.Z which checks a number of common problems with
compilers. Kahan's paranoia test, found in netlib/paranoia on
netlib.att.com, strenuously tests a C implementation's floating
point capabilities.
17.25: Where can I get a YACC grammar for C?
A: The definitive grammar is of course the one in the ANSI
standard. Another grammar, by Jim Roskind, is in pub/*grammar*
at ics.uci.edu . A fleshed-out, working instance of the ANSI
grammar (due to Jeff Lee) is on uunet (see question 17.12) in
usenet/net.sources/ansi.c.grammar.Z (including a companion
lexer). The FSF's GNU C compiler contains a grammar, as does
the appendix to K&R II.
References: ANSI Sec. A.2 .
17.26: I need code to parse and evaluate expressions.
A: Two available packages are "defunc," posted to comp.source.misc
in December, 1993 (V41 i32,33), to alt.sources in January, 1994,
and available from sunsite.unc.edu in
pub/packages/development/libraries/defunc-1.3.tar.Z; and
"parse," at lamont.ldgo.columbia.edu.
17.27: I need a sort of an "approximate" strcmp routine, for comparing
two strings for close, but not necessarily exact, equality.
A: The traditional routine for doing this sort of thing involves
the "soundex" algorithm, which maps similar-sounding words to
the same numeric codes. Soundex is described in the Searching
and Sorting volume of Donald Knuth's classic _The Art of
Computer Programming_.
17.28: How can I find the day of the week given the date?
A: Use mktime (see questions 12.6 and 12.7), or Zeller's
congruence. Here is one quick implementation posted by Tomohiko
Sakamoto:
dayofweek(y, m, d) /* 0 = Sunday */
int y, m, d; /* 1 <= m <= 12, y > 1752 or so */
{
static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
y -= m < 3;
return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
}
17.29: Will 2000 be a leap year? Is (year % 4 == 0) an accurate test
for leap years?
A: Yes and no, respectively. The full expression for the Gregorian
calendar is
year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
See a good astronomical almanac or other reference for details.
17.30: How do you pronounce "char"?
A: You can pronounce the C keyword "char" in at least three ways:
like the English words "char," "care," or "car;" the choice is
arbitrary.
17.31: What's a good book for learning C?
A: Mitch Wright maintains an annotated bibliography of C and Unix
books; it is available for anonymous ftp from ftp.rahul.net in
directory pub/mitch/YABL.
This FAQ list's editor maintains a collection of previous
answers to this question, which is available upon request.
17.32: Are there any C tutorials on the net?
A: There are at least two of them:
"Notes for C programmers," by Christopher Sawtell,
available from:
svr-ftp.eng.cam.ac.uk:misc/sawtell_C.shar
garbo.uwasa.fi:/pc/c/c-lesson.zip
oak.oakland.edu:pub/msdos/c/LEARN-C.ZIP
paris7.jussieu.fr:/contributions/docs
Tim Love's "C for Programmers,"
available from svr-ftp.eng.cam.ac.uk in the misc directory.
17.33: 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 Expires: line
which should keep it around all month. An abridged version is
also available (and posted), as is a list of changes
accompanying each significantly updated version. These lists
can also be found in the newsgroups comp.answers and
news.answers . Several sites archive news.answers postings and
other FAQ lists, including this one: two sites are rtfm.mit.edu
(directories pub/usenet/news.answers/C-faq/ and
pub/usenet/comp.lang.c/ ) and ftp.uu.net (directory
usenet/news.answers/C-faq/ ). The archie server should help you
find others; query it for "prog C-faq". See the meta-FAQ list
in news.answers for more information; see also question 17.12.
This list is an evolving document of questions which have been
Frequent since before the Great Renaming, 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 5.2).
JLB 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.
EoPS Brian W. Kernighan and P.J. Plauger, 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.
Knuth Donald E. Knuth, The Art of Computer Programming, (3 vols.),
Addison-Wesley, 1981.
CT&P Andrew Koenig, C Traps and Pitfalls, Addison-Wesley, 1989,
ISBN 0-201-17928-8.
P.J. Plauger, The Standard C Library, Prentice Hall, 1992,
ISBN 0-13-131509-9.
Harry Rabinowitz and Chaim Schaap, Portable C, Prentice-Hall,
1990, ISBN 0-13-685967-4.
There is a more extensive bibliography in the revised Indian Hill style
guide (see question 14.3). See also question 17.31.
Steve Summit
scs@eskimo.com
This article is Copyright 1988, 1990-1994 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.