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From: [email protected] (Steve Summit)
Newsgroups: comp.lang.c,news.answers
Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List)
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[Last modified February 1, 1992 by scs.]
Certain topics come up again and again on this newsgroup. They are good
questions, and the answers may not be immediately obvious, but each time
they recur, much net bandwidth and reader time is wasted on repetitive
responses, and on tedious corrections to the incorrect answers which are
inevitably posted.
This article, which is posted monthly, attempts to answer these common
questions definitively and succinctly, so that net discussion can move
on to more constructive topics without continual regression to first
principles.
No mere newsgroup article can substitute for thoughtful perusal of a
full-length language reference manual. Anyone interested enough in C to
be following this newsgroup should also be interested enough to read and
study one or more such manuals, preferably several times. Some vendors'
compiler manuals are unfortunately inadequate; a few even perpetuate
some of the myths which this article attempts to refute. Several
noteworthy books on C are listed in this article's bibliography. Many
of the questions and answers are cross-referenced to these books, for
further study by the interested and dedicated reader.
If you have a question about C which is not answered in this article,
please try to answer it by checking a few of the referenced books, or by
asking knowledgeable colleagues, before posing your question to the net
at large. There are many people on the net who are happy to answer
questions, but the volume of repetitive answers posted to one question,
as well as the growing number of questions as the net attracts more
readers, can become oppressive. If you have questions or comments
prompted by this article, please reply by mail rather than following up
-- this article is meant to decrease net traffic, not increase it.
Besides listing frequently-asked questions, this article also summarizes
frequently-posted answers. Even if you know all the answers, it's worth
skimming through this list once in a while, so that when you see one of
its questions unwittingly posted, you won't have to waste time
answering.
This article is always being improved. Your input is welcomed. Send
your comments to [email protected], scs%[email protected], and/or
mit-eddie!adam!scs; this article's From: line may be unusable.
The questions answered here are divided into several categories:
1. Null Pointers
2. Arrays and Pointers
3. Expressions
4. ANSI C
5. C Preprocessor
6. Variable-Length Argument Lists
7. Lint
8. Memory Allocation
9. Structures
10. Declarations
11. Boolean Expressions and Variables
12. Operating System Dependencies
13. Stdio
14. Library Subroutines
15. Style
16. 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. 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
8.1, 8.9, and 16.1.
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:
execl("/bin/sh", "sh", "-c", "ls", (char *)0);
If the (char *) cast were omitted, the compiler would not know
to pass a null pointer, and would pass an integer 0 instead.
(Note that many Unix manuals get this example wrong.)
When function prototypes are in scope, argument passing becomes
an "assignment context," and most casts may safely be omitted,
since the prototype tells the compiler that a pointer is
required, and of which type, enabling it to correctly cast
unadorned 0's. Function prototypes cannot provide the types for
variable arguments in variable-length argument lists, however,
so explicit casts are still required for those arguments. It is
safest always to cast null pointer function arguments, to guard
against varargs functions or those without prototypes, to allow
interim use of non-ANSI compilers, and to demonstrate that you
know what you are doing.
Summary:
Unadorned 0 okay: Explicit cast required:
initialization function call,
no prototype in scope
assignment
variable argument in
comparison varargs function call
function call,
prototype in scope,
fixed argument
References: K&R I Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R II
Sec. A7.10 p. 207, Sec. A7.17 p. 209; H&S Sec. 4.6.3 p. 72; ANSI
Sec. 3.2.2.3 .
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
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).
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.
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 11.2.
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.
Reference: 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 relation to the null pointer except in
name. Since this character terminates strings in C, an
empty string may be called...
6. The "null string," an ambiguous term which might mean
either a string consisting only of the terminating '\0',
or (more properly) a null pointer of type char *.
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 ANSI C refinements, or with the
internal representation of null pointers, which you shouldn't
need to know. 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 8.9). 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 addresses (char *'s) than word addresses (int *'s).
Some Honeywell-Bull mainframes use the bit pattern 06000 for
(internal) null pointers.
The Symbolics Lisp Machine, a tagged architecture, does not even
have conventional numeric pointers; it uses the pair
(basically a nonexistent
pointer.
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 remember 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 question 16.11.)
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" does not by any means imply that they
are interchangeable.
"Equivalence" refers to the following key definition:
An lvalue of type array-of-T which appears in an
expression decays (with three exceptions) into a
pointer to its first element; the type of the
resultant pointer is pointer-to-T.
(The exceptions are when the array is the operand of the
sizeof() or & operators, or is a literal string initializer for
a character array.)
As a consequence of this definition, there is not really any
difference in the behavior of the "array subscripting" operator
[] as it applies to arrays and pointers. In an expression of
the form a[i], the array reference "a" decays into a pointer,
following the rule above, and is then subscripted exactly as
would be a pointer variable in the expression p[i]. In either
case, the expression x[i] (where x is an array or a pointer) is,
by definition, exactly equivalent to *((x)+(i)).
References: K&R I Sec. 5.3 pp. 93-6; K&R II Sec. 5.3 p. 99; H&S
Sec. 5.4.1 p. 93; ANSI Sec. 3.2.2.1, 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. Therefore, any parameter
declarations which "look like" arrays, e.g.
f(a)
char a[];
are treated by the compiler as if they were pointers, since that
is what the function will receive if an array is passed:
f(a)
char *a;
This conversion holds only within function formal parameter
declarations, nowhere else. If this conversion bothers you,
avoid it; many people have concluded that the confusion it
causes outweighs the small advantage of having the declaration
"look like" the call and/or the uses within the function.
References: K&R I Sec. 5.3 p. 95, Sec. A10.1 p. 205; K&R II
Sec. 5.3 p. 100, Sec. A8.6.3 p. 218, Sec. A10.1 p. 226; H&S
Sec. 5.4.3 p. 96; ANSI Sec. 3.5.4.3, Sec. 3.7.1, CT&P Sec. 3.3
pp. 33-4.
2.5: Someone explained to me that arrays were really just constant
pointers.
A: That person did you a disservice. An array name is "constant"
in that it cannot be assigned to, but an array is _not_ a
pointer, as the discussion and pictures in question 2.2 should
make clear.
2.6: I came across some "joke" code containing the "expression"
5["abcdef"] . How can this be legal C?
A: Yes, Virginia, array subscripting is commutative in C. This
curious fact follows from the pointer definition of array
subscripting, namely that a[e] is exactly equivalent to
*((a)+(e)), for _any_ expression e and primary expression a, as
long as one of them is a pointer expression and one is integral.
This unsuspected commutativity is often mentioned in C texts as
if it were something to be proud of, but it finds no useful
application outside of the Obfuscated C Contest (see question
16.7).
References: ANSI Rationale Sec. 3.3.2.1 p. 41.
2.7: My compiler complained when I passed a two-dimensional array to
a routine expecting a pointer to a pointer.
A: The rule by which arrays decay into pointers is not applied
recursively. An array of arrays (i.e. a two-dimensional array
in C) decays into a pointer to an array, not a pointer to a
pointer. Pointers to arrays can be confusing, and must be
treated carefully. (The confusion is heightened by the
existence of incorrect compilers, including some versions of pcc
and pcc-derived lint's, which improperly accept assignments of
multi-dimensional arrays to multi-level pointers.) If you are
passing a two-dimensional array to a function:
int array[YSIZE][XSIZE];
f(array);
the function's declaration should match:
f(int a[][XSIZE]) {...}
or
f(int (*ap)[XSIZE]) {...} /* ap is a pointer to an array */
In the first declaration, the compiler performs the usual
implicit parameter rewriting of "array of array" to "pointer to
array;" in the second form the pointer declaration is explicit.
Since the called function does not allocate space for the array,
it does not need to know the overall size, so the number of
"rows," YSIZE, can be omitted. The "shape" of the array is
still important, so the "column" dimension XSIZE (and, for 3- or
more dimensional arrays, the intervening ones) must be included.
If a function is already declared as accepting a pointer to a
pointer, it is probably incorrect to pass a two-dimensional
array directly to it.
2.8: How do I declare a pointer to an array?
A: Usually, you don't want to. Consider using a pointer to one of
the array's elements instead. Arrays of type T decay into
pointers to type T (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 2.7 above.) When people speak casually of a
pointer to an array, they usually mean a pointer to its first
element.
If you really need to declare a pointer to an entire array, use
something like "int (*ap)[N];" where N is the size of the array.
(See also question 10.3.) If the size of the array is unknown,
N can be omitted, but the resulting type, "pointer to array of
unknown size," is useless.
2.9: How can I dynamically allocate a multidimensional array?
A: It is usually best to allocate an array of pointers, and then
initialize each pointer to a dynamically-allocated "row." The
resulting "ragged" array can save space, although it is not
necessarily contiguous in memory as a real array would be. Here
is a two-dimensional example:
int **array = (int **)malloc(nrows * sizeof(int *));
for(i = 0; i < nrows; i++)
array[i] = (int *)malloc(ncolumns * sizeof(int));
(In "real" code, of course, malloc 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 **array = (int **)malloc(nrows * sizeof(int *));
array[0] = (int *)malloc(nrows * ncolumns * sizeof(int));
for(i = 1; i < nrows; i++)
array[i] = array[0] + i * ncolumns;
In either case, the elements of the dynamic array can be
accessed with normal-looking array subscripts: array[i][j].
If the double indirection implied by the above schemes is for
some reason unacceptable, you can simulate a two-dimensional
array with a single, dynamically-allocated one-dimensional
array:
int *array = (int *)malloc(nrows * ncolumns * sizeof(int));
However, you must now perform subscript calculations manually,
accessing the i,jth element with array[i * ncolumns + j]. (A
macro can hide the explicit calculation, but invoking it then
requires parentheses and commas which don't look exactly like
multidimensional array subscripts.)
2.10: Can I simulate a non-0-based array with a pointer?
A: Not if the pointer points outside of the block of memory it is
intended to access, contrary to an idiom used in Numerical
Recipes in C. If a pointer points outside of an allocated block
of memory, the behavior is undefined, _even if the pointer is
not dereferenced_. (Explicit dispensation is granted, however,
for a pointer that points to the location one past an array's
last element.)
References: ANSI Rationale Sec. 3.2.2.3 p. 38.
2.11: 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);
References: ANSI Sec. 3.3.4, Rationale Sec. 3.3.2.4 p. 43.
Section 3. Expressions
3.1: Under my compiler, the code
int i = 7;
printf("%d\n", i++ * i++);
prints 49. Regardless of the order of evaluation, shouldn't it
print 56?
A: Although the postincrement and postdecrement operators ++ and --
perform the operations after yielding the former value, many
people misunderstand the implication of "after." It is _not_
guaranteed that the operation is performed immediately after
giving up the previous value and before any other part of the
expression is evaluated. It is merely guaranteed that the
update will be performed sometime before the expression is
considered "finished" (before the next "sequence point," in ANSI
C's terminology). In the example, the compiler chose to
multiply the previous value by itself and to perform both
increments afterwards.
The behavior of code which contains ambiguous or undefined side
effects (including ambiguous embedded assignments) has always
been undefined. 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.)
3.2: 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.
3.3: 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. Some
people will tell you that in the old days one form was preferred
over the other because it utilized a PDP-11 autoincrement
addressing mode, but those people are confused.
Section 4. ANSI C
4.1: What is the "ANSI C Standard?"
A: In 1983, the American National Standards Institute commissioned
a committee, X3J11, to standardize the C language. After a
long, arduous process, including several widespread public
reviews, the committee's work was finally ratified as an
American National Standard, X3.159-1989, on December 14, 1989,
and published in the spring of 1990. For the most part, ANSI C
standardizes existing practice, with a few additions from C++
(most notably function prototypes) and support for multinational
character sets (including the much-lambasted trigraph
sequences). The ANSI C standard also formalizes the C run-time
library support routines.
The published Standard includes a "Rationale," which explains
many of its decisions, and discusses a number of subtle points,
including several of those covered here. (The Rationale is "not
part of ANSI Standard X3.159-1989, but is included for
information only.")
The Standard has been adopted as an international standard,
ISO/IEC 9899:1990, although the Rationale is currently not
included.
4.2: How can I get a copy of the Standard?
A: Copies are available from
American National Standards Institute
1430 Broadway
New York, NY 10018 USA
(+1) 212 642 4900
or
Global Engineering Documents
2805 McGaw Avenue
Irvine, CA 92714 USA
(+1) 714 261 1455
(800) 854 7179 (U.S. & Canada)
The cost from ANSI is $50.00, plus $6.00 shipping. Quantity
discounts are available. (Note that ANSI derives revenues to
support its operations from the sale of printed standards, so
electronic copies are _not_ available.)
The Rationale, by itself, has been printed by Silicon Press,
ISBN 0-929306-07-4.
4.3: Does anyone have a tool for converting old-style C programs to
ANSI C, or for automatically generating prototypes?
A: Two programs, protoize and unprotoize, convert back and forth
between prototyped and "old style" function definitions and
declarations. (These programs do _not_ handle full-blown
translation between "Classic" C and ANSI C.) These programs
exist as patches to the FSF GNU C compiler, gcc. Look for the
file protoize-1.39.0 in pub/gnu at prep.ai.mit.edu (18.71.0.38),
or at several other FSF archive sites.
Several prototype generators exist, many as modifications to
lint. (See also question 16.6.)
4.4: 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.
4.5: 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
10.3.)
References: ANSI Rationale Sec. 3.5.4.1 p. 54.
4.6: My ANSI compiler complains about a mismatch when it sees
extern int func(float);
int func(x)
float x;
{...
A: You have mixed the new-style prototype declaration
"extern int func(float);" with the old-style definition
"int func(x) float x;". Old C (and ANSI C, in the absence of
prototypes) silently promotes floats to doubles when passing
them as arguments, and arranges that doubles being passed are
coerced back to floats if the formal parameters are declared
that way.
The problem can be fixed either by using new-style syntax
consistently in the definition:
int func(float x) { ... }
or by changing the new-style prototype declaration to match the
old-style definition:
extern int func(double);
(In this case, it would be clearest to change the old-style
definition to use double as well, as long as the address of that
parameter is not taken.)
Reference: ANSI Sec. 3.3.2.2 .
4.7: 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 16.8.)
References: ANSI Sec. 2.1.1.2 p. 6, Sec. 3.1 p. 19 line 37.
4.8: 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).
References: ANSI Sec. 2.1.2.2.1 pp. 7-8.
4.9: 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 4.1), 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.
4.10: 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 .
4.11: What are #pragmas and what are they good for?
A: The #pragma directive provides a single, well-defined "escape
hatch" which can be used for all sorts of implementation-
specific controls and extensions: source listing control,
structure packing, warning suppression (like the old lint
/* NOTREACHED */ comments), etc.
References: ANSI Sec. 3.8.6 .
Section 5. C Preprocessor
5.1: 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, (and it will not work if the two values are the same
variable, and the "obvious" supercompressed implementation for
integral types a^=b^=a^=b is, strictly speaking, illegal due to
multiple side-effects, and...). If the macro is intended to be
used on values of arbitrary type (the usual goal), it cannot use
a temporary, since it does not know what type of temporary it
needs, and standard C does not provide a typeof operator.
The best all-around solution is probably to forget about using a
macro, unless you don't mind passing in the type as a third
argument.
5.2: 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 4.4.)
Reference: ANSI Sec. 3.8.3.3 p. 91, Rationale pp. 66-7.
5.3: 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. (This technique also allows a value to be
"returned.")
Reference: CT&P Sec. 6.3 pp. 82-3.
5.4: How can I write a cpp macro which takes a variable number of
arguments?
A: One popular trick is to define the macro with a single argument,
and call it with a double set of parentheses, which appear to
the preprocessor to indicate a single argument:
#define DEBUG(args) {printf("DEBUG: "); printf args;}
if(n != 0) DEBUG(("n is %d\n", n));
The obvious disadvantage is that the caller must always remember
to use the extra parentheses. (It is often better to use a
bona-fide function, which can take a variable number of
arguments in a well-defined way. See questions 6.1 and 6.2
below.)
Section 6. Variable-Length Argument Lists
6.1: How can I write a function that takes a variable number of
arguments?
A: Use the
Here is a function which concatenates an arbitrary number of
strings into malloc'ed memory:
#include
#include
#include
#include
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
of size_t. You may also have to delete the (void) casts, and
use the older varargs package instead of stdarg. See the next
question for hints.
References: K&R II Sec. 7.3 p. 155, Sec. B7 p. 254; H&S
Sec. 13.4 pp. 286-9; ANSI Secs. 4.8 through 4.8.1.3 .
6.2: 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
#include
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
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 .
6.3: How can I discover how many arguments a function was actually
called with?
A: This information is not available to a portable program. Some
systems provide a nonstandard nargs() function, but its use is
questionable, since it typically returns the number of words
passed, not the number of arguments. (Floating point values and
structures are usually passed as several words.)
Any function which takes a variable number of arguments must be
able to determine from the arguments themselves how many of them
there are. printf-like functions do this by looking for
formatting specifiers (%d and the like) in the format string
(which is why these functions fail badly if the format string
does not match the argument list). Another common technique
(useful when the arguments are all of the same type) is to use a
sentinel value (often 0, -1, or an appropriately-cast null
pointer) at the end of the list (see the execl and vstrcat
examples under questions 1.2 and 6.1 above).
6.4: 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.)
Section 7. Lint
7.1: I just typed in this program, and it's acting strangely. Can
you see anything wrong with it?
A: Try running lint first. Many C compilers are really only half-
compilers, electing not to diagnose numerous source code
difficulties which would not actively preclude code generation.
7.2: 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.
7.3: 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 8. Memory Allocation
8.1: 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 16.1.)
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
char answer[100], *p;
printf("Type something:\n");
fgets(answer, 100, stdin);
if((p = strchr(answer, '\n')) != NULL)
*p = '\0';
printf("You typed \"%s\"\n", answer);
Note that this example also uses fgets instead of gets (always a
good idea), so that the size of the array can be specified, so
that fgets will not overwrite the end of the array if the user
types an overly-long line. (Unfortunately for this example,
fgets does not automatically delete the trailing \n, as gets
would.) It would also be possible to use malloc to allocate the
answer buffer, and/or to parameterize its size
(#define ANSWERSIZE 100).
8.2: 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.
Reference: CT&P Sec. 3.2 p. 32.
8.3: 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().
8.4: I have a function that returns a string, and it seems to work
fine under the debugger, 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 array.
8.5: You can't use dynamically-allocated memory after you free it,
can you?
A: No. Some early man pages for malloc stated that the contents of
freed memory was "left undisturbed;" this ill-advised guarantee
was never universal and is not required by ANSI.
Few programmers would use the contents of freed memory
deliberately, but it is easy to do so accidentally. Consider
the following (correct) code for freeing a singly-linked list:
struct list *listp, *nextp;
for(listp = base; listp != NULL; listp = nextp) {
nextp = listp->next;
free((char *)listp);
}
and notice what would happen if the more-obvious loop iteration
expression listp = listp->next were used, without the temporary
nextp pointer.
References: ANSI Rationale Sec. 4.10.3.2 p. 102; CT&P Sec. 7.10
p. 95.
8.6: 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.
8.7: So can I query the malloc package to find out how big an
allocated block is?
A: Not portably.
8.8: 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 .
8.9: 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) 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 .
8.10: What is alloca and why is its use discouraged?
A: alloca allocates memory which is automatically freed when the
function which called alloca returns. That is, memory allocated
with alloca is local to a particular function's "stack frame" or
context.
alloca cannot be written portably, and is difficult to implement
on machines without a stack. Its use is problematical (and the
obvious implementation on a stack-based machine fails) when its
return value is passed directly to another function, as in
fgets(alloca(100), 100, stdin).
For these reasons, alloca cannot be used in programs which must
be widely portable, no matter how useful it might be.
Section 9. Structures
9.1: 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 .
9.2: 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 typically 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.
Reference: ANSI Sec. 2.2.3 p. 13.
9.3: 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 16.14.
Reference: CT&P Sec. 2.3 pp. 21-2.
9.4: Why can't you compare structs?
A: There is no reasonable way for a compiler to implement struct
comparison which is consistent with C's low-level flavor. A
byte-by-byte comparison could be invalidated by random bits
present in unused "holes" in the structure (such padding is used
to keep the alignment of later fields correct). A field-by-
field comparison would require unacceptable amounts of
repetitive, in-line code for large structures.
If you want to compare two structures, you must write your own
function to do so. C++ would let you arrange for the ==
operator to map to your function.
References: K&R II Sec. 6.2 p. 129; H&S Sec. 5.6.2 p. 103; ANSI
Rationale Sec. 3.3.9 p. 47.
9.5: I came across some code that declared a structure like this:
struct name
{
int namelen;
char name[1];
};
and then did some tricky allocation to make the name array act
like it had several elements. Is this legal and/or portable?
A: This technique is popular, although Dennis Ritchie has called it
"unwarranted chumminess with the compiler." The ANSI C standard
allows it only implicitly. It seems to be portable to all known
implementations. (Compilers which check array bounds carefully
might issue warnings.)
9.6: 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
implementation is
#define offsetof(type, mem) ((size_t) \
((char *)&((type *) 0)->mem - (char *)((type *) 0)))
This implementation is not 100% portable; some compilers may
legitimately refuse to accept it.
See the next question for a usage hint.
Reference: ANSI Sec. 4.1.5 .
9.7: How can I access structure fields by name at run time?
A: Build a table of names and offsets, using the offsetof() macro.
The offset of field b in struct a is
offsetb = offsetof(struct a, b)
If structp is a pointer to an instance of this structure, and b
is an int field with offset as computed above, b's value can be
set indirectly with
*(int *)((char *)structp + offsetb) = value;
Section 10. Declarations
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 mixtures of signed and unsigned.) 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 compiled code bloating.
These rules obviously don't apply if the address of a variable
is taken and must have a particular type.
10.2: 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 fixed 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 recursive
structures.
References: K&R I Sec. 6.5 p. 101; K&R II Sec. 6.5 p. 139; H&S
Sec. 5.6.1 p. 102; ANSI Sec. 3.5.2.3 .
10.3: How do I declare an array of pointers to functions returning
pointers to functions returning pointers to characters?
A: This question can be answered in at least three ways (all
declare the hypothetical array with 5 elements):
1. char *(*(*a[5])())();
2. Build the declaration up in stages, using typedefs:
typedef char *pc; /* pointer to char */
typedef pc fpc(); /* function returning pointer to char */
typedef fpc *pfpc; /* pointer to above */
typedef pfpc fpfpc(); /* function returning... */
typedef fpfpc *pfpfpc; /* pointer to... */
pfpfpc a[5]; /* array of... */
3. Use the cdecl program, which turns English into C and vice
versa:
cdecl> declare a as array 5 of pointer to function returning
pointer to function returning pointer to char
char *(*(*a[5])())()
cdecl can also explain complicated declarations, help with
casts, and indicate which set of parentheses the arguments
go in (for complicated function definitions, like the
above).
Any good book on C should explain how to read these complicated
C declarations "inside out" to understand them ("declaration
mimics use").
Reference: H&S Sec. 5.10.1 p. 116.
10.4: So where can I get cdecl?
A: Several public-domain versions are available. One is in volume
14 of comp.sources.unix . (See question 16.6.)
Reference: K&R II Sec. 5.12 .
10.5: 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: ANSI Sec. 3.1.2.2 (esp. Rationale), Secs. 3.7,
3.7.2, Sec. F.5.11 .
10.6: 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 "(").
10.7: I've seen different methods used for calling through pointers to
functions. What's the story?
A: Originally, a pointer to a function had to be "turned into" a
"real" function, with the * operator (and an extra pair of
parentheses, to keep the precedence straight), before calling:
int r, f(), (*fp)() = f;
r = (*fp)();
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.
Section 11. Boolean Expressions and Variables
11.1: 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. Use any of
#define TRUE 1 #define YES 1
#define FALSE 0 #define NO 0
enum bool {false, true}; enum bool {no, yes};
or use raw 1 and 0, as long as you are consistent within one
program or project. (An enum may be preferable if your debugger
expands enum values when examining variables.)
Some people prefer variants like
#define TRUE (1==1)
#define FALSE (!TRUE)
or define "helper" macros such as
#define Istrue(e) ((e) != 0)
These don't buy anything (see below).
11.2: 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 (and, of course, NULL)
are used for code readability, not because the underlying values
might ever change. (See also question 1.7.)
References: K&R I Sec. 2.7 p. 41; K&R II Sec. 2.6 p. 42,
Sec. A7.4.7 p. 204, Sec. A7.9 p. 206; ANSI Secs. 3.3.3.3, 3.3.8,
3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, 3.6.5; Achilles and the
Tortoise.
11.3: What is the difference between an enum and a series of
preprocessor #defines?
A: At the present time, there is little difference. Although many
people might have wished otherwise, the ANSI standard says that
enumerations may be freely intermixed with integral types,
without errors. (If such intermixing were disallowed without
explicit casts, judicious use of enums could catch certain
programming errors.)
The primary advantages of enums are that the numeric values are
automatically assigned, and that a debugger may be able to
display the symbolic values when enum variables are examined.
(A compiler may also generate nonfatal warnings when enums and
ints are indiscriminately mixed, since doing so can still be
considered bad style even though it is not strictly illegal). A
disadvantage is that the programmer has little control over the
size (or over those nonfatal warnings).
References: K&R II Sec. 2.3 p. 39, Sec. A4.2 p. 196; H&S
Sec. 5.5 p. 100; ANSI Secs. 3.1.2.5, 3.5.2, 3.5.2.2 .
Section 12. Operating System Dependencies
12.1: 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. The delivery of characters from a
"keyboard" to a C program is a function of the operating system
in use, and cannot be standardized by the C language. Some
versions of curses have a cbreak() function which does what you
want. Under UNIX, use ioctl to play with the terminal driver
modes (CBREAK or RAW under "classic" versions; ICANON,
c_cc[VMIN] and c_cc[VTIME] under System V or Posix systems).
Under MS-DOS, use getch(). Under VMS, try the Screen Management
(SMG$) routines. 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.
12.2: 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().
12.3: 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.
12.4: 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.
12.5: 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, but there is no truly portable solution.
Section 13. Stdio
13.1: Why does errno contain ENOTTY after a call to printf?
A: Many implementations of the stdio package adjust their behavior
slightly if stdout is a terminal. To make the determination,
these implementations perform an operation which fails (with
ENOTTY) if stdout is not a terminal. Although the output
operation goes on to complete successfully, errno still contains
ENOTTY.
Reference: CT&P Sec. 5.4 p. 73.
13.2: 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 13.1.)
13.3: 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 means, not to expect a newline, but to read and
discard characters as long as each is a whitespace character.
It is usually better to use fgets() to read a whole line, and
then use sscanf() or other string functions to parse the line
buffer.
13.4: How can I recover the file name given an open file descriptor?
A: This problem is, in general, insoluble. Under Unix, for
instance, a scan of the entire disk, (perhaps requiring special
permissions) would theoretically be required, and would fail if
the file descriptor was a pipe or referred to a deleted file
(and could give a misleading answer for a file with multiple
links). It is best to remember the names of open files yourself
(perhaps with a wrapper function around fopen).
Section 14. Library Subroutines
14.1: 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," void * or char *. They must be cast 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; /* void * for ANSI C */
{
return strcmp(*(char **)p1, *(char **)p2);
}
14.2: 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 casts must be in the comparison function, which must be
declared as accepting "generic pointers" (void * or char *) as
discussed above.
14.3: Can someone tell me how to write itoa (the inverse of atoi)?
A: Just use sprintf. (You'll have to allocate space for the result
somewhere anyway; see questions 8.1 and 8.2. 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.
14.4: How can I get the 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
#include
main()
{
time_t now;
(void)time(&now);
printf("It's %.24s\n.", ctime(&now));
}
References: ANSI Sec. 4.12 .
14.5: I know that the library routine localtime will convert a time_t
into a broken-down struct tm, and that ctime will convert a
time_t to a printable string. How can I perform the inverse
operations of converting a struct tm or a string into a time_t?
A: ANSI C specifies a library routine, mktime, which converts a
struct tm to a time_t. Several public-domain versions of this
routine are available in case your compiler does not support it
yet.
Converting a string to a time_t is harder, because of the wide
variety of date and time formats which should be parsed.
Public-domain routines have been written for performing this
function (see, for example, the file partime.c, widely
distributed with the RCS package), but they are less likely to
become standardized.
References: K&R II Sec. B10 p. 256; H&S Sec. 20.4 p. 361; ANSI
Sec. 4.12.2.3 .
14.6: 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.
14.7: Each time I run my program, I get the same sequence of random
numbers.
A: You can call srand() to seed the pseudo-random number generator
with a more random value. Popular random initial seeds are the
time of day, or the elapsed time before the user presses a key.
References: ANSI Sec. 4.10.2.2 p. 154.
14.8: 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.
14.9-13: I'm trying to port this old A: These routines are variously
program. Why do I get obsolete; you should
"undefined external" errors instead:
for:
14.9: index? A: use strchr.
14.10: rindex? A: use strrchr.
14.11: bcopy? A: use memmove, after
interchanging the first and
second arguments (see also
question 4.10).
14.12: bcmp? A: use memcmp.
14.13: bzero? A: use memset, with a second
argument of 0.
Section 15. Style
15.1: Here's a neat trick:
if(!strcmp(s1, s2))
Is this good style?
A: No, although it's a common idiom. This is a classic example of
C minimalism carried to an obnoxious degree. The test succeeds
if the two strings are equal, but its form strongly suggests
that it tests for inequality.
A much better solution is to use a macro:
#define Streq(s1, s2) (strcmp(s1, s2) == 0)
15.2: 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.)
Reference: K&R Sec. 1.2 p. 10.
15.3: 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 16. Miscellaneous
16.1: 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 to zero, as if the
programmer had typed "= 0". Therefore, such variables are
initialized to the null pointer (of the correct type) if they
are pointers, and to 0.0 if they are floating-point.
Variables with "automatic" duration (i.e. local variables
without the static storage class) start out containing garbage,
unless they are explicitly initialized. Nothing useful can be
predicted about the garbage.
Dynamically-allocated memory obtained with malloc and realloc is
also likely to contain garbage, and must be initialized by the
calling program, as appropriate. Memory obtained with calloc
contains all-bits-0, but this is not necessarily useful for
pointer or floating-point values (see section 1 and question
8.9).
16.2: How can I write data files which can be read on other machines
with different word size, byte order, or floating point formats?
A: The best solution is to use text files (usually ASCII), written
with fprintf and read with fscanf or the like. (Similar advice
also applies to network protocols.) Be skeptical of arguments
which imply that text files are too big, or that reading and
writing them is too slow. Not only is their efficiency
frequently acceptable in practice, but the advantages of being
able to manipulate them with standard tools can be overwhelming.
If you must use a binary format, you can improve portability,
and perhaps take advantage of prewritten I/O libraries, by
making use of standardized formats such as Sun's XDR, OSI's
ASN.1, or CCITT's X.409 .
16.3: I seem to be missing the system header file
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).
16.4: How can I call Fortran (BASIC, Pascal, ADA, lisp) functions from
C? (And vice versa?)
A: The answer is entirely dependent on the machine and the specific
calling sequences of the various compilers in use, and may not
be possible at all. Read your compiler documentation very
carefully; sometimes there is a "mixed-language programming
guide," although the techniques for passing arguments and
ensuring correct run-time startup are often arcane.
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).
16.5: Does anyone know of a program for converting Pascal (Fortran,
lisp, "Old" C, ...) to C?
A: Several public-domain programs are available:
p2c written by Dave Gillespie, and posted to
comp.sources.unix in March, 1990 (Volume 21); also
available by anonymous ftp from csvax.cs.caltech.edu,
file pub/p2c-1.20.tar.Z .
ptoc another comp.sources.unix contribution, this one written
in Pascal (comp.sources.unix, Volume 10, also patches in
Volume 13?).
f2c jointly developed by people from Bell Labs, Bellcore,
and Carnegie Mellon. To find about f2c, send the mail
message "send index from f2c" to [email protected]
or research!netlib. (It is also available via anonymous
ftp on research.att.com, in directory dist/f2c.)
A PL/M to C converter was posted to alt.sources in April, 1991.
The following companies sell various translation tools and
services:
Cobalt Blue
2940 Union Ave., Suite C
San Jose, CA 95124 USA
(+1) 408 723 0474
Promula Development Corp.
3620 N. High St., Suite 301
Columbus, OH 43214 USA
(+1) 614 263 5454
Micro-Processor Services Inc.
92 Stone Hurst Lane
Dix Hills, NY 11746 USA
(+1) 519 499 4461
See also question 4.3.
16.6: Where can I get copies of all these public-domain programs?
A: If you have access to Usenet, see the regular postings in the
comp.sources.unix and comp.sources.misc newsgroups, which
describe, in some detail, the archiving policies and how to
retrieve copies. The usual approach is to use anonymous ftp
and/or uucp from a central, public-spirited site, such as
uunet.uu.net (192.48.96.2). However, this article cannot track
or list all of the available archive sites and how to access
them. The comp.archives newsgroup contains numerous
announcements of anonymous ftp availability of various items.
The "archie" mailserver can tell you which anonymous ftp sites
have which packages; send the mail message "help" to
[email protected] 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.
16.7: When will the next International Obfuscated C Contest (IOCCC) be
held? How can I get a copy of the current and previous winning
entries?
A: The contest typically runs from early March through mid-May. To
obtain a current copy of the rules, send email to:
{pacbell,uunet,utzoo}!hoptoad!judges or [email protected]
Contest winners are first announced at the Summer Usenix
Conference in mid-June, and posted to the net in July. Previous
winners are available on uunet (see question 16.6) under the
directory ~/pub/ioccc.
16.8: Why don't C comments nest? Are they legal inside quoted
strings?
A: Nested comments would cause more harm than good, mostly because
of the possibility of accidentally leaving comments unclosed by
including the characters "/*" within them. For this reason, it
is usually better to "comment out" large sections of code, which
might contain comments, with #ifdef or #if 0 (but see question
4.7).
The character sequences /* and */ are not special within
double-quoted strings, and do not therefore introduce comments,
because a program (particularly one which is generating C code
as output) might want to print them.
Reference: ANSI Rationale Sec. 3.1.9 p. 33.
16.9: 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).
16.10: How can I make this code more efficient?
A: Efficiency, though a favorite comp.lang.c topic, is not
important nearly as often as people tend to think it is. Most
of the code in most programs is not time-critical. When code is
not time-critical, it is far more important that it be written
clearly and portably than that it be written maximally
efficiently. (Remember that computers are very, very fast, and
that even "inefficient" code can run without apparent delay.)
It is notoriously difficult to predict what the "hot spots" in a
program will be. When efficiency is a concern, it is important
to use profiling software to determine which parts of the
program deserve attention. Often, actual computation time is
swamped by peripheral tasks such as I/O and memory allocation,
which can be sped up by using buffering and cacheing techniques.
For the small fraction of code that is time-critical, it is
vital to pick a good algorithm; it is less important to
"microoptimize" the coding details. Many of the "efficient
coding tricks" which are frequently suggested (e.g. substituting
shift operators for multiplication by powers of two) are
performed automatically by even simpleminded compilers.
Heavyhanded "optimization" attempts can make code so bulky that
performance is degraded.
For more discussion of efficiency tradeoffs, as well as good
advice on how to increase efficiency when it is important, see
chapter 7 of Kernighan and Plauger's The Elements of Programming
Style, and Jon Bentley's Writing Efficient Programs.
16.11: 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. A good compiler will generate identical code for
++i, i += 1, and i = i + 1. The reasons for using ++i or i += 1
over i = i + 1 have to do with style, not efficiency.
16.12: My floating-point calculations are acting strangely and giving
me different answers on different machines.
A: First, make sure that you have #included
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. Among
other things, the associative and distributive laws do not hold
completely (i.e. order of operation may be important, and
repeated addition is not necessarily equivalent to
multiplication). Underflow or cumulative precision loss is
often a problem.
Don't assume that floating-point results will be exact, and
especially don't assume that floating-point values can be
compared for equality. (Don't throw haphazard "fuzz factors"
in, either.)
These problems are no worse for C than they are for any other
computer language. Floating-point semantics are usually defined
as "however the processor does them;" otherwise a compiler for a
machine without the "right" model would have to do prohibitively
expensive emulations.
This article cannot begin to list the pitfalls associated with,
and workarounds appropriate for, floating-point work. A good
programming text should cover the basics.
References: K&P Sec. 6 pp. 115-8.
16.13: I'm having trouble with a Turbo C program which crashes and says
something like "floating point not loaded."
A: Some compilers for small machines, including Turbo C (and
Ritchie's original PDP-11 compiler), leave out floating point
support if it looks like it will not be needed. In particular,
the non-floating-point versions of printf and scanf save space
by not including code to handle %e, %f, and %g. It happens that
Turbo C's heuristics for determining whether the program uses
floating point are occasionally insufficient, and the programmer
must sometimes insert a dummy explicit floating-point call to
force loading of floating-point support.
In general, questions about a particular compiler are
inappropriate for comp.lang.c . Problems with PC compilers, for
instance, will find a more receptive audience in a PC newsgroup
(e.g. comp.os.msdos.programmer).
16.14: 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 9.3.)
16.15: Does anyone have a C compiler test suite I can use?
A: Plum Hall (1 Spruce Ave., Cardiff, NJ 08232, USA), among others,
sells one.
16.16: Where can I get a YACC grammar for C?
A: The definitive grammar is of course the one in the ANSI
standard. Several copies are floating around; keep your eyes
open. There is one on uunet.uu.net (192.48.96.2) in
net.sources/ansi.c.grammar.Z . The FSF's GNU C compiler
contains a grammar, as does the appendix to K&R II.
References: ANSI Sec. A.2 .
16.17: How do you pronounce "char"? What's that funny name for the "#"
character?
A: Depending on your dialect, you can pronounce the C keyword
"char" in at least three ways: like the English words "char,"
"care," or "car;" the choice is arbitrary. Bell Labs once
proposed the (now obsolete) term "octothorpe" for the "#"
character.
Trivia questions like these aren't any more pertinent for
comp.lang.c than they are for most of the other groups they
frequently come up in. You can find lots of information in the
net.announce.newusers frequently-asked questions postings, the
"jargon file" (also published as _The Hacker's Dictionary_), and
the old Usenet ASCII pronunciation list.
16.18: Where can I get extra copies of this list? What about back
issues?
A: For now, just pull it off the net; it is normally posted to
comp.lang.c on the first of each month, with an Expiration: line
which should keep it around all month. It can also be found in
the newsgroup news.answers . Several sites archive news.answers
postings and other FAQ lists, including this one; the archie
server (see question 16.6) should help you find them.
This list is an evolving document, not just a collection of this
month's interesting questions. Older copies are obsolete and
don't contain much, except the occasional typo, that the current
list doesn't.
Bibliography
ANSI American National Standard for Information Systems --
Programming Language -- C, ANSI X3.159-1989 (see question 4.2).
Jon Louis Bentley, Writing Efficient Programs, Prentice-Hall,
1982, ISBN 0-13-970244-X.
H&S Samuel P. Harbison and Guy L. Steele, C: A Reference Manual,
Second Edition, Prentice-Hall, 1987, ISBN 0-13-109802-0. (A
third edition has recently been released.)
PCS Mark R. Horton, Portable C Software, Prentice Hall, 1990, ISBN
0-13-868050-7.
K&P Brian W. Kernighan and P.J. 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.
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.
H. Rabinowitz and Chaim Schaap, Portable C, Prentice-Hall, 1990.
There is a more extensive bibliography in the revised Indian Hill style
guide (see question 15.3).
Acknowledgements
Thanks to Jamshid Afshar, Sudheer Apte, Dan Bernstein, Joe Buehler,
[email protected], Raymond Chen, Christopher Calabrese, James Davies,
Norm Diamond, Ray Dunn, Stephen M. Dunn, Bjorn Engsig, Dave Gillespie,
Ron Guilmette, Doug Gwyn, Tony Hansen, Joe Harrington, Guy Harris, Blair
Houghton, Kirk Johnson, Peter Klausler, Andrew Koenig, John Lauro,
Christopher Lott, Tim McDaniel, Evan Manning, Mark Moraes, Darren Morby,
Francois Pinard, randall@virginia, Pat Rankin, Rich Salz, Chip
Salzenberg, Paul Sand, Doug Schmidt, Patricia Shanahan, Peter da Silva,
Joshua Simons, Henry Spencer, Erik Talvola, Clarke Thatcher, Chris
Torek, Goran Uddeborg, Ed Vielmetti, Larry Virden, Freek Wiedijk, and
Dave Wolverton, who have contributed, directly or indirectly, to this
article. Special thanks to Karl Heuer, and particularly to Mark Brader,
who (to borrow a line from Steve Johnson) have goaded me beyond my
inclination, and occasionally beyond my endurance, in relentless pursuit
of a better FAQ list.
Steve Summit
[email protected]
scs%[email protected]
mit-eddie!adam!scs
This article is Copyright 1988, 1990-1992 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.
Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!
This is so awesome! 😀 I’d be cool if you could download an entire archive of this at once, though.
But one thing that puzzles me is the “mtswslnkmcjklsdlsbdmMICROSOFT” string. There is an article about it here. It is definitely worth a read: http://www.os2museum.com/wp/mtswslnk/