C interview questions and answers

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C - FAQ
The questions answered here are divided into several categories:
1. Null Pointers
2. Arrays and Pointers
3. Order of Evaluation
4. ANSI C
5. C Preprocessor
6. Variable-Length Argument Lists
7. Lint
8. Memory Allocation
9. Structures
10. Declarations
11. Boolean Expressions and Variables
12. Operating System Dependencies
13. Stdio
14. Style
15. Miscellaneous (Fortran to C converters, YACC grammars, etc.)
Section 1. Null Pointers
1. What is this infamous null pointer, anyway?
A: The language definition states that for each pointer type, there is a special value -- the "null
pointer" -- which is distinguishable from all other pointer values and which is not the address of
any object. That is the address-of operator & will never yield a null pointer, nor will a successful
call to malloc. (malloc returns a null pointer when it fails, and this is a typical use of null pointers:
as a "special" pointer value with some other meaning, usually "not allocated" or "not pointing
anywhere yet.")
A null pointer is conceptually different from an uninitialized pointer. A null pointer is known not to
point to any object; an uninitialized pointer might point anywhere. See also questions 49, 55, and
85.
As mentioned in the definition above, there is a null pointer for each pointer type, and the internal
values of null pointers for different types may be different. Although programmers need not know
the internal values, the compiler must always be informed which type of null pointer is required, so
it can make the distinction if necessary (see below).

References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S Sec. 5.3 p. 91; ANSI Sec.
3.2.2.3 p. 38.
2. How do I "get" a null pointer in my programs?
A: According to the language definition, a constant 0 in a pointer context is converted into a null
pointer at compile time. That is, in an initialization, assignment, or comparison when one side is a
variable or expression of pointer type, the compiler can tell that a constant 0 on the other side
requests a null pointer, and generate the correctly-typed null pointer value. Therefore, the following
fragments are perfectly legal:
char *p = 0;
if(p != 0)
However, an argument being passed to a function is not necessarily recognizable as a pointer
context, and the compiler may not be able to tell that an unadorned 0 "means" a null pointer. For
instance, the Unix system call "execl" takes a variable-length, null-pointer-terminated list of
character pointer arguments. To generate a null pointer in a function call context, an explicit cast is
typically required:
execl("/bin/sh", "sh", "-c", "ls", (char *)0);
If the (char *) cast were omitted, the compiler would not know to pass a null pointer, and would
pass an integer 0 instead. (Note that many Unix manuals get this example wrong.)
When function prototypes are in scope, argument passing becomes an "assignment context," and
most casts may safely be omitted, since the prototype tells the compiler that a pointer is required,
and of which type, enabling it to correctly cast unadorned 0's. Function prototypes cannot provide
the types for variable arguments in variable-length argument lists, however, so explicit casts are
still required for those arguments. It is safest always to cast null pointer function arguments, to
guard against varargs functions or those without prototypes, to allow interim use of non-ANSI
compilers, and to demonstrate that you know what you are doing.
Summary:
Unadorned 0 okay: Explicit cast required:
initialization function call,
no prototype in scope
assignment
variable argument in
comparison varargs function call
function call,
prototype in scope,
fixed argument

References: K&R I Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R II Sec. A7.10 p. 207, Sec. A7.17 p.
209; H&S Sec. 4.6.3 p. 72; ANSI Sec. 3.2.2.3.
3. What is NULL and how is it #defined?
A: As a matter of style, many people prefer not to have unadorned 0's scattered throughout their
programs. For this reason, the preprocessor macro NULL is #defined (by or ), with value 0 (or
(void *)0, about which more later). A programmer who wishes to make explicit the distinction
between 0 the integer and 0 the null pointer can then use NULL whenever a null pointer is required.
This is a stylistic convention only; the preprocessor turns NULL back to 0 which is then recognized
by the compiler (in pointer contexts) as before. In particular, a cast may still be necessary before
NULL (as before 0) in a function call argument.
(The table under question 2 above applies for NULL as well as 0.)
NULL should _only_ be used for pointers; see question 8.
References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S Sec. 13.1 p. 283; ANSI Sec.
4.1.5 p. 99, Sec. 3.2.2.3 p. 38, Rationale Sec. 4.1.5 p. 74.
4. How should NULL be #defined on a machine which uses a nonzero bit pattern as the
internal representation of a null pointer?
A: Programmers should never need to know the internal representation(s) of null pointers, because
they are normally taken care of by the compiler. If a machine uses a nonzero bit pattern for null
pointers, it is the compiler's responsibility to generate it when the programmer requests, by writing
"0" or "NULL," a null pointer. Therefore, #defining NULL as 0 on a machine for which internal
null pointers are nonzero is as valid as on any other, because the compiler must (and can) still
generate the machine's correct null pointers in response to unadorned 0's seen in pointer contexts.
5. If NULL were defined as follows:
#define NULL (char *)0
wouldn't that make function calls which pass an uncast NULL work?
A: Not in general. The problem is that there are machines, which use different internal
representations for pointers to different types of data. The suggested #definition would make uncast
NULL arguments to functions expecting pointers to characters to work correctly, but pointer
arguments to other types would still be problematical, and legal constructions such as
FILE *fp = NULL;
could fail.
Nevertheless, ANSI C allows the alternate
#define NULL (void *)0

definition for NULL. Besides helping incorrect programs to work (but only on machines with
homogeneous pointers, thus questionably valid assistance) this definition may catch programs
which use NULL incorrectly (e.g. when the ASCII NUL character was really intended).
6. I use the preprocessor macro
#define Nullptr(type) (type *)0
to help me build null pointers of the correct type.
A: This trick, though popular in some circles, does not buy much. It is not needed in assignments
and comparisons; see question 2. It does not even save keystrokes. Its use suggests to the reader
that the author is shaky on the subject of null pointers, and requires the reader to check the
#definition of the macro, its invocations, and _all_ other pointer usages much more carefully.
7. Is the abbreviated pointer comparison "if(p)" to test for non-null pointers valid? What if
the internal representation for null pointers is nonzero?
A: When C requires the boolean value of an expression (in the if, while, for, and do statements, and
with the &&, ||,!, and ?: operators), a false value is produced when the expression compares equal
to zero, and a true value otherwise. That is, whenever one writes
if(expr)
where "expr" is any expression at all, the compiler essentially acts as if it had been written as
if(expr != 0)
Substituting the trivial pointer expression "p" for "expr," we have
if(p) is equivalent to if(p != 0)
and this is a comparison context, so the compiler can tell that the (implicit) 0 is a null pointer, and
use the correct value. There is no trickery involved here; compilers do work this way, and generate
identical code for both statements. The internal representation of a pointer does _not_ matter.
The Boolean negation operator, !, can be described as follows:
!expr is essentially equivalent to expr?0:1
It is left as an exercise for the reader to show that
if(!p) is equivalent to if(p == 0)
"Abbreviations" such as if(p), though perfectly legal, are considered by some to be bad style.
See also question 71.
References: K&R II Sec. A7.4.7 p. 204; H&S Sec. 5.3 p. 91; ANSI Secs. 3.3.3.3, 3.3.9, 3.3.13,
3.3.14, 3.3.15, 3.6.4.1, and 3.6.5 .

8. If "NULL" and "0" are equivalent, which should I use?
A: Many programmers believe that "NULL" should be used in all pointer contexts, as a reminder
that the value is to be thought of as a pointer. Others feel that the confusion surrounding "NULL"
and "0" is only compounded by hiding "0" behind a #definition, and prefer to use unadorned "0"
instead. There is no one right answer.
C programmers must understand that "NULL" and "0" are interchangeable and that an uncast "0" is
perfectly acceptable in initialization, assignment, and comparison contexts. Any usage of "NULL"
(as opposed to "0") should be considered a gentle reminder that a pointer is involved; programmers
should not depend on it (either for their own understanding or the compiler's) for
distinguishing pointer 0's from integer 0's.
NULL should _not_ be used when another kind of 0 is required, even though it might work,
because doing so sends the wrong stylistic message. (ANSI allows the #definition of NULL to be
(void *)0, which will not work in non-pointer contexts.) In particular, do not use NULL when the
ASCII null character (NUL) is desired.
Provide your own definition
#define NUL '0'
if you must.
Reference: K&R II Sec. 5.4 p. 102.
9. But wouldn't it be better to use NULL (rather than 0) in case the value of NULL changes,
perhaps on a machine with nonzero null pointers?
A: No. Although symbolic constants are often used in place of numbers because the numbers might
change, this is _not_ the reason that NULL is used in place of 0. Once again, the language
guarantees that source-code 0's (in pointer contexts) generate null pointers.
NULL is used only as a stylistic convention.
10. I'm confused. NULL is guaranteed to be 0, but the null pointer is not?
A: When the term "null" or "NULL" is casually used, one of several things may be meant:
1. The conceptual null pointer, the abstract language concept defined in question 1. It is
implemented with...
2. The internal (or run-time) representation of a null pointer, which may or may not be all-bits-0
and which may be different for different pointer types. The actual values should be of concern only
to compiler writers. Authors of C programs never see them, since they use...

3. The source code syntax for null pointers, which is the single character "0". It is often hidden
behind...
4. The NULL macro, which is #defined to be "0" or "(void *)0". Finally, as a red herring, we
have...
5. The ASCII null character (NUL), which does have all bits zero, but has no relation to the null
pointer except in name.
This article always uses the phrase "null pointer" (in lower case) for sense 1, the character "0" for
sense 3, and the capitalized word "NULL" for sense 4.
11. Why is there so much confusion surrounding null pointers? Why do these questions come
up so often?
A: C programmers traditionally like to know more than they need to about the underlying machine
implementation. The fact that null pointers are represented both in source code, and internally to
most machines, as zero invites unwarranted assumptions. The use of a preprocessor macro (NULL)
suggests that the value might change later, or on some weird machine. 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 (as it does with
0's in reality), or complain when it could not. Now, in fact, in C the keyword for a null pointer is
not "nil" but "0", which works almost as well, except that an uncast "0" in a non-pointer context
generates an integer zero instead of an error message, and if that uncast 0 was supposed to be a null
pointer, the code may not work.
12. I'm still confused. I just can't understand all this null pointer stuff.
A: Follow these two simple rules:
1. When you want to refer to a null pointer in source code, use "0" or "NULL".
2. If the usage of "0" or "NULL" is an argument in a function call, cast it to the pointer type
expected by the function being called.
The rest of the discussion has to do with other people's misunderstandings, or with the internal
representation of null pointers, which you shouldn't need to know. Understand questions 1, 2, and 3
and consider 8 and 11, and you'll do fine.
13. Given all the confusion surrounding null pointers, wouldn't it be easier simply to require
them to be represented internally by zeroes?
A: If for no other reason, doing so would be ill-advised because it would unnecessarily constrain
implementations which would otherwise naturally represent null pointers by special, nonzero bit

patterns, particularly when those values would trigger automatic hardware traps for invalid
accesses.
Besides, what would this requirement really accomplish? Proper understanding of null pointers
does not require knowledge of the internal representation, whether zero or nonzero. Assuming that
null pointers are internally zero does not make any code easier to write (except for a certain ill-
advised usage of calloc; see question 55). Known-zero internal pointers would not obviate casts in
function calls, because the _size_ of the pointer might still be different from that of an int. (If "nil"
were used to request null pointers rather than "0," as mentioned in question 11, the urge to assume
an internal zero representation would not even arise.)
14. Seriously, have any actual machines really used nonzero null pointers?
A: "Certain Prime computers use a value different from all-bits-0 to encode the null pointer. Also,
some large Honeywell-Bull machines use the bit pattern 06000 to encode the null pointer."
-- Portable C, by H. Rabinowitz and Chaim Schaap, Prentice-Hall, 1990, page 147.
The "certain Prime computers" were the segmented 50 series, which used segment 07777, offset 0
for the null pointer, at least for PL/I. Later models used segment 0, offset 0 for null pointers in C,
necessitating new instructions such as TCNP (Test C Null Pointer), evidently as a sop to all the
extant poorly-written C code which made incorrect assumptions.
The Symbolics Lisp Machine, a tagged architecture, does not even have conventional numeric
pointers; it uses the pair (basically a nonexistent handle) as a C null pointer.
Section 2. Arrays and Pointers
15. I had the definition char x[6] in one source file, and in another I declared extern char *x.
Why didn't it work?
A: The declaration extern char *x simply does not match the actual definition. The type "pointer-to-
type-T" is not the same as "array-of-type-T." Use extern char x[].
References: CT&P Sec. 3.3 pp. 33-4, Sec. 4.5 pp. 64-5.
16. But I heard that char x[] was identical to char *x.
A: Not at all. (What you heard has to do with formal parameters to functions; see question 19.)
Arrays are not pointers. The 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 |
+-----+ +---+---+---+---+---+---+
17. You mean that a reference like x[3] generates different code depending on whether x is an
array or a pointer?
A: Precisely. Referring back to the sample declarations in the previous question, when the compiler
sees the expression a[3], it emits code to start at the location "a," move three past it, and fetch the
character there. When it sees the expression p[3], it emits code to start at the location "p," fetch the
pointer value there, add three to the pointer, and finally fetch the character pointed to. In the
example above, both a[3] and p[3] happen to be the character 'l', but the compiler gets there
differently. (See also question 98.)
18. So what is meant by the "equivalence of pointers and arrays" in C?
A: Much of the confusion surrounding pointers in C can be traced to a misunderstanding of this
statement. Saying that arrays and pointers are "equivalent" does not by any means imply that they
are interchangeable.
"Equivalence" refers to the following key definition:
An lvalue of type array-of-T which appears in an expression decays (with three exceptions) into a
pointer to its first element; the type of the resultant pointer is pointer-to-T.
(The exceptions are when the array is the operand of the sizeof() operator or of the & operator, or is
a literal string initializer for a character array.)
As a consequence of this definition, there is not really any difference in the behavior of the "array
subscripting" operator [] as it applies to arrays and pointers. In an expression of the form a[i], the
array reference "a" decays into a pointer, following the rule above, and is then subscripted exactly
as would be a pointer variable in the expression p[i]. In either case, the expression
x[i] (where x is an array or a pointer) is, by definition, exactly equivalent to *((x)+(i)).
References: K&R I Sec. 5.3 pp. 93-6; K&R II Sec. 5.3 p. 99; H&S Sec. 5.4.1 p. 93; ANSI Sec.
3.3.2.1, Sec. 3.3.6 .

19. Then why are array and pointer declarations interchangeable as function formal
parameters?
A: Since arrays decay immediately into pointers, an array is never actually passed to a function.
Therefore, any parameter declarations which "look like" arrays, e.g.
f(a)
char a[];
are treated by the compiler as if they were pointers, since that is what the function will receive if an
array is passed:
f(a)
char *a;
Th1is conversion holds only within function formal parameter declarations, nowhere else. If this
conversion bothers you, avoid it; many people have concluded that the confusion it causes
outweighs the small advantage of having the declaration "look like" the call and/or the uses within
the function.
References: K&R I Sec. 5.3 p. 95, Sec. A10.1 p. 205; K&R II Sec. 5.3 p. 100, Sec. A8.6.3 p. 218,
Sec. A10.1 p. 226; H&S Sec. 5.4.3 p. 96; ANSI Sec. 3.5.4.3, Sec. 3.7.1, CT&P Sec. 3.3 pp. 33-4.
20. Someone explained to me that arrays were really just constant pointers.
A: That person did you a disservice. An array name is "constant" in that it cannot be assigned to,
but an array is _not_ a pointer, as the discussion and pictures in question 16 should make clear.
21. I came across some "joke" code containing the "expression" 5["abcdef"] . How can this
be legal C?
A: Yes, Virginia, array subscripting is commutative in C. This curious fact follows from the pointer
definition of array subscripting, namely that a[e] is exactly equivalent to *((a)+(e)), for _any_
expression e and primary expression a, as long as one of them is a pointer expression and one is
integral. This unsuspected commutativity is often mentioned in C texts as if it were something
to be proud of, but it finds no useful application outside of the Obfuscated C Contest (see question
95).
22. My compiler complained when I passed a two-dimensional array to a routine expecting a
pointer to a pointer.
A: The rule by which arrays decay into pointers is not applied recursively. An array of arrays (i.e. a
two-dimensional array in C) decays into a pointer to an array, not a pointer to a pointer. Pointers to
arrays can be confusing, and must be treated carefully. (The confusion is heightened by the
existence of incorrect compilers, including some versions of pcc and pcc-derived lint's, which

improperly accept assignments of multi-dimensional arrays to multi-level pointers.) If you are
passing a two-dimensional array to a function:
int array[YSIZE][XSIZE];
f(array);
the function's declaration should match:
f(int a[][XSIZE]) {...}
or
f(int (*ap)[XSIZE]) {...} /* ap is a pointer to an array */
In the first declaration, the compiler performs the usual implicit parameter rewriting of "array of
array" to "pointer to array;" in the second form the pointer declaration is explicit. Since the called
function does not allocate space for the array, it does not need to know the overall size, so the
number of "rows," YSIZE, can be omitted. The "shape" of the array is still important, so the
"column" dimension XSIZE (and, for 3- or more dimensional arrays, the intervening ones) must be
included.
If a function is already declared as accepting a pointer to a pointer, it is probably incorrect to pass a
two-dimensional array directly to it.
23. How do I declare a pointer to an array?
A: Usually, you don't want to. Consider using a pointer to one of the array's elements instead.
Arrays of type T decay into pointers to type T, which is convenient; subscripting or incrementing
the resultant pointer access the individual members of the array. True pointers to arrays, when
subscripted or incremented, step over entire arrays, and are generally only useful when operating on
multidimensional arrays, if at all. (See question 22 above.) When people speak casually of a pointer
to an array, they usually mean a pointer to its first element.
If you really need to declare a pointer to an entire array, use something like "int (*ap)[N];" where N
is the size of the array. (See also question 66.) If the size of the array is unknown, N can be omitted,
but the resulting type, "pointer to array of unknown size," is useless.
24. How can I dynamically allocate a multidimensional array?
A: It is usually best to allocate an array of pointers, and then initialize each pointer to a dynamically
allocated "row." The resulting "ragged" array can save space, although it is not necessarily
contiguous in memory as a real array would be. Here is a two-dimensional example:
int **array = (int **)malloc(nrows * sizeof(int *));
for(i = 0; i < nrows; i++)
array[i] = (int *)malloc(ncolumns * sizeof(int));
(In "real" code, of course, malloc should be declared correctly, and each return value checked.)

You can keep the array's contents contiguous, while making later reallocation of individual rows
difficult, with a bit of explicit pointer arithmetic:
int **array = (int **)malloc(nrows * sizeof(int *));
array[0] = (int *)malloc(nrows * ncolumns * sizeof(int));
for(i = 1; i < nrows; i++)
array[i] = array[0] + i * ncolumns;
In either case, the elements of the dynamic array can be accessed with normal-looking array
subscripts: array[i][j].
If the double indirection implied by the above schemes is for some reason unacceptable, you can
simulate a two-dimensional array with a single, dynamically allocated one-dimensional array:
int *array = (int *)malloc(nrows * ncolumns * sizeof(int));
However, you must now perform subscript calculations manually, accessing the i,jth element with
array[i * ncolumns + j]. (A macro can hide the explicit calculation, but invoking it then requires
parentheses and commas, which don't look exactly like multidimensional array subscripts.)
25. 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 was an anomaly in certain versions of pcc that expressions
such as the above were ever accepted.) Say what you mean: use
p = (char *)((int *)p + 1);
Section 3. Order of Evaluation
26. Under my compiler, the code
int i = 7;
printf("%dn", i++ * i++);
prints 49. Regardless of the order of evaluation, shouldn't it print 56?
A: Although the postincrement and postdecrement operators ++ and -- perform the operations after
yielding the former value, many people misunderstand the implication of "after." It is _not_
guaranteed that the operation is performed immediately after giving up the previous value and
before any other part of the expression is evaluated. It is merely guaranteed that the update will be
performed sometime before the expression is considered "finished" (before the next "sequence

point," in ANSI C's terminology). In the example, the compiler chose to multiply the previous
value by itself and to perform both increments afterwards.
The behavior of code, which contains ambiguous or undefined side effects (including ambiguous
embedded assignments), has always been undefined. (Note, too, that a compiler's choice, especially
under ANSI rules, for "undefined behavior" may be to refuse to compile the code.) Don't even try
to find out how your compiler implements such things (contrary to the ill-advised exercises in
many C textbooks); as K&R wisely point out, "if you don't know _how_ they are done on various
machines, that innocence may help to protect you."
References: K&R I Sec. 2.12 p. 50; K&R II Sec. 2.12 p. 54; ANSI Sec. 3.3 p. 39; CT&P Sec. 3.7 p.
47; PCS Sec. 9.5 pp. 120-1. (Ignore H&S Sec. 7.12 pp. 190-1, which is obsolete.)
27. But what about the &&, ||, and comma operators? I see code like "if((c = getchar()) ==
EOF || c == 'n')" ...
A: There is a special exception for those operators, (as well as ?: ); each of them does imply a
sequence point (i.e. left-to-right evaluation is guaranteed). Any book on C should make this clear.
References: K&R I Sec. 2.6 p. 38, Secs. A7.11-12 pp. 190-1; K&R II Sec. 2.6 p. 41, Secs. A7.14-
15 pp. 207-8; ANSI Secs. 3.3.13 p. 52, 3.3.14 p. 52, 3.3.15 p. 53, 3.3.17 p. 55, CT&P Sec. 3.7 pp.
46-7.
Section 4. ANSI C
28. 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.
29. 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.)
Silicon Press, ISBN 0-929306-07-4, has printed the Rationale, by itself.
30. 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 94.)
31. 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 66.)
32. 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 .
33. 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 96.)
References: ANSI Sec. 2.1.1.2 p. 6, Sec. 3.1 p. 19 line 37.
34. 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 a "notreached" directive, if available).
References: ANSI Sec. 2.1.2.2.1 pp. 7-8.
35. 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 those 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 28), 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.
36. What was noalias and what ever happened to it?
A: noalias was another type qualifier, in the same syntactic class as const and volatile, which was
intended to assert that the object pointed to was not also pointed to ("aliased") by other pointers.
It was phenomenally difficult to define precisely and explain coherently, and sparked widespread,
acrimonious debate. Because of the criticism and the difficulty of defining noalias well, the
Committee wisely declined to adopt it, in spite of its superficial attractions.
References: ANSI Sec. 3.9.6.
37. 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
38. 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.
39. I have some old code that tries to construct identifiers with a macro like
#define Paste(a, b) a/**/b
but it doesn't work any more.
A: That comments disappeared entirely and could therefore be used for token pasting was an
undocumented feature of some early preprocessor implementations, notably Reiser's. ANSI affirms

(as did K&R) that comments are replaced with white space. However, since the need for pasting
tokens was demonstrated and real, ANSI introduced a well-defined token-pasting operator, ##,
which can be used like this:
#define Paste(a, b) a##b
Reference: ANSI Sec. 3.8.3.3 p. 91, Rationale pp. 66-7.
40. 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.
41. 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 %dn", n));
The obvious disadvantage is that the caller must always remember to use the extra parentheses. (It
is often best to use a bona-fide function, which can take a variable number of arguments in a well-
defined way, rather than a macro. See questions 42 and 43 below.)
Section 6. Variable-Length Argument Lists

42. How can I write a function that takes a variable number of arguments?
A: Use the header (or, if you must, the older).
Here is a function, which concatenates an arbitrary number of strings into malloc'ed memory:
#include /* for NULL, size_t */
#include /* for va_ stuff */
#include /* for strcat et al */
#include /* for malloc */
char *vstrcat(char *first, ...)
{
size_t len = 0;
char *retbuf;
va_list argp;
char *p;
if(first == NULL)
return NULL;
len = strlen(first);
va_start(argp, first);
while((p = va_arg(argp, char *)) != NULL)
len += strlen(p);
va_end(argp);
retbuf = malloc(len + 1); /* +1 for trailing 0 */
if(retbuf == NULL)
return NULL; /* error */
(void)strcpy(retbuf, first);
va_start(argp, first);
while((p = va_arg(argp, char *)) != NULL)
(void)strcat(retbuf, p);
va_end(argp);
return retbuf;

}
Usage is something like
char *str = vstrcat("Hello, ", "world!", (char *)NULL);
Note the cast on the last argument. (Also note that the caller must free the returned, malloc'ed
storage.)
Under a pre-ANSI compiler, rewrite the function definition without a prototype ("char
*vstrcat(first) char *first; {"), include rather than , replace "#include " with "extern char
*malloc();", and use int instead of size_t. You may also have to delete the (void) casts, and use the
older varargs package instead of stdarg. See the next question for hints.
References: K&R II Sec. 7.3 p. 155, Sec. B7 p. 254; H&S Sec. 13.4 pp. 286-9; ANSI Secs. 4.8
through 4.8.1.3.
43. 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 package, instead of, 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.
44. How can I discover how many arguments a function was actually called with?
A: This information is not available to a portable program. Some systems provide a nonstandard
nargs() function, but its use is questionable, since it typically returns the number of words pushed,
not the number of arguments. (Floating point values and structures are usually passed as several
words.)
Any function which takes a variable number of arguments must be able to determine from the
arguments themselves how many of them there are. printf-like functions do this by looking for
formatting specifiers (%d and the like) in the format string (which is why these functions fail badly
if the format string does not match the argument list). Another common technique (useful when the
arguments are all of the same type) is to use a sentinel value (often 0, -1, or an appropriately-cast
null pointer) at the end of the list (see the execl and vstrcat examples under questions 2 and
42 above).
45. 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
46. 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.
47. 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.
48. 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
49. 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 85.)
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).
50. 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.
51. But the man page for strcat says that it takes two char *'s as arguments. How am I
supposed to know to allocate things?
A: In general, when using pointers you _always_ have to consider memory allocation, at least to
make sure that the compiler is doing it for you. If a library routine's documentation does not
explicitly mention allocation, it is usually the caller's problem.
The Synopsis section at the top of a Unix-style man page can be misleading. The code fragments
presented there are closer to the function definition used by the call's implementor than the

invocation used by the caller. In particular, many routines which accept pointers (e.g. to structs or
strings), are usually called with the address of some object (a struct, or an array – see questions 18
and 19.) Another common example is stat().
52. 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.
53. 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.
54. 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 .
55. What is the difference between calloc and malloc? Is it safe to use calloc's zero-fill
guarantee for pointer and floating-point values? Does free work on memory allocated with
calloc, or do you need a cfree?
A: calloc(m, n) is essentially equivalent to
p = malloc(m * n);

memset(p, 0, m * n);
The zero fill is all-bits-zero, and does not therefore guarantee useful zero values for pointers (see
questions 1-14) or floating- point values. free can (and should) be used to free the memory
allocated by calloc.
References: ANSI Secs. 4.10.3 to 4.10.3.2.
56. 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
57. 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 compilers 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.
58. How does struct passing and returning work?
A: When structures are passed as arguments to functions, the entire struct is typically pushed on the
stack, using as many words as are required. (Pointers to structures are often chosen precisely to
avoid this overhead.)
Structures are typically returned from functions in a location pointed to by an extra, compiler-
supplied "hidden" argument to the function. Older compilers often used a special, static location for

structure returns, although this made struct-valued functions nonreentrant, which ANSI C
disallows.
Reference: ANSI Sec. 2.2.3 p. 13.
59. 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 return a struct list. (The
connection is hard to see because of the intervening comment.) Since struct-valued functions are
usually implemented by adding a hidden return pointer, the generated code for main() actually
expects three arguments, although only two were passed (in this case, by the C start-up code). See
also question 101.
Reference: CT&P Sec. 2.3 pp. 21-2.
60. 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.
61. 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.)
62. 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 . If you don't have it,
a suggested implementation is
#define offsetof(type, mem) ((size_t)
((char *)&((type *) 0)->mem - (char *)((type *) 0)))
This implementation is not 100% portable; some compilers may legitimately refuse to accept it.
See the next question for a usage hint.
Reference: ANSI Sec. 4.1.5.
63. 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
64. 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. Exceptions apply if the address
of a variable is taken and must have a particular type.
Although char or unsigned char can be used as a "tiny" int type, doing so is often more trouble than
it is worth.
65. 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 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.
66. How do I declare an array of pointers to functions returning pointers to functions
returning pointers to characters?
A: This question can be answered in at least three ways (all assume the hypothetical array is to have
5 elements):

1. char *(*(*a[5])())();
2. Build the declaration up in stages, using typedefs:
typedef char *pc; /* pointer to char */
typedef pc fpc(); /* function returning pointer to char */
typedef fpc *pfpc; /* pointer to above */
typedef pfpc fpfpc(); /* function returning... */
typedef fpfpc *pfpfpc; /* pointer to... */
pfpfpc a[5]; /* array of... */
3. Use the cdecl program, which turns English into C and vice versa:
cdecl> declare a as array 5 of pointer to function returning pointer to function returning pointer to
char
char *(*(*a[5])())()
cdecl can also explain complicated declarations, help with casts, and indicate which set of
parentheses the arguments go in (for complicated function definitions, like the above).
Any good book on C should explain how to read these complicated C declarations "inside out" to
understand them ("declaration mimics use").
Reference: H&S Sec. 5.10.1 p. 116.
67. So where can I get cdecl?
A: Several public-domain versions are available. One is in volume 14 of comp.sources.unix . (See
question 94.)
Reference: K&R II Sec. 5.12 .
68. 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 "(").
69. I've seen different methods used for calling through pointers to functions. What's the
story?
A: Originally, a pointer to a function had to be "turned into" a "real" function, with the * operator
(and an extra pair of parentheses, to keep the precedence straight), before calling:
int r, f(), (*fp)() = f;
r = (*fp)();
Another analysis holds that functions are always called through pointers, but that "real" functions
decay implicitly into pointers (in expressions, as they do in initializations) and so cause no
trouble. This reasoning, which was adopted in the ANSI standard, means that
r = fp();
is legal and works correctly, whether fp is a function or a pointer to one. (The usage has always
been unambiguous; there is nothing you ever could have done with a function pointer followed by
an argument list except call through it.) An explicit * is harmless, and still allowed (and
recommended, if portability to older compilers is important).
References: ANSI Sec. 3.3.2.2 p. 41, Rationale p. 41.
Section 11. Boolean Expressions and Variables
70. 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).
71. Isn't #defining TRUE to be 1 dangerous, since any nonzero value is considered "true" in
C? What if a built-in boolean or relational operator "returns" something other than 1?
A: It is true (sic) that any nonzero value is considered true in C, but this applies only "on input", i.e.
where a boolean value is expected. When a boolean value is generated by a built-in operator, it is
guaranteed to be 1 or 0. Therefore, the test
if((a == b) == TRUE)
will work as expected (as long as TRUE is 1), but it is obviously silly. In general, explicit tests
against TRUE and FALSE are undesirable, because some library functions (notably isupper,
isalpha, etc.) return, on success, a nonzero value which is _not_ necessarily 1. (Besides, if you
believe that "if((a == b) == TRUE)" is an improvement over "if(a == b)", why stop there? Why not
use "if(((a == b) == TRUE) == TRUE)"?) A good rule of thumb is to use TRUE and FALSE (or
the like) only for assignment to a Boolean variable, or as the return value from a Boolean function,
never in a comparison.
The preprocessor macros TRUE and FALSE (and, of course, NULL) are used for code readability,
not because the underlying values might ever change. That "true" is 1 and "false" 0 is guaranteed
by the language. (See also question 7.)
References: K&R I Sec. 2.7 p. 41; K&R II Sec. 2.6 p. 42, Sec. A7.4.7 p. 204, Sec. A7.9 p. 206;
ANSI Secs. 3.3.3.3, 3.3.8,3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, 3.6.5; Achilles and the Tortoise.
72. 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
73. 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 question postings in such groups as comp.unix.questions and
comp.sys.ibm.pc.misc . Note that the answers are often not unique even across different variants of
a system. Bear in mind when answering system-specific questions that the answer that applies to
your system may not apply to everyone else's.
References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1.
74. 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().
75. 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.

76. 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.
77. 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
78. 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.
79. 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 78.)
80. 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 fgets() to read a whole line, and then use sscanf() or other string functions to
parse the line buffer.
81. How can I recover the file name given an open file descriptor?

A: This problem is, in general, insoluble. Under Unix, for instance, a scan of the entire disk,
(perhaps requiring special permissions) would theoretically be required, and would fail if the file
descriptor was a pipe or referred to a deleted file (and could give a misleading answer for a file
with multiple links). It is best to remember the names of open files yourself (perhaps with a
wrapper function around fopen).
Section 14. Style
82. Here's a neat trick:
if(!strcmp(s1, s2))
Is this good style?
A: No. This is a classic example of C minimalism carried to an obnoxious degree. The test
succeeds if the two strings are equal, but its form strongly suggests that it tests for inequality.
A much better solution is to use a macro:
#define Streq(s1, s2) (strcmp(s1, s2) == 0)
83. 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.
84. Where can I get the "Indian Hill Style Guide" and other coding standards?
A: Various documents are available for anonymous ftp from:
Site: File or directory:
cs.washington.edu ~ftp/pub/cstyle.tar.Z
(128.95.1.4) (the updated Indian Hill guide)

cs.toronto.edu doc/programming
giza.cis.ohio-state.edu pub/style-guide
Section 15. Miscellaneous
85. What can I safely assume about the initial values of variables which are not explicitly
initialized? If global variables start out as "zero," is that good enough for null pointers and
floating-point zeroes?
A: Variables with "static" duration (that is, those declared outside of functions, and those declared
with the storage class static), are guaranteed initialized to zero, as if the programmer had typed
"= 0". Therefore, such variables are initialized to the null pointer (of the correct type) if they are
pointers, and to 0.0 if they are floating-point.
Variables with "automatic" duration (i.e. local variables without the static storage class) start out
containing garbage, unless they are explicitly initialized. Nothing useful can be predicted about the
garbage.
Dynamically allocated memory obtained with malloc and realloc is also likely to contain garbage,
and must be initialized by the calling program, as appropriate. Memory obtained with calloc
contains all-bits-0, but this is not necessarily useful for pointer or floating-point values (see
question 55).
86. 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. The 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);
}
87. Now I'm trying to sort an array of structures with qsort. My comparison routine takes
pointers to structures, but the compiler complains it's 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 *).
88. 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
49 and 50. 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.
89. 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 .
90. 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 .
91. I seem to be missing the system header file . Can someone send me a copy?
A: Standard headers exist in part so that definitions appropriate to your compiler, operating system,
and processor can be supplied. You cannot just pick up a copy of someone else's header file and

expect it to work, unless that person is using exactly the same environment. Ask your compiler
vendor why the file was not provided (or to send a replacement copy).
92. How can I call Fortran (BASIC, Pascal, ADA and 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.
93. Does anyone know of a program for converting Pascal (Fortran, lisp, "Old" C, ...) to C?
A: Several public-domain programs are available:
p2c written by Dave Gillespie, and posted to comp.sources.unix in March, 1990 (Volume 21).
ptoc another comp.sources.unix contribution, this one written in Pascal (comp.sources.unix,
Volume 10, also patches in Volume13?).
f2c jointly developed by people from Bell Labs, Bellcore, and Carnegie Mellon. To find about f2c,
send the mail message "send index from f2c" to netlib@research.att.com or research!netlib. (It is
also available via anonymous ftp on research.att.com, in directory dist/f2c.)
A PL/M to C converter was posted to alt.sources in April, 1991.
The following companies sell various translation tools and services:
Cobalt Blue
2940 Union Ave., Suite C
San Jose, CA 95124 USA
(+1) 408 723 0474
Promula Development Corp.
3620 N. High St., Suite 301
Columbus, OH 43214 USA
(+1) 614 263 5454
Micro-Processor Services Inc
92 Stone Hurst Lane
Dix Hills, NY 11746 USA
(+1) 519 499 4461
See also question 30.

94. Where can I get copies of all these public-domain programs?
A: If you have access to Usenet, see the regular postings in the comp.sources.unix and
comp.sources.misc newsgroups, which describe, in some detail, the archiving policies and how to
retrieve copies. The usual approach is to use anonymous ftp and/or uucp from a central, public-
spirited site, such as uunet.uu.net (192.48.96.2). However, this article cannot track or list all of the
available archive sites and how to access them. The comp.archives newsgroup contains numerous
announcements of anonymous ftp availability of various items. The "archie" mailserver can tell you
which anonymous ftp sites have which packages; send the mail message "help" to
archie@quiche.cs.mcgill.ca for information.
95. When will the next International Obfuscated C Contest (IOCCC) be held? How can I get
a copy of the current and previous winning entries?
A: The contest typically runs from early March through mid-May. To obtain a current copy of the
rules, send email to:
{pacbell,uunet,utzoo}!hoptoad!judges or judges@toad.com
Contest winners are first announced at the Summer Usenix Conference in mid-June, and posted to
the net in July. Previous winners are available on uunet (see question 94) under the directory
~/pub/ioccc.
96. 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 33).
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.
97. 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.
98. Are pointers really faster than arrays? Do function calls really slow things down? Is ++i
faster than i = i + 1?
A: Precise answers to these and many similar questions depend of course on the processor and
compiler in use. If you simply must know, you'll have to time test programs carefully. (Often the
differences are so slight that hundreds of thousands of iterations are required even to see them.
Check the compiler's assembly language output, if available, to see if two purported alternatives
aren't compiled identically.)
It is "usually" faster to march through large arrays with pointers rather than array subscripts, but for
some processors the reverse is true.
Function calls, though obviously incrementally slower than in-line code, contribute so much to
modularity and code clarity that there is rarely good reason to avoid them.
Before rearranging expressions such as i = i + 1, remember that you are dealing with a C compiler,
not a keystroke-programmable calculator. A good compiler will generate identical code for ++i,
i += 1, and i = i + 1. The reasons for using ++i or i += 1 over i = i + 1 have to do with style, not
efficiency.
99. My floating-point calculations are acting strangely and giving me different answers on
different machines.
A: Most digital computers use floating-point formats which provide a close but by no means exact
simulation of real number arithmetic. Among other things, the associative and distributive laws do
not hold completely (i.e. order of operation may be important, repeated addition is not necessarily
equivalent to multiplication).

Underflow or cumulative precision loss is often a problem.
Don't assume that floating-point results will be exact, and especially don't assume that floating-
point values can be compared for equality. (Don't throw haphazard "fuzz factors" in, either.)
These problems are no worse for C than they are for any other computer language. Floating-point
semantics are usually defined as "however the processor does them;" otherwise a compiler for a
machine without the "right" model would have to do prohibitively expensive emulations.
This article cannot begin to list the pitfalls associated with, and workarounds appropriate for,
floating-point work. A good programming text should cover the basics. Do make sure that you
have #included , and correctly declared other functions returning double.
References: K&P Sec. 6 pp. 115-8.
100. 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).
101. 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 59.)
102. 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.

103. 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 .
104. How do you pronounce "char"? What's that funny name for the "#" character?
A: You can pronounce the C keyword "char" like the English words "char," "care," or "car;" the
choice is arbitrary. Bell Labs once proposed the (now obsolete) term "octothorpe" for the "#"
character.
Trivia questions like these aren't any more pertinent for comp.lang.c than they are for any of the
other groups they frequently come up in. You can find lots of information in the
net.announce.newusers frequently asked questions postings, the "jargon file" (also published as
_The Hacker's Dictionary_), and the Usenet ASCII pronunciation list.
105. Where can I get extra copies of this list? What about back issues?
A: For now, just pull it off the net; it is normally posted to comp.lang.c on the first of each month,
with an Expiration: line which should keep it around all month. Eventually, it may be available for
anonymous ftp, or via a mailserver.
This list is an evolving document, not just a collection of this month's interesting questions. Older
copies are obsolete and don't contain much, except the occasional typo, that the current list
doesn't.
Bibliography
ANSI American National Standard for Information Systems -- Programming Language -- C, ANSI
X3.159-1989 (see question 29).
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.
There is a more extensive bibliography in the revised Indian Hill style guide (see question 84).
Acknowledgements
Thanks to Jamshid Afshar, Sudheer Apte, Dan Bernstein, Joe Buehler, Raymond Chen, Christopher
Calabrese, James Davies, Norm Diamond, Ray Dunn, Stephen M. Dunn, Bjorn Engsig, Ron
Guilmette, Doug Gwyn, Tony Hansen, Joe Harrington, Guy Harris, Blair Houghton, Kirk Johnson,
Andrew Koenig, John Lauro, Christopher Lott, Tim McDaniel, Evan Manning, Mark Moraes,
Francois Pinard, randall@virginia, Pat Rankin, Rich Salz, Chip Salzenberg, Paul Sand, Doug
Schmidt, Patricia Shanahan, Peter da Silva, Joshua Simons, Henry Spencer, Erik Talvola, Clarke
Thatcher, Chris Torek, Ed Vielmetti, Larry Virden, Freek Wiedijk, and Dave Wolverton, who have
contributed, directly or indirectly, to this article. Special thanks to Karl Heuer, and particularly to
Mark Brader, who (to borrow a line from Steve Johnson) have goaded me beyond my inclination,
and frequently beyond my endurance, in relentless pursuit of a better FAQ list.
Steve Summit
scs@adam.mit.edu
scs%adam.mit.edu@mit.edu
mit-eddie!adam!scs
This article is Copyright 1988, 1990, 1991 by Steve Summit. It may be freely redistributed so long
as the author's name, and this notice, are retained. The C code in this article (vstrcat, error, etc.) is
public domain and may be used without restriction.

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