7/17/08

C FAQ's

Question 1.1
How do you decide which integer type to use?
If you might need large values (above 32,767 or below -32,767), use long. Otherwise, if space is
very important (i.e. if there are large arrays or many structures), use short. Otherwise, use int. If well-defined overflow characteristics are important and negative values are not, or if you want to steer clear of sign-extension problems when manipulating bits or bytes, use one of the
corresponding unsigned types. (Beware when mixing signed and unsigned values in expressions,
though.)
Although character types (especially unsigned char) can be used as ``tiny'' integers, doing so is
sometimes more trouble than it's worth, due to unpredictable sign extension and increased code
size. (Using unsigned char can help; see question 12.1 for a related problem.)
A similar space/time tradeoff applies when deciding between float and double. None of the above
rules apply if the address of a variable is taken and must have a particular type.
If for some reason you need to declare something with an exact size (usually the only good reason for doing so is when attempting to conform to some externally-imposed storage layout, but see question 20.5), be sure to encapsulate the choice behind an appropriate typedef.
References: K&R1 Sec. 2.2 p. 34
K&R2 Sec. 2.2 p. 36, Sec. A4.2 pp. 195-6, Sec. B11 p. 257
ANSI Sec. 2.2.4.2.1, Sec. 3.1.2.5
ISO Sec. 5.2.4.2.1, Sec. 6.1.2.5
H&S Secs. 5.1,5.2 pp. 110-114

Question 1.4
What should the 64-bit type on new, 64-bit machines be?
Some vendors of C products for 64-bit machines support 64-bit long ints. Others fear that too
much existing code is written to assume that ints and longs are the same size, or that one or the
other of them is exactly 32 bits, and introduce a new, nonstandard, 64-bit long long (or
__longlong) type instead.
Programmers interested in writing portable code should therefore insulate their 64-bit type needs behind appropriate typedefs. Vendors who feel compelled to introduce a new, longer integral type should advertise it as being ``at least 64 bits'' (which is truly new, a type traditional C does not have), and not ``exactly 64 bits.''
References: ANSI Sec. F.5.6
ISO Sec. G.5.6

Question 1.7
What's the best way to declare and define global variables?
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.) The best arrangement is to place each definition in some relevant .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 same header file, so that the compiler can
check that the definition matches the declarations.
This rule promotes a high degree of portability: it is consistent with the requirements of the ANSI C Standard, and is also consistent with most pre-ANSI compilers and linkers. (Unix compilers and linkers typically use a ``common model'' which allows multiple definitions, as long as at most one is initialized; this behavior is mentioned as a ``common extension'' by the ANSI Standard, no pun intended. 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 a line like
DEFINE(int, i);
need only be entered once in one header file, and turned into a definition or a declaration
depending on the setting of some macro, but it's not clear if this is worth the trouble.
It's especially important to put global declarations in header files if you want the compiler to
catch inconsistent declarations for you. In particular, never place a prototype for an external
function in a .c file: it wouldn't generally be checked for consistency with the definition, and an
incompatible prototype is worse than useless.
See also questions 10.6 and 18.8.
References: K&R1 Sec. 4.5 pp. 76-7
K&R2 Sec. 4.4 pp. 80-1
ANSI Sec. 3.1.2.2, Sec. 3.7, Sec. 3.7.2, Sec. F.5.11
ISO Sec. 6.1.2.2, Sec. 6.7, Sec. 6.7.2, Sec. G.5.11
Rationale Sec. 3.1.2.2
H&S Sec. 4.8 pp. 101-104, Sec. 9.2.3 p. 267
CT&P Sec. 4.2 pp. 54-56

Question 1.11
What does extern mean in a function declaration?
It can be used as a stylistic hint to indicate that the function's definition is probably in another
source file, but there is no formal difference between
extern int f();
and
int f();
References: ANSI Sec. 3.1.2.2, Sec. 3.5.1
ISO Sec. 6.1.2.2, Sec. 6.5.1
Rationale Sec. 3.1.2.2
H&S Secs. 4.3,4.3.1 pp. 75-6

Question 1.12
What's the auto keyword good for?
Nothing; it's archaic. See also question 20.37.
References: K&R1 Sec. A8.1 p. 193
ANSI Sec. 3.1.2.4, Sec. 3.5.1
ISO Sec. 6.1.2.4, Sec. 6.5.1
H&S Sec. 4.3 p. 75, Sec. 4.3.1 p. 76
Question 1.14
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 structure in C contain a pointer to itself?
Structures 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 the NODEPTR example is that the typedef
has not been defined at the point where the next field is declared. To fix this code, first give the
structure a tag (``struct node''). Then, declare the next field as a simple struct node *, or
disentangle the typedef declaration from the structure definition, or both. One corrected version
would be
struct node {
char *item;
struct node *next;
};
typedef struct node *NODEPTR;
and there are at least three other equivalently correct ways of arranging it.
A similar problem, with a similar solution, can arise when attempting to declare a pair of
typedef'ed mutually referential structures.
See also question 2.1.
References: K&R1 Sec. 6.5 p. 101
K&R2 Sec. 6.5 p. 139
ANSI Sec. 3.5.2, Sec. 3.5.2.3, esp. examples
ISO Sec. 6.5.2, Sec. 6.5.2.3
H&S Sec. 5.6.1 pp. 132-3

Question 1.21
How do I declare an array of N pointers to functions returning pointers to functions returning
pointers to characters?
The first part of this question can be answered in at least three ways:
1. char *(*(*a[N])())();
2. Build the declaration up incrementally, using typedefs:
typedef char *pc; /* pointer to char */
typedef pc fpc(); /* function returning pointer to char */
typedef fpc *pfpc; /* pointer to above */
typedef pfpc fpfpc(); /* function returning... */
typedef fpfpc *pfpfpc; /* pointer to... */
pfpfpc a[N]; /* array of... */
3. Use the cdecl program, which turns English into C and vice versa:
cdecl> declare a as array of pointer to function returning
pointer to function returning pointer to char
char *(*(*a[])())()
cdecl can also explain complicated declarations, help with casts, and indicate which set of
parentheses the arguments go in (for complicated function definitions, like the one above).
Versions of cdecl are in volume 14 of comp.sources.unix (see question 18.16) and K&R2.
Any good book on C should explain how to read these complicated C declarations ``inside out'' to
understand them (``declaration mimics use'').
The pointer-to-function declarations in the examples above have not included parameter type
information. When the parameters have complicated types, declarations can really get messy.
(Modern versions of cdecl can help here, too.)
References: K&R2 Sec. 5.12 p. 122
ANSI Sec. 3.5ff (esp. Sec. 3.5.4)
ISO Sec. 6.5ff (esp. Sec. 6.5.4)
H&S Sec. 4.5 pp. 85-92, Sec. 5.10.1 pp. 149-50

Question 1.22
How can I declare a function that can return a pointer to a function of the same type? I'm building
a state machine with one function for each state, each of which returns a pointer to the function
for the next state. But I can't find a way to declare the functions.
You can't quite do it directly. Either have the function return a generic function pointer, with
some judicious casts to adjust the types as the pointers are passed around; or have it return a
structure containing only a pointer to a function returning that structure.
Question 1.25
My compiler is complaining about an invalid redeclaration of a function, but I only define it once
and call it once.
Functions which are called without a declaration in scope (perhaps because the first call precedes
the function's definition) are assumed to be declared as returning int (and without any argument
type information), leading to discrepancies if the function is later declared or defined otherwise.
Non-int functions must be declared before they are called.
Another possible source of this problem is that the function has the same name as another one
declared in some header file.
See also questions 11.3 and 15.1.
References: K&R1 Sec. 4.2 p. 70
K&R2 Sec. 4.2 p. 72
ANSI Sec. 3.3.2.2
ISO Sec. 6.3.2.2
H&S Sec. 4.7 p. 101

Question 1.30
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?
Variables with static duration (that is, those declared outside of functions, and those declared
with the storage class static), are guaranteed initialized (just once, at program startup) to zero, as if the programmer had typed ``= 0''. Therefore, such variables are initialized to the null pointer (of the correct type; see also section 5) 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 is
all-bits-0, but this is not necessarily useful for pointer or floating-point values (see question 7.31,
and section 5).
References: K&R1 Sec. 4.9 pp. 82-4
K&R2 Sec. 4.9 pp. 85-86
ANSI Sec. 3.5.7, Sec. 4.10.3.1, Sec. 4.10.5.3
ISO Sec. 6.5.7, Sec. 7.10.3.1, Sec. 7.10.5.3
H&S Sec. 4.2.8 pp. 72-3, Sec. 4.6 pp. 92-3, Sec. 4.6.2 pp. 94-5, Sec. 4.6.3 p. 96, Sec. 16.1 p. 386

Question 1.31
This code, straight out of a book, isn't compiling:
f()
{
char a[] = "Hello, world!";
}
Perhaps you have a pre-ANSI compiler, which doesn't allow initialization of ``automatic
aggregates'' (i.e. non-static local arrays, structures, and unions). As a workaround, you can make
the array global or static (if you won't need a fresh copy during any subsequent calls), or replace it
with a pointer (if the array won't be written to). (You can always initialize local char * variables
to point to string literals, but see question 1.32.) If neither of these conditions hold, you'll have to
initialize the array by hand with strcpy when f is called. See also question 11.29.
Question 1.32
What is the difference between these initializations?
char a[] = "string literal";
char *p = "string literal";
My program crashes if I try to assign a new value to p[i].
A string literal can be used in two slightly different ways. As an array initializer (as in the
declaration of char a[]), it specifies the initial values of the characters in that array. Anywhere
else, it turns into an unnamed, static array of characters, which may be stored in read-only
memory, which is why you can't safely modify it. In an expression context, the array is converted
at once to a pointer, as usual (see section 6), so the second declaration initializes p to point to the
unnamed array's first element.
(For compiling old code, some compilers have a switch controlling whether strings are writable
or not.)
See also questions 1.31, 6.1, 6.2, and 6.8.
References: K&R2 Sec. 5.5 p. 104
ANSI Sec. 3.1.4, Sec. 3.5.7
ISO Sec. 6.1.4, Sec. 6.5.7
Rationale Sec. 3.1.4
H&S Sec. 2.7.4 pp. 31-2
Question 1.34
I finally figured out the syntax for declaring pointers to functions, but now how do I initialize
one?
Use something like
extern int func();
int (*fp)() = func;
When the name of a function appears in an expression like this, it ``decays'' into a pointer (that is,
it has its address implicitly taken), much as an array name does.
An explicit declaration for the function is normally needed, since implicit external function
declaration does not happen in this case (because the function name in the initialization is not part
of a function call).
See also question 4.12.
Question 2.1
What's the difference between these two declarations?
struct x1 { ... };
typedef struct { ... } x2;
The first form declares a structure tag; the second declares a typedef. The main difference is that
the second declaration is of a slightly more abstract type--its users don't necessarily know that it
is a structure, and the keyword struct is not used when declaring instances of it.
Question 2.2
Why doesn't
struct x { ... };
x thestruct;
work?
C is not C++. Typedef names are not automatically generated for structure tags. See also question
2.1.
Question 2.3
Can a structure contain a pointer to itself?
Most certainly. See question 1.14.
Question 2.4
What's the best way of implementing opaque (abstract) data types in C?
One good way is for clients to use structure pointers (perhaps additionally hidden behind
typedefs) which point to structure types which are not publicly defined.
Question 2.6
I came across some code that declared a structure like this:
struct name {
int namelen;
char namestr[1];
};
and then did some tricky allocation to make the namestr array act like it had several elements. Is
this legal or portable?
This technique is popular, although Dennis Ritchie has called it ``unwarranted chumminess with
the C implementation.'' An official interpretation has deemed that it is not strictly conforming
with the C Standard. (A thorough treatment of the arguments surrounding the legality of the
technique is beyond the scope of this list.) It does seem to be portable to all known
implementations. (Compilers which check array bounds carefully might issue warnings.)
Another possibility is to declare the variable-size element very large, rather than very small; in
the case of the above example:
...
char namestr[MAXSIZE];
...
where MAXSIZE is larger than any name which will be stored. However, it looks like this
technique is disallowed by a strict interpretation of the Standard as well.
References: Rationale Sec. 3.5.4.2
Question 2.7
I heard that structures could be assigned to variables and passed to and from functions, but K&R1
says not.
What K&R1 said was that the restrictions on structure operations would be lifted in a
forthcoming version of the compiler, and in fact structure assignment and passing were fully
functional in Ritchie's compiler even as K&R1 was being published. Although a few early C
compilers lacked these operations, all modern compilers support them, and they are part of the
ANSI C standard, so there should be no reluctance to use them. [footnote]
(Note that when a structure is assigned, passed, or returned, the copying is done monolithically;
anything pointed to by any pointer fields is not copied.)
References: K&R1 Sec. 6.2 p. 121
K&R2 Sec. 6.2 p. 129
ANSI Sec. 3.1.2.5, Sec. 3.2.2.1, Sec. 3.3.16
ISO Sec. 6.1.2.5, Sec. 6.2.2.1, Sec. 6.3.16
H&S Sec. 5.6.2 p. 133
Question 2.8
Why can't you compare structures?
There is no single, good way for a compiler to implement structure comparison which is
consistent with C's low-level flavor. A simple byte-by-byte comparison could founder on random
bits present in unused ``holes'' in the structure (such padding is used to keep the alignment of later
fields correct; see question 2.12). A field-by-field comparison might require unacceptable
amounts of repetitive code for large structures.
If you need to compare two structures, you'll have to write your own function to do so, field by
field.
References: K&R2 Sec. 6.2 p. 129
ANSI Sec. 4.11.4.1 footnote 136
Rationale Sec. 3.3.9
H&S Sec. 5.6.2 p. 133
Question 2.9
How are structure passing and returning implemented?
When structures are passed as arguments to functions, the entire structure 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.) Some compilers merely pass a pointer to the
structure, though they may have to make a local copy to preserve pass-by-value semantics.
Structures are often returned from functions in a location pointed to by an extra, compilersupplied
``hidden'' argument to the function. Some older compilers used a special, static location
for structure returns, although this made structure-valued functions non-reentrant, which ANSI C
disallows.
References: ANSI Sec. 2.2.3
ISO Sec. 5.2.3
Question 2.10
How can I pass constant values to functions which accept structure arguments?
C has no way of generating anonymous structure values. You will have to use a temporary
structure variable or a little structure-building function; see question 14.11 for an example. (gcc
provides structure constants as an extension, and the mechanism will probably be added to a
future revision of the C Standard.) See also question 4.10.
Question 2.11
How can I read/write structures from/to data files?
It is relatively straightforward to write a structure out using fwrite:
fwrite(&somestruct, sizeof somestruct, 1, fp);
and a corresponding fread invocation can read it back in. (Under pre-ANSI C, a (char *) cast on
the first argument is required. What's important is that fwrite receive a byte pointer, not a
structure pointer.) However, data files so written will not be portable (see questions 2.12 and
20.5). Note also that if the structure contains any pointers, only the pointer values will be written,
and they are most unlikely to be valid when read back in. Finally, note that for widespread
portability you must use the "b" flag when fopening the files; see question 12.38.
A more portable solution, though it's a bit more work initially, is to write a pair of functions for
writing and reading a structure, field-by-field, in a portable (perhaps even human-readable) way.
References: H&S Sec. 15.13 p. 381
Question 2.12
My compiler is leaving holes in structures, which is wasting space and preventing ``binary'' I/O to
external data files. Can I turn off the padding, or otherwise control the alignment of structure
fields?
Your compiler may provide an extension to give you this control (perhaps a #pragma; see
question 11.20), but there is no standard method.
See also question 20.5.
References: K&R2 Sec. 6.4 p. 138
H&S Sec. 5.6.4 p. 135
Question 2.13
Why does sizeof report a larger size than I expect for a structure type, as if there were padding at
the end?
Structures may have this padding (as well as internal padding), if necessary, to ensure that
alignment properties will be preserved when an array of contiguous structures is allocated. Even
when the structure is not part of an array, the end padding remains, so that sizeof can always
return a consistent size. See question 2.12.
References: H&S Sec. 5.6.7 pp. 139-40
Question 2.14
How can I determine the byte offset of a field within a structure?
ANSI C defines the offsetof() macro, which should be used if available; see . If you
don't have it, one possible 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 question 2.15 for a usage hint.
References: ANSI Sec. 4.1.5
ISO Sec. 7.1.6
Rationale Sec. 3.5.4.2
H&S Sec. 11.1 pp. 292-3
Question 2.15
How can I access structure fields by name at run time?
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 field b is an int (with offset as computed
above), b's value can be set indirectly with
*(int *)((char *)structp + offsetb) = value;
Question 2.18
This 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 missing semicolon causes main to be declared as returning a structure. (The connection is hard
to see because of the intervening comment.) Since structure-valued functions are usually
implemented by adding a hidden return pointer (see question 2.9), 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 questions 10.9 and 16.4.
References: CT&P Sec. 2.3 pp. 21-2
Question 2.20
Can I initialize unions?
ANSI Standard C allows an initializer for the first member of a union. There is no standard way
of initializing any other member (nor, under a pre-ANSI compiler, is there generally any way of
initializing a union at all).
References: K&R2 Sec. 6.8 pp. 148-9
ANSI Sec. 3.5.7
ISO Sec. 6.5.7
H&S Sec. 4.6.7 p. 100
Question 2.22
What is the difference between an enumeration and a set of preprocessor #defines?
At the present time, there is little difference. Although many people might have wished
otherwise, the C Standard says that enumerations may be freely intermixed with other integral
types, without errors. (If such intermixing were disallowed without explicit casts, judicious use of
enumerations could catch certain programming errors.)
Some advantages of enumerations are that the numeric values are automatically assigned, that a
debugger may be able to display the symbolic values when enumeration variables are examined,
and that they obey block scope. (A compiler may also generate nonfatal warnings when
enumerations and integers 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 those nonfatal warnings; some programmers also resent not having control over the sizes of
enumeration variables.
References: K&R2 Sec. 2.3 p. 39, Sec. A4.2 p. 196
ANSI Sec. 3.1.2.5, Sec. 3.5.2, Sec. 3.5.2.2, Appendix E
ISO Sec. 6.1.2.5, Sec. 6.5.2, Sec. 6.5.2.2, Annex F
H&S Sec. 5.5 pp. 127-9, Sec. 5.11.2 p. 153
Question 2.24
Is there an easy way to print enumeration values symbolically?
No. You can write a little function to map an enumeration constant to a string. (If all you're
worried about is debugging, a good debugger should automatically print enumeration constants
symbolically.)
Question 3.1
Why doesn't this code:
a[i] = i++;
work?
The subexpression i++ causes a side effect--it modifies i's value--which leads to undefined
behavior since i is also referenced elsewhere in the same expression. (Note that although the
language in K&R suggests that the behavior of this expression is unspecified, the C Standard
makes the stronger statement that it is undefined--see question 11.33.)
References: K&R1 Sec. 2.12
K&R2 Sec. 2.12
ANSI Sec. 3.3
ISO Sec. 6.3
Question 3.2
Under my compiler, the code
int i = 7;
printf("%d\n", i++ * i++);
prints 49. Regardless of the order of evaluation, shouldn't it print 56?
Although the postincrement and postdecrement operators ++ and -- perform their operations after
yielding the former value, the implication of ``after'' is often misunderstood. It is not guaranteed
that an increment or decrement 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; see question 3.8). In the example, the compiler chose to
multiply the previous value by itself and to perform both increments afterwards.
The behavior of code which contains multiple, ambiguous side effects has always been
undefined. (Loosely speaking, by ``multiple, ambiguous side effects'' we mean any combination
of ++, --, =, +=, -=, etc. in a single expression which causes the same object either to be modified
twice or modified and then inspected. This is a rough definition; see question 3.8 for a precise
one, and question 11.33 for the meaning of ``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&R1 Sec. 2.12 p. 50
K&R2 Sec. 2.12 p. 54
ANSI Sec. 3.3
ISO Sec. 6.3
CT&P Sec. 3.7 p. 47
PCS Sec. 9.5 pp. 120-1
Question 3.3
I've experimented with the code
int i = 3;
i = i++;
on several compilers. Some gave i the value 3, some gave 4, but one gave 7. I know the behavior
is undefined, but how could it give 7?
Undefined behavior means anything can happen. See questions 3.9 and 11.33. (Also, note that
neither i++ nor ++i is the same as i+1. If you want to increment i, use i=i+1 or i++ or ++i, not
some combination. See also question 3.12.)
Question 3.4
Can I use explicit parentheses to force the order of evaluation I want? Even if I don't, doesn't
precedence dictate it?
Not in general.
Operator precedence and explicit parentheses impose only a partial ordering on the evaluation of
an expression. In the expression
f() + g() * h()
although we know that the multiplication will happen before the addition, there is no telling
which of the three functions will be called first.
When you need to ensure the order of subexpression evaluation, you may need to use explicit
temporary variables and separate statements.
References: K&R1 Sec. 2.12 p. 49, Sec. A.7 p. 185
K&R2 Sec. 2.12 pp. 52-3, Sec. A.7 p. 200
Question 3.5
But what about the && and || operators?
I see code like ``while((c = getchar()) != EOF && c != '\n')'' ...
There is a special exception for those operators (as well as the ?: operator): left-to-right
evaluation is guaranteed (as is an intermediate sequence point, see question 3.8). Any book on C
should make this clear.
References: K&R1 Sec. 2.6 p. 38, Secs. A7.11-12 pp. 190-1
K&R2 Sec. 2.6 p. 41, Secs. A7.14-15 pp. 207-8
ANSI Sec. 3.3.13, Sec. 3.3.14, Sec. 3.3.15
ISO Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15
H&S Sec. 7.7 pp. 217-8, Sec. 7.8 pp. 218-20, Sec. 7.12.1 p. 229
CT&P Sec. 3.7 pp. 46-7
Question 3.8
How can I understand these complex expressions? What's a ``sequence point''?
A sequence point is the point (at the end of a full expression, or at the ||, &&, ?:, or comma
operators, or just before a function call) at which the dust has settled and all side effects are
guaranteed to be complete. The ANSI/ISO C Standard states that
Between the previous and next sequence point an object shall have its stored value
modified at most once by the evaluation of an expression. Furthermore, the prior
value shall be accessed only to determine the value to be stored.
The second sentence can be difficult to understand. It says that if an object is written to within a
full expression, any and all accesses to it within the same expression must be for the purposes of
computing the value to be written. This rule effectively constrains legal expressions to those in
which the accesses demonstrably precede the modification.
See also question 3.9.
References: ANSI Sec. 2.1.2.3, Sec. 3.3, Appendix B
ISO Sec. 5.1.2.3, Sec. 6.3, Annex C
Rationale Sec. 2.1.2.3
H&S Sec. 7.12.1 pp. 228-9
Question 3.9
So given
a[i] = i++;
we don't know which cell of a[] gets written to, but i does get incremented by one.
No. Once an expression or program becomes undefined, all aspects of it become undefined. See
questions 3.2, 3.3, 11.33, and 11.35.
Question 3.12
If I'm not using the value of the expression, should I use i++ or ++i to increment a variable?
Since the two forms differ only in the value yielded, they are entirely equivalent when only their
side effect is needed.
See also question 3.3.
References: K&R1 Sec. 2.8 p. 43
K&R2 Sec. 2.8 p. 47
ANSI Sec. 3.3.2.4, Sec. 3.3.3.1
ISO Sec. 6.3.2.4, Sec. 6.3.3.1
H&S Sec. 7.4.4 pp. 192-3, Sec. 7.5.8 pp. 199-200
Question 3.14
Why doesn't the code
int a = 1000, b = 1000;
long int c = a * b;
work?
Under C's integral promotion rules, the multiplication is carried out using int arithmetic, and the
result may overflow or be truncated before being promoted and assigned to the long int left-hand
side. Use an explicit cast to force long arithmetic:
long int c = (long int)a * b;
Note that (long int)(a * b) would not have the desired effect.
A similar problem can arise when two integers are divided, with the result assigned to a floatingpoint
variable.
References: K&R1 Sec. 2.7 p. 41
K&R2 Sec. 2.7 p. 44
ANSI Sec. 3.2.1.5
ISO Sec. 6.2.1.5
H&S Sec. 6.3.4 p. 176
CT&P Sec. 3.9 pp. 49-50
Question 3.16
I have a complicated expression which I have to assign to one of two variables, depending on a
condition. Can I use code like this?
((condition) ? a : b) = complicated_expression;
No. The ?: operator, like most operators, yields a value, and you can't assign to a value. (In other
words, ?: does not yield an lvalue.) If you really want to, you can try something like
*((condition) ? &a : &b) = complicated_expression;
although this is admittedly not as pretty.
References: ANSI Sec. 3.3.15 esp. footnote 50
ISO Sec. 6.3.15
H&S Sec. 7.1 pp. 179-180
Question 4.2
I'm trying to declare a pointer and allocate some space for it, but it's not working. What's wrong
with this code?
char *p;
*p = malloc(10);
The pointer you declared is p, not *p. To make a pointer point somewhere, you just use the name
of the pointer:
p = malloc(10);
It's when you're manipulating the pointed-to memory that you use * as an indirection operator:
*p = 'H';
See also questions 1.21, 7.1, and 8.3.
References: CT&P Sec. 3.1 p. 28
Question 4.3
Does *p++ increment p, or what it points to?
Unary operators like *, ++, and -- all associate (group) from right to left. Therefore, *p++
increments p (and returns the value pointed to by p before the increment). To increment the value
pointed to by p, use (*p)++ (or perhaps ++*p, if the order of the side effect doesn't matter).
References: K&R1 Sec. 5.1 p. 91
K&R2 Sec. 5.1 p. 95
ANSI Sec. 3.3.2, Sec. 3.3.3
ISO Sec. 6.3.2, Sec. 6.3.3
H&S Sec. 7.4.4 pp. 192-3, Sec. 7.5 p. 193, Secs. 7.5.7,7.5.8 pp. 199-200
Question 4.5
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?
In C, a cast operator does not mean ``pretend these bits have a different type, and treat them
accordingly''; it is a conversion operator, and by definition it yields an rvalue, which cannot be
assigned to, or incremented with ++. (It is an anomaly in pcc-derived compilers, and an extension
in gcc, that expressions such as the above are ever accepted.) Say what you mean: use
p = (char *)((int *)p + 1);
or (since p is a char *) simply
p += sizeof(int);
Whenever possible, you should choose appropriate pointer types in the first place, instead of
trying to treat one type as another.
References: K&R2 Sec. A7.5 p. 205
ANSI Sec. 3.3.4 (esp. footnote 14)
ISO Sec. 6.3.4
Rationale Sec. 3.3.2.4
H&S Sec. 7.1 pp. 179-80
Question 4.8
I have a function which accepts, and is supposed to initialize, a pointer:
void f(ip)
int *ip;
{
static int dummy = 5;
ip = &dummy;
}
But when I call it like this:
int *ip;
f(ip);
the pointer in the caller remains unchanged.
Are you sure the function initialized what you thought it did? Remember that arguments in C are
passed by value. The called function altered only the passed copy of the pointer. You'll either
want to pass the address of the pointer (the function will end up accepting a pointer-to-a-pointer),
or have the function return the pointer.
See also questions 4.9 and 4.11.
Question 4.9
Can I use a void ** pointer to pass a generic pointer to a function by reference?
Not portably. There is no generic pointer-to-pointer type in C. void * acts as a generic pointer
only because conversions are applied automatically when other pointer types are assigned to and
from void *'s; these conversions cannot be performed (the correct underlying pointer type is not
known) if an attempt is made to indirect upon a void ** value which points at something other
than a void *.
Question 4.10
I have a function
extern int f(int *);
which accepts a pointer to an int. How can I pass a constant by reference? A call like
f(&5);
doesn't seem to work.
You can't do this directly. You will have to declare a temporary variable, and then pass its address
to the function:
int five = 5;
f(&five);
See also questions 2.10, 4.8, and 20.1.
Question 4.11
Does C even have ``pass by reference''?
Not really. Strictly speaking, C always uses pass by value. You can simulate pass by reference
yourself, by defining functions which accept pointers and then using the & operator when calling,
and the compiler will essentially simulate it for you when you pass an array to a function (by
passing a pointer instead, see question 6.4 et al.), but C has nothing truly equivalent to formal
pass by reference or C++ reference parameters. (However, function-like preprocessor macros do
provide a form of ``call by name''.)
See also questions 4.8 and 20.1.
References: K&R1 Sec. 1.8 pp. 24-5, Sec. 5.2 pp. 91-3
K&R2 Sec. 1.8 pp. 27-8, Sec. 5.2 pp. 91-3
ANSI Sec. 3.3.2.2, esp. footnote 39
ISO Sec. 6.3.2.2
H&S Sec. 9.5 pp. 273-4
Question 4.12
I've seen different methods used for calling functions via pointers. What's the story?
Originally, a pointer to a function had to be ``turned into'' a ``real'' function, with the * operator
(and an extra pair of parentheses, to keep the precedence straight), before calling:
int r, func(), (*fp)() = func;
r = (*fp)();
It can also be argued that functions are always called via pointers, and that ``real'' function names
always decay implicitly into pointers (in expressions, as they do in initializations; see question
1.34). This reasoning, made widespread through pcc and adopted in the ANSI standard, means
that
r = fp();
is legal and works correctly, whether fp is the name of 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 the function pointed to.) An explicit * is still allowed
(and recommended, if portability to older compilers is important).
See also question 1.34.
References: K&R1 Sec. 5.12 p. 116
K&R2 Sec. 5.11 p. 120
ANSI Sec. 3.3.2.2
ISO Sec. 6.3.2.2
Rationale Sec. 3.3.2.2
H&S Sec. 5.8 p. 147, Sec. 7.4.3 p. 190
Question 5.1
What is this infamous null pointer, anyway?
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 ``guaranteed to
compare unequal to a pointer to any object or function.'' That is, the address-of operator & will
never yield a null pointer, nor will a successful call to malloc. (malloc does return 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 or function; an uninitialized pointer might point anywhere. See also
questions 1.30, 7.1, and 7.31.
As mentioned 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 that it can
make the distinction if necessary (see questions 5.2, 5.5, and 5.6).
References: K&R1 Sec. 5.4 pp. 97-8
K&R2 Sec. 5.4 p. 102
ANSI Sec. 3.2.2.3
ISO Sec. 6.2.2.3
Rationale Sec. 3.2.2.3
H&S Sec. 5.3.2 pp. 121-3
Question 5.2
How do I get a null pointer in my programs?
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)
(See also question 5.3.)
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. To
generate a null pointer in a function call context, an explicit cast may be required, to force the 0
to be recognized as a pointer. For example, the Unix system call execl takes a variable-length,
null-pointer-terminated list of character pointer arguments, and is correctly called like this:
execl("/bin/sh", "sh", "-c", "date", (char *)0);
If the (char *) cast on the last argument were omitted, the compiler would not know to pass a null
pointer, and would pass an integer 0 instead. (Note that many Unix manuals get this example
wrong .)
When function prototypes are in scope, argument passing becomes an ``assignment context,'' and
most casts may safely be omitted, since the prototype tells the compiler that a pointer is required,
and of which type, enabling it to correctly convert an unadorned 0. Function prototypes cannot
provide the types for variable arguments in variable-length argument lists however, so explicit
casts are still required for those arguments. (See also question 15.3.) It is safest to properly cast
all null pointer constants in function calls: to guard against varargs functions or those without
prototypes, to allow interim use of non-ANSI compilers, and to demonstrate that you know what
you are doing. (Incidentally, it's also a simpler rule to remember.)
Summary:
Unadorned 0 okay: Explicit cast required:
initialization function call,
no prototype in scope
assignment
variable argument in
comparison varargs function call
function call,
prototype in scope,
fixed argument
References: K&R1 Sec. A7.7 p. 190, Sec. A7.14 p. 192
K&R2 Sec. A7.10 p. 207, Sec. A7.17 p. 209
ANSI Sec. 3.2.2.3
ISO Sec. 6.2.2.3
H&S Sec. 4.6.3 p. 95, Sec. 6.2.7 p. 171
Question 5.3
Is the abbreviated pointer comparison ``if(p)'' to test for non-null pointers valid? What if the
internal representation for null pointers is nonzero?
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 inferred 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 actually a null
pointer constant, and use the correct null pointer value. There is no trickery involved here;
compilers do work this way, and generate identical code for both constructs. The internal
representation of a null pointer does not matter.
The boolean negation operator, !, can be described as follows:
!expr is essentially equivalent to (expr)?0:1
or to ((expr) == 0)
which leads to the conclusion 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 (and
by others to be good style; see question 17.10).
See also question 9.2.
References: K&R2 Sec. A7.4.7 p. 204
ANSI Sec. 3.3.3.3, Sec. 3.3.9, Sec. 3.3.13, Sec. 3.3.14, Sec. 3.3.15, Sec. 3.6.4.1, Sec. 3.6.5
ISO Sec. 6.3.3.3, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15, Sec. 6.6.4.1, Sec. 6.6.5
H&S Sec. 5.3.2 p. 122
Question 5.4
What is NULL and how is it #defined?
As a matter of style, many programmers prefer not to have unadorned 0's scattered through their
programs. Therefore, the preprocessor macro NULL is #defined (by or )
with the value 0, possibly cast to (void *) (see also question 5.6). A programmer who wishes to
make explicit the distinction between 0 the integer and 0 the null pointer constant can then use
NULL whenever a null pointer is required.
Using NULL is a stylistic convention only; the preprocessor turns NULL back into 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 5.2
above applies for NULL as well as 0 (an unadorned NULL is equivalent to an unadorned 0).
NULL should only be used for pointers; see question 5.9.
References: K&R1 Sec. 5.4 pp. 97-8
K&R2 Sec. 5.4 p. 102
ANSI Sec. 4.1.5, Sec. 3.2.2.3
ISO Sec. 7.1.6, Sec. 6.2.2.3
Rationale Sec. 4.1.5
H&S Sec. 5.3.2 p. 122, Sec. 11.1 p. 292
Question 5.5
How should NULL be defined on a machine which uses a nonzero bit pattern as the internal
representation of a null pointer?
The same as on any other machine: as 0 (or ((void *)0)).
Whenever a programmer requests a null pointer, either by writing ``0'' or ``NULL,'' it is the
compiler's responsibility to generate whatever bit pattern the machine uses for that null pointer.
Therefore, #defining NULL as 0 on a machine for which internal null pointers are nonzero is as
valid as on any other: the compiler must always be able to generate the machine's correct null
pointers in response to unadorned 0's seen in pointer contexts. See also questions 5.2, 5.10, and
5.17.
References: ANSI Sec. 4.1.5
ISO Sec. 7.1.6
Rationale Sec. 4.1.5
Question 5.6
If NULL were defined as follows:
#define NULL ((char *)0)
wouldn't that make function calls which pass an uncast NULL work?
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 work correctly, but pointer
arguments of other types would still be problematical, and legal constructions such as
FILE *fp = NULL;
could fail.
Nevertheless, ANSI C allows the alternate definition
#define NULL ((void *)0)
for NULL. Besides potentially 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 5.9).
References: Rationale Sec. 4.1.5
Question 5.9
If NULL and 0 are equivalent as null pointer constants, which should I use?
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 macro, and prefer to use unadorned 0 instead. There is
no one right answer. (See also questions 9.2 and 17.10.) C programmers must understand that
NULL and 0 are interchangeable in pointer contexts, and that an uncast 0 is perfectly acceptable.
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. (Furthermore, ANSI allows the definition of NULL
to be ((void *)0), which will not work at all in non-pointer contexts.) In particular, do not use
NULL when the ASCII null character (NUL) is desired. Provide your own definition
#define NUL '\0'
if you must.
References: K&R1 Sec. 5.4 pp. 97-8
K&R2 Sec. 5.4 p. 102
Question 5.10
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 internal null pointers?
No. (Using NULL may be preferable, but not for this reason.) 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. See questions 5.5
and 9.2.
Question 5.12
I use the preprocessor macro
#define Nullptr(type) (type *)0
to help me build null pointers of the correct type.
This trick, though popular and superficially attractive, does not buy much. It is not needed in
assignments and comparisons; see question 5.2. It does not even save keystrokes. Its use may
suggest to the reader that the program's author is shaky on the subject of null pointers, requiring
that the #definition of the macro, its invocations, and all other pointer usages be checked. See
also questions 9.1 and 10.2.
Question 5.13
This is strange. NULL is guaranteed to be 0, but the null pointer is not?
When the term ``null'' or ``NULL'' is casually used, one of several things may be meant:
1. 1. The conceptual null pointer, the abstract language concept defined in question 5.1. It is
implemented with...
2. 2. The internal (or run-time) representation of a null pointer, which may or may not be allbits-
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. 3. The null pointer constant, which is a constant integer 0 (see question 5.2). It is often
hidden behind...
4. 4. The NULL macro, which is #defined to be 0 or ((void *)0) (see question 5.4). Finally,
as red herrings, we have...
5. 5. The ASCII null character (NUL), which does have all bits zero, but has no necessary
relation to the null pointer except in name; and...
6. 6. The ``null string,'' which is another name for the empty string (""). Using the term ``null
string'' can be confusing in C, because an empty string involves a null ('\0') character, but
not a null pointer, which brings us full circle...
This article uses the phrase ``null pointer'' (in lower case) for sense 1, the character ``0'' or the
phrase ``null pointer constant'' for sense 3, and the capitalized word ``NULL'' for sense 4.
Question 5.14
Why is there so much confusion surrounding null pointers? Why do these questions come up so
often?
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) may seem to suggest that the value could change some day, 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 in question 5.13) is often overlooked.
One good way to wade out of the confusion is to imagine that C used a keyword (perhaps nil, like
Pascal) as a null pointer constant. 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 constant 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 constant, the code may not work.
Question 5.15
I'm confused. I just can't understand all this null pointer stuff.
Follow these two simple rules:
1. When you want a null pointer constant 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, with the internal
representation of null pointers (which you shouldn't need to know), and with ANSI C
refinements. Understand questions 5.1, 5.2, and 5.4, and consider 5.3, 5.9, 5.13, and 5.14, and
you'll do fine.
Question 5.16
Given all the confusion surrounding null pointers, wouldn't it be easier simply to require them to
be represented internally by zeroes?
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 such a 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 7.31). 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, as mentioned in question 5.14, the urge to assume an
internal zero representation would not even arise.)
Question 5.17
Seriously, have any actual machines really used nonzero null pointers, or different representations
for pointers to different types?
The Prime 50 series used segment 07777, offset 0 for the null pointer, at least for PL/I. Later
models used segment 0, offset 0 for null pointers in C, necessitating new instructions such as
TCNP (Test C Null Pointer), evidently as a sop to all the extant poorly-written C code which
made incorrect assumptions. Older, word-addressed Prime machines were also notorious for
requiring larger byte pointers (char *'s) than word pointers (int *'s).
The Eclipse MV series from Data General has three architecturally supported pointer formats
(word, byte, and bit pointers), two of which are used by C compilers: byte pointers for char * and
void *, and word pointers for everything else.
Some Honeywell-Bull mainframes use the bit pattern 06000 for (internal) null pointers.
The CDC Cyber 180 Series has 48-bit pointers consisting of a ring, segment, and offset. Most
users (in ring 11) have null pointers of 0xB00000000000. It was common on old CDC onescomplement
machines to use an all-one-bits word as a special flag for all kinds of data, including
invalid addresses.
The old HP 3000 series uses a different addressing scheme for byte addresses than for word
addresses; like several of the machines above it therefore uses different representations for char *
and void * pointers than for other pointers.
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.
Depending on the ``memory model'' in use, 8086-family processors (PC compatibles) may use
16-bit data pointers and 32-bit function pointers, or vice versa.
Some 64-bit Cray machines represent int * in the lower 48 bits of a word; char * additionally uses
the upper 16 bits to indicate a byte address within a word.
References: K&R1 Sec. A14.4 p. 211
Question 5.20
What does a run-time ``null pointer assignment'' error mean? How do I track it down?
This message, which typically occurs with MS-DOS compilers (see, therefore, section 19) means
that you've written, via a null (perhaps because uninitialized) pointer, to location 0. (See also
question 16.8.)
A debugger may let you set a data breakpoint or watchpoint or something on location 0.
Alternatively, you could write a bit of code to stash away a copy of 20 or so bytes from location
0, and periodically check that the memory at location 0 hasn't changed.
Question 6.1
I had the definition char a[6] in one source file, and in another I declared extern char *a. Why
didn't it work?
The declaration extern char *a simply does not match the actual definition. The type pointer-totype-
T is not the same as array-of-type-T. Use extern char a[].
References: ANSI Sec. 3.5.4.2
ISO Sec. 6.5.4.2
CT&P Sec. 3.3 pp. 33-4, Sec. 4.5 pp. 64-5
Question 6.2
But I heard that char a[] was identical to char *a.
Not at all. (What you heard has to do with formal parameters to functions; see question 6.4.)
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, to be known by the name ``p.'' This pointer can point almost anywhere: to any
char, or to any contiguous array of chars, or nowhere (see also questions 5.1 and 1.30).
As usual, a picture is worth a thousand words. The declarations
char a[] = "hello";
char *p = "world";
would initialize data structures which could be represented like this:
+---+---+---+---+---+---+
a: | h | e | l | l | o |\0 |
+---+---+---+---+---+---+
+-----+ +---+---+---+---+---+---+
p: | *======> | w | o | r | l | d |\0 |
+-----+ +---+---+---+---+---+---+
It is important to realize that a reference like x[3] generates different code depending on whether
x is an array or a pointer. Given the declarations above, when the compiler sees the expression a
[3], it emits code to start at the location ``a,'' move three past it, and fetch the character there.
When it sees the expression p[3], it emits code to start at the location ``p,'' fetch the pointer value
there, add three to the pointer, and finally fetch the character pointed to. In other words, a[3] is
three places past (the start of) the object named a, while p[3] is three places past the object
pointed to by p. In the example above, both a[3] and p[3] happen to be the character 'l', but the
compiler gets there differently.
References: K&R2 Sec. 5.5 p. 104
CT&P Sec. 4.5 pp. 64-5
Question 6.3
So what is meant by the ``equivalence of pointers and arrays'' in C?
Much of the confusion surrounding arrays and pointers in C can be traced to a misunderstanding
of this statement. Saying that arrays and pointers are ``equivalent'' means neither that they are
identical nor even 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 a sizeof or & operator, or is a string literal
initializer for a character array.)
As a consequence of this definition, the compiler doesn't apply the array subscripting operator []
that differently to arrays and pointers, after all. In an expression of the form a[i], the array decays
into a pointer, following the rule above, and is then subscripted just as would be a pointer
variable in the expression p[i] (although the eventual memory accesses will be different, as
explained in question 6.2). If you were to assign the array's address to the pointer:
p = a;
then p[3] and a[3] would access the same element.
See also question 6.8.
References: K&R1 Sec. 5.3 pp. 93-6
K&R2 Sec. 5.3 p. 99
ANSI Sec. 3.2.2.1, Sec. 3.3.2.1, Sec. 3.3.6
ISO Sec. 6.2.2.1, Sec. 6.3.2.1, Sec. 6.3.6
H&S Sec. 5.4.1 p. 124
Question 6.4
Then why are array and pointer declarations interchangeable as function formal parameters?
It's supposed to be a convenience.
Since arrays decay immediately into pointers, an array is never actually passed to a function.
Allowing pointer parameters to be declared as arrays is a simply a way of making it look as
though the array was being passed--a programmer may wish to emphasize that a parameter is
traditionally treated as if it were an array, or that an array (strictly speaking, the address) is
traditionally passed. As a convenience, 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 the
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 or the uses within
the function.
See also question 6.21.
References: K&R1 Sec. 5.3 p. 95, Sec. A10.1 p. 205
K&R2 Sec. 5.3 p. 100, Sec. A8.6.3 p. 218, Sec. A10.1 p. 226
ANSI Sec. 3.5.4.3, Sec. 3.7.1, Sec. 3.9.6
ISO Sec. 6.5.4.3, Sec. 6.7.1, Sec. 6.9.6
H&S Sec. 9.3 p. 271
CT&P Sec. 3.3 pp. 33-4
Question 6.7
How can an array be an lvalue, if you can't assign to it?
The ANSI C Standard defines a ``modifiable lvalue,'' which an array is not.
References: ANSI Sec. 3.2.2.1
ISO Sec. 6.2.2.1
Rationale Sec. 3.2.2.1
H&S Sec. 7.1 p. 179
Question 6.8
Practically speaking, what is the difference between arrays and pointers?
Arrays automatically allocate space, but can't be relocated or resized. Pointers must be explicitly
assigned to point to allocated space (perhaps using malloc), but can be reassigned (i.e. pointed at
different objects) at will, and have many other uses besides serving as the base of blocks of
memory.
Due to the so-called equivalence of arrays and pointers (see question 6.3), arrays and pointers
often seem interchangeable, and in particular a pointer to a block of memory assigned by malloc
is frequently treated (and can be referenced using []) exactly as if it were a true array. See
questions 6.14 and 6.16. (Be careful with sizeof, though.)
See also questions 1.32 and 20.14.
Question 6.9
Someone explained to me that arrays were really just constant pointers.
This is a bit of an oversimplification. An array name is ``constant'' in that it cannot be assigned to,
but an array is not a pointer, as the discussion and pictures in question 6.2 should make clear. See
also questions 6.3 and 6.8.
Question 6.11
I came across some ``joke'' code containing the ``expression'' 5["abcdef"] . How can this be legal
C?
Yes, Virginia, array subscripting is commutative in C. This curious fact follows from the pointer
definition of array subscripting, namely that a[e] is identical to *((a)+(e)), for any two
expressions a and e, 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 20.36).
References: Rationale Sec. 3.3.2.1
H&S Sec. 5.4.1 p. 124, Sec. 7.4.1 pp. 186-7
Question 6.12
Since array references decay into pointers, if arr is an array, what's the difference between arr and
&arr?
The type.
In Standard C, &arr yields a pointer, of type pointer-to-array-of-T, to the entire array. (In pre-
ANSI C, the & in &arr generally elicited a warning, and was generally ignored.) Under all C
compilers, a simple reference (without an explicit &) to an array yields a pointer, of type pointerto-
T, to the array's first element. (See also questions 6.3, 6.13, and 6.18.)
References: ANSI Sec. 3.2.2.1, Sec. 3.3.3.2
ISO Sec. 6.2.2.1, Sec. 6.3.3.2
Rationale Sec. 3.3.3.2
H&S Sec. 7.5.6 p. 198
Question 6.13
How do I declare a pointer to an array?
Usually, you don't want to. When people speak casually of a pointer to an array, they usually
mean a pointer to its first element.
Instead of a pointer to an array, consider using a pointer to one of the array's elements. Arrays of
type T decay into pointers to type T (see question 6.3), which is convenient; subscripting or
incrementing the resultant pointer will access the individual members of the array. True pointers
to arrays, when subscripted or incremented, step over entire arrays, and are generally useful only
when operating on arrays of arrays, if at all. (See question 6.18.)
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 1.21.) If the size of the array is unknown, N can in
principle be omitted, but the resulting type, ``pointer to array of unknown size,'' is useless.
See also question 6.12.
References: ANSI Sec. 3.2.2.1
ISO Sec. 6.2.2.1
Question 6.14
How can I set an array's size at run time?
How can I avoid fixed-sized arrays?
The equivalence between arrays and pointers (see question 6.3) allows a pointer to malloc'ed
memory to simulate an array quite effectively. After executing
#include
int *dynarray = (int *)malloc(10 * sizeof(int));
(and if the call to malloc succeeds), you can reference dynarray[i] (for i from 0 to 9) just as if
dynarray were a conventional, statically-allocated array (int a[10]). See also question 6.16.
Question 6.15
How can I declare local arrays of a size matching a passed-in array?
You can't, in C. Array dimensions must be compile-time constants. (gcc provides parameterized
arrays as an extension.) You'll have to use malloc, and remember to call free before the function
returns. See also questions 6.14, 6.16, 6.19, 7.22, and maybe 7.32.
References: ANSI Sec. 3.4, Sec. 3.5.4.2
ISO Sec. 6.4, Sec. 6.5.4.2
Question 6.16
How can I dynamically allocate a multidimensional array?
It is usually best to allocate an array of pointers, and then initialize each pointer to a dynamicallyallocated
``row.'' Here is a two-dimensional example:
#include
int **array1 = (int **)malloc(nrows * sizeof(int *));
for(i = 0; i < array2 =" (int" i =" 1;" array3 =" (int" array =" &realarray[-1];">=
NCOLUMNS.
gcc allows local arrays to be declared having sizes which are specified by a function's arguments,
but this is a nonstandard extension.
When you want to be able to use a function on multidimensional arrays of various sizes, one
solution is to simulate all the arrays dynamically, as in question 6.16.
See also questions 6.18, 6.20, and 6.15.
References: ANSI Sec. 3.3.6
ISO Sec. 6.3.6
Question 6.20
How can I use statically- and dynamically-allocated multidimensional arrays interchangeably
when passing them to functions?
There is no single perfect method. Given the declarations
int array[NROWS][NCOLUMNS];
int **array1; /* ragged */
int **array2; /* contiguous */
int *array3; /* "flattened" */
int (*array4)[NCOLUMNS];
with the pointers initialized as in the code fragments in question 6.16, and functions declared as
f1(int a[][NCOLUMNS], int nrows, int ncolumns);
f2(int *aryp, int nrows, int ncolumns);
f3(int **pp, int nrows, int ncolumns);
where f1 accepts a conventional two-dimensional array, f2 accepts a ``flattened'' two-dimensional
array, and f3 accepts a pointer-to-pointer, simulated array (see also questions 6.18 and 6.19), the
following calls should work as expected:
f1(array, NROWS, NCOLUMNS);
f1(array4, nrows, NCOLUMNS);
f2(&array[0][0], NROWS, NCOLUMNS);
f2(*array, NROWS, NCOLUMNS);
f2(*array2, nrows, ncolumns);
f2(array3, nrows, ncolumns);
f2(*array4, nrows, NCOLUMNS);
f3(array1, nrows, ncolumns);
f3(array2, nrows, ncolumns);
The following two calls would probably work on most systems, but involve questionable casts,
and work only if the dynamic ncolumns matches the static NCOLUMNS:
f1((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);
f1((int (*)[NCOLUMNS])array3, nrows, ncolumns);
It must again be noted that passing &array[0][0] (or, equivalently, *array) to f2 is not strictly
conforming; see question 6.19.
If you can understand why all of the above calls work and are written as they are, and if you
understand why the combinations that are not listed would not work, then you have a very good
understanding of arrays and pointers in C.
Rather than worrying about all of this, one approach to using multidimensional arrays of various
sizes is to make them all dynamic, as in question 6.16. If there are no static multidimensional
arrays--if all arrays are allocated like array1 or array2 in question 6.16--then all functions can be
written like f3.
Question 6.21
Why doesn't sizeof properly report the size of an array when the array is a parameter to a
function?
The compiler pretends that the array parameter was declared as a pointer (see question 6.4), and
sizeof reports the size of the pointer.
References: H&S Sec. 7.5.2 p. 195
Question 7.1
Why doesn't this fragment work?
char *answer;
printf("Type something:\n");
gets(answer);
printf("You typed \"%s\"\n", answer);
The pointer variable answer, which is handed to gets() 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 questions 1.30 and 5.1.)
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
#include
char answer[100], *p;
printf("Type something:\n");
fgets(answer, sizeof answer, stdin);
if((p = strchr(answer, '\n')) != NULL)
*p = '\0';
printf("You typed \"%s\"\n", answer);
This example also uses fgets() instead of gets(), so that the end of the array cannot be overwritten.
(See question 12.23. Unfortunately for this example, fgets() does not automatically delete the
trailing \n, gets() would.) It would also be possible to use malloc() to allocate the answer buffer.
Question 7.2
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.
As in question 7.1, the main problem here 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 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:
char s1[20] = "Hello, ";
Since strcat returns the value of its first argument (s1, in this case), the variable s3 is superfluous.
The original call to strcat in the question actually has two problems: the string literal pointed to
by s1, besides not being big enough for any concatenated text, is not necessarily writable at all.
See question 1.32.
References: CT&P Sec. 3.2 p. 32
Question 7.3
But the man page for strcat says that it takes two char *'s as arguments. How am I supposed to
know to allocate things?
In general, when using pointers you always have to consider memory allocation, if only to make
sure that the compiler is doing it for you. If a library function'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 or in the ANSI C standard can be
misleading. The code fragments presented there are closer to the function definitions used by an
implementor than the invocations used by the caller. In particular, many functions which accept
pointers (e.g. to structures or strings) are usually called with the address of some object (a
structure, or an array--see questions 6.3 and 6.4). Other common examples are time (see question
13.12) and stat.
Question 7.5
I have a function that is supposed to return a string, but when it returns to its caller, the returned
string is garbage.
Make sure that the pointed-to memory is properly allocated. The returned pointer should be to a
statically-allocated buffer, or to a buffer passed in by the caller, or to memory obtained with
malloc, but not to a local (automatic) array. In other words, never do something like
char *itoa(int n)
{
char retbuf[20]; /* WRONG */
sprintf(retbuf, "%d", n);
return retbuf; /* WRONG */
}
One fix (which is imperfect, especially if the function in question is called recursively, or if
several of its return values are needed simultaneously) would be to declare the return buffer as
static char retbuf[20];
See also questions 12.21 and 20.1.
References: ANSI Sec. 3.1.2.4
ISO Sec. 6.1.2.4
Question 7.6
Why am I getting ``warning: assignment of pointer from integer lacks a cast'' for calls to malloc?
Have you #included , or otherwise arranged for malloc to be declared properly?
References: H&S Sec. 4.7 p. 101
Question 7.7
Why does some code carefully cast the values returned by malloc to the pointer type being
allocated?
Before ANSI/ISO Standard C introduced the void * generic pointer type, these casts were
typically required to silence warnings (and perhaps induce conversions) when assigning between
incompatible pointer types. (Under ANSI/ISO Standard C, these casts are no longer necessary.)
References: H&S Sec. 16.1 pp. 386-7
Question 7.8
I see code like
char *p = malloc(strlen(s) + 1);
strcpy(p, s);
Shouldn't that be malloc((strlen(s) + 1) * sizeof(char))?
It's never necessary to multiply by sizeof(char), since sizeof(char) is, by definition, exactly 1. (On
the other hand, multiplying by sizeof(char) doesn't hurt, and may help by introducing a size_t into
the expression.) See also question 8.9.
References: ANSI Sec. 3.3.3.4
ISO Sec. 6.3.3.4
H&S Sec. 7.5.2 p. 195
Question 7.14
I've heard that some operating systems don't actually allocate malloc'ed memory until the
program tries to use it. Is this legal?
It's hard to say. The Standard doesn't say that systems can act this way, but it doesn't explicitly say
that they can't, either.
References: ANSI Sec. 4.10.3
ISO Sec. 7.10.3
Question 7.16
I'm allocating a large array for some numeric work, using the line
double *array = malloc(256 * 256 * sizeof(double));
malloc isn't returning null, but the program is acting strangely, as if it's overwriting memory, or
malloc isn't allocating as much as I asked for, or something.
Notice that 256 x 256 is 65,536, which will not fit in a 16-bit int, even before you multiply it by
sizeof(double). If you need to allocate this much memory, you'll have to be careful. If size_t (the
type accepted by malloc) is a 32-bit type on your machine, but int is 16 bits, you might be able to
get away with writing 256 * (256 * sizeof(double)) (see question 3.14). Otherwise, you'll have to
break your data structure up into smaller chunks, or use a 32-bit machine, or use some
nonstandard memory allocation routines. See also question 19.23.
Question 7.17
I've got 8 meg of memory in my PC. Why can I only seem to malloc 640K or so?
Under the segmented architecture of PC compatibles, it can be difficult to use more than 640K
with any degree of transparency. See also question 19.23.
Question 7.19
My program is crashing, apparently somewhere down inside malloc, but I can't see anything
wrong with it.
It is unfortunately very easy to corrupt malloc's internal data structures, and the resulting
problems can be stubborn. The most common source of problems is writing more to a malloc'ed
region than it was allocated to hold; a particularly common bug is to malloc(strlen(s)) instead of
strlen(s) + 1. Other problems may involve using pointers to freed storage, freeing pointers twice,
freeing pointers not obtained from malloc, or trying to realloc a null pointer (see question 7.30).
See also questions 7.26, 16.8, and 18.2.
Question 7.20
You can't use dynamically-allocated memory after you free it, can you?
No. Some early documentation for malloc stated that the contents of freed memory were ``left
undisturbed,'' but this ill-advised guarantee was never universal and is not required by the C
Standard.
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((void *)listp);
}
and notice what would happen if the more-obvious loop iteration expression listp = listp->next
were used, without the temporary nextp pointer.
References: K&R2 Sec. 7.8.5 p. 167
ANSI Sec. 4.10.3
ISO Sec. 7.10.3
Rationale Sec. 4.10.3.2
H&S Sec. 16.2 p. 387
CT&P Sec. 7.10 p. 95
Question 7.21
Why isn't a pointer null after calling free?
How unsafe is it to use (assign, compare) a pointer value after it's been freed?
When you call free, the memory pointed to by the passed pointer is freed, but the value of the
pointer in the caller remains unchanged, because C's pass-by-value semantics mean that called
functions never permanently change the values of their arguments. (See also question 4.8.)
A pointer value which has been freed is, strictly speaking, invalid, and any use of it, even if is not
dereferenced can theoretically lead to trouble, though as a quality of implementation issue, most
implementations will probably not go out of their way to generate exceptions for innocuous uses
of invalid pointers.
References: ANSI Sec. 4.10.3
ISO Sec. 7.10.3
Rationale Sec. 3.2.2.3
Question 7.22
When I call malloc to allocate memory for a local pointer, do I have to explicitly free it?
Yes. Remember that a pointer is different from what it points to. Local variables are deallocated
when the function returns, but in the case of a pointer variable, this means that the pointer is
deallocated, not what it points to. Memory allocated with malloc always persists until you
explicitly free it. In general, for every call to malloc, there should be a corresponding call to free
Question 7.23
I'm allocating structures which contain pointers to other dynamically-allocated objects. When I
free a structure, do I have to free each subsidiary pointer first?
Yes. In general, you must arrange that each pointer returned from malloc be individually passed
to free, exactly once (if it is freed at all).
A good rule of thumb is that for each call to malloc in a program, you should be able to point at
the call to free which frees the memory allocated by that malloc call.
See also question 7.24.
Question 7.24
Must I free allocated memory before the program exits?
You shouldn't have to. A real operating system definitively reclaims all memory when a program
exits. Nevertheless, some personal computers are said not to reliably recover memory, and all that
can be inferred from the ANSI/ISO C Standard is that this is a ``quality of implementation issue.''
References: ANSI Sec. 4.10.3.2
ISO Sec. 7.10.3.2
Question 7.25
I have a program which mallocs and later frees a lot of memory, but memory usage (as reported
by ps) doesn't seem to go back down.
Most implementations of malloc/free do not return freed memory to the operating system (if there
is one), but merely make it available for future malloc calls within the same program.
Question 7.26
How does free know how many bytes to free?
The malloc/free implementation remembers the size of each block allocated and returned, so it is
not necessary to remind it of the size when freeing.
Question 7.27
So can I query the malloc package to find out how big an allocated block is?
Not portably.
Question 7.30
Is it legal to pass a null pointer as the first argument to realloc? Why would you want to?
ANSI C sanctions this usage (and the related realloc(..., 0), which frees), although several earlier
implementations do not support it, so it may not be fully 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
ISO Sec. 7.10.3.4
H&S Sec. 16.3 p. 388
Question 7.31
What's the difference between calloc and malloc? Is it safe to take advantage of calloc's zerofilling?
Does free work on memory allocated with calloc, or do you need a cfree?
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 null pointer values (see
section 5 of this list) or floating-point zero values. free is properly used to free the memory
allocated by calloc.
References: ANSI Sec. 4.10.3 to 4.10.3.2
ISO Sec. 7.10.3 to 7.10.3.2
H&S Sec. 16.1 p. 386, Sec. 16.2 p. 386
PCS Sec. 11 pp. 141,142
Question 7.32
What is alloca and why is its use discouraged?
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
conventional 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 is not Standard and cannot be used in programs which must be widely
portable, no matter how useful it might be.
See also question 7.22.
References: Rationale Sec. 4.10.3
Question 8.1
Why doesn't
strcat(string, '!');
work?
There is a very real difference between characters and strings, and strcat concatenates strings.
Characters in C are represented by small integers corresponding to their character set values (see
also question 8.6). Strings are represented by arrays of characters; you usually manipulate a
pointer to the first character of the array. It is never correct to use one when the other is expected.
To append a ! to a string, use
strcat(string, "!");
See also questions 1.32, 7.2, and 16.6.
References: CT&P Sec. 1.5 pp. 9-10
Question 8.2
I'm checking a string to see if it matches a particular value. Why isn't this code working?
char *string;
...
if(string == "value") {
/* string matches "value" */
...
}
Strings in C are represented as arrays of characters, and C never manipulates (assigns, compares,
etc.) arrays as a whole. The == operator in the code fragment above compares two pointers--the
value of the pointer variable string and a pointer to the string literal "value"--to see if they are
equal, that is, if they point to the same place. They probably don't, so the comparison never
succeeds.
To compare two strings, you generally use the library function strcmp:
if(strcmp(string, "value") == 0) {
/* string matches "value" */
...
}
Question 8.3
If I can say
char a[] = "Hello, world!";
why can't I say
char a[14];
a = "Hello, world!";
Strings are arrays, and you can't assign arrays directly. Use strcpy instead:
strcpy(a, "Hello, world!");
See also questions 1.32, 4.2, and 7.2.
Question 8.6
How can I get the numeric (character set) value corresponding to a character, or vice versa?
In C, characters are represented by small integers corresponding to their values (in the machine's
character set), so you don't need a conversion routine: if you have the character, you have its
value.
Question 8.9
I think something's wrong with my compiler: I just noticed that sizeof('a') is 2, not 1 (i.e. not
sizeof(char)).
Perhaps surprisingly, character constants in C are of type int, so sizeof('a') is sizeof(int) (though
it's different in C++). See also question 7.8.
References: ANSI Sec. 3.1.3.4
ISO Sec. 6.1.3.4
H&S Sec. 2.7.3 p. 29
Question 9.1
What is the right type to use for Boolean values in C? Why isn't it a standard type? Should I use
#defines or enums for the true and false values?
C does not provide a standard Boolean type, in part because picking one involves a space/time
tradeoff which can best be decided by the programmer. (Using an int may be faster, while using
char may save data space. Smaller types may make the generated code bigger or slower, though,
if they require lots of conversions to and from int.)
The choice between #defines and enumeration constants for the true/false values is arbitrary and
not terribly interesting (see also questions 2.22 and 17.10). 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 enumeration
may be preferable if your debugger shows the names of enumeration constants when examining
variables.)
Some people prefer variants like
#define TRUE (1==1)
#define FALSE (!TRUE)
or define ``helper'' macros such as
#define Istrue(e) ((e) != 0)
These don't buy anything (see question 9.2; see also questions 5.12 and 10.2). Question 9.2
Isn't #defining TRUE to be 1 dangerous, since any nonzero value is considered ``true'' in C? What
if a built-in logical or relational operator ``returns'' something other than 1?
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)
would work as expected (as long as TRUE is 1), but it is obviously silly. In general, explicit tests
against TRUE and FALSE are inappropriate, 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 function parameter, or as the return
value from a Boolean function, but 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 questions 5.3 and
5.10.)
On the other hand, Boolean values and definitions can evidently be confusing, and some
programmers feel that TRUE and FALSE macros only compound the confusion. (See also
question 5.9.)
References: K&R1 Sec. 2.6 p. 39, Sec. 2.7 p. 41
K&R2 Sec. 2.6 p. 42, Sec. 2.7 p. 44, Sec. A7.4.7 p. 204, Sec. A7.9 p. 206
ANSI Sec. 3.3.3.3, Sec. 3.3.8, Sec. 3.3.9, Sec. 3.3.13, Sec. 3.3.14, Sec. 3.3.15, Sec. 3.6.4.1, Sec.
3.6.5
ISO Sec. 6.3.3.3, Sec. 6.3.8, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15, Sec. 6.6.4.1, Sec.
6.6.5
H&S Sec. 7.5.4 pp. 196-7, Sec. 7.6.4 pp. 207-8, Sec. 7.6.5 pp. 208-9, Sec. 7.7 pp. 217-8, Sec. 7.8
pp. 218-9, Sec. 8.5 pp. 238-9, Sec. 8.6 pp. 241-4
``What the Tortoise Said to Achilles''
Question 9.3
Is if(p), where p is a pointer, a valid conditional?
Yes. See question 5.3.
Question 10.2
Here are some cute preprocessor macros:
#define begin {
#define end }
What do y'all think?
Bleah. See also section 17.
Question 10.3
How can I write a generic macro to swap two values?
There is no good answer to this question. If the values are integers, a well-known trick using
exclusive-OR could perhaps be used, but it will not work for floating-point values or pointers, or
if the two values are the same variable (and the ``obvious'' supercompressed implementation for
integral types a^=b^=a^=b is illegal due to multiple side-effects; see question 3.2). If the macro is
intended to be used on values of arbitrary type (the usual goal), it cannot use a temporary, since it
does not know what type of temporary it needs (and would have a hard time naming it if it did),
and standard C does not provide a typeof operator.
The best all-around solution is probably to forget about using a macro, unless you're willing to
pass in the type as a third argument.
Question 10.4
What's the best way to write a multi-statement macro?
The usual goal is to write a macro that can be invoked as if it were a statement consisting of a
single function call. This means that the ``caller'' will be supplying the final semicolon, so the
macro body should not. The macro body cannot therefore be a simple brace-enclosed 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, therefore, is to use
#define MACRO(arg1, arg2) 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. (For an example, see the first DEBUG() macro in question 10.26.) This technique also
allows a value to be ``returned.''
References: H&S Sec. 3.3.2 p. 45
CT&P Sec. 6.3 pp. 82-3
Question 10.6
I'm splitting up a program into multiple source files for the first time, and I'm wondering what to
put in .c files and what to put in .h files. (What does ``.h'' mean, anyway?)
As a general rule, you should put these things in header (.h) files:
macro definitions (preprocessor #defines)
structure, union, and enumeration declarations
typedef declarations
external function declarations (see also question 1.11)
global variable declarations
It's especially important to put a declaration or definition in a header file when it will be shared
between several other files. (In particular, never put external function prototypes in .c files. See
also question 1.7.)
On the other hand, when a definition or declaration should remain private to one source file, it's
fine to leave it there.
See also questions 1.7 and 10.7.
References: K&R2 Sec. 4.5 pp. 81-2
H&S Sec. 9.2.3 p. 267
CT&P Sec. 4.6 pp. 66-7
Question 10.7
Is it acceptable for one header file to #include another?
It's a question of style, and thus receives considerable debate. Many people believe that ``nested
#include files'' are to be avoided: the prestigious Indian Hill Style Guide (see question 17.9)
disparages them; they can make it harder to find relevant definitions; they can lead to multipledefinition
errors if a file is #included twice; and they make manual Makefile maintenance very
difficult. On the other hand, they make it possible to use header files in a modular way (a header
file can #include what it needs itself, rather than requiring each #includer to do so); a tool like
grep (or a tags file) makes it easy to find definitions no matter where they are; a popular trick
along the lines of:
#ifndef HFILENAME_USED
#define HFILENAME_USED
...header file contents...
#endif
(where a different bracketing macro name is used for each header file) makes a header file
``idempotent'' so that it can safely be #included multiple times; and automated Makefile
maintenance tools (which are a virtual necessity in large projects anyway; see question 18.1)
handle dependency generation in the face of nested #include files easily. See also question 17.10.
References: Rationale Sec. 4.1.2
Question 10.8
Where are header (``#include'') files searched for?
The exact behavior is implementation-defined (which means that it is supposed to be
documented; see question 11.33). Typically, headers named with <> syntax are searched for in
one or more standard places. Header files named with "" syntax are first searched for in the
``current directory,'' then (if not found) in the same standard places.
Traditionally (especially under Unix compilers), the current directory is taken to be the directory
containing the file containing the #include directive. Under other compilers, however, the current
directory (if any) is the directory in which the compiler was initially invoked. Check your
compiler documentation.
References: K&R2 Sec. A12.4 p. 231
ANSI Sec. 3.8.2
ISO Sec. 6.8.2
H&S Sec. 3.4 p. 55
Question 10.9
I'm getting strange syntax errors on the very first declaration in a file, but it looks fine.
Perhaps there's a missing semicolon at the end of the last declaration in the last header file you're
#including. See also questions 2.18 and 11.29.
Question 10.11
I seem to be missing the system header file . Can someone send me a copy?
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).
Question 10.12
How can I construct preprocessor #if expressions which compare strings?
You can't do it directly; preprocessor #if arithmetic uses only integers. You can #define several
manifest constants, however, and implement conditionals on those.
See also question 20.17.
References: K&R2 Sec. 4.11.3 p. 91
ANSI Sec. 3.8.1
ISO Sec. 6.8.1
H&S Sec. 7.11.1 p. 225
Question 10.13
Does the sizeof operator work in preprocessor #if directives?
No. Preprocessing happens during an earlier phase of compilation, before type names have been
parsed. Instead of sizeof, consider using the predefined constants in ANSI's , if
applicable, or perhaps a ``configure'' script. (Better yet, try to write code which is inherently
insensitive to type sizes.)
References: ANSI Sec. 2.1.1.2, Sec. 3.8.1 footnote 83
ISO Sec. 5.1.1.2, Sec. 6.8.1
H&S Sec. 7.11.1 p. 225
Question 10.14
Can I use an #ifdef in a #define line, to define something two different ways?
No. You can't ``run the preprocessor on itself,'' so to speak. What you can do is use one of two
completely separate #define lines, depending on the #ifdef setting.
References: ANSI Sec. 3.8.3, Sec. 3.8.3.4
ISO Sec. 6.8.3, Sec. 6.8.3.4
H&S Sec. 3.2 pp. 40-1
Question 10.15
Is there anything like an #ifdef for typedefs?
Unfortunately, no. (See also question 10.13.)
References: ANSI Sec. 2.1.1.2, Sec. 3.8.1 footnote 83
ISO Sec. 5.1.1.2, Sec. 6.8.1
H&S Sec. 7.11.1 p. 225
Question 10.16
How can I use a preprocessor #if expression to tell if a machine is big-endian or little-endian?
You probably can't. (Preprocessor arithmetic uses only long integers, and there is no concept of
addressing. ) Are you sure you need to know the machine's endianness explicitly? Usually it's
better to write code which doesn't care ). See also question 20.9.
References: ANSI Sec. 3.8.1
ISO Sec. 6.8.1
H&S Sec. 7.11.1 p. 225
Question 10.18
I inherited some code which contains far too many #ifdef's for my taste. How can I preprocess the
code to leave only one conditional compilation set, without running it through the preprocessor
and expanding all of the #include's and #define's as well?
There are programs floating around called unifdef, rmifdef, and scpp (``selective C preprocessor'')
which do exactly this. See question 18.16.
Question 10.19
How can I list all of the pre#defined identifiers?
There's no standard way, although it is a common need. If the compiler documentation is
unhelpful, the most expedient way is probably to extract printable strings from the compiler or
preprocessor executable with something like the Unix strings utility. Beware that many
traditional system-specific pre#defined identifiers (e.g. ``unix'') are non-Standard (because they
clash with the user's namespace) and are being removed or renamed.
Question 10.20
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.
It was an undocumented feature of some early preprocessor implementations (notably John
Reiser's) that comments disappeared entirely and could therefore be used for token pasting. ANSI
affirms (as did K&R1) 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 11.17.
References: ANSI Sec. 3.8.3.3
ISO Sec. 6.8.3.3
Rationale Sec. 3.8.3.3
H&S Sec. 3.3.9 p. 52
Question 10.22
Why is the macro
#define TRACE(n) printf("TRACE: %d\n", n)
giving me the warning ``macro replacement within a string literal''? It seems to be expanding
TRACE(count);
as
printf("TRACE: %d\count", count);
See question 11.18.
Question 10.23
How can I use a macro argument inside a string literal in the macro expansion?
See question 11.18.
Question 10.25
I've got this tricky preprocessing I want to do and I can't figure out a way to do it.
C's preprocessor is not intended as a general-purpose tool. (Note also that it is not guaranteed to
be available as a separate program.) Rather than forcing it to do something inappropriate,
consider writing your own little special-purpose preprocessing tool, instead. You can easily get a
utility like make(1) to run it for you automatically.
If you are trying to preprocess something other than C, consider using a general-purpose
preprocessor. (One older one available on most Unix systems is m4.)
Question 10.26
How can I write a macro which takes a variable number of arguments?
One popular trick is to define and invoke the macro with a single, parenthesized ``argument''
which in the macro expansion becomes the entire argument list, parentheses and all, for a
function such as printf:
#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.
gcc has an extension which allows a function-like macro to accept a variable number of
arguments, but it's not standard. Other possible solutions are to use different macros (DEBUG1,
DEBUG2, etc.) depending on the number of arguments, to play games with commas:
#define DEBUG(args) (printf("DEBUG: "), printf(args))
#define _ ,
DEBUG("i = %d" _ i)
It is often better to use a bona-fide function, which can take a variable number of arguments in a
well-defined way. See questions 15.4 and 15.5.
Question 11.1
What is the ``ANSI C Standard?''
In 1983, the American National Standards Institute (ANSI) commissioned a committee, X3J11,
to standardize the C language. After a long, arduous process, including several widespread public
reviews, the committee's work was finally ratified as ANS X3.159-1989 on December 14, 1989,
and published in the spring of 1990. For the most part, ANSI C standardizes existing practice,
with a few additions from C++ (most notably function prototypes) and support for multinational
character sets (including the controversial trigraph sequences). The ANSI C standard also
formalizes the C run-time library support routines.
More recently, the Standard has been adopted as an international standard, ISO/IEC 9899:1990,
and this ISO Standard replaces the earlier X3.159 even within the United States. Its sections are
numbered differently (briefly, ISO sections 5 through 7 correspond roughly to the old ANSI
sections 2 through 4). As an ISO Standard, it is subject to ongoing revision through the release of
Technical Corrigenda and Normative Addenda.
In 1994, Technical Corrigendum 1 amended the Standard in about 40 places, most of them minor
corrections or clarifications. More recently, Normative Addendum 1 added about 50 pages of new
material, mostly specifying new library functions for internationalization. The production of
Technical Corrigenda is an ongoing process, and a second one is expected in late 1995. In
addition, both ANSI and ISO require periodic review of their standards. This process is beginning
in 1995, and will likely result in a completely revised standard (nicknamed ``C9X'' on the
assumption of completion by 1999).
The original ANSI Standard included a ``Rationale,'' explaining many of its decisions, and
discussing a number of subtle points, including several of those covered here. (The Rationale was
``not part of ANSI Standard X3.159-1989, but... included for information only,'' and is not
included with the ISO Standard.)
Question 11.2
How can I get a copy of the Standard?
[Late-breaking news: I've been told that copies of the new C99 can be obtained directly from
www.ansi.org; the price for an electronic document is only US $18.00.]
Copies are available in the United States from
American National Standards Institute
11 W. 42nd St., 13th floor
New York, NY 10036 USA
(+1) 212 642 4900
and
Global Engineering Documents
15 Inverness Way E
Englewood, CO 80112 USA
(+1) 303 397 2715
(800) 854 7179 (U.S. & Canada)
In other countries, contact the appropriate national standards body, or ISO in Geneva at:
ISO Sales
Case Postale 56
CH-1211 Geneve 20
Switzerland
(or see URL http://www.iso.ch or check the comp.std.internat FAQ list, Standards.Faq).
At the time of this writing, the cost is $130.00 from ANSI or $410.00 from Global. Copies of the
original X3.159 (including the Rationale) may still be available at $205.00 from ANSI or $162.50
from Global. Note that ANSI derives revenues to support its operations from the sale of printed
standards, so electronic copies are not available.
In the U.S., it may be possible to get a copy of the original ANSI X3.159 (including the
Rationale) as ``FIPS PUB 160'' from
National Technical Information Service (NTIS)
U.S. Department of Commerce
Springfield, VA 22161
703 487 4650
The mistitled Annotated ANSI C Standard, with annotations by Herbert Schildt, contains most of
the text of ISO 9899; it is published by Osborne/McGraw-Hill, ISBN 0-07-881952-0, and sells in
the U.S. for approximately $40. It has been suggested that the price differential between this work
and the official standard reflects the value of the annotations: they are plagued by numerous
errors and omissions, and a few pages of the Standard itself are missing. Many people on the net
recommend ignoring the annotations entirely. A review of the annotations (``annotated
annotations'') by Clive Feather can be found on the web at http://www.lysator.liu.se/c/schildt.html
.
The text of the Rationale (not the full Standard) can be obtained by anonymous ftp from
ftp.uu.net (see question 18.16) in directory doc/standards/ansi/X3.159-1989, and is also available
on the web at http://www.lysator.liu.se/c/rat/title.html . The Rationale has also been printed by
Silicon Press, ISBN 0-929306-07-4.
Question 11.3
My ANSI compiler complains about a mismatch when it sees
extern int func(float);
int func(x)
float x;
{ ...
You have mixed the new-style prototype declaration ``extern int func(float);'' with the old-style
definition ``int func(x) float x;''. It is usually safe to mix the two styles (see question 11.4), but
not in this case.
Old C (and ANSI C, in the absence of prototypes, and in variable-length argument lists; see
question 15.2) ``widens'' certain arguments when they are passed to functions. floats are promoted
to double, and characters and short integers are promoted to int. (For old-style function
definitions, the values are automatically converted back to the corresponding narrower types
within the body of the called function, if they are declared that way there.)
This problem can be fixed either by using new-style syntax consistently in the definition:
int func(float x) { ... }
or by changing the new-style prototype declaration to match the old-style definition:
extern int func(double);
(In this case, it would be clearest to change the old-style definition to use double as well, as long
as the address of that parameter is not taken.)
It may also be safer to avoid ``narrow'' (char, short int, and float) function arguments and return
types altogether.
See also question 1.25.
References: K&R1 Sec. A7.1 p. 186
K&R2 Sec. A7.3.2 p. 202
ANSI Sec. 3.3.2.2, Sec. 3.5.4.3
ISO Sec. 6.3.2.2, Sec. 6.5.4.3
Rationale Sec. 3.3.2.2, Sec. 3.5.4.3
H&S Sec. 9.2 pp. 265-7, Sec. 9.4 pp. 272-3
Question 11.4
Can you mix old-style and new-style function syntax?
Doing so is perfectly legal, as long as you're careful (see especially question 11.3). Note however
that old-style syntax is marked as obsolescent, so official support for it may be removed some
day.
References: ANSI Sec. 3.7.1, Sec. 3.9.5
ISO Sec. 6.7.1, Sec. 6.9.5
H&S Sec. 9.2.2 pp. 265-7, Sec. 9.2.5 pp. 269-70
Question 11.5
Why does the declaration
extern f(struct x *p);
give me an obscure warning message about ``struct x introduced in prototype scope''?
In a quirk of C's normal block scoping rules, a structure declared (or even mentioned) for the first
time within a prototype cannot be compatible with other structures declared in the same source
file (it goes out of scope at the end of the prototype).
To resolve the problem, precede the prototype with the vacuous-looking declaration
struct x;
which places an (incomplete) declaration of struct x at file scope, so that all following
declarations involving struct x can at least be sure they're referring to the same struct x.
References: ANSI Sec. 3.1.2.1, Sec. 3.1.2.6, Sec. 3.5.2.3
ISO Sec. 6.1.2.1, Sec. 6.1.2.6, Sec. 6.5.2.3
Question 11.8
I don't understand why I can't use const values in initializers and array dimensions, as in
const int n = 5;
int a[n];
The const qualifier really means ``read-only;'' an object so qualified is a run-time object which
cannot (normally) be assigned to. The value of a const-qualified object is therefore not a constant
expression in the full sense of the term. (C is unlike C++ in this regard.) When you need a true
compile-time constant, use a preprocessor #define.
References: ANSI Sec. 3.4
ISO Sec. 6.4
H&S Secs. 7.11.2,7.11.3 pp. 226-7
Question 11.9
What's the difference between const char *p and char * const p?
const char *p declares a pointer to a constant character (you can't change the character); char *
const p declares a constant pointer to a (variable) character (i.e. you can't change the pointer).
Read these ``inside out'' to understand them; see also question 1.21.
References: ANSI Sec. 3.5.4.1 examples
ISO Sec. 6.5.4.1
Rationale Sec. 3.5.4.1
H&S Sec. 4.4.4 p. 81
Question 11.10
Why can't I pass a char ** to a function which expects a const char **?
You can use a pointer-to-T (for any type T) where a pointer-to-const-T is expected. However, the
rule (an explicit exception) which permits slight mismatches in qualified pointer types is not
applied recursively, but only at the top level.
You must use explicit casts (e.g. (const char **) in this case) when assigning (or passing) pointers
which have qualifier mismatches at other than the first level of indirection.
References: ANSI Sec. 3.1.2.6, Sec. 3.3.16.1, Sec. 3.5.3
ISO Sec. 6.1.2.6, Sec. 6.3.16.1, Sec. 6.5.3
H&S Sec. 7.9.1 pp. 221-2
Question 11.12
Can I declare main as void, to shut off these annoying ``main returns no value'' messages?
No. main must be declared as returning an int, and as taking either zero or two arguments, of the
appropriate types. If you're calling exit() but still getting warnings, you may have to insert a
redundant return statement (or use some kind of ``not reached'' directive, if available).
Declaring a function as void does not merely shut off or rearrange warnings: it may also result in
a different function call/return sequence, incompatible with what the caller (in main's case, the C
run-time startup code) expects.
(Note that this discussion of main pertains only to ``hosted'' implementations; none of it applies
to ``freestanding'' implementations, which may not even have main. However, freestanding
implementations are comparatively rare, and if you're using one, you probably know it. If you've
never heard of the distinction, you're probably using a hosted implementation, and the above rules
apply.)
References: ANSI Sec. 2.1.2.2.1, Sec. F.5.1
ISO Sec. 5.1.2.2.1, Sec. G.5.1
H&S Sec. 20.1 p. 416
CT&P Sec. 3.10 pp. 50-51
Question 11.13
But what about main's third argument, envp?
It's a non-standard (though common) extension. If you really need to access the environment in
ways beyond what the standard getenv function provides, though, the global variable environ is
probably a better avenue (though it's equally non-standard).
References: ANSI Sec. F.5.1
ISO Sec. G.5.1
H&S Sec. 20.1 pp. 416-7
Question 11.14
I believe that declaring void main() can't fail, since I'm calling exit instead of returning, and
anyway my operating system ignores a program's exit/return status.
It doesn't matter whether main returns or not, or whether anyone looks at the status; the problem
is that when main is misdeclared, its caller (the runtime startup code) may not even be able to call
it correctly (due to the potential clash of calling conventions; see question 11.12). Your operating
system may ignore the exit status, and void main() may work for you, but it is not portable and
not correct
Question 11.15
The book I've been using, C Programing for the Compleat Idiot, always uses void main().
Perhaps its author counts himself among the target audience. Many books unaccountably use void
main() in examples. They're wrong.
Question 11.16
Is exit(status) truly equivalent to returning the same status from main?
Yes and no. The Standard says that they are equivalent. However, a few older, nonconforming
systems may have problems with one or the other form. Also, a return from main cannot be
expected to work if data local to main might be needed during cleanup; see also question 16.4.
(Finally, the two forms are obviously not equivalent in a recursive call to main.)
References: K&R2 Sec. 7.6 pp. 163-4

ANSI Sec. 2.1.2.2.3
ISO Sec. 5.1.2.2.3
Question 11.17
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.
You can use something like the following two-step procedure to force a 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 code 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
ISO Sec. 6.8.3.2, Sec. 6.8.3.5
Question 11.18
What does the message ``warning: macro replacement within a string literal'' mean?
Some pre-ANSI compilers/preprocessors interpreted macro definitions like
#define TRACE(var, fmt) printf("TRACE: var = fmt\n", var)
such that invocations like
TRACE(i, %d);
were expanded as
printf("TRACE: i = %d\n", i);
In other words, macro parameters were expanded even inside string literals and character
constants.
Macro expansion is not defined in this way by K&R or by Standard C. When you do want to turn
macro arguments into strings, you can use the new # preprocessing operator, along with string
literal concatenation (another new ANSI feature):
#define TRACE(var, fmt) \
printf("TRACE: " #var " = " #fmt "\n", var)
See also question 11.17.
References: H&S Sec. 3.3.8 p. 51
Question 11.19
I'm getting strange syntax errors inside lines I've #ifdeffed out.
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 newlines inside quotes, and no
unterminated comments or quotes (note particularly that an apostrophe within a contracted word
looks like the beginning of a character constant). Therefore, natural-language comments and
pseudocode should always be written between the ``official'' comment delimiters /* and */. (But
see question 20.20, and also 10.25.)
References: ANSI Sec. 2.1.1.2, Sec. 3.1
ISO Sec. 5.1.1.2, Sec. 6.1
H&S Sec. 3.2 p. 40
Question 11.20
What are #pragmas and what are they good for?
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 lint's old /* NOTREACHED */ comments), etc.
References: ANSI Sec. 3.8.6
ISO Sec. 6.8.6
H&S Sec. 3.7 p. 61
Question 11.21
What does ``#pragma once'' mean? I found it in some header files.
It is an extension implemented by some preprocessors to help make header files idempotent; it is
essentially equivalent to the #ifndef trick mentioned in question 10.7.
Question 11.22
Is char a[3] = "abc"; legal? What does it mean?
It is legal in ANSI C (and perhaps in a few pre-ANSI systems), though useful only in rare
circumstances. It declares an array of size three, initialized with the three characters 'a', 'b', and 'c',
without the usual terminating '\0' character. The array is therefore not a true C string and cannot
be used with strcpy, printf %s, etc.
Most of the time, you should let the compiler count the initializers when initializing arrays (in the
case of the initializer "abc", of course, the computed size will be 4).
References: ANSI Sec. 3.5.7
ISO Sec. 6.5.7
H&S Sec. 4.6.4 p. 98
Question 11.24
Why can't I perform arithmetic on a void * pointer?
The compiler doesn't know the size of the pointed-to objects. Before performing arithmetic,
convert the pointer either to char * or to the pointer type you're trying to manipulate (but see also
question 4.5).
References: ANSI Sec. 3.1.2.5, Sec. 3.3.6
ISO Sec. 6.1.2.5, Sec. 6.3.6
H&S Sec. 7.6.2 p. 204
Question 11.25
What's the difference between memcpy and memmove?
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: K&R2 Sec. B3 p. 250
ANSI Sec. 4.11.2.1, Sec. 4.11.2.2
ISO Sec. 7.11.2.1, Sec. 7.11.2.2
Rationale Sec. 4.11.2
H&S Sec. 14.3 pp. 341-2
PCS Sec. 11 pp. 165-6
Question 11.26
What should malloc(0) do? Return a null pointer or a pointer to 0 bytes?
The ANSI/ISO Standard says that it may do either; the behavior is implementation-defined (see
question 11.33).
References: ANSI Sec. 4.10.3
ISO Sec. 7.10.3
PCS Sec. 16.1 p. 386
Question 11.27
Why does the ANSI Standard not guarantee more than six case-insensitive characters of external
identifier significance?
The problem is older linkers which are under the control of neither the ANSI/ISO 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 11.1), which discusses
several such schemes and explains why they could not be mandated.
References: ANSI Sec. 3.1.2, Sec. 3.9.1
ISO Sec. 6.1.2, Sec. 6.9.1
Rationale Sec. 3.1.2
H&S Sec. 2.5 pp. 22-3
Question 11.29
My compiler is rejecting the simplest possible test programs, with all kinds of syntax errors.
Perhaps it is a pre-ANSI compiler, unable to accept function prototypes and the like.
See also questions 1.31, 10.9, and 11.30.
Question 11.30
Why are some ANSI/ISO Standard library routines showing up as undefined, even though I've got
an ANSI compiler?
It's possible to have a compiler available which accepts ANSI syntax, but not to have ANSIcompatible
header files or run-time libraries installed. (In fact, this situation is rather common
when using a non-vendor-supplied compiler such as gcc.) See also questions 11.29, 13.25, and
13.26.
Question 11.31
Does anyone have a tool for converting old-style C programs to ANSI C, or vice versa, or for
automatically generating prototypes?
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 are part of the FSF's GNU C compiler
distribution; see question 18.3.
The unproto program (/pub/unix/unproto5.shar.Z on ftp.win.tue.nl) is a filter which sits between
the preprocessor and the next compiler pass, converting most of ANSI C to traditional C on-thefly.
The GNU GhostScript package comes with a little program called ansi2knr.
Before converting ANSI C back to old-style, beware that such a conversion cannot always be
made both safely and automatically. ANSI C introduces new features and complexities not found
in K&R C. You'll especially need to be careful of prototyped function calls; you'll probably need
to insert explicit casts. See also questions 11.3 and 11.29.
Several prototype generators exist, many as modifications to lint. A program called CPROTO
was posted to comp.sources.misc in March, 1992. There is another program called ``cextract.''
Many vendors supply simple utilities like these with their compilers. See also question 18.16.
(But be careful when generating prototypes for old functions with ``narrow'' parameters; see
question 11.3.)
Finally, are you sure you really need to convert lots of old code to ANSI C? The old-style
function syntax is still acceptable, and a hasty conversion can easily introduce bugs. (See
question 11.3.)
Question 11.32
Why won't the Frobozz Magic C Compiler, which claims to be ANSI compliant, accept this
code? I know that the code is ANSI, because gcc accepts it.
Many compilers support a few non-Standard extensions, gcc more so than most. Are you sure
that the code being rejected doesn't rely on such an extension? It is usually a bad idea to perform
experiments with a particular compiler to determine properties of a language; the applicable
standard may permit variations, or the compiler may be wrong. See also question 11.35.
Question 11.33
People seem to make a point of distinguishing between implementation-defined, unspecified, and
undefined behavior. What's the difference?
Briefly: implementation-defined means that an implementation must choose some behavior and
document it. Unspecified means that an implementation should choose some behavior, but need
not document it. Undefined means that absolutely anything might happen. In no case does the
Standard impose requirements; in the first two cases it occasionally suggests (and may require a
choice from among) a small set of likely behaviors.
Note that since the Standard imposes no requirements on the behavior of a compiler faced with
an instance of undefined behavior, the compiler can do absolutely anything. In particular, there is
no guarantee that the rest of the program will perform normally. It's perilous to think that you can
tolerate undefined behavior in a program; see question 3.2 for a relatively simple example.
If you're interested in writing portable code, you can ignore the distinctions, as you'll want to
avoid code that depends on any of the three behaviors.
See also questions 3.9, and 11.34.
References: ANSI Sec. 1.6
ISO Sec. 3.10, Sec. 3.16, Sec. 3.17
Rationale Sec. 1.6
Question 11.34
I'm appalled that the ANSI Standard leaves so many issues undefined. Isn't a Standard's whole job
to standardize these things?
It has always been a characteristic of C that certain constructs behaved in whatever way a
particular compiler or a particular piece of hardware chose to implement them. This deliberate
imprecision often allows compilers to generate more efficient code for common cases, without
having to burden all programs with extra code to assure well-defined behavior of cases deemed to
be less reasonable. Therefore, the Standard is simply codifying existing practice.
A programming language standard can be thought of as a treaty between the language user and
the compiler implementor. Parts of that treaty consist of features which the compiler implementor
agrees to provide, and which the user may assume will be available. Other parts, however, consist
of rules which the user agrees to follow and which the implementor may assume will be
followed. As long as both sides uphold their guarantees, programs have a fighting chance of
working correctly. If either side reneges on any of its commitments, nothing is guaranteed to
work.
See also question 11.35.
References: Rationale Sec. 1.1
Question 11.35
People keep saying that the behavior of i = i++ is undefined, but I just tried it on an ANSIconforming
compiler, and got the results I expected.
A compiler may do anything it likes when faced with undefined behavior (and, within limits, with
implementation-defined and unspecified behavior), including doing what you expect. It's unwise
to depend on it, though. See also questions 11.32, 11.33, and 11.34.
Question 12.1
What's wrong with this code?
char c;
while((c = getchar()) != EOF) ...
For one thing, the variable to hold getchar's return value must be an int. getchar can return all
possible character values, as well as EOF. By passing getchar's return value through a char, either
a normal character might be misinterpreted as EOF, or the EOF might be altered (particularly if
type char is unsigned) and so never seen.
References: K&R1 Sec. 1.5 p. 14
K&R2 Sec. 1.5.1 p. 16
ANSI Sec. 3.1.2.5, Sec. 4.9.1, Sec. 4.9.7.5
ISO Sec. 6.1.2.5, Sec. 7.9.1, Sec. 7.9.7.5
H&S Sec. 5.1.3 p. 116, Sec. 15.1, Sec. 15.6
CT&P Sec. 5.1 p. 70
PCS Sec. 11 p. 157
Question 12.2
Why does the code while(!feof(infp)) { fgets(buf, MAXLINE, infp); fputs(buf, outfp); } copy the
last line twice?
In C, EOF is only indicated after an input routine has tried to read, and has reached end-of-file.
(In other words, C's I/O is not like Pascal's.) Usually, you should just check the return value of the
input routine (fgets in this case); often, you don't need to use feof at all.
References: K&R2 Sec. 7.6 p. 164
ANSI Sec. 4.9.3, Sec. 4.9.7.1, Sec. 4.9.10.2
ISO Sec. 7.9.3, Sec. 7.9.7.1, Sec. 7.9.10.2
H&S Sec. 15.14 p. 382
Question 12.4
My program's prompts and intermediate output don't always show up on the screen, especially
when I pipe the output through another program.
It's 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 an interactive terminal. (See also question 12.24.)
References: ANSI Sec. 4.9.5.2
ISO Sec. 7.9.5.2
Question 12.5
How can I read one character at a time, without waiting for the RETURN key?
See question 19.1.
Question 12.6
How can I print a '%' character in a printf format string? I tried \%, but it didn't work.
Simply double the percent sign: %% .
\% can't work, because the backslash \ is the compiler's escape character, while here our problem
is that the % is printf's escape character.
See also question 19.17.
References: K&R1 Sec. 7.3 p. 147
K&R2 Sec. 7.2 p. 154
ANSI Sec. 4.9.6.1
ISO Sec. 7.9.6.1
Question 12.9
Someone told me it was wrong to use %lf with printf. How can printf use %f for type double, if
scanf requires %lf?
It's true that printf's %f specifier works with both float and double arguments. Due to the ``default
argument promotions'' (which apply in variable-length argument lists such as printf's, whether or
not prototypes are in scope), values of type float are promoted to double, and printf therefore sees
only doubles. See also questions 12.13 and 15.2.
References: K&R1 Sec. 7.3 pp. 145-47, Sec. 7.4 pp. 147-50
K&R2 Sec. 7.2 pp. 153-44, Sec. 7.4 pp. 157-59
ANSI Sec. 4.9.6.1, Sec. 4.9.6.2
ISO Sec. 7.9.6.1, Sec. 7.9.6.2
H&S Sec. 15.8 pp. 357-64, Sec. 15.11 pp. 366-78
CT&P Sec. A.1 pp. 121-33
Question 12.10
How can I implement a variable field width with printf? That is, instead of %8d, I want the width
to be specified at run time.
printf("%*d", width, n) will do just what you want. See also question 12.15.
References: K&R1 Sec. 7.3
K&R2 Sec. 7.2
ANSI Sec. 4.9.6.1
ISO Sec. 7.9.6.1
H&S Sec. 15.11.6
CT&P Sec. A.1
Question 12.11
How can I print numbers with commas separating the thousands?
What about currency formatted numbers?
The routines in begin to provide some support for these operations, but there is no
standard routine for doing either task. (The only thing printf does in response to a custom locale
setting is to change its decimal-point character.)
References: ANSI Sec. 4.4
ISO Sec. 7.4
H&S Sec. 11.6 pp. 301-4
Question 12.12
Why doesn't the call scanf("%d", i) work?
The arguments you pass to scanf must always be pointers. To fix the fragment above, change it to
scanf("%d", &i) .
Question 12.13
Why doesn't this code:
double d;
scanf("%f", &d);
work?
Unlike printf, scanf uses %lf for values of type double, and %f for float. See also question 12.9.
Question 12.15
How can I specify a variable width in a scanf format string?
You can't; an asterisk in a scanf format string means to suppress assignment. You may be able to
use ANSI stringizing and string concatenation to accomplish about the same thing, or to construct
a scanf format string on-the-fly.
Question 12.17
When I read numbers from the keyboard with scanf "%d\n", it seems to hang until I type one
extra line of input.
Perhaps surprisingly, \n in a scanf format string does not mean to expect a newline, but rather to
read and discard characters as long as each is a whitespace character. See also question 12.20.
References: K&R2 Sec. B1.3 pp. 245-6
ANSI Sec. 4.9.6.2
ISO Sec. 7.9.6.2
H&S Sec. 15.8 pp. 357-64
Question 12.18
I'm reading a number with scanf %d and then a string with gets(), but the compiler seems to be
skipping the call to gets()!
scanf %d won't consume a trailing newline. If the input number is immediately followed by a
newline, that newline will immediately satisfy the gets().
As a general rule, you shouldn't try to interlace calls to scanf with calls to gets() (or any other
input routines); scanf's peculiar treatment of newlines almost always leads to trouble. Either use
scanf to read everything or nothing.
See also questions 12.20 and 12.23.
References: ANSI Sec. 4.9.6.2
ISO Sec. 7.9.6.2
H&S Sec. 15.8 pp. 357-64
Question 12.19
I figured I could use scanf more safely if I checked its return value to make sure that the user
typed the numeric values I expect, but sometimes it seems to go into an infinite loop.
When scanf is attempting to convert numbers, any non-numeric characters it encounters terminate
the conversion and are left on the input stream. Therefore, unless some other steps are taken,
unexpected non-numeric input ``jams'' scanf again and again: scanf never gets past the bad
character(s) to encounter later, valid data. If the user types a character like `x' in response to a
numeric scanf format such as %d or %f, code that simply re-prompts and retries the same scanf
call will immediately reencounter the same `x'.
See also question 12.20.
References: ANSI Sec. 4.9.6.2
ISO Sec. 7.9.6.2
H&S Sec. 15.8 pp. 357-64
Question 12.20
Why does everyone say not to use scanf? What should I use instead?
scanf has a number of problems--see questions 12.17, 12.18, and 12.19. Also, its %s format has
the same problem that gets() has (see question 12.23)--it's hard to guarantee that the receiving
buffer won't overflow.
More generally, scanf is designed for relatively structured, formatted input (its name is in fact
derived from ``scan formatted''). If you pay attention, it will tell you whether it succeeded or
failed, but it can tell you only approximately where it failed, and not at all how or why. It's nearly
impossible to do decent error recovery with scanf; usually it's far easier to read entire lines (with
fgets or the like), then interpret them, either using sscanf or some other techniques. (Routines like
strtol, strtok, and atoi are often useful; see also question 13.6.) If you do use sscanf, don't forget
to check the return value to make sure that the expected number of items were found.
References: K&R2 Sec. 7.4 p. 159
Question 12.21
How can I tell how much destination buffer space I'll need for an arbitrary sprintf call? How can I
avoid overflowing the destination buffer with sprintf?
There are not (yet) any good answers to either of these excellent questions, and this represents
perhaps the biggest deficiency in the traditional stdio library.
When the format string being used with sprintf is known and relatively simple, you can usually
predict a buffer size in an ad-hoc way. If the format consists of one or two %s's, you can count the
fixed characters in the format string yourself (or let sizeof count them for you) and add in the
result of calling strlen on the string(s) to be inserted. You can conservatively estimate the size
that %d will expand to with code like:
#include
char buf[(sizeof(int) * CHAR_BIT + 2) / 3 + 1 + 1];
sprintf(buf, "%d", n);
(This code computes the number of characters required for a base-8 representation of a number; a
base-10 expansion is guaranteed to take as much room or less.)
When the format string is more complicated, or is not even known until run time, predicting the
buffer size becomes as difficult as reimplementing sprintf, and correspondingly error-prone (and
inadvisable). A last-ditch technique which is sometimes suggested is to use fprintf to print the
same text to a bit bucket or temporary file, and then to look at fprintf's return value or the size of
the file (but see question 19.12).
If there's any chance that the buffer might not be big enough, you won't want to call sprintf
without some guarantee that the buffer will not overflow and overwrite some other part of
memory. Several stdio's (including GNU and 4.4bsd) provide the obvious snprintf function,
which can be used like this:
snprintf(buf, bufsize, "You typed \"%s\"", answer);
and we can hope that a future revision of the ANSI/ISO C Standard will include this function.
Question 12.23
Why does everyone say not to use gets()?
Unlike fgets(), gets() cannot be told the size of the buffer it's to read into, so it cannot be
prevented from overflowing that buffer. As a general rule, always use fgets(). See question 7.1 for
a code fragment illustrating the replacement of gets() with fgets().
References: Rationale Sec. 4.9.7.2
H&S Sec. 15.7 p. 356
Question 12.24
Why does errno contain ENOTTY after a call to printf?
Many implementations of the stdio package adjust their behavior slightly if stdout is a terminal.
To make the determination, these implementations perform some operation which happens to fail
(with ENOTTY) if stdout is not a terminal. Although the output operation goes on to complete
successfully, errno still contains ENOTTY. (Note that it is only meaningful for a program to
inspect the contents of errno after an error has been reported.)
References: ANSI Sec. 4.1.3, Sec. 4.9.10.3
ISO Sec. 7.1.4, Sec. 7.9.10.3
CT&P Sec. 5.4 p. 73
PCS Sec. 14 p. 254
Question 12.25
What's the difference between fgetpos/fsetpos and ftell/fseek?
What are fgetpos and fsetpos good for?
fgetpos and fsetpos use a special typedef, fpos_t, for representing offsets (positions) in a file. The
type behind this typedef, if chosen appropriately, can represent arbitrarily large offsets, allowing
fgetpos and fsetpos to be used with arbitrarily huge files. ftell and fseek, on the other hand, use
long int, and are therefore limited to offsets which can be represented in a long int. See also
question 1.4.
References: K&R2 Sec. B1.6 p. 248
ANSI Sec. 4.9.1, Secs. 4.9.9.1,4.9.9.3
ISO Sec. 7.9.1, Secs. 7.9.9.1,7.9.9.3
H&S Sec. 15.5 p. 252
Question 12.26
How can I flush pending input so that a user's typeahead isn't read at the next prompt? Will fflush
(stdin) work?
fflush is defined only for output streams. Since its definition of ``flush'' is to complete the writing
of buffered characters (not to discard them), discarding unread input would not be an analogous
meaning for fflush on input streams.
There is no standard way to discard unread characters from a stdio input stream, nor would such a
way be sufficient unread characters can also accumulate in other, OS-level input buffers.
References: ANSI Sec. 4.9.5.2
ISO Sec. 7.9.5.2
H&S Sec. 15.2
Question 12.30
I'm trying to update a file in place, by using fopen mode "r+", reading a certain string, and writing
back a modified string, but it's not working.
Be sure to call fseek before you write, both to seek back to the beginning of the string you're
trying to overwrite, and because an fseek or fflush is always required between reading and writing
in the read/write "+" modes. Also, remember that you can only overwrite characters with the
same number of replacement characters; see also question 19.14.
References: ANSI Sec. 4.9.5.3
ISO Sec. 7.9.5.3
Question 12.33
How can I redirect stdin or stdout to a file from within a program?
Use freopen (but see question 12.34).
References: ANSI Sec. 4.9.5.4
ISO Sec. 7.9.5.4
H&S Sec. 15.2
Question 12.34
Once I've used freopen, how can I get the original stdout (or stdin) back?
There isn't a good way. If you need to switch back, the best solution is not to have used freopen in
the first place. Try using your own explicit output (or input) stream variable, which you can
reassign at will, while leaving the original stdout (or stdin) undisturbed.
Question 12.38
How can I read a binary data file properly? I'm occasionally seeing 0x0a and 0x0d values getting
garbled, and it seems to hit EOF prematurely if the data contains the value 0x1a.
When you're reading a binary data file, you should specify "rb" mode when calling fopen, to
make sure that text file translations do not occur. Similarly, when writing binary data files, use
"wb".
Note that the text/binary distinction is made when you open the file: once a file is open, it doesn't
matter which I/O calls you use on it. See also question 20.5.
References: ANSI Sec. 4.9.5.3
ISO Sec. 7.9.5.3
H&S Sec. 15.2.1 p. 348
Question 13.1
How can I convert numbers to strings (the opposite of atoi)? Is there an itoa function?
Just use sprintf. (Don't worry that sprintf may be overkill, potentially wasting run time or code
space; it works well in practice.) See the examples in the answer to question 7.5; see also
question 12.21.
You can obviously use sprintf to convert long or floating-point numbers to strings as well (using
%ld or %f).
References: K&R1 Sec. 3.6 p. 60
K&R2 Sec. 3.6 p. 64
Question 13.2
Why does strncpy not always place a '\0' terminator in the destination string?
strncpy was first designed to handle a now-obsolete data structure, the fixed-length, notnecessarily-\
0-terminated ``string.'' (A related quirk of strncpy's is that it pads short strings with
multiple \0's, out to the specified length.) strncpy is admittedly a bit cumbersome to use in other
contexts, since you must often append a '\0' to the destination string by hand. You can get around
the problem by using strncat instead of strncpy: if the destination string starts out empty, strncat
does what you probably wanted strncpy to do. Another possibility is sprintf(dest, "%.*s", n,
source) .
When arbitrary bytes (as opposed to strings) are being copied, memcpy is usually a more
appropriate routine to use than strncpy.
Question 13.5
Why do some versions of toupper act strangely if given an upper-case letter?
Why does some code call islower before toupper?
Older versions of toupper and tolower did not always work correctly on arguments which did not
need converting (i.e. on digits or punctuation or letters already of the desired case). In ANSI/ISO
Standard C, these functions are guaranteed to work appropriately on all character arguments.
References: ANSI Sec. 4.3.2
ISO Sec. 7.3.2
H&S Sec. 12.9 pp. 320-1
PCS p. 182
Question 13.6
How can I split up a string into whitespace-separated fields?
How can I duplicate the process by which main() is handed argc and argv?
The only Standard routine available for this kind of ``tokenizing'' is strtok, although it can be
tricky to use and it may not do everything you want it to. (For instance, it does not handle
quoting.)
References: K&R2 Sec. B3 p. 250
ANSI Sec. 4.11.5.8
ISO Sec. 7.11.5.8
H&S Sec. 13.7 pp. 333-4
PCS p. 178
Question 13.7
I need some code to do regular expression and wildcard matching.
Make sure you recognize the difference between classic regular expressions (variants of which
are used in such Unix utilities as ed and grep), and filename wildcards (variants of which are used
by most operating systems).
There are a number of packages available for matching regular expressions. Most packages use a
pair of functions, one for ``compiling'' the regular expression, and one for ``executing'' it (i.e.
matching strings against it). Look for header files named or , and functions
called regcmp/regex, regcomp/regexec, or re_comp/re_exec. (These functions may exist in a
separate regexp library.) A popular, freely-redistributable regexp package by Henry Spencer is
available from ftp.cs.toronto.edu in pub/regexp.shar.Z or in several other archives. The GNU
project has a package called rx. See also question 18.16.
Filename wildcard matching (sometimes called ``globbing'') is done in a variety of ways on
different systems. On Unix, wildcards are automatically expanded by the shell before a process is
invoked, so programs rarely have to worry about them explicitly. Under MS-DOS compilers,
there is often a special object file which can be linked in to a program to expand wildcards while
argv is being built. Several systems (including MS-DOS and VMS) provide system services for
listing or opening files specified by wildcards. Check your compiler/library documentation.
Question 13.8
I'm trying to sort an array of strings with qsort, using strcmp as the comparison function, but it's
not working.
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, however, accepts simple pointers to char. Therefore, strcmp can't be used directly.
Write an intermediate comparison function like this:
/* compare strings via pointers */
int pstrcmp(const void *p1, const void *p2)
{
return strcmp(*(char * const *)p1, *(char * const *)p2);
}
The comparison function's arguments are expressed as ``generic pointers,'' const void *. They are
converted back to what they ``really are'' (char **) and dereferenced, yielding char *'s which can
be passed to strcmp. (Under a pre-ANSI compiler, declare the pointer parameters as char *
instead of void *, and drop the consts.)
(Don't be misled by the discussion in K&R2 Sec. 5.11 pp. 119-20, which is not discussing the
Standard library's qsort).
References: ANSI Sec. 4.10.5.2
ISO Sec. 7.10.5.2
H&S Sec. 20.5 p. 419
Question 13.9
Now I'm trying to sort an array of structures with qsort. My comparison function 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?
The conversions must be in the comparison function, which must be declared as accepting
``generic pointers'' (const void *) as discussed in question 13.8 above. The comparison function
might look like
int mystructcmp(const void *p1, const void *p2)
{
const struct mystruct *sp1 = p1;
const struct mystruct *sp2 = p2;
/* now compare sp1->whatever and sp2-> ... */
(The conversions from generic pointers to struct mystruct pointers happen in the initializations
sp1 = p1 and sp2 = p2; the compiler performs the conversions implicitly since p1 and p2 are void
pointers. Explicit casts, and char * pointers, would be required under a pre-ANSI compiler. See
also question 7.7.)
If, on the other hand, you're sorting pointers to structures, you'll need indirection, as in question
13.8: sp1 = *(struct mystruct **)p1 .
In general, it is a bad idea to insert casts just to ``shut the compiler up.'' Compiler warnings are
usually trying to tell you something, and unless you really know what you're doing, you ignore or
muzzle them at your peril. See also question 4.9.
References: ANSI Sec. 4.10.5.2
ISO Sec. 7.10.5.2
H&S Sec. 20.5 p. 419
Question 13.10
How can I sort a linked list?
Sometimes it's easier to keep the list in order as you build it (or perhaps to use a tree instead).
Algorithms like insertion sort and merge sort lend themselves ideally to use with linked lists. If
you want to use a standard library function, you can allocate a temporary array of pointers, fill it
in with pointers to all your list nodes, call qsort, and finally rebuild the list pointers based on the
sorted array.
References: Knuth Sec. 5.2.1 pp. 80-102, Sec. 5.2.4 pp. 159-168
Sedgewick Sec. 8 pp. 98-100, Sec. 12 pp. 163-175
Question 13.11
How can I sort more data than will fit in memory?
You want an ``external sort,'' which you can read about in Knuth, Volume 3. The basic idea is to
sort the data in chunks (as much as will fit in memory at one time), write each sorted chunk to a
temporary file, and then merge the files. Your operating system may provide a general-purpose
sort utility, and if so, you can try invoking it from within your program: see questions 19.27 and
19.30.
References: Knuth Sec. 5.4 pp. 247-378
Sedgewick Sec. 13 pp. 177-187
Question 13.12
How can I get the current date or time of day in a C program?
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;
time(&now);
printf("It's %.24s.\n", ctime(&now));
return 0;
}
References: K&R2 Sec. B10 pp. 255-7
ANSI Sec. 4.12
ISO Sec. 7.12
H&S Sec. 18
Question 13.13
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?
ANSI C specifies a library routine, mktime, which converts a struct tm to a time_t.
Converting a string to a time_t is harder, because of the wide variety of date and time formats
which might be encountered. Some systems provide a strptime function, which is basically the
inverse of strftime. Other popular routines are partime (widely distributed with the RCS package)
and getdate (and a few others, from the C news distribution). See question 18.16.
References: K&R2 Sec. B10 p. 256
ANSI Sec. 4.12.2.3
ISO Sec. 7.12.2.3
H&S Sec. 18.4 pp. 401-2
Question 13.14
How can I add n days to a date? How can I find the difference between two dates?
The ANSI/ISO Standard C mktime and difftime functions provide some support for both
problems. mktime accepts non-normalized dates, so it is straightforward to take a filled-in struct
tm, add or subtract from the tm_mday field, and call mktime to normalize the year, month, and
day fields (and incidentally convert to a time_t value). difftime computes the difference, in
seconds, between two time_t values; mktime can be used to compute time_t values for two dates
to be subtracted.
These solutions are only guaranteed to work correctly for dates in the range which can be
represented as time_t's. The tm_mday field is an int, so day offsets of more than 32,736 or so may
cause overflow. Note also that at daylight saving time changeovers, local days are not 24 hours
long.
Another approach to both problems is to use ``Julian day'' numbers. Implementations of Julian
day routines can be found in the file JULCAL10.ZIP from the Simtel/Oakland archives (see
question 18.16) and the ``Date conversions'' article mentioned in the References.
See also questions 13.13, 20.31, and 20.32.
References: K&R2 Sec. B10 p. 256
ANSI Secs. 4.12.2.2,4.12.2.3
ISO Secs. 7.12.2.2,7.12.2.3
H&S Secs. 18.4,18.5 pp. 401-2
David Burki, ``Date Conversions''
Question 13.15
I need a random number generator.
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.
If you do find yourself needing to implement your own random number generator, there is plenty
of literature out there; see the References. There are also any number of packages on the net: look
for r250, RANLIB, and FSULTRA (see question 18.16).
References: K&R2 Sec. 2.7 p. 46, Sec. 7.8.7 p. 168
ANSI Sec. 4.10.2.1
ISO Sec. 7.10.2.1
H&S Sec. 17.7 p. 393
PCS Sec. 11 p. 172
Knuth Vol. 2 Chap. 3 pp. 1-177
Park and Miller, ``Random Number Generators: Good Ones are hard to Find''
Question 13.16
How can I get random integers in a certain range?
The obvious way,
rand() % N /* POOR */
(which tries to return numbers from 0 to N-1) is poor, because the low-order bits of many random
number generators are distressingly non-random. (See question 13.18.) A better method is
something like
(int)((double)rand() / ((double)RAND_MAX + 1) * N)
If you're worried about using floating point, you could use
rand() / (RAND_MAX / N + 1)
Both methods obviously require knowing RAND_MAX (which ANSI #defines in ),
and assume that N is much less than RAND_MAX.
(Note, by the way, that RAND_MAX is a constant telling you what the fixed range of the C
library rand function is. You cannot set RAND_MAX to some other value, and there is no way of
requesting that rand return numbers in some other range.)
If you're starting with a random number generator which returns floating-point values between 0
and 1, all you have to do to get integers from 0 to N-1 is multiply the output of that generator by
N.
References: K&R2 Sec. 7.8.7 p. 168
PCS Sec. 11 p. 172
Question 13.17
Each time I run my program, I get the same sequence of numbers back from rand().
You can call srand to seed the pseudo-random number generator with a truly random initial value.
Popular seed values are the time of day, or the elapsed time before the user presses a key
(although keypress times are hard to determine portably; see question 19.37). (Note also that it's
rarely useful to call srand more than once during a run of a program; in particular, don't try
calling srand before each call to rand, in an attempt to get ``really random'' numbers.)
References: K&R2 Sec. 7.8.7 p. 168
ANSI Sec. 4.10.2.2
ISO Sec. 7.10.2.2
H&S Sec. 17.7 p. 393
Question 13.18
I need a random true/false value, so I'm just taking rand() % 2, but it's alternating 0, 1, 0, 1, 0...
Poor pseudorandom number generators (such as the ones unfortunately supplied with some
systems) are not very random in the low-order bits. Try using the higher-order bits: see question
13.16.
References: Knuth Sec. 3.2.1.1 pp. 12-14
Question 13.20
How can I generate random numbers with a normal or Gaussian distribution?
Here is one method, by Box and Muller, and recommended by Knuth:
#include
#include
double gaussrand()
{
static double V1, V2, S;
static int phase = 0;
double X;
if(phase == 0) {
do {
double U1 = (double)rand() / RAND_MAX;
double U2 = (double)rand() / RAND_MAX;
V1 = 2 * U1 - 1;
V2 = 2 * U2 - 1;
S = V1 * V1 + V2 * V2;
} while(S >= 1 || S == 0);
X = V1 * sqrt(-2 * log(S) / S);
} else
X = V2 * sqrt(-2 * log(S) / S);
phase = 1 - phase;
return X;
}
See the extended versions of this list (see question 20.40) for other ideas.
References: Knuth Sec. 3.4.1 p. 117
Box and Muller, ``A Note on the Generation of Random Normal Deviates''
Press et al., Numerical Recipes in C Sec. 7.2 pp. 288-290
Question 13.24
I'm trying to port this old program. Why do I get ``undefined external'' errors for some library
functions?
Some old or semistandard functions have been renamed or replaced over the years;
if you need:/you should instead:
index
use strchr.
rindex
use strrchr.
bcopy
use memmove, after interchanging the first and second arguments (see also question 11.25).
bcmp
use memcmp.
bzero
use memset, with a second argument of 0.
Contrariwise, if you're using an older system which is missing the functions in the second
column, you may be able to implement them in terms of, or substitute, the functions in the first.
References: PCS Sec. 11
Question 13.25
I keep getting errors due to library functions being undefined, but I'm #including all the right
header files.
In some cases (especially if the functions are nonstandard) you may have to explicitly ask for the
correct libraries to be searched when you link the program. See also questions 11.30, 13.26, and
14.3.
Question 13.26
I'm still getting errors due to library functions being undefined, even though I'm explicitly
requesting the right libraries while linking.
Many linkers make one pass over the list of object files and libraries you specify, and extract
from libraries only those modules which satisfy references which have so far come up as
undefined. Therefore, the order in which libraries are listed with respect to object files (and each
other) is significant; usually, you want to search the libraries last. (For example, under Unix, put
any -l options towards the end of the command line.) See also question 13.28.
Question 13.28
What does it mean when the linker says that _end is undefined?
That message is a quirk of the old Unix linkers. You get an error about _end being undefined
only when other things are undefined, too--fix the others, and the error about _end will disappear.
(See also questions 13.25 and 13.26.)
Question 14.1
When I set a float variable to, say, 3.1, why is printf printing it as 3.0999999?
Most computers use base 2 for floating-point numbers as well as for integers. In base 2, 1/1010
(that is, 1/10 decimal) is an infinitely-repeating fraction: its binary representation is
0.0001100110011... . Depending on how carefully your compiler's binary/decimal conversion
routines (such as those used by printf) have been written, you may see discrepancies when
numbers (especially low-precision floats) not exactly representable in base 2 are assigned or read
in and then printed (i.e. converted from base 10 to base 2 and back again). See also question 14.6.
Question 14.2
I'm trying to take some square roots, but I'm getting crazy numbers.
Make sure that you have #included , and correctly declared other functions returning
double. (Another library routine to be careful with is atof, which is declared in .) See
also question 14.3.
References: CT&P Sec. 4.5 pp. 65-6
Question 14.3
I'm trying to do some simple trig, and I am #including , but I keep getting ``undefined:
sin'' compilation errors.
Make sure you're actually linking with the math library. For instance, under Unix, you usually
need to use the -lm option, at the end of the command line, when compiling/linking. See also
questions 13.25 and 13.26.
Question 14.4
My floating-point calculations are acting strangely and giving me different answers on different
machines.
First, see question 14.2.
If the problem isn't that simple, recall that digital computers usually use floating-point formats
which provide a close but by no means exact simulation of real number arithmetic. Underflow,
cumulative precision loss, and other anomalies are often troublesome.
Don't assume that floating-point results will be exact, and especially don't assume that floatingpoint
values can be compared for equality. (Don't throw haphazard ``fuzz factors'' in, either; see
question 14.5.)
These problems are no worse for C than they are for any other computer language. Certain
aspects of floating-point are usually defined as ``however the processor does them'' (see also
question 11.34), 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 numerical programming text should cover the basics; see also the
references below.
References: Kernighan and Plauger, The Elements of Programming Style Sec. 6 pp. 115-8
Knuth, Volume 2 chapter 4
David Goldberg, ``What Every Computer Scientist Should Know about Floating-Point
Arithmetic''
Question 14.5
What's a good way to check for ``close enough'' floating-point equality?
Since the absolute accuracy of floating point values varies, by definition, with their magnitude,
the best way of comparing two floating point values is to use an accuracy threshold which is
relative to the magnitude of the numbers being compared. Rather than
double a, b;
...
if(a == b) /* WRONG */
use something like
#include
if(fabs(a - b) <= epsilon * fabs(a)) for some suitably-chosen epsilon. References: Knuth Sec. 4.2.2 pp. 217-8 Question 14.6 How do I round numbers? The simplest and most straightforward way is with code like (int)(x + 0.5) This technique won't work properly for negative numbers, though. Question 14.7 Why doesn't C have an exponentiation operator? Because few processors have an exponentiation instruction. C has a pow function, declared in , although explicit multiplication is often better for small positive integral exponents.
References: ANSI Sec. 4.5.5.1
ISO Sec. 7.5.5.1
H&S Sec. 17.6 p. 393
Question 14.8
The pre-#defined constant M_PI seems to be missing from my machine's copy of .
That constant (which is apparently supposed to be the value of pi, accurate to the machine's
precision), is not standard. If you need pi, you'll have to #define it yourself.
References: PCS Sec. 13 p. 237
Question 14.9
How do I test for IEEE NaN and other special values?
Many systems with high-quality IEEE floating-point implementations provide facilities (e.g.
predefined constants, and functions like isnan(), either as nonstandard extensions in or
perhaps in or ) to deal with these values cleanly, and work is being done to
formally standardize such facilities. A crude but usually effective test for NaN is exemplified by
#define isnan(x) ((x) != (x))
although non-IEEE-aware compilers may optimize the test away.
Another possibility is to format the value in question using sprintf: on many systems it generates
strings like "NaN" and "Inf" which you could compare for in a pinch.
See also question 19.39.
Question 14.11
What's a good way to implement complex numbers in C?
It is straightforward to define a simple structure and some arithmetic functions to manipulate
them. See also questions 2.7, 2.10, and 14.12.
Question 14.12
I'm looking for some code to do:
Fast Fourier Transforms (FFT's)
matrix arithmetic (multiplication, inversion, etc.)
complex arithmetic
Ajay Shah maintains an index of free numerical software; it is posted periodically, and available
where this FAQ list is archived (see question 20.40). See also question 18.16.
Question 14.13
I'm having trouble with a Turbo C program which crashes and says something like ``floating
point formats not linked.''
Some compilers for small machines, including Borland's (and Ritchie's original PDP-11
compiler), leave out certain 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 Borland's heuristics for determining whether the program
uses floating point are insufficient, and the programmer must sometimes insert an extra, explicit
call to a floating-point library routine to force loading of floating-point support. (See the
comp.os.msdos.programmer FAQ list for more information.)
Question 15.1
I heard that you have to #include before calling printf. Why?
So that a proper prototype for printf will be in scope.
A compiler may use a different calling sequence for functions which accept variable-length
argument lists. (It might do so if calls using variable-length argument lists were less efficient than
those using fixed-length.) Therefore, a prototype (indicating, using the ellipsis notation ``...'', that
the argument list is of variable length) must be in scope whenever a varargs function is called, so
that the compiler knows to use the varargs calling mechanism.
References: ANSI Sec. 3.3.2.2, Sec. 4.1.6
ISO Sec. 6.3.2.2, Sec. 7.1.7
Rationale Sec. 3.3.2.2, Sec. 4.1.6
H&S Sec. 9.2.4 pp. 268-9, Sec. 9.6 pp. 275-6
Question 15.2
How can %f be used for both float and double arguments in printf? Aren't they different types?
In the variable-length part of a variable-length argument list, the ``default argument promotions''
apply: types char and short int are promoted to int, and float is promoted to double. (These are the
same promotions that apply to function calls without a prototype in scope, also known as ``old
style'' function calls; see question 11.3.) Therefore, printf's %f format always sees a double.
(Similarly, %c always sees an int, as does %hd.) See also questions 12.9 and 12.13.
References: ANSI Sec. 3.3.2.2
ISO Sec. 6.3.2.2
H&S Sec. 6.3.5 p. 177, Sec. 9.4 pp. 272-3
Question 15.3
I had a frustrating problem which turned out to be caused by the line
printf("%d", n);
where n was actually a long int. I thought that ANSI function prototypes were supposed to guard
against argument type mismatches like this.
When a function accepts a variable number of arguments, its prototype does not (and cannot)
provide any information about the number and types of those variable arguments. Therefore, the
usual protections do not apply in the variable-length part of variable-length argument lists: the
compiler cannot perform implicit conversions or (in general) warn about mismatches.
See also questions 5.2, 11.3, 12.9, and 15.2.
Question 15.4
How can I write a function that takes a variable number of arguments?
Use the facilities of the header.
Here is a function which concatenates an arbitrary number of strings into malloc'ed memory:
#include /* for malloc, NULL, size_t */
#include /* for va_ stuff */
#include /* for strcat et al. */
char *vstrcat(char *first, ...)
{
size_t len;
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); /* restart for second scan */
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; see questions 5.2 and 15.3. (Also note that the caller must free
the returned, malloc'ed storage.)
See also question 15.7.
References: K&R2 Sec. 7.3 p. 155, Sec. B7 p. 254
ANSI Sec. 4.8
ISO Sec. 7.8
Rationale Sec. 4.8
H&S Sec. 11.4 pp. 296-9
CT&P Sec. A.3 pp. 139-141
PCS Sec. 11 pp. 184-5, Sec. 13 p. 242
Question 15.5
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?
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");
}
See also question 15.7.
References: K&R2 Sec. 8.3 p. 174, Sec. B1.2 p. 245
ANSI Secs. 4.9.6.7,4.9.6.8,4.9.6.9
ISO Secs. 7.9.6.7,7.9.6.8,7.9.6.9
H&S Sec. 15.12 pp. 379-80
PCS Sec. 11 pp. 186-7
Question 15.6
How can I write a function analogous to scanf, that calls scanf to do most of the work?
Unfortunately, vscanf and the like are not standard. You're on your own
Question 15.7
I have a pre-ANSI compiler, without . What can I do?
There's an older header, , which offers about the same functionality.
References: H&S Sec. 11.4 pp. 296-9
CT&P Sec. A.2 pp. 134-139
PCS Sec. 11 pp. 184-5, Sec. 13 p. 250
Question 15.8
How can I discover how many arguments a function was actually called with?
This information is not available to a portable program. Some old systems provided a
nonstandard nargs function, but its use was always questionable, since it typically returned the
number of words passed, not the number of arguments. (Structures, long ints, and floating point
values are usually passed as several words.)
Any function which takes a variable number of arguments must be able to determine from the
arguments themselves how many of them there are. printf-like functions do this by looking for
formatting specifiers (%d and the like) in the format string (which is why these functions fail
badly if the format string does not match the argument list). Another common technique,
applicable 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 in
questions 5.2 and 15.4). Finally, if their types are predictable, you can pass an explicit count of
the number of variable arguments (although it's usually a nuisance for the caller to generate).
References: PCS Sec. 11 pp. 167-8
Question 15.9
My compiler isn't letting me declare a function
int f(...)
{
}
i.e. with no fixed arguments.
Standard C requires at least one fixed argument, in part so that you can hand it to va_start.
References: ANSI Sec. 3.5.4, Sec. 3.5.4.3, Sec. 4.8.1.1
ISO Sec. 6.5.4, Sec. 6.5.4.3, Sec. 7.8.1.1
H&S Sec. 9.2 p. 263
Question 15.10
I have a varargs function which accepts a float parameter. Why isn't
va_arg(argp, float)
working?
In the variable-length part of variable-length argument lists, the old ``default argument
promotions'' apply: arguments of type float are always promoted (widened) to type double, and
types char and short int are promoted to int. Therefore, it is never correct to invoke va_arg(argp,
float); instead you should always use va_arg(argp, double). Similarly, use va_arg(argp, int) to
retrieve arguments which were originally char, short, or int. See also questions 11.3 and 15.2.
References: ANSI Sec. 3.3.2.2
ISO Sec. 6.3.2.2
Rationale Sec. 4.8.1.2
H&S Sec. 11.4 p. 297
Question 15.11
I can't get va_arg to pull in an argument of type pointer-to-function.
The type-rewriting games which the va_arg macro typically plays are stymied by overlycomplicated
types such as pointer-to-function. If you use a typedef for the function pointer type,
however, all will be well. See also question 1.21.
References: ANSI Sec. 4.8.1.2
ISO Sec. 7.8.1.2
Rationale Sec. 4.8.1.2
Question 15.12
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)?
In general, you cannot. Ideally, you should provide a version of that other function which accepts
a va_list pointer (analogous to vfprintf; see question 15.5). If the arguments must be passed
directly as actual arguments, or if you do not have the option of rewriting the second function to
accept a va_list (in other words, if the second, called function must accept a variable number of
arguments, not a va_list), no portable solution is possible. (The problem could perhaps be solved
by resorting to machine-specific assembly language; see also question 15.13.)
Question 15.13
How can I call a function with an argument list built up at run time?
There is no guaranteed or portable way to do this. If you're curious, however, this list's editor has
a few wacky ideas you could try...
Instead of an actual argument list, you might consider passing an array of generic (void *)
pointers. The called function can then step through the array, much like main() might step
through argv. (Obviously this works only if you have control over all the called functions.)
(See also question 19.36.)

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