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[Last modified February 7, 1999 by scs.]
This article is Copyright 1990-1999 by Steve Summit. Content from the
book _C Programming FAQs: Frequently Asked Questions_ is made available
here by permission of the author and the publisher as a service to the
community. It is intended to complement the use of the published text
and is protected by international copyright laws. The content is made
available here and may be accessed freely for personal use but may not
be republished without permission.
Certain topics come up again and again on this newsgroup. They are good
questions, and the answers may not be immediately obvious, but each time
they recur, much net bandwidth and reader time is wasted on repetitive
responses, and on tedious corrections to the incorrect answers which are
inevitably posted.
This article, which is posted monthly, attempts to answer these common
questions definitively and succinctly, so that net discussion can move
on to more constructive topics without continual regression to first
principles.
No mere newsgroup article can substitute for thoughtful perusal of a
full-length tutorial or language reference manual. Anyone interested
enough in C to be following this newsgroup should also be interested
enough to read and study one or more such manuals, preferably several
times. Some C books and compiler manuals are unfortunately inadequate;
a few even perpetuate some of the myths which this article attempts to
refute. Several noteworthy books on C are listed in this article's
bibliography; see also questions 18.9 and 18.10. Many of the questions
and answers are cross-referenced to these books, for further study by
the interested and dedicated reader.
If you have a question about C which is not answered in this article,
first try to answer it by checking a few of the referenced books, or by
asking knowledgeable colleagues, before posing your question to the net
at large. There are many people on the net who are happy to answer
questions, but the volume of repetitive answers posted to one question,
as well as the growing number of questions as the net attracts more
readers, can become oppressive. If you have questions or comments
prompted by this article, please reply by mail rather than following up --
this article is meant to decrease net traffic, not increase it.
Besides listing frequently-asked questions, this article also summarizes
frequently-posted answers. Even if you know all the answers, it's worth
skimming through this list once in a while, so that when you see one of
its questions unwittingly posted, you won't have to waste time
answering. (However, this is a large and heavy document, so don't
assume that everyone on the newsgroup has managed to read all of it in
detail, and please don't roll it up and thwack people over the head with
it just because they missed their answer in it.)
This article was last modified on February 7, 1999, and its travels may
have taken it far from its original home on Usenet. It may, however,
be out-of-date, particularly if you are looking at a printed copy
or one retrieved from a tertiary archive site or CD-ROM. You should
be able to obtain the most up-to-date copy on the web at
http://www.eskimo.com/~scs/C-faq/top.html or http://www.faqs.org/faqs/ ,
or from one of the ftp sites mentioned in question 20.40. Since this
list is modified from time to time, its question numbers may not match
those in older or newer copies which are in circulation; be careful when
referring to FAQ list entries by number alone.
This article was produced for free redistribution. You should not need
to pay anyone for a copy of it.
Other versions of this document are also available. Posted along with
it are an abridged version and (when there are changes) a list of
differences with respect to the previous version. A hypertext version
is available on the web at the aforementioned URL. Finally, for those
who might prefer a bound, hardcopy version (and even longer answers to
even more questions!), a book-length version has been published by
Addison-Wesley (ISBN 0-201-84519-9).
This article is always being improved. Your input is welcomed. Send
your comments to [email protected] .
The questions answered here are divided into several categories:
1. Declarations and Initializations
2. Structures, Unions, and Enumerations
3. Expressions
4. Pointers
5. Null Pointers
6. Arrays and Pointers
7. Memory Allocation
8. Characters and Strings
9. Boolean Expressions and Variables
10. C Preprocessor
11. ANSI/ISO Standard C
12. Stdio
13. Library Functions
14. Floating Point
15. Variable-Length Argument Lists
16. Strange Problems
17. Style
18. Tools and Resources
19. System Dependencies
20. Miscellaneous
Bibliography
Acknowledgements
(The question numbers within each section are not always continuous,
because they are aligned with the aforementioned book-length version,
which contains even more questions.)
Herewith, some frequently-asked questions and their answers:
Section 1. Declarations and Initializations
1.1: How do you decide which integer type to use?
A: 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; ISO Sec. 5.2.4.2.1, Sec. 6.1.2.5;
H&S Secs. 5.1,5.2 pp. 110-114.
1.4: What should the 64-bit type on a machine that can support it?
A: The forthcoming revision to the C Standard (C9X) specifies type
long long as effectively being at least 64 bits, and this type
has been implemented by a number of compilers for some time.
(Others have implemented extensions such as __longlong.)
On the other hand, there's no theoretical reason why a compiler
couldn't implement type short int as 16, int as 32, and long int
as 64 bits, and some compilers do indeed choose this
arrangement.
See also question 18.15d.
References: C9X Sec. 5.2.4.2.1, Sec. 6.1.2.5.
1.7: What's the best way to declare and define global variables
and functions?
A: First, though there can be many "declarations" (and in many
translation units) of a single "global" (strictly speaking,
"external") variable or function, there must be exactly one
"definition". (The definition is the declaration that actually
allocates space, and provides an initialization value, if any.)
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; 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.
1.11: What does extern mean in a function declaration?
A: It can be used as a stylistic hint to indicate that the
function's definition is probably in another source file, but
there is no formal difference between
extern int f();
and
int f();
References: 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.
1.12: What's the auto keyword good for?
A: Nothing; it's archaic. See also question 20.37.
References: K&R1 Sec. A8.1 p. 193; ISO Sec. 6.1.2.4, Sec. 6.5.1;
H&S Sec. 4.3 p. 75, Sec. 4.3.1 p. 76.
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?
A: 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; ISO
Sec. 6.5.2, Sec. 6.5.2.3; H&S Sec. 5.6.1 pp. 132-3.
1.21: How do I declare an array of N pointers to functions returning
pointers to functions returning pointers to characters?
A: 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). See question 18.1.
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; ISO Sec. 6.5ff (esp.
Sec. 6.5.4); H&S Sec. 4.5 pp. 85-92, Sec. 5.10.1 pp. 149-50.
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.
A: 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.
1.25: My compiler is complaining about an invalid redeclaration of a
function, but I only define it once and call it once.
A: 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; ISO
Sec. 6.3.2.2; H&S Sec. 4.7 p. 101.
1.25b: What's the right declaration for main()?
Is void main() correct?
A: See questions 11.12a to 11.15. (But no, it's not correct.)
1.30: What am I allowed to assume about the initial values
of variables which are not explicitly initialized?
If global variables start out as "zero", is that good
enough for null pointers and floating-point zeroes?
A: Uninitialized variables with "static" duration (that is, those
declared outside of functions, and those declared with the
storage class static), are guaranteed to start out as zero, as
if the programmer had typed "= 0". Therefore, such variables
are implicitly 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; 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.
1.31: This code, straight out of a book, isn't compiling:
int f()
{
char a[] = "Hello, world!";
}
A: Perhaps you have a pre-ANSI compiler, which doesn't allow
initialization of "automatic aggregates" (i.e. non-static
local arrays, structures, and unions). (As a workaround, and
depending on how the variable a is used, you may be able to make
it global or static, or replace it with a pointer, or initialize
it by hand with strcpy() when f() is called.) See also
question 11.29.
1.31b: What's wrong with this initialization?
char *p = malloc(10);
My compiler is complaining about an "invalid initializer",
or something.
A: Is the declaration of a static or non-local variable? Function
calls are allowed only in initializers for automatic variables
(that is, for local, non-static variables).
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: 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; ISO Sec. 6.1.4, Sec. 6.5.7;
Rationale Sec. 3.1.4; H&S Sec. 2.7.4 pp. 31-2.
1.34: I finally figured out the syntax for declaring pointers to
functions, but now how do I initialize one?
A: Use something like
extern int func();
int (*fp)() = func;
When the name of a function appears in an expression 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 questions 1.25 and 4.12.
Section 2. Structures, Unions, and Enumerations
2.1: What's the difference between these two declarations?
struct x1 { ... };
typedef struct { ... } x2;
A: The first form declares a "structure tag"; the second declares a
"typedef". The main difference is that you subsequently refer
to the first type as "struct x1" and the second simply as "x2".
That is, 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.
2.2: Why doesn't
struct x { ... };
x thestruct;
work?
A: C is not C++. Typedef names are not automatically generated for
structure tags. See also question 2.1 above.
2.3: Can a structure contain a pointer to itself?
A: Most certainly. See question 1.14.
2.4: What's the best way of implementing opaque (abstract) data types
in C?
A: 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.
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?
A: 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, although it does seem to work under 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. Furthermore, either of
these "chummy" structures must be used with care, since the
programmer knows more about their size than the compiler does.
(In particular, they can generally only be manipulated via
pointers.)
C9X will introduce the concept of a "flexible array member",
which will allow the size of an array to be omitted if it is
the last member in a structure, thus providing a well-defined
solution.
References: Rationale Sec. 3.5.4.2; C9X Sec. 6.5.2.1.
2.7: I heard that structures could be assigned to variables and
passed to and from functions, but K&R1 says not.
A: What K&R1 said (though this was quite some time ago by now) 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. A few ancient C
compilers may have lacked these operations, but all modern
compilers support them, and they are part of the ANSI C
standard, so there should be no reluctance to use them.
(Note that when a structure is assigned, passed, or returned,
the copying is done monolithically; the data pointed to by any
pointer fields is *not* copied.)
References: K&R1 Sec. 6.2 p. 121; K&R2 Sec. 6.2 p. 129; ISO
Sec. 6.1.2.5, Sec. 6.2.2.1, Sec. 6.3.16; H&S Sec. 5.6.2 p. 133.
2.8: Is there a way to compare structures automatically?
A: No. There is no single, good way for a compiler to implement
implicit structure comparison (i.e. to support the == operator
for structures) 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; Rationale Sec. 3.3.9; H&S
Sec. 5.6.2 p. 133.
2.10: How can I pass constant values to functions which accept
structure arguments?
A: As of this writing, C has no way of generating anonymous
structure values. You will have to use a temporary structure
variable or a little structure-building function.
The C9X Standard will introduce "compound literals"; one form of
compound literal will allow structure constants. For example,
to pass a constant coordinate pair to a plotpoint() function
which expects a struct point, you will be able to call
plotpoint((struct point){1, 2});
Combined with "designated initializers" (another C9X feature),
it will also be possible to specify member values by name:
plotpoint((struct point){.x=1, .y=2});
See also question 4.10.
References: C9X Sec. 6.3.2.5, Sec. 6.5.8.
2.11: How can I read/write structures from/to data files?
A: 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.
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.
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?
A: 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.
2.13: Why does sizeof report a larger size than I expect for a
structure type, as if there were padding at the end?
A: 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 also question 2.12 above.
References: H&S Sec. 5.6.7 pp. 139-40.
2.14: How can I determine the byte offset of a field within a
structure?
A: ANSI C defines the offsetof() macro, which should be used if
available; see <stddef.h>. If you don't have it, 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 below for a usage hint.
References: ISO Sec. 7.1.6; Rationale Sec. 3.5.4.2; H&S
Sec. 11.1 pp. 292-3.
2.15: How can I access structure fields by name at run time?
A: Build a table of names and offsets, using the offsetof() macro.
The offset of field b in struct a is
offsetb = offsetof(struct a, b)
If structp is a pointer to an instance of this structure, and
field b is an int (with offset as computed above), b's value can
be set indirectly with
*(int *)((char *)structp + offsetb) = value;
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: 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, 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.
2.20: Can I initialize unions?
A: The current C Standard allows an initializer for the first-named
member of a union. C9X will introduce "designated initializers"
which can be used to initialize any member.
References: K&R2 Sec. 6.8 pp. 148-9; ISO Sec. 6.5.7; C9X
Sec. 6.5.8; H&S Sec. 4.6.7 p. 100.
2.22: What is the difference between an enumeration and a set of
preprocessor #defines?
A: At the present time, there is little difference. The C Standard
says that enumerations may be freely intermixed with other
integral types, without errors. (If, on the other hand, 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; 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.
2.24: Is there an easy way to print enumeration values symbolically?
A: No. You can write a little function to map an enumeration
constant to a string. (For debugging purposes, a good debugger
should automatically print enumeration constants symbolically.)
Section 3. Expressions
3.1: Why doesn't this code:
a[i] = i++;
work?
A: The subexpression i++ causes a side effect -- it modifies i's
value -- which leads to undefined behavior since i is also
referenced elsewhere in the same expression, and there's no way
to determine whether the reference (in a[i] on the left-hand
side) should be to the old or the new value. (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; ISO Sec. 6.3; H&S
Sec. 7.12 pp. 227-9.
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?
A: 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; ISO
Sec. 6.3; H&S Sec. 7.12 pp. 227-9; CT&P Sec. 3.7 p. 47; PCS
Sec. 9.5 pp. 120-1.
3.3: I've experimented with the code
int i = 3;
i = i++;
on several compilers. Some gave i the value 3, and some gave 4.
Which compiler is correct?
A: There is no correct answer; the expression is undefined. See
questions 3.1, 3.8, 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, i+=1, i++, or ++i, not some combination. See also
question 3.12.)
3.3b: Here's a slick expression:
a ^= b ^= a ^= b
It swaps a and b without using a temporary.
A: Not portably, it doesn't. It attempts to modify the variable a
twice between sequence points, so its behavior is undefined.
For example, it has been reported that when given the code
int a = 123, b = 7654;
a ^= b ^= a ^= b;
the SCO Optimizing C compiler (icc) sets b to 123 and a to 0.
See also questions 3.1, 3.8, 10.3, and 20.15c.
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?
A: 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.
3.5: But what about the && and || operators?
I see code like "while((c = getchar()) != EOF && c != '\n')" ...
A: There is a special "short-circuiting" exception for those
operators. The right-hand side is not evaluated if the left-
hand side determines the outcome (i.e. is true for || or false
for &&). Therefore, left-to-right evaluation is guaranteed, as
it also is for the comma operator. Furthermore, all of these
operators (along with ?:) introduce an extra internal sequence
point (see question 3.8).
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; 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.
3.8: How can I understand these complex expressions? What's a
"sequence point"?
A: A sequence point is a point in time (at the end of the
evaluation 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 below.
References: ISO Sec. 5.1.2.3, Sec. 6.3, Sec. 6.6, Annex C;
Rationale Sec. 2.1.2.3; H&S Sec. 7.12.1 pp. 228-9.
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, right?
A: *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.
3.12: If I'm not using the value of the expression, should I use i++
or ++i to increment a variable?
A: Since the two forms differ only in the value yielded, they are
entirely equivalent when only their side effect is needed.
(However, the prefix form is preferred in C++.) See also
question 3.3.
References: K&R1 Sec. 2.8 p. 43; K&R2 Sec. 2.8 p. 47; 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.
3.14: Why doesn't the code
int a = 1000, b = 1000;
long int c = a * b;
work?
A: Under C's integral promotion rules, the multiplication is
carried out using int arithmetic, and the result may overflow 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 floating-point variable; the solution
is similar, too.
References: K&R1 Sec. 2.7 p. 41; K&R2 Sec. 2.7 p. 44; ISO
Sec. 6.2.1.5; H&S Sec. 6.3.4 p. 176; CT&P Sec. 3.9 pp. 49-50.
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;
A: 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: ISO Sec. 6.3.15; H&S Sec. 7.1 pp. 179-180.
Section 4. Pointers
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);
A: 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, 7.3c, and 8.3.
References: CT&P Sec. 3.1 p. 28.
4.3: Does *p++ increment p, or what it points to?
A: Postfix ++ essentially has higher precedence than the prefix
unary operators. Therefore, *p++ is equivalent to *(p++); it
increments p, and returns the value which p pointed to before p
was incremented. 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; 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.
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?
A: In C, a cast operator does not mean "pretend these bits have a
different type, and treat them accordingly"; it is a conversion
operator, and by definition it yields an rvalue, which cannot be
assigned to, or incremented with ++. (It is either an accident
or a delibrate but nonstandard extension if a particular
compiler accepts expressions such as the above.) 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; ISO Sec. 6.3.4; Rationale
Sec. 3.3.2.4; H&S Sec. 7.1 pp. 179-80.
4.8: I have a function which accepts, and is supposed to initialize,
a pointer:
void f(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.
A: 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.
4.9: Can I use a void ** pointer as a parameter so that a function
can accept a generic pointer by reference?
A: Not portably. There is no generic pointer-to-pointer type in C.
void * acts as a generic pointer only because conversions are
applied automatically when other pointer types are assigned to
and from void *'s; these conversions cannot be performed (the
correct underlying pointer type is not known) if an attempt is
made to indirect upon a void ** value which points at a pointer
type other than void *.
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.
A: 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.
4.11: Does C even have "pass by reference"?
A: 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.). However, C has nothing truly
equivalent to formal pass by reference or C++ reference
parameters. (On the other hand, function-like preprocessor
macros can provide a form of "pass 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. 95-7; ISO Sec. 6.3.2.2; H&S
Sec. 9.5 pp. 273-4.
4.12: I've seen different methods used for calling functions via
pointers. What's the story?
A: Originally, a pointer to a function had to be "turned into" a
"real" function, with the * operator (and an extra pair of
parentheses, to keep the precedence straight), before calling:
int r, func(), (*fp)() = func;
r = (*fp)();
It can also be argued that functions are always called 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 (which is in fact used 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.
See also question 1.34.
References: K&R1 Sec. 5.12 p. 116; K&R2 Sec. 5.11 p. 120; 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.
Section 5. Null Pointers
5.1: What is this infamous null pointer, anyway?
A: The language definition states that for each pointer type, there
is a special value -- the "null pointer" -- which is
distinguishable from all other pointer values and which is
"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 below).
References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102; ISO
Sec. 6.2.2.3; Rationale Sec. 3.2.2.3; H&S Sec. 5.3.2 pp. 121-3.
5.2: How do I get a null pointer in my programs?
A: According to the language definition, a constant 0 in a pointer
context is converted into a null pointer at compile time. That
is, in an initialization, assignment, or comparison when one
side is a variable or expression of pointer type, the compiler
can tell that a constant 0 on the other side requests a null
pointer, and generate the correctly-typed null pointer value.
Therefore, the following fragments are perfectly legal:
char *p = 0;
if(p != 0)
(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 probably safest to properly cast
all null pointer constants in function calls, to guard against
varargs functions or those without prototypes.
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; ISO Sec. 6.2.2.3; H&S
Sec. 4.6.3 p. 95, Sec. 6.2.7 p. 171.
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?
A: When C requires the Boolean value of an expression, 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; 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.
5.4: What is NULL and how is it #defined?
A: 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 <stdio.h> and several
other headers) 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; 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.
5.5: How should NULL be defined on a machine which uses a nonzero bit
pattern as the internal representation of a null pointer?
A: The same as on any other machine: as 0 (or some version of 0;
see question 5.4).
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: ISO Sec. 7.1.6; Rationale Sec. 4.1.5.
5.6: If NULL were defined as follows:
#define NULL ((char *)0)
wouldn't that make function calls which pass an uncast NULL
work?
A: Not in general. The complication 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.
5.9: If NULL and 0 are equivalent as null pointer constants, which
should I use?
A: Many programmers believe that NULL should be used in all pointer
contexts, as a reminder that the value is to be thought of as a
pointer. Others feel that the confusion surrounding NULL and 0
is only compounded by hiding 0 behind a 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.
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?
A: 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.
5.12: I use the preprocessor macro
#define Nullptr(type) (type *)0
to help me build null pointers of the correct type.
A: This trick, though popular and superficially attractive, does
not buy much. It is not needed in assignments or comparisons;
see question 5.2. (It does not even save keystrokes.) See also
questions 9.1 and 10.2.
5.13: This is strange. NULL is guaranteed to be 0, but the null
pointer is not?
A: When the term "null" or "NULL" is casually used, one of several
things may be meant:
1. The conceptual null pointer, the abstract language concept
defined in question 5.1. It is implemented with...
2. The internal (or run-time) representation of a null
pointer, which may or may not be all-bits-0 and which may
be different for different pointer types. The actual
values should be of concern only to compiler writers.
Authors of C programs never see them, since they use...
3. The null pointer constant, which is a constant integer 0
(see question 5.2). It is often hidden behind...
4. The NULL macro, which is #defined to be 0 (see question
5.4). Finally, as red herrings, we have...
5. The ASCII null character (NUL), which does have all bits
zero, but has no necessary relation to the null pointer
except in name; and...
6. The "null string," which is another name for 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.
5.14: Why is there so much confusion surrounding null pointers? Why
do these questions come up so often?
A: C programmers traditionally like to know more than they might
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 above) 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
appropriate type of null pointer when it could unambiguously
determine that 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.
5.15: I'm confused. I just can't understand all this null pointer
stuff.
A: Here are two simple rules you can follow:
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 the
complexities of function prototypes. (Taking those complexities
into account, we find that rule 2 is conservative, of course;
but it doesn't hurt.) 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.
5.16: Given all the confusion surrounding null pointers, wouldn't it
be easier simply to require them to be represented internally by
zeroes?
A: If for no other reason, doing so would be ill-advised because it
would unnecessarily constrain implementations which would
otherwise naturally represent null pointers by special, nonzero
bit patterns, particularly when those values would trigger
automatic hardware traps for invalid accesses.
Besides, what would 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 above, the urge to assume an internal zero
representation would not even arise.)
5.17: Seriously, have any actual machines really used nonzero null
pointers, or different representations for pointers to different
types?
A: The Prime 50 series used segment 07777, offset 0 for the null
pointer, at least for PL/I. Later models used segment 0, offset
0 for null pointers in C, necessitating new instructions such as
TCNP (Test C Null Pointer), evidently as a sop to all the extant
poorly-written C code which made incorrect assumptions. Older,
word-addressed Prime machines were also notorious for requiring
larger byte pointers (char *'s) than word pointers (int *'s).
The Eclipse MV series from Data General has three
architecturally supported pointer formats (word, byte, and bit
pointers), two of which are used by C compilers: byte pointers
for char * and void *, and word pointers for everything else.
Some Honeywell-Bull mainframes use the bit pattern 06000 for
(internal) null pointers.
The CDC Cyber 180 Series has 48-bit pointers consisting of a
ring, segment, and offset. Most users (in ring 11) have null
pointers of 0xB00000000000. It was common on old CDC ones-
complement 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 <NIL, 0>
(basically a nonexistent <object, offset> 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.
5.20: What does a run-time "null pointer assignment" error mean?
How can I track it down?
A: This message, which typically occurs with MS-DOS compilers, means
that you've written, via a null (perhaps because uninitialized)
pointer, to an invalid location (probably offset 0 in the
default data segment).
A debugger may let you set a data watchpoint 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. See also question
16.8.
Section 6. Arrays and Pointers
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?
A: In one source file you defind an array of characters and in the
other you declared a pointer to characters. The declaration
extern char *a simply does not match the actual definition.
The type pointer-to-type-T is not the same as array-of-type-T.
Use extern char a[].
References: ISO Sec. 6.5.4.2; CT&P Sec. 3.3 pp. 33-4, Sec. 4.5
pp. 64-5.
6.2: But I heard that char a[] was identical to char *a.
A: Not at all. (What you heard has to do with formal parameters to
functions; see question 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. (The essential
difference is that the values of an array like a and a pointer
like p are computed differently *whenever* they appear in
expressions, whether or not they are being subscripted, as
explained further in the next question.)
References: K&R2 Sec. 5.5 p. 104; CT&P Sec. 4.5 pp. 64-5.
6.3: So what is meant by the "equivalence of pointers and arrays" in
C?
A: 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. What it means is that array
and pointer arithmetic is defined such that a pointer can be
conveniently used to access an array or to simulate an array.
Specifically, the cornerstone of the equivalence is this 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.
That is, whenever an array appears in an expression,
the compiler implicitly generates a pointer to the array's
first element, just as if the programmer had written &a[0].
(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 questions 6.8 and 6.14.
References: K&R1 Sec. 5.3 pp. 93-6; K&R2 Sec. 5.3 p. 99; ISO
Sec. 6.2.2.1, Sec. 6.3.2.1, Sec. 6.3.6; H&S Sec. 5.4.1 p. 124.
6.4: Then why are array and pointer declarations interchangeable as
function formal parameters?
A: 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 an array was being passed, perhaps because the parameter
will be used within the function as if it were an array.
Specifically, any parameter declarations which "look like"
arrays, e.g.
void f(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:
void f(char *a)
{ ... }
This conversion holds only within function formal parameter
declarations, nowhere else. If the conversion bothers you,
avoid it; many programmers 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; 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.
6.7: How can an array be an lvalue, if you can't assign to it?
A: The ANSI C Standard defines a "modifiable lvalue," which an
array is not.
References: ISO Sec. 6.2.2.1; Rationale Sec. 3.2.2.1; H&S
Sec. 7.1 p. 179.
6.8: Practically speaking, what is the difference between arrays and
pointers?
A: Arrays automatically allocate space, but can't be relocated or
resized. Pointers must be explicitly assigned to point to
allocated space (perhaps using malloc), but can be reassigned
(i.e. pointed at different objects) at will, and have many other
uses besides serving as the base of blocks of memory.
Due to the so-called equivalence of arrays and pointers (see
question 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.
6.9: Someone explained to me that arrays were really just constant
pointers.
A: This is a bit of an oversimplification. An array name is
"constant" in that it cannot be assigned to, but an array is
*not* a pointer, as the discussion and pictures in question 6.2
should make clear. See also questions 6.3 and 6.8.
6.11: I came across some "joke" code containing the "expression"
5["abcdef"] . How can this be legal C?
A: Yes, Virginia, array subscripting is commutative in C. This
curious fact follows from the pointer definition of array
subscripting, namely that a[e] is identical to *((a)+(e)), for
*any* 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.
6.12: Since array references decay into pointers, if arr is an array,
what's the difference between arr and &arr?
A: 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 pointer-to-T, to the array's
first element. (See also questions 6.3, 6.13, and 6.18.)
References: 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.
6.13: How do I declare a pointer to an array?
A: Usually, you don't want to. When people speak casually of a
pointer to an array, they usually mean a pointer to its first
element.
Instead of a pointer to an array, consider using a pointer to
one of the array's elements. Arrays of type T decay into
pointers to type T (see question 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 above.
References: ISO Sec. 6.2.2.1.
6.14: How can I set an array's size at run time?
How can I avoid fixed-sized arrays?
A: 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 <stdlib.h>
int *dynarray;
dynarray = malloc(10 * sizeof(int));
(and if the call to malloc succeeds), you can reference
dynarray[i] (for i from 0 to 9) almost as if dynarray were a
conventional, statically-allocated array (int a[10]). The only
difference is that sizeof will not give the size of the "array".
See also questions 1.31b, 6.16, and 7.7.
6.15: How can I declare local arrays of a size matching a passed-in
array?
A: Until recently, you couldn't. Array dimensions in C
traditionally had to be compile-time constants. C9X will
introduce variable-length arrays (VLA's) which will solve this
problem; local arrays may have sizes set by variables or other
expressions, perhaps involving function parameters. (gcc has
provided parameterized arrays as an extension for some time.)
If you can't use C9X or gcc, 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: ISO Sec. 6.4, Sec. 6.5.4.2; C9X Sec. 6.5.5.2.
6.16: How can I dynamically allocate a multidimensional array?
A: The traditional solution is to allocate an array of pointers,
and then initialize each pointer to a dynamically-allocated
"row." Here is a two-dimensional example:
#include <stdlib.h>
int **array1 = malloc(nrows * sizeof(int *));
for(i = 0; i < nrows; i++)
array1[i] = malloc(ncolumns * sizeof(int));
(In real code, of course, all of malloc's return values would
be checked.)
You can keep the array's contents contiguous, at the cost of
making later reallocation of individual rows more difficult,
with a bit of explicit pointer arithmetic:
int **array2 = malloc(nrows * sizeof(int *));
array2[0] = malloc(nrows * ncolumns * sizeof(int));
for(i = 1; i < nrows; i++)
array2[i] = array2[0] + i * ncolumns;
In either case, the elements of the dynamic array can be
accessed with normal-looking array subscripts: arrayx[i][j]
(for 0 <= i < nrows and 0 <= j < ncolumns).
If the double indirection implied by the above schemes is for
some reason unacceptable, you can simulate a two-dimensional
array with a single, dynamically-allocated one-dimensional
array:
int *array3 = malloc(nrows * ncolumns * sizeof(int));
However, you must now perform subscript calculations manually,
accessing the i,jth element with array3[i * ncolumns + j]. (A
macro could hide the explicit calculation, but invoking it would
require parentheses and commas which wouldn't look exactly like
multidimensional array syntax, and the macro would need access
to at least one of the dimensions, as well. See also question
6.19.)
Yet another option is to use pointers to arrays:
int (*array4)[NCOLUMNS] = malloc(nrows * sizeof(*array4));
but the syntax starts getting horrific and at most one dimension
may be specified at run time.
With all of these techniques, you may of course need to remember
to free the arrays (which may take several steps; see question
7.23) when they are no longer needed, and you cannot necessarily
intermix dynamically-allocated arrays with conventional,
statically-allocated ones (see question 6.20, and also question
6.18).
Finally, in C9X you can use a variable-length array.
All of these techniques can also be extended to three or more
dimensions.
References: C9X Sec. 6.5.5.2.
6.17: Here's a neat trick: if I write
int realarray[10];
int *array = &realarray[-1];
I can treat "array" as if it were a 1-based array.
A: Although this technique is attractive (and was used in old
editions of the book _Numerical Recipes in C_), it is not
strictly conforming to the C Standard. Pointer arithmetic
is defined only as long as the pointer points within the same
allocated block of memory, or to the imaginary "terminating"
element one past it; otherwise, the behavior is undefined,
*even if the pointer is not dereferenced*. The code above
could fail if, while subtracting the offset, an illegal
address were generated (perhaps because the address tried
to "wrap around" past the beginning of some memory segment).
References: K&R2 Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3, Sec. A7.7
pp. 205-6; ISO Sec. 6.3.6; Rationale Sec. 3.2.2.3.
6.18: My compiler complained when I passed a two-dimensional array to
a function expecting a pointer to a pointer.
A: The rule (see question 6.3) by which arrays decay into pointers
is not applied recursively. An array of arrays (i.e. a two-
dimensional array in C) decays into a pointer to an array, not a
pointer to a pointer. Pointers to arrays can be confusing, and
must be treated carefully; see also question 6.13.
If you are passing a two-dimensional array to a function:
int array[NROWS][NCOLUMNS];
f(array);
the function's declaration must match:
void f(int a[][NCOLUMNS])
{ ... }
or
void f(int (*ap)[NCOLUMNS]) /* ap is a pointer to an array */
{ ... }
In the first declaration, the compiler performs the usual
implicit parameter rewriting of "array of array" to "pointer to
array" (see questions 6.3 and 6.4); in the second form the
pointer declaration is explicit. Since the called function does
not allocate space for the array, it does not need to know the
overall size, so the number of rows, NROWS, can be omitted. The
width of the array is still important, so the column dimension
NCOLUMNS (and, for three- or more dimensional arrays, the
intervening ones) must be retained.
If a function is already declared as accepting a pointer to a
pointer, it is almost certainly meaningless to pass a two-
dimensional array directly to it.
See also questions 6.12 and 6.15.
References: K&R1 Sec. 5.10 p. 110; K&R2 Sec. 5.9 p. 113; H&S
Sec. 5.4.3 p. 126.
6.19: How do I write functions which accept two-dimensional arrays
when the width is not known at compile time?
A: It's not always easy. One way is to pass in a pointer to the
[0][0] element, along with the two dimensions, and simulate
array subscripting "by hand":
void f2(int *aryp, int nrows, int ncolumns)
{ ... array[i][j] is accessed as aryp[i * ncolumns + j] ... }
This function could be called with the array from question 6.18
as
f2(&array[0][0], NROWS, NCOLUMNS);
It must be noted, however, that a program which performs
multidimensional array subscripting "by hand" in this way is not
in strict conformance with the ANSI C Standard; according to an
official interpretation, the behavior of accessing
(&array[0][0])[x] is not defined for x >= NCOLUMNS.
C9X will allow variable-length arrays, and once compilers which
accept C9X's extensions become widespread, this will probably
become the preferred solution. (gcc has supported variable-
sized arrays for some time.)
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: ISO Sec. 6.3.6; C9X Sec. 6.5.5.2.
6.20: How can I use statically- and dynamically-allocated
multidimensional arrays interchangeably when passing them to
functions?
A: 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
void f1a(int a[][NCOLUMNS], int nrows, int ncolumns);
void f1b(int (*a)[NCOLUMNS], int nrows, int ncolumns);
void f2(int *aryp, int nrows, int ncolumns);
void f3(int **pp, int nrows, int ncolumns);
where f1a() and f1b() accept conventional two-dimensional
arrays, 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:
f1a(array, NROWS, NCOLUMNS);
f1b(array, NROWS, NCOLUMNS);
f1a(array4, nrows, NCOLUMNS);
f1b(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 calls would probably work on most systems, but
involve questionable casts, and work only if the dynamic
ncolumns matches the static NCOLUMNS:
f1a((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);
f1a((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);
f1b((int (*)[NCOLUMNS])array3, nrows, ncolumns);
f1b((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().
6.21: Why doesn't sizeof properly report the size of an array when the
array is a parameter to a function?
A: 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.
Section 7. Memory Allocation
7.1: Why doesn't this fragment work?
char *answer;
printf("Type something:\n");
gets(answer);
printf("You typed \"%s\"\n", answer);
A: The pointer variable answer, which is handed to 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 <stdio.h>
#include <string.h>
char answer[100], *p;
printf("Type something:\n");
fgets(answer, sizeof answer, stdin);
if((p = strchr(answer, '\n')) != NULL)
*p = '\0';
printf("You typed \"%s\"\n", answer);
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, as gets() would.) It would also be
possible to use malloc() to allocate the answer buffer.
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.
A: As in question 7.1 above, 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; after the call to
strcat(), s1 contains the result.
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.
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?
A: 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 a pointer to some
object (a structure, or an array -- see questions 6.3 and 6.4)
which the caller has allocated. Other common examples are
time() (see question 13.12) and stat().
7.3b: I just tried the code
char *p;
strcpy(p, "abc");
and it worked. How? Why didn't it crash?
A: You got lucky, I guess. The memory pointed to by the
unitialized pointer p happened to be writable by you,
and apparently was not already in use for anything vital.
7.3c: How much memory does a pointer variable allocate?
A: That's a pretty misleading question. When you declare
a pointer variable, as in
char *p;
you (or, more properly, the compiler) have allocated only enough
memory to hold the pointer itself; that is, in this case you
have allocated sizeof(char *) bytes of memory. But you have
not yet allocated *any* memory for the pointer to point to.
See also questions 7.1 and 7.2.
7.5a: I have a function that is supposed to return a string, but when
it returns to its caller, the returned string is garbage.
A: Make sure that the pointed-to memory is properly allocated.
For example, make sure you have *not* done 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 7.5b, 12.21, and 20.1.
References: ISO Sec. 6.1.2.4.
7.5b: So what's the right way to return a string or other aggregate?
A: 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.
See also question 20.1.
7.6: Why am I getting "warning: assignment of pointer from integer
lacks a cast" for calls to malloc()?
A: Have you #included <stdlib.h>, or otherwise arranged for
malloc() to be declared properly? See also question 1.25.
References: H&S Sec. 4.7 p. 101.
7.7: Why does some code carefully cast the values returned by malloc
to the pointer type being allocated?
A: Before ANSI/ISO Standard C introduced the void * generic pointer
type, these casts were typically required to silence warnings
(and perhaps induce conversions) when assigning between
incompatible pointer types.
Under ANSI/ISO Standard C, these casts are no longer necessary,
and in fact modern practice discourages them, since they can
camouflage important warnings which would otherwise be generated
if malloc() happened not to be declared correctly; see question
7.6 above. (However, the casts are typically seen in C code
which for one reason or another is intended to be compatible
with C++, where explicit casts from void * are required.)
References: H&S Sec. 16.1 pp. 386-7.
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))?
A: 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 in some
circumstances may help by introducing a size_t into the
expression.) See also question 8.9.
References: ISO Sec. 6.3.3.4; H&S Sec. 7.5.2 p. 195.
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?
A: 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: ISO Sec. 7.10.3.
7.16: I'm allocating a large array for some numeric work, using the
line
double *array = malloc(300 * 300 * 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.
A: Notice that 300 x 300 is 90,000, 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 300 * (300 * 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 compiler, or use
some nonstandard memory allocation functions. See also
question 19.23.
7.17: I've got 8 meg of memory in my PC. Why can I only seem to
malloc 640K or so?
A: Under the segmented architecture of PC compatibles, it can be
difficult to use more than 640K with any degree of transparency,
especially under MS-DOS. See also question 19.23.
7.19: My program is crashing, apparently somewhere down inside malloc,
but I can't see anything wrong with it. Is there a bug in
malloc()?
A: 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 memory that has been
freed, 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.
7.20: You can't use dynamically-allocated memory after you free it,
can you?
A: No. Some early documentation for malloc() stated that the
contents of freed memory 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(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; 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.
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?
A: When you call free(), the memory pointed to by the passed
pointer is freed, but the value of the pointer in the caller
probably 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: ISO Sec. 7.10.3; Rationale Sec. 3.2.2.3.
7.22: When I call malloc() to allocate memory for a pointer which is
local to a function, do I have to explicitly free() it?
A: 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().
7.23: I'm allocating structures which contain pointers to other
dynamically-allocated objects. When I free a structure, do I
also have to free each subsidiary pointer?
A: Yes. In general, you must arrange that each pointer returned
from malloc() be individually passed to free(), exactly once (if
it is freed at all). 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.
7.24: Must I free allocated memory before the program exits?
A: You shouldn't have to. A real operating system definitively
reclaims all memory and other resources 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: ISO Sec. 7.10.3.2.
7.25: I have a program which mallocs and later frees a lot of memory,
but I can see from the operating system that memory usage
doesn't actually go back down.
A: Most implementations of malloc/free do not return freed memory
to the operating system, but merely make it available for future
malloc() calls within the same program.
7.26: How does free() know how many bytes to free?
A: The malloc/free implementation remembers the size of each block
as it is allocated, so it is not necessary to remind it of the
size when freeing.
7.27: So can I query the malloc package to find out how big an
allocated block is?
A: Unfortunately, there is no standard or portable way.
7.30: Is it legal to pass a null pointer as the first argument to
realloc()? Why would you want to?
A: ANSI C sanctions this usage (and the related realloc(..., 0),
which frees), 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: ISO Sec. 7.10.3.4; H&S Sec. 16.3 p. 388.
7.31: What's the difference between calloc() and malloc()? Is it safe
to take advantage of calloc's zero-filling? Does free() work
on memory allocated with calloc(), or do you need a cfree()?
A: calloc(m, n) is essentially equivalent to
p = malloc(m * n);
memset(p, 0, m * n);
The zero fill is all-bits-zero, and does *not* therefore
guarantee useful 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: 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.
7.32: What is alloca() and why is its use discouraged?
A: alloca() allocates memory which is automatically freed when the
function which called alloca() returns. That is, memory
allocated with alloca is local to a particular function's "stack
frame" or context.
alloca() cannot be written portably, and is difficult to
implement on machines without a 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.
Section 8. Characters and Strings
8.1: Why doesn't
strcat(string, '!');
work?
A: 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 below).
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.
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" */
...
}
A: 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" */
...
}
8.3: If I can say
char a[] = "Hello, world!";
why can't I say
char a[14];
a = "Hello, world!";
A: 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.
8.6: How can I get the numeric (character set) value corresponding to
a character, or vice versa?
A: In C, characters are represented by small integers corresponding
to their values (in the machine's character set), so you don't
need a conversion function: if you have the character, you have
its value.
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)).
A: Perhaps surprisingly, character constants in C are of type int,
so sizeof('a') is sizeof(int) (though this is another area
where C++ differs). See also question 7.8.
References: ISO Sec. 6.1.3.4; H&S Sec. 2.7.3 p. 29.
Section 9. Boolean Expressions and Variables
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?
A: 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 below; see also
questions 5.12 and 10.2).
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?
A: It is true (sic) that any nonzero value is considered true in C,
but this applies only "on input", i.e. where a Boolean value is
expected. When a Boolean value is generated by a built-in
operator, it is guaranteed to be 1 or 0. Therefore, the test
if((a == b) == TRUE)
would work as expected (as long as TRUE is 1), but it is
obviously silly. In fact, explicit tests against TRUE and
FALSE are generally 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.)
Although the use of macros like TRUE and FALSE (or YES
and NO) seems clearer, Boolean values and definitions can
be sufficiently confusing in C that some programmers feel
that TRUE and FALSE macros only compound the confusion, and
prefer to use raw 1 and 0 instead. (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; 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".
9.3: Is if(p), where p is a pointer, a valid conditional?
A: Yes. See question 5.3.
Section 10. C Preprocessor
10.2: Here are some cute preprocessor macros:
#define begin {
#define end }
What do y'all think?
A: Bleah. See also section 17.
10.3: How can I write a generic macro to swap two values?
A: There is no good answer to this question. If the values are
integers, a well-known trick using exclusive-OR could perhaps
be used, but it will not work for floating-point values or
pointers, or if the two values are the same variable. (See
questions 3.3b and 20.15c.) 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 picking a name for 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.
10.4: What's the best way to write a multi-statement macro?
A: 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.
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?)
A: 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 .c 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.
10.7: Is it acceptable for one header file to #include another?
A: It's a question of style, and thus receives considerable debate.
Many people believe that "nested #include files" are to be
avoided: the prestigious Indian Hill Style Guide (see question
17.9) disparages them; they can make it harder to find relevant
definitions; they can lead to multiple-definition 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.
10.8a: What's the difference between #include <> and #include "" ?
A: The <> syntax is typically used with Standard or system-supplied
headers, while "" is typically used for a program's own header
files.
10.8b: What are the complete rules for header file searching?
A: 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; ISO Sec. 6.8.2; H&S Sec. 3.4
p. 55.
10.9: I'm getting strange syntax errors on the very first declaration
in a file, but it looks fine.
A: 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, 11.29, and 16.1b.
10.10b: I'm #including the right header file for the library function
I'm using, but the linker keeps saying it's undefined.
A: See question 13.25.
10.11: I seem to be missing the system header file <sgtty.h>.
Can someone send me a copy?
A: Standard headers exist in part so that definitions appropriate
to your compiler, operating system, and processor can be
supplied. You cannot just pick up a copy of someone else's
header file and expect it to work, unless that person is using
exactly the same environment. Ask your compiler vendor why the
file was not provided (or to send a replacement copy).
10.12: How can I construct preprocessor #if expressions which compare
strings?
A: You can't do it directly; preprocessor #if arithmetic uses only
integers. An alternative is to #define several macros with
symbolic names and distinct integer values, and implement
conditionals on those.
See also question 20.17.
References: K&R2 Sec. 4.11.3 p. 91; ISO Sec. 6.8.1; H&S
Sec. 7.11.1 p. 225.
10.13: Does the sizeof operator work in preprocessor #if directives?
A: 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
<limits.h>, if applicable, or perhaps a "configure" script.
(Better yet, try to write code which is inherently insensitive
to type sizes; see also question 1.1.)
References: ISO Sec. 5.1.1.2, Sec. 6.8.1; H&S Sec. 7.11.1 p.
225.
10.14: Can I use an #ifdef in a #define line, to define something two
different ways?
A: 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: ISO Sec. 6.8.3, Sec. 6.8.3.4; H&S Sec. 3.2 pp. 40-1.
10.15: Is there anything like an #ifdef for typedefs?
A: Unfortunately, no. You may have to keep sets of preprocessor
macros (e.g. MY_TYPE_DEFINED) recording whether certain typedefs
have been declared. (See also question 10.13.)
References: ISO Sec. 5.1.1.2, Sec. 6.8.1; H&S Sec. 7.11.1 p.
225.
10.16: How can I use a preprocessor #if expression to tell if a machine
is big-endian or little-endian?
A: You probably can't. (Preprocessor arithmetic uses only long
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: ISO Sec. 6.8.1; H&S Sec. 7.11.1 p. 225.
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?
A: There are programs floating around called unifdef, rmifdef,
and scpp ("selective C preprocessor") which do exactly this.
See question 18.16.
10.19: How can I list all of the predefined identifiers?
A: There's no standard way, although it is a common need. gcc
provides a -dM option which works with -E, and other compilers
may provide something similar. 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 predefined identifiers (e.g. "unix")
are non-Standard (because they clash with the user's namespace)
and are being removed or renamed.
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.
A: 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: ISO Sec. 6.8.3.3; Rationale Sec. 3.8.3.3; H&S
Sec. 3.3.9 p. 52.
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);
A: See question 11.18.
10.23-4: I'm having trouble using macro arguments inside string
literals, using the `#' operator.
A: See questions 11.17 and 11.18.
10.25: I've got this tricky preprocessing I want to do and I can't
figure out a way to do it.
A: 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.)
10.26: How can I write a macro which takes a variable number of
arguments?
A: 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, or to play
tricky games with commas:
#define DEBUG(args) (printf("DEBUG: "), printf(args))
#define _ ,
DEBUG("i = %d" _ i)
C9X will introduce formal support for function-like macros with
variable-length argument lists. The notation ... will appear at
the end of the macro "prototype" (just as it does for varargs
functions), and the pseudomacro __VA_ARGS__ in the macro
definition will be replaced by the variable arguments during
invocation.
Finally, you can always 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. (If you needed a macro
replacement, try using a function plus a non-function-like
macro, e.g. #define printf myprintf .)
References: C9X Sec. 6.8.3, Sec. 6.8.3.1.
Section 11. ANSI/ISO Standard C
11.1: What is the "ANSI C Standard?"
A: In 1983, the American National Standards Institute (ANSI)
commissioned a committee, X3J11, to standardize the C language.
After a long, arduous process, including several widespread
public reviews, the committee's work was finally ratified as ANS
X3.159-1989 on December 14, 1989, and published in the spring of
1990. For the most part, ANSI C standardizes existing practice,
with a few additions from C++ (most notably function prototypes)
and support for multinational character sets (including the
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 (where it is known
as ANSI/ISO 9899-1990 [1992]). As an ISO Standard, it is
subject to ongoing revision through the release of Technical
Corrigenda and Normative Addenda.
In 1994, Technical Corrigendum 1 (TC1) amended the Standard
in about 40 places, most of them minor corrections or
clarifications, and Normative Addendum 1 (NA1) added about 50
pages of new material, mostly specifying new library functions
for internationalization. In 1995, TC2 added a few more minor
corrections.
As of this writing, a complete revision of the Standard is in
its final stages. The new Standard is nicknamed "C9X" on the
assumption that it will be finished by the end of 1999. (Many
of this article's answers have been updated to reflect new C9X
features.)
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.
A new one is being prepared for C9X.)
11.2: How can I get a copy of the Standard?
A: 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).
The last time I checked, the cost was $130.00 from ANSI or
$400.50 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.
Public review drafts of C9X are available from ISO/IEC
JTC1/SC22/WG14's web site, http://www.dkuug.dk/JTC1/SC22/WG14/ .
See also question 11.2b below.
11.2b: Where can I get information about updates to the Standard?
A: You can find information (including C9X drafts) at
the web sites http://www.lysator.liu.se/c/index.html,
http://www.dkuug.dk/JTC1/SC22/WG14/, and http://www.dmk.com/ .
11.3: My ANSI compiler complains about a mismatch when it sees
extern int func(float);
int func(x)
float x;
{ ...
A: You have mixed the new-style prototype declaration
"extern int func(float);" with the old-style definition
"int func(x) float x;". It is usually possible 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, if possible.)
It is arguably much 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; 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.
11.4: Can you mix old-style and new-style function syntax?
A: Doing so is legal, but requires a certain amount of care (see
especially question 11.3). Modern practice, however, is to
use the prototyped form in both declarations and definitions.
(The old-style syntax is marked as obsolescent, so official
support for it may be removed some day.)
References: 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.
11.5: Why does the declaration
extern int f(struct x *p);
give me an obscure warning message about "struct x introduced in
prototype scope"?
A: In a quirk of C's normal block scoping rules, a 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: ISO Sec. 6.1.2.1, Sec. 6.1.2.6, Sec. 6.5.2.3.
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];
A: 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 (or perhaps an enum).
References: ISO Sec. 6.4; H&S Secs. 7.11.2,7.11.3 pp. 226-7.
11.9: What's the difference between "const char *p" and
"char * const p"?
A: "const char *p" (which can also be written "char const *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: ISO Sec. 6.5.4.1; Rationale Sec. 3.5.4.1; H&S
Sec. 4.4.4 p. 81.
11.10: Why can't I pass a char ** to a function which expects a
const char **?
A: You can use a pointer-to-T (for any type T) where a pointer-to-
const-T is expected. 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: ISO Sec. 6.1.2.6, Sec. 6.3.16.1, Sec. 6.5.3; H&S
Sec. 7.9.1 pp. 221-2.
11.12a: What's the correct declaration of main()?
A: Either int main(), int main(void), or int main(int argc,
char *argv[]) (with alternate spellings of argc and *argv[]
obviously allowed). See also questions 11.12b to 11.15 below.
References: 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.
11.12b: Can I declare main() as void, to shut off these annoying
"main returns no value" messages?
A: 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: 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.
11.13: But what about main's third argument, envp?
A: 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: ISO Sec. G.5.1; H&S Sec. 20.1 pp. 416-7.
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.
A: 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 11.12b).
It has been reported that programs using void main() and
compiled using BC++ 4.5 can crash. Some compilers (including
DEC C V4.1 and gcc with certain warnings enabled) will complain
about void main().
Your operating system may ignore the exit status, and
void main() may work for you, but it is not portable and not
correct.
11.15: The book I've been using, _C Programing for the Compleat Idiot_,
always uses void main().
A: Perhaps its author counts himself among the target audience.
Many books unaccountably use void main() in examples, and assert
that it's correct. They're wrong.
11.16: Is exit(status) truly equivalent to returning the same status
from main()?
A: Yes and no. The Standard says that they are equivalent.
However, a return from main() cannot be expected to work if
data local to main() might be needed during cleanup; see also
question 16.4. A few very old, nonconforming systems may once
have had problems with one or the other form. (Finally, the
two forms are obviously not equivalent in a recursive call to
main().)
References: K&R2 Sec. 7.6 pp. 163-4; ISO Sec. 5.1.2.2.3.
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.
A: 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: ISO Sec. 6.8.3.2, Sec. 6.8.3.5.
11.18: What does the message "warning: macro replacement within a
string literal" mean?
A: 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 above.
References: H&S Sec. 3.3.8 p. 51.
11.19: I'm getting strange syntax errors inside lines I've #ifdeffed
out.
A: Under ANSI C, the text inside a "turned off" #if, #ifdef, or
#ifndef must still consist of "valid preprocessing tokens."
This means that the characters " and ' must each be paired just
as in real C code, and the pairs mustn't cross line boundaries.
(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: ISO Sec. 5.1.1.2, Sec. 6.1; H&S Sec. 3.2 p. 40.
11.20: What are #pragmas and what are they good for?
A: The #pragma directive provides a single, well-defined "escape
hatch" which can be used for all sorts of (nonportable)
implementation-specific controls and extensions: source listing
control, structure packing, warning suppression (like lint's old
/* NOTREACHED */ comments), etc.
References: ISO Sec. 6.8.6; H&S Sec. 3.7 p. 61.
11.21: What does "#pragma once" mean? I found it in some header files.
A: It is an extension implemented by some preprocessors to help
make header files idempotent; it is equivalent to the #ifndef
trick mentioned in question 10.7, though less portable.
11.22: Is char a[3] = "abc"; legal? What does it mean?
A: It is legal in ANSI C (and perhaps in a few pre-ANSI systems),
though 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: ISO Sec. 6.5.7; H&S Sec. 4.6.4 p. 98.
11.24: Why can't I perform arithmetic on a void * pointer?
A: The compiler doesn't know the size of the pointed-to objects.
Before performing arithmetic, convert the pointer either to
char * or to the pointer type you're trying to manipulate (but
see also question 4.5).
References: ISO Sec. 6.1.2.5, Sec. 6.3.6; H&S Sec. 7.6.2 p. 204.
11.25: What's the difference between memcpy() and memmove()?
A: memmove() offers guaranteed behavior if the source and
destination arguments overlap. memcpy() makes no such
guarantee, and may therefore be more efficiently implementable.
When in doubt, it's safer to use memmove().
References: K&R2 Sec. B3 p. 250; 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.
11.26: What should malloc(0) do? Return a null pointer or a pointer to
0 bytes?
A: The ANSI/ISO Standard says that it may do either; the behavior
is implementation-defined (see question 11.33).
References: ISO Sec. 7.10.3; PCS Sec. 16.1 p. 386.
11.27: Why does the ANSI Standard not guarantee more than six case-
insensitive characters of external identifier significance?
A: 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 marked in the Standard as "obsolescent", and will
be removed in C9X.
References: ISO Sec. 6.1.2, Sec. 6.9.1; Rationale Sec. 3.1.2;
C9X Sec. 6.1.2; H&S Sec. 2.5 pp. 22-3.
11.29: My compiler is rejecting the simplest possible test programs,
with all kinds of syntax errors.
A: Perhaps it is a pre-ANSI compiler, unable to accept function
prototypes and the like.
See also questions 1.31, 10.9, 11.30, and 16.1b.
11.30: Why are some ANSI/ISO Standard library functions showing up as
undefined, even though I've got an ANSI compiler?
A: It's possible to have a compiler available which accepts ANSI
syntax, but not to have ANSI-compatible 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.
11.31: Does anyone have a tool for converting old-style C programs to
ANSI C, or vice versa, or for automatically generating
prototypes?
A: Two programs, protoize and unprotoize, convert back and forth
between prototyped and "old style" function definitions and
declarations. (These programs do *not* handle full-blown
translation between "Classic" C and ANSI C.) These programs 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-the-fly.
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.)
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.
A: 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.
11.33: People seem to make a point of distinguishing between
implementation-defined, unspecified, and undefined behavior.
What's the difference?
A: Briefly: implementation-defined means that an implementation
must choose some behavior and document it. Unspecified means
that an implementation should choose some behavior, but need not
document it. Undefined means that absolutely anything might
happen. In no case does the Standard impose requirements; in
the first two cases it occasionally suggests (and may require a
choice from among) a small set of likely behaviors.
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: ISO Sec. 3.10, Sec. 3.16, Sec. 3.17; Rationale
Sec. 1.6.
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?
A: 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.
11.35: People keep saying that the behavior of i = i++ is undefined,
but I just tried it on an ANSI-conforming compiler, and got the
results I expected.
A: A compiler may do anything it likes when faced with undefined
behavior (and, within limits, with implementation-defined and
unspecified behavior), including doing what you expect. It's
unwise to depend on it, though. See also questions 11.32,
11.33, and 11.34.
Section 12. Stdio
12.1: What's wrong with this code?
char c;
while((c = getchar()) != EOF) ...
A: For one thing, the variable to hold getchar's return value must
be an int. getchar() can return all possible character values,
as well as EOF. By squeezing getchar's return value into 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; 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.
12.2: Why does the code
while(!feof(infp)) {
fgets(buf, MAXLINE, infp);
fputs(buf, outfp);
}
copy the last line twice?
A: In C, end-of-file is only indicated *after* an input routine has
tried to read, and failed. (In other words, C's I/O is not like
Pascal's.) Usually, you should just check the return value of
the input routine (in this case, fgets() will return NULL on end-
of-file); often, you don't need to use feof() at all.
References: K&R2 Sec. 7.6 p. 164; ISO Sec. 7.9.3, Sec. 7.9.7.1,
Sec. 7.9.10.2; H&S Sec. 15.14 p. 382.
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.
A: It's best to use an explicit fflush(stdout) whenever output
should definitely be visible (and especially if the text does
not end with \n). 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: ISO Sec. 7.9.5.2.
12.5: How can I read one character at a time, without waiting for the
RETURN key?
A: See question 19.1.
12.6: How can I print a '%' character in a printf format string? I
tried \%, but it didn't work.
A: 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
essentially printf's escape character.
See also question 19.17.
References: K&R1 Sec. 7.3 p. 147; K&R2 Sec. 7.2 p. 154; ISO
Sec. 7.9.6.1.
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?
A: 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. (printf() does accept %Lf, for long double.)
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; 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.
12.9b: What printf format should I use for a typedef like size_t
when I don't know whether it's long or some other type?
A: Use a cast to convert the value to a known, conservatively-
sized type, then use the printf format matching that type.
For example, to print the size of a type, you might use
printf("%lu", (unsigned long)sizeof(thetype));
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.
A: printf("%*d", width, x) will do just what you want.
See also question 12.15.
References: K&R1 Sec. 7.3; K&R2 Sec. 7.2; ISO Sec. 7.9.6.1; H&S
Sec. 15.11.6; CT&P Sec. A.1.
12.11: How can I print numbers with commas separating the thousands?
What about currency formatted numbers?
A: The functions in <locale.h> 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: ISO Sec. 7.4; H&S Sec. 11.6 pp. 301-4.
12.12: Why doesn't the call scanf("%d", i) work?
A: The arguments you pass to scanf() must always be pointers.
To fix the fragment above, change it to scanf("%d", &i) .
12.13: Why doesn't this code:
double d;
scanf("%f", &d);
work?
A: Unlike printf(), scanf() uses %lf for values of type double, and
%f for float. See also question 12.9.
12.15: How can I specify a variable width in a scanf() format string?
A: 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
you can construct the scanf format string at run time.
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.
A: 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; ISO Sec. 7.9.6.2; H&S
Sec. 15.8 pp. 357-64.
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()!
A: 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: ISO Sec. 7.9.6.2; H&S Sec. 15.8 pp. 357-64.
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.
A: 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: ISO Sec. 7.9.6.2; H&S Sec. 15.8 pp. 357-64.
12.20: Why does everyone say not to use scanf()? What should I use
instead?
A: 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. (Functions like strtol(), strtok(), and atoi() are
often useful; see also question 13.6.) If you do use any scanf
variant, be sure to check the return value to make sure that the
expected number of items were found. Also, if you use %s, be
sure to guard against buffer overflow.
References: K&R2 Sec. 7.4 p. 159.
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()?
A: When the format string being used with sprintf() is known and
relatively simple, you can sometimes 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. For integers, the
number of characters produced by %d is no more than
((sizeof(int) * CHAR_BIT + 2) / 3 + 1) /* +1 for '-' */
(CHAR_BIT is in <limits.h>), though this computation may be over-
conservative. (It 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, and worry
about write errors).
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. If the format string is known, you can limit %s
expansion by using %.Ns for some N, or %.*s (see also question
12.10).
The "obvious" solution to the overflow problem is a length-
limited version of sprintf(), namely snprintf(). It would be
used like this:
snprintf(buf, bufsize, "You typed \"%s\"", answer);
snprintf() has been available in several stdio libraries
(including GNU and 4.4bsd) for several years. It will be
standardized in C9X.
When the C9X snprintf() arrives, it will also be possible to use
it to predict the size required for an arbitrary sprintf() call.
C9X snprintf() will return the number of characters it would
have placed in the buffer, not just how many it did place.
Furthermore, it may be called with a buffer size of 0 and a
null pointer as the destination buffer. Therefore, the call
nch = snprintf(NULL, 0, fmtstring, /* other arguments */ );
will compute the number of characters required for the fully-
formatted string.
References: C9X Sec. 7.13.6.6.
12.23: Why does everyone say not to use gets()?
A: 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.
12.24: Why does errno contain ENOTTY after a call to printf()?
A: Many implementations of the stdio package adjust their behavior
slightly if stdout is a terminal. To make the determination,
these implementations perform 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; errno is not guaranteed to be 0 otherwise.)
References: ISO Sec. 7.1.4, Sec. 7.9.10.3; CT&P Sec. 5.4 p. 73;
PCS Sec. 14 p. 254.
12.25: What's the difference between fgetpos/fsetpos and ftell/fseek?
What are fgetpos() and fsetpos() good for?
A: ftell() and fseek() use type long int to represent offsets
(positions) in a file, and may therefore be limited to offsets
of about 2 billion (2**31-1). The newer fgetpos() and fsetpos()
functions, on the other hand, use a special typedef, fpos_t, to
represent the offsets. The type behind this typedef, if chosen
appropriately, can represent arbitrarily large offsets, so
fgetpos() and fsetpos() can be used with arbitrarily huge files.
fgetpos() and fsetpos() also record the state associated with
multibyte streams. See also question 1.4.
References: K&R2 Sec. B1.6 p. 248; ISO Sec. 7.9.1,
Secs. 7.9.9.1,7.9.9.3; H&S Sec. 15.5 p. 252.
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?
A: fflush() is defined only for output streams. Since its
definition of "flush" is to complete the writing of buffered
characters (not to discard them), discarding unread input would
not be an analogous meaning for fflush on input streams.
There is no standard way to discard unread characters from a
stdio input stream, nor would such a way necessarily be
sufficient, since unread characters can also accumulate in
other, OS-level input buffers. You may be able to read and
discard characters until \n, or use the curses flushinp()
function, or use some system-specific technique. See also
questions 19.1 and 19.2.
References: ISO Sec. 7.9.5.2; H&S Sec. 15.2.
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.
A: Be sure to call fseek before you write, both to seek back to the
beginning of the string you're trying to overwrite, and because
an fseek or fflush is always required between reading and
writing in the read/write "+" modes. Also, remember that you
can only overwrite characters with the same number of
replacement characters, and that overwriting in text mode may
truncate the file at that point. See also question 19.14.
References: ISO Sec. 7.9.5.3.
12.33: How can I redirect stdin or stdout to a file from within a
program?
A: Use freopen() (but see question 12.34 below).
References: ISO Sec. 7.9.5.4; H&S Sec. 15.2.
12.34: Once I've used freopen(), how can I get the original stdout (or
stdin) back?
A: 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.
It is barely possible to save away information about a stream
before calling freopen(), such that the original stream can
later be restored, but the methods involve system-specific calls
such as dup(), or copying or inspecting the contents of a FILE
structure, which is exceedingly nonportable and unreliable.
12.36b: How can I arrange to have output go two places at once,
e.g. to the screen and to a file?
A: You can't do this directly, but you could write your
own printf variant which printed everything twice.
See question 15.5.
12.38: How can I read a binary data file properly? I'm occasionally
seeing 0x0a and 0x0d values getting garbled, and I seem to hit
EOF prematurely if the data contains the value 0x1a.
A: 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: ISO Sec. 7.9.5.3; H&S Sec. 15.2.1 p. 348.
Section 13. Library Functions
13.1: How can I convert numbers to strings (the opposite of atoi)?
Is there an itoa() function?
A: 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
7.5a; 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.
13.2: Why does strncpy() not always place a '\0' terminator in the
destination string?
A: strncpy() was first designed to handle a now-obsolete data
structure, the fixed-length, not-necessarily-\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 function to use than
strncpy().
13.5: Why do some versions of toupper() act strangely if given an
upper-case letter?
Why does some code call islower() before toupper()?
A: 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: ISO Sec. 7.3.2; H&S Sec. 12.9 pp. 320-1; PCS p. 182.
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?
A: The only Standard function 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; ISO Sec. 7.11.5.8; H&S
Sec. 13.7 pp. 333-4; PCS p. 178.
13.7: I need some code to do regular expression and wildcard matching.
A: 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
<regex.h> or <regexp.h>, 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. See
also questions 19.20 and 20.3.
13.8: I'm trying to sort an array of strings with qsort(), using
strcmp() as the comparison function, but it's not working.
A: By "array of strings" you probably mean "array of pointers to
char." The arguments to qsort's comparison function are
pointers to the objects being sorted, in this case, pointers to
pointers to char. strcmp(), 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" (pointers to pointers to char) and dereferenced,
yielding char *'s which can be passed to strcmp().
(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: ISO Sec. 7.10.5.2; H&S Sec. 20.5 p. 419.
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?
A: 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.)
If, on the other hand, you're sorting pointers to structures,
you'll need indirection, as in question 13.8:
sp1 = *(struct mystruct * const *)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: ISO Sec. 7.10.5.2; H&S Sec. 20.5 p. 419.
13.10: How can I sort a linked list?
A: 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.
13.11: How can I sort more data than will fit in memory?
A: 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.
13.12: How can I get the current date or time of day in a C program?
A: Just use the time(), ctime(), localtime() and/or strftime()
functions. Here is a simple example:
#include <stdio.h>
#include <time.h>
int main()
{
time_t now;
time(&now);
printf("It's %.24s.\n", ctime(&now));
return 0;
}
References: K&R2 Sec. B10 pp. 255-7; ISO Sec. 7.12; H&S Sec. 18.
13.13: I know that the library function localtime() will convert a
time_t into a broken-down struct tm, and that ctime() will
convert a time_t to a printable string. How can I perform the
inverse operations of converting a struct tm or a string into a
time_t?
A: ANSI C specifies a library function, 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 functions 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; ISO Sec. 7.12.2.3; H&S
Sec. 18.4 pp. 401-2.
13.14: How can I add N days to a date? How can I find the difference
between two dates?
A: The ANSI/ISO Standard C mktime() and difftime() functions
provide 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 (so don't assume
that division by 86400 will be exact).
Another approach to both problems is to use "Julian day
numbers". Code for handling Julian day numbers can be found
in the Snippets collection (see question 18.15c), the
Simtel/Oakland archives (file JULCAL10.ZIP, 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; ISO Secs. 7.12.2.2,7.12.2.3;
H&S Secs. 18.4,18.5 pp. 401-2; David Burki, "Date Conversions".
13.14b: Does C have any Year 2000 problems?
A: No, although poorly-written C programs do.
The tm_year field of struct tm holds the value of the year minus
1900; this field will therefore contain the value 100 for the
year 2000. Code that uses tm_year correctly (by adding or
subtracting 1900 when converting to or from human-readable
4-digit year representations) will have no problems at the turn
of the millennium. Any code that uses tm_year incorrectly,
however, such as by using it directly as a human-readable
2-digit year, or setting it from a 4-digit year with code like
tm.tm_year = yyyy % 100; /* WRONG */
or printing it as an allegedly human-readable 4-digit year with
code like
printf("19%d", tm.tm_year); /* WRONG */
will have grave y2k problems indeed. See also question 20.32.
References: K&R2 Sec. B10 p. 255; ISO Sec. 7.12.1; H&S Sec. 18.4
p. 401.
13.15: I need a random number generator.
A: The Standard C library has one: rand(). The implementation on
your system may not be perfect, but writing a better one isn't
necessarily easy, either.
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; 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".
13.16: How can I get random integers in a certain range?
A: 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 <stdlib.h>), 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.
13.17: Each time I run my program, I get the same sequence of numbers
back from rand().
A: You can call srand() to seed the pseudo-random number generator
with a 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; ISO Sec. 7.10.2.2; H&S
Sec. 17.7 p. 393.
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...
A: Poor pseudorandom number generators (such as the ones
unfortunately supplied with some systems) are not very random in
the low-order bits. Try using the higher-order bits: see
question 13.16.
References: Knuth Sec. 3.2.1.1 pp. 12-14.
13.20: How can I generate random numbers with a normal or Gaussian
distribution?
A: Here is one method, recommended by Knuth and due originally to
Marsaglia:
#include <stdlib.h>
#include <math.h>
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; Marsaglia and Bray,
"A Convenient Method for Generating Normal Variables";
Press et al., _Numerical Recipes in C_ Sec. 7.2 pp. 288-290.
13.24: I'm trying to port this A: Those functions are variously
old program. Why do I obsolete; you should
get "undefined external" instead:
errors for:
index? use strchr.
rindex? use strrchr.
bcopy? use memmove, after
interchanging the first and
second arguments (see also
question 11.25).
bcmp? use memcmp.
bzero? use memset, with a second
argument of 0.
References: PCS Sec. 11.
13.25: I keep getting errors due to library functions being undefined,
but I'm #including all the right header files.
A: In general, a header file contains only declarations. In some
cases (especially if the functions are nonstandard) obtaining
the actual *definitions* may require explicitly asking for the
correct libraries to be searched when you link the program.
(#including the header doesn't do that.) See also questions
11.30, 13.26, and 14.3.
13.26: I'm still getting errors due to library functions being
undefined, even though I'm explicitly requesting the right
libraries while linking.
A: Many linkers make one pass over the list of object files and
libraries you specify, and extract from libraries only those
modules which satisfy references which have so far come up as
undefined. Therefore, the order in which libraries are listed
with respect to object files (and each other) is significant;
usually, you want to search the libraries last. (For example,
under Unix, put any -l options towards the end of the command
line.) See also question 13.28.
13.28: What does it mean when the linker says that _end is undefined?
A: That message is a quirk of the old Unix linkers. You get an
error about _end being undefined only when other symbols are
undefined, too -- fix the others, and the error about _end will
disappear. (See also questions 13.25 and 13.26.)
Section 14. Floating Point
14.1: When I set a float variable to, say, 3.1, why is printf printing
it as 3.0999999?
A: Most computers use base 2 for floating-point numbers as well as
for integers. In base 2, one divided by ten is an infinitely-
repeating fraction (0.0001100110011...), so fractions such as
3.1 (which look like they can be exactly represented in decimal)
cannot be represented exactly in binary. 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.
14.2: I'm trying to take some square roots, but I'm getting crazy
numbers.
A: Make sure that you have #included <math.h>, and correctly
declared other functions returning double. (Another library
function to be careful with is atof(), which is declared in
<stdlib.h>.) See also question 14.3 below.
References: CT&P Sec. 4.5 pp. 65-6.
14.3: I'm trying to do some simple trig, and I am #including <math.h>,
but I keep getting "undefined: sin" compilation errors.
A: Make sure you're 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, 13.26, and 14.2.
14.4: My floating-point calculations are acting strangely and giving
me different answers on different machines.
A: First, see question 14.2 above.
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 floating-point 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".
14.5: What's a good way to check for "close enough" floating-point
equality?
A: 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 <math.h>
if(fabs(a - b) <= epsilon * fabs(a))
for some suitably-chosen degree of closeness epsilon (as long as
a is nonzero!).
References: Knuth Sec. 4.2.2 pp. 217-8.
14.6: How do I round numbers?
A: The simplest and most straightforward way is with code like
(int)(x + 0.5)
This technique won't work properly for negative numbers,
though (for which you could use something like
(int)(x < 0 ? x - 0.5 : x + 0.5)).
14.7: Why doesn't C have an exponentiation operator?
A: Because few processors have an exponentiation instruction.
C has a pow() function, declared in <math.h>, although explicit
multiplication is usually better for small positive integral
exponents.
References: ISO Sec. 7.5.5.1; H&S Sec. 17.6 p. 393.
14.8: The predefined constant M_PI seems to be missing from my
machine's copy of <math.h>.
A: 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, or compute it
with 4*atan(1.0).
References: PCS Sec. 13 p. 237.
14.9: How do I test for IEEE NaN and other special values?
A: Many systems with high-quality IEEE floating-point
implementations provide facilities (e.g. predefined constants,
and functions like isnan(), either as nonstandard extensions in
<math.h> or perhaps in <ieee.h> or <nan.h>) 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.
C9X will provide isnan(), fpclassify(), and several other
classification routines.
Another possibility is to 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.
References: C9X Sec. 7.7.3.
14.11: What's a good way to implement complex numbers in C?
A: It is straightforward to define a simple structure and some
arithmetic functions to manipulate them. C9X will support
complex as a standard type. See also questions 2.7, 2.10, and
14.12.
References: C9X Sec. 6.1.2.5, Sec. 7.8.
14.12: I'm looking for some code to do:
Fast Fourier Transforms (FFT's)
matrix arithmetic (multiplication, inversion, etc.)
complex arithmetic
A: Ajay Shah has prepared a nice index of free numerical
software which has been archived pretty widely; one URL
is ftp://ftp.math.psu.edu/pub/FAQ/numcomp-free-c .
See also questions 18.13, 18.15c, and 18.16.
14.13: I'm having trouble with a Turbo C program which crashes and says
something like "floating point formats not linked."
A: Some compilers for small machines, including 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 a dummy call to a
floating-point library function (such as sqrt(); any will
do) to force loading of floating-point support. (See the
comp.os.msdos.programmer FAQ list for more information.)
Section 15. Variable-Length Argument Lists
15.1: I heard that you have to #include <stdio.h> before calling
printf(). Why?
A: 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: 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.
15.2: How can %f be used for both float and double arguments in
printf()? Aren't they different types?
A: 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: ISO Sec. 6.3.2.2; H&S Sec. 6.3.5 p. 177, Sec. 9.4
pp. 272-3.
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.
A: 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.
15.4: How can I write a function that takes a variable number of
arguments?
A: Use the facilities of the <stdarg.h> header.
Here is a function which concatenates an arbitrary number of
strings into malloc'ed memory:
#include <stdlib.h> /* for malloc, NULL, size_t */
#include <stdarg.h> /* for va_ stuff */
#include <string.h> /* for strcat et al. */
char *vstrcat(char *first, ...)
{
size_t len;
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; 2nd 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; 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.
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?
A: Use vprintf(), vfprintf(), or vsprintf().
Here is an error() function which prints an error message,
preceded by the string "error: " and terminated with a newline:
#include <stdio.h>
#include <stdarg.h>
void error(char *fmt, ...)
{
va_list argp;
fprintf(stderr, "error: ");
va_start(argp, fmt);
vfprintf(stderr, fmt, argp);
va_end(argp);
fprintf(stderr, "\n");
}
See also question 15.7.
References: K&R2 Sec. 8.3 p. 174, Sec. B1.2 p. 245; 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.
15.6: How can I write a function analogous to scanf(), that calls
scanf() to do most of the work?
A: C9X will support vscanf(), vfscanf(), and vsscanf().
(Until then, you may be on your own.)
References: C9X Secs. 7.3.6.12-14.
15.7: I have a pre-ANSI compiler, without <stdarg.h>. What can I do?
A: There's an older header, <varargs.h>, 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.
15.8: How can I discover how many arguments a function was actually
called with?
A: This information is not available to a portable program. Some
old systems provided a nonstandard nargs() function, but its use
was always questionable, since it typically returned the number
of words passed, not the number of arguments. (Structures, 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 supply).
References: PCS Sec. 11 pp. 167-8.
15.9: My compiler isn't letting me declare a function
int f(...)
{
}
i.e. with no fixed arguments.
A: Standard C requires at least one fixed argument, in part so that
you can hand it to va_start(). See also question 15.10.
References: ISO Sec. 6.5.4, Sec. 6.5.4.3, Sec. 7.8.1.1; H&S
Sec. 9.2 p. 263.
15.10: I have a varargs function which accepts a float parameter. Why
isn't
va_arg(argp, float)
working?
A: 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.
(For analogous reasons, the last "fixed" argument, as handed to
va_start(), should not be widenable, either.) See also
questions 11.3 and 15.2.
References: ISO Sec. 6.3.2.2; Rationale Sec. 4.8.1.2; H&S
Sec. 11.4 p. 297.
15.11: I can't get va_arg() to pull in an argument of type pointer-to-
function.
A: The type-rewriting games which the va_arg() macro typically
plays are stymied by overly-complicated types such as pointer-to-
function. If you use a typedef for the function pointer type,
however, all will be well. See also question 1.21.
References: ISO Sec. 7.8.1.2; Rationale Sec. 4.8.1.2.
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)?
A: 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 above). 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 below.)
15.13: How can I call a function with an argument list built up at run
time?
A: There is no guaranteed or portable way to do this. If you're
curious, ask this list's editor, who has a few wacky ideas you
could try...
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.)
Section 16. Strange Problems
16.1b: I'm getting baffling syntax errors which make no sense at all,
and it seems like large chunks of my program aren't being
compiled.
A: Check for unclosed comments or mismatched #if/#ifdef/#ifndef/
#else/#endif directives; remember to check header files, too.
(See also questions 2.18, 10.9, and 11.29.)
16.1c: Why isn't my procedure call working? The compiler seems to skip
right over it.
A: Does the code look like this?
myprocedure;
C has only functions, and function calls always require
parenthesized argument lists, even if empty. Use
myprocedure();
16.3: This program crashes before it even runs! (When single-stepping
with a debugger, it dies before the first statement in main().)
A: You probably have one or more very large (kilobyte or more)
local arrays. Many systems have fixed-size stacks, and those
which perform dynamic stack allocation automatically (e.g. Unix)
can be confused when the stack tries to grow by a huge chunk all
at once. It is often better to declare large arrays with static
duration (unless of course you need a fresh set with each
recursive call, in which case you could dynamically allocate
them with malloc(); see also question 1.31).
(See also questions 11.12b, 16.4, 16.5, and 18.4.)
16.4: I have a program that seems to run correctly, but it crashes as
it's exiting, *after* the last statement in main(). What could
be causing this?
A: Look for a misdeclared main() (see questions 2.18 and 10.9), or
local buffers passed to setbuf() or setvbuf(), or problems in
cleanup functions registered by atexit(). See also questions
7.5a and 11.16.
References: CT&P Sec. 5.3 pp. 72-3.
16.5: This program runs perfectly on one machine, but I get weird
results on another. Stranger still, adding or removing a
debugging printout changes the symptoms...
A: Lots of things could be going wrong; here are a few of the more
common things to check:
uninitialized local variables (see also question 7.1)
integer overflow, especially on 16-bit machines,
especially of an intermediate result when doing things
like a * b / c (see also question 3.14)
undefined evaluation order (see questions 3.1 through 3.4)
omitted declaration of external functions, especially
those which return something other than int, or have
"narrow" or variable arguments (see questions 1.25, 11.3,
14.2, and 15.1)
dereferenced null pointers (see section 5)
improper malloc/free use: assuming malloc'ed memory
contains 0, assuming freed storage persists, freeing
something twice, corrupting the malloc arena (see also
questions 7.19 and 7.20)
pointer problems in general (see also question 16.8)
mismatch between printf() format and arguments, especially
trying to print long ints using %d (see question 12.9)
trying to allocate more memory than an unsigned int can
count, especially on machines with limited memory (see
also questions 7.16 and 19.23)
array bounds problems, especially of small, temporary
buffers, perhaps used for constructing strings with
sprintf() (see also questions 7.1 and 12.21)
invalid assumptions about the mapping of typedefs,
especially size_t
floating point problems (see questions 14.1 and 14.4)
anything you thought was a clever exploitation of the way
you believe code is generated for your specific system
Proper use of function prototypes can catch several of these
problems; lint would catch several more. See also questions
16.3, 16.4, and 18.4.
16.6: Why does this code:
char *p = "hello, world!";
p[0] = 'H';
crash?
A: String literals are not necessarily modifiable, except (in
effect) when they are used as array initializers. Try
char a[] = "hello, world!";
See also question 1.32.
References: ISO Sec. 6.1.4; H&S Sec. 2.7.4 pp. 31-2.
16.8: What do "Segmentation violation" and "Bus error" mean?
A: These generally mean that your program tried to access memory it
shouldn't have, invariably as a result of stack corruption or
improper pointer use. Likely causes are overflow of local
("automatic," stack-allocated) arrays; inadvertent use of null
pointers (see also questions 5.2 and 5.20) or uninitialized,
misaligned, or otherwise improperly allocated pointers (see
questions 7.1 and 7.2); corruption of the malloc arena (see
question 7.19); and mismatched function arguments, especially
involving pointers; two possibilities are scanf() (see question
12.12) and fprintf() (make sure it receives its first FILE *
argument).
See also questions 16.3 and 16.4.
Section 17. Style
17.1: What's the best style for code layout in C?
A: K&R, while providing the example most often copied, also supply
a good excuse for disregarding it:
The position of braces is less important,
although people hold passionate beliefs.
We have chosen one of several popular styles.
Pick a style that suits you, then use it
consistently.
It is more important that the layout chosen be consistent (with
itself, and with nearby or common code) than that it be
"perfect." If your coding environment (i.e. local custom or
company policy) does not suggest a style, and you don't feel
like inventing your own, just copy K&R. (The tradeoffs between
various indenting and brace placement options can be
exhaustively and minutely examined, but don't warrant repetition
here. See also the Indian Hill Style Guide.)
The elusive quality of "good style" involves much more than mere
code layout details; don't spend time on formatting to the
exclusion of more substantive code quality issues.
See also question 10.6.
References: K&R1 Sec. 1.2 p. 10; K&R2 Sec. 1.2 p. 10.
17.3: Here's a neat trick for checking whether two strings are equal:
if(!strcmp(s1, s2))
Is this good style?
A: It is not particularly good style, although it is a popular
idiom. The test succeeds if the two strings are equal, but the
use of ! ("not") suggests that it tests for inequality.
Another option is to use a macro:
#define Streq(s1, s2) (strcmp((s1), (s2)) == 0)
See also question 17.10.
17.4: Why do some people write if(0 == x) instead of if(x == 0)?
A: It's a trick to guard against the common error of writing
if(x = 0)
If you're in the habit of writing the constant before the ==,
the compiler will complain if you accidentally type
if(0 = x)
Evidently it can be easier for some people to remember to
reverse the test than to remember to type the doubled = sign.
(Of course, the trick only helps when comparing to a constant.)
References: H&S Sec. 7.6.5 pp. 209-10.
17.5: I came across some code that puts a (void) cast before each call
to printf(). Why?
A: printf() does return a value, though few programs bother to
check the return values from each call. Since some compilers
(and lint) will warn about discarded return values, an explicit
cast to (void) is a way of saying "Yes, I've decided to ignore
the return value from this call, but please continue to warn me
about other (perhaps inadvertently) ignored return values."
It's also common to use void casts on calls to strcpy() and
strcat(), since the return value is never surprising.
References: K&R2 Sec. A6.7 p. 199; Rationale Sec. 3.3.4; H&S
Sec. 6.2.9 p. 172, Sec. 7.13 pp. 229-30.
17.8: What is "Hungarian Notation"? Is it worthwhile?
A: Hungarian Notation is a naming convention, invented by Charles
Simonyi, which encodes information about a variable's type (and
perhaps its intended use) in its name. It is well-loved in some
circles and roundly castigated in others. Its chief advantage
is that it makes a variable's type or intended use obvious from
its name; its chief disadvantage is that type information is not
necessarily a worthwhile thing to carry around in the name of a
variable.
References: Simonyi and Heller, "The Hungarian Revolution" .
17.9: Where can I get the "Indian Hill Style Guide" and other coding
standards?
A: Various documents are available for anonymous ftp from:
Site: File or directory:
ftp.cs.washington.edu pub/cstyle.tar.Z
(the updated Indian Hill guide)
ftp.cs.toronto.edu doc/programming
(including Henry Spencer's
"10 Commandments for C Programmers")
ftp.cs.umd.edu pub/style-guide
You may also be interested in the books _The Elements of
Programming Style_, _Plum Hall Programming Guidelines_, and _C
Style: Standards and Guidelines_; see the Bibliography.
See also question 18.9.
17.10: Some people say that goto's are evil and that I should never use
them. Isn't that a bit extreme?
A: Programming style, like writing style, is somewhat of an art and
cannot be codified by inflexible rules, although discussions
about style often seem to center exclusively around such rules.
In the case of the goto statement, it has long been observed
that unfettered use of goto's quickly leads to unmaintainable
spaghetti code. However, a simple, unthinking ban on the goto
statement does not necessarily lead immediately to beautiful
programming: an unstructured programmer is just as capable of
constructing a Byzantine tangle without using any goto's
(perhaps substituting oddly-nested loops and Boolean control
variables, instead).
Most observations or "rules" about programming style usually
work better as guidelines than rules, and work much better if
programmers understand what the guidelines are trying to
accomplish. Blindly avoiding certain constructs or following
rules without understanding them can lead to just as many
problems as the rules were supposed to avert.
Furthermore, many opinions on programming style are just that:
opinions. It's usually futile to get dragged into "style wars,"
because on certain issues (such as those referred to in
questions 9.2, 5.3, 5.9, and 10.7), opponents can never seem to
agree, or agree to disagree, or stop arguing.
Section 18. Tools and Resources
18.1: I need: A: Look for programs (see also
question 18.16) named:
a C cross-reference cflow, cxref, calls, cscope,
generator xscope, or ixfw
a C beautifier/pretty- cb, indent, GNU indent, or
printer vgrind
a revision control or CVS, RCS, or SCCS
configuration management
tool
a C source obfuscator obfus, shroud, or opqcp
(shrouder)
a "make" dependency makedepend, or try cc -M or
generator cpp -M
tools to compute code ccount, Metre, lcount, or csize,
metrics or see URL http://www.qucis.queensu.ca/
Software-Engineering/Cmetrics.html ;
there is also a package sold
by McCabe and Associates
a C lines-of-source this can be done very
counter crudely with the standard
Unix utility wc, and
somewhat better with
grep -c ";"
a C declaration aid check volume 14 of
(cdecl) comp.sources.unix (see
question 18.16) and K&R2
a prototype generator see question 11.31
a tool to track down
malloc problems see question 18.2
a "selective" C
preprocessor see question 10.18
language translation see questions 11.31 and
tools 20.26
C verifiers (lint) see question 18.7
a C compiler! see question 18.3
(This list of tools is by no means complete; if you know of
tools not mentioned, you're welcome to contact this list's
maintainer.)
Other lists of tools, and discussion about them, can be found in
the Usenet newsgroups comp.compilers and comp.software-eng.
See also questions 18.3 and 18.16.
18.2: How can I track down these pesky malloc problems?
A: A number of debugging packages exist to help track down malloc
problems; one popular one is Conor P. Cahill's "dbmalloc",
posted to comp.sources.misc in 1992, volume 32. Others are
"leak", available in volume 27 of the comp.sources.unix
archives; JMalloc.c and JMalloc.h in the "Snippets" collection;
and MEMDEBUG from ftp.crpht.lu in pub/sources/memdebug . See
also question 18.16.
A number of commercial debugging tools exist, and can be
invaluable in tracking down malloc-related and other stubborn
problems:
Bounds-Checker for DOS, from Nu-Mega Technologies,
P.O. Box 7780, Nashua, NH 03060-7780, USA, 603-889-2386.
CodeCenter (formerly Saber-C) from Centerline Software,
10 Fawcett Street, Cambridge, MA 02138, USA, 617-498-3000.
Insight, from ParaSoft Corporation, 2500 E. Foothill
Blvd., Pasadena, CA 91107, USA, 818-792-9941,
[email protected] .
Purify, from Pure Software, 1309 S. Mary Ave., Sunnyvale,
CA 94087, USA, 800-224-7873, http://www.pure.com ,
[email protected] .
(I believe Pure was recently acquired by Rational.)
Final Exam Memory Advisor, from PLATINUM Technology
(formerly Sentinel from AIB Software), 1815 South Meyers
Rd., Oakbrook Terrace, IL 60181, USA, 630-620-5000,
800-442-6861, [email protected], www.platinum.com .
ZeroFault, from The Kernel Group, 1250 Capital of Texas
Highway South, Building Three, Suite 601, Austin,
TX 78746, 512-433-3333, http://www.tkg.com, [email protected] .
18.3: What's a free or cheap C compiler I can use?
A: A popular and high-quality free C compiler is the FSF's GNU C
compiler, or gcc. It is available by anonymous ftp from
prep.ai.mit.edu in directory pub/gnu, or at several other FSF
archive sites. An MS-DOS port, djgpp, is also available;
see the djgpp home page at http://www.delorie.com/djgpp/ .
There is a shareware compiler called PCC, available as
PCC12C.ZIP .
A very inexpensive MS-DOS compiler is Power C from Mix Software,
1132 Commerce Drive, Richardson, TX 75801, USA, 214-783-6001.
Another recently-developed compiler is lcc, available for
anonymous ftp from ftp.cs.princeton.edu in pub/lcc/.
A shareware MS-DOS C compiler is available from
ftp.hitech.com.au/hitech/pacific. Registration is optional for
non-commercial use.
There are currently no viable shareware compilers for the
Macintosh.
Archives associated with comp.compilers contain a great deal of
information about available compilers, interpreters, grammars,
etc. (for many languages). The comp.compilers archives
(including an FAQ list), maintained by the moderator, John R.
Levine, are at iecc.com . A list of available compilers and
related resources, maintained by Mark Hopkins, Steven Robenalt,
and David Muir Sharnoff, is at ftp.idiom.com in pub/compilers-
list/. (See also the comp.compilers directory in the
news.answers archives at rtfm.mit.edu and ftp.uu.net; see
question 20.40.)
See also question 18.16.
18.4: I just typed in this program, and it's acting strangely. Can
you see anything wrong with it?
A: See if you can run lint first (perhaps with the -a, -c, -h, -p
or other options). Many C compilers are really only half-
compilers, electing not to diagnose numerous source code
difficulties which would not actively preclude code generation.
See also questions 16.5, 16.8, and 18.7.
References: Ian Darwin, _Checking C Programs with lint_ .
18.5: How can I shut off the "warning: possible pointer alignment
problem" message which lint gives me for each call to malloc()?
A: The problem is that traditional versions of lint do not know,
and cannot be told, that malloc() "returns a pointer to space
suitably aligned for storage of any type of object." It is
possible to provide a pseudoimplementation of malloc(), using a
#define inside of #ifdef lint, which effectively shuts this
warning off, but a simpleminded definition will also suppress
meaningful messages about truly incorrect invocations. It may
be easier simply to ignore the message, perhaps in an automated
way with grep -v. (But don't get in the habit of ignoring too
many lint messages, otherwise one day you'll overlook a
significant one.)
18.7: Where can I get an ANSI-compatible lint?
A: Products called PC-Lint and FlexeLint (in "shrouded source
form," for compilation on 'most any system) are available from
Gimpel Software
3207 Hogarth Lane
Collegeville, PA 19426 USA
(+1) 610 584 4261
[email protected]
The Unix System V release 4 lint is ANSI-compatible, and is
available separately (bundled with other C tools) from UNIX
Support Labs or from System V resellers.
Another ANSI-compatible lint (which can also perform higher-
level formal verification) is LCLint, available via anonymous
ftp from larch.lcs.mit.edu in pub/Larch/lclint/.
In the absence of lint, many modern compilers do attempt to
diagnose almost as many problems as lint does. (Many netters
recommend gcc -Wall -pedantic .)
18.8: Don't ANSI function prototypes render lint obsolete?
A: No. First of all, prototypes work only if they are present and
correct; an inadvertently incorrect prototype is worse than
useless. Secondly, lint checks consistency across multiple
source files, and checks data declarations as well as functions.
Finally, an independent program like lint will probably always
be more scrupulous at enforcing compatible, portable coding
practices than will any particular, implementation-specific,
feature- and extension-laden compiler.
If you do want to use function prototypes instead of lint for
cross-file consistency checking, make sure that you set the
prototypes up correctly in header files. See questions 1.7 and
10.6.
18.9: Are there any C tutorials or other resources on the net?
A: There are several of them:
Tom Torfs has a nice tutorial at
http://members.xoom.com/tomtorfs/cintro.html .
"Notes for C programmers," by Christopher Sawtell, are
available from svr-ftp.eng.cam.ac.uk in misc/sawtell_C.shar and
garbo.uwasa.fi in /pc/c-lang/c-lesson.zip .
Tim Love's "C for Programmers" is available by ftp from svr-
ftp.eng.cam.ac.uk in the misc directory. An html version is at
http://www-h.eng.cam.ac.uk/help/tpl/languages/C/teaching_C/
teaching_C.html .
The Coronado Enterprises C tutorials are available on Simtel
mirrors in pub/msdos/c or on the web at http://www.swcp.com/~dodrill .
Rick Rowe has a tutorial which is available from ftp.netcom.com
as pub/rowe/tutorde.zip or ftp.wustl.edu as
pub/MSDOS_UPLOADS/programming/c_language/ctutorde.zip .
There is evidently a web-based course at
http://www.strath.ac.uk/CC/Courses/CCourse/CCourse.html .
Martin Brown has C course material on the web at
http://www-isis.ecs.soton.ac.uk/computing/c/Welcome.html .
On some Unix machines you can try typing "learn c" at the shell
prompt (but the lessons may be quite dated).
Finally, the author of this FAQ list teaches a C class
and has placed its notes on the web; they are at
http://www.eskimo.com/~scs/cclass/cclass.html .
[Disclaimer: I have not reviewed many of these tutorials, and
I gather that they tend to contain errors. With the exception
of the one with my name on it, I can't vouch for any of them.
Also, this sort of information rapidly becomes out-of-date;
these addresses may not work by the time you read this and
try them.]
Several of these tutorials, plus a great deal of other
information about C, are accessible via the web at
http://www.lysator.liu.se/c/index.html .
Vinit Carpenter maintains a list of resources for learning C and
C++; it is posted to comp.lang.c and comp.lang.c++, and archived
where this FAQ list is (see question 20.40), or on the web at
http://www.cyberdiem.com/vin/learn.html .
See also questions 18.10 and 18.15c.
18.10: What's a good book for learning C?
A: There are far too many books on C to list here; it's impossible
to rate them all. Many people believe that the best one was
also the first: _The C Programming Language_, by Kernighan and
Ritchie ("K&R," now in its second edition). Opinions vary on
K&R's suitability as an initial programming text: many of us did
learn C from it, and learned it well; some, however, feel that
it is a bit too clinical as a first tutorial for those without
much programming background. Several sets of annotations and
errata are available on the net, see e.g.
http://www.csd.uwo.ca/~jamie/.Refs/.Footnotes/C-annotes.html,
http://www.eskimo.com/~scs/cclass/cclass.html, and
http://www.lysator.liu.se/c/c-errata.html#main .
Many comp.lang.c regulars recommend _C: A Modern Approach_,
by K.N. King.
An excellent reference manual is _C: A Reference Manual_, by
Samuel P. Harbison and Guy L. Steele, now in its fourth edition.
Though not suitable for learning C from scratch, this FAQ list
has been published in book form; see the Bibliography.
Mitch Wright maintains an annotated bibliography of C and Unix
books; it is available for anonymous ftp from ftp.rahul.net in
directory pub/mitch/YABL/.
Scott McMahon has a nice set of reviews at
http://www.skwc.com/essent/cyberreviews.html .
The Association of C and C++ Users (ACCU) maintains a
comprehensive set of bibliographic reviews of C/C++ titles, at
http://bach.cis.temple.edu/accu/bookcase or
http://www.accu.org/accu .
This FAQ list's editor has a large collection of assorted
old recommendations which various people have posted; it
is available upon request. See also question 18.9 above.
18.13: Where can I find the sources of the standard C libraries?
A: One source (though not public domain) is _The Standard C
Library_, by P.J. Plauger (see the Bibliography).
Implementations of all or part of the C library have been
written and are readily available as part of the NetBSD and GNU
(also Linux) projects. See also questions 18.15c and 18.16.
18.13b: Is there an on-line C reference manual?
A: Two possibilities are
http://www.cs.man.ac.uk/standard_c/_index.html and
http://www.dinkumware.com/htm_cl/index.html .
18.13c: Where can I get a copy of the ANSI/ISO C Standard?
A: See question 11.2.
18.14: I need code to parse and evaluate expressions.
A: Two available packages are "defunc," posted to comp.sources.misc
in December, 1993 (V41 i32,33), to alt.sources in January, 1994,
and available from sunsite.unc.edu in
pub/packages/development/libraries/defunc-1.3.tar.Z, and
"parse," at lamont.ldgo.columbia.edu. Other options include the
S-Lang interpreter, available via anonymous ftp from
amy.tch.harvard.edu in pub/slang, and the shareware Cmm ("C-
minus-minus" or "C minus the hard stuff"). See also questions
18.16 and 20.6.
There is also some parsing/evaluation code in _Software
Solutions in C_ (chapter 12, pp. 235-55).
18.15: Where can I get a BNF or YACC grammar for C?
A: The definitive grammar is of course the one in the ANSI
standard; see question 11.2. Another grammar (along with
one for C++) by Jim Roskind is in pub/c++grammar1.1.tar.Z
at ics.uci.edu (or perhaps ftp.ics.uci.edu, or perhaps
OLD/pub/c++grammar1.1.tar.Z), or at ftp.eskimo.com in
u/s/scs/roskind_grammar.Z . A fleshed-out, working instance
of the ANSI grammar (due to Jeff Lee) is on ftp.uu.net
(see question 18.16) in usenet/net.sources/ansi.c.grammar.Z
(including a companion lexer). The FSF's GNU C compiler
contains a grammar, as does the appendix to K&R2.
The comp.compilers archives contain more information about
grammars; see question 18.3.
References: K&R1 Sec. A18 pp. 214-219; K&R2 Sec. A13 pp. 234-
239; ISO Sec. B.2; H&S pp. 423-435 Appendix B.
18.15b: Does anyone have a C compiler test suite I can use?
A: Plum Hall (formerly in Cardiff, NJ; now in Hawaii) sells one;
other packages are Ronald Guilmette's RoadTest(tm) Compiler Test
Suites (ftp to netcom.com, pub/rfg/roadtest/announce.txt for
information) and Nullstone's Automated Compiler Performance
Analysis Tool (see http://www.nullstone.com). The FSF's GNU C
(gcc) distribution includes a c-torture-test which checks a
number of common problems with compilers. Kahan's paranoia
test, found in netlib/paranoia on netlib.att.com, strenuously
tests a C implementation's floating point capabilities.
18.15c: Where are some collections of useful code fragments and
examples?
A: Bob Stout's popular "SNIPPETS" collection is available from
ftp.brokersys.com in directory pub/snippets or on the web at
http://www.brokersys.com/snippets/ .
Lars Wirzenius's "publib" library is available from ftp.funet.fi
in directory pub/languages/C/Publib/.
See also questions 14.12, 18.9, 18.13, and 18.16.
18.15d: I need code for performing multiple precision arithmetic.
A: Some popular packages are the "quad" functions within the BSD
Unix libc sources (ftp.uu.net, /systems/unix/bsd-sources/..../
/src/lib/libc/quad/*), the GNU MP library, the MIRACL package
(see http://indigo.ie/~mscott/ ), and the old Unix libmp.a.
See also questions 14.12 and 18.16.
References: Schumacher, ed., _Software Solutions in C_ Sec. 17
pp. 343-454.
18.16: Where and how can I get copies of all these freely distributable
programs?
A: As the number of available programs, the number of publicly
accessible archive sites, and the number of people trying to
access them all grow, this question becomes both easier and more
difficult to answer.
There are a number of large, public-spirited archive sites out
there, such as ftp.uu.net, archive.umich.edu, oak.oakland.edu,
sumex-aim.stanford.edu, and wuarchive.wustl.edu, which have huge
amounts of software and other information all freely available.
For the FSF's GNU project, the central distribution site is
prep.ai.mit.edu . These well-known sites tend to be extremely
busy and hard to reach, but there are also numerous "mirror"
sites which try to spread the load around.
On the connected Internet, the traditional way to retrieve files
from an archive site is with anonymous ftp. For those without
ftp access, there are also several ftp-by-mail servers in
operation. More and more, the world-wide web (WWW) is being
used to announce, index, and even transfer large data files.
There are probably yet newer access methods, too.
Those are some of the easy parts of the question to answer. The
hard part is in the details -- this article cannot begin to
track or list all of the available archive sites or all of the
various ways of accessing them. If you have access to the net
at all, you probably have access to more up-to-date information
about active sites and useful access methods than this FAQ list
does.
The other easy-and-hard aspect of the question, of course, is
simply *finding* which site has what you're looking for. There
is a tremendous amount of work going on in this area, and there
are probably new indexing services springing up every day. One
of the first was "archie", and of course there are a number of
high-profile commercial net indexing and searching services such
as Alta Vista, Excite, and Yahoo.
If you have access to Usenet, see the regular postings in the
comp.sources.unix and comp.sources.misc newsgroups, which
describe the archiving policies for those groups and how to
access their archives, two of which are
ftp://gatekeeper.dec.com/pub/usenet/comp.sources.unix/ and
ftp://ftp.uu.net/usenet/comp.sources.unix/. The comp.archives
newsgroup contains numerous announcements of anonymous ftp
availability of various items. Finally, the newsgroup
comp.sources.wanted is generally a more appropriate place to
post queries for source availability, but check *its* FAQ list,
"How to find sources," before posting there.
See also questions 14.12, 18.13, and 18.15c.
Section 19. System Dependencies
19.1: How can I read a single character from the keyboard without
waiting for the RETURN key? How can I stop characters from
being echoed on the screen as they're typed?
A: Alas, there is no standard or portable way to do these things in
C. Concepts such as screens and keyboards are not even
mentioned in the Standard, which deals only with simple I/O
"streams" of characters.
At some level, interactive keyboard input is usually collected
and presented to the requesting program a line at a time. This
gives the operating system a chance to support input line
editing (backspace/delete/rubout, etc.) in a consistent way,
without requiring that it be built into every program. Only
when the user is satisfied and presses the RETURN key (or
equivalent) is the line made available to the calling program.
Even if the calling program appears to be reading input a
character at a time (with getchar() or the like), the first call
blocks until the user has typed an entire line, at which point
potentially many characters become available and many character
requests (e.g. getchar() calls) are satisfied in quick
succession.
When a program wants to read each character immediately as it
arrives, its course of action will depend on where in the input
stream the line collection is happening and how it can be
disabled. Under some systems (e.g. MS-DOS, VMS in some modes),
a program can use a different or modified set of OS-level input
calls to bypass line-at-a-time input processing. Under other
systems (e.g. Unix, VMS in other modes), the part of the
operating system responsible for serial input (often called the
"terminal driver") must be placed in a mode which turns off line-
at-a-time processing, after which all calls to the usual input
routines (e.g. read(), getchar(), etc.) will return characters
immediately. Finally, a few systems (particularly older, batch-
oriented mainframes) perform input processing in peripheral
processors which cannot be told to do anything other than line-
at-a-time input.
Therefore, when you need to do character-at-a-time input (or
disable keyboard echo, which is an analogous problem), you will
have to use a technique specific to the system you're using,
assuming it provides one. Since comp.lang.c is oriented towards
those topics that the C language has defined support for, you
will usually get better answers to other questions by referring
to a system-specific newsgroup such as comp.unix.questions or
comp.os.msdos.programmer, and to the FAQ lists for these groups.
Note that the answers are often not unique even across different
variants of a system; bear in mind when answering system-
specific questions that the answer that applies to your system
may not apply to everyone else's.
However, since these questions are frequently asked here, here
are brief answers for some common situations.
Some versions of curses have functions called cbreak(),
noecho(), and getch() which do what you want. If you're
specifically trying to read a short password without echo, you
might try getpass(). Under Unix, you can use ioctl() to play
with the terminal driver modes (CBREAK or RAW under "classic"
versions; ICANON, c_cc[VMIN] and c_cc[VTIME] under System V or
POSIX systems; ECHO under all versions), or in a pinch, system()
and the stty command. (For more information, see <sgtty.h> and
tty(4) under classic versions, <termio.h> and termio(4) under
System V, or <termios.h> and termios(4) under POSIX.) Under
MS-DOS, use getch() or getche(), or the corresponding BIOS
interrupts. Under VMS, try the Screen Management (SMG$)
routines, or curses, or issue low-level $QIO's with the
IO$_READVBLK function code (and perhaps IO$M_NOECHO, and others)
to ask for one character at a time. (It's also possible to set
character-at-a-time or "pass through" modes in the VMS terminal
driver.) Under other operating systems, you're on your own.
(As an aside, note that simply using setbuf() or setvbuf() to
set stdin to unbuffered will *not* generally serve to allow
character-at-a-time input.)
If you're trying to write a portable program, a good approach is
to define your own suite of three functions to (1) set the
terminal driver or input system into character-at-a-time mode
(if necessary), (2) get characters, and (3) return the terminal
driver to its initial state when the program is finished.
(Ideally, such a set of functions might be part of the C
Standard, some day.) The extended versions of this FAQ list
(see question 20.40) contain examples of such functions for
several popular systems.
See also question 19.2.
References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1; POSIX
Sec. 7.
19.2: How can I find out if there are characters available for reading
(and if so, how many)? Alternatively, how can I do a read that
will not block if there are no characters available?
A: These, too, are entirely operating-system-specific. Some
versions of curses have a nodelay() function. Depending on your
system, you may also be able to use "nonblocking I/O", or a
system call named "select" or "poll", or the FIONREAD ioctl, or
c_cc[VTIME], or kbhit(), or rdchk(), or the O_NDELAY option to
open() or fcntl(). See also question 19.1.
19.3: How can I display a percentage-done indication that updates
itself in place, or show one of those "twirling baton" progress
indicators?
A: These simple things, at least, you can do fairly portably.
Printing the character '\r' will usually give you a carriage
return without a line feed, so that you can overwrite the
current line. The character '\b' is a backspace, and will
usually move the cursor one position to the left.
References: ISO Sec. 5.2.2.
19.4: How can I clear the screen?
How can I print text in color?
How can I move the cursor to a specific x, y position?
A: Such things depend on the terminal type (or display) you're
using. You will have to use a library such as termcap,
terminfo, or curses, or some system-specific routines, to
perform these operations. On MS-DOS systems, two functions
to look for are clrscr() and gotoxy().
For clearing the screen, a halfway portable solution is to print
a form-feed character ('\f'), which will cause some displays to
clear. Even more portable (albeit even more gunky) might be to
print enough newlines to scroll everything away. As a last
resort, you could use system() (see question 19.27) to invoke
an operating system clear-screen command.
References: PCS Sec. 5.1.4 pp. 54-60, Sec. 5.1.5 pp. 60-62.
19.5: How do I read the arrow keys? What about function keys?
A: Terminfo, some versions of termcap, and some versions of curses
have support for these non-ASCII keys. Typically, a special key
sends a multicharacter sequence (usually beginning with ESC,
'\033'); parsing these can be tricky. (curses will do the
parsing for you, if you call keypad() first.)
Under MS-DOS, if you receive a character with value 0 (*not*
'0'!) while reading the keyboard, it's a flag indicating that
the next character read will be a code indicating a special key.
See any DOS programming guide for lists of keyboard scan codes.
(Very briefly: the up, left, right, and down arrow keys are 72,
75, 77, and 80, and the function keys are 59 through 68.)
References: PCS Sec. 5.1.4 pp. 56-7.
19.6: How do I read the mouse?
A: Consult your system documentation, or ask on an appropriate
system-specific newsgroup (but check its FAQ list first). Mouse
handling is completely different under the X window system, MS-
DOS, the Macintosh, and probably every other system.
References: PCS Sec. 5.5 pp. 78-80.
19.7: How can I do serial ("comm") port I/O?
A: It's system-dependent. Under Unix, you typically open, read,
and write a device file in /dev, and use the facilities of the
terminal driver to adjust its characteristics. (See also
questions 19.1 and 19.2.) Under MS-DOS, you can use the
predefined stream stdaux, or a special file like COM1, or some
primitive BIOS interrupts, or (if you require decent
performance) any number of interrupt-driven serial I/O packages.
Several netters recommend the book _C Programmer's Guide to
Serial Communications_, by Joe Campbell.
19.8: How can I direct output to the printer?
A: Under Unix, either use popen() (see question 19.30) to write to
the lp or lpr program, or perhaps open a special file like
/dev/lp. Under MS-DOS, write to the (nonstandard) predefined
stdio stream stdprn, or open the special files PRN or LPT1.
References: PCS Sec. 5.3 pp. 72-74.
19.9: How do I send escape sequences to control a terminal or other
device?
A: If you can figure out how to send characters to the device at
all (see question 19.8 above), it's easy enough to send escape
sequences. In ASCII, the ESC code is 033 (27 decimal), so code
like
fprintf(ofd, "\033[J");
sends the sequence ESC [ J .
19.10: How can I do graphics?
A: Once upon a time, Unix had a fairly nice little set of device-
independent plot functions described in plot(3) and plot(5).
The GNU libplot package maintains the same spirit and supports
many modern plot devices;
see http://www.gnu.org/software/plotutils/plotutils.html .
If you're programming for MS-DOS, you'll probably want to use
libraries conforming to the VESA or BGI standards.
If you're trying to talk to a particular plotter, making it draw
is usually a matter of sending it the appropriate escape
sequences; see also question 19.9. The vendor may supply a C-
callable library, or you may be able to find one on the net.
If you're programming for a particular window system (Macintosh,
X windows, Microsoft Windows), you will use its facilities; see
the relevant documentation or newsgroup or FAQ list.
References: PCS Sec. 5.4 pp. 75-77.
19.11: How can I check whether a file exists? I want to warn the user
if a requested input file is missing.
A: It's surprisingly difficult to make this determination reliably
and portably. Any test you make can be invalidated if the file
is created or deleted (i.e. by some other process) between the
time you make the test and the time you try to open the file.
Three possible test functions are stat(), access(), and fopen().
(To make an approximate test using fopen(), just open for
reading and close immediately, although failure does not
necessarily indicate nonexistence.) Of these, only fopen() is
widely portable, and access(), where it exists, must be used
carefully if the program uses the Unix set-UID feature.
Rather than trying to predict in advance whether an operation
such as opening a file will succeed, it's often better to try
it, check the return value, and complain if it fails.
(Obviously, this approach won't work if you're trying to avoid
overwriting an existing file, unless you've got something like
the O_EXCL file opening option available, which does just what
you want in this case.)
References: PCS Sec. 12 pp. 189,213; POSIX Sec. 5.3.1,
Sec. 5.6.2, Sec. 5.6.3.
19.12: How can I find out the size of a file, prior to reading it in?
A: If the "size of a file" is the number of characters you'll be
able to read from it in C, it is difficult or impossible to
determine this number exactly.
Under Unix, the stat() call will give you an exact answer.
Several other systems supply a Unix-like stat() which will give
an approximate answer. You can fseek() to the end and then use
ftell(), or maybe try fstat(), but these tend to have the same
sorts of problems: fstat() is not portable, and generally tells
you the same thing stat() tells you; ftell() is not guaranteed
to return a byte count except for binary files. Some systems
provide functions called filesize() or filelength(), but these
are obviously not portable, either.
Are you sure you have to determine the file's size in advance?
Since the most accurate way of determining the size of a file as
a C program will see it is to open the file and read it, perhaps
you can rearrange the code to learn the size as it reads.
References: ISO Sec. 7.9.9.4; H&S Sec. 15.5.1; PCS Sec. 12 p.
213; POSIX Sec. 5.6.2.
19.12b: How can I find the modification date and time of a file?
A: The Unix and POSIX function is stat(), which several other
systems supply as well. (See also question 19.12.)
19.13: How can a file be shortened in-place without completely clearing
or rewriting it?
A: BSD systems provide ftruncate(), several others supply chsize(),
and a few may provide a (possibly undocumented) fcntl option
F_FREESP. Under MS-DOS, you can sometimes use write(fd, "", 0).
However, there is no portable solution, nor a way to delete
blocks at the beginning. See also question 19.14.
19.14: How can I insert or delete a line (or record) in the middle of a
file?
A: Short of rewriting the file, you probably can't. The usual
solution is simply to rewrite the file. (Instead of deleting
records, you might consider simply marking them as deleted, to
avoid rewriting.) Another possibility, of course, is to use a
database instead of a flat file. See also questions 12.30 and
19.13.
19.15: How can I recover the file name given an open stream or file
descriptor?
A: This problem is, in general, insoluble. Under Unix, for
instance, a scan of the entire disk (perhaps involving special
permissions) would theoretically be required, and would fail if
the descriptor were connected to a pipe or referred to a deleted
file (and could give a misleading answer for a file with
multiple links). It is best to remember the names of files
yourself as you open them (perhaps with a wrapper function
around fopen()).
19.16: How can I delete a file?
A: The Standard C Library function is remove(). (This is therefore
one of the few questions in this section for which the answer is
*not* "It's system-dependent.") On older, pre-ANSI Unix
systems, remove() may not exist, in which case you can try
unlink().
References: K&R2 Sec. B1.1 p. 242; ISO Sec. 7.9.4.1; H&S
Sec. 15.15 p. 382; PCS Sec. 12 pp. 208,220-221; POSIX
Sec. 5.5.1, Sec. 8.2.4.
19.16b: How do I copy files?
A: Either use system() to invoke your operating system's copy
utility (see question 19.27), or open the source and destination
files (using fopen() or some lower-level file-opening system call),
read characters or blocks of characters from the source file,
and write them to the destination file.
References: K&R Sec. 1, Sec. 7.
19.17: Why can't I open a file by its explicit path? The call
fopen("c:\newdir\file.dat", "r")
is failing.
A: The file you actually requested -- with the characters \n and \f
in its name -- probably doesn't exist, and isn't what you
thought you were trying to open.
In character constants and string literals, the backslash \ is
an escape character, giving special meaning to the character
following it. In order for literal backslashes in a pathname to
be passed through to fopen() (or any other function) correctly,
they have to be doubled, so that the first backslash in each
pair quotes the second one:
fopen("c:\\newdir\\file.dat", "r")
Alternatively, under MS-DOS, it turns out that forward slashes
are also accepted as directory separators, so you could use
fopen("c:/newdir/file.dat", "r")
(Note, by the way, that header file names mentioned in
preprocessor #include directives are *not* string literals, so
you may not have to worry about backslashes there.)
19.18: I'm getting an error, "Too many open files". How can I increase
the allowable number of simultaneously open files?
A: There are typically at least two resource limitations on the
number of simultaneously open files: the number of low-level
"file descriptors" or "file handles" available in the operating
system, and the number of FILE structures available in the stdio
library. Both must be sufficient. Under MS-DOS systems, you
can control the number of operating system file handles with a
line in CONFIG.SYS. Some compilers come with instructions (and
perhaps a source file or two) for increasing the number of stdio
FILE structures.
19.20: How can I read a directory in a C program?
A: See if you can use the opendir() and readdir() functions, which
are part of the POSIX standard and are available on most Unix
variants. Implementations also exist for MS-DOS, VMS, and other
systems. (MS-DOS also has FINDFIRST and FINDNEXT routines which
do essentially the same thing.) readdir() only returns file
names; if you need more information about the file, try calling
stat(). To match filenames to some wildcard pattern, see
question 13.7.
References: K&R2 Sec. 8.6 pp. 179-184; PCS Sec. 13 pp. 230-1;
POSIX Sec. 5.1; Schumacher, ed., _Software Solutions in C_
Sec. 8.
19.22: How can I find out how much memory is available?
A: Your operating system may provide a routine which returns this
information, but it's quite system-dependent.
19.23: How can I allocate arrays or structures bigger than 64K?
A: A reasonable computer ought to give you transparent access to
all available memory. If you're not so lucky, you'll either
have to rethink your program's use of memory, or use various
system-specific techniques.
64K is (still) a pretty big chunk of memory. No matter how much
memory your computer has available, it's asking a lot to be able
to allocate huge amounts of it contiguously. (The C Standard
does not guarantee that single objects can be 32K or larger,
or 64K for C9X.) Often it's a good idea to use data
structures which don't require that all memory be contiguous.
For dynamically-allocated multidimensional arrays, you can
use pointers to pointers, as illustrated in question 6.16.
Instead of a large array of structures, you can use a linked
list, or an array of pointers to structures.
If you're using a PC-compatible (8086-based) system, and running
up against a 64K or 640K limit, consider using "huge" memory
model, or expanded or extended memory, or malloc variants such
as halloc() or farmalloc(), or a 32-bit "flat" compiler (e.g.
djgpp, see question 18.3), or some kind of a DOS extender, or
another operating system.
References: ISO Sec. 5.2.4.1; C9X Sec. 5.2.4.1.
19.24: What does the error message "DGROUP data allocation exceeds 64K"
mean, and what can I do about it? I thought that using large
model meant that I could use more than 64K of data!
A: Even in large memory models, MS-DOS compilers apparently toss
certain data (strings, some initialized global or static
variables) into a default data segment, and it's this segment
that is overflowing. Either use less global data, or, if you're
already limiting yourself to reasonable amounts (and if the
problem is due to something like the number of strings), you may
be able to coax the compiler into not using the default data
segment for so much. Some compilers place only "small" data
objects in the default data segment, and give you a way (e.g.
the /Gt option under Microsoft compilers) to configure the
threshold for "small."
19.25: How can I access memory (a memory-mapped device, or graphics
memory) located at a certain address?
A: Set a pointer, of the appropriate type, to the right number
(using an explicit cast to assure the compiler that you really
do intend this nonportable conversion):
unsigned int *magicloc = (unsigned int *)0x12345678;
Then, *magicloc refers to the location you want. (Under MS-DOS,
you may find a macro like MK_FP() handy for working with
segments and offsets.)
References: K&R1 Sec. A14.4 p. 210; K&R2 Sec. A6.6 p. 199; ISO
Sec. 6.3.4; Rationale Sec. 3.3.4; H&S Sec. 6.2.7 pp. 171-2.
19.27: How can I invoke another program (a standalone executable,
or an operating system command) from within a C program?
A: Use the library function system(), which does exactly that.
Note that system's return value is at best the command's exit
status (although even that is not guaranteed), and usually has
nothing to do with the output of the command. Note also that
system() accepts a single string representing the command to be
invoked; if you need to build up a complex command line, you can
use sprintf(). See also question 19.30.
References: K&R1 Sec. 7.9 p. 157; K&R2 Sec. 7.8.4 p. 167,
Sec. B6 p. 253; ISO Sec. 7.10.4.5; H&S Sec. 19.2 p. 407; PCS
Sec. 11 p. 179.
19.30: How can I invoke another program or command and trap its output?
A: Unix and some other systems provide a popen() function, which
sets up a stdio stream on a pipe connected to the process
running a command, so that the output can be read (or the input
supplied). (Also, remember to call pclose().)
If you can't use popen(), you may be able to use system(), with
the output going to a file which you then open and read.
If you're using Unix and popen() isn't sufficient, you can learn
about pipe(), dup(), fork(), and exec().
(One thing that probably would *not* work, by the way, would be
to use freopen().)
References: PCS Sec. 11 p. 169.
19.31: How can my program discover the complete pathname to the
executable from which it was invoked?
A: argv[0] may contain all or part of the pathname, or it may
contain nothing. You may be able to duplicate the command
language interpreter's search path logic to locate the
executable if the name in argv[0] is present but incomplete.
However, there is no guaranteed solution.
References: K&R1 Sec. 5.11 p. 111; K&R2 Sec. 5.10 p. 115; ISO
Sec. 5.1.2.2.1; H&S Sec. 20.1 p. 416.
19.32: How can I automatically locate a program's configuration files
in the same directory as the executable?
A: It's hard; see also question 19.31 above. Even if you can
figure out a workable way to do it, you might want to consider
making the program's auxiliary (library) directory configurable,
perhaps with an environment variable. (It's especially
important to allow variable placement of a program's
configuration files when the program will be used by several
people, e.g. on a multiuser system.)
19.33: How can a process change an environment variable in its caller?
A: It may or may not be possible to do so at all. Different
operating systems implement global name/value functionality
similar to the Unix environment in different ways. Whether the
"environment" can be usefully altered by a running program, and
if so, how, is system-dependent.
Under Unix, a process can modify its own environment (some
systems provide setenv() or putenv() functions for the purpose),
and the modified environment is generally passed on to child
processes, but it is *not* propagated back to the parent
process. Under MS-DOS, it's possible to manipulate the master
copy of the environment, but the required techniques are arcane.
(See an MS-DOS FAQ list.)
19.36: How can I read in an object file and jump to locations in it?
A: You want a dynamic linker or loader. It may be possible to
malloc some space and read in object files, but you have to know
an awful lot about object file formats, relocation, etc. Under
BSD Unix, you could use system() and ld -A to do the linking for
you. Many versions of SunOS and System V have the -ldl library
which allows object files to be dynamically loaded. Under VMS,
use LIB$FIND_IMAGE_SYMBOL. GNU has a package called "dld". See
also question 15.13.
19.37: How can I implement a delay, or time a user's response, with sub-
second resolution?
A: Unfortunately, there is no portable way. V7 Unix, and derived
systems, provided a fairly useful ftime() function with
resolution up to a millisecond, but it has disappeared from
System V and POSIX. Other routines you might look for on your
system include clock(), delay(), gettimeofday(), msleep(),
nap(), napms(), nanosleep(), setitimer(), sleep(), times(), and
usleep(). (A function called wait(), however, is at least under
Unix *not* what you want.) The select() and poll() calls (if
available) can be pressed into service to implement simple
delays. On MS-DOS machines, it is possible to reprogram the
system timer and timer interrupts.
Of these, only clock() is part of the ANSI Standard. The
difference between two calls to clock() gives elapsed execution
time, and may even have subsecond resolution, if CLOCKS_PER_SEC
is greater than 1. However, clock() gives elapsed processor time
used by the current program, which on a multitasking system may
differ considerably from real time.
If you're trying to implement a delay and all you have available
is a time-reporting function, you can implement a CPU-intensive
busy-wait, but this is only an option on a single-user, single-
tasking machine as it is terribly antisocial to any other
processes. Under a multitasking operating system, be sure to
use a call which puts your process to sleep for the duration,
such as sleep() or select(), or pause() in conjunction with
alarm() or setitimer().
For really brief delays, it's tempting to use a do-nothing loop
like
long int i;
for(i = 0; i < 1000000; i++)
;
but resist this temptation if at all possible! For one thing,
your carefully-calculated delay loops will stop working properly
next month when a faster processor comes out. Perhaps worse, a
clever compiler may notice that the loop does nothing and
optimize it away completely.
References: H&S Sec. 18.1 pp. 398-9; PCS Sec. 12 pp. 197-8,215-
6; POSIX Sec. 4.5.2.
19.38: How can I trap or ignore keyboard interrupts like control-C?
A: The basic step is to call signal(), either as
#include <signal.h>
signal(SIGINT, SIG_IGN);
to ignore the interrupt signal, or as
extern void func(int);
signal(SIGINT, func);
to cause control to transfer to function func() on receipt of an
interrupt signal.
On a multi-tasking system such as Unix, it's best to use a
slightly more involved technique:
extern void func(int);
if(signal(SIGINT, SIG_IGN) != SIG_IGN)
signal(SIGINT, func);
The test and extra call ensure that a keyboard interrupt typed
in the foreground won't inadvertently interrupt a program
running in the background (and it doesn't hurt to code calls to
signal() this way on any system).
On some systems, keyboard interrupt handling is also a function
of the mode of the terminal-input subsystem; see question 19.1.
On some systems, checking for keyboard interrupts is only
performed when the program is reading input, and keyboard
interrupt handling may therefore depend on which input routines
are being called (and *whether* any input routines are active at
all). On MS-DOS systems, setcbrk() or ctrlbrk() functions may
also be involved.
References: ISO Secs. 7.7,7.7.1; H&S Sec. 19.6 pp. 411-3; PCS
Sec. 12 pp. 210-2; POSIX Secs. 3.3.1,3.3.4.
19.39: How can I handle floating-point exceptions gracefully?
A: On many systems, you can define a function matherr() which will
be called when there are certain floating-point errors, such as
errors in the math routines in <math.h>. You may also be able
to use signal() (see question 19.38 above) to catch SIGFPE. See
also question 14.9.
References: Rationale Sec. 4.5.1.
19.40: How do I... Use sockets? Do networking? Write client/server
applications?
A: All of these questions are outside of the scope of this list and
have much more to do with the networking facilities which you
have available than they do with C. Good books on the subject
are Douglas Comer's three-volume _Internetworking with TCP/IP_
and W. R. Stevens's _UNIX Network Programming_. (There is also
plenty of information out on the net itself, including the
"Unix Socket FAQ" at http://kipper.york.ac.uk/~vic/sock-faq/ .)
19.40b: How do I... Use BIOS calls? Write ISR's? Create TSR's?
A: These are very particular to specific systems (PC compatibles
running MS-DOS, most likely). You'll get much better
information in a specific newsgroup such as
comp.os.msdos.programmer or its FAQ list; another excellent
resource is Ralf Brown's interrupt list.
19.40c: I'm trying to compile this program, but the compiler is
complaining that "union REGS" is undefined, and the linker
is complaining that int86() is undefined.
A: Those have to do with MS-DOS interrupt programming. They don't
exist on other systems.
19.41: But I can't use all these nonstandard, system-dependent
functions, because my program has to be ANSI compatible!
A: You're out of luck. Either you misunderstood your requirement,
or it's an impossible one to meet. ANSI/ISO Standard C simply
does not define ways of doing these things; it is a language
standard, not an operating system standard. An international
standard which does address many of these issues is POSIX
(IEEE 1003.1, ISO/IEC 9945-1), and many operating systems (not
just Unix) now have POSIX-compatible programming interfaces.
It is possible, and desirable, for *most* of a program to be
ANSI-compatible, deferring the system-dependent functionality to
a few routines in a few files which are rewritten for each
system ported to.
Section 20. Miscellaneous
20.1: How can I return multiple values from a function?
A: Either pass pointers to several locations which the function can
fill in, or have the function return a structure containing the
desired values, or (in a pinch) consider global variables. See
also questions 2.7, 4.8, and 7.5a.
20.3: How do I access command-line arguments?
A: They are pointed to by the argv array with which main() is
called. See also questions 8.2, 13.7, and 19.20.
References: K&R1 Sec. 5.11 pp. 110-114; K&R2 Sec. 5.10 pp. 114-
118; ISO Sec. 5.1.2.2.1; H&S Sec. 20.1 p. 416; PCS Sec. 5.6 pp.
81-2, Sec. 11 p. 159, pp. 339-40 Appendix F; Schumacher, ed.,
_Software Solutions in C_ Sec. 4 pp. 75-85.
20.5: How can I write data files which can be read on other machines
with different word size, byte order, or floating point formats?
A: The most portable solution is to use text files (usually ASCII),
written with fprintf() and read with fscanf() or the like.
(Similar advice also applies to network protocols.) Be
skeptical of arguments which imply that text files are too big,
or that reading and writing them is too slow. Not only is their
efficiency frequently acceptable in practice, but the advantages
of being able to interchange them easily between machines, and
manipulate them with standard tools, can be overwhelming.
If you must use a binary format, you can improve portability,
and perhaps take advantage of prewritten I/O libraries, by
making use of standardized formats such as Sun's XDR (RFC 1014),
OSI's ASN.1 (referenced in CCITT X.409 and ISO 8825 "Basic
Encoding Rules"), CDF, netCDF, or HDF. See also questions 2.12
and 12.38.
References: PCS Sec. 6 pp. 86, 88.
20.6: If I have a char * variable pointing to the name of a function,
how can I call that function?
A: The most straightforward thing to do is to maintain a
correspondence table of names and function pointers:
int func(), anotherfunc();
struct { char *name; int (*funcptr)(); } symtab[] = {
"func", func,
"anotherfunc", anotherfunc,
};
Then, search the table for the name, and call via the associated
function pointer. See also questions 2.15, 18.14, and 19.36.
References: PCS Sec. 11 p. 168.
20.8: How can I implement sets or arrays of bits?
A: Use arrays of char or int, with a few macros to access the
desired bit at the proper index. Here are some simple macros to
use with arrays of char:
#include <limits.h> /* for CHAR_BIT */
#define BITMASK(b) (1 << ((b) % CHAR_BIT))
#define BITSLOT(b) ((b) / CHAR_BIT)
#define BITSET(a, b) ((a)[BITSLOT(b)] |= BITMASK(b))
#define BITTEST(a, b) ((a)[BITSLOT(b)] & BITMASK(b))
(If you don't have <limits.h>, try using 8 for CHAR_BIT.)
References: H&S Sec. 7.6.7 pp. 211-216.
20.9: How can I determine whether a machine's byte order is big-endian
or little-endian?
A: One way is to use a pointer:
int x = 1;
if(*(char *)&x == 1)
printf("little-endian\n");
else printf("big-endian\n");
It's also possible to use a union.
See also question 10.16.
References: H&S Sec. 6.1.2 pp. 163-4.
20.10: How can I convert integers to binary or hexadecimal?
A: Make sure you really know what you're asking. Integers are
stored internally in binary, although for most purposes it is
not incorrect to think of them as being in octal, decimal, or
hexadecimal, whichever is convenient. The base in which a
number is expressed matters only when that number is read in
from or written out to the outside world.
In source code, a non-decimal base is indicated by a leading 0
or 0x (for octal or hexadecimal, respectively). During I/O, the
base of a formatted number is controlled in the printf and scanf
family of functions by the choice of format specifier (%d, %o,
%x, etc.) and in the strtol() and strtoul() functions by the
third argument. If you need to output numeric strings in
arbitrary bases, you'll have to supply your own function to do
so (it will essentially be the inverse of strtol). During
*binary* I/O, however, the base again becomes immaterial.
For more information about "binary" I/O, see question 2.11.
See also questions 8.6 and 13.1.
References: ISO Secs. 7.10.1.5,7.10.1.6.
20.11: Can I use base-2 constants (something like 0b101010)?
Is there a printf() format for binary?
A: No, on both counts. You can convert base-2 string
representations to integers with strtol(). See also question
20.10.
20.12: What is the most efficient way to count the number of bits which
are set in an integer?
A: Many "bit-fiddling" problems like this one can be sped up and
streamlined using lookup tables (but see question 20.13 below).
20.13: What's the best way of making my program efficient?
A: By picking good algorithms, implementing them carefully, and
making sure that your program isn't doing any extra work. For
example, the most microoptimized character-copying loop in the
world will be beat by code which avoids having to copy
characters at all.
When worrying about efficiency, it's important to keep several
things in perspective. First of all, although efficiency is an
enormously popular topic, it is not always as important as
people tend to think it is. Most of the code in most programs
is not time-critical. When code is not time-critical, it is
usually more important that it be written clearly and portably
than that it be written maximally efficiently. (Remember that
computers are very, very fast, and that seemingly "inefficient"
code may be quite efficiently compilable, and run without
apparent delay.)
It is notoriously difficult to predict what the "hot spots" in a
program will be. When efficiency is a concern, it is important
to use profiling software to determine which parts of the
program deserve attention. Often, actual computation time is
swamped by peripheral tasks such as I/O and memory allocation,
which can be sped up by using buffering and caching techniques.
Even for code that *is* time-critical, one of the least
effective optimization techniques is to fuss with the coding
details. Many of the "efficient coding tricks" which are
frequently suggested (e.g. substituting shift operators for
multiplication by powers of two) are performed automatically by
even simpleminded compilers. Heavyhanded optimization attempts
can make code so bulky that performance is actually degraded,
and are rarely portable (i.e. they may speed things up on one
machine but slow them down on another). In any case, tweaking
the coding usually results in at best linear performance
improvements; the big payoffs are in better algorithms.
For more discussion of efficiency tradeoffs, as well as good
advice on how to improve efficiency when it is important, see
chapter 7 of Kernighan and Plauger's _The Elements of
Programming Style_, and Jon Bentley's _Writing Efficient
Programs_.
20.14: Are pointers really faster than arrays? How much do function
calls slow things down? Is ++i faster than i = i + 1?
A: Precise answers to these and many similar questions depend of
course on the processor and compiler in use. If you simply must
know, you'll have to time test programs carefully. (Often the
differences are so slight that hundreds of thousands of
iterations are required even to see them. Check the compiler's
assembly language output, if available, to see if two purported
alternatives aren't compiled identically.)
It is "usually" faster to march through large arrays with
pointers rather than array subscripts, but for some processors
the reverse is true.
Function calls, though obviously incrementally slower than in-
line code, contribute so much to modularity and code clarity
that there is rarely good reason to avoid them.
Before rearranging expressions such as i = i + 1, remember that
you are dealing with a compiler, not a keystroke-programmable
calculator. Any decent compiler will generate identical code
for ++i, i += 1, and i = i + 1. The reasons for using ++i or
i += 1 over i = i + 1 have to do with style, not efficiency.
(See also question 3.12.)
20.15b: People claim that optimizing compilers are good and that we no
longer have to write things in assembler for speed, but my
compiler can't even replace i/=2 with a shift.
A: Was i signed or unsigned? If it was signed, a shift is not
equivalent (hint: think about the result if i is negative and
odd), so the compiler was correct not to use it.
20.15c: How can I swap two values without using a temporary?
A: The standard hoary old assembly language programmer's trick is:
a ^= b;
b ^= a;
a ^= b;
But this sort of code has little place in modern, HLL
programming. Temporary variables are essentially free,
and the idiomatic code using three assignments, namely
int t = a;
a = b;
b = t;
is not only clearer to the human reader, it is more likely to be
recognized by the compiler and turned into the most-efficient
code (e.g. using a swap instruction, if available). The latter
code is obviously also amenable to use with pointers and
floating-point values, unlike the XOR trick. See also questions
3.3b and 10.3.
20.17: Is there a way to switch on strings?
A: Not directly. Sometimes, it's appropriate to use a separate
function to map strings to integer codes, and then switch on
those. Otherwise, of course, you can fall back on strcmp() and
a conventional if/else chain. See also questions 10.12, 20.18,
and 20.29.
References: K&R1 Sec. 3.4 p. 55; K&R2 Sec. 3.4 p. 58; ISO
Sec. 6.6.4.2; H&S Sec. 8.7 p. 248.
20.18: Is there a way to have non-constant case labels (i.e. ranges or
arbitrary expressions)?
A: No. The switch statement was originally designed to be quite
simple for the compiler to translate, therefore case labels are
limited to single, constant, integral expressions. You *can*
attach several case labels to the same statement, which will let
you cover a small range if you don't mind listing all cases
explicitly.
If you want to select on arbitrary ranges or non-constant
expressions, you'll have to use an if/else chain.
See also question 20.17.
References: K&R1 Sec. 3.4 p. 55; K&R2 Sec. 3.4 p. 58; ISO
Sec. 6.6.4.2; Rationale Sec. 3.6.4.2; H&S Sec. 8.7 p. 248.
20.19: Are the outer parentheses in return statements really optional?
A: Yes.
Long ago, in the early days of C, they were required, and just
enough people learned C then, and wrote code which is still in
circulation, that the notion that they might still be required
is widespread.
(As it happens, parentheses are optional with the sizeof
operator, too, under certain circumstances.)
References: K&R1 Sec. A18.3 p. 218; ISO Sec. 6.3.3, Sec. 6.6.6;
H&S Sec. 8.9 p. 254.
20.20: Why don't C comments nest? How am I supposed to comment out
code containing comments? Are comments legal inside quoted
strings?
A: C comments don't nest mostly because PL/I's comments, which C's
are borrowed from, don't either. Therefore, it is usually
better to "comment out" large sections of code, which might
contain comments, with #ifdef or #if 0 (but see question 11.19).
The character sequences /* and */ are not special within double-
quoted strings, and do not therefore introduce comments, because
a program (particularly one which is generating C code as
output) might want to print them.
Note also that // comments, as in C++, are not yet legal in C,
so it's not a good idea to use them in C programs (even if your
compiler supports them as an extension).
References: K&R1 Sec. A2.1 p. 179; K&R2 Sec. A2.2 p. 192; ISO
Sec. 6.1.9, Annex F; Rationale Sec. 3.1.9; H&S Sec. 2.2 pp. 18-
9; PCS Sec. 10 p. 130.
20.20b: Is C a great language, or what? Where else could you write
something like a+++++b ?
A: Well, you can't meaningfully write it in C, either.
The rule for lexical analysis is that at each point during a
straightforward left-to-right scan, the longest possible token
is determined, without regard to whether the resulting sequence
of tokens makes sense. The fragment in the question is
therefore interpreted as
a ++ ++ + b
and cannot be parsed as a valid expression.
References: K&R1 Sec. A2 p. 179; K&R2 Sec. A2.1 p. 192; ISO
Sec. 6.1; H&S Sec. 2.3 pp. 19-20.
20.24: Why doesn't C have nested functions?
A: It's not trivial to implement nested functions such that they
have the proper access to local variables in the containing
function(s), so they were deliberately left out of C as a
simplification. (gcc does allow them, as an extension.) For
many potential uses of nested functions (e.g. qsort comparison
functions), an adequate if slightly cumbersome solution is to
use an adjacent function with static declaration, communicating
if necessary via a few static variables. (A cleaner solution,
though unsupported by qsort(), is to pass around a pointer to
a structure containing the necessary context.)
20.24b: What is assert() and when would I use it?
A: It is a macro, defined in <assert.h>, for testing "assertions".
An assertion essentially documents an assumption being made by
the programmer, an assumption which, if violated, would indicate
a serious programming error. For example, a function which was
supposed to be called with a non-null pointer could write
assert(p != NULL);
A failed assertion terminates the program. Assertions should
*not* be used to catch expected errors, such as malloc() or
fopen() failures.
References: K&R2 Sec. B6 pp. 253-4; ISO Sec. 7.2; H&S Sec. 19.1
p. 406.
20.25: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions
from C? (And vice versa?)
A: The answer is entirely dependent on the machine and the specific
calling sequences of the various compilers in use, and may not
be possible at all. Read your compiler documentation very
carefully; sometimes there is a "mixed-language programming
guide," although the techniques for passing arguments and
ensuring correct run-time startup are often arcane. More
information may be found in FORT.gz by Glenn Geers, available
via anonymous ftp from suphys.physics.su.oz.au in the src
directory.
cfortran.h, a C header file, simplifies C/FORTRAN interfacing on
many popular machines. It is available via anonymous ftp from
zebra.desy.de or at http://www-zeus.desy.de/~burow .
In C++, a "C" modifier in an external function declaration
indicates that the function is to be called using C calling
conventions.
References: H&S Sec. 4.9.8 pp. 106-7.
20.26: Does anyone know of a program for converting Pascal or FORTRAN
(or LISP, Ada, awk, "Old" C, ...) to C?
A: Several freely distributable programs are available:
p2c A Pascal to C converter written by Dave Gillespie,
posted to comp.sources.unix in March, 1990 (Volume 21);
also available by anonymous ftp from
csvax.cs.caltech.edu, file pub/p2c-1.20.tar.Z .
ptoc Another Pascal to C converter, this one written in
Pascal (comp.sources.unix, Volume 10, also patches in
Volume 13?).
f2c A FORTRAN to C converter jointly developed by people
from Bell Labs, Bellcore, and Carnegie Mellon. To find
out more about f2c, send the mail message "send index
from f2c" to [email protected] or research!netlib.
(It is also available via anonymous ftp on
netlib.att.com, in directory netlib/f2c.)
This FAQ list's maintainer also has available a list of a few
other commercial translation products, and some for more obscure
languages.
See also questions 11.31 and 18.16.
20.27: Is C++ a superset of C? Can I use a C++ compiler to compile C
code?
A: C++ was derived from C, and is largely based on it, but there
are some legal C constructs which are not legal C++.
Conversely, ANSI C inherited several features from C++,
including prototypes and const, so neither language is really a
subset or superset of the other; the two also define the meaning
of some common constructs differently. In spite of the
differences, many C programs will compile correctly in a C++
environment, and many recent compilers offer both C and C++
compilation modes. See also questions 8.9 and 20.20.
References: H&S p. xviii, Sec. 1.1.5 p. 6, Sec. 2.8 pp. 36-7,
Sec. 4.9 pp. 104-107.
20.28: I need a sort of an "approximate" strcmp routine, for comparing
two strings for close, but not necessarily exact, equality.
A: Some nice information and algorithms having to do with
approximate string matching, as well as a useful bibliography,
can be found in Sun Wu and Udi Manber's paper "AGREP -- A Fast
Approximate Pattern-Matching Tool."
Another approach involves the "soundex" algorithm, which maps
similar-sounding words to the same codes. Soundex was designed
for discovering similar-sounding names (for telephone directory
assistance, as it happens), but it can be pressed into service
for processing arbitrary words.
References: Knuth Sec. 6 pp. 391-2 Volume 3; Wu and Manber,
"AGREP -- A Fast Approximate Pattern-Matching Tool" .
20.29: What is hashing?
A: Hashing is the process of mapping strings to integers, usually
in a relatively small range. A "hash function" maps a string
(or some other data structure) to a bounded number (the "hash
bucket") which can more easily be used as an index in an array,
or for performing repeated comparisons. (Obviously, a mapping
from a potentially huge set of strings to a small set of
integers will not be unique. Any algorithm using hashing
therefore has to deal with the possibility of "collisions.")
Many hashing functions and related algorithms have been
developed; a full treatment is beyond the scope of this list.
References: K&R2 Sec. 6.6; Knuth Sec. 6.4 pp. 506-549 Volume 3;
Sedgewick Sec. 16 pp. 231-244.
20.31: How can I find the day of the week given the date?
A: Use mktime() or localtime() (see questions 13.13 and 13.14, but
beware of DST adjustments if tm_hour is 0), or Zeller's
congruence (see the sci.math FAQ list), or this elegant code by
Tomohiko Sakamoto:
int dayofweek(int y, int m, int d) /* 0 = Sunday */
{
static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
y -= m < 3;
return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
}
See also questions 13.14 and 20.32.
References: ISO Sec. 7.12.2.3.
20.32: Will 2000 be a leap year? Is (year % 4 == 0) an accurate test
for leap years?
A: Yes and no, respectively. The full expression for the present
Gregorian calendar is
year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
See a good astronomical almanac or other reference for details.
(To forestall an eternal debate: references which claim the
existence of a 4000-year rule are wrong.) See also questions
13.14 and 13.14b.
20.34: Here's a good puzzle: how do you write a program which produces
its own source code as output?
A: It is actually quite difficult to write a self-reproducing
program that is truly portable, due particularly to quoting and
character set difficulties.
Here is a classic example (which ought to be presented on one
line, although it will fix itself the first time it's run):
char*s="char*s=%c%s%c;main(){printf(s,34,s,34);}";
main(){printf(s,34,s,34);}
(This program, like many of the genre, neglects to #include
<stdio.h>, and assumes that the double-quote character " has the
value 34, as it does in ASCII.)
20.35: What is "Duff's Device"?
A: It's a devastatingly deviously unrolled byte-copying loop,
devised by Tom Duff while he was at Lucasfilm. In its "classic"
form, it looks like:
register n = (count + 7) / 8; /* count > 0 assumed */
switch (count % 8)
{
case 0: do { *to = *from++;
case 7: *to = *from++;
case 6: *to = *from++;
case 5: *to = *from++;
case 4: *to = *from++;
case 3: *to = *from++;
case 2: *to = *from++;
case 1: *to = *from++;
} while (--n > 0);
}
where count bytes are to be copied from the array pointed to by
from to the memory location pointed to by to (which is a memory-
mapped device output register, which is why to isn't
incremented). It solves the problem of handling the leftover
bytes (when count isn't a multiple of 8) by interleaving a
switch statement with the loop which copies bytes 8 at a time.
(Believe it or not, it *is* legal to have case labels buried
within blocks nested in a switch statement like this. In his
announcement of the technique to C's developers and the world,
Duff noted that C's switch syntax, in particular its "fall
through" behavior, had long been controversial, and that "This
code forms some sort of argument in that debate, but I'm not
sure whether it's for or against.")
20.36: When will the next International Obfuscated C Code Contest
(IOCCC) be held? How can I get a copy of the current and
previous winning entries?
A: The contest is in a state of flux; see
http://www.ioccc.org/index.html for current details.
Contest winners are usually announced at a Usenix conference,
and are posted to the net sometime thereafter. Winning entries
from previous years (back to 1984) are archived at ftp.uu.net
(see question 18.16) under the directory pub/ioccc/; see also
http://www.ioccc.org/index.html .
20.37: What was the entry keyword mentioned in K&R1?
A: It was reserved to allow the possibility of having functions
with multiple, differently-named entry points, a la FORTRAN. It
was not, to anyone's knowledge, ever implemented (nor does
anyone remember what sort of syntax might have been imagined for
it). It has been withdrawn, and is not a keyword in ANSI C.
(See also question 1.12.)
References: K&R2 p. 259 Appendix C.
20.38: Where does the name "C" come from, anyway?
A: C was derived from Ken Thompson's experimental language B, which
was inspired by Martin Richards's BCPL (Basic Combined
Programming Language), which was a simplification of CPL
(Cambridge Programming Language). For a while, there was
speculation that C's successor might be named P (the third
letter in BCPL) instead of D, but of course the most visible
descendant language today is C++.
20.39: How do you pronounce "char"?
A: You can pronounce the C keyword "char" in at least three ways:
like the English words "char," "care," or "car" (or maybe even
"character"); the choice is arbitrary.
20.39b: What do "lvalue" and "rvalue" mean?
A: Simply speaking, an "lvalue" is an expression that could appear
on the left-hand sign of an assignment; you can also think of it
as denoting an object that has a location. (But see question
6.7 concerning arrays.) An "rvalue" is any expression that has
a value (and that can therefore appear on the right-hand sign of
an assignment).
20.40: Where can I get extra copies of this list?
What about back issues?
A: An up-to-date copy may be obtained from ftp.eskimo.com in
directory u/s/scs/C-faq/. You can also just pull it off the
net; it is normally posted to comp.lang.c on the first of each
month, with an Expires: line which should keep it around all
month. A parallel, abridged version is available (and posted),
as is a list of changes accompanying each significantly updated
version.
The various versions of this list are also posted to the
newsgroups comp.answers and news.answers . Several sites
archive news.answers postings and other FAQ lists, including
this one; two sites are rtfm.mit.edu (directories
pub/usenet/news.answers/C-faq/ and pub/usenet/comp.lang.c/) and
ftp.uu.net (directory usenet/news.answers/C-faq/). If you don't
have ftp access, a mailserver at rtfm.mit.edu can mail you FAQ
lists: send a message containing the single word "help" to
[email protected] . See the meta-FAQ list in
news.answers for more information.
A hypertext (HTML) version of this FAQ list is available on the
World-Wide Web; the URL is http://www.eskimo.com/~scs/C-faq/top.html .
A comprehensive site which references all Usenet FAQ lists is
http://www.faqs.org/faqs/ .
An extended version of this FAQ list has been published by
Addison-Wesley as _C Programming FAQs: Frequently Asked
Questions_ (ISBN 0-201-84519-9). An errata list is at
http://www.eskimo.com/~scs/C-faq/book/Errata.html and on
ftp.eskimo.com in u/s/scs/ftp/C-faq/book/Errata .
This list is an evolving document containing questions which
have been Frequent since before the Great Renaming; it is not
just a collection of this month's interesting questions. Older
copies are obsolete and don't contain much, except the
occasional typo, that the current list doesn't.
Bibliography
American National Standards Institute, _American National Standard for
Information Systems -- Programming Language -- C_, ANSI X3.159-1989
(see question 11.2). [ANSI]
American National Standards Institute, _Rationale for American National
Standard for Information Systems -- Programming Language -- C_
(see question 11.2). [Rationale]
Jon Bentley, _Writing Efficient Programs_, Prentice-Hall, 1982,
ISBN 0-13-970244-X.
David Burki, "Date Conversions," _The C Users Journal_, February 1993,
pp. 29-34.
Ian F. Darwin, _Checking C Programs with lint_, O'Reilly, 1988,
ISBN 0-937175-30-7.
David Goldberg, "What Every Computer Scientist Should Know about
Floating-Point Arithmetic," _ACM Computing Surveys_, Vol. 23 #1,
March, 1991, pp. 5-48.
Samuel P. Harbison and Guy L. Steele, Jr., _C: A Reference Manual_,
Fourth Edition, Prentice-Hall, 1995, ISBN 0-13-326224-3. [H&S]
Mark R. Horton, _Portable C Software_, Prentice Hall, 1990,
ISBN 0-13-868050-7. [PCS]
Institute of Electrical and Electronics Engineers, _Portable Operating
System Interface (POSIX) -- Part 1: System Application Program Interface
(API) [C Language]_, IEEE Std. 1003.1, ISO/IEC 9945-1.
International Organization for Standardization, ISO 9899:1990
(see question 11.2). [ISO]
International Organization for Standardization, WG14/N794 Working Draft
(see questions 11.1 and 11.2b). [C9X]
Brian W. Kernighan and P.J. Plauger, _The Elements of Programming
Style_, Second Edition, McGraw-Hill, 1978, ISBN 0-07-034207-5.
Brian W. Kernighan and Dennis M. Ritchie, _The C Programming Language_,
Prentice-Hall, 1978, ISBN 0-13-110163-3. [K&R1]
Brian W. Kernighan and Dennis M. Ritchie, _The C Programming Language_,
Second Edition, Prentice Hall, 1988, ISBN 0-13-110362-8, 0-13-110370-9.
(See also question 18.10.) [K&R2]
Donald E. Knuth, _The Art of Computer Programming_. Volume 1:
_Fundamental Algorithms_, Second Edition, Addison-Wesley, 1973, ISBN
0-201-03809-9. Volume 2: _Seminumerical Algorithms_, Second Edition,
Addison-Wesley, 1981, ISBN 0-201-03822-6. Volume 3: _Sorting and
Searching_, Addison-Wesley, 1973, ISBN 0-201-03803-X. (New editions
are coming out!) [Knuth]
Andrew Koenig, _C Traps and Pitfalls_, Addison-Wesley, 1989,
ISBN 0-201-17928-8. [CT&P]
G. Marsaglia and T.A. Bray, "A Convenient Method for Generating Normal
Variables," _SIAM Review_, Vol. 6 #3, July, 1964.
Stephen K. Park and Keith W. Miller, "Random Number Generators: Good
Ones are Hard to Find," _Communications of the ACM_, Vol. 31 #10,
October, 1988, pp. 1192-1201 (also technical correspondence August,
1989, pp. 1020-1024, and July, 1993, pp. 108-110).
P.J. Plauger, _The Standard C Library_, Prentice Hall, 1992,
ISBN 0-13-131509-9.
Thomas Plum, _C Programming Guidelines_, Second Edition, Plum Hall,
1989, ISBN 0-911537-07-4.
William H. Press, Saul A. Teukolsky, William T. Vetterling, and Brian P.
Flannery, _Numerical Recipes in C_, Second Edition, Cambridge University
Press, 1992, ISBN 0-521-43108-5.
Dale Schumacher, Ed., _Software Solutions in C_, AP Professional, 1994,
ISBN 0-12-632360-7.
Robert Sedgewick, _Algorithms in C_, Addison-Wesley, 1990,
ISBN 0-201-51425-7. (A new edition is being prepared;
the first half is ISBN 0-201-31452-5.)
Charles Simonyi and Martin Heller, "The Hungarian Revolution," _Byte_,
August, 1991, pp.131-138.
David Straker, _C Style: Standards and Guidelines_, Prentice Hall,
ISBN 0-13-116898-3.
Steve Summit, _C Programming FAQs: Frequently Asked Questions_, Addison-
Wesley, 1995, ISBN 0-201-84519-9. [The book version of this FAQ list;
see also http://www.eskimo.com/~scs/C-faq/book/Errata.html .]
Sun Wu and Udi Manber, "AGREP -- A Fast Approximate Pattern-Matching
Tool," USENIX Conference Proceedings, Winter, 1992, pp. 153-162.
There is another bibliography in the revised Indian Hill style guide
(see question 17.9). See also question 18.10.
Acknowledgements
Thanks to Jamshid Afshar, Lauri Alanko, David Anderson, Tanner Andrews,
Sudheer Apte, Joseph Arceneaux, Randall Atkinson, Rick Beem, Peter
Bennett, Wayne Berke, Dan Bernstein, Tanmoy Bhattacharya, John Bickers,
Gary Blaine, Yuan Bo, Mark J. Bobak, Dave Boutcher, Alan Bowler, Michael
Bresnahan, Walter Briscoe, Vincent Broman, Robert T. Brown, Stan Brown,
John R. Buchan, Joe Buehler, Kimberley Burchett, Gordon Burditt, Scott
Burkett, Burkhard Burow, Conor P. Cahill, D'Arcy J.M. Cain, Christopher
Calabrese, Ian Cargill, Vinit Carpenter, Paul Carter, Mike Chambers,
Billy Chambless, C. Ron Charlton, Franklin Chen, Jonathan Chen, Raymond
Chen, Richard Cheung, Avinash Chopde, Steve Clamage, Ken Corbin, Dann
Corbit, Ian Cottam, Russ Cox, Jonathan Coxhead, Lee Crawford, Nick
Cropper, Steve Dahmer, Andrew Daviel, James Davies, John E. Davis, Ken
Delong, Norm Diamond, Bob Dinse, Jeff Dunlop, Ray Dunn, Stephen M. Dunn,
Michael J. Eager, Scott Ehrlich, Arno Eigenwillig, Yoav Eilat, Dave
Eisen, Joe English, Bjorn Engsig, David Evans, Clive D.W. Feather,
Dominic Feeley, Simao Ferraz, Chris Flatters, Rod Flores, Alexander
Forst, Steve Fosdick, Jeff Francis, Ken Fuchs, Tom Gambill, Dave
Gillespie, Samuel Goldstein, Tim Goodwin, Alasdair Grant, Ron Guilmette,
Doug Gwyn, Michael Hafner, Darrel Hankerson, Tony Hansen, Douglas
Wilhelm Harder, Elliotte Rusty Harold, Joe Harrington, Des Herriott,
Guy Harris, John Hascall, Ger Hobbelt, Jos Horsmeier, Syed Zaeem Hosain,
Blair Houghton, James C. Hu, Chin Huang, David Hurt, Einar Indridason,
Vladimir Ivanovic, Jon Jagger, Ke Jin, Kirk Johnson, Larry Jones, Arjan
Kenter, Bhaktha Keshavachar, James Kew, Darrell Kindred, Lawrence Kirby,
Kin-ichi Kitano, Peter Klausler, John Kleinjans, Andrew Koenig, Tom
Koenig, Adam Kolawa, Jukka Korpela, Ajoy Krishnan T, Jon Krom, Markus
Kuhn, Deepak Kulkarni, Oliver Laumann, John Lauro, Felix Lee, Mike Lee,
Timothy J. Lee, Tony Lee, Marty Leisner, Dave Lewis; Don Libes, Brian
Liedtke, Philip Lijnzaad, Keith Lindsay, Yen-Wei Liu, Paul Long,
Christopher Lott, Tim Love, Tim McDaniel, J. Scott McKellar, Kevin
McMahon, Stuart MacMartin, John R. MacMillan, Robert S. Maier, Andrew
Main, Bob Makowski, Evan Manning, Barry Margolin, George Marsaglia,
George Matas, Brad Mears, Wayne Mery, De Mickey, Rich Miller, Roger
Miller, Bill Mitchell, Mark Moraes, Darren Morby, Bernhard Muenzer,
David Murphy, Walter Murray, Ralf Muschall, Ken Nakata, Todd Nathan,
Taed Nelson, Landon Curt Noll, Tim Norman, Paul Nulsen, David O'Brien,
Richard A. O'Keefe, Adam Kolawa, Keith Edward O'hara, James Ojaste, Max
Okumoto, Hans Olsson, Bob Peck, Andrew Phillips, Christopher Phillips,
Francois Pinard, Nick Pitfield, Wayne Pollock, [email protected], Dan Pop,
Claudio Potenza, Lutz Prechelt, Lynn Pye, Kevin D. Quitt, Pat Rankin,
Arjun Ray, Eric S. Raymond, Peter W. Richards, James Robinson, Eric
Roode, Manfred Rosenboom, J. M. Rosenstock, Rick Rowe, Erkki Ruohtula,
John Rushford, Kadda Sahnine, Tomohiko Sakamoto, Matthew Saltzman, Rich
Salz, Chip Salzenberg, Matthew Sams, Paul Sand, DaviD W. Sanderson,
Frank Sandy, Christopher Sawtell, Jonas Schlein, Paul Schlyter, Doug
Schmidt, Rene Schmit, Russell Schulz, Dean Schulze, Jens Schweikhardt,
Chris Sears, Peter Seebach, Patricia Shanahan, Aaron Sherman, Raymond
Shwake, Nathan Sidwell, Peter da Silva, Joshua Simons, Ross Smith, Henri
Socha, Leslie J. Somos, Henry Spencer, David Spuler, Frederic Stark,
James Stern, Zalman Stern, Michael Sternberg, Geoff Stevens, Alan
Stokes, Bob Stout, Dan Stubbs, Steve Sullivan, Melanie Summit, Erik
Talvola, Dave Taylor, Clarke Thatcher, Wayne Throop, Chris Torek, Steve
Traugott, Nikos Triantafillis, Ilya Tsindlekht, Andrew Tucker, Goran
Uddeborg, Rodrigo Vanegas, Jim Van Zandt, Wietse Venema, Tom Verhoeff,
Ed Vielmetti, Larry Virden, Chris Volpe, Mark Warren, Alan Watson, Kurt
Watzka, Larry Weiss, Martin Weitzel, Howard West, Tom White, Freek
Wiedijk, Tim Wilson, Dik T. Winter, Lars Wirzenius, Dave Wolverton,
Mitch Wright, Conway Yee, Ozan S. Yigit, and Zhuo Zang, who have
contributed, directly or indirectly, to this article. Thanks to the
reviewers of the book-length version: Mark Brader, Vinit Carpenter,
Stephen Clamage, Jutta Degener, Doug Gwyn, Karl Heuer, and Joseph Kent.
Thanks to Debbie Lafferty and Tom Stone at Addison-Wesley for
encouragement, and permission to cross-pollinate this list with new text
from the book. Special thanks to Karl Heuer, Jutta Degener, and
particularly to Mark Brader, who (to borrow a line from Steve Johnson)
have goaded me beyond my inclination, and occasionally beyond my
endurance, in relentless pursuit of a better FAQ list.
Steve Summit
[email protected]
This article is Copyright 1990-1999 by Steve Summit.
Content from the book _C Programming FAQs: Frequently Asked Questions_
is made available here by permission of the author and the publisher as
a service to the community. It is intended to complement the use of the
published text and is protected by international copyright laws. The
content is made available here and may be accessed freely for personal
use but may not be republished without permission.
With the exception of the examples by other, cited authors (i.e. in
questions 20.31 and 20.35) the C code in this article is public domain
and may be used without restriction.
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