1.1: How do you decide which integer type to use?
A: If you might need large values (tens of thousands), use long.
Otherwise, if space is very important, use short. Otherwise,
use int.
1.4: What should the 64-bit type on new, 64-bit machines be?
A: There are arguments in favor of long int and long long int,
among other options.
1.7: What's the best way to declare and define global variables?
A: The best arrangement is to place each definition in some
relevant .c file, with an external declaration in a header file.
1.11: What does extern mean in a function declaration?
A: Nothing, really.
1.12: What's the auto keyword good for?
A: Nothing.
1.14: I can't seem to define a linked list node which contains 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. Problems arise if an attempt is made to define (and use)
a typedef in the midst of such a declaration; avoid this.
1.21: How do I declare an array of N pointers to functions returning
pointers to functions returning pointers to characters?
A: char *(*(*a[N])())();
Using a chain of typedefs, or the cdecl program, makes these
declarations easier.
1.22: How can I declare a function that returns a pointer to a
function of its own type?
A: You can't quite do it directly. Use a cast, or wrap a struct
around the pointer and return that.
1.25: My compiler is complaining about an invalid redeclaration of a
function, but I only define it once.
A: Calling an undeclared function declares it implicitly as
returning int.
1.30: What can I safely assume about the initial values of variables
which are not explicitly initialized?
A: Uninitialized variables with "static" duration start out as 0,
as if the programmer had initialized them. Variables with
"automatic" duration, and dynamically-allocated memory, start
out containing garbage (with the exception of calloc).
1.31: Why can't I initialize a local array with a string?
A: Perhaps you have a pre-ANSI compiler.
1.31a: What's wrong with "char *p = malloc(10);" ?
A: Function calls are not allowed in initializers for global or
static variables.
1.32: What is the difference between char a[] = "string"; and
char *p = "string"; ?
A: The first declares an initialized and modifiable array; the
second declares a pointer initialized to a not-necessarily-
modifiable constant string.
1.34: How do I initialize a pointer to a function?
A: Use something like "extern int func(); int (*fp)() = func;" .
Section 2. Structures, Unions, and Enumerations
2.1: What's the difference between struct x1 { ... }; and
typedef struct { ... } x2; ?
A: The first structure is named by a tag, the second by a typedef
name.
2.2: Why doesn't "struct x { ... }; x thestruct;" work?
A: C is not C++.
2.3: Can a structure contain a pointer to itself?
A: See question 1.14.
2.4: What's the best way of implementing opaque (abstract) data types
in C?
A: One good way is to use structure pointers which point to
structure types which are not publicly defined.
2.6: I came across some code that declared a structure with the last
member an array of one element, and then did some tricky
allocation to make it act like the array had several elements.
Is this legal or portable?
A: An official interpretation has deemed that it is not strictly
conforming with the C Standard.
2.7: I heard that structures could be assigned to variables and
passed to and from functions, but K&R1 says not.
A: These operations are supported by all modern compilers.
2.8: Why can't you compare structures?
A: There is no single, good way for a compiler to implement
structure comparison which is consistent with C's low-level
flavor.
2.9: How are structure passing and returning implemented?
A: If you really need to know, see the unabridged list.
2.10: Can I pass constant values to functions which accept structure
arguments?
A: No. C has no way of generating anonymous structure values.
2.11: How can I read/write structures from/to data files?
A: It is relatively straightforward to use fread and fwrite.
2.12: How can I turn off structure padding?
A: There is no standard method.
2.13: Why does sizeof report a larger size than I expect for a
structure type?
A: The alignment of arrays of structures must be preserved.
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.
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.
2.18: I have a program which works correctly, but dumps core after it
finishes. Why?
A: Check to see if a structure type declaration just before main()
is missing its trailing semicolon, causing main() to be declared
as returning a structure. See also questions 10.9 and 16.4.
2.20: Can I initialize unions?
A: ANSI Standard C allows an initializer for the first-named
member.
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
states that enumerations are compatible with integral types.
2.24: Is there an easy way to print enumeration values symbolically?
A: No.
Section 3. Expressions
3.1: Why doesn't the code "a[i] = i++;" work?
A: The variable i is both referenced and modified in the same
expression.
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: The operations implied by the postincrement and postdecrement
operators ++ and -- are performed at some time after the
operand's former values are yielded and before the end of the
expression, but not necessarily immediately after, or before
other parts of the expression are evaluated.
3.3: How could the code "int i = 3; i = i++;" ever give 7?
A: Undefined behavior means *anything* can happen.
3.4: Don't precedence and parentheses dictate order of evaluation?
A: Operator precedence and explicit parentheses impose only a
partial ordering on the evaluation of an expression, which does
not generally include the order of side effects.
3.5: But what about the && and || operators?
A: There is a special exception for those operators: left-to-right
evaluation is guaranteed.
3.8: What's a "sequence point"?
A: The point (at the end of a full expression, or at the ||, &&,
?:, or comma operators, or just before a function call) at which
all side effects are guaranteed to be complete.
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.
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.
3.14: Why doesn't the code "int a = 1000, b = 1000;
long int c = a * b;" work?
A: You must manually cast one of the operands to (long).
3.16: Can I use ?: on the left-hand side of an assignment expression?
A: No.
Section 4. Pointers
4.2: What's wrong with "char *p; *p = malloc(10);"?
A: The pointer you declared is p, not *p.
4.3: Does *p++ increment p, or what it points to?
A: *p++ increments p. To increment the value pointed to by p, use
(*p)++ .
N.Raj: THIS QUESTION 4.5 is wrong.
4.5: I want to use a char * pointer to step over some ints. Why
doesn't "((int *)p)++;" work?
A: In C, a cast operator is a conversion operator, and by
definition it yields an rvalue, which cannot be assigned to, or
incremented with ++.
4.8: I have a function which accepts, and is supposed to initialize,
a pointer, but the pointer in the caller remains unchanged.
A: The called function probably altered only the passed copy of the
pointer.
4.9: Can I use a void ** pointer to pass a generic pointer to a
function by reference?
A: Not portably.
4.10: I have a function which accepts a pointer to an int. How can I
pass a constant like 5 to it?
A: You will have to declare a temporary variable.
4.11: Does C even have "pass by reference"?
A: Not really, though it can be simulated.
4.12: I've seen different methods used for calling functions via
pointers.
A: The extra parentheses and explicit * are now officially
optional, although some older implementations require them.
Section 5. Null Pointers
5.1: What is this infamous null pointer, anyway?
A: For each pointer type, there is a special value -- the "null
pointer" -- which is distinguishable from all other pointer
values and which is not the address of any object or function.
5.2: How do I get a null pointer in my programs?
A: A constant 0 in a pointer context is converted into a null
pointer at compile time. A "pointer context" is an
initialization, assignment, or comparison with one side a
variable or expression of pointer type, and (in ANSI standard C)
a function argument which has a prototype in scope declaring a
certain parameter as being of pointer type. In other contexts
(function arguments without prototypes, or in the variable part
of variadic function calls) a constant 0 with an appropriate
explicit cast is required.
5.3: Is the abbreviated pointer comparison "if(p)" to test for non-
null pointers valid?
A: Yes. The construction "if(p)" works, regardless of the internal
representation of null pointers, because the compiler
essentially rewrites it as "if(p != 0)" and goes on to convert 0
into the correct null pointer.
5.4: What is NULL and how is it #defined?
A: NULL is simply a preprocessor macro, #defined as 0 (or
((void *)0)), which is used (as a stylistic convention, in
preference to unadorned 0's) to generate null pointers.
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 ((void *)0)). (The
compiler makes the translation, upon seeing a 0, not the
preprocessor.)
5.6: If NULL were defined as "((char *)0)," wouldn't that make
function calls which pass an uncast NULL work?
A: Not in general. The problem is that there are machines which
use different internal representations for pointers to different
types of data. A cast is still required to tell the compiler
which kind of null pointer is required, since it may be
different from (char *)0.
5.9: If NULL and 0 are equivalent as null pointer constants, which
should I use?
A: Either; the distinction is entirely stylistic.
5.10: But wouldn't it be better to use NULL, in case the value of NULL
changes?
A: No. NULL is a constant zero, so a constant zero is equally
sufficient.
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 valid, does not buy much.
5.13: This is strange. NULL is guaranteed to be 0, but the null
pointer is not?
A: A "null pointer" is a language concept whose particular internal
value does not matter. A null pointer is requested in source
code with the character "0". "NULL" is a preprocessor macro,
which is always #defined as 0 (or ((void *)0)).
5.14: Why is there so much confusion surrounding null pointers?
A: 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.
5.15: I'm confused. I just can't understand all this null pointer
stuff.
A: A simple rule is, "Always use `0' or `NULL' for null pointers,
and always cast them when they are used as arguments in function
calls."
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: Such a requirement would accomplish little.
5.17: Seriously, have any actual machines really used nonzero null
pointers?
A: Machines manufactured by Prime, Honeywell-Bull, and CDC, as well
as Symbolics Lisp Machines, have done so.
5.20: What does a run-time "null pointer assignment" error mean?
A: It means that you've written, via a null pointer, to an invalid
location. (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: The declaration extern char *a simply does not match the actual
definition. Use extern char a[].
6.2: But I heard that char a[] was identical to char *a.
A: Not at all. Arrays are not pointers. A reference like x[3]
generates different code depending on whether x is an array or a
pointer.
6.3: So what is meant by the "equivalence of pointers and arrays" in
C?
A: An lvalue of type array-of-T which appears in an expression
decays into a pointer to its first element; the type of the
resultant pointer is pointer-to-T. So for an array a and
pointer p, you can say "p = a;" and then p[3] and a[3] will
access the same element.
6.4: Why are array and pointer declarations interchangeable as
function formal parameters?
A: It's supposed to be a convenience.
6.7: How can an array be an lvalue, if you can't assign to it?
A: An array is not a "modifiable lvalue."
6.8: What is the real difference between arrays and pointers?
A: Arrays automatically allocate space which is fixed in size and
location; pointers are dynamic.
6.9: Someone explained to me that arrays were really just constant
pointers.
A: An array name is "constant" in that it cannot be assigned to,
but an array is *not* a pointer.
6.11: I came across some "joke" code containing the "expression"
5["abcdef"] . How can this be legal C?
A: Yes, array subscripting is commutative in C. The array
subscripting operation a[e] is defined as being identical to
*((a)+(e)).
6.12: What's the difference between array and &array?
A: The type.
6.13: How do I declare a pointer to an array?
A: Usually, you don't want to. Consider using a pointer to one of
the array's elements instead.
6.14: How can I set an array's size at run time?
A: It's straightforward to use malloc() and a pointer.
6.15: How can I declare local arrays of a size matching a passed-in
array?
A: You can't; array dimensions must be compile-time constants.
6.16: How can I dynamically allocate a multidimensional array?
A: It is usually best to allocate an array of pointers, and then
initialize each pointer to a dynamically-allocated "row." See
the full list for code samples.
6.17: Can I simulate a non-0-based array with a pointer?
A: Not if the pointer points outside of the block of memory it is
intended to access.
6.18: My compiler complained when I passed a two-dimensional array to
a function expecting a pointer to a pointer.
A: The rule by which arrays decay into pointers is not applied
recursively. An array of arrays (i.e. a two-dimensional array
in C) decays into a pointer to an array, not a pointer to a
pointer.
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 particularly easy.
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, but see the full list for
some ideas.
6.21: Why doesn't sizeof properly report the size of an array which is
a parameter to a function?
A: The sizeof operator reports the size of the pointer parameter
which the function actually receives.
Section 7. Memory Allocation
7.1: Why doesn't the code "char *answer; gets(answer);" work?
A: The pointer variable answer() has not been set to point to any
valid storage. The simplest way to correct this fragment is to
use a local array, instead of a pointer.
7.2: I can't get strcat() to work. I tried "char *s3 =
strcat(s1, s2);" but I got strange results.
A: Again, the main problem here is that space for the concatenated
result is not properly allocated.
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.
7.5: I have a function that is supposed to return a string, but when
it returns to its caller, the returned string is garbage.
A: Make sure that the pointed-to memory is properly (i.e. not
locally) allocated.
7.6: Why am I getting "warning: assignment of pointer from integer
lacks a cast" for calls to malloc()?
A: Have you #included
7.7: Why does some code carefully cast the values returned by malloc
to the pointer type being allocated?
A: Before ANSI/ISO C, these casts were required to silence certain
warnings.
7.8: Why does so much code leave out the multiplication by
sizeof(char) when allocating strings?
A: Because sizeof(char) is, by definition, exactly 1.
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.
7.16: I'm allocating a large array for some numeric work, but malloc()
is acting strangely.
A: Make sure the number you're trying to pass to malloc() isn't
bigger than a size_t can hold.
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.
See also question 19.23.
7.19: My program is crashing, apparently somewhere down inside malloc.
A: Make sure you aren't using more memory than you malloc'ed,
especially for strings (which need strlen(str) + 1 bytes).
7.20: You can't use dynamically-allocated memory after you free it,
can you?
A: No. Some early documentation implied otherwise, but the claim
is no longer valid.
7.21: Why isn't a pointer null after calling free()?
A: C's pass-by-value semantics mean that called functions can never
permanently change the values of their arguments.
7.22: When I call malloc() to allocate memory for a local pointer, do
I have to explicitly free() it?
A: Yes.
7.23: When I free a dynamically-allocated structure containing
pointers, do I also have to free each subsidiary pointer?
A: Yes.
7.24: Must I free allocated memory before the program exits?
A: You shouldn't have to.
7.25: Why doesn't my program's memory usage go down when I free
memory?
A: Most implementations of malloc/free do not return freed memory
to the operating system.
7.26: How does free() know how many bytes to free?
A: The malloc/free implementation remembers the size of each block
allocated and returned.
7.27: So can I query the malloc package to find out how big an
allocated block is?
A: Not portably.
7.30: Is it legal to pass a null pointer as the first argument to
realloc()?
A: ANSI C sanctions this usage, although several earlier
implementations do not support it.
7.31: What's the difference between calloc() and malloc()?
A: calloc() takes two arguments, and initializes the allocated
memory to all-bits-0.
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. alloca() cannot be
written portably, is difficult to implement on machines without
a stack, and fails under certain conditions if implemented
simply.
Section 8. Characters and Strings
8.1: Why doesn't "strcat(string, '!');" work?
A: strcat() concatenates *strings*, not characters.
8.2: Why won't the test if(string == "value") correctly compare
string against the value?
A: It's comparing pointers. To compare two strings, use strcmp().
8.3: Why can't I assign strings to character arrays?
A: Strings are arrays, and you can't assign arrays directly. Use
strcpy() instead.
8.6: How can I get the numeric (character set) value corresponding to
a character?
A: In C, if you have the character, you have its value.
8.9: Why is sizeof('a') not 1?
A: Character constants in C are of type int.
Section 9. Boolean Expressions and Variables
9.1: What is the right type to use for Boolean values in C?
A: There's no one right answer; see the full list for some
discussion.
9.2: What if a built-in logical or relational operator "returns"
something other than 1?
A: When a Boolean value is generated by a built-in operator, it is
guaranteed to be 1 or 0. (This is *not* true for some library
routines such as isalpha.)
9.3: Is if(p), where p is a pointer, valid?
A: Yes. See question 5.3.
Section 10. C Preprocessor
10.2: I've got some cute preprocessor macros that let me write C code
that looks more like Pascal. What do y'all think?
A: Bleah.
10.3: How can I write a generic macro to swap two values?
A: There is no good answer to this question. The best all-around
solution is probably to forget about using a macro.
10.4: What's the best way to write a multi-statement macro?
A: #define Func() do {stmt1; stmt2; ... } while(0) /* (no trailing ;) */
10.6: What are .h files and what should I put in them?
A: Header files (also called ".h files") should generally contain
common declarations and macro, structure, and typedef
definitions, but not variable or function definitions.
10.7: Is it acceptable for one header file to #include another?
A: It's a question of style, and thus receives considerable debate.
10.8: Where are header ("#include") files searched for?
A: The exact behavior is implementation-defined; see the full list
for some discussion.
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.
10.11: Where can I get a copy of a missing header file?
A: Contact your vendor, or see question 18.16 or the full list.
10.12: How can I construct preprocessor #if expressions which compare
strings?
A: You can't do it directly; try #defining several manifest
constants and implementing conditionals on those.
10.13: Does the sizeof operator work in preprocessor #if directives?
A: No.
10.14: Can I use an #ifdef in a #define line, to define something two
different ways?
A: No.
10.15: Is there anything like an #ifdef for typedefs?
A: Unfortunately, no.
10.16: How can I use a preprocessor #if expression to detect
endianness?
A: You probably can't.
10.18: How can I preprocess some code to remove selected conditional
compilations, without preprocessing everything?
A: Look for a program called unifdef, rmifdef, or scpp.
10.19: How can I list all of the pre#defined identifiers?
A: If the compiler documentation is unhelpful, try extracting
printable strings from the compiler or preprocessor executable.
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: Try the ANSI token-pasting operator ##.
10.22: What does the message "warning: macro replacement within a
string literal" mean?
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: Consider writing your own little special-purpose preprocessing
tool, instead.
10.26: How can I write a macro which takes a variable number of
arguments?
A: Here is one popular trick. Note that the parentheses around
printf's argument list are in the macro call, not the
definition.
#define DEBUG(args) (printf("DEBUG: "), printf args)
if(n != 0) DEBUG(("n is %d\n", n));
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