mirror of
https://sourceware.org/git/glibc.git
synced 2024-12-22 10:50:07 +00:00
82 lines
3.6 KiB
Plaintext
82 lines
3.6 KiB
Plaintext
@node Common Definitions, Memory Allocation, Error Reporting, Top
|
|
@chapter Common Definitions
|
|
|
|
There are some miscellaneous data types and macros that are not part of
|
|
the C language kernel but are nonetheless almost universally used, such
|
|
as the macro @code{NULL}. In order to use these type and macro
|
|
definitions, your program should include the header file
|
|
@file{stddef.h}.
|
|
@pindex stddef.h
|
|
|
|
@comment stddef.h
|
|
@comment ANSI
|
|
@deftp {Data Type} ptrdiff_t
|
|
This is the signed integer type of the result of subtracting two
|
|
pointers. For example, with the declaration @code{char *p1, *p2;}, the
|
|
expression @code{p2 - p1} is of type @code{ptrdiff_t}. This will
|
|
probably be one of the standard signed integer types (@code{short int},
|
|
@code{int} or @code{long int}), but might be a nonstandard type that
|
|
exists only for this purpose.
|
|
@end deftp
|
|
|
|
@comment stddef.h
|
|
@comment ANSI
|
|
@deftp {Data Type} size_t
|
|
This is an unsigned integer type used to represent the sizes of objects.
|
|
The result of the @code{sizeof} operator is of this type, and functions
|
|
such as @code{malloc} (@pxref{Unconstrained Allocation}) and
|
|
@code{memcpy} (@pxref{Copying and Concatenation}) that manipulate
|
|
objects of arbitrary sizes accept arguments of this type to specify
|
|
object sizes.
|
|
@end deftp
|
|
|
|
In the GNU system @code{size_t} is equivalent to one of the types
|
|
@code{unsigned int} and @code{unsigned long int}. These types have
|
|
identical properties on the GNU system, and for most purposes, you
|
|
can use them interchangeably. However, they are distinct types,
|
|
and in certain contexts, you may not treat them as identical. For
|
|
example, when you specify the type of a function argument in a
|
|
function prototype, it makes a difference which one you use. If
|
|
the system header files declare @code{malloc} with an argument
|
|
of type @code{size_t} and you declare @code{malloc} with an argument
|
|
of type @code{unsigned int}, you will get a compilation error if
|
|
@code{size_t} happens to be @code{unsigned long int} on your system.
|
|
To avoid any possibility of error, when a function argument is
|
|
supposed to have type @code{size_t}, always write the type as
|
|
@code{size_t}, and make no assumptions about what that type might
|
|
actually be.
|
|
|
|
@strong{Compatibility Note:} Types such as @code{size_t} are new
|
|
features of ANSI C. Older, pre-ANSI C implementations have
|
|
traditionally used @code{unsigned int} for representing object sizes
|
|
and @code{int} for pointer subtraction results.
|
|
|
|
@comment stddef.h
|
|
@comment ANSI
|
|
@deftypevr Macro {void *} NULL
|
|
@cindex null pointer
|
|
This is a null pointer constant. It can be assigned to any pointer
|
|
variable since it has type @code{void *}, and is guaranteed not to
|
|
point to any real object. This macro is the best way to get a null
|
|
pointer value. You can also use @code{0} or @code{(void *)0} as a null
|
|
pointer constant, but using @code{NULL} makes the purpose of the
|
|
constant more evident.
|
|
|
|
When passing a null pointer as an argument to a function for which there
|
|
is no prototype declaration in scope, you should explicitly cast
|
|
@code{NULL} or @code{0} into a pointer of the appropriate type. Again,
|
|
this is because the default argument promotions may not do the right
|
|
thing.
|
|
@end deftypevr
|
|
|
|
@comment stddef.h
|
|
@comment ANSI
|
|
@deftypefn {Macro} size_t offsetof (@var{type}, @var{member})
|
|
This expands to a integer constant expression that is the offset of the
|
|
structure member named @var{member} in a @code{struct} of type
|
|
@var{type}. For example, @code{offsetof (struct s, elem)} is the
|
|
offset, in bytes, of the member @code{elem} in a @code{struct s}. This
|
|
macro won't work if @var{member} is a bit field; you get an error from
|
|
the C compiler in that case.
|
|
@end deftypefn
|