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