The result of subtracting two pointers in C is always an integer, but the
precise data type varies from C compiler to C compiler. Likewise, the
data type of the result of sizeof also varies between compilers.
ISO defines standard aliases for these two types, so you can refer to
them in a portable fashion. They are defined in the header file
`stddef.h'.
char *p1, *p2;, the
expression p2 - p1 is of type ptrdiff_t. This will
probably be one of the standard signed integer types (short
int, int or long int), but might be a nonstandard
type that exists only for this purpose.
sizeof operator is of this type, and functions
such as malloc (see section Unconstrained Allocation) and
memcpy (see section Copying and Concatenation) accept arguments of
this type to specify object sizes.
Usage Note: size_t is the preferred way to declare any
arguments or variables that hold the size of an object.
In the GNU system size_t is equivalent to either
unsigned int or 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 as data types,
which makes a difference in certain contexts.
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 malloc with an argument of type
size_t and you declare malloc with an argument of type
unsigned int, you will get a compilation error if size_t
happens to be unsigned long int on your system. To avoid any
possibility of error, when a function argument or value is supposed to
have type size_t, never declare its type in any other way.
Compatibility Note: Implementations of C before the advent of
ISO C generally used unsigned int for representing object sizes
and int for pointer subtraction results. They did not
necessarily define either size_t or ptrdiff_t. Unix
systems did define size_t, in `sys/types.h', but the
definition was usually a signed type.
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