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
This is the signed integer type of the result of subtracting two
pointers. For example, with the declaration
char *p1, *p2;, the
p2 - p1 is of type
ptrdiff_t. This will
probably be one of the standard signed integer types (
long int), but might be a nonstandard
type that exists only for this purpose.
This is an unsigned integer type used to represent the sizes of objects.
The result of the
sizeof operator is of this type, and functions
malloc (see Unconstrained Allocation) and
memcpy (see Copying Strings and Arrays) accept arguments of
this type to specify object sizes. On systems using the GNU C Library, this
unsigned int or
unsigned long int.
size_t is the preferred way to declare any
arguments or variables that hold the size of an object.
Compatibility Note: Implementations of C before the advent of
ISO C generally used
unsigned int for representing object sizes
int for pointer subtraction results. They did not
necessarily define either
systems did define
size_t, in sys/types.h, but the
definition was usually a signed type.