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### A.4 Important Data Types

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`.

— Data Type: ptrdiff_t

> This is the signed integer type of the result of subtracting two pointers. For example, with the declaration `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.

— Data Type: size_t

> 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 such as `malloc` (see Unconstrained Allocation) and `memcpy` (see Copying and Concatenation) accept arguments of this type to specify object sizes. On systems using the GNU C Library, this will be `unsigned int` or `unsigned long int`.

> Usage Note: `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 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.