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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.

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.