# 34. Common definitions <stddef.h>¶

The following types and macros are defined in the standard header <stddef.h>. Some are also defined in other headers, as noted in their respective subclauses.

The types are

ptrdiff_t

which is the signed integer type of the result of subtracting two pointers;

size_t

which is the unsigned integer type of the result of the sizeof operator; and

wchar_t

which is an integer type whose range of values can represent distinct codes for all members of the largest extended character set specified among the supported locales; the null character shall have the code value zero.

The macros are

NULL

which expands to an implementation-defined null pointer constant; and

offsetof (type, member-designator)

which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The type and member designator shall be such that given

static type t;

then the expression & (t.*member-designator*) evaluates to an address constant. (If the specified member is a bit-field, the behavior is undefined.)

Recommended practice

The types used for size_t and ptrdiff_t should not have an integer conversion rank greater than that of signed long int unless the implementation supports objects large enough to make this necessary.