The
<stddef.h> header defines the following types and macros:
1.
ptrdiff_t, a signed integer type of the result of subtracting two pointers;
2.
size_t, an unsigned integer type of the result of the sizeof() operator;
3.
wchar_t, an integer type whose range of values can represent distinct wide-character codes for all members of the largest character set specified among the supported locales: the null character has the code value 0 and each member of the character set has a code value equal to its value when used as the lone character in an integer character constant;
4.
NULL, which expands to an implementation-defined null pointer constant; and
5.
offsetof(), a macro that expands to an integer constant as described in
offsetof(3).
Some of the described types and macros may appear also in other headers.