Standard library header <stddef.h>

This header is part of types support library, in particular, it provides additional basic types and convenience macros.

Types
ptrdiff_t	signed integer type returned when subtracting two pointers (typedef)
nullptr_t (C23)	the type of the predefined null pointer constant nullptr (typedef)
max_align_t (C11)	a type with alignment requirement as great as any other scalar type (typedef)
size_t	unsigned integer type returned by the sizeof operator (typedef)
Constants
NULL	implementation-defined null pointer constant (macro constant)
Macros
offsetof	byte offset from the beginning of a struct type to specified member (function macro)

Synopsis

#define __STDC_VERSION_STDDEF_H__ 202311L

typedef /* see description */ ptrdiff_t;
typedef /* see description */ nullptr_t;
typedef /* see description */ max_align_t;
typedef /* see description */ wchar_t;
typedef /* see description */ size_t;

#define NULL /* see description */
#define unreachable() /* see description */
#define offsetof(P, D) /* see description */

Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code defines __STDC_WANT_LIB_EXT1__ before any inclusion of <stddef.h>:

#if defined(__STDC_WANT_LIB_EXT1__)
typedef /* see description */ rsize_t;
#endif