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Standard library header <stddef.h>

From cppreference.net
C
Standard Library headers

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

Contents

Types

signed integer type returned when subtracting two pointers
(typedef)
(C23)
the type of the predefined null pointer constant nullptr
(typedef)
(C11)
a type with alignment requirement as great as any other scalar type
(typedef)
unsigned integer type returned by the sizeof operator
(typedef)

Constants

implementation-defined null pointer constant
(macro constant)

Macros

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