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`<stddef.h>`

Include the standard header <stddef.h> to define several types and macros that are of general use throughout the program. The standard header <stddef.h> is available even in a freestanding implementation.

    /* MACROS */
#define NULL <either 0, 0L, or (void *)0> [0 in C++]
#define offsetof(s-type, mbr) <size_t constant expression>

    /* TYPES */
typedef si-type ptrdiff_t;
typedef ui-type size_t;
typedef i-type wchar_t; [keyword in C++]

`NULL`

#define NULL <either 0, 0L, or (void *)0> [0 in C++]

The macro yields a null pointer constant that is usable as an address constant expression.

`offsetof`

#define offsetof(s-type, mbr) <size_t constant expression>

The macro yields the offset in bytes, of type size_t, of member mbr from the beginning of structure type s-type, where for X of type s-type, &X.mbr is an address constant expression.

`ptrdiff_t`

typedef si-type ptrdiff_t;

The type is the signed integer type si-type of an object that you declare to store the result of subtracting two pointers.

`size_t`

typedef ui-type size_t;

The type is the unsigned integer type ui-type of an object that you declare to store the result of the sizeof operator.

`wchar_t`

typedef i-type wchar_t; [keyword in C++]

The type is the integer type i-type of a wide-character constant, such as L'X'. You declare an object of type wchar_t to hold a wide character.

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