STDDEF - standard definitions header.

#include <stddef.h>

<stddef.h> contains definitions for several widely used symbols.

ptrdiff_t

the signed integral type of the result of subtracting one pointer from another.

size_t

the unsigned integral type of the result of "sizeof".

NULL

the (void *) null pointer.

offsetof(TYPE,NAME)

a macro obtaining the offset of the structure field NAME in the structure referred to by type. For example,

offsetof(struct X,fieldY)

calculates the (integral) offset of "fieldY" in a structure with tag X. The offset is given in bytes, with the type "size_t".

errno

refers to a modifiable Lvalue used in reporting errors.