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2.11 stddef.h

The stddef header defines several standard definitions. Many of these definitions also appear in other headers.

Macros:

NULL
offsetof();


typedef ptrdiff_t
typedef size_t
typedef wchar_t


2.11.1 Variables and Definitions

ptrdiff_t is the result of subtracting two pointers.
size_t is the unsigned integer result of the sizeof keyword.
wchar_t is an integer type of the size of a wide character constant.

NULL is the value of a null pointer constant.

offsetof(type, member-designator)

This results in a constant integer of type size_t which is the offset in bytes of a structure member from the beginning of the structure. The member is given by member-designator, and the name of the structure is given in type.

Example:

#include<stddef.h>
#include<stdio.h>

int main(void)
{
 struct user{
 char name[50];
 char alias[50];
 int level;
 };

 printf("level is the %d byte in the user structure.\n"),
  offsetof(struct user,level));
}

level is the 100 byte in the user structure.

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