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2.12 stdio.h

2.11 stddef.h

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

Macros:

NULL
offsetof();
Variables:
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));
}
The output should be:
level is the 100 byte in the user structure.
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2.12 stdio.h