offsetof(S)

offsetof -- offset of structure member

Synopsis

   #include <stddef.h>
   
   size_t offsetof (type, member-designator);

Description

offsetof is a macro defined in stddef.h which expands to an integral constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type).