Full prototype:
size_t offsetof(structure-type, member-of-structure);
Purpose and Notes:
A function-like macro that determines how far away the member of the structure is from the beggining of the structure itself in bytes.
Returns:
size_t- The number of bytes away the begining of the structure member passed in as the second argument is from the begining of the structure passed in as the first argument.
Argument 1:
type: Never defined, but is widely assumed to be struct and Union types exclusivly.
Argument 2:
member-designator: also never defined, but often assumed to only mean struct and unions members (the identifiers used to distinguish different blocks of memory within the struct or union)