The getgid system call returns the real group ID of the calling process, getegid returns the effective group ID of the calling process.
The real group ID is specified at login time.
The real group ID is the group of the user who invoked the program. As the effective group ID gives the process additional permissions during the execution of "set-group-ID" mode processes, getgid is used to determine the real-user-id of the calling process.
The getgid and getegid system calls are always successful, and no return value is reserved to indicate an error.
getuid(2), issetugid(2), setgid(2), setregid(2)