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