Figure 2 uid-related functions in HP-UX

geteuid(2)   : get the effective uid (user ID)
getegid(2)   : get the effective gid (group ID)
getuid(2)    : get the real uid
getgid(2)    : get the real gid (group ID)
setuid(2)    : set both real and effective uid  (see the above rules)
setreuid(2)  : set real and effective user IDs
setresuid(2) : set real, effective and saved uid (HPUX)
setresgid(2) : set real, effective and saved gid (HPUX)