redhat man page for setregid

SETREUID(2)						     Linux Programmer's Manual						       SETREUID(2)

NAME
       setreuid, setregid - set real and/or effective user or group ID

SYNOPSIS
       #include <sys/types.h>
       #include <unistd.h>

       int setreuid(uid_t ruid, uid_t euid);
       int setregid(gid_t rgid, gid_t egid);

DESCRIPTION
       setreuid  sets real and effective user IDs of the current process.  Unprivileged users may only set the real user ID to the real user ID or
       the effective user ID, and may only set the effective user ID to the real user ID, the effective user ID or the saved user ID.

       Supplying a value of -1 for either the real or effective user ID forces the system to leave that ID unchanged.

       If the real user ID is set or the effective user ID is set to a value not equal to the previous real user ID, the saved user ID will be set
       to the new effective user ID.

       Completely analogously, setregid sets real and effective group ID's of the current process, and all of the above holds with "group" instead
       of "user".

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       EPERM  The current process is not the super-user and changes other than (i) swapping the effective user	(group)  ID  with  the	real  user
	      (group)  ID,  or (ii) setting one to the value of the other or (iii) setting the effective user (group) ID to the value of the saved
	      user (group) ID was specified.

NOTES
       Setting the effective user (group) ID to the saved user ID is possible since Linux 1.1.37 (1.1.38).

CONFORMING TO
       BSD 4.3 (the setreuid and setregid function calls first appeared in 4.2BSD).

SEE ALSO
       getuid(2), getgid(2), setuid(2), setgid(2), seteuid(2), setresuid(2)

Linux 1.1.38							    1994-08-02							       SETREUID(2)