Setresgid

Name
setresuid, setresgid - set real, effective and saved user or group ID

Synopsis
int setresuid(uid_t ruid, uid_t euid, uid_t suid); int setresgid(gid_t rgid, gid_t egid, gid_t sgid);
 * 1) define _GNU_SOURCE
 * 2) include 

Description
setresuid sets the real user ID, the effective user ID, and the saved set-user-ID of the current process.

Unprivileged user processes may change the real UID, effective UID, and saved set-user-ID, each to one of: the current real UID, the current effective UID or the current saved set-user-ID.

Privileged processes (on Linux, those having the CAP_SETUID capability) may set the real UID, effective UID, and saved set-user-ID to arbitrary values.

If one of the parameters equals -1, the corresponding value is not changed.

Regardless of what changes are made to the real UID, effective UID, and saved set-user-ID, the file system UID is always set to the same value as the (possibly new) effective UID.

Completely analogously, setresgid sets the real GID, effective GID, and saved set-group-ID of the current process (and always modifies the file system GID to be the same as the effective GID), with the same restrictions for non-privileged processes.

Return Value
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

Errors
EAGAIN uid does not match the current UID and this call would bring that user ID over its NPROC rlimit. EPERM The calling process is not privileged (did not have the CAP_SETUID capability) and tried to change the IDs to values that are not permitted.

Conforming to
These calls are non-standard; they also appear on HP-UX and some of the BSDs.

History
This system call was first introduced in HP-UX. It is available under Linux since Linux 2.1.44. These days it is also found in FreeBSD (for emulation of Linux binaries).