SETGID(2) Linux Programmer's Manual SETGID(2)
setgid - set group identity
int setgid(gid_t gid);
setgid() sets the effective group ID of the calling process. If the caller is the superuser, the real GID and saved set-group-ID are also
Under Linux, setgid() is implemented like the POSIX version with the _POSIX_SAVED_IDS feature. This allows a set-group-ID program that is
not set-user-ID-root to drop all of its group privileges, do some un-privileged work, and then reengage the original effective group ID in
a secure manner.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EPERM The calling process is not privileged (does not have the CAP_SETGID capability), and gid does not match the real group ID or saved
set-group-ID of the calling process.
The original Linux setgid() system call supported only 16-bit group IDs. Subsequently, Linux 2.4 added setgid32() supporting 32-bit IDs.
The glibc setgid() wrapper function transparently deals with the variation across kernel versions.
getgid(2), setegid(2), setregid(2), capabilities(7), credentials(7)
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Linux 2010-11-22 SETGID(2)