int setgid(gid_t gid);
setgid() sets the effective group ID of the calling process. If the
caller is privileged (has the CAP_SETGID capability), the real GID and
saved set-group-ID are also set.
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
On success, zero is returned. On error, -1 is returned, and errno is
EINVAL The group ID specified in gid is not valid in this user names-
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.
POSIX.1-2001, POSIX.1-2008, SVr4.
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 varia-
tion across kernel versions.
C library/kernel differences
At the kernel level, user IDs and group IDs are a per-thread attribute.
However, POSIX requires that all threads in a process share the same
credentials. The NPTL threading implementation handles the POSIX
requirements by providing wrapper functions for the various system
calls that change process UIDs and GIDs. These wrapper functions
(including the one for setgid()) employ a signal-based technique to
ensure that when one thread changes credentials, all of the other
threads in the process also change their credentials. For details, see
getgid(2), setegid(2), setregid(2), capabilities(7), credentials(7),
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