setgid - set group identity
Standard C library (
libc,
-lc)
#include <unistd.h>
int setgid(gid_t gid);
setgid() sets the effective group ID of the calling process. If the
calling process is privileged (more precisely: has the
CAP_SETGID
capability in its user namespace), 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
secure manner.
On success, zero is returned. On error, -1 is returned, and
errno is set
to indicate the error.
- EINVAL
- The group ID specified in gid is not valid in this
user namespace.
- EPERM
- The calling process is not privileged (does not have the
CAP_SETGID capability in its user namespace), 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 variation
across kernel versions.
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
nptl(7).
getgid(2),
setegid(2),
setregid(2),
capabilities(7),
credentials(7),
user_namespaces(7)