setgid - set group identity
#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).
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