set group identity
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.
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
zero is returned. On error, −1 is returned, and
errno is set appropriately.
The group ID specified in
gid is not valid in this user namespace.
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
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
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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