.\" Copyright (C), 1994, Graeme W. Wilford. (Wilf.)
.\" and Copyright (C) 2010, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Fri Jul 29th 12:56:44 BST 1994  Wilf. <G.Wilford@ee.surrey.ac.uk>
.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2002-03-09 by aeb
.\"
.TH setgid 2 2024-05-02 "Linux man-pages 6.9.1"
.SH NAME
setgid \- set group identity
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.P
.BI "int setgid(gid_t " gid );
.fi
.SH DESCRIPTION
.BR setgid ()
sets the effective group ID of the calling process.
If the calling process is privileged (more precisely: has the
.B CAP_SETGID
capability in its user namespace),
the real GID and saved set-group-ID are also set.
.P
Under Linux,
.BR setgid ()
is implemented like the POSIX version with the
.B _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.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
The group ID specified in
.I gid
is not valid in this user namespace.
.TP
.B EPERM
The calling process is not privileged (does not have the
\fBCAP_SETGID\fP capability in its user namespace), and
.I gid
does not match the real group ID or saved set-group-ID of
the calling process.
.SH VERSIONS
.SS 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
.BR 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
.BR nptl (7).
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4.
.P
The original Linux
.BR setgid ()
system call supported only 16-bit group IDs.
Subsequently, Linux 2.4 added
.BR setgid32 ()
supporting 32-bit IDs.
The glibc
.BR setgid ()
wrapper function transparently deals with the variation across kernel versions.
.SH SEE ALSO
.BR getgid (2),
.BR setegid (2),
.BR setregid (2),
.BR capabilities (7),
.BR credentials (7),
.BR user_namespaces (7)