.\" Copyright (C), 1994, Graeme W. Wilford. (Wilf.)
.\" and Copyright (C) 2010, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.\" 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 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
setgid \- set group identity
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.PP
.BI "int setgid(gid_t " gid );
.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.
.PP
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 appropriately.
.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 CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
.SH NOTES
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.
.\"
.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 SEE ALSO
.BR getgid (2),
.BR setegid (2),
.BR setregid (2),
.BR capabilities (7),
.BR credentials (7),
.BR user_namespaces (7)
.SH COLOPHON
This page is part of release 5.10 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.