.\" Copyright (C), 1994, Graeme W. Wilford. (Wilf.)

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.\" 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 2017-09-15 "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 (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), 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 4.15 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/.