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.\" Copyright Michael Haardt (michael@cantor.informatik.rwth-aachen.de)
.\" Sat Aug 27 20:43:50 MET DST 1994
.\" and Copyright (C) 2014, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.\" Modified Sun Sep 11 19:19:05 1994 <faith@cs.unc.edu>
.\" Modified Mon Mar 25 10:19:00 1996 <aeb@cwi.nl> (merged a few
.\" tiny changes from a man page by Charles Livingston).
.\" Modified Sun Jul 21 14:45:46 1996 <aeb@cwi.nl>
.\"
.TH setsid 2 2023-03-30 "Linux man-pages 6.04"
.SH NAME
setsid \- creates a session and sets the process group ID
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.B pid_t setsid(void);
.fi
.SH DESCRIPTION
.BR setsid ()
creates a new session if the calling process is not a
process group leader.
The calling process is the leader of the new session
(i.e., its session ID is made the same as its process ID).
The calling process also becomes
the process group leader of a new process group in the session
(i.e., its process group ID is made the same as its process ID).
.PP
The calling process will be the only process in
the new process group and in the new session.
.PP
Initially, the new session has no controlling terminal.
For details of how a session acquires a controlling terminal, see
.BR credentials (7).
.SH RETURN VALUE
On success, the (new) session ID of the calling process is returned.
On error,
.I "(pid_t)\ \-1"
is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EPERM
The process group ID of any process equals the PID of the calling process.
Thus, in particular,
.BR setsid ()
fails if the calling process is already a process group leader.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4.
.SH NOTES
A child created via
.BR fork (2)
inherits its parent's session ID.
The session ID is preserved across an
.BR execve (2).
.PP
A process group leader is a process whose process group ID equals its PID.
Disallowing a process group leader from calling
.BR setsid ()
prevents the possibility that a process group leader places itself
in a new session while other processes in the process group remain
in the original session;
such a scenario would break the strict
two-level hierarchy of sessions and process groups.
In order to be sure that
.BR setsid ()
will succeed, call
.BR fork (2)
and have the parent
.BR _exit (2),
while the child (which by definition can't be a process group leader) calls
.BR setsid ().
.PP
If a session has a controlling terminal, and the
.B CLOCAL
flag for that terminal is not set,
and a terminal hangup occurs, then the session leader is sent a
.B SIGHUP
signal.
.PP
If a process that is a session leader terminates, then a
.B SIGHUP
signal is sent to each process in the foreground
process group of the controlling terminal.
.SH SEE ALSO
.BR setsid (1),
.BR getsid (2),
.BR setpgid (2),
.BR setpgrp (2),
.BR tcgetsid (3),
.BR credentials (7),
.BR sched (7)
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