man2/setsid.2

   1 .\" Copyright Michael Haardt (michael@cantor.informatik.rwth-aachen.de)
   2 .\"     Sat Aug 27 20:43:50 MET DST 1994
   3 .\" and Copyright (C) 2014, Michael Kerrisk <mtk.manpages@gmail.com>
   4 .\"
   5 .\" SPDX-License-Identifier: GPL-2.0-or-later
   6 .\"
   7 .\" Modified Sun Sep 11 19:19:05 1994 <faith@cs.unc.edu>
   8 .\" Modified Mon Mar 25 10:19:00 1996 <aeb@cwi.nl> (merged a few
   9 .\"     tiny changes from a man page by Charles Livingston).
  10 .\" Modified Sun Jul 21 14:45:46 1996 <aeb@cwi.nl>
  11 .\"
  12 .TH setsid 2 (date) "Linux man-pages (unreleased)"
  13 .SH NAME
  14 setsid \- creates a session and sets the process group ID
  15 .SH LIBRARY
  16 Standard C library
  17 .RI ( libc ", " \-lc )
  18 .SH SYNOPSIS
  19 .nf
  20 .B #include <unistd.h>
  21 .PP
  22 .B pid_t setsid(void);
  23 .fi
  24 .SH DESCRIPTION
  25 .BR setsid ()
  26 creates a new session if the calling process is not a
  27 process group leader.
  28 The calling process is the leader of the new session
  29 (i.e., its session ID is made the same as its process ID).
  30 The calling process also becomes
  31 the process group leader of a new process group in the session
  32 (i.e., its process group ID is made the same as its process ID).
  33 .PP
  34 The calling process will be the only process in
  35 the new process group and in the new session.
  36 .PP
  37 Initially, the new session has no controlling terminal.
  38 For details of how a session acquires a controlling terminal, see
  39 .BR credentials (7).
  40 .SH RETURN VALUE
  41 On success, the (new) session ID of the calling process is returned.
  42 On error,
  43 .I "(pid_t)\ \-1"
  44 is returned, and
  45 .I errno
  46 is set to indicate the error.
  47 .SH ERRORS
  48 .TP
  49 .B EPERM
  50 The process group ID of any process equals the PID of the calling process.
  51 Thus, in particular,
  52 .BR setsid ()
  53 fails if the calling process is already a process group leader.
  54 .SH STANDARDS
  55 POSIX.1-2001, POSIX.1-2008, SVr4.
  56 .SH NOTES
  57 A child created via
  58 .BR fork (2)
  59 inherits its parent's session ID.
  60 The session ID is preserved across an
  61 .BR execve (2).
  62 .PP
  63 A process group leader is a process whose process group ID equals its PID.
  64 Disallowing a process group leader from calling
  65 .BR setsid ()
  66 prevents the possibility that a process group leader places itself
  67 in a new session while other processes in the process group remain
  68 in the original session;
  69 such a scenario would break the strict
  70 two-level hierarchy of sessions and process groups.
  71 In order to be sure that
  72 .BR setsid ()
  73 will succeed, call
  74 .BR fork (2)
  75 and have the parent
  76 .BR _exit (2),
  77 while the child (which by definition can't be a process group leader) calls
  78 .BR setsid ().
  79 .PP
  80 If a session has a controlling terminal, and the
  81 .B CLOCAL
  82 flag for that terminal is not set,
  83 and a terminal hangup occurs, then the session leader is sent a
  84 .B SIGHUP
  85 signal.
  86 .PP
  87 If a process that is a session leader terminates, then a
  88 .B SIGHUP
  89 signal is sent to each process in the foreground
  90 process group of the controlling terminal.
  91 .SH SEE ALSO
  92 .BR setsid (1),
  93 .BR getsid (2),
  94 .BR setpgid (2),
  95 .BR setpgrp (2),
  96 .BR tcgetsid (3),
  97 .BR credentials (7),
  98 .BR sched (7)