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 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
   6 .\" This is free documentation; you can redistribute it and/or
   7 .\" modify it under the terms of the GNU General Public License as
   8 .\" published by the Free Software Foundation; either version 2 of
   9 .\" the License, or (at your option) any later version.
  10 .\"
  11 .\" The GNU General Public License's references to "object code"
  12 .\" and "executables" are to be interpreted as the output of any
  13 .\" document formatting or typesetting system, including
  14 .\" intermediate and printed output.
  15 .\"
  16 .\" This manual is distributed in the hope that it will be useful,
  17 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
  18 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  19 .\" GNU General Public License for more details.
  20 .\"
  21 .\" You should have received a copy of the GNU General Public
  22 .\" License along with this manual; if not, see
  23 .\" <http://www.gnu.org/licenses/>.
  24 .\" %%%LICENSE_END
  25 .\"
  26 .\" Modified Sun Sep 11 19:19:05 1994 <faith@cs.unc.edu>
  27 .\" Modified Mon Mar 25 10:19:00 1996 <aeb@cwi.nl> (merged a few
  28 .\"     tiny changes from a man page by Charles Livingston).
  29 .\" Modified Sun Jul 21 14:45:46 1996 <aeb@cwi.nl>
  30 .\"
  31 .TH SETSID 2 2016-10-08 "Linux" "Linux Programmer's Manual"
  32 .SH NAME
  33 setsid \- creates a session and sets the process group ID
  34 .SH SYNOPSIS
  35 .ad l
  36 .B #include <unistd.h>
  37 .sp
  38 .B pid_t setsid(void);
  39 .br
  40 .ad b
  41 .SH DESCRIPTION
  42 .BR setsid ()
  43 creates a new session if the calling process is not a
  44 process group leader.
  45 The calling process is the leader of the new session
  46 (i.e., its session ID is made the same as its process ID).
  47 The calling process also becomes
  48 the process group leader of a new process group in the session
  49 (i.e., its process group ID is made the same as its process ID).
  50
  51 The calling process will be the only process in
  52 the new process group and in the new session.
  53
  54 Initially, the new session has no controlling terminal.
  55 For details of how a session acquires a controlling terminal, see
  56 .BR credentials (7).
  57 .SH RETURN VALUE
  58 On success, the (new) session ID of the calling process is returned.
  59 On error,
  60 .I "(pid_t)\ \-1"
  61 is returned, and
  62 .I errno
  63 is set to indicate the error.
  64 .SH ERRORS
  65 .TP
  66 .B EPERM
  67 The process group ID of any process equals the PID of the calling process.
  68 Thus, in particular,
  69 .BR setsid ()
  70 fails if the calling process is already a process group leader.
  71 .SH CONFORMING TO
  72 POSIX.1-2001, POSIX.1-2008, SVr4.
  73 .SH NOTES
  74 A child created via
  75 .BR fork (2)
  76 inherits its parent's session ID.
  77 The session ID is preserved across an
  78 .BR execve (2).
  79
  80 A process group leader is a process whose process group ID equals its PID.
  81 Disallowing a process group leader from calling
  82 .BR setsid ()
  83 prevents the possibility that a process group leader places itself
  84 in a new session while other processes in the process group remain
  85 in the original session;
  86 such a scenario would break the strict
  87 two-level hierarchy of sessions and process groups.
  88 In order to be sure that
  89 .BR setsid ()
  90 will succeed, call
  91 .BR fork (2)
  92 and have the parent
  93 .BR _exit (2),
  94 while the child (which by definition can't be a process group leader) calls
  95 .BR setsid ().
  96
  97 If a session has a controlling terminal, and the
  98 .B CLOCAL
  99 flag for that terminal is not set,
 100 and a terminal hangup occurs, then the session leader is sent a
 101 .BR SIGHUP
 102 signal.
 103
 104 If a process that is a session leader terminates, then a
 105 .B SIGHUP
 106 signal is sent to each process in the foreground
 107 process group of the controlling terminal.
 108 .SH SEE ALSO
 109 .BR setsid (1),
 110 .BR getsid (2),
 111 .BR setpgid (2),
 112 .BR setpgrp (2),
 113 .BR tcgetsid (3),
 114 .BR credentials (7)