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>
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  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 2017-09-15 "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 <sys/types.h>
  37 .br
  38 .B #include <unistd.h>
  39 .PP
  40 .B pid_t setsid(void);
  41 .br
  42 .ad b
  43 .SH DESCRIPTION
  44 .BR setsid ()
  45 creates a new session if the calling process is not a
  46 process group leader.
  47 The calling process is the leader of the new session
  48 (i.e., its session ID is made the same as its process ID).
  49 The calling process also becomes
  50 the process group leader of a new process group in the session
  51 (i.e., its process group ID is made the same as its process ID).
  52 .PP
  53 The calling process will be the only process in
  54 the new process group and in the new session.
  55 .PP
  56 Initially, the new session has no controlling terminal.
  57 For details of how a session acquires a controlling terminal, see
  58 .BR credentials (7).
  59 .SH RETURN VALUE
  60 On success, the (new) session ID of the calling process is returned.
  61 On error,
  62 .I "(pid_t)\ \-1"
  63 is returned, and
  64 .I errno
  65 is set to indicate the error.
  66 .SH ERRORS
  67 .TP
  68 .B EPERM
  69 The process group ID of any process equals the PID of the calling process.
  70 Thus, in particular,
  71 .BR setsid ()
  72 fails if the calling process is already a process group leader.
  73 .SH CONFORMING TO
  74 POSIX.1-2001, POSIX.1-2008, SVr4.
  75 .SH NOTES
  76 A child created via
  77 .BR fork (2)
  78 inherits its parent's session ID.
  79 The session ID is preserved across an
  80 .BR execve (2).
  81 .PP
  82 A process group leader is a process whose process group ID equals its PID.
  83 Disallowing a process group leader from calling
  84 .BR setsid ()
  85 prevents the possibility that a process group leader places itself
  86 in a new session while other processes in the process group remain
  87 in the original session;
  88 such a scenario would break the strict
  89 two-level hierarchy of sessions and process groups.
  90 In order to be sure that
  91 .BR setsid ()
  92 will succeed, call
  93 .BR fork (2)
  94 and have the parent
  95 .BR _exit (2),
  96 while the child (which by definition can't be a process group leader) calls
  97 .BR setsid ().
  98 .PP
  99 If a session has a controlling terminal, and the
 100 .B CLOCAL
 101 flag for that terminal is not set,
 102 and a terminal hangup occurs, then the session leader is sent a
 103 .BR SIGHUP
 104 signal.
 105 .PP
 106 If a process that is a session leader terminates, then a
 107 .B SIGHUP
 108 signal is sent to each process in the foreground
 109 process group of the controlling terminal.
 110 .SH SEE ALSO
 111 .BR setsid (1),
 112 .BR getsid (2),
 113 .BR setpgid (2),
 114 .BR setpgrp (2),
 115 .BR tcgetsid (3),
 116 .BR credentials (7),
 117 .BR sched (7)