setsid(2)

1setsid(2)                     System Calls Manual                    setsid(2)
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NAME

6       setsid - creates a session and sets the process group ID
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <unistd.h>
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14       pid_t setsid(void);
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DESCRIPTION

17       setsid()  creates a new session if the calling process is not a process
18       group leader.  The calling process is the leader  of  the  new  session
19       (i.e., its session ID is made the same as its process ID).  The calling
20       process also becomes the process group leader of a new process group in
21       the session (i.e., its process group ID is made the same as its process
22       ID).
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24       The calling process will be the only process in the new  process  group
25       and in the new session.
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27       Initially, the new session has no controlling terminal.  For details of
28       how a session acquires a controlling terminal, see credentials(7).
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RETURN VALUE

31       On success, the (new) session ID of the calling  process  is  returned.
32       On  error, (pid_t) -1 is returned, and errno is set to indicate the er‐
33       ror.
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ERRORS

36       EPERM  The process group ID of any process equals the PID of the  call‐
37              ing process.  Thus, in particular, setsid() fails if the calling
38              process is already a process group leader.
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STANDARDS

41       POSIX.1-2008.
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HISTORY

44       POSIX.1-2001, SVr4.
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NOTES

47       A child created via fork(2) inherits its parent's session ID.  The ses‐
48       sion ID is preserved across an execve(2).
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50       A  process  group leader is a process whose process group ID equals its
51       PID.  Disallowing a process group leader from calling setsid() prevents
52       the possibility that a process group leader places itself in a new ses‐
53       sion while other processes in the process group remain in the  original
54       session;  such a scenario would break the strict two-level hierarchy of
55       sessions and process groups.  In order to be sure  that  setsid()  will
56       succeed,  call  fork(2)  and  have the parent _exit(2), while the child
57       (which by definition can't be a process group leader) calls setsid().
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59       If a session has a controlling terminal, and the CLOCAL flag  for  that
60       terminal  is  not  set,  and a terminal hangup occurs, then the session
61       leader is sent a SIGHUP signal.
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63       If a process that is a session leader terminates, then a SIGHUP  signal
64       is sent to each process in the foreground process group of the control‐
65       ling terminal.
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