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setsid.2: Rewrite some pieces and add some details 

Among other changes, add an explanation of why setsid() can't
be called from a process group leader

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2014-11-25 23:31:55 +01:00
parent a00ca3d5a0
commit d6c9f01b64
1 changed files with 17 additions and 8 deletions
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@ -1,5 +1,6 @@
.\" 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>
.\"
.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
.\" This is free documentation; you can redistribute it and/or
@ -41,13 +42,15 @@ setsid \- creates a session and sets the process group ID
.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, the process group leader of the new process group, and has no
controlling terminal.
The process group ID and session ID of the calling
process are set to the PID of the calling process.
The calling process is the leader of the new session
(i.e., its session ID is made the same as it 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 it process ID).
The calling process will be the only process in
this new process group and in this new session.
the new process group and in the new session.
The new session has no controlling terminal.
.SH RETURN VALUE
On success, the (new) session ID of the calling process is returned.
On error,
@ -71,8 +74,14 @@ inherits its parent's session ID.
The session ID is preserved across an
.BR execve (2).
A process group leader is a process with process group ID equal
to its PID.
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,