The setsid() function shall create a new session, if the calling process
is not a process group leader. Upon return the calling process shall be the
session leader of this new session, shall be the process group leader of a new
process group, and shall have no controlling terminal. The process group ID of
the calling process shall be set equal to the process ID of the calling
process. The calling process shall be the only process in the new process
group and the only process in the new session.
The setsid() function is similar to the setpgrp() function of
System V. System V, without job control, groups processes into process groups
and creates new process groups via setpgrp(); only one process group
may be part of a login session.
Job control allows multiple process groups within a login session. In order to
limit job control actions so that they can only affect processes in the same
login session, this volume of IEEE Std 1003.1-2001 adds the
concept of a session that is created via setsid(). The setsid()
function also creates the initial process group contained in the session.
Additional process groups can be created via the setpgid() function. A
System V process group would correspond to a POSIX System Interfaces session
containing a single POSIX process group. Note that this function requires that
the calling process not be a process group leader. The usual way to ensure
this is true is to create a new process with fork() and have it call
setsid(). The fork() function guarantees that the process ID of
the new process does not match any existing process group ID.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html