SETSID(P) POSIX Programmer's Manual SETSID(P)
setsid - create session and set process group ID
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 ses-
sion, shall be the process group leader of a new process group, and shall have no control-
ling 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.
Upon successful completion, setsid() shall return the value of the new process group ID of
the calling process. Otherwise, it shall return (pid_t)-1 and set errno to indicate the
The setsid() function shall fail if:
EPERM The calling process is already a process group leader, or the process group ID of a
process other than the calling process matches the process ID of the calling
The following sections are informative.
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 setp-
grp(); 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.
getsid() , setpgid() , setpgrp() , the Base Definitions volume of IEEE Std 1003.1-2001,
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SETSID(P)