setsid - creates a session and sets the process group ID
Standard C library (
libc,
-lc)
#include <unistd.h>
pid_t setsid(void);
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 (i.e., its
session ID is made the same as its 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 its process ID).
The calling process will be the only process in the new process group and in the
new session.
Initially, the new session has no controlling terminal. For details of how a
session acquires a controlling terminal, see
credentials(7).
On success, the (new) session ID of the calling process is returned. On error,
(pid_t) -1 is returned, and
errno is set to indicate the
error.
- EPERM
- The process group ID of any process equals the PID of the
calling process. Thus, in particular, setsid() fails if the calling
process is already a process group leader.
POSIX.1-2001, POSIX.1-2008, SVr4.
A child created via
fork(2) inherits its parent's session ID. The session
ID is preserved across an
execve(2).
A process group leader is a process whose process group ID equals its PID.
Disallowing a process group leader from calling
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
setsid() will succeed, call
fork(2) and have the parent
_exit(2), while the child (which by
definition can't be a process group leader) calls
setsid().
If a session has a controlling terminal, and the
CLOCAL flag for that
terminal is not set, and a terminal hangup occurs, then the session leader is
sent a
SIGHUP signal.
If a process that is a session leader terminates, then a
SIGHUP signal is
sent to each process in the foreground process group of the controlling
terminal.
setsid(1),
getsid(2),
setpgid(2),
setpgrp(2),
tcgetsid(3),
credentials(7),
sched(7)