setsid - creates a session and sets the process group ID
() 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
Initially, the new session has no controlling terminal. For details of how a
session acquires a controlling terminal, see credentials
On success, the (new) session ID of the calling process is returned. On error,
is returned, and errno
is set to indicate the
- 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
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
(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
If a process that is a session leader terminates, then a SIGHUP
sent to each process in the foreground process group of the controlling