setsid - creates a session and sets the process group ID
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 tty. The pro
cess group ID and session ID of the calling process are
set to the PID of the calling process. The calling pro
cess will be the only process in this new process group
and in this new session.
The session ID of the calling process.
On error, -1 will be returned. The only error which can
happen is EPERM. It is returned when 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.
A process group leader is a process with process group ID
equal to its PID. In order to be sure that setsid will
succeed, fork and exit, and have the child do setsid().