Section: Linux Programmer's Manual (2)
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 new session.
Initially, the new session has no controlling terminal.
For details of how a session acquires a controlling terminal, see
On success, the (new) session ID of the calling process is returned.
is returned, and
is set to indicate the error.
The process group ID of any process equals the PID of the calling process.
Thus, in particular,
fails if the calling process is already a process group leader.
POSIX.1-2001, POSIX.1-2008, SVr4.
A child created via
inherits its parent's session ID.
The session ID is preserved across an
A process group leader is a process whose process group ID equals its PID.
Disallowing a process group leader from calling
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
will succeed, call
and have the parent
while the child (which by definition can't be a process group leader) calls
If a session has a controlling terminal, and the
flag for that terminal is not set,
and a terminal hangup occurs, then the session leader is sent a
If a process that is a session leader terminates, then a
signal is sent to each process in the foreground
process group of the controlling terminal.
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