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SETSID(2) Linux Programmer's Manual SETSID(2)
NAME
setsid - creates a session and sets the process group ID
SYNOPSIS
#include <unistd.h>
pid_t setsid(void);
DESCRIPTION
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).
RETURN VALUE
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.
ERRORS
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.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
NOTES
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.
SEE ALSO
setsid(1), getsid(2), setpgid(2), setpgrp(2), tcgetsid(3), credentials(7), sched(7)
Linux 2021-03-22 SETSID(2)