setsid(2) System Calls Manual setsid(2)
NAME
setsid - creates a session and sets the process group ID
LIBRARY
Standard C library (libc, -lc)
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.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4.
NOTES
A child created via fork(2) inherits its parent's session ID. The ses-
sion 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 ses-
sion 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 control-
ling terminal.
SEE ALSO
setsid(1), getsid(2), setpgid(2), setpgrp(2), tcgetsid(3), creden-
tials(7), sched(7)
Linux man-pages 6.9.1 2024-05-02 setsid(2)
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