setsid(2) — Linux manual page

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 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)

COLOPHON

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Linux man-pages 6.9.1          2024-05-02                      setsid(2)

Pages that refer to this page: setsid(1), getsid(2), setpgid(2), syscalls(2), daemon(3), posix_spawn(3), tcgetpgrp(3), credentials(7), pthreads(7), pty(7), sched(7), signal-safety(7)

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