setsid(2)

Purpose

setsid() creates a new session. The calling process becomes the session leader (session ID = its PID), the leader of a new process group (PGID = its PID), and the only member of both. This is how daemons detach from a controlling terminal and how shells start background jobs in their own session.

Synopsis

#include <unistd.h>
pid_t setsid(void);

Arguments and behavior

No arguments. If the calling process is not a process group leader, setsid() creates a new session with the caller as leader and sole member; the new session initially has no controlling terminal (setsid.2, DESCRIPTION). A process group leader is a process whose PGID equals its PID. If that is already true of the caller, the call fails with EPERM – disallowed so a group leader cannot slip into a new session while the rest of its group stays behind, breaking the strict two-level session/process-group hierarchy (setsid.2, NOTES). To guarantee success, fork(2), have the parent _exit(2), and let the child – which cannot be a group leader – call setsid(). A child inherits its parent’s session ID, and the session ID survives execve(2) (setsid.2, NOTES).

Return value and errors

On success the new session ID is returned; on error (pid_t) -1 with errno (setsid.2, RETURN VALUE). Errors: EPERM, the process group ID of any process equals the PID of the calling process – in particular, the caller is already a process group leader (setsid.2, ERRORS).

Flags and constants

None.

Security and notes

Controlling-terminal consequences (setsid.2, NOTES): if the session’s terminal hangs up and CLOCAL is not set, the session leader is sent SIGHUP; when a session leader terminates, SIGHUP goes to the foreground process group of the controlling terminal. The setsid(1) utility wraps this for launching commands in new sessions. Cross-reference: setpgid, getsid, fork, _exit.

References

• https://man7.org/linux/man-pages/man2/setsid.2.html
• /usr/include/unistd.h (setsid prototype)
• worklog Task 1-a (session family facts)