setsid(2) -- Linux man page
NAMEsetsid - creates a session and sets the process group ID
DESCRIPTIONsetsid() creates a new session if the calling process is not a process group leader. The calling process is the leader of the new session, the process group leader of the new process group, and has no controlling tty. The process group ID and session ID of the calling process are set to the PID of the calling process. The calling process will be the only process in this new process group and in this new session.
RETURN VALUEThe session ID of the calling process.
ERRORSOn error, -1 will be returned. The only error which can happen is EPERM. It is returned when 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.
NOTESA process group leader is a process with process group ID equal to its PID. In order to be sure that setsid will succeed, fork and exit, and have the child do setsid().
CONFORMING TOPOSIX, SVr4.
SEE ALSOsetpgid(2), setpgrp(2)