The setsid() function creates a new session, if the calling process is not a process group leader. Upon return the calling process will be the session leader of this new session, will be the process group leader of a new process group, and will have no controlling terminal. The process group ID of the calling process will be set equal to the process ID of the calling process. The calling process will be the only process in the new process group and the only process in the new session.

Upon successful completion, setsid() returns the value of the process group ID of the calling process. Otherwise it returns (pid_t)-1 and sets errno to indicate the error.

The setsid() function will fail if:

EPERM

The calling process is already a process group leader, or the process group ID of a process other than the calling process matches the process ID of the calling process.

See attributes(5) for descriptions of the following attributes:

getsid(2), setpgid(2), setpgrp(2), attributes(5)

A call to setsid() by a process that is a process group leader will fail. A process can become a process group leader by being the last member of a pipeline started by a job control shell. Thus, a process that expects to be part of a pipeline, and that calls setsid(), should always first fork; the parent should exit and the child should call setsid(). This will ensure that the calling process will work reliably when started by both job control shells and non-job control shells.