What happens to a unix session when the session leader exits?

Question

I'm trying to properly emulate POSIX signals handling and job control for my pet operating system, but it's not clear to me what should happen to a session after the session leader exits.

I cannot find documentation related to what happens to the session and its process if, for example, a child kills the session leader while several background processes and a different foreground process are running.

My tests show that all the process in the session are killed, but how?
Do they receive a specific signal?
Is this case specified in the POSIX standard? And if so, can you provide some references?

Answer 1

You are not the only one puzzled by POSIX sessions; Lennart Poettering (he of systemd fame) is puzzled too.

As far as anybody can tell, when a session leader dies, init inherits the orphaned session and

• All session member processes in the foreground process group (if any) receive a SIGHUP.

• Session member processes who are not in the foreground group don't receive any signal.

See also:

• notes.shichao.io/apue/ch9
• Chapter 10 "Processes" in The Linux Kernel by Andries Brouwer (2003).

If the terminal goes away by modem hangup, and the line was not local, then a SIGHUP is sent to the session leader. [...] When the session leader dies, a SIGHUP is sent to all processes in the foreground process group. [...] Thus, if the terminal goes away and the session leader is a job control shell, then it can handle things for its descendants, e.g. by sending them again a SIGHUP. If on the other hand the session leader is an innocent process that does not catch SIGHUP, it will die, and all foreground processes get a SIGHUP.

Andries Brower, The Linux Kernel, section 10.3 "Sessions".