Handling ungraceful shutdowns when using fork and sockets

Make your server the leader of a process group. In that case children are terminated when the group leader exits.

Where a textual user interface is being used on a Unix-like system, sessions are used to implement login sessions. A single process, the session leader, interacts with the controlling terminal in order to ensure that all programs are terminated when a user "hangs up" the terminal connection. (Where a session leader is absent, the processes in the terminal's foreground process group are expected to handle hangups.)

Use this on your socket before you call listen():

int on = 1;setsockopt (sockfd_wan, SOL_SOCKET, SO_REUSEADDR, &on, sizeof (on));

It allows your programm to use that socket, even it was randomly picked before by another outgoing TCP-connection (cannot happen for ports <1024). But it should also help directly with your problem!!

Unrelated:

There is another bad thing that can happen: If your childs are forked, they inherit EVERY open filedescriptor. If they simply fork and launch another long running programm, those will also have an open handle to your listen-socket, so it stays in use (find out with lsof and netstat command!)

So one should call this:

int close_on_exec_on(int fd){  return fcntl(fd, F_SETFD, FD_CLOEXEC);}close_on_exec_on(sockfd);

But I never tried it in the main programm if it forks off childs and it clearly will not help you because the childs are forked, not run with exec.

But keep it in mind and call it on your listen socket in the main programm anyway! Just in case you run an external programm

Perhaps when you fork, disown the child, so that the parent process isn't the parent registered with the OS. Does the parent really need to communicate with the child? If not this may be an option.

You can keep track of child processes, but in a different way. You won't get SIGCHLD events anymore.