What happens when a process is forked?

Question

I've read about fork and from what I understand, the process is cloned but which process? The script itself or the process that launched the script?

For example:

I'm running rTorrent on my machine and when a torrent completes, I have a script run against it. This script fetches data from the web so it takes a few seconds to complete. During this time, my rtorrent process is frozen. So I made the script fork using the following

my $pid = fork();
if ($pid == 0) { blah blah blah; exit 0; }

If I run this script from the CLI, it comes back to the shell within a second while it runs in the background, exactly as I intended. However, when I run it from rTorrent, it seems to be even slower than before. So what exactly was forked? Did the rtorrent process clone itself and my script ran in that, or did my script clone itself? I hope this makes sense.

Answer 1

The fork() function returns TWICE! Once in the parent process, and once in the child process. In general, both processes are IDENTICAL in every way, as if EACH one had just returned from fork(). The only difference is that in one, the return value from fork() is 0, and in the other it is non-zero (the PID of the child process).

So whatever process was running your Perl script (if it is an embedded Perl interpreter inside rTorrent then rTorrent would be the process) would be duplicated at exactly the point that the fork() happened.

Answer 2

I believe I found the problem by looking through rTorrent's source. For some processes, it will read all of the output sent to stdout before continuing. If this is happening to your process, rTorrent will block until you close the stdout process. Because you're forking, your child process shares the same stdout as the parent. Your parent process will exit, but the pipe remains open (because your child process is still running). If you did an strace of rTorrent, I'd bet that it'd be blocked on this read() call while executing your command.

Try closing/redirecting stdout in your perl script before the fork().