I'm afraid that I cannot rely upon a database being present as this is some very simple standalone code. (Which also runs under Windows).

 The basic code is something that forks to serve HTTP like requests and I wish to have a means of determining which requests are "active". As the files served are very large it's very likely that requests take a significant amount of time to process.

 I think that I could use a seperate file for each slave - so that when the child forms it will write to a file "/var/server/child-ip.$$" and the master parent can read these .

 That's a more basic idea than I'd considered but I see nothing wrong with it, and there's no synchronisation issues to deal with at all.