intro to semaphores   (link)

http://www.ecst.csuchico.edu/~beej/guide/ipc/semaphores.html
Remember file locking? Well, semaphores can be thought of as really generic advisory locking mechanisms. You can use them to control access to files, shared memory, and, well, just about anything you want. The basic functionality of a semaphore is that you can either set it, check it, or wait until it clears then set it ("test-n-set"). No matter how complex the stuff that follows gets, remember those three operations.

This document will provide an overview of semaphore functionality, and will end with a program that uses semaphores to control access to a file. (This task, admittedly, could easily be handled with file locking, but it makes a good example since it's easier to wrap your head around than, say, shared memory.)