Some recent tasks at work highlighted some obvious gaps in my working knowledge of Linux inter-process communication (IPC). I spent some personal time brushing up on things I had used before and learning about some mechanisms that I had not used before. I am not discussing pipes or sockets here; I am discussing message queues, semaphores, and shared memory. The System V and POSIX specifications have different APIs for using basically the same three IPC mechanisms. I am not exploring every nuance or feature of each API and I am not comparing performance. My goal was simply to demonstrate each IPC mechanism for each API in a set of very simple programs. I show the programs side by side where appropriate, but the code is also available in here.