In a university class, we had an assignment that asked us to write a program that would communicate with its copies over POSIX message queues. I remember grepping through the source of all the software I had on my Gentoo installation, and couldn’t find a single one that had any sort of usage or support for POSIX message queues. Does anyone know of even a single app that uses them?

They tend to be used more in real time systems I think.

There are two things: SysV IPC and POSIX IPC, both of which have message queues. In the SysV case this feature probably made sense because it was designed at a time when BSD sockets were not yet the universaly accepted API. POSIX equivalent probably exists mostly for completeness.

One might say that the whole of POSIX IPC is simply SysV IPC with sane and "unix-like" API (ie. names instead of "IPC keys").

One of the problems with POSIX IPC is that you cannot really use it if you want to be portable, because on BSD and derived systems the implementation is best described as also existing only for completeness.

I don't know of anything other than one I wrote back in the late 2000s that let me coordinate a web service with process pool of ERP instances. When I wrote it I remember thinking that ftok could have real issues on a large filesystem. I never ran into any problems so I sort of put the issue out of my mind.