About

psmq is publish ssubscribe message queue. It's a set of programs and libraries to implement publish/subscribe way of inter process communication on top of POSIX message queue. This program targets mainly small embedded systems, which do not have access to some more sophisticated IPC (like unix domain socket). It could also prove useful when you want to realy heavly on message priority, since every message has set a priority and broker processes messages from highest ones first. It's main focues is memory and runtime safety, speed is treated as second class citizen - but of course is not ignored completely, it's just memory and safety comes first. Always.

Why?

Good, question. Why would you want to add more code, to your project, when you can do all this thing using ordinary queues - after all psmq is implemented on top of mqueue. Well, if you only communicate processes in 1:1 way, you won't find this library useful - it's easy enough to create 2 queues and send/receive from them. No, publish subscribe shines best when you need to communicate N:N processes. That is for example, you have process 1 that handles CAN hardware and reads battery level. Now couple of processes might be interested in battery level, so you simply publish that information on "/battery/level" topic and any other processes might subscribe to it and read it. Some examples who could be interested in such messages:

graphical user interface - for nice displaying battery status
engine control unit - to shut down engine to prevent battery damage
disgnostic module for monitoring battery live and performance.

Note that these processes could also receive multiple battery information (if there are more than 1 battery). So instead of creating bunch of queues yourself and sending multiple messages to different processes, remembering who wants what, you can shift responsibility to receiving end, since application knows best what it needs, so it can just subscribe to messages, and your client will just send frame to broker and forget about it. Of course, you could do all of this in one process, or multiple threads that share such information easily, but it's poor design.

Documentation

For detailed information about using broker and library, check out man pages.

Examples

Example of receiving message is in psmq_receive(3) manual page, and example of publishing message is in psmq_publish(3) manual page.

You can also study psmq-pub and psmq-sub source code.

Dependencies

All code follows ANSI C and POSIX standard (version 200112). External dependencies are:

>=embedlog-0.5

psmq-0.1.0 requires * =embedlog-0.4

Test results

NOTE: these represent test results from master branch and may, from time to time, show failures. Tagged versions always pass all tests on all architectures. No exceptions.