Are message queues obsolete in linux?

Question

I've been playing with message queues (System V, but POSIX should be ok too) in Linux recently and they seem perfect for my application, but after reading The Art of Unix Programming I'm not sure if they are really a good choice.

http://www.faqs.org/docs/artu/ch07s02.html#id2922148

The upper, message-passing layer of System V IPC has largely fallen out of use. The lower layer, which consists of shared memory and semaphores, still has significant applications under circumstances in which one needs to do mutual-exclusion locking and some global data sharing among processes running on the same machine. These System V shared memory facilities evolved into the POSIX shared-memory API, supported under Linux, the BSDs, MacOS X and Windows, but not classic MacOS.

http://www.faqs.org/docs/artu/ch07s03.html#id2923376

The System V IPC facilities are present in Linux and other modern Unixes. However, as they are a legacy feature, they are not exercised very often. The Linux version is still known to have bugs as of mid-2003. Nobody seems to care enough to fix them.

Are the System V message queues still buggy in more recent Linux versions? I'm not sure if the author means that POSIX message queues should be ok?

It seems that sockets are the preferred IPC for almost anything(?), but I cannot see how it would be very simple to implement message queues with sockets or something else. Or am I thinking too complexly?

I don't know if it's relevant that I'm working with embedded Linux?

Answer 1

I haven't actually used POSIX message queues because I always want to leave open the option to distribute my messages across a network. With that in mind, you might look at a more robust message-passing interface like zeromq or something that implements AMQP

One of the nice things about 0mq is that when used from the same process space in a multithreaded app, it uses a lockless zero-copy mechanism that is quite fast. Still, you can use the same interface to pass messages over a network as well.

Answer 2

Personally I am quite fond of message queues and think they are arguably the most under-utilized IPC in the unix world. They are fast and easy to use.

A couple of thoughts:

• Some of this is just fashion. Old things become new again. Add a shiny do-dad on message queues and they may be next year's newest and hottest thing. Look at Google's Chrome using separate processes instead of threads for its tabs. Suddenly people are thrilled that when one tab locks up it doesn't bring down the entire browser.

• Shared memory has something of a He-man halo about it. You're not a "real" programmer if you aren't squeezing that last cycle out of the machine and MQs are marginally less efficient. For many, if not most apps, it is utter nonsense but sometimes it is hard to break a mindset once it takes hold.

• MQs really aren't appropriate for applications with unbounded data. Stream oriented mechanisms like pipes or sockets are just easier to use for that.

• The System V variants really have fallen out of favor. As a general rule go with POSIX versions of IPC when you can.

Answer 3

Yes, I think that message queues are appropriate for some applications. POSIX message queues provide a nicer interface, in particular, you get to give your queues names rather than IDs, which is very useful for fault diagnosis (makes it easier to see which is which).

Linux allows you to mount the posix message queues as a filesystem and see them with "ls", delete them with "rm" which is quite handy too (System V depends on the clunky "ipcs" and "ipcrm" commands)