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linuxunixipcmessage-queue
Dave Clarke
Dave Clarke

Reputation: 2696

Linux Message Queues - Multiple receivers

I've recently been investigating and playing around with linux message queues and have come across something that I don't quite understand why it happens!

If we have two programs running that are both using msgrcv() in an infinite for loop to check for messages and then send two messages, the first program running will receive the 1st message, and the second program the 2nd message? If you keep sending messages it then alternates between each receiver.

Obviously, I understand that as soon as one program has read the message it is removed from the queue but who/how is it decided who will receive the message if they are all infinitely checking?

Any help would be appreciated!

Upvotes: 2

Views: 1695

Answers (1)

hrunting
hrunting

Reputation: 3947

The short answer is that the kernel decides.

The long answer is that this is handled by the do_msgrcv() call within the Linux kernel. If there is no message available, the caller gets put on a queue until a message is available. It's not guaranteed to go back and forth like you describe, since it all depends on the timing of each msgrcv() call, but in your case, it will probably behave that way virtually all of the time.

Upvotes: 2

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