Multiple subscriber queue allocation using the same callback

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The queue_size is specific to each subscriber. It only controls the 'number of incoming messages to queue up for processing (messages in excess of this queue capacity will be discarded).' ( http://docs.ros.org/api/roscpp/html/c... ).

You can have multiple subscribers using the same callback function but with different queue sizes.

e.g sub1 and sub2 both use myCallBack() and subscribe to "/mytopic1" and "/mytopic2" respectively, sub1 has queue_size=1, sub2 has queue_size=10

If I publish 15 identical messages "instantaneously" on both topics (faster than the time it takes to complete myCallBack()), you should see 2 calls to myCallBack() corresponding to sub1 for messages 1 (assuming sub1's callback is processed first) and 15, and 10 calls to myCallBack() corresponding to sub2 for messages 6-15.

Also remember that by default, all the callbacks within a node are invoked from a single global callback queue. You can assign separate callback queues for each of your subscribers if that is what you are looking for, but it takes a bit more work ( http://wiki.ros.org/roscpp/Overview/C... ).