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I would to only processing the msg in A when the subscriber B got all messages from topic "b"(i.e. until there is no new message from b), is it possible to do it?

No. Not with just rostopic alone.

I'm also wondering what you mean by "processing the msg": rostopic does not really process messages. It can only print them to the console, or publish new ones.

And something to consider: when are "all messages on a topic" received? How would you know?