ok don't mind elaborating my problem again, gladly, but just didn't want to bore you guys more.
What I want to do is to measure BER of my receiver built with USRP1 and WBX board. Here is the diagram, it is probably much better than written explanation:
TX path consists of random source and a block that makes digital signal out of pseudorandom bit sequence.
RX path consists of my receiver.
The idea is that top block code sets the number of symbole that will be transmitted, power level, etc ...
My main problem was how to stop RX path after certain number of received symbols. I had following ideas:
1) TX path informs RX path somehow that the sequence has been transmitted. [i didn't know how to proceed with this one]
2) I made a file sink block with limited number written elements which returns -1 after that number is reached.
The practical problem that I had was how to inform upstream blocks from RC path that they should stop. In one moment I figured out that my GUI based top_block also makes troubles cause it blocks in infinite loop and I didn't know how to call some callback in order to "restart" flowgraph.
So this is basically the idea. I am going now with 2) approach. I realized that sink block can also return -1 and the top_block will go out of wait(). If you have any suggestion I would be happy to hear.