title: MPI recv from an unknown source
body: <p>I am implementing in MPI a program in which the main process (with rank=0) should be able to receive requests from the other processes who ask for values of variables that are only known by the root. 
  If I make MPI_Recv(...) by the rank 0, I have to specify the rank of the process which sends request to the root, but i cannot control that since the processes don't run in the order 1,2,3,....
  How can I receive the request from any rank and use the number of the emitting process to send it the necessary information?</p>