MPI recv from an unknown source

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 t开发者_如何转开发he 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?

This assumes you are using C. There are similar concepts in C++ and Fortran. You would just specify MPI_ANY_SOURCE as the source in the MPI_recv(). The status struct contains the actual source of the message.

int buf[32];
MPI_Status status;
// receive message from any source
MPI_recv(buf, 32, MPI_INT, MPI_ANY_SOURCE, MPI_ANY_TAG, MPI_COMM_WORLD, &status);
int replybuf[];
// send reply back to sender of the message received above
MPI_send(buf, 32, MPI_INT, status.MPI_SOURCE, tag, MPI_COMM_WORLD);

MPI_ANY_SOURCE is the obvious answer.

However, if all the ranks will be sending a request to rank 0, then MPI_Irecv combined with MPI_Testall might also work as a pattern. This will allow the MPI_Send calls to be executed in any order, and the information can be received and processed in the order that the MPI_Irecv calls are matched.