How to send a message without a specific destination in MPI?

Question

I want to send a message to one of the ranks receiving a message with a specific tag. If there is any rank received the message and the message is consumed. In MPI_Recv() we can receive a message using MPI_ANY_SOURCE/MPI_ANY_TAG but the MPI_Send() can't do this. How can I send an message with unknown destination? The MPI_Bcast() can't do it because after receiving, I have to reply to the source process. Thanks.

Answer 1

What I would do is have the worker processes signal to the master that they're ready to receive. The master would keep track of which ranks are ready, pick one (lowest rank first, random, round robin, however you like), send to it, and clear its "ready" flag.

Answer 2

Do you just want to send a message it to a random rank?

 MPI_Comm_size(MPI_COMM_WORLD, &size);
 sendto = rand() % size;
 MPI_Send(buffer, count, MPI_CHAR, sendto, 0, MPI_COMM_WORLD);