MPI recv from an unknown source

Question

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 o

Answer 1

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.