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

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);