How can I check receive message in MPI - parallel-processing

Question

I am trying to write MPI Send/Receive code but I have a problem. I want to check receive message. If I have a message from any source, read this. Otherwise, continue code.
MPI_Comm_size(MPI_COMM_WORLD,&p);
MPI_Comm_rank(MPI_COMM_WORLD,&id);
MPI_Status status;
int source,ask,request,answer,tag=10;
for(i=first; i<last; i++){
if(there is a message for my id){
MPI_Recv(&request,1,MPI_INT,MPI_ANY_SOURCE,tag,MPI_COMM_WORLD,&status);
answer = request*request;
source = status.MPI_SOURCE;
MPI_Send(&answer,1,MPI_INT,source,tag+1,MPI_COMM_WORLD);
}
square=i*i;
if(last < square){
MPI_Send(&square,1,MPI_INT,id-1,tag,MPI_COMM_WORLD);
MPI_Recv(&square,1,MPI_INT,id-1,tag+1,MPI_COMM_WORLD,&status);
}
local[i]=square;
}
If I can't check receive message with if statement, Can I check with timeout or wait expression?
*I know this code trivial but I hope it can help us.
Thanks

Answer 1

It's a little hard to tell exactly what you want, but it sounds like what you might be looking for is either a non-blocking receive paired with a test or an MPI_IMPROBE paired with an MPI_MRECV.
So for the non-blocking receive version, your code would end up looking more like:
int flag = 0;
for (i=first; i<last; i++) {
MPI_Irecv(&data, 1, MPI_INT, MPI_ANY_SOURCE, tag, MPI_COMM_WORLD, &mpi_request);
MPI_Test(&mpi_request, &flag, &status);
while (!flag) {
// Do other stuff
MPI_Test(&mpi_request, &flag, &status);
}
// Do stuff with message
}
The version with probe would be slightly different, but in the end, pretty similar:
MPI_Message message;
int flag;
for (i=first; i<last; i++) {
MPI_Improbe(MPI_ANY_SOURCE, tag, MPI_COMM_WORLD, &flag, &message, &status);
while (!flag) {
// Do other stuff
MPI_Test(&mpi_request, &flag, &status);
}
MPI_Mrecv(&data, 1, MPI_INT, &message, &status);
// Do stuff with message
}