How to Send/Receive in MPI using all processors

Question

This program written using C Lagrange and MPI. I am new to MPI and want to use all processors to do some calculations, including process 0. To learn this concept, I have written

Answer 1

Actually, processes 1-3 are indeed sending the result back to processor 0. However, processor 0 is stuck in the first iteration of this loop:

for(i=0; i<4; i++)
{      
    MPI_Recv(&number, 1, MPI_INT, i, 99, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    printf("Process 0 received number %d from i:%d\n", number, i);
}

In the first MPI_Recv call, processor 0 will block waiting to receive a message from itself with tag 99, a message that 0 did not send yet.

Generally, it is a bad idea for a processor to send/receive messages to itself, especially using blocking calls. 0 already have the value in memory. It does not need to send it to itself.

However, a workaround is to start the receive loop from i=1

for(i=1; i<4; i++)
{           
    MPI_Recv(&number, 1, MPI_INT, i, 99, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    printf("Process 0 received number %d from i:%d\n", number, i);
}

Running the code now will give you:

Process 1 received number -2 from process 0
Process 2 received number -2 from process 0
Process 3 received number -2 from process 0
Process 0 received number 2 from i:1
Process 0 received number 3 from i:2
Process 0 received number 4 from i:3
Process 0 received number -2 from process 0

Note that using MPI_Bcast and MPI_Gather as mentioned by Gilles is a much more efficient and standard way for data distribution/collection.