Query in MPI initialization

Question

If we call MPI_Init() we know that multiple copies of the same executable run on different machines. Suppose MPI_Init() is in a function f(), then will multiple copies of main() function exist too?

The main problem that I am facing is of taking inputs. In effect, what is happening is that input is being taken once but the main function is running several times. The processor with rank 0 always seems to have the input, rest of them have random values. So to send the values do we have to broadcast the input from processor 0 to all the other processors?

Answer 1

MPI_Init() doesn't create multiple copies, it just initializes in-process MPI library. Multiple copies of your process are created before that, most probably with some kind of mpirun command (that is how you run your MPI application). All processes are independent from the beginning, so answering the first part of your question — yes, multiple copies of main() will exist, and they will exist even if you don't call MPI_Init.

The answer to your question about inputs depends on nature of the inputs: if it's typed in from console, then you have to input the values only in one process (e.g. rank 0) and then broadcast them. If the inputs are in some file or specified as a command-line argument, then all processes can access them.