I am completely new to MPI so I hope I’m not asking anything too silly. I have two computers, one running Ubuntu and the other running Scientific Linux. They recognise each other as hosts, I can ssh from each without a password and have disabled any firewalls. Although I can run a simple Hello World application, the trouble starts when I try to use MPI_SEND and MPI_RECV. I am trying a simple program to send an int variable from one node to the other (see source code below). If I run the code with two processes on the same computer I get the correct output: dan@siriusa:~/mpich2/tutorial2$ mpiexec -n 2 ./result2 Process 0 sending number 4 Process 1 received number 4 from process 0 However, if I try to run the code using both computers, I get: dan@siriusa:~/mpich2/tutorial2$ mpiexec -n 2 -f hosts ./result2 Process 0 sending number 4 Process 1 received number -1078477356 from process 0 My host file: dan@siriusa:~/mpich2/tutorial2$ cat hosts 192.168.88.1 192.168.88.2 So there is some sort of problem with using the send/recieve functions over the network. As I said I am a beginner at this, so any help will be greatly appreciated. Thanks in advance. Code: #include <mpi.h> #include <stdio.h> #include <stdlib.h> int main(int argc, char** argv) { // Initialize the MPI environment MPI_Init(NULL, NULL); // Find out rank, size int world_rank; MPI_Comm_rank(MPI_COMM_WORLD, &world_rank); int world_size; MPI_Comm_size(MPI_COMM_WORLD, &world_size); // Get the name of the processor char processor_name[MPI_MAX_PROCESSOR_NAME]; int name_len; MPI_Get_processor_name(processor_name, &name_len); // We are assuming at least 2 processes for this task if (world_size < 2) { fprintf(stderr, "World size must be greater than 1 for %s\n", argv[0]); MPI_Abort(MPI_COMM_WORLD, 1); } int number; if (world_rank == 0) { // If we are rank 0, set the number to -1 and send it to process 1 number = 4; printf("Process 0 sending number %d\n",number); MPI_Send(&number, 1, MPI_INT, 1, 0, MPI_COMM_WORLD); } else if (world_rank == 1) { MPI_Recv(&number, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE); printf("Process 1 received number %d from process 0\n", number); } MPI_Finalize(); }