MPI and global variables

Question

I have to implement an MPI program. There are some global variables (4 arrays of float numbers and other 6 single float variables) which are first inizialized by the main pr

Answer 1

I fear you have misunderstood.

It is probably best to think of each MPI process as an independent program, albeit one with the same source code as every other process in the computation. Operations that process 0 carries out on variables in its address space have no impact on the contents of the address spaces of other processes.

I'm not sure that the MPI standard even requires process 0 to have values for variables which were declared and initialised prior to the call to mpi_init, that is before process 0 really exists.

Whether it does or not you will have to write code to get the values into the variables in the address space of the other processes. One way to do this would be to have process 0 send the values to the other processes, either one by one or using a broadcast. Another way would be for all processes to read the values from the input files; if you choose this option watch out for contention over i/o resources.

In passing, I don't think it is common for MPI implementations to create processes by forking at the call to mpi_init, forking is more commonly used for creating threads. I think that most MPI implementations actually create the processes when you make a call to mpiexec, the call to mpi_init is the formality which announces that your program is starting its parallel computations.