Initialized variable before MPI_Init() and cout after MPI_Finanlize()
c++, linux, mpi
Solution
It's perfectly valid to execute code before `MPI_Init` and after `MPI_Finalize`. Of course you are not allowed to use MPI in that code, but otherwise it's just normal C++.
`MPI_Init` and `MPI_Finalize` are just library calls, they are not supposed to change control flow or remove values assigned prior to init (and how would that be possible at all?).
`MPI_Init` doesn't cancel initialization of variables.
The question you're referring to is saying just that initialization in process 0 doesn't initialize values in other processes. In that question, data were read from file which probably existed only for process 0.
Note that `MPI_Init` doesn't create new process. It's not like `fork`. All the processes are created before your program starts (most likely by `mpirun`). And in your case, each process initializes its array.
`MPI_Finalize` doesn't terminate the process. It only shuts down the MPI library. Processes still continue running after that, although they cannot interact any more.
Problem
I have been testing how exactly mpi works with the following code ``` #include <iostream> #include <mpi.h> using namespace std; int main(int argc, char *argv[]){ r = 3.0; int id; int p; int a[100]; for(int i=0;i<100;++i){a[i]=i+5; } MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &id); MPI_Comm_size(MPI_COMM_WORLD, &p); cout<<id<<" "<<r<<" "<<a[id]<<endl; MPI_Finalize(); cout<< "Hello world " <<endl; return 0; } ``` I am using 30 cores to run the code. but the output is somehow surprising on 2 aspects, - Here this question says that if you initialize a variable before MPI_Init(), the other process but 0 will be blind about the initialized value, but in my output file all the process of different ids output the r=3 and the correct value for `a[i]`, so does this mean that I can initialize a variable or an array before calling MPI_Init() and all processes will share the same value for that variable? - The output has a lot of lines of "Hello world " which apparently indicates that each process is printing "Hello world " even after calling MPI_Finalize(), why is that? By the way, I am using mpicc for compiling the code.