How do I read arguments from the command line with an MPI program?

Question

How do I read arguments from the command line in C++?

I currently have this code:

int data_size = 0;
std::cout << "Please enter an integer value: ";
std::cin >> data_size;
std::cout << "The value you entered is " << data_size;

Main :

int main(int argc, char** argv) {

        int data_size = 0;
        std::cout << "Please enter an integer value: ";
        std::cin >> data_size;
        std::cout << "The value you entered is " << data_size; 


    // initialise the MPI library
    MPI_Init(NULL, NULL);

    // determine the world size
    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);

    // determine our rank in the world
    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);

    std::cout << "rank " << world_rank << " size " << world_size << std::endl;

    if (world_rank == 0){
        coordinator(world_size);
    }
    else{
        participant(world_rank, world_size);
    }

    MPI_Finalize();

    return 0;
}

It works but it keeps asking me to enter an integer value 4 times then when I enter it a number it the command line freezes.

here is what i get in the command line

C:\Users\Roland\Documents\Visual Studio 2013\Projects\DistributedSystems\Debug>m
piexec -n 4 .\DistributedSystems.exe
Please enter an integer value:
Please enter an integer value:
Please enter an integer value:
Please enter an integer value: 

Answer 1

With MPI programs, reading stuff with std::cin is not a good idea. I have no idea how you could make it work that way and you just shouldn't.

Here are your alternatives though:

If the input to your code is small enough to be passed as command line argument, do so. In your example, your input code block would change to

// Do some error handling if needed, then
int data_size = std::atoi(argv[1]);

and the start the job like

mpiexec -n 4 .\DistributedSystems.exe k

with k being the number you want data_size to be.

If you should get to a point where the amount input is to large for convenient use like this, write it in a file and pass the input filename as above. Then, every process can open that file in its own std::ifstream and read the data from there.

According to Rob Latham , this working is an implementation specific behavior. You can however generally expect this to work if your system uses a command line interface.