int i, j, sum, columns, rows, mem_size, key, shmid, sem_key, semid, *start;
    struct sigaction act = {{0}};
    struct sembuf op;

    rows = atoi(argv[1]); //get the main arguments
    columns = atoi(argv[2]);

    //MEMORY CREATION
    mem_size = (1 + 1 + 1 + 1 + 1 + rows + (rows * columns));
    shmid = mem_create(mem_size);
    start = (int *)shmat(shmid, NULL, 0); //start points to the beggining of the shared memory

    //FILL MEMORY
    mem_fill(rows, columns, start); //grafei ton pinaka tis diastaseis ktl