Why is my MPI program outputting incorrectly

I am a newbie in MPI and I have a homework, I am not asking for you to solve it, I only need a hint on why my program is malfunctioning.

Here is the problem

Write a MPI C program that simulates a ping pong table game. 2 processes should only be used. Processes use MPI_Send and MPI_Recv to continually bounce messages off to each other, a specific number of times. The message is formed of an integer count variable that is incremented by each process before being sent. The count variable is initialized to zero prior to starting the game.

Output

A text file called ping_pong_output.txt formed as the following example: if count = 5

Process 0 started the game and initialized the count Process 0 incremented the count ( 1 ) and sent it to process 1

Process received the count Process 0 incremented the count ( 2 ) & sent it back to process 0

Process received the count Process 0 incremented the count ( 3 ) & sent it back to process 1

Process received the count Process 0 incremented the count ( 4 ) & sent it back to process 0

Process received the count Process 0 incremented the count ( 5 ) & sent it back to process 1

Note the following:

The processes take turns being the sender and receiver (need to use %)
The program should allow only 2 processes to play the game

The code I wrote

#include <stdlib.h>
#include <stdio.h>
#include <mpi.h>
#include <time.h>

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

        int size=0,my_rank=0,tag=1,count;
        MPI_Status status;
        MPI_Init(&argc,&argv);
        MPI_Comm_size(MPI_COMM_WORLD,&size);
        MPI_Comm_rank(MPI_COMM_WORLD,&my_rank);
        MPI_Barrier(MPI_COMM_WORLD);

        if (my_rank==0){
                count=0;
                count++;
         printf("Process 0 started the game and initialized the count\nProcess 0 incremented the count and sent it to process 1\n");

         MPI_Send(&count,1,MPI_INT,1,tag,MPI_COMM_WORLD);


                }
        MPI_Barrier(MPI_COMM_WORLD);
        while (count<=5){
                MPI_Barrier(MPI_COMM_WORLD);

                MPI_Recv(&count,1,MPI_INT,(my_rank+1)%2,tag,MPI_COMM_WORLD,&status);
                printf("Process %d received the count\n",my_rank);

                count++;
                MPI_Send(&count,1,MPI_INT,(my_rank+1)%2,tag,MPI_COMM_WORLD);
                printf("process %d incremented the count (%d) & sent it back to process %d\n",my_rank,count,(my_rank+1)%2);
        }

                MPI_Finalize();
                return 0;
}

The output I got is :

Process 1 received the count

process 1 incremented the count (2) & sent it back to process 0

Process 1 received the count

process 1 incremented the count (4) & sent it back to process 0

Process 1 received the count

process 1 incremented the count (6) & sent it back to process 0

Process 0 started the game and initialized the count

Process 0 incremented the count and sent it to process 1

Process 0 received the count

process 0 incremented the count (3) & sent it back to process 1

Process 0 received the count

process 0 incremented the count (5) & sent it back to process 1

Process 0 received the count

process 0 incremented the count (7) & sent it back to process 1

I don't understand why process 1 is running first, the loop is even running before the if statement. All those MPI_Barrier are probably useless but they came out of desperation. Any help is appreciated.

回答1:

For anyone who might be looking for the answer, i managed to find out that you should let one process handle all the printing.

回答2:

That is one approach. The problem you're encountering is related to output flushing. Using printf will send data to stdout, but stdout doesn't flush immediately. Since each rank will flush at its own time (in this case most likely at the end of execution), you'll get everything at the end, and both ranks will be grouped together. The easiest solution I know which will maintain the original structure of your program is to add fflush(stdout) after each printf call. This forces the buffer to flush which will display the output.