Thread: Need some help with shared memory and fork system call

Hello, I'm trying to make a program that will multiply 2 matrices using shmget() and fork(). For example, I would need to multiply a 64 x 64 matrix using 4 processes or 16 processes, and the multi-processes will be created using fork. Each process will calculate a partition of the final Matrix

Now, I'm not sure whether I should write all the matrices to the shared memory or just a single integer to keep track of what partition to calculate. I'm also not sure if I'm using fork correctly, as my print commands print out twice each instead of once.

Code:

#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>


#define DIM 4
#define NUM_OF_FORK_CALLS 2
#define NUM_OF_PROC 4

int main()
{
    pid_t pid;
    int segment_id;
    int *partition;
    *partition = 0;
    int i;
    int j;
    int matrixA[DIM][DIM];
    int matrixB[DIM][DIM];
    int matrixC[DIM][DIM];

    //allocating a shared memory segment
    segment_id = shmget(IPC_PRIVATE, sizeof(int)*(DIM*DIM)*3, IPC_CREAT|0666);

    //attach the shared memory segment to the partition variable
    partition = (int *) shmat(segment_id, NULL,0);

    //fill the matrices
    for(i = 0; i < DIM; i++)
    {
	for(j = 0; j < DIM; j++)
	{
		matrixA[i][j] = i + j;
		matrixB[i][j] = i + 3;
	}
    }

    for(i = 0; i < NUM_OF_FORK_CALLS; i++)
    {
   	pid = fork();
    }


    //error occurred
    if(pid < 0)
    {
        fprintf(stderr, "Fork Failed");
        return 1;
    }

    //child process
    else if(pid==0)
    {

        int start = (*partition * DIM)/NUM_OF_PROC;
        /*The start position will tell which row to start calculating */

        int end = ((*partition+1) * DIM)/NUM_OF_PROC;
        /*The end position wil tell which row to stop calculating*/

        int i;
        int j;
        int k;

         //These for loops will calculate a partition of C
        for(i = start; i<end; i++)
        {
            for(j = 0; j < DIM; j++)
            {
                matrixC[i][j] = 0;

                for(k = 0; k < DIM; k++)
                {
                     matrixC[i][j] += matrixA[i][k] * matrixB[k][j];
                }
             }
        }

    }

    //parent process
    else
    {

        wait(NULL);

        int i;
        int j;

        //print matrix A
        for(i = 0; i < DIM; i++)
        {
            for(j = 0; j<DIM; j++) 
            {
                printf("%d ",matrixA[i][j]);  
            }

            printf("\n");  
        }

        printf("-----------\n\n");

        //print matrix B
        for(i = 0; i < DIM; i++)
        {
            for(j = 0; j<DIM; j++)  
            {
                printf("%d ",matrixB[i][j]);  
            }

            printf("\n");  
        }

         printf("-----------\n\n");

        //print matrix C
        for(i = 0; i < DIM; i++)
        {
            for(j = 0; j<DIM; j++) 
            {
                printf("%d ",matrixC[i][j]);  
            }

            printf("\n"); 
        }
         printf("-----------\n\n");

    }

    return 0;
}

Any help would be appreciated, thanks in advance

Cross-posted