multiplying 2 2D arrays using fork()

Question

i am trying to multiply a[2][3] with b[3][4] and put the answer in c[2][4]. I have to use fork() and make 2 processes, each child for c[0][j] and c[1][j], parent will wait. So each child process will calculate a row for c. This is my code :

//multiply a[2][3] * b[3][4] = c[2][4]
//with fork
//each child completes one c[2][4]'s row
#include<stdio.h>
#include<stdlib.h>

main()
{
  int a[2][3]={1,2,3, 
           4,5,6};
  int b[3][4]={1,2,3,4,
           5,6,7,8,
           9,10,11,12};
  int c[2][4];
      int i,j;
  int m;    //for the calculation
  int n;    //common 3
  int pid;  //fork

  for(i=0;i<2;i++)
  {
    pid=fork();
    if(pid==-1)
    {
      printf("Can't fork\n");
    }
    if(pid==0)//child
    {
      for(j=0;j<4;j++)
      {
    for(n=0;n<3;n++)
    {
      m=m+a[i][n]*b[n][j];
    }
    c[i][j]=m;
    m=0;
      }
      exit(EXIT_SUCCESS);
    }
    if(pid>0)//parent
    {
      wait(0);
    }
  }
  printf("C:\n");
  for(i=0;i<2;i++)
  {
    for(j=0;j<4;j++)
    {
      printf("%d\t",c[i][j]);
    }
    printf("\n");
  }        
}

I have no errors at compiling, but i get wrong numbers...awkward big numbers ... Any ideas?

Answer 1

Q: I have no errors at compiling, but i get wrong numbers...awkward big numbers ... Any ideas?

A: Yes - the moment you fork, you have two different processes with two completely independent address spaces :)

One good solution is to use threads instead of fork():

https://computing.llnl.gov/tutorials/pthreads/