/*  Use pipes to sum N rows concurrently. */

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

#define   N   3

int    add_vector(int v[]);
void   error_exit(char *s);
int    fork(void);
int    pipe(int pd[2]);
int    read(int fd, void *buf, unsigned len);
int    write(int fd, void *buf, unsigned len);

int main(void)
{
   int   a[N][N] = {{1, 1, 1}, {2, 2, 2}, {3, 3, 3}},
         i, row_sum, sum = 0,
         pd[2];                                    /* pipe descriptors */

   if (pipe(pd) == -1)                                /* create a pipe */
      error_exit("pipe() failed");
   for (i = 0; i < N; ++i)
      if (fork() == 0) {                              /* child process */
         row_sum = add_vector(a[i]);
         if (write(pd[1], &row_sum, sizeof(int)) == -1) 
            error_exit("write() failed");
         return 0;                                /* return from child */
      }
   for (i = 0; i < N; ++i) {
      if (read(pd[0], &row_sum, sizeof(int)) == -1) 
         error_exit("read() failed");
      sum += row_sum;
   }
   printf("Sum of the array = %d\n", sum);
   return 0;
}

int add_vector(int v[])
{
   int i, vector_sum = 0;

   for (i = 0; i < N; ++i)
      vector_sum += v[i];
   return vector_sum;
}

void error_exit(char *s)
{
   fprintf(stderr, "\nERROR: %s - bye!\n", s);
   exit(1);
}