/* p. 509, A Book on C, by Al Kelley and Ira Pohl */

/*Uses 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);

main()
{
  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;                             /*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);
}

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);
}