// test pipe() and dup2()
//
#include <stdio.h>
#include <unistd.h>

int main( void)
{
  int fd[2];

  if( pipe(fd) < 0) { perror("pipe"); return 1; }

  int pid = fork(); if( pid < 0) { perror("fork"); return 1; }

  if( pid == 0) // child
  {
    dup2( fd[0], 0); // make stdin come from pipe

    close(fd[0]); close(fd[1]); // fd no longer needed, close it

    int x = 0; scanf("%d",&x); // read from parent

    printf( "x = %d\n", x);
  }
  else // parent
  {
    dup2( fd[1], 1); // make stdout go to pipe

    close(fd[0]); close(fd[1]); // fd no longer needed, close it

    printf( "%d", 123); // write to child
  }

  return 0;
} 

/* output:

x = 123

*/