text: /** pipe.c
   *	* Demonstrates how to create a pipeline from one process to another
   *	* Takes two args, each a command, and connects
   *	  av[1]'s output to input of av[2]
   *	* usage: pipe command1 command2
   *	  effect: command1 | command2
   *	* Limitations: commands do not take arguments
   *	* uses execlp() since known number of args
   *	* Note: exchange child and parent and watch fun
   */
  #include	<stdio.h>
  #include	<unistd.h>
  
  #define	oops(m,x)	{ perror(m); exit(x); }
  
  main(int ac, char **av)
  {
      int	thepipe[2],			/* two file descriptors	*/
              newfd,				/* useful for pipes	*/
              pid;				/* and the pid		*/
  
      if ( ac != 3 ){
          fprintf(stderr, "usage: pipe cmd1 cmd2\n");
          exit(1);
      }
      if ( pipe( thepipe ) == -1 )		/* get a pipe		*/
      oops("Cannot get a pipe", 1);
  
      /* ------------------------------------------------------------ */
      /*	now we have a pipe, now let's get two processes		*/
  
      if ( (pid = fork()) == -1 )			/* get a proc	*/
      oops("Cannot fork", 2);
  
      /* ------------------------------------------------------------ */
      /* 	Right Here, there are two processes			*/
      /*             parent will read from pipe			*/
  
      if ( pid > 0 ){			/* parent will exec av[2]	*/
          close(thepipe[1]);	/* parent doesn't write to pipe	*/
  
          if ( dup2(thepipe[0], 0) == -1 )
          oops("could not redirect stdin",3);
  
          close(thepipe[0]);	/* stdin is duped, close pipe	*/
          execlp( av[2], av[2], NULL);
          oops(av[2], 4);
      }
  
      /*	 child execs av[1] and writes into pipe			*/
  
      close(thepipe[0]);		/* child doesn't read from pipe	*/
  
      if ( dup2(thepipe[1], 1) == -1 )
      oops("could not redirect stdout", 4);
  
      close(thepipe[1]);		/* stdout is duped, close pipe	*/
      execlp( av[1], av[1], NULL);
      oops(av[1], 5);
  }