Programming

  if(pipe(p_stdin) != 0 || pipe(p_stdout) != 0) {
    fprintf(stderr, "Aufruf von pipe() schlug fehl.\n");
    exit(1);
  }

  if((pid = fork()) < 0) {
    fprintf(stderr, "Aufruf von fork() schlug fehl.\n");
    exit(1);

  // kinderprozess
  } else if(pid == 0) {
    close(p_stdin[WRITE]);
    dup2(p_stdin[READ], READ);
    close(p_stdout[READ]);
    dup2(p_stdout[WRITE], WRITE);

    // factor aurufen
    execl("/bin/sh", "sh", "-c", "factor", NULL);
    perror("execl");
    exit(1);
  }

  close(p_stdin[READ]);
  close(p_stdout[WRITE]);

  // zahlen einlesen
  do {
    assert(fgets(buf, 128, stdin) != NULL);
    
    if((number = atoi(buf)) > 0) {
      // zahl an factor übergeben
      buf[length = strlen(buf)] = '\n';
      assert(write(p_stdin[WRITE], buf, length + 1) != -1);

      // ergebniss auslesen
      close(p_stdin[WRITE]); // <- this musst be removed
      assert(read(p_stdout[READ], buf, 128) != -1);
      printf("Die Zufallszahl %d hat die Primfaktoren %s", number, buf + length + 1);
    }
  } while(number > 0);

 // kinderprozess
  } else if(pid == 0) {
    close(p_stdin[WRITE]);
    dup2(p_stdin[READ], READ);
    close(p_stdout[READ]);
    dup2(p_stdout[WRITE], WRITE);
    setvbuf(stdout, NULL, _IONBF, 0);
    ...

factor | more
123
456
(no output)
x
factor: `x' is not a valid positive integer
123: 3 41
456: 2 2 2 3 19