The tree to file program "treeo" is changed slightly.
It doesn't open a file, but uses the pre-existing fd 1 (for stdout) instead.

This is its main now:

  int main(int argc, char * argv[])
  { struct node * t = NULL;
    int x, number, i, f;
    int * nums;
    if (argc != 2)
    { fprintf(stderr, "usage: %s number-of-numbers\n", argv[0]);
      exit(1); }
    number = atol(argv[1]); 
  
    srandomdev();
    nums = (int *)malloc(number * sizeof(int));
    fill_with_randoms(nums, number);
    for (i=0; i<number; i+=1)
      t = insert_node(t, nums[i]);
    print_tree(1, t); }  

cc treeo.c -o treeo
    

The file content displayer is also changed in a similar way.
It doesn't read from a file, but fd 0 (for stdin) instead.

This is its main now:

  int main(int argc, char * argv[])
  { while (1)
    { int x;
      int r = read(0, & x, sizeof(x));
      if (r <= 0)
        break;
      printf("%d ", x); }
    printf("\n"); }

cc display.c -o display

The key trick is to "pipe" one into the other, sothey run concurrently, with
treeo's stdout connected to display's standard input, so they communicate
directly.

treeo 16 | display