XOpenDisplay return display, but set errno to 11

Question

I have a function,that open X11 display, do some stuff, and closes it. I use errno to catch errors throughout the code. After sometime debuffing I learned, that even though XOpenDisplay return the actual pointer (not NULL), errno is set to 11 in this function.

int
take_screenshot()    
{
  /* Get display */
  Display *display = XOpenDisplay(NULL);

  /* XOpenDisplay may set errno to 11
   * even though display is not NULL
   */
  if (!display)
  {
    fprintf(stderr, "Error opening display: %s", strerror(errno));
    return 1;
  }
  XCloseDisplay(display);
  return 0;
}

It works fine, if add just errno = 0 after if statement, but this looks like a bad idea. Also in man page it said that errno initial state is platform-dependent, but i check and it's value is 0 before calling XOpenDisplay. Thank you in advance!

Answer 1

As @Some_programmer_dude metioned, I have mistread errno, which I used as an error indicator, so I should simply check docs and use errno only after NULL check