XOpenDisplay fails in OS X

I am trying to run an applicatoin that uses X11. The application tries to make a call to XOpenDisplay(NULL), but开发者_如何学Go it always returns NULL. I have set the DISPLAY environment variable set to :0 and the XAUTHORITY environment variable set to ~/.Xauthority. I am not familiar with X11 - is there anything else I need to do to make this call work?

Have you started the X server? /Applications/Utilities/X11.app

It's been a while since I used X, but perhaps "xhost +localhost" or just "xhost +" as a sanity check, but that latter has security issues.