How to programmatically (C/C++) verify if X11 forwarding is enabled?

Question

The application is run remotely on target hardware. User can choose to enable visual debug output (using command line switches) in that case a ssh's X11 forwarding is used to view it.

The problem starts if user forgets to establish the connection before running the application like this: ssh -YC remote.host. The application will crash.

I would like to detect this situation beforehand and print proper message while starting the application. Is it possible to do it in C/C++? In the worst case I can execute some shell commands in the background.

Answer 1

You could use XOpenDisplay man page

#include <X11/Xlib.h>
#include <cstdlib>
#include <iostream>

bool displayIsValid()
{
    char* disp = getenv("DISPLAY");
    if ( disp==nullptr ) return false;
    Display* dpy = XOpenDisplay(disp);  
    if ( dpy==nullptr ) return false;
    XCloseDisplay(dpy);
    return true;
}

int main() {
    if ( displayIsValid() ) {
        std::cout << "Display is valid" << std::endl;
    }
}

Answer 2

If you have an X11 display (real or forwarded by ssh), you have a DISPLAY environment variable. Otherwise it should not be present.

The least invasive check is probably:

if (!getenv("DISPLAY")) {
    fprintf(stderr, "missing X11 display, have you forgotten something?\n");
    exit(1);
}

This should trigger just fine if someone forgets X forwarding in ssh.