Xlib: capture screen in separate xserver session

[Alex Sviridov]

Hi all.

I need to run some-program which makes screen capturing via xlib library in separate xserver session. So, I start my ubuntu 14 and `origin` xserver session (lets call it this way) starts. After that I run

    startx some-program
After running this command a black screen appears and on this black screen I see my program and it works as expected. That's ok - this is what I need. 

However, firstly I want to continue using my computer and secondly I must run N such programs in N separate xserver sessions. The problem is that when I try to return to my `origin` session by pressing ctrl+alt+f7 some-program continues working however, its screen captures become simple black rectangles (no any content).

This is my code of some-program using which I capture screen:

    Display* display = XOpenDisplay(NULL);
    Window root = DefaultRootWindow(display);
    XWindowAttributes attributes = {0};
    XGetWindowAttributes(display, root, &attributes);
    int width, height;
    width = attributes.width;
    height = attributes.height;
    XImage* img = XGetImage(display, root, 0, 0 , width, height, AllPlanes, ZPixmap);
    ...
    XDestroyImage(img);

Could anyone say what should I do in order to capture screen in separate xserver session while it is not active (the user is using another xsession).


-- 
Alex Sviridov
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