CODEWITHSUNDEEP
c++linuxopengldebianx11
MichaelMitchell
MichaelMitchell

Reputation: 1167

Make OpenGL context with No screens

I am trying to make a opengl context with glx on a debian server. The problem is that I can't make the display and it seems the reason is because there is no X server running and I cannot start an X server with sudo startx because it says there are no screens.

The server is offsite and there is no way of adding a display onto it and I need to make an opengl application that can run on it and render things and some stuff.

This is my current c++ test code:

#include <cstdio>
#include <X11/Xutil.h>
#include <GL/gl.h>
#include <GL/glx.h>

typedef GLXContext (*glXCreateContextAttribsARBProc) (Display*, GLXFBConfig, GLXContext, Bool, const int*);
typedef Bool (*glXMakeContextCurrentARBProc) (Display*, GLXDrawable, GLXDrawable, GLXContext);
static glXCreateContextAttribsARBProc glXCreateContextAttribsARB = NULL;
static glxMakeContextCurrentARBProc glxMakeContextCurrentARB = NULL;

int main(){
    printf("tacos\n");
    glXCreateContextAttribsARB = (glXCreateContextAttribsARBProc) glXGetProcAddressARB((const GLubyte*) "glXCreateContextAttribsARB");
    glXMakeContextCurrentARB = (glXMakeContextCurrentARBProc) glXGetProcAddressARB((const GLubyte*) "glXMakeContextCurrent");

    [ ... ] // Check if the two funcs are null, they are not when I run the program.

    const char* display_name = NULL;
    Display* display = XOpenDisplay(display_name);
    if (display == NULL){
        printf("failed to open display\n"); // outputs this and ends program
        return 0;
    }

    printf("Great Success\n"); // does not get this far ^
    return 0;
}

I check if X Server is running with this:

if ! xset q &>/dev/null; then
    echo "No X server at \$DISPLAY [$DISPLAY]" >&2;
fi

Which outputs the following:

No X server at $DISPLAY []

Which leads me to think the $DISPLAY var is not set, though I don't know how to check if it is set.

I then ran 'sudo startx' and got the following:

Fatal server error:
(EE) no screens found(EE)

Upvotes: 0

Views: 531

Answers (1)

datenwolf
datenwolf

Reputation: 162164

Well, GLX is the X11 OpenGL transport protocol. So you absolutely need an X server running (do you have a GPU at the off-site location available?).

The later versions of the Xorg server in the default configuration will refuse to start if no monitors are connected. However using the right xorg.conf placed in /etc/X11 and with the right command line options you can coax the server into starting even then. However you will either have to start a redirecting composite manager or rewrite your OpenGL programs to use a Framebuffer Objects, otherwise you'll not get a framebuffer to draw to (I highly recommend going the Framebuffer Object route).

Upvotes: 2

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