How to create a "core" OpenGL context with GLX?

Question

I want to create a OpenGL context using GLX with "core" profile.

For comparison's sake, QOpenGLContext can be created with QGLFormat::CoreProfile.

I have found these instructions: Creating a modern OpenGL context but I see no mention of core/compatibility profiles.

/* gcc this.c -lGL -lX11 */

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <GL/glx.h>
#include <X11/Xlib.h>

/*
    License: Public domain

    Contents
    --------
    - Create_display_and_window
    - Create_the_modern_OpenGL_context
    - Show_the_window
    - Application_loop
*/

typedef GLXContext (*glXCreateContextAttribsARBProc)
    (Display*, GLXFBConfig, GLXContext, Bool, const int*);

int main()
{
    Display* disp = 0;
    Window win = 0;

    /* Create_display_and_window
       -------------------------
       Skip if you already have a display and window */
    disp = XOpenDisplay(0);
    win = XCreateSimpleWindow(disp, DefaultRootWindow(disp),
                              10, 10,   /* x, y */
                              800, 600, /* width, height */
                              0, 0,     /* border_width, border */
                              0);       /* background */

    /* Create_the_modern_OpenGL_context
       -------------------------------- */
    static int visual_attribs[] = {
        GLX_RENDER_TYPE, GLX_RGBA_BIT,
        GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT,
        GLX_DOUBLEBUFFER, true,
        GLX_RED_SIZE, 1,
        GLX_GREEN_SIZE, 1,
        GLX_BLUE_SIZE, 1,
        None
    };

    int num_fbc = 0;
    GLXFBConfig *fbc = glXChooseFBConfig(disp,
                                         DefaultScreen(disp),
                                         visual_attribs, &num_fbc);
    if (!fbc) {
        printf("glXChooseFBConfig() failed\n");
        exit(1);
    }

    /* If we were on Windows (i.e. WGL), we would need to create an old
       dummy OpenGL context here, before calling GetProcAddress(). This is
       unnecessary on Linux (GLX).

       For details, refer to the spec
       (https://www.khronos.org/registry/OpenGL/extensions/ARB/GLX_ARB_get_proc_address.txt)
       which says:
           > Are function pointers context-independent? Yes. The pointer to an
           > extension function can be used with any context [...]

       This is in direct contrast to WGL's wglGetProcAddress. */

    glXCreateContextAttribsARBProc glXCreateContextAttribsARB = 0;
    glXCreateContextAttribsARB =
        (glXCreateContextAttribsARBProc)
        glXGetProcAddress((const GLubyte*)"glXCreateContextAttribsARB");

    /* If we were on Winows, we would destroy the dummy context here. Again,
       this is unnecessary on Linux.*/

    if (!glXCreateContextAttribsARB) {
        printf("glXCreateContextAttribsARB() not found\n");
        exit(1);
    }

    /* Set desired minimum OpenGL version */
    static int context_attribs[] = {
        GLX_CONTEXT_MAJOR_VERSION_ARB, 4,
        GLX_CONTEXT_MINOR_VERSION_ARB, 2,
        None
    };
    /* Create modern OpenGL context */
    GLXContext ctx = glXCreateContextAttribsARB(disp, fbc[0], NULL, true,
                                                context_attribs);
    if (!ctx) {
        printf("Failed to create OpenGL context. Exiting.\n");
        exit(1);
    }

    /* Show_the_window
       --------------- */
    XMapWindow(disp, win);
    glXMakeCurrent(disp, win, ctx);

    int major = 0, minor = 0;
    glGetIntegerv(GL_MAJOR_VERSION, &major);
    glGetIntegerv(GL_MINOR_VERSION, &minor);
    printf("OpenGL context created.\nVersion %d.%d\nVendor %s\nRenderer %s\n",
           major, minor,
           glGetString(GL_VENDOR),
           glGetString(GL_RENDERER));

    /* Application_loop
       ---------------- */
    while(1) {}

    return 0;
}

The khronos.org documentation says

You can detect which profile the context supports with this query:

glGetIntegerv(GL_CONTEXT_PROFILE_MASK, *);

This can contain the bits GL_CONTEXT_CORE_PROFILE_BIT or GL_CONTEXT_COMPATIBILITY_PROFILE_BIT, but not both at the same time.

but this is querying the capabilities of a context which was already created.

How do I create a core context using GLX?

Answer 1

Your context_attribs should include GLX_CONTEXT_PROFILE_MASK_ARB parameter (like in this answer GLX Context Creation Error: GLXBadFBConfig):

    static int context_attribs[] = {
        GLX_CONTEXT_MAJOR_VERSION_ARB, 4,
        GLX_CONTEXT_MINOR_VERSION_ARB, 2,
        GLX_CONTEXT_PROFILE_MASK_ARB,  GLX_CONTEXT_CORE_PROFILE_BIT_ARB,
        None
    };

    GLXContext ctx = glXCreateContextAttribsARB(disp, fbc[0], NULL, true,
                                                context_attribs);