Specifying your OpenGL context version on Linux

References

OpenGL added features from 1.0 through 3.0. At that point, it was decided that the older fixed-function API should be deprecated. In 3.1, many features were removed, and from 3.2 onward two profiles differentiated these differences: core and compatibility. Core profile only has the new API, and compatibility still supports the deprecated API. When you request a specific version, the graphics driver will give you a version compatible with the version you requested. These days that most often means a 4.6 compatibility or core profile context.

Build with:

gcc main.c -lX11 -lGL

    const int fb_attributes[] = {
        GLX_X_RENDERABLE, True,
        GLX_DOUBLEBUFFER, True,
        GLX_RED_SIZE, 8, GLX_GREEN_SIZE, 8, GLX_BLUE_SIZE, 8, GLX_DEPTH_SIZE, 24,
        None
    };

    int fb_count;
    GLXFBConfig* fb_configs = glXChooseFBConfig(display, screen, fb_attributes, &fb_count); assert(fb_configs && fb_count > 0);
    GLXFBConfig fb_config = fb_configs[0];
    XFree (fb_configs);

    XVisualInfo* visual = glXGetVisualFromFBConfig(display, fb_config); assert(visual);

gl_attributes and glXChooseVisual have been replaced with fb_attributes, glXChooseFBConfig and glXGetVisualFromFBConfig. These are needed to get a frame buffer configuration for use later when specifying the OpenGL context version. For a list of fb_attributes options, see the glXChooseFBConfig specification.

auto gl_context_temp = glXCreateContext (display, visual, 0, true); assert (gl_context_temp);

You may be surprised to see that we're creating a context without specifying the version! In order to use the GL functions required to create a specific context version, we need to already have a valid OpenGL context, so we create one the non-specific way initially.

typedef GLXContext (*glXCreateContextAttribsARB_t) (Display *dpy, GLXFBConfig config, GLXContext share_context, Bool direct, const int *attrib_list); glXCreateContextAttribsARB_t glXCreateContextAttribsARB;

    glXCreateContextAttribsARB = (glXCreateContextAttribsARB_t)glXGetProcAddress ((const GLubyte*)"glXCreateContextAttribsARB"); assert (glXCreateContextAttribsARB);

We can't link to glXCreateContextAttribsARB in the normal way, since it may or may not be present depending on the GPU driver. Instead, we have to retrieve a pointer to the function with glXGetProcAddress. When retrieving OpenGL functions, I like to create a typedef of the function pointer type.

    const int context_attributes[] = {
        GLX_RENDER_TYPE, GLX_RGBA_TYPE,
        GLX_CONTEXT_MAJOR_VERSION_ARB, 3,
        GLX_CONTEXT_MINOR_VERSION_ARB, 2,
        GLX_CONTEXT_PROFILE_MASK_ARB, GLX_CONTEXT_CORE_PROFILE_BIT_ARB,
        None
    };

    const auto gl_context = glXCreateContextAttribsARB (display, fb_config, 0, True, context_attributes); assert(gl_context);

Context attributes are created as a NULL-terminated array of ints. Here I've set the major.minor version to 3.2 and specified the core profile. All available attributes are below.

GLX_CONTEXT_MAJOR_VERSION_ARB
GLX_CONTEXT_MINOR_VERSION_ARB

These together must specify a valid OpenGL version, all of which are: 1.0-1.5, 2.0-2.1, 3.0-3.3, 4.0-4.6.

GLX_CONTEXT_PROFILE_MASK_ARB

GLX_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB: Compatibility profile.
GLX_CONTEXT_CORE_PROFILE_BIT_ARB: Core profile. Only valid when the requested version is 3.2 or above.

GLX_CONTEXT_FLAGS_ARB

GLX_CONTEXT_DEBUG_BIT_ARB: Create a debug context. Provide additional runtime checking, validation, and logging.
GLX_CONTEXT_FORWARD_COMPATIBLE_BIT_ARB: Do not support deprecated functionality. Only valid when requested version is 3.0 or above.

GLX_RENDER_TYPE

GLX_RGBA_TYPE
GLX_COLOR_INDEX_TYPE. Not valid for version 3.0 or later.

Note that according to the specification, GLX_RENDER_TYPE must be set to a valid value. In my experience, it seems to default to RGBA, but this is not guaranteed behaviour, so it should always be specified.