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Colleague Riley
Colleague Riley

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RGFW Under the Hood: OpenGL Context Creation

#opengl #tutorial #lowlevel #api

Tutorial for creating an accelerated opengl context for X11, Windows, and MacOS. Based on my project: RGFW.

Introduction

In my experience working on RGFW, one of the most annoying parts of working with low-level APIs is creating an OpenGL context.
This is not because it's hard, but because there are many not-so-obvious steps that you must do correctly to create your OpenGL context.
This tutorial explains how to create an OpenGL context for Windows, MacOS and Linux so that way others don't have to struggle with figuring it out.

NOTE: MacOS code will be written with a Cocoa C Wrapper in mind (see the RGFW.h or Silicon.h)

Overview

A quick overview of the steps required
1) Load OpenGL context creation functions (if you need to)
2) Create an OpenGL pixel format (or Visual on X11) using an attribute list
3) Create your OpenGL context using an attribute array to set the OpenGL version

4) Free the OpenGL context

On MacOS steps 2 and 3 are one step.
EGL will not be included in this article because the setup is far easier.

Step 1 (Load OpenGL context creation functions)

First RGFW needs to load these functions

//X11 (GLX):
    glXCreateContextAttribsARB
    glXSwapIntervalEXT //(optional)

//Windows (WGL):
    wglCreateContextAttribsARB
    wglChoosePixelFormatARB
    wglSwapIntervalEXT //(optional)

//Cocoa (NSOpenGL)
    (none)
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It needs to load these functions because they're extension functions provided by the hardware vendor. By default, wglCreateContext or glXCreateContext will create an OpenGL ~1.0 context that probably uses software rendering.

To load the extension functions RGFW has to start by defining them.

//X11 (GLX):
    typedef GLXContext(*glXCreateContextAttribsARBProc)(Display*, GLXFBConfig, GLXContext, Bool, const int*);
    static glXCreateContextAttribsARBProc glXCreateContextAttribsARB = 0;

    //(optional)
    typedef void ( *PFNGLXSWAPINTERVALEXTPROC) (Display *dpy, GLXDrawable drawable, int interval);
    PFNGLXSWAPINTERVALEXTPROC glXSwapIntervalEXT = NULL;

//Windows (WGL):
    typedef HGLRC (WINAPI *PFNWGLCREATECONTEXTATTRIBSARBPROC)(HDC hdc, HGLRC hglrc, const int *attribList);
    PFNWGLCREATECONTEXTATTRIBSARBPROC wglCreateContextAttribsARB = NULL;

    typedef HRESULT (APIENTRY* PFNWGLCHOOSEPIXELFORMATARBPROC)(HDC hdc, const int* piAttribIList, const FLOAT* pfAttribFList, UINT nMaxFormats, int* piFormats, UINT* nNumFormats);
    static PFNWGLCHOOSEPIXELFORMATARBPROC wglChoosePixelFormatARB = NULL;

    //(optional)
    typedef BOOL(APIENTRY* PFNWGLSWAPINTERVALEXTPROC)(int interval);
    static PFNWGLSWAPINTERVALEXTPROC wglSwapIntervalEXT = NULL;
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Once the functions are defined, RGFW loads the functions with these API calls:

/*X11 (GLX):*/ glXGetProcAddress aad glXGetProcAddressARB
/*Windows (WGL):*/ wglGetProcAddress
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For example using GLX,

glXCreateContextAttribsARB = glXGetProcAddressARB((GLubyte*) "glXCreateContextAttribsARB");;

//(optional)
glXSwapIntervalEXT = (PFNGLXSWAPINTERVALEXTPROC) glXGetProcAddress((GLubyte*) "glXSwapIntervalEXT");
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WGL is a little bit more complicated because it needs to start by creating a dummy context.
This dummy context is used by WGL to load the functions.

WGL dummy

First, RGFW has to create a dummy window and device context, RGFW also uses this dummy window to get the height offset for the title bar.

HWND dummyWin = CreateWindowA(Class.lpszClassName, name, window_style, x, y, w, h, 0, 0, inh, 0);

HDC dummy_dc = GetDC(dummyWin);
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After that, RGFW creates a dummy pixel format for the dummy window and dummy OpenGL context.

u32 pfd_flags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;

PIXELFORMATDESCRIPTOR pfd = {
    sizeof(pfd),
    1, /* version */
    pfd_flags,
    PFD_TYPE_RGBA, /* ipixel type */
    24, /* color bits */
    0, 0, 0, 0, 0, 0,
    8, /* alpha bits */
    0, 0, 0, 0, 0, 0,
    32, /* depth bits */
    8, /* stencil bits */ 
    0,
    PFD_MAIN_PLANE, /* Layer type */
    0, 0, 0, 0
};

int pixel_format = ChoosePixelFormat(dummy_dc, &pfd);
SetPixelFormat(dummy_dc, pixel_format, &pfd);
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Now RGFW can create a dummy context and set it to be the current context.

This context will be using the default OpenGL ~1.0 context, OpenGL.

HGLRC dummy_context = wglCreateContext(dummy_dc);
wglMakeCurrent(dummy_dc, dummy_context);
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Now RGFW can load the functions and delete the dummy

wglCreateContextAttribsARB = (PFNWGLCREATECONTEXTATTRIBSARBPROC) (void*) wglGetProcAddress("wglCreateContextAttribsARB");
wglChoosePixelFormatARB = (PFNWGLCHOOSEPIXELFORMATARBPROC) (void*)wglGetProcAddress("wglChoosePixelFormatARB");

//(optional)
wglSwapIntervalEXT = (PFNWGLSWAPINTERVALEXTPROC) wglGetProcAddress("wglSwapIntervalEXT");

wglMakeCurrent(dummy_dc, 0);
wglDeleteContext(dummy_context);
ReleaseDC(dummyWin, dummy_dc);
DestroyWindow(dummyWin);
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Step 2 (Create an OpenGL pixel format (or Visual on X11) using an attribute list)

RGFW needs to create a pixel format/visual so the window knows how to draw the data it gets and so OpenGL knows the draw format we want.

Step 2.1: creating the attribute list

To create an OpenGL pixel format using an attribute list, RGFW has a function that creates an attribute list, RGFW_initFormatAttribs for the pixel format.

This function uses macros based on the target OS's API, supporting all of the APIs with a single function.

For this tutorial, I'll be separating an array for each OS rather than using one big array.

//linux:
    static u32 attribs[] = {
                            GLX_X_VISUAL_TYPE,      GLX_TRUE_COLOR,
                            GLX_DEPTH_SIZE,         24,                            
                            GLX_X_RENDERABLE,       1,
                            GLX_RED_SIZE,           8,
                            GLX_GREEN_SIZE,         8,
                            GLX_BLUE_SIZE,          8,
                            GLX_ALPHA_SIZE,         8,
                            GLX_RENDER_TYPE,        GLX_RGBA_BIT,
                            GLX_DRAWABLE_TYPE,      GLX_WINDOW_BIT,

                            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
    };

//windows:

    /// windows makes you define the macros yourself (or you can use wglext.h (which may or may not be installed on your system)
    // https://registry.khronos.org/OpenGL/api/GL/wglext.h
    // so I'll be using the hardcoded instead 
    static u32 attribs[] = {
                                    0x2003, // WGL_ACCELERATION_ARB
                                    0x2027, // WGL_FULL_ACCELERATION_ARB
                                    0x201b, 8, // WGL_ALPHA_BITS_ARB
                                    0x2022, 24, // WGL_DEPTH_BITS_ARB
                                    0x2001, 1, // WGL_DRAW_TO_WINDOW_ARB
                                    0x2015, 8, // WGL_RED_BITS_ARB
                                    0x2017, 8, // WGL_GREEN_BITS_ARB
                                    0x2019, 8, // WGL_BLUE_BITS_ARB
                                    0x2013, 0x202B, // WGL_PIXEL_TYPE_ARB,  WGL_TYPE_RGBA_ARB
                                    0x2010,     1, // WGL_SUPPORT_OPENGL_ARB
                                    0x2014,  32, // WGL_COLOR_BITS_ARB
                                    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
            };

//macos:
        static u32 attribs[] = {
                                11      , 8, // alpha size
                                24      , 24, // depth size
                                72, // NSOpenGLPFANoRecovery
                                8, 24, // NSOpenGLPFAColorSize
                                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
        };
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You may notice the extra 0s at the bottom of the array, that's for optional arguments.

To fill in the optional arguments, first RGFW sets the index to the first 0.

size_t index = (sizeof(attribs) / sizeof(attribs[0])) - 13;
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RGFW uses a macro to fill in optional arguments

#define RGFW_GL_ADD_ATTRIB(attrib, attVal) \
        if (attVal) { \
            attribs[index] = attrib;\
            attribs[index + 1] = attVal;\
            index += 2;\
        }
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RGFW defines variables that can be changed by the user to fill in these arguments.

i32 RGFW_STENCIL = 0, RGFW_SAMPLES = 0, RGFW_STEREO = GL_FALSE, RGFW_AUX_BUFFERS = 0, RGFW_DOUBLE_BUFFER = 1;
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I'm going to split the arguments up between each platform.

linux:
    if (RGFW_DOUBLE_BUFFER)
        RGFW_GL_ADD_ATTRIB(GLX_DOUBLEBUFFER, 1);
    RGFW_GL_ADD_ATTRIB(GLX_STENCIL_SIZE, RGFW_STENCIL);
    RGFW_GL_ADD_ATTRIB(GLX_STEREO, RGFW_STEREO);
    RGFW_GL_ADD_ATTRIB(GLX_AUX_BUFFERS, RGFW_AUX_BUFFERS);
    // samples are handled by GLX later

windows:
    if (RGFW_DOUBLE_BUFFER)
        RGFW_GL_ADD_ATTRIB(0x2011, 1);  // double buffer
    RGFW_GL_ADD_ATTRIB(0x2023, RGFW_STENCIL);
    RGFW_GL_ADD_ATTRIB(0x2012, RGFW_STEREO);
    RGFW_GL_ADD_ATTRIB(0x2024, RGFW_AUX_BUFFERS);
    RGFW_GL_ADD_ATTRIB(0x2042, RGFW_SAMPLES);

macOS:
    if (RGFW_DOUBLE_BUFFER)
        RGFW_GL_ADD_ATTRIB(5, 1); // double buffer
    RGFW_GL_ADD_ATTRIB(13, RGFW_STENCIL);
    RGFW_GL_ADD_ATTRIB(6, RGFW_STEREO);
    RGFW_GL_ADD_ATTRIB(7, RGFW_AUX_BUFFERS);
    RGFW_GL_ADD_ATTRIB(55, RGFW_SAMPLES);

    /* this is here because macOS has a specific way to handle using software rendering */

    if (useSoftware) {
        RGFW_GL_ADD_ATTRIB(70, kCGLRendererGenericFloatID);
    } else {
        attribs[index] = 73;
        index += 1;
    }
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On macOS RGFW also sets the version here.

attribs[index] = 99;
attribs[index + 1] = 0x1000;

if (RGFW_majorVersion >= 4 || RGFW_majorVersion >= 3) {
    attribs[index + 1] = (u32) ((RGFW_majorVersion >= 4) ? 0x4100 : 0x3200);
}
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Make sure the final two arguments are set to 0, this is how OpenGL/WGL/GLX/NSOpenGL knows to stop reading.

RGFW_GL_ADD_ATTRIB(0, 0);
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Step 2.2 creating the pixel format

Now that the list is created, it can be used to create the pixel format.

GLX

GLX Handles this by creating an array of GLXFBConfig based on the attributes.

i32 fbcount;
GLXFBConfig* fbc = glXChooseFBConfig((Display*) display, DefaultScreen(display), (i32*) attribs, &fbcount);

i32 best_fbc = -1;

if (fbcount == 0) {
    printf("Failed to find any valid GLX visual configs\n");
    return NULL;
}
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Then it uses the generated array to find the closest matching FBConfig object. (This is where RGFW_SAMPLES comes in)

u32 i;
for (i = 0; i < (u32)fbcount; i++) {
    XVisualInfo* vi = glXGetVisualFromFBConfig((Display*) display, fbc[i]);
                if (vi == NULL)
        continue;

    XFree(vi);

    i32 samp_buf, samples;
    glXGetFBConfigAttrib((Display*) display, fbc[i], GLX_SAMPLE_BUFFERS, &samp_buf);
    glXGetFBConfigAttrib((Display*) display, fbc[i], GLX_SAMPLES, &samples);

    if ((best_fbc < 0 || samp_buf) && (samples == RGFW_SAMPLES || best_fbc == -1)) {
        best_fbc = i;
    }
}

if (best_fbc == -1) {
    printf("Failed to get a valid GLX visual\n");
    return NULL;
}

GLXFBConfig bestFbc = fbc[best_fbc];
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Once it finds the closest matching object, it gets the X11 visual (or pixel format) from the array and frees the array.

/* Get a visual */
XVisualInfo* vi = glXGetVisualFromFBConfig((Display*) display, bestFbc);

XFree(fbc);
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Now this Visual can be used to create a window and/or colormap.

XSetWindowAttributes swa;
Colormap cmap;

swa.colormap = cmap = XCreateColormap((Display*) display,
    DefaultRootWindow(display),
    vi->visual, AllocNone);

swa.background_pixmap = None;
swa.border_pixel = 0;
swa.event_mask = event_mask;

swa.background_pixel = 0;

Window window = XCreateWindow((Display*) display, DefaultRootWindow((Display*) display), x, y, w, h,
    0, vi->depth, InputOutput, vi->visual,
    CWColormap | CWBorderPixel | CWBackPixel | CWEventMask, &swa);
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WGL

RGFW needs some WGL defines for creating the context:

#define WGL_CONTEXT_CORE_PROFILE_BIT_ARB 0x00000001
#define WGL_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB 0x00000002
#define WGL_CONTEXT_MAJOR_VERSION_ARB             0x2091
#define WGL_CONTEXT_MINOR_VERSION_ARB             0x2092
#define WGL_CONTEXT_PROFILE_MASK_ARB              0x9126
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For WGL, RGFW has to first create a win32 pixel format.

PIXELFORMATDESCRIPTOR pfd2 = (PIXELFORMATDESCRIPTOR){ sizeof(pfd2), 1, pfd_flags, PFD_TYPE_RGBA, 32, 8, PFD_MAIN_PLANE, 24, 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
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Then RGFW can create a WGL pixel format based on the attribs array.

int pixel_format2;
UINT num_formats;
wglChoosePixelFormatARB(hdc, attribs, 0, 1, &pixel_format2, &num_formats);
if (!num_formats) {
    printf("Failed to create a pixel format for WGL.\n");
}
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Now RGFW can merge the two as one big format and set it as the format for your window.

DescribePixelFormat(hdc, pixel_format2, sizeof(pfd2), &pfd2);
if (!SetPixelFormat(hdc, pixel_format2, &pfd2)) {
    printf("Failed to set the WGL pixel format.\n");
}
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NSOpenGL

For MacOS, RGFW just has to create the pixel format and then init a view based on the format for OpenGL.

void* format = NSOpenGLPixelFormat_initWithAttributes(attribs);

/* the pixel format can be passed directly to opengl context creation to create a context 
    this is because the format also includes information about the opengl version (which may be a bad thing) */
view = NSOpenGLView_initWithFrame((NSRect){{0, 0}, {w, h}}, format);
objc_msgSend_void(view, sel_registerName("prepareOpenGL"));
ctx = objc_msgSend_id(view, sel_registerName("openGLContext"))
objc_msgSend_void(ctx, sel_registerName("makeCurrentContext"));
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If you're following along for MacOS, you can skip step 3.

Step 3 (Create your OpenGL context using an attribute array to set the OpenGL version)

NOTE: RGFW defines this enum and these variables so the user can control the OpenGL version:

typedef u8 RGFW_GL_profile; enum { RGFW_GL_CORE = 0,  RGFW_GL_COMPATIBILITY  };
i32 RGFW_majorVersion = 0, RGFW_minorVersion = 0;
b8 RGFW_profile = RGFW_GL_CORE;
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glx

Now it's time to create the attribute array for the GL context creation and load the OpenGL version you want:

i32 context_attribs[7] = { 0, 0, 0, 0, 0, 0, 0 };
context_attribs[0] = GLX_CONTEXT_PROFILE_MASK_ARB;
if (RGFW_profile == RGFW_GL_CORE) 
    context_attribs[1] = GLX_CONTEXT_CORE_PROFILE_BIT_ARB;
else 
    context_attribs[1] = GLX_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB;

if (RGFW_majorVersion || RGFW_minorVersion) {
    context_attribs[2] = GLX_CONTEXT_MAJOR_VERSION_ARB;
    context_attribs[3] = RGFW_majorVersion;
    context_attribs[4] = GLX_CONTEXT_MINOR_VERSION_ARB;
    context_attribs[5] = RGFW_minorVersion;
}
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Finally, the context can be created using the context_attribs array:

ctx = glXCreateContextAttribsARB((Display*) display, bestFbc, NULL, True, context_attribs);
glXMakeCurrent(display, window, ctx); // make the window the current so it can be rendered to in the same thread
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WGL

First WGL needs to create an attribs array for setting the OpenGL Version

It also uses a helper macro called SET_ATTRIB

#define SET_ATTRIB(a, v) { \
    assert(((size_t) index + 1) < sizeof(context_attribs) / sizeof(context_attribs[0])); \
    context_attribs[index++] = a; \
    context_attribs[index++] = v; \
}

/* create opengl/WGL context for the specified version */ 
u32 index = 0;
i32 context_attribs[40];

if (RGFW_profile == RGFW_GL_CORE) {
    SET_ATTRIB(WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_CORE_PROFILE_BIT_ARB);
}
else {
    SET_ATTRIB(WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB);
}

if (RGFW_majorVersion || RGFW_minorVersion) {
    SET_ATTRIB(WGL_CONTEXT_MAJOR_VERSION_ARB, RGFW_majorVersion);
    SET_ATTRIB(WGL_CONTEXT_MINOR_VERSION_ARB, RGFW_minorVersion);
}

SET_ATTRIB(0, 0);
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Now the context can be created:

ctx = (HGLRC)wglCreateContextAttribsARB(hdc, NULL, context_attribs);
wglMakeCurrent(hdc, ctx); // make the window the current so it can be rendered to in the same thread
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Step 4 (Free the OpenGL context)

Now that RGFW has created its OpenGL context, it has to free the context when it's done using it.

This is the easy part.

Linux (GLX):
    glXDestroyContext((Display*) display, ctx);

windows (WGL):
    wglDeleteContext((HGLRC) ctx);

macOS (NSOpenGL):
    // I think macOS NSOpenGL stuff is freed automatically when everything else is freed
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Full code examples

X11 

// compile with gcc x11.c -lX11 -lGL

#include <X11/Xlib.h>
#include <GL/glx.h>

#include <stdio.h>
#include <stdint.h>

typedef GLXContext(*glXCreateContextAttribsARBProc)(Display*, GLXFBConfig, GLXContext, Bool, const int*);
typedef void ( *PFNGLXSWAPINTERVALEXTPROC) (Display *dpy, GLXDrawable drawable, int interval);

#define GL_ADD_ATTRIB(attrib, attVal) \
        if (attVal) { \
            attribs[index] = attrib;\
            attribs[index + 1] = attVal;\
            index += 2;\
        }

typedef uint8_t GL_profile; enum  { GL_CORE = 0,  GL_COMPATIBILITY  };
int32_t majorVersion = 0, minorVersion = 0;
Bool profile = GL_CORE;

int32_t STENCIL = 0, SAMPLES = 0, STEREO = GL_FALSE, AUX_BUFFERS = 0, DOUBLE_BUFFER = 1;

int main(void) {
    typedef void ( *PFNGLXSWAPINTERVALEXTPROC) (Display *dpy, GLXDrawable drawable, int interval);
    PFNGLXSWAPINTERVALEXTPROC glXSwapIntervalEXT = NULL;

    glXCreateContextAttribsARBProc glXCreateContextAttribsARB = (glXCreateContextAttribsARBProc)glXGetProcAddressARB((GLubyte*) "glXCreateContextAttribsARB");
    glXSwapIntervalEXT = (PFNGLXSWAPINTERVALEXTPROC) glXGetProcAddress((GLubyte*) "glXSwapIntervalEXT");

    static uint32_t attribs[] = {
               GLX_X_VISUAL_TYPE,      GLX_TRUE_COLOR,
               GLX_DEPTH_SIZE,         24,                            
               GLX_X_RENDERABLE,       1,
               GLX_RED_SIZE,           8,
               GLX_GREEN_SIZE,         8,
               GLX_BLUE_SIZE,          8,
               GLX_ALPHA_SIZE,         8,
               GLX_RENDER_TYPE,        GLX_RGBA_BIT,
               GLX_DRAWABLE_TYPE,      GLX_WINDOW_BIT,
               0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
    };

    size_t index = (sizeof(attribs) / sizeof(attribs[0])) - 13;

    if (DOUBLE_BUFFER)
        GL_ADD_ATTRIB(GLX_DOUBLEBUFFER, 1);
    GL_ADD_ATTRIB(GLX_STENCIL_SIZE, STENCIL);
    GL_ADD_ATTRIB(GLX_STEREO, STEREO);
    GL_ADD_ATTRIB(GLX_AUX_BUFFERS, AUX_BUFFERS);

    Display *display;
    XEvent event;

    display = XOpenDisplay(NULL);
    if (display == NULL) {
        fprintf(stderr, "Cannot open display\n");
        return -1;
    }

    int s = DefaultScreen(display);

    int32_t fbcount;
    GLXFBConfig* fbc = glXChooseFBConfig((Display*) display, DefaultScreen(display), (int32_t*) attribs, &fbcount);

    int32_t best_fbc = -1;

    if (fbcount == 0) {
        printf("Failed to find any valid GLX visual configs\n");
        return -1;
    }

    uint32_t i;
    for (i = 0; i < (uint32_t)fbcount; i++) {
        XVisualInfo* vi = glXGetVisualFromFBConfig((Display*) display, fbc[i]);
        if (vi == NULL)
            continue;

        XFree(vi);

        int32_t samp_buf, samples;
        glXGetFBConfigAttrib((Display*) display, fbc[i], GLX_SAMPLE_BUFFERS, &samp_buf);
        glXGetFBConfigAttrib((Display*) display, fbc[i], GLX_SAMPLES, &samples);

        if ((best_fbc < 0 || samp_buf) && (samples == SAMPLES || best_fbc == -1)) {
            best_fbc = i;
        }
    }

    if (best_fbc == -1) {
        printf("Failed to get a valid GLX visual\n");
        return -1;
    }

    GLXFBConfig bestFbc = fbc[best_fbc];

    XVisualInfo* vi = glXGetVisualFromFBConfig((Display*) display, bestFbc);

    XFree(fbc);

    XSetWindowAttributes swa;
    Colormap cmap;

    swa.colormap = cmap = XCreateColormap((Display*) display, DefaultRootWindow(display), vi->visual, AllocNone);

    swa.background_pixmap = None;
    swa.border_pixel = 0;
    swa.event_mask = CWColormap | CWBorderPixel | CWBackPixel | CWEventMask;

    swa.background_pixel = 0;

    Window window = XCreateWindow((Display*) display, DefaultRootWindow((Display*) display), 400, 400, 200, 200, 0, vi->depth, InputOutput, vi->visual, CWColormap | CWBorderPixel | CWBackPixel | CWEventMask, &swa);

    XSelectInput(display, window, ExposureMask | KeyPressMask);

    int32_t context_attribs[7] = { 0, 0, 0, 0, 0, 0, 0 };
    context_attribs[0] = GLX_CONTEXT_PROFILE_MASK_ARB;
    if (profile == GL_CORE) 
        context_attribs[1] = GLX_CONTEXT_CORE_PROFILE_BIT_ARB;
    else 
        context_attribs[1] = GLX_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB;

    if (majorVersion || minorVersion) {
        context_attribs[2] = GLX_CONTEXT_MAJOR_VERSION_ARB;
        context_attribs[3] = majorVersion;
        context_attribs[4] = GLX_CONTEXT_MINOR_VERSION_ARB;
        context_attribs[5] = minorVersion;
    }

    GLXContext ctx = glXCreateContextAttribsARB((Display*) display, bestFbc, NULL, True, context_attribs);
    glXMakeCurrent(display, window, ctx);

    XMapWindow(display, window);

    for (;;) {
        XNextEvent(display, &event);

        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        glXSwapBuffers(display, window);
    }

    glXDestroyContext((Display*) display, ctx);
    XCloseDisplay(display);

    return 0;
 }
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Windows 


// compile with gcc winapi.c -lopengl32 -lgdi32
#include <windows.h>
#include <GL/gl.h>

#include <stdio.h>
#include <stdint.h>
#include <assert.h>

typedef uint8_t     u8;
typedef int8_t      i8;
typedef uint16_t   u16;
typedef int16_t    i16;
typedef uint32_t   u32;
typedef int32_t    i32;
typedef uint64_t   u64;
typedef int64_t    i64;
typedef u8 b8;

#define WGL_CONTEXT_CORE_PROFILE_BIT_ARB 0x00000001
#define WGL_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB 0x00000002
#define WGL_CONTEXT_MAJOR_VERSION_ARB             0x2091
#define WGL_CONTEXT_MINOR_VERSION_ARB             0x2092
#define WGL_CONTEXT_PROFILE_MASK_ARB              0x9126

int main() {
    typedef HGLRC (WINAPI *PFNWGLCREATECONTEXTATTRIBSARBPROC)(HDC hdc, HGLRC hglrc, const int *attribList);
    PFNWGLCREATECONTEXTATTRIBSARBPROC wglCreateContextAttribsARB = NULL;

    typedef HRESULT (APIENTRY* PFNWGLCHOOSEPIXELFORMATARBPROC)(HDC hdc, const int* piAttribIList, const FLOAT* pfAttribFList, UINT nMaxFormats, int* piFormats, UINT* nNumFormats);
    static PFNWGLCHOOSEPIXELFORMATARBPROC wglChoosePixelFormatARB = NULL;

    typedef BOOL(APIENTRY* PFNWGLSWAPINTERVALEXTPROC)(int interval);
    static PFNWGLSWAPINTERVALEXTPROC wglSwapIntervalEXT = NULL;

    WNDCLASS wc = {0};
    wc.lpfnWndProc   = DefWindowProc; // Default window procedure
    wc.hInstance     = GetModuleHandle(NULL);
    wc.lpszClassName = "SampleWindowClass";

    RegisterClass(&wc);

    HWND dummyWin = CreateWindowA(wc.lpszClassName, "Sample Window", 0, 200, 200, 300, 300, 0, 0, wc.hInstance, 0);
    HDC dummy_dc = GetDC(dummyWin);

    u32 pfd_flags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;

    PIXELFORMATDESCRIPTOR pfd = {
        sizeof(pfd),
        1, /* version */
        pfd_flags,
        PFD_TYPE_RGBA, /* ipixel type */
        24, /* color bits */
        0, 0, 0, 0, 0, 0,
        8, /* alpha bits */
        0, 0, 0, 0, 0, 0,
        32, /* depth bits */
        8, /* stencil bits */ 
        0,
        PFD_MAIN_PLANE, /* Layer type */
        0, 0, 0, 0
    };

    int pixel_format = ChoosePixelFormat(dummy_dc, &pfd);
    SetPixelFormat(dummy_dc, pixel_format, &pfd);

    HGLRC dummy_context = wglCreateContext(dummy_dc);
    wglMakeCurrent(dummy_dc, dummy_context);

    wglCreateContextAttribsARB = (PFNWGLCREATECONTEXTATTRIBSARBPROC) (void*) wglGetProcAddress("wglCreateContextAttribsARB");
    wglChoosePixelFormatARB = (PFNWGLCHOOSEPIXELFORMATARBPROC) (void*)wglGetProcAddress("wglChoosePixelFormatARB");

    wglSwapIntervalEXT = (PFNWGLSWAPINTERVALEXTPROC) wglGetProcAddress("wglSwapIntervalEXT");

    wglMakeCurrent(dummy_dc, 0);
    wglDeleteContext(dummy_context);
    ReleaseDC(dummyWin, dummy_dc);
    DestroyWindow(dummyWin);

    static u32 attribs[] = {
              0x2003, // WGL_ACCELERATION_ARB
              0x2027, // WGL_FULL_ACCELERATION_ARB
              0x201b, 8, // WGL_ALPHA_BITS_ARB
              0x2022, 24, // WGL_DEPTH_BITS_ARB
              0x2001, 1, // WGL_DRAW_TO_WINDOW_ARB
              0x2015, 8, // WGL_RED_BITS_ARB
              0x2017, 8, // WGL_GREEN_BITS_ARB
              0x2019, 8, // WGL_BLUE_BITS_ARB
              0x2013, 0x202B, // WGL_PIXEL_TYPE_ARB, WGL_TYPE_RGBA_ARB
              0x2010, 1, // WGL_SUPPORT_OPENGL_ARB
              0x2014,    32, // WGL_COLOR_BITS_ARB
              0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
            };

    size_t index = (sizeof(attribs) / sizeof(attribs[0])) - 13;
    #define RGFW_GL_ADD_ATTRIB(attrib, attVal) \
        if (attVal) { \
            attribs[index] = attrib;\
            attribs[index + 1] = attVal;\
            index += 2;\
        }

    i32 RGFW_STENCIL = 0, RGFW_SAMPLES = 0, RGFW_STEREO = GL_FALSE, RGFW_AUX_BUFFERS = 0, RGFW_DOUBLE_BUFFER = 1;

    if (RGFW_DOUBLE_BUFFER)
        RGFW_GL_ADD_ATTRIB(0x2011, 1);  // double buffer
    RGFW_GL_ADD_ATTRIB(0x2023, RGFW_STENCIL);
    RGFW_GL_ADD_ATTRIB(0x2012, RGFW_STEREO);
    RGFW_GL_ADD_ATTRIB(0x2024, RGFW_AUX_BUFFERS);
    RGFW_GL_ADD_ATTRIB(0x2042, RGFW_SAMPLES);
    RGFW_GL_ADD_ATTRIB(0, 0);

    typedef u8 RGFW_GL_profile; enum { RGFW_GL_CORE = 0,  RGFW_GL_COMPATIBILITY  };
    i32 RGFW_majorVersion = 0, RGFW_minorVersion = 0;
    b8 RGFW_profile = RGFW_GL_CORE;

    HWND hwnd = CreateWindowA(wc.lpszClassName, "Sample Window",
                                0,
                                400, 400, 300, 300,
                                NULL, NULL, wc.hInstance, NULL);

    HDC hdc = GetDC(hwnd);


    PIXELFORMATDESCRIPTOR pfd2 = (PIXELFORMATDESCRIPTOR){ sizeof(pfd2), 1, pfd_flags, PFD_TYPE_RGBA, 32, 8, PFD_MAIN_PLANE, 24, 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    int pixel_format2;
    UINT num_formats;
    wglChoosePixelFormatARB(hdc, attribs, 0, 1, &pixel_format2, &num_formats);
    if (!num_formats) {
        printf("Failed to create a pixel format for WGL.\n");
    }

    DescribePixelFormat(hdc, pixel_format2, sizeof(pfd2), &pfd2);
    if (!SetPixelFormat(hdc, pixel_format2, &pfd2)) {
        printf("Failed to set the WGL pixel format.\n");
    }

    #define SET_ATTRIB(a, v) { \
        assert(((size_t) index + 1) < sizeof(context_attribs) / sizeof(context_attribs[0])); \
        context_attribs[index++] = a; \
        context_attribs[index++] = v; \
    }

    /* create opengl/WGL context for the specified version */ 
    index = 0;
    i32 context_attribs[40];

    if (RGFW_profile == RGFW_GL_CORE) {
        SET_ATTRIB(WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_CORE_PROFILE_BIT_ARB);
    }
    else {
        SET_ATTRIB(WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB);
    }

    if (RGFW_majorVersion || RGFW_minorVersion) {
        SET_ATTRIB(WGL_CONTEXT_MAJOR_VERSION_ARB, RGFW_majorVersion);
        SET_ATTRIB(WGL_CONTEXT_MINOR_VERSION_ARB, RGFW_minorVersion);
    }

    SET_ATTRIB(0, 0);

    HGLRC ctx = (HGLRC)wglCreateContextAttribsARB(hdc, NULL, context_attribs);
    wglMakeCurrent(hdc, ctx);

    ShowWindow(hwnd, SW_SHOW);
    UpdateWindow(hwnd);

    MSG msg;

    BOOL running = TRUE;
    while (running) {
        if (PeekMessageA(&msg, hwnd, 0u, 0u, PM_REMOVE)) {
            switch (msg.message) {
                case WM_CLOSE:
                case WM_QUIT:
                    running = FALSE;
                    break;
            }
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }

        running = IsWindow(hwnd);


        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        SwapBuffers(hdc);
    }

    DeleteDC(hdc);
    DestroyWindow(hwnd);
    return 0;
}
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