GL Debug Output Message Callback Guide

GLDebugMessageCallback.c

// This is free and unencumbered software released into the public domain.
// 
// Anyone is free to copy, modify, publish, use, compile, sell, or distribute
// this software, either in source code form or as a compiled binary, for any
// purpose, commercial or non-commercial, and by any means.
// 
// In jurisdictions that recognize copyright laws, the author or authors of this
// software dedicate any and all copyright interest in the software to the
// public domain. We make this dedication for the benefit of the public at large
// and to the detriment of our heirs and successors. We intend this dedication
// to be an overt act of relinquishment in perpetuity of all present and future
// rights to this software under copyright law.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
// AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
// ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
// 
// For more information, please refer to <http://unlicense.org/>

// REQUIREMENTS: OpenGL version with the KHR_debug extension available.

// Callback function for printing debug statements
void APIENTRY GLDebugMessageCallback(GLenum source, GLenum type, GLuint id,
                            GLenum severity, GLsizei length,
                            const GLchar *msg, const void *data)
{
    char* _source;
    char* _type;
    char* _severity;

    switch (source) {
        case GL_DEBUG_SOURCE_API:
        _source = "API";
        break;

        case GL_DEBUG_SOURCE_WINDOW_SYSTEM:
        _source = "WINDOW SYSTEM";
        break;

        case GL_DEBUG_SOURCE_SHADER_COMPILER:
        _source = "SHADER COMPILER";
        break;

        case GL_DEBUG_SOURCE_THIRD_PARTY:
        _source = "THIRD PARTY";
        break;

        case GL_DEBUG_SOURCE_APPLICATION:
        _source = "APPLICATION";
        break;

        case GL_DEBUG_SOURCE_OTHER:
        _source = "UNKNOWN";
        break;

        default:
        _source = "UNKNOWN";
        break;
    }

    switch (type) {
        case GL_DEBUG_TYPE_ERROR:
        _type = "ERROR";
        break;

        case GL_DEBUG_TYPE_DEPRECATED_BEHAVIOR:
        _type = "DEPRECATED BEHAVIOR";
        break;

        case GL_DEBUG_TYPE_UNDEFINED_BEHAVIOR:
        _type = "UDEFINED BEHAVIOR";
        break;

        case GL_DEBUG_TYPE_PORTABILITY:
        _type = "PORTABILITY";
        break;

        case GL_DEBUG_TYPE_PERFORMANCE:
        _type = "PERFORMANCE";
        break;

        case GL_DEBUG_TYPE_OTHER:
        _type = "OTHER";
        break;

        case GL_DEBUG_TYPE_MARKER:
        _type = "MARKER";
        break;

        default:
        _type = "UNKNOWN";
        break;
    }

    switch (severity) {
        case GL_DEBUG_SEVERITY_HIGH:
        _severity = "HIGH";
        break;

        case GL_DEBUG_SEVERITY_MEDIUM:
        _severity = "MEDIUM";
        break;

        case GL_DEBUG_SEVERITY_LOW:
        _severity = "LOW";
        break;

        case GL_DEBUG_SEVERITY_NOTIFICATION:
        _severity = "NOTIFICATION";
        break;

        default:
        _severity = "UNKNOWN";
        break;
    }

    printf("%d: %s of %s severity, raised from %s: %s\n",
            id, _type, _severity, _source, msg);
}

// =============== INIT DEBUG OUTPUT ================
// The following function calls should be made directly after OpenGL
// initialization.

// Enable the debugging layer of OpenGL
//
// GL_DEBUG_OUTPUT - Faster version but not useful for breakpoints
// GL_DEBUG_OUTPUT_SYNCHRONUS - Callback is in sync with errors, so a breakpoint
// can be placed on the callback in order to get a stacktrace for the GL error.

glEnable(GL_DEBUG_OUTPUT_SYNCHRONOUS);

// Set the function that will be triggered by the callback, the second parameter
// is the data parameter of the callback, it can be useful for different
// contexts but isn't necessary for our simple use case.
glDebugMessageCallback(GLDebugMessageCallback, NULL);

THANK YOU!

Hello! Beginner programmer here... I don't understand how this part of the code works ?

char* _source;
char* _type;
char* _severity;

Shouldn't these be allocated, why are they just pointers ?

I believe the "API" etc parts are already allocated, meaning the pointer just references those literals.
(actually not entirely sure in what part of memory (data segment or text segment?))

Hello! Beginner programmer here... I don't understand how this part of the code works ?

char* _source;
char* _type;
char* _severity;

Shouldn't these be allocated, why are they just pointers ?

pointers are just pointers, they just refer to somewhere in memory. it doesn't matter if that memory was dynamically-allocated, or if it was just a variable living on the stack, or if it was built into the binary itself. in the case of string literals, they're guaranteed to be valid for the entire lifetime of the program, usually implemented as references into the read-only data section of the binary.

// this does not dynamically-allocate the string
char *myString1 = "hello!";

// this is equivalent to the code above
char *myString2; // note, the contents of this variable are initially undefined (could be a pointer that points anywhere)
myString2 = "hello!"; // ok, now it points to the contents of this string literal

// both of the branches assign a statically-allocated string to myString
char *myString3;
if (foo) { myString3 = "successfully foo'd!!"; }
else { myString3 = "aw, the foo failed... ;w;"; }

// and there is nothing to "clean up" at the end here! yay!

Forked and reformatted with more sane linting (83 lines vs 139) and variable naming, +const char *source_,...: https://gist.github.com/Challanger524/cdf90cf11809749363fb638646225773
Big thanks to original author @liam-middlebrook!