Why does glewInit() result in GL_INVALID_ENUM after making some calls to glfwWindowHint()

Question

For some reason when I call glfwWindowHint():

glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);

and later make a call to:

glewInit()

I end up getting a glError: GL_INVALID_ENUM. When I leave out all of the glfwWindowHint() calls everything works fine and no glError is set. Am I using these libraries incorrectly, or if this is a bug in glfw, or glew?

Note that I'm using glew-1.10.0 and glfw-3.0.3

Here's a simple program to illustrate the issue I'm seeing:

#include <iostream>

#include "GL/glew.h"
#include "GLFW/glfw3.h"

int main(char* argc, char* argv[])
{
    GLFWwindow* window;

    if (!glfwInit())
    {
        return -1;
    }

    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    window = glfwCreateWindow(800, 600, "Hello World", NULL, NULL);
    if (!window)
    {
        glfwTerminate();
        return -1;
    }

    glfwMakeContextCurrent(window);

    //glewExperimental = GL_TRUE; tried commenting this out but I still get the error
    if(glewInit() != GLEW_OK)
    {
        return -1;
    } 
    switch(glGetError())
    {
    case GL_INVALID_ENUM:
        std::cout << "why is this happening?";
    }
}

Answer 1

Because in a core profile context, it is invalid to query the extension string using glGetString (...). You have to use glGetStringi (...) and query each extension one-by-one. Frankly this is terrible design, since glGetStringi has to be loaded through the extension loading mechanism on most platforms. It is a chicken and egg sort of situation, the proper behavior in a core profile is definitely to use glewExperimental = TRUE before initialization and ignore an invalid enum error immediately following glewInit (...).

I should point out that this is nothing that you are doing wrong. It is a problem with how GLEW is implemented behind the scenes and a somewhat questionable decision by the OpenGL ARB. I would have left GL_EXTENSIONS a valid thing to query with glGetString (...) but defined some special string (e.g. "GL_CORE_PROFILE") to return in a core profile. Everybody wins then, the way I see it.

Incidentally, it is not theglfwWindowHint for major.minor 3.3 that causes this issue to show up. It is actually your use of GLFW_OPENGL_CORE_PROFILE. Core profiles are only valid for OpenGL 3.2+, so this issue will only manifest itself with a combination of major.minor ≥ 3.2 and core.