John P John P - 2 months ago 20
C++ Question

Why is OpenGL version 0.0?

I was troubleshooting an OpenGL application on a new computer when I discovered that GLFW could not create a window with the specified version of OpenGL. I created a minimal version of the application to test the version of OpenGL created, and no matter what version I hint, the version I get is 0.0. Do I simply not have OpenGL? This seems impossible, since glxgears runs and glxinfo suggests that I have version 2.1.

#include <iostream>
#include <GLFW/glfw3.h>

int main(int argc, const char *argv[]) {
if(!glfwInit()) {
return 1;
}
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 1);
auto win = glfwCreateWindow(640, 480, "", NULL, NULL);
if(!win) {
return 1;
}
int major = 0, minor = 0;
glfwMakeContextCurrent(win);
glGetIntegerv(GL_MAJOR_VERSION, &major);
glGetIntegerv(GL_MINOR_VERSION, &minor);
std::cout << "Initialized with OpenGL "
<< major << "." << minor << std::endl;
glfwDestroyWindow(win);
glfwTerminate();
}


The output of the application is "Initialized with OpenGL 0.0". A window briefly opens and closes and the application terminates without errors.

Answer

The GL_MAJOR_VERSION and GL_MINOR_VERSION queries were introduced in GL 3.0. Prior to that, this will just generate an GL_INVALID_ENUM error during the glGetIntegerv call, and leave your variables untouched.

You have to use glGetString(GL_VERSION) to reliably get the verison number if you can't make sure that you are on a >= 3.0 context. If you need those as numbers, you'll have to manually parse the string.

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