Why does glGetString(GL_VERSION) return null / zero instead of the OpenGL version?

Question

I'm on Linux Mint 13 XFCE. My problem is that when I run in terminal the command:

glxinfo | grep "OpenGL version"

I get the following output:

OpenGL version string: 3.3.0 NVIDIA 295.40

But when I run the glGetString(GL_VERSION) in my application then the result is null. Why doesn't this code get the gl_version?

#include <stdio.h>
#include <GL/glew.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <GL/glext.h>

int main(int argc, char **argv) {

    glutInit(&argc, argv);
    glewInit();

    printf("OpenGL version supported by this platform (%s): \n",
        glGetString(GL_VERSION));
}

Answer 1

You can also use glfw in order to create GL context and then query the version:

Include this files:

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

And then you can do:

    // Initialise GLFW
    glewExperimental = true; // Needed for core profile
    if (!glfwInit())
    {
        return "";
    }

    // We are rendering off-screen, but a window is still needed for the context
    // creation. There are hints that this is no longer needed in GL 3.3, but that
    // windows still wants it. So just in case.
    glfwWindowHint(GLFW_VISIBLE, GL_FALSE); //dont show the window

    // Open a window and create its OpenGL context
    GLFWwindow* window;
    window = glfwCreateWindow(100, 100, "Dummy window", NULL, NULL);
    if (window == NULL) {
        return "";
    }
    glfwMakeContextCurrent(window); // Initialize GLEW
    if (glewInit() != GLEW_OK)
    {
        return "";
    }

    std::string versionString = std::string((const char*)glGetString(GL_VERSION));

Answer 2

glutInit() doesn't create a GL context or make one current. You need a current GL context for glewInit() and glGetString() to work.

Try this:

#include <GL/glew.h>
#include <GL/glut.h>
#include <cstdio>

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutCreateWindow("GLUT");

    glewInit();
    printf("OpenGL version supported by this platform (%s): \n", glGetString(GL_VERSION));
}