OpenGL draw buffer problems

Question

I've begun doing a bit of OpenGL, and I've just discovered alternatives to the fixed function pipeline, so I went searching tutorials. Sadly, most good tutorials use C++, and I would like to stick to C if possible. So I wasn't able to just copy-paste stuff and try it.

Copy-pasting and merging code gave errors though, and I'm not sure how I should deal with them. The error in question was "Segmentation fault", and here is the code:

#include <GL/glfw.h>
#include <GL/glu.h>
#include <stdlib.h>

#define SCREEN_WIDTH 1600
#define SCREEN_HEIGHT 1000

void main(int argc, char **argv)
{
    float mouse_x, mouse_y;

    GLuint positionBufferObject;
    const float vertexPositions[] = {
    0.75f, 0.75f, 0.0f, 1.0f,
    0.75f, -0.75f, 0.0f, 1.0f,
    -0.75f, -0.75f, 0.0f, 1.0f,
};
    glGenBuffers(1, &positionBufferObject);

    glBindBuffer(GL_ARRAY_BUFFER, positionBufferObject);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertexPositions), vertexPositions, GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    int running = GL_TRUE;
    // Initialize GLFW
    if( !glfwInit() )
    {
        exit( EXIT_FAILURE );
    }

    // Open an OpenGL window
    if( !glfwOpenWindow(SCREEN_WIDTH,SCREEN_HEIGHT, 8,8,8,8,8,8, GLFW_WINDOW ) )
    {
        glfwTerminate();
        exit( EXIT_FAILURE );
    }

    while (running)
    {
        glBindBuffer(GL_ARRAY_BUFFER, positionBufferObject);
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);

        glDrawArrays(GL_TRIANGLES, 0, 3);
    }
}

Of course, a lot of code is missing here, because it (apparently) didn't influence the error. Commenting out "glDrawArrays" fixes the error, and OpenGL runs normally (without drawing anything, logically).

Any help?

Robinson · Accepted Answer · 2012-03-16 13:39:43Z

4

You should initialise gl before you start binding buffers and so on. The initialisation process fetches and assigns the pointers to these functions. Otherwise they're null.

answered Mar 16, 2012 at 13:39

Robinson

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4 Comments

Orpheon Over a year ago

Doesn't glfwInit initialize OpenGL? I tried putting that before. On this short code, it subsequently crashed my computer, and on the full code, still ended with a segmentation fault.

Robinson Over a year ago

Well, apparently glfwInit is what you need to do first, before other stuff, so that at least is wrong in the code sample you've given here. Strange that you say removing DrawArrays fixes the error when there are quite a few other functions called before that that shouldn't work!

Nicol Bolas Over a year ago

@Orpheon: No, glfwInit doesn't initialize OpenGL. It initializes GLFW. Before you can use OpenGL, you must initialize GLFW and open a window.

Robinson Over a year ago

Possibly. I'm not sure what glfwinit actually is. I do know that you need to initialise GL before trying to give it commands, however :p.

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