When should a VBO be unbind or not?

Question

Searching about VAOs and VBOs I read here and here that I can optionally unbind them to avoid accidental modification so I gave it a try. The problem is that unbinding the VBO causes an access violation when calling glDrawElements and I don't understand why am I getting this behavior.

int main(int argc, char *argv[]) {
    if (!glfwInit()) {
        return -1;
    }

    GLFWwindow* window = glfwCreateWindow(800, 600, "OpenGL", nullptr, nullptr);
    glfwMakeContextCurrent(window);
    glewExperimental = GL_TRUE;

    if (glewInit() != GLEW_OK) {
        return -1;
    }

    GLfloat vertices[] = {
        -0.5f, 0.5f, 1.0f, 0.0f, 0.0f, // Top-left
        0.5f, 0.5f, 0.0f, 1.0f, 0.0f, // Top-right
        0.5f, -0.5f, 0.0f, 0.0f, 1.0f, // Bottom-right
        -0.5f, -0.5f, 1.0f, 1.0f, 1.0f  // Bottom-left
    };

    // Create Vertex Array Object
    GLuint vao;
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);

    //Vertex Buffer Object
    GLuint vbo;
    glGenBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    // Index Buffer Object
    GLuint elements[] = {
        0, 1, 2,
        2, 3, 0
    };

    GLuint ibo;
    glGenBuffers(1, &ibo);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(elements), elements, GL_STATIC_DRAW);

    (...) // Shader-related stuff

    // Specify the layout of the vertex data
    GLint posAttrib = glGetAttribLocation(shaderProgram, "position");
    glEnableVertexAttribArray(posAttrib);
    glVertexAttribPointer(posAttrib, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), 0);

    GLint colAttrib = glGetAttribLocation(shaderProgram, "color");
    glEnableVertexAttribArray(colAttrib);
    glVertexAttribPointer(colAttrib, 3, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), (void*)(2 * sizeof(GLfloat)));

    glBindVertexArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

    glBindVertexArray(vao);
    while (!glfwWindowShouldClose(window)) {
        glClearColor(1.0f, 1.0f, 1.0f, 0.0f);
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);    

        if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
            glfwSetWindowShouldClose(window, GL_TRUE);

        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    // Prepare to close the application
    glDeleteProgram(shaderProgram);
    glDeleteShader(fragmentShader);
    glDeleteShader(vertexShader);
    glDeleteBuffers(1, &vbo);
    glDeleteBuffers(1, &vao);
    glDeleteBuffers(1, &eab);

    return 0;
}

I'm getting the following error and it happens only when I unbind the VBO. Unbinding the VAO and the IBO doesn't throw any exception.

Unhandled exception at 0x0282D434 (ig4icd32.dll) in main.exe: 0xC0000005: Access violation reading location 0x00000000.

Is there a problem with the way I did the unbinding?

Answer 1

Before glfwCreateWindow() call these functions:

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

And after glfwMakeContextCurrent(window) call:

printf("OpenGL version: %s\n", glGetString(GL_VERSION));