OpenGL + GLFW simple

#include <iostream>
#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <glm/glm.hpp>
using namespace std;

void error_callback(int error, const char *description);
void key_callback(GLFWwindow *window, int key, int scancode, int action, int mods);
void framebuffer_size_callback(GLFWwindow *window, int w, int h);

GLFWwindow *window = NULL;
int width, height;
void init_glfw();

const GLfloat vertex_data[] = {
    -0.75f, -0.75f, 0.0f, 1.0f,
     0.0f,   0.75f, 0.0f, 1.0f,
     0.75f, -0.75f, 0.0f, 1.0f
};

int main()
{
    init_glfw();

    GLuint positionBO;
    glGenBuffers(1, &positionBO);
    glBindBuffer(GL_ARRAY_BUFFER, positionBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertex_data), vertex_data, GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    while(!glfwWindowShouldClose(window)) {
        glClear(GL_COLOR_BUFFER_BIT);
        glViewport(0, 0, width, height);

        glBindBuffer(GL_ARRAY_BUFFER, positionBO);
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);

        glDrawArrays(GL_TRIANGLES, 0, 3);


        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glfwDestroyWindow(window);
    glfwTerminate();
    return 0;
}

void key_callback(GLFWwindow *window, int key, int scancode, int action, int mods)
{
    if (key == GLFW_KEY_ESCAPE and action == GLFW_PRESS)
        glfwSetWindowShouldClose(window, GL_TRUE);
}

void init_glfw()
{
    glfwSetErrorCallback(error_callback);
    if (!glfwInit())
        exit(EXIT_FAILURE);

    window = glfwCreateWindow(640, 480, "OpenGL", NULL, NULL);
    if (!window) {
        glfwTerminate();
        exit(EXIT_FAILURE);
    }

    glfwMakeContextCurrent(window);
    glewInit();
    glfwSetKeyCallback(window, key_callback);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);
    glfwGetFramebufferSize(window, &width, &height);
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
}

void framebuffer_size_callback(GLFWwindow *window, int w, int h)
{
    cout << "Resized to: " << w << "x" << h << endl;
    width  = w;
    height = h;
    //glViewport(0, 0, width, height);
}

void error_callback(int error, const char *description)
{
    cerr << description;
}