OpenGL - second version

void windowSizeCallback(int width, int height)
{
	glViewport(0, 0, width, height);
}

int main()
{
	if (!glfwInit()) {
		std::cout << "Couldn't initialize GLFW\n";
		return 1;
	}

	if (!glfwOpenWindow(400, 300, 0, 0, 0, 0, 0, 0, GLFW_WINDOW)) {
		std::cout << "Couldn't open window\n";
		return 1;
	}

	if (glewInit() != GLEW_OK) {
		std::cout << "Couldn't initialize glew\n";
		return 1;
	}

	glfwSetWindowTitle("Test");
	glfwSetWindowSizeCallback(windowSizeCallback);

	glfwOpenWindowHint(GLFW_OPENGL_VERSION_MAJOR, 4);
	glfwOpenWindowHint(GLFW_OPENGL_VERSION_MINOR, 0);
	glfwOpenWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

	const char* vertexShaderSource =
		"#version 330\n"
		"in vec4 inPosition;\n"
		"in vec4 inColor;\n"
		"out vec4 vertexColor;\n"
		"void main()\n"
		"{\n"
		"    vertexColor = inColor;\n"
		"    gl_Position = inPosition;\n"
		"}";
	const char* fragmentShaderSource =
		"#version 330\n"
		"in vec4 vertexColor;\n"
		"out vec4 fragColor;\n"
		"void main()\n"
		"{\n"
		"    fragColor = vertexColor;\n"
		"    //fragColor = vec4(1.0, 1.0, 1.0, 1.0);\n"
		"}";

	GLuint vertexShader(glCreateShader(GL_VERTEX_SHADER))
		 , fragmentShader(glCreateShader(GL_FRAGMENT_SHADER))
		 , shaderProgram(glCreateProgram());

	glShaderSource(vertexShader, 1, &vertexShaderSource, 0);
	glCompileShader(vertexShader);
	glShaderSource(fragmentShader, 1, &fragmentShaderSource, 0);
	glCompileShader(fragmentShader);
	glAttachShader(shaderProgram, vertexShader);
	glAttachShader(shaderProgram, fragmentShader);
	glLinkProgram(shaderProgram);

	GLuint positionLoc(glGetAttribLocation(shaderProgram, "inPosition"))
		 , colorLoc(glGetAttribLocation(shaderProgram, "inColor"));

	float positionData[] =
		{ -0.5, -0.5, 0.0, 1.0
		,  0.5, -0.5, 0.0, 1.0
		, -0.5,  0.5, 0.0, 1.0
		,  0.5,  0.5, 0.0, 1.0
		};

	float colorData [] =
		{ 1.0, 1.0, 1.0, 1.0
		, 1.0, 1.0, 1.0, 1.0
		, 1.0, 1.0, 1.0, 1.0
		, 1.0, 1.0, 1.0, 1.0
		};

	GLuint vao, positionBuffer, colorBuffer;
	glGenBuffers(1, &positionBuffer);
	glGenBuffers(1, &colorBuffer);

	glGenVertexArrays(1, &vao);
	glBindVertexArray(vao);

	glEnableVertexAttribArray(positionLoc);
	glBindBuffer(GL_ARRAY_BUFFER, positionBuffer);
	glBufferData(GL_ARRAY_BUFFER, sizeof(positionData), positionData, GL_STATIC_DRAW);
	glVertexAttribPointer(positionLoc, 4, GL_FLOAT, GL_FALSE, 0, 0);
	glBindBuffer(GL_ARRAY_BUFFER, 0);

	glEnableVertexAttribArray(colorLoc);
	glBindBuffer(GL_ARRAY_BUFFER, colorBuffer);
	glBufferData(GL_ARRAY_BUFFER, sizeof(colorData), colorData, GL_STATIC_DRAW);
	glVertexAttribPointer(colorLoc, 4, GL_FLOAT, GL_FALSE, 0, 0);
	glBindBuffer(GL_ARRAY_BUFFER, 0);

	//glBindVertexArray(0);
	glClearColor(0.5, 0.5, 0.5, 1.0);

	// Main loop
	while (true) {
		glClear(GL_COLOR_BUFFER_BIT);

		//glBindVertexArray(vao);
		glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
		//glBindVertexArray(0);

		glfwSwapBuffers();
		if (glfwGetKey(GLFW_KEY_ESC) || !glfwGetWindowParam(GLFW_OPENED)) {
			break;
		}
	}
	glfwTerminate();
	return 0;
}