Why is my GLFW window so slow? - performance

Question

For some reason the following code produces a result such as this:
I have no idea why it takes a second or two to render each frame. The code seems to be normal to me, but I have no idea why it renders it so slowly.
#define GLFW_INCLUDE_GLU
#define GLEW_STATIC
#include"GL/glew.c"
#include"GLFW/glfw3.h"
#include<cmath>
#include<ctime>
#include<stdlib.h>
using namespace std;
int main( ) {
// init glfw
if ( !glfwInit( ) ) return 0;
// create window and set active context
GLFWwindow* window = glfwCreateWindow( 640, 480, "Test", NULL, NULL );
glfwMakeContextCurrent( window );
// init glew
if ( glewInit( ) != GLEW_OK ) return 0;
// set swap interval
glfwSwapInterval( 1 );
// render loop
while ( !glfwWindowShouldClose( window ) ) {
srand( time( NULL ) );
float r = ( rand( ) % 100 ) / 100.0;
float ratio;
int width, height;
glfwGetFramebufferSize( window, &width, &height );
ratio = width / ( float ) height;
glViewport( 0, 0, width, height );
// render
glClear( GL_COLOR_BUFFER_BIT );
glClearColor( r, 0, 0, 1 );
// swap
glfwSwapBuffers( window );
// events
glfwPollEvents( );
}
// terminate glfw
glfwTerminate( );
return 0;
}

Answer 1

Your GLFW code is correct, and it is performing much faster than you think.
This is incorrect usage of rand and srand. More concretely, you srand with the current time (measured in seconds) every time you render, which will, within the same second, produce exactly the same value for r every time.
You therefore clear the screen to the same color several hundred times per second. This looks like it only renders at one frame per second, but it really isn't.
There are a few other problems with your code (such as using rand()%100 which gives a biased distribution. and using some OpenGL commands redundantly), but the one thing you need to fix for your immediate problem is: srand only once, not every time you render.