Where can I find frustum code?

OpenGL OpenGL: Basic Coding
1

I am looking for the implementation of frustum. My google search hasn’t helped so direct code or a link would be great!

mat4 frustum(GLfloat left, GLfloat right, GLfloat bottom, GLfloat top,
GLfloat near, GLfloat far);
2

[ul]
[li]OpenGL Spec, Version 2.1[/li][li]OpenGL Projection Matrix (Ahn)[/li][li]GLM - OpenGL Mathematics Library (Riccio)[/li][/ul]