Storing state in Lua

There are two ways to store state in Lua for lua_CFunction: upvalues and the registry. Let’s recap them and dig deeper into upvalues.

Upvalues

To introduce the complete definition for upvalues, we need to introduce Lua C closures at the same time. To quote the Lua reference manual:

When a C function is created, it is possible to associate some values with it, thus creating a C closure; these values are called upvalues and are accessible to the function whenever it is called.

To put it simply, the closure is still our old friend lua_CFunction. When you associate some values with it, it becomes a closure, and the values become upvalues.

It is important to note that Lua C closures and upvalues are inseparable.

To create a closure, use the following library function:

void lua_pushcclosure(
    lua_State *L, lua_CFunction fn, int n);

This creates a closure from lua_CFunction and associates n values with it.

To see...