Function pushcclosure 

pub unsafe extern "C" fn pushcclosure(
    state: lua_State,
    func: lua_CFunction,
    upvalues: i32,
)

Pushes a new C closure onto the stack.

When a C function is created, it is possible to associate some values with it, thus creating a C closure (see §3.4); these values are then accessible to the function whenever it is called. To associate values with a C function, first these values should be pushed onto the stack (when there are multiple values, the first value is pushed first). Then pushcclosure (lua_pushcclosure) is called to create and push the C function onto the stack, with the argument n telling how many values should be associated with the function. pushcclosure (lua_pushcclosure) also pops these values from the stack.

The maximum value for upvalues is 255.