Question

const char *lua_setupvalue (lua_State *L, int funcindex, int n);

Sets the value of a closure's upvalue. Parameters funcindex and n are as in lua_getupvalue (see lua_getupvalue). It assigns the value at the top of the stack to the upvalue and returns its name. It also pops the value from the stack.

Returns NULL (and pops nothing) when the index is greater than the number of upvalues.