Re: Install __newindex for _G in C
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- Subject: Re: Install __newindex for _G in C
- From: Sean Conner <sean@...>
- Date: 2021年10月14日 18:17:50 -0400
It was thus said that the Great Flyer31 Test once stated:
> Hi,
> I would like to install a metatable for global environment _G from C code.
>
> http://lua-users.org/wiki/DetectingUndefinedVariables
>
> gives a way to do this in Lua code (see the function GLOBAL_lock) on this page.
>
> function GLOBAL_lock(t)
> local mt = getmetatable(t) or {}
> mt.__newindex = lock_new_index
> setmetatable(t, mt)
> end
>
> I tried to use the following code in my C program:
>
> void GLOBAL_lock (lua_State *L) {
> luaL_newmetatable( L, "_G"));
> lua_pushvalue(L, -1); /* duplicate the metatable */
> lua_setfield(L, -2, "__index");
> lua_pushcfunction( L, lock_new_index);
> lua_setfield( L, -2, "__newindex");
> luaL_setmetatable( L, "_G");
> }
>
> This compiles fine, and if I check lua_gettop at end of this function,
> also all fine, also luaL_getmetatable will then return me some table
> for "_G"... . So it looks as if it has worked.
>
> But my function lock_new_index (int lock_new_index( lua_State *L) {
> ... } ) is never invoked.
>
> Can anybody give me some hint, what code in my C program might be
> wrong / missing to get this run?
The solution you want is (assuming Lua 5.2 or higher here):
static int foo___index (lua_State *L);
static int foo___newindex(lua_State *L);
static luaL_Reg const metatable[] =
{
{ "__index" , foo___index } ,
{ "__newindex" , foo___newindex } ,
/* other metatable methods as required */
{ NULL , NULL }
};
/*-----------------------------------------------------------------
; Obtain the global table. We don't use "_G" as this is just a
; convenience value and one I wouldn't fully rely upon. Following
; the Lua manual, a reference to the global table is stored in the
; registry. So we get it from there.
;------------------------------------------------------------------*/
lua_geti(L,LUA_REGISTRYINDEX,LUA_RIDX_GLOBALS); /* obtain _G */
luaL_newlib(L,metatable); /* create metatable */
lua_setmetatable(L,-2); /* and set it */
The functions luaL_setmetatable() and lual_getmetatable() are really meant
to be used for userdata. The name parameter isn't the name of a global
variable, but instead a name indentifying that particular metatable. For
example, I create a new userdata type foo and associate a metatable for it:
static int foo___index (lua_State *L);
static int foo___newindex (lua_State *L);
static int foo___len (lua_State *L);
static luaL_Reg const foo_metatable[] =
{
{ "__index" , foo___index } ,
{ "__newindex" , foo___newindex } ,
{ "__len" , foo___len } ,
{ NULL , NULL }
};
luaL_newmetatable(L,"TYPE_FOO");
luaL_setfuncs(L,foo_metatable,0);
Then, when a new foo is created:
Foo *foo = luaL_newuserdata(L,sizeof(Foo));
luaL_setmetatable(L,"TYPE_FOO");
Alternatively (to remain compatible with Lua 5.1):
Foo * foo = luaL_newuserdata(L,sizeof(Foo));
luaL_getmetatable(L,"TYPE_FOO");
lua_setmetatable(L,-2);
-spc (Hope that clears it up some)