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Lyte _ wrote:
When I run this the function I added to the metatable does not get called. What am I doing wrong here?

Looks fine. But could you please inform us what Lua version you are using. How you check whether __gc metamethod gets called or not. Can is it possible that CreateObject leves reference to return value somewhere. Could it be that you modified metatable of global environment? Try inserting some assertions just after luaL_getmetatable (for example ASSERT(lua_istable(L, -1))), etc. Also try to provide complete minimal example representing your problem. Because the following example works fine with Lua 5.1.1:

// begin code
#include "lua.h"
#include "lauxlib.h"
#include <stdio.h>

static int gc_function (lua_State *L) {
 return 0;

static void registermt(lua_State *L) {
 luaL_newmetatable(L, "LuaBook.gc_event");

 /* set its __gc field */
 lua_pushstring(L, "__gc");
 lua_pushcfunction(L, gc_function);
 lua_settable(L, -3);

 lua_pop(L, 1); /* pop mt */

static int create(lua_State *L) {
 lua_newuserdata(L, sizeof(char));
 luaL_getmetatable(L, "LuaBook.gc_event");
 lua_setmetatable(L, -2);
 return 1;

int main() {
 lua_State *L = luaL_newstate();
lua_register(L, "create", create); luaL_openlibs(L);
 luaL_dostring(L, "o = create();"                  \
                  "o = nil;"                       \
                  "print 'before collectgarbage';" \
                  "collectgarbage();"                \
"print 'after collectgarbage';"); lua_close(L);
 return 0;
// end code