Observe the following complete, compile-able (and mostly C) code, which stores a simple "luaL_ref" wrapper in a struct named "co_test" and links up a simple creation method ("new") and a simple deletion method ("__gc"):
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <assert.h>
typedef struct co_test {
lua_State* L;
int ref;
} co_test;
void co_test_make(co_test* p, lua_State* L, int stack_target) {
lua_pushvalue(L, stack_target); // copy value
p->L = L;
p->ref = luaL_ref(L, LUA_REGISTRYINDEX); // save in registry
}
void co_test_destroy(co_test* p) {
luaL_unref(p->L, LUA_REGISTRYINDEX, p->ref);
p->ref = 0;
p->L = NULL;
}
int co_test_new(lua_State* L) {
co_test* p = (co_test*)lua_newuserdata(L, sizeof(co_test));
luaL_setmetatable(L, "co_test"); // bless userdata with metatable
co_test_make(p, L, 1);
return 1;
}
int co_test___gc(lua_State* L) {
co_test* p = (co_test*)lua_touserdata(L, 1);
co_test_destroy(p);
return 0;
}
int main(int, char*[]) {
const char code[] = R"(
local co = coroutine.wrap(
function()
local t = co_test.new({ "SOME_TABLE" })
-- STUFF
end
)
co() -- call
co = nil -- destroy coroutine/underlying thread
collectgarbage() -- collect garbage
)";
lua_State* L = luaL_newstate();
luaL_Reg registration[3] = { 0 };
int pre_stack_check = 0;
int post_stack_check = 0;
luaL_requiref(L, "base", luaopen_base, 1);
lua_pop(L, 1);
luaL_requiref(L, "coroutine", luaopen_coroutine, 1);
lua_pop(L, 1);
pre_stack_check = lua_gettop(L);
if (luaL_newmetatable(L, "co_test") == 1) {
registration[0] = luaL_Reg{ "new", &co_test_new };
registration[1] = luaL_Reg{ "__gc", &co_test___gc };
luaL_setfuncs(L, registration, 0);
lua_pushvalue(L, -1); // duplicate metatable
lua_setfield(L, -1, "__index"); // loop-back lookup
}
lua_setglobal(L, "co_test");
post_stack_check = lua_gettop(L);
// sanity check
assert(pre_stack_check == post_stack_check);
if (luaL_dostring(L, code)) {
assert(false);
}
return 0;
}
In "co_test_destroy", the code segfaults (g++, clang++) / "Read Access Violation" (VC++) when doing "luaL_unref" when this code executes. I found that the reason is that the "co" variable, which holds the coroutine and thread represented by the "L" that the "co_test" was created with, dies. If you do not explicitly "nil" the "co" variable in the Lua code, the thread/state that "co_test" is created with stays alive.
