CODEWITHSUNDEEP
crashluastack-overflow
Patillac
Patillac

Reputation: 313

Lua: How to properly clean up errors returned by lua_pcall

I've been having a crash problem with Lua for a little while now, and I finally discovered what I believe to be the problem. I'm allowing the script to omit functions that are attempted to be called for convenience. I want my application to attempt to invoke TestFun (as an example), and if it exists then execute it, otherwise gracefully and silently fail.

The problem I was having was that I simply invoked lua_pcall(L, 0, 0, 0) and ignored the return value because it didn't matter to me. What I discovered was that when Lua generates the error "attempt to call nil" it places this on its stack and I was not popping this off. The code below exhibits a crash shortly after being run due to the stacksize growing too large:

int _tmain(int argc, _TCHAR* argv[])
{
    std::string script = "";
    lua_State* L = luaL_newstate();
    luaL_openlibs(L);
    luaL_loadstring(L, script.c_str());
    lua_pcall(L, 0, LUA_MULTRET, 0);

    while (true)
    {
        lua_getglobal(L, "TestFunc");
        lua_pcall(L, 0, 0, 0);
    }

    return 0;
}

I simply modified my code inside the while loop to be:

while (true)
{
    lua_getglobal(L, "TestFunc");
    if (lua_pcall(L, 0, 0, 0))
        lua_pop(L, -1);
}

And this solved my crash. My question is whether or not this is valid in all cases of lua_pcall() error results, or if I'm setting myself up for another bug/crash by not specifically checking if I should pop -1 (or possibly others?). Perhaps I should only pop if -1 is a string, or perhaps there's a standard 'cleanup the stack' function I could call?

Thanks

Upvotes: 3

Views: 1663

Answers (1)

lhf
lhf

Reputation: 72312

Your modified code is fine except that you need to pop 1, not -1.

Upvotes: 3

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