Lua: Can this cause a segfault

Question

I'm developing a program which uses Lua for scripting, and sometimes it would crash. With GDB I think I found the problem, but I don't know if it solved it, as the segfault would only occur sporadically. So, the old code was this:

void Call(std::string func){
    lua_getglobal(L, func.c_str()); //This is the line GDB mentioned in a backtrace
    if( lua_isfunction(L,lua_gettop(L)) ) {
        int err = lua_pcall(L, 0, 0,0 );
        if(err != 0){
            std::cout << "Lua error: " << luaL_checkstring(L, -1) << std::endl;
        }
    }
}

The thing is, that this function would be called a few times per second, but the function it needs to call isn't always defined, so I thought that the stack would overflow. I added the following line:

lua_pop(L,lua_gettop(L));

And the segfault hasn't occurred anymore. Could this have been the problem?

Answer 1

Using lua_gettop(L) as the argument for lua_pop will clear the entire Lua API stack (it is equivalent to lua_settop(0)), which is probably not what you want. But indeed your problem is that lua_getglobal always pushes something; if no global with the given name exists, it pushes nil, just like the equivalent Lua expression would do. However, lua_pcall pops the function and all arguments (if any; in your case you specified zero), so you won't get a problem if the function exists. What you should do is adding lua_pop(L, 1) to an else clause of the outer if. This way, your function will always be balanced (i.e. leave the stack as it was).

You can see these things in the Lua manual: For every function, it is spelled out in the description and also indicated in grey, in brackets beside the functions prototype. For example, lua_getglobal has [-0, +1, e] meaning that it will pop no elements and (always) push one element (and the e meanst that it may cause errors).