Marcus Irven

February 19, 2009

Dangers of lua_error when using C++

By default Lua uses longjmp() when lua_error() or luaL_error() is called, this can produce subtle bugs or memory leaks in C++ if you are not careful.

Take the following code, which simply calls the foo function which creates an object on the stack and throws an error:

   1  #include <iostream>
   2  
   3  extern "C" {
   4      #include "lua.h"
   5      #include "lualib.h"
   6      #include "lauxlib.h"
   7  }
   8  
   9  class Obj {
  10  public:
  11      Obj() {
  12          std::cout << "object create" << std::endl;
  13      }
  14  
  15      ~Obj() {
  16          std::cout << "object destroy" << std::endl;
  17      }
  18  };
  19  
  20  int foo(lua_State*L) {
  21      Obj obj;
  22      return luaL_error(L, "error in foo");   
  23  }
  24  
  25  
  26  int main(int argc, char** argv) {
  27      lua_State* L = lua_open();
  28  
  29      lua_pushcfunction(L, foo);
  30      int r = lua_pcall(L, 0,0,0);
  31  
  32      if (r == LUA_ERRRUN) {
  33          std::cout << lua_tostring(L, -1) << std::endl;
  34      }
  35  
  36      lua_close(L);
  37      return 0;
  38  }

When compiled against the default build of the Lua library and run you get the following output:

$ g++ main.cpp -llua -o example
$ ./example 
object create
error in foo

Notice the destructor was never called on obj. However if you build Lua as C++ then lua_error() will be implemented using C++ exceptions which will cause the stack to clean up as expected.

On my Mac I built Lua as C++ with the following command:

make macosx -e CC=g++

I’ve built Lua on Windows as C++ as well, in Visual Studio there is a setting to compile sources as C++ rather than by file extensions which is the default. Now when building and running you get the output you would expected:

$ g++ main.cpp -llua -o example
$ ./example
object create
object destroy
error in foo

One other thing to note is that the extern “C” around the Lua includes is no longer needed after building the Lua library as C++. If you can’t rebuild the Lua library for some reason or another than you just need to be aware and make sure everything is cleaned up before calling lua_error() or luaL_error().