luaL_dofile fails on known-good bytecode, works with uncompiled version - c++

Question

I've put together a very simple Lua engine but it seems to reject bytecode which works in the lua console. The uncompiled version works in the engine. Am I using luac wrong somehow?
I compile using the given command and run as './a.out'.
res/default.lua:
print("Setting up world structure.")
luac command:
luac -o res/default.lux res/default.lua
MWE:
#define SCRIPTDIR "res/"
#define THROW_IF_NONZERO(x,m) if((x)!=0) throw std::runtime_error(m);
#define THROW_IF_ZERO(x,m) if((x)==0) throw std::runtime_error(m);
extern "C" {
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"
}
#include "sys/stat.h"
#include <string>
#include <system_error>
#include <iostream>
using std::string;
class Entity {
private:
lua_State *m_lua;
public:
Entity() : Entity(nullptr) { }
Entity(lua_State *lua) : m_lua{lua} { }
virtual ~Entity() { }
void load_and_run(string);
};
class WorldEntity : public Entity {
public:
WorldEntity(lua_State *lua) : Entity(lua) {
luaL_openlibs(lua);
}
~WorldEntity() { }
};
int main() {
lua_State *lua{nullptr};
try {
lua = luaL_newstate();
WorldEntity eWorld{lua};
eWorld.load_and_run("default"); // load default.lua/lux
} catch(std::exception &e) {
if (lua != nullptr) {
lua_close(lua);
}
std::cout << "Error: " << e.what() << std::endl;
}
return 0;
}
void Entity::load_and_run(string filename) {
THROW_IF_ZERO(m_lua, "Lua not started.");
filename = SCRIPTDIR + filename + ".lux";
struct stat sb;
int rc = stat(filename.c_str(), &sb);
if (rc == -1) {
filename.pop_back();
filename += "a";
rc = stat(filename.c_str(), &sb);
THROW_IF_NONZERO(rc, "File not found!");
}
std::cout << "File: " << filename << std::endl;
// Currently won't run compiled Lua scripts, not sure why.
rc = luaL_dofile(m_lua, filename.c_str());
THROW_IF_NONZERO(rc, "Could not load lua file.");
}
compile command:
gcc src/bug001mwe.cpp -std=c++14 -llua -lstdc++
correct output from script:
File: res/default.lua
Setting up world structure.
wrong output from bytecode:
File: res/default.lux
Error: Could not load lua file.
both files, output from lua console:
Setting up world structure.

Answer 1

What confused me was that it worked in the lua console but not in my program. I added a call to lua_tostring after the call to luaL_dofile, like this:
rc = luaL_dofile(m_lua, filename.c_str());
std::ostringstream ostr;
ostr << "Could not load lua file. ";
ostr << lua_tostring(m_lua, -1);
THROW_IF_NONZERO(rc, ostr.str());
The error string became:
Error: Could not load lua file. res/default.lux: version mismatch in precompiled chunk
What the heck?
Long story short, I had a previous version of Lua installed due to out of date package dependencies in some unrelated stuff. The older luac was intercepting the luac command and compiling to valid but incompatible bytecode. Uninstalled the unrelated packages which I didn't really need, and now everything works.
Moral of the story: always check for an error string on the Lua stack, it will (probably) tell you what's wrong.