Compiling module for lua with gcc - c++

Question

I have the following c++ code for testing:
#include <lua.hpp>
#include <iostream>
static int dummy(lua_State * L)
{
std::cout << "Test";
return 0;
}
int luaopen_testlib(lua_State * L)
{
lua_register(L,"dummy",dummy);
return 0;
}
I compile it with commands and it gives me no errors:
g++ -Wextra -O2 -c -o testlib.o main.cpp
g++ -shared -o testlib.so testlib.o
But when i try to load it in lua i get undefined symbol error as this:
Lua 5.1.5 Copyright (C) 1994-2012 Lua.org, PUC-Rio
> require"testlib"
error loading module 'testlib' from file './testlib.so':
./testlib.so: undefined symbol: _Z16lua_pushcclosureP9lua_StatePFiS0_Ei
It seems for me that there is something missing in the g++ commands, but i have been searching solution for whole morning and can't get this simple example to compile.
EDIT:
after few recompilations it returned to:
error loading module 'testlib' from file './testlib.so':
./testlib.so: undefined symbol: luaopen_testlib
which was solved by adding :
extern "C"
{
int luaopen_testlib(lua_State *L)
{
lua_register(L,"dummy",dummy);
return 0;
}
}

Answer 1

The Lua binary is compiled as C code, the library tries to use it as C++. That will not work as C++ does name mangling to support overloading. As C does not support overloading it does not need the name mangling and will not understand mangled names.
The solution to this is to tell the C++ compiler that the Lua functions it is going to interact with are straight C and they need no name mangling.
Also the luaopen_testlib function must be extern "C" as it will be called from C code with no mangling.
extern "C" {
#include <lua.h>
}
#include <iostream>
static int dummy(lua_State * L)
{
(void)L;
std::cout << "Test"<<std::endl;
return 0;
}
extern "C"
int luaopen_testlib(lua_State * L)
{
lua_register(L,"dummy",dummy);
return 0;
}
I ran my test with Lua 5.4.2 and used the following commands to build the library:
g++ -Wall -Wextra -O2 -Isrc -c -fPIC -o testlib.o testlib.cpp
g++ -shared -o testlib.so testlib.o
Note the -Isrc is needed to find lua.h in my test setup and -fPIC was required to use cout in the library (but that may depend on the compiler version used).
and the result is:
Lua 5.4.2 Copyright (C) 1994-2020 Lua.org, PUC-Rio
> require 'testlib'
true ./testlib.so
> dummy
function: 0x7ff07d2a0aa0
> dummy()
Test
>
The Lua version used will, in this case, not make any difference.

Answer 2

Try to use Luabind. Here is the Hello World example
#include <iostream>
#include <luabind/luabind.hpp>
void greet()
{
std::cout << "hello world!\n";
}
extern "C" int init(lua_State* L)
{
using namespace luabind;
open(L);
module(L)
[
def("greet", &greet)
];
return 0;
}

Answer 3

This is how i would compile lua, is not exacly gcc but cmake can use gcc.
.
├── CMakeList.txt (A)
├── main.cpp
├── lua_535
│ ├── CMakeLists.txt (B)
│ └── * lua_content *
main.cpp | Just checks if it works
#include <iostream>
#include <string>
#include "lua.hpp"
int main(){
lua_State * lua = luaL_newstate();
std::string str_acction = "a = 5";
int res = luaL_dostring(lua, str_acction.c_str());
std::cout << "DS State > " << res << std::endl;
lua_close(lua);
return 0;
}
CMakeList.txt (A) | Creates the executable and links the library
cmake_minimum_required(VERSION 3.12)
project(lua_test)
add_executable(main main.cpp)
add_subdirectory("lua_535")
target_link_libraries(main PUBLIC lua_lib)
CMakeList.txt (B) | Joins Lua files into a library
Get the latest lua source files
Extract the content into a sub folder
Add this file into the folder
cmake_minimum_required(VERSION 3.12)
project( lua_lib )
set ( LUA_EMBEDDED ON )
set ( LUA_RUNTIME_MAIN "src/luac.c" )
set (LUA_RUNTIME_SOURCES
"src/lapi.c"
"src/lapi.h"
"src/lauxlib.c"
"src/lauxlib.h"
"src/lbaselib.c"
"src/lbitlib.c"
"src/lcode.c"
"src/lcode.h"
"src/lcorolib.c"
"src/lctype.c"
"src/lctype.h"
"src/ldblib.c"
"src/ldebug.c"
"src/ldebug.h"
"src/ldo.c"
"src/ldo.h"
"src/ldump.c"
"src/lfunc.c"
"src/lfunc.h"
"src/lgc.c"
"src/lgc.h"
"src/linit.c"
"src/liolib.c"
"src/llex.c"
"src/llex.h"
"src/llimits.h"
"src/lmathlib.c"
"src/lmem.c"
"src/lmem.h"
"src/loadlib.c"
"src/lobject.c"
"src/lobject.h"
"src/lopcodes.c"
"src/lopcodes.h"
"src/loslib.c"
"src/lparser.c"
"src/lparser.h"
"src/lprefix.h"
"src/lstate.c"
"src/lstate.h"
"src/lstring.c"
"src/lstring.h"
"src/lstrlib.c"
"src/ltable.c"
"src/ltable.h"
"src/ltablib.c"
"src/ltm.c"
"src/ltm.h"
"src/lua.c"
"src/lua.h"
"src/lua.hpp"
"src/luaconf.h"
"src/lualib.h"
"src/lundump.c"
"src/lundump.h"
"src/lutf8lib.c"
"src/lvm.c"
"src/lvm.h"
"src/lzio.c"
"src/lzio.h"
)
add_library( lua_lib "${LUA_RUNTIME_SOURCES}" )
if( NOT LUA_EMBEDDED)
add_library( lua_lib "${LUA_RUNTIME_MAIN}")
endif()
target_include_directories ( lua_lib PUBLIC "${PROJECT_SOURCE_DIR}/src")
If lua is enbedded src/luac.c should be excluded because conteins a int main(){}