How to use LuaJIT's ffi module when embedding? - c

Question

I'm trying to embed LuaJIT into a C application. The code is like this:
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <stdlib.h>
#include <stdio.h>
int barfunc(int foo)
{
/* a dummy function to test with FFI */
return foo + 1;
}
int
main(void)
{
int status, result;
lua_State *L;
L = luaL_newstate();
luaL_openlibs(L);
/* Load the file containing the script we are going to run */
status = luaL_loadfile(L, "hello.lua");
if (status) {
fprintf(stderr, "Couldn't load file: %s\n", lua_tostring(L, -1));
exit(1);
}
/* Ask Lua to run our little script */
result = lua_pcall(L, 0, LUA_MULTRET, 0);
if (result) {
fprintf(stderr, "Failed to run script: %s\n", lua_tostring(L, -1));
exit(1);
}
lua_close(L); /* Cya, Lua */
return 0;
}
the Lua code is like this:
-- Test FFI
local ffi = require("ffi")
ffi.cdef[[
int barfunc(int foo);
]]
local barreturn = ffi.C.barfunc(253)
io.write(barreturn)
io.write('\n')
It reports error like this:
Failed to run script: hello.lua:6: cannot resolve symbol 'barfunc'.
I've searched around and found that there're really little document on the ffi module. Thanks a lot.

Answer 1

ffi library requires luajit, so you must run lua code with luajit.
From the doc:
"The FFI library is tightly integrated into LuaJIT (it's not available as a separate module)".
How to embed luajit?
Look here http://luajit.org/install.html under "Embedding LuaJIT"
Under mingw your example run if i add
__declspec(dllexport) int barfunc(int foo)
at the barfunc function.
Under Windows luajit is linked as a dll.

Answer 2

As misianne pointed out, you need to export the function, which you can do by using extern if you are using GCC:
extern "C" int barfunc(int foo)
{
/* a dummy function to test with FFI */
return foo + 1;
}
If you are experiencing problems with undefined symbols under Linux using GCC, take care to have the linker add all symbols to the dynamic symbol table, by passing the -rdynamic flag to GCC:
g++ -o application soure.cpp -rdynamic -I... -L... -llua

Answer 3

For those of you trying to make this work on Windows with VC++ (2012 or later), using the C++ compiler:
make sure you use the .cpp extension, as this will do C++ compilation
make the function have external C linkage so that ffi can link to it, with extern "C" { ... }
export the function from the executable, with __declspec(dllexport)
optionally specify the calling convention __cdecl, not required because should be it by default and not portable
wrap the Lua headers in an extern "C" { include headers }, or better just #include "lua.hpp"
#include "lua.hpp"
extern "C" {
__declspec(dllexport) int __cdecl barfunc(int foo) {
return foo + 1;
}}