Error loading module undefined symbol: luaL_setfuncs

Question

I am trying to create a C module to be called from a lua script. I am working on debian linux. I am using mysql-proxy and lua 5.2. I have created (copied from a tutorial) some example functions to be called.

The loader is defined like this:

int luaopen_luacall(lua_State* l)
{

    luaL_newlibtable(l, luacall);
    luaL_setfuncs(l, luacall, 0);
    return 1;
}

To call this from lua I use this code:

luacall = require("luacall")
local f = luacall.fun1()

I have compiled it with this command:

g++ -shared -Wl,-E,-soname,libluacall.so -o luacall.so luacall.c  -fPIC -llua -ldl

When I try to run the script I get the following error on the require command:

 error loading module 'luacall' from file '/usr/lib/mysql-proxy/lua/luacall.so':
        /usr/lib/mysql-proxy/lua/luacall.so: undefined symbol: luaL_setfuncs

I am really lost on what I am doing wrong.

Answer 1

Never use -llua when building Lua modules. The Lua interpreter itself is already linked with liblua and satisfies those symbols when the module is loaded. Linking your module against liblua is clashing with the interpreter.

Answer 2

I think I found the problem (not yet the solution): Mysql-proxy runs internally an embended lua library.

mysql-proxy -V

gives as result

mysql-proxy 0.8.1
  chassis: mysql-proxy 0.8.1
  glib2: 2.30.2
  libevent: 2.0.19-stable
  LUA: Lua 5.1.4
    package.path: /usr/lib/mysql-proxy/lua/?.lua
    package.cpath: /usr/lib/mysql-proxy/lua/?.so
-- modules
  admin: 0.8.1
  proxy: 0.8.1

So I am running the wrong lua version. I think that this explains the luaL_setfuncs error. I have seen that even the 0.8.4 version includes this version of lua, so I will have to rewrite the C library.

the final code of the module ends like this (and runs!!!):

static const struct luaL_Reg my_luacall[] = {
    {"trasnquery", trasnquery},
    {"fun2", function_2},
    {NULL, NULL}
};

int luaopen_luacall(lua_State* l)
{
    luaL_register(l, "luacall", my_luacall);
    return 1;
}