在lua中加载C ++模块时出现“尝试为字符串值建立索引”错误

Question

I'm trying to use a function written in C++ from lua. Given below is the cpp file:

extern "C"
{
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"
}

static int  add_5(lua_State *L)
{
  double d = lua_tonumber(L, 1); /* get argument */
  d=d+5;
  lua_pushnumber(L, d); /* push result */
  return 1; /* number of results */
}

static const struct luaL_Reg mylib [] =
{
  {"add_5", add_5},
  {NULL, NULL} /* sentinel */
};

extern "C"
{
  int luaopen_mylib (lua_State *L)
  {
    //luaL_newlib(L, mylib);
    luaL_register(L, NULL, mylib);
    return 1;
  }
}

I compiled the above code by g++ using the following command:

g++ -shared -o mylib.so test.cpp -fPIC

I'm getting the following error on the lua interpreter:

Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> local temp = require "mylib"
attempt to index a string value
stack traceback:
        [C]: ?
        [C]: in function 'require'
        stdin:1: in main chunk
        [C]: ?

Please note that I can't upgrade the version of Lua due to some reasons.

Answer 1

The second argument to luaL_register is the library name. You can leave it as NULL , but if you do, luaL_register will try to insert the registered functions into the table it expects to find on the top of the stack (and in your code there's no table on top of the stack). For the general case of registering a library, it's easiest to pass your library name as the second parameter.

Note that LHF suggests not doing it that way, since it automatically puts the libary's table into the global table, whereas the user of the library might want to have it only as a local variable. The alternative is to create your own table with lua_newtable before calling luaL_register (with a null name).