Re: Arguments for a module loader

[Sean Conner <sean@...>]

It was thus said that the Great Matthias Kluwe once stated:
> Hi!
> 
> I must admit that I don't really understand the documentation of
> require (as given by
> http://www.lua.org/manual/5.2/manual.html#pdf-require).
> 
> After reading it several times I gave up and decided to help myself by
> some example code. I wrote the following simple module:
> 
> #include <stdio.h>
> #include <lua5.2/lua.h>
> 
> int luaopen_mod( lua_State *L ) {
>    printf( "stack size  : %d\n", lua_gettop( L ) );
>    printf( "typename   1: %s\n", lua_typename( L, 1 ) );
>    printf( "typename   2: %s\n", lua_typename( L, 2 ) );
>    printf( "value      1: %d\n", lua_toboolean( L, 1 ) );
>    printf( "isuserdata 2: %d\n", lua_isuserdata( L, 2 ) );
>    printf( "isstring   2: %d\n", lua_isstring( L, 2 ) );
>    printf( "value      2: %s\n", lua_tolstring( L, 2, 0 ) );
>    lua_newtable( L );
>    return 1;
> }
> 
> and compiled it with gcc -shared -o mod.so mod.c.
> 
> When called by `require 'mod'` the output is
> 
> stack size  : 2
> typename   1: boolean
> typename   2: userdata
> value      1: 1
> isuserdata 2: 0
> isstring   2: 1
> value      2: ./mod.so

  The major issue with your code is the use of lua_typename()---it doesn't
do what you think it does.  lua_typename() returns the name of the given
type, such that 1 is LUA_TBOOLEAN, 2 is LUA_TUSERDATA.  To get the type at
the given stack index, use luaL_typename().  If you try this:

#include <stdio.h>
#include <lua5.2/lua.h>
#include <lua5.2/lauxlib.h>

int luaopen_mod( lua_State *L ) {
   printf( "stack size  : %d\n", lua_gettop( L ) );
   printf( "typename   1: %s\n", luaL_typename( L, 1 ) );
   printf( "typename   2: %s\n", luaL_typename( L, 2 ) );
   printf( "value      1: %s\n", lua_tostring( L, 1 ) );
   printf( "isuserdata 2: %d\n", lua_isuserdata( L, 2 ) );
   printf( "isstring   2: %d\n", lua_isstring( L, 2 ) );
   printf( "value      2: %s\n", lua_tolstring( L, 2, NULL ) ); /* [1] */

   /*-----------------------------------------------------------------------
   ; [1] - please, please, please, ifyou are writing in C, use NULL instead
   ;       of a 0.  Even though NULL is 0, using NULL portrays the intent
   ;       better than a 0.  It also stands out more.
   ;
   ;       If you are writing in C++, you have my condolences. [2]
   ;
   ; [2] - Okay, my bias is showing through.  Sorry.
   ;----------------------------------------------------------------------*/

   lua_newtable( L );
   return 1;
}

You should see:

stack size  : 2
typename   1: string
typename   2: string
value      1: mod
isuserdata 2: 0
isstring   2: 1
value      2: ./mod.so

  The first parameter is the string given to require() (compatible with Lua
5.1) while the second paramter is the filename that contains the module (new
for Lua 5.2).  

  -spc