Re: wrapping c lib

[Philipp Janda <siffiejoe@...>]

Am 17.07.2015 um 16:30 schröbte Gregg Reynolds:
> Hi list,

Hi!

> I'm just starting on a lua wrapper of libmraa for the intel Edison (
> http://iotdk.intel.com/docs/master/mraa/).  I've got a basic implementation
> working. My question is whether it is possible to split the code into
> multiple c files.

Sure it is.

> libmraa is split into seven modules so I'd like to keep
> the wrapper code distinct.  But my understanding is that the interface
> functions must be static.

C functions exported to Lua don't *have* to be static. Lua uses them by 
pointer only, so they usually *can* be static, and it's good practice to 
limit symbol visibility as much as possible. But nobody prevents you 
from using non-static functions if you want to reference those symbols 
from another file (e.g. to put them into a `luaL_Reg` array). I usually 
don't do that though, because it gets inconvenient when the number of 
exported functions is large. Instead, I do something like the following:

    LUAMRAA_API int luaopen_luamraa( lua_State* L ) {
      lua_newtable( L );
      luamraa_mod1_setup( L );
      luamraa_mod2_setup( L );
      luamraa_mod3_setup( L );
      return 1;
    }

The `luamraa_modX_setup` functions are defined in separate files and 
register all functions, metatables, etc. in that file. They can't be 
`static`, but they don't have to be exported from the shared library 
either (on Unices you can have hidden visibility, on Windows you don't 
need `__declspec(dllexport)` stuff).

> I can create seven wrappers but I'd like to have
> one "luamraa" module as well.  Is the Right Way to do this to just use a
> lua module to import the seven wrappers?  (My lua is a little rusty,
> haven't worked with it for several years.)

You can also use separate modules per C file and collect those modules 
in a single shared object/DLL (e.g. module `luamraa.modX` defined in 
function `luaopen_luamraa_modX` exported from DLL `luamraa.dll`). This 
makes sense if the modules are more or less independent from each other. 
For convenience you may load sub-modules from the "main" module using 
`require` in Lua or `luaL_requiref` in C. There are also some modules 
out there that do some form of auto-loading via metatables ...

> Thanks,
>
> Gregg

HTH,
Philipp