Re: easy sharing globals between C and Lua

["Paulo Lopes" <pmlopes@...>]

#include <lualib.h>
#include <lauxlib.h>

static int lua_setclocal(lua_State *L)
{
	printf("Setting var\n");
	return 0;
}

static int lua_getclocal(lua_State *L)
{
	printf("Getting var\n");
	return 0;
}

int main()
{
	lua_State *L = lua_open();
	luaL_openlibs(L);
	
	float foo = 100.0f;
	
	printf("  C --> %f\n", foo);
	
	lua_register(L, "getclocal", lua_getclocal);
	lua_register(L, "setclocal", lua_setclocal);
	
    lua_newtable(L);
	lua_pushstring(L, "foo");
	lua_pushlightuserdata(L, (void *)&foo);
	lua_settable(L, -3);
	lua_setglobal(L, "global");
	
	// main execution
    if(luaL_dofile(L, "test.lua") != 0)
	{
		printf("%s\n", lua_tostring(L, -1));
		lua_pop(L, 1);
	}
	
	printf("  C --> %f\n", foo);
	
    lua_close(L);
	return 0;
}

OK i'm trying to make a table on C side called global and for now I'm
putting 1 entry called foo with a pointer to my global variable on C.
What i can't do now is to write on the C side again a metatable to the
global table to override the __index and __newindex metafunctions with
my lua_getclocal and lua_setclocal.

You explained on the Lua side how to do it, but can i do it on the C side?

Thanks,
Paulo

On Nov 19, 2007 8:48 PM, Thomas Lefort <thomas.lefort@gmail.com> wrote:
> On Nov 19, 2007 10:50 AM, Paulo Lopes <pmlopes@gmail.com> wrote:
> > Hi,
> >
> > I'm trying to make my C library Lua aware but the problem is that it
> > relies on lots of global variables to change all its internal state. I
> > diged the list and found some code that was supposed to help me out,
> > but now that i compiled it it fails because I think it was old Lua 4.x
> > code and it isn't compatible with Lua 5.x
>
> You shouldn't rely on global variables to create a library. They are a
> generally messy way to program, they are not shared between programs
> that link with the library, and if you ever use threads someone will
> have to write extra functions to avoid race conditions. An ideal
> library should consist mostly of reentrant functions that take a state
> as their first argument, like the Lua library itself.
>
> That being said, in Lua 5 (actually since Lua 4.1) metatables and
> metamethods replace tags and fallbacks. An example:
>
>   -- Expose library variables as get/set pairs.
>   -- This assumes the getSomeVar and setSomeVar lua_CFunctions were exposed
>   -- from C, typically by using luaL_register.
>
>   local libVariables = {
>     someVar = {
>       get = getSomeVar,
>       set = setSomeVar
>     }
>   }
>
>   -- This metatable will define fallback behaviour for our Lua environment.
>   local libMetatable = {}
>
>   -- The __index metamethod.
>   -- Triggered when reading a free table index (or indexing a userdatum).
>   function libMetatable:__index(name)
>     local var = libVariables[name]
>     if var then
>       return var.get()
>     else
>       error("attempt to get an undefined library variable: ", var)
>     end
>   end
>
>   -- The __newindex metamethod is the counterpart to __index for write access.
>   function libMetatable:__newindex(name, value)
>     local var = libVariables[name]
>     if var then
>       var.set(value)
>     else
>       error("attempt to set an undefined library variable: ", var)
>     end
>   end
>
>   -- Let's use the library in a dedicated environment.
>   -- The print function is included to for test purposes.
>   -- Other indexes are resolved by the metatable.
>   setfenv(1, setmetatable({print = print}, libMetatable))
>
>   someVar = 42      -- triggers __newindex("someVar", 42) --> setSomeVar(42)
>   print(someVar)    -- triggers __index("someVar") --> return getSomeVar()
>
> You will find extensive developments in the must-read book by Roberto,
> Programming in Lua, at http://www.lua.org/pil/ .
>
> Happy learning !
>
> --
> -- Thomas
>



-- 
Paulo Lopes
www.scratchydreams.com