OpenEuphoria

Re: sdl2 and ffi

• Posted by ChrisB (moderator) Dec 05, 2022
• 930 views

Hi

At the risk of sounding like a troll (seriously not my intention, Andy has always done great work on all the libraries he has created, and wrapping all those functions must be some sort of tedium), but I think you've missed my point Gregg.

Let's also take SDL_GetVersion as an example. This is how Andy originally presented it

--SDL_Version is a type set up in sdl_version.e 
public constant SDL_version = define_c_type({ 
	C_UINT, --major 
	C_UINT, --minor 
	C_UINT --patch 
}) 
 
 
--and then in sdl_version.e 
 
export constant xSDL_GetVersion = define_c_proc(sdl,"+SDL_GetVersion",{SDL_version})  
public procedure SDL_GetVersion(sequence ver)  
	c_proc(xSDL_GetVersion,{ver})  
end procedure  

As you can see, and is passing a type to the c function, not a pointer, and the eu procedure is expecting a sequence

The correct way is

export constant xSDL_GetVersion = define_c_proc(sdl,"+SDL_GetVersion",{C_POINTER})  
public procedure SDL_GetVersion(atom ver_ptr)  
	c_proc(xSDL_GetVersion,{ver_ptr})  
end procedure  

Now the c function gets a pointer value, and the eu procedure gets an atom, created with

atom DLL_vers  
DLL_vers = allocate_struct(SDL_version, {0,0,0})   --allocate the memory, and return the pointer to that memory  
SDL_GetVersion(DLL_vers)                           --an atom, a pointer, is passed to the eu function 
printf(1, "DLL version : %d.%d.%d\n)", {  
        peek(DLL_vers),                     --single byte, 8 bit numbers  
        peek(DLL_vers + 1),  
        peek(DLL_vers + 2)  
        }) 
--the more ''ffi' way 
printf(1, "DLL version : %d.%d.%d\n", peek_struct( DLL_vers, SDL_VERSION ))  --just tried this - doesn't work, peek_struct is 'out' 
 

My point is that all sdl functions need pointers to structures, as far as I can tell, and passing a type, created using allocate structure, does not contain the pointer to that allocated memory

I don't doubt that ffi will make it easier to manipulate structures, it already has done so for me, and I know I'm a pest, but the SDL library as Andy has presented just doesn't work. Now, correct me if I'm wrong, and I'm misunderstanding something here, but is passing a type the same as passing pointer?

Cheers, Chris

ps - see the code above - peek_struct isn't returning the correct values - is C_UINT in euphoria 1 byte, 8 bits only, as that's what SDL_GetVersion returns, and that's why the peek version returns the correct values. Change the SDL_version type C_UINTs to C_UINT8s, and it works.

Cheers

Chris