OpenEuphoria

1. accessing C union members...

• Posted by ssallen Jan 10, 2013
• 1260 views

Hello, I am trying to access a C union in a dll. The Union looks like this:

typedef union{ 
  Uint8 type; 
  SDL_ActiveEvent active; 
  SDL_KeyboardEvent key; 
  SDL_MouseMotionEvent motion; 
  SDL_MouseButtonEvent button; 
  SDL_JoyAxisEvent jaxis; 
  SDL_JoyBallEvent jball; 
  SDL_JoyHatEvent jhat; 
  SDL_JoyButtonEvent jbutton; 
  SDL_ResizeEvent resize; 
  SDL_ExposeEvent expose; 
  SDL_QuitEvent quit; 
  SDL_UserEvent user; 
  SDL_SywWMEvent syswm; 
} SDL_Event; 

First, I want to check that type equals a certain value then I need to access the SDL_MouseButtonEvent member. It looks like so:

typedef struct{ 
  Uint8 type; 
  Uint8 which; 
  Uint8 button; 
  Uint8 state; 
  Uint16 x, y; 
} SDL_MouseButtonEvent; 

Here is my code:

global function viewScrollWheel() 
 
	atom sdl_event 
	integer mwheel, etype, button 
	 
	etype = 0 
	button = 0 
		 
	sdl_event = allocate(100)	-- allocate enough memory for event structure 
	 
	while(SDL_PollEvent(sdl_event)) do   -- loop thru events 
	 
		etype = peek(sdl_event)   --get type member from event? 
		 
		if etype = SDL_MOUSEBUTTONDOWN then  -- if type = MBD then  
			button = 1		    -- making sure we are here...	 
		        exit					 
		end if 
	 
	end while 
	 
	free(sdl_event) 
 
	return button 
	 
end function 

I want to check the type field in the event union and if it equals SDL_MOUSEBUTTONDOWN then I want to check the button field that is part of the struct. Any help is appreciated!

3. Re: accessing C union members...

• Posted by gbonvehi Jan 10, 2013
• 1089 views

Adding to what Matt said:

The union type tells basically that it can contain any of the types specified in it's declaration. Don't get confused, it will only hold one value at a time, either a type (Uint8), a key (SDL_KeyboardEvent), a jaxis (SDL_JoyAxisEvent), etc.

A Uint8 type uses 1 byte in memory (8-bits). Knowing this and that the initial offset is 0, you can calculate the offsets you will need for each var in the event structure:

typedef struct{  
  Uint8 type;  
  Uint8 which;  
  Uint8 button;  
  Uint8 state;  
  Uint16 x, y;  
} SDL_MouseButtonEvent;  

peek(sdl_event) -- type 
peek(sdl_event+1) -- which 
peek(sdl_event+2) -- button 
peek(sdl_event+3) -- state 
peek2u(sdl_event+4) -- x 
peek2u(sdl_event+6) -- y (Uint16 takes 2 bytes in memory) 

Cheers,
Guillermo