OpenEuphoria

Re: accessing C union members...

• Posted by gbonvehi Jan 10, 2013
• 1092 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