OpenEuphoria

1. FFI question

• Posted by ChrisB (moderator) Dec 28, 2022
• 563 views

Hi

In Andy's SDL, events, the event union is a bunch of stuctures within a structure - a union. I understand that ffi cannot access members of a particular union, but I think we can 'flatten' these, as each member of the union is a structure with only one level deep.

So, very simply, Andy has defined each of the members of the union as a structure already. Simplified, the event union looks like this.

public constant SDL_Event = define_c_type({ 
	C_UINT32, --type [1] 
	SDL_CommonEvent, --[2] 
	SDL_DisplayEvent, --[3] 
	SDL_WindowEvent, --[4] 
	SDL_KeyboardEvent, --[5] 
        --and so on 
{) 

and exh of these members of the union has been defined like this (for instance)

public constant SDL_CommonEvent = define_c_type({ 
	C_UINT32, --type 
	C_UINT32 --timestamp 
}) 
 
public constant SDL_DisplayEvent = define_c_type({ 
	C_UINT32, --type 
	C_UINT32, --timestamp 
	C_UINT32, --display 
	C_UINT8, --event 
	C_UINT8, --padding1 
	C_UINT8, --padding2 
	C_UINT8, --padding3 
	C_INT32 --data1 
 
--and so on 
 
}) 
 

Now, from what I can tell, FFI allocates a block of memory for the structure depending on the size of the elements, and returns a pointer to the start of the structure. Easy enough with C_UINT32 and C_UINT8 etc, but in the event union does FFI allocate a block of memory depending on the size of the structure elements? Ie does it know the size of a SDL_CommonEvent and the size of an SDL_DisplayEvent and so on, so that it allocates the correct amount / block of memory for the entire union?

Would make life easier, instead of 'flattening' it.

Cheers

Chris