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Externs and Byte Ptr

BlitzMax Forums/BlitzMax Programming/Externs and Byte Ptr

Grey Alien(Posted 2007) [#1]
Why use Byte Ptr and not say Int Ptr for this?

Extern "win32"
	Function GlobalMemoryStatus(lpBuffer:Byte Ptr)
End Extern


lpBuffer is supposed to be this structure:

Type TMemoryStatus 'for use with GlobalMemoryStatus
	Field dwLength:Int
	Field dwMemoryLoad:Int
	Field dwTotalPhys:Int
	Field dwAvailPhys:Int
	Field dwTotalPageFile:Int
	Field dwAvailPageFile:Int
	Field dwTotalVirtual:Int
	Field dwAvailVirtual:Int
	
	Method New()
		'Must set the length.
		dwLength=SizeOf(TMemoryStatus) 
	End Method
EndType


And if I create an instance of TMemoryStatus with new and pass it into GlobalMemoryStatus, it works totally fine.

Thing is I don't really understand what's going on. Why is Byte Ptr is used in the function declaration instead of say Int Ptr? Is Byte pointer the normal thing you use when you point to a Type (structure) because it's a pointer to an area of memory that can be read as bytes, or what?

Thanks for any insight, hope I'm making myself clear. I still haven't had that cup of tea...


Brucey(Posted 2007) [#2]
Is Byte pointer the normal thing you use when you point to a Type

Byte Ptr is what I use all the time for referring to a point in memory for structs and the likes.
If it happened to be an array of Ints, you could refer to it either by a Byte Ptr, or more usefully as an Int Ptr, with which you could then do something like - i = Int Ptr(mypointer)[0] - to get the first integer in the array.

Byte Ptr is just convenient.

If you need the real address you can cast it to Int - Int(myPtr).


Grey Alien(Posted 2007) [#3]
Ah very handy thanks!

Did you see this thread?

http://www.blitzbasic.com/Community/posts.php?topic=73289

I'm trying to pass an array of types (structures) into a WinAPI function but failing badly...


Azathoth(Posted 2007) [#4]
Seems like objects can only be cast to Byte Ptr because it doesn't demand how the data is aligned. Int Ptr doesn't work.


Grey Alien(Posted 2007) [#5]
Ah I see, yes that makes sense.