[fpc-devel] GetDiskFreeSpaceExW implementation

[Tomas Hajny]

On 2025-11-25 23:27, Bart via fpc-devel wrote:


Hi Bart,

> MS defines GetDiskFreeSpaceExW as follows:
> (https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getdiskfreespaceexw)
> 
> BOOL GetDiskFreeSpaceExW(
>   [in, optional]  LPCWSTR         lpDirectoryName,
>   [out, optional] PULARGE_INTEGER lpFreeBytesAvailableToCaller,
>   [out, optional] PULARGE_INTEGER lpTotalNumberOfBytes,
>   [out, optional] PULARGE_INTEGER lpTotalNumberOfFreeBytes
> );
> 
> And ULARGE_INTEGER is a union representing a 64-bit unsigned integer 
> value.
> 
> However in rtl/win/wininc/redef.inc we define 2 overloads for this 
> function:
> 
> function GetDiskFreeSpaceExW(lpDirectoryName: LPWSTR; var
> lpFreeBytesAvailableToCaller, lpTotalNumberOfBytes: TLargeInteger;
> lpTotalNumberOfFreeBytes: PLargeInteger): BOOL;external 'kernel32'
> name 'GetDiskFreeSpaceExW';
> function GetDiskFreeSpaceExW(lpDirectoryName: LPWSTR;
> lpFreeBytesAvailableToCaller, lpTotalNumberOfBytes: pLargeInteger;
> lpTotalNumberOfFreeBytes: PLargeInteger): BOOL;external 'kernel32'
> name 'GetDiskFreeSpaceExW';
> 
> I have 2 questions about this:
> 1. why do we use signed parameters instead of unsigned
> (TULargeInteger/PULargeInteger)?
>  (well, High(Int64) > 9223372 Tera, so it'll take some time before we
> have disks where this will overflow)
> 2. since all out params are of the same type, why does the overload
> with var parameters onlu use "var" for the first 2 params and not for
> the last one?
>  -> now I have to call this like GetDiskFreeSpaceExW(PWideChar(Drive),
> Avail, Size, @FreeSize);
> 
> I know that having the third param being a pointer, you can set it to
> nil if you're not interested in FreeSize, but the same argument can be
> made for Size param.
> Just use the overload with only pointer params for that IMO.

I'd guess that Delphi compatibility is the answer to both questions 
(https://docwiki.embarcadero.com/Libraries/Sydney/en/System.SysUtils.GetDiskFreeSpaceEx). 
Possibly due to Delphi not having 64-bit unsigned integer at some stage? 
Obviously, it would make sense to have an unsigned overload as well if 
nothing else.

Tomas