Delphi: add constant to TFileTime

Question

I am new in Delphi and I need in my project add some constant to TFileTime which is record of lower and upper part of 64bit value. How to do this in Delphi? I have found only code in C++, but i dont know how make in Delphi unsigned int64 (ULONGLONG) and also I dont know how to cast this to longword (DWORD):

ULONGLONG qwResult;

// Copy the time into a quadword.
qwResult = (((ULONGLONG) ft.dwHighDateTime) << 32) + ft.dwLowDateTime;

// Add constant
qwResult += constant;

// Copy the result back into the FILETIME structure.
ft.dwLowDateTime  = (DWORD) (qwResult & 0xFFFFFFFF );
ft.dwHighDateTime = (DWORD) (qwResult >> 32 );

Thanks

Answer 1

The FILETIME struct is defined as:

typedef struct _FILETIME {
  DWORD dwLowDateTime;
  DWORD dwHighDateTime;
} FILETIME, *PFILETIME;

So, because Windows runs on little Endian, the layout of this struct is compatible with a 64 bit integer value.

So, you can cast TFileTime to UInt64, do the arithmetic, and cast back. Like this:

function IncrementedFileTime(const Value: TFileTime; const Incr: UInt64): TFileTime;
begin
  Result := TFileTime(UInt64(Value) + Incr);
end;

---

Now, the documentation for the FILETIME record says:

It is not recommended that you add and subtract values from the FILETIME structure to obtain relative times. Instead, you should copy the low- and high-order parts of the file time to a ULARGE_INTEGER structure, perform 64-bit arithmetic on the QuadPart member, and copy the LowPart and HighPart members into the FILETIME structure.

Do not cast a pointer to a FILETIME structure to either a ULARGE_INTEGER* or __int64* value because it can cause alignment faults on 64-bit Windows.

This is an issue on targets where alignment errors result in hard faults. For instance Itanium. However, on x86 and x64 the code in my answer is fine because those architectures do not issue hard faults for alignment errors. Which is just as well because the Delphi compiler isn't very good at alignment.