How can I set a file's compression attribute in Delphi?

Question

How can I compact files (set the 'c' attribute) from Delphi? I am speaking about the "compress contents to save disk space" function available under NTFS.

It seems that FileSetAttr does not allow me to set the 'c' attribute for a file.

Answer 1

The documentation for SetFileAttributes() explains that the FILE_ATTRIBUTE_COMPRESSED flag is not accepted by that function (although it is for GetFileAttributes). Instead it states:

To set a file's compression state, use the DeviceIoControl function with the FSCTL_SET_COMPRESSION operation.

The FSCTL_SET_COMPRESSION link in particular explains precisely how to do it. It goes something like this:

const
  COMPRESSION_FORMAT_NONE = 0;
  COMPRESSION_FORMAT_DEFAULT = 1;
  COMPRESSION_FORMAT_LZNT1 = 2;

procedure SetCompressionAttribute(const FileName: string; const CompressionFormat: USHORT);
const
  FSCTL_SET_COMPRESSION = $9C040;
var
  Handle: THandle;
  Flags: DWORD;
  BytesReturned: DWORD;
begin
  if DirectoryExists(FileName) then
    Flags := FILE_FLAG_BACKUP_SEMANTICS
  else if FileExists(FileName) then
    Flags := 0
  else
    raise Exception.CreateFmt('%s does not exist', [FileName]);

  Handle := CreateFile(PChar(FileName), GENERIC_READ or GENERIC_WRITE, 0, nil, OPEN_EXISTING, Flags, 0);
  Win32Check(Handle <> INVALID_HANDLE_VALUE);
  try
    if not DeviceIoControl(Handle, FSCTL_SET_COMPRESSION, @CompressionFormat, SizeOf(Comp), nil, 0, BytesReturned, nil) then
      RaiseLastOSError;
  finally
    CloseHandle(Handle);
  end;
end;