CODEWITHSUNDEEP
delphidelphi-xe2
Marcello Impastato
Marcello Impastato

Reputation: 2281

FileSize, what difference from this procedure and what is better use?

about get size of a file, i have this two functions:

function GetFileSize1(const FileName: TFileName): Int64;
var
 iTmp: Int64;
 SearchRec: TSearchRec;
begin
  iTmp := -1;
  if FindFirst(FileName, faAnyFile, SearchRec) = 0 then
  begin
    iTmp := SearchRec.Size;
    System.SysUtils.FindClose(SearchRec);
  end;
  Result := iTmp;
end;

And:

function GetFileSize2(const FileName: TFileName): Int64;
var
 FileStream: TFileStream;
begin
  FileStream := TFileStream.Create(FileName, fmOpenRead);
  try
    Result := FileStream.Size; 
  finally  
    FileStream.Free;
  end; 
end;

In practise, what is the difference of it? Both return same result of course but what is more affidable, more fast, more secure? Or better, what is preferible use? First or second? Thanks very much.

Upvotes: 7

Views: 2280

Answers (2)

David Heffernan
David Heffernan

Reputation: 612944

Well, the obvious difference is that GetFileSize2 opens the file, using the CreateFile API to obtain a file handle. In contrast, GetFileSize1 does not because it reads the size from the file meta data.

So I would expect GetFileSize1 to perform better. Although, for many applications that performance difference would not matter. Much more significantly, GetFileSize2 can fail due to a sharing violation in situations that GetFileSize1 will succeed. So you really should not use GetFileSize2.

Note also that the two functions you present behave differently in case of an error: GetFileSize1 returns -1, and GetFileSize2 raises an exception.

Personally I prefer this version:

function GetFileSize3(const FileName: string): Int64;
var
  fad: TWin32FileAttributeData;
begin
  if not GetFileAttributesEx(PChar(FileName), GetFileExInfoStandard, @fad) then
    RaiseLastOSError;
  Int64Rec(Result).Lo := fad.nFileSizeLow;
  Int64Rec(Result).Hi := fad.nFileSizeHigh;
end;

Or, if you prefer to return -1 in case of error you would write it like this:

function GetFileSize3(const FileName: string): Int64;
var
  fad: TWin32FileAttributeData;
begin
  if not GetFileAttributesEx(PChar(FileName), GetFileExInfoStandard, @fad) then
    exit(-1);
  Int64Rec(Result).Lo := fad.nFileSizeLow;
  Int64Rec(Result).Hi := fad.nFileSizeHigh;
end;

Some how this feels more natural than calling FindFirstFile, but that's perhaps just personal preference. There's really nothing wrong with the FindFirstFile approach. Although it doesn't need that iTmp variable. You can write it more clearly like this:

function GetFileSize1(const FileName: TFileName): Int64;
var
 SearchRec: TSearchRec;
begin
  if FindFirst(FileName, faAnyFile, SearchRec) = 0 then
  begin
    Result := SearchRec.Size;
    System.SysUtils.FindClose(SearchRec);
  end
  else
    Result := -1;
end;

Update: @CodeInChaos makes a good point about the approaches that don't open a file handle. These approaches can give inaccurate results for hard linked files.

Upvotes: 10

Sir Rufo
Sir Rufo

Reputation: 19096

The difference is, that GetFileSize1 reads the Meta Information of a file (Windows-API-Call) and GetFileSize2 has a direct touch to the file (getting FileHandle, walk through until the end to calculate the size).

So GetFileSize1 consumes less performance/ressources than GetFileSize2

UPDATE

I forgot to mention, if the file is already in use, you maybe not allowed to get access with a TFileStream, but the Meta Information will be available.

UPDATE (just another Variant of Davids suggestion)

function GetFileSize1(const FileName: TFileName): Int64;
var
 SearchRec: TSearchRec;
begin
  if FindFirst( FileName, faAnyFile, SearchRec ) = 0 then
    try
      Exit( SearchRec.Size );
    finally
      System.SysUtils.FindClose(SearchRec);
    end;
  Result := -1;
end;

Upvotes: 2

Related Questions