Fast read/write from file in delphi

Question

I am loading a file into a array in binary form this seems to take a while is there a better faster more efficent way to do this. i am using a similar method for writing back to the file.

procedure openfile(fname:string);
var
    myfile: file;
    filesizevalue,i:integer;
begin
  assignfile(myfile,fname);
  filesizevalue:=GetFileSize(fname); //my method
  SetLength(dataarray, filesizevalue);
  i:=0;
  Reset(myFile, 1);
  while not Eof(myFile) do
    begin
      BlockRead(myfile,dataarray[i], 1);
      i:=i+1;
    end;
  CloseFile(myfile);
end;

Answer 1

If your really want to read a binary file fast, let windows worry about buffering ;-) by using Memory Mapped Files. Using this you can simple map a file to a memory location an read like it's an array.

Your function would become:

procedure openfile(fname:string);
var
    InputFile: TMappedFile;
begin
  InputFile := TMappedFile.Create;
  try
    InputFile.MapFile(fname);
    SetLength(dataarray, InputFile.Size);
    Move(PByteArray(InputFile.Content)[0], Result[0], InputFile.Size);
  finally
    InputFile.Free;
  end;
end;

But I would suggest not using the global variable dataarray, but either pass it as a var in the parameter, or use a function which returns the resulting array.

procedure ReadBytesFromFile(const AFileName : String; var ADestination : TByteArray);
var
    InputFile : TMappedFile;
begin
  InputFile := TMappedFile.Create;
  try
    InputFile.MapFile(AFileName);
    SetLength(ADestination, InputFile.Size);
    Move(PByteArray(InputFile.Content)[0], ADestination[0], InputFile.Size);
  finally
    InputFile.Free;
  end;
end;

The TMappedFile is from my article Fast reading of files using Memory Mapping, this article also contains an example of how to use it for more "advanced" binary files.

Answer 2

You generally shouldn't read files byte for byte. Use BlockRead with a larger value (512 or 1024 often are best) and use its return value to find out how many bytes were read.

If the size isn't too large (and your use of SetLength seems to support this), you can also use one BlockRead call reading the complete file at once. So, modifying your approach, this would be:

AssignFile(myfile,fname);
filesizevalue := GetFileSize(fname);
Reset(myFile, 1);
SetLength(dataarray, filesizevalue);
BlockRead(myFile, dataarray[0], filesizevalue);
CloseFile(myfile);

Perhaps you could also change the procedure to a boolean function named OpenAndReadFile and return false if the file couldn't be opened or read.