CODEWITHSUNDEEP
delphi
nguyentu
nguyentu

Reputation: 45

How to ReadBuffer several times for WriteBuffer

Although the value of the bbuf and hbuff is different but nothing happening for the hdevi.

var
  bfile:    TFileStream;
  hdevi:    TFileStream;
  bbuff:    array[0..511] of byte;
  hbuff:    array[0..87039] of byte;
  curr:     string;
  i:        integer;
begin
  curr:=GetCurrentDir;
  hdevi := TFileStream.Create(PChar(deviceno), fmOpenReadWrite);
  try
    bfile := TFileStream.Create(PChar(curr+'\bfile'), fmOpenReadWrite);
    try
      hdevi.ReadBuffer(hbuff[0],length(hbuff));
      bfile.ReadBuffer(bbuff[0],length(bbuff));
      hdevi.WriteBuffer(bbuff[0],length(bbuff));
      //for i:=0 to length(bbuff)-1 do
      //ShowMessage(IntToHex(hbuff[i],2)+'-'+IntToHex(bbuff[i],2));
    finally
      bfile.Free;
    end
  finally
    hdevi.Free;
  end;
end;

But it work after the following line is removed

hdevi.ReadBuffer(hbuff[0],length(hbuff));

or add this line hdevi.Position:=0; before

hdevi.WriteBuffer(bbuff[0],length(bbuff));

I don't know why, can someone explain for me?

Upvotes: 0

Views: 350

Answers (2)

Tom Brunberg
Tom Brunberg

Reputation: 21045

I don't know why, can someone explain for me?

Every TStream descendant has a Position property which indicates at what byte position in the stream a following read or write operation will take place. For each read or write operation the Position is advanced with the number of bytes read or written. To change the Position you can either assign to Position or call the Seek() function directly (Position uses internally the Seek() function).

Let's look at these three lines of your code:

  // At this point both hdevi.Position and bfile.Position are 0
  hdevi.ReadBuffer(hbuff[0],length(hbuff));
  // At this point hdevi.Position is 87040
  bfile.ReadBuffer(bbuff[0],length(bbuff));
  // now bfile.Position is 512
  // hdevi.Position is of course still 87040, alas, that's where the bbuff is written
  hdevi.WriteBuffer(bbuff[0],length(bbuff));
  // hdevi.Position is now 87040 + 512 = 87552

Obviously you wanted to write the 512 bytes from bbuff to the beginning of hdevi. As you have found out you can either skip the reading into hbuff altogether (in which case hdevi.Position is at 0) or if you need to read the hbuff buffer first, you must reset the hdevi.Position to 0.

Upvotes: 2

FlorianSchunke
FlorianSchunke

Reputation: 581

Each time you call WriteBuffer or ReadBuffer the pointer to the position you are currently at the stream is moved forward by length(...). Therefore moving the position back to 0 (Have a look at the Seek method of TStream, see documentation here) your code is working.

Upvotes: 3

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