Why are strings truncated when using direct printing?

Question

I'm trying to print directly to a printer using esc/p commands (EPSON TM-T70) without using printer driver. Code found here.

However, if I try to print any strings, they are truncated. For example:

MyPrinter := TRawPrint.Create(nil);
try
  MyPrinter.DeviceName := 'EPSON TM-T70 Receipt';
  MyPrinter.JobName := 'MyJob';
  if MyPrinter.OpenDevice then
  begin
    MyPrinter.WriteString('This is page 1');
    MyPrinter.NewPage;
    MyPrinter.WriteString('This is page 2');
    MyPrinter.CloseDevice;
  end;
finally
  MyPrinter.Free;
end;

Would print only "This isThis is"! I wouldn't ordinarily use MyPrinter.NewPage to send a line break command, but regardless, why does it truncates the string?

Also notice in RawPrint unit WriteString function:

Result := False;
if IsOpenDevice then begin
  Result := True;
  if not WritePrinter(hPrinter, PChar(Text), Length(Text), WrittenChars) then begin
    RaiseError(GetLastErrMsg);
    Result := False;
  end;
end;

If I put a breakpoint there and step through the code, then WrittenChars is set to 14, which is correct. Why does it act like that?

Cosmin Prund · Accepted Answer · 2011-05-09T07:52:29.333

4

You are using a unicode-enabled version of Delphi. Chars are 2 bytes long. When you call your function with Length(s) you're sending the number of chars, but the function probably expects the size of the buffer. Replace it with ~~SizeOf(s)~~ Length(s)*SizeOf(Char).

Since the size of one unicode char is exactly 2 bytes, when you're sending Length when buffer size is required, you're essentially telling the API to only use half the buffer. Hence all strings are aproximately split in half.

edited May 09 '11 at 07:52

answered May 09 '11 at 07:33

Cosmin Prund

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Thank you for your answer. SizeOf gave me value 4 (size of string?), so the string was even more truncated. But when i mutliplied Length with 2, then everything is ok. I accept your answer as that led me to solution. Thanks again – Peacelyk May 09 '11 at 07:46

score 4 · Answer 2 · answered May 09 '11 at 07:57

4

Maybe you can use the ByteLength function which gives the length of a string in bytes.

answered May 09 '11 at 07:57

Toto

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+1 That's what I was looking for but failed to locate it. Of course, it's implemented as `Length(S) * SizeOf(Char)`, but it does look cleaner and does express the concept of buffer length much better. – Cosmin Prund May 09 '11 at 08:07

Why are strings truncated when using direct printing?

2 Answers2