Why?
lazarus / 34-Windows

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typo · 2013-05-10 11:20

Could someone explain me what is going on here:

Code: [Select]    SendMessage(lzRichEdit.Handle, EM_GETSELTEXT, 0,
LongInt(PChar((Result))));
    showmessage(Result);        // returns selected text string
    showmessage(Result + 'a');  // returns "a"

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Martin_fr · 2013-05-10 11:51

I am not 100% sure, but I take a ***guess***:
Code: [Select]LongInt(PChar((Result)))Result contains a memory
address.
The address is either nil, or pointing at something like (in32 bit):

Addr - 8: ref count
addr -4: len
addr: text + #0

http://msdn.microsoft.com/en-
gb/library/windows/desktop/bb774190%28v=vs.85%29.aspx
QuotewParam    This parameter is not used; it must be zero.

lParam    Pointer to a buffer that receives the selected text. The
calling application must ensure that the buffer is large enough to
hold the selected text.

So I assume Result does contain some string before. because if not,
you pass nil as lparam, and that would not fit the above requirement.

Then the memory at addr will be changed to:
addr: newtext + #0

but no ref count, no len are touched. So your string is not a valid
string anymore (and had a lot of other potential site effects, like
overwriting random memory after the string, or altering other strings
(if ref count > 1))

showmessage probably internally handles it as pchar (does not use the
len).

Result + 'a' looks at the ref count and len. And it dis-behaves.

----------------------
If you do
  string := somepchar;
fpc adds the conversion.

But if you typecast a string to a function param, it seems that is not
added.

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typo · 2013-05-10 13:57

So what should I do in order to "extract" the string from Result?

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Leledumbo · 2013-05-10 14:41

Typecast Result to PChar and extract using StrPas.

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typo · 2013-05-10 14:50

Code: [Select]    SendMessage(lzrichedit.Handle, EM_GETSELTEXT, 0,
LongInt(PChar(Result)));
    result := strpas(pchar(result));
    ShowMessage(IntToStr(Length(Result)));  // returns 1
    ShowMessage(Result);                    // returns first letter
    ShowMessage(Result + 'a');

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Leledumbo · 2013-05-10 15:18

Err... shouldn't you be using another variable to store result of
strpas?  I don't know the internal implementation, but it might
overwrite result.

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typo · 2013-05-10 15:25

Code: [Select]var
  p :PChar;
begin
  SendMessage(re.Handle, EM_GETSELTEXT, 0, LongInt(PChar(Result)));
  p := pchar(result);
  Result := StrPas(p);
  ShowMessage(IntToStr(Length(Result)));  // returns 1
  ShowMessage(Result);                    // returns first letter
  ShowMessage(Result + 'a');
end;  

No change.

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Martin_fr · 2013-05-10 15:34

Use an array of byte, or an array of char.

You must set it's length before calling the function

QuotelParam    Pointer to a buffer that receives the selected text.
The calling application must ensure that the buffer is large enough to
hold the selected text.

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Jurassic Pork · 2013-05-10 16:13

hello,
it seems that the returned string is a wideString ? ???
try this :
Code: [Select]  MyResult : Widestring;
  CharRange: TCharRange;
  nbCar,TheLength : Integer;
begin
  // number of selected char
  SendMessage(lzrichedit1.Handle, EM_EXGETSEL, 0,
Longint(@CharRange));
  NbCar := CharRange.cpMax - CharRange.cpMin;
  SetLength(MyResult, NbCar + 1);
  TheLength := SendMessage(lzrichedit1.Handle, EM_GETSELTEXT, 0,
LongInt(PChar(MyResult)));
  // adjust String Length
  SetLength(MyResult, TheLength );
  ShowMessage(IntToStr(Length(MyResult)));
  ShowMessage(MyResult);                   
  ShowMessage(MyResult + 'a');
 // easy way :-)
  ShowMessage(lzrichedit1.SelText);
Friendly , J.P

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ChrisF · 2013-05-10 16:23

Code: [Select]  SetLength(result,256);             <-----------

  SendMessage(lzRichEdit.Handle, EM_GETSELTEXT, 0,
LongInt(PChar((Result))));
  showmessage(Result);        // returns selected text string
  showmessage(Result + 'a');  // returns "a"

Anyway, it's highly risky to use it like this.

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typo · 2013-05-10 16:41

Code: [Select]var
  TheLength: integer;
  NbCar: Integer;
  CharRange: TCharRange;
begin
  // number of selected char
  SendMessage(lzrichedit.Handle, EM_EXGETSEL, 0, Longint(@CharRange));
  NbCar := CharRange.cpMax - CharRange.cpMin;
  //SetLength(Result, NbCar + 1);

  SetLength(Result, 256);
  TheLength := SendMessage(lzrichedit.Handle, EM_GETSELTEXT, 0,
LongInt(PChar(Result)));

  ShowMessage(IntToStr(TheLength));  // returns correct length

  // adjust String Length
  SetLength(Result, TheLength );
  ShowMessage(Result);                  // returns first letter
  ShowMessage(Result + 'a');            // returns first letter
end;

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Jurassic Pork · 2013-05-10 16:45

result is a wideString ?

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typo · 2013-05-10 17:01

I have just changed Result to a widestring and this worked. Thanks.