R – How to return a PChar from a DLL function to a VB6 application without risking crashes or memory leaks

delphidllstringvb6

I have to create a DLL which is used by a VB6 application. This DLL has to provide several functions, some of them must return strings.

This is the VB6 declaration:

Declare Function MyProc Lib "mylib.dll" (ByVal Param As String) As String

And this the Delphi implementation stub in mylib.dll:

function MyProc(AParam: PChar): PChar; stdcall;
var
  ReturnValue: string;
begin
  ReturnValue := GetReturnValue(AParam);
  Result := ???;
end;

What do I have to return here? Who will free the memory of the returnd PChar string?

EDIT: I'm asking about Delphi 2005 (PChar = PAnsiChar)

Best Answer

You need to craft a BSTR instead. VB6 strings are actually BSTRs. Call SysAllocString() on the Delphi side and return the BSTR to the VB6 side. The VB6 side will have to call SysFreeString() to free the string - it will do it automatically.

If PChar corresponds to an ANSI string (your case) you have to manually convert it to Unicode - use MultiByteToWideChar() for that. See this answer for how to better use SysAllocStringLen() and MultiByteToWideChar() together.

Related Solutions

Delphi – Get string return value from C DLL in Delphi

Consider rewriting your test code as follows:

var
  p1, p2 : pchar;
begin
  GetMem( p1, 255 ); // initialize
  GetMem( p2, 255 );
  StrPLCopy( p2, 'some string to decrypt', 255 ); // prevent buffer overrun
  StrPLCopy( p1, DecryptStr( p2 ), 255); // make a copy since dll will free its internal buffer
end;

If still fails within a call to DecryptStr, then read http://support.microsoft.com/kb/187912 carefully.

Python – How to check if a string represents an int, without using try/except

with positive integers you could use .isdigit:

>>> '16'.isdigit()
True

it doesn't work with negative integers though. suppose you could try the following:

>>> s = '-17'
>>> s.startswith('-') and s[1:].isdigit()
True

it won't work with '16.0' format, which is similar to int casting in this sense.

edit:

def check_int(s):
    if s[0] in ('-', '+'):
        return s[1:].isdigit()
    return s.isdigit()

Best Answer

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Python – How to check if a string represents an int, without using try/except

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