Get string return value from C DLL in Delphi

I have a legacy DLL written in C that contains a function that returns a string, and I need to access this function from Delphi. The only info I have about the DLL is the VB declare for accessing the function:

Public Declare Function DecryptStr Lib "strlib" (Str As String) As String

I've tried the following without success:

Declaration:

function DecryptStr(s: PChar): PChar; cdecl; external 'strlib.dll';

Usage:

var
  p1, p2 : pchar;
begin
  GetMem( p1, 255 );
  StrPCopy( p2, 'some string to decrypt' );
  p1 := DecryptStr( p2 );
end;

This consistently crashes the DLL with an Access Violation. I'm at a loss.

Any suggestions ?

From stackoverflow Kevin McBrearty

• I'm guessing here, but are you sure it's cdecl? If the VB declare isn't mentioning it, I'd assume it's in fact a STDCALL function (STDCALL is quite common on Windows, as almost all of its native API uses it). Calling a function of one calling convention as if it were of another calling convention can really mess up the stack, usually leading to a crash.

  Also, be sure to check whether the string is ANSI (LPSTR/LPCSTR) or UNICODE (LPWSTR/LPCWSTR). I don't know VB or Delphi, so I don't know what each one uses by default.

  From CesarB

• Actually I've tried both, cdecl and stdcall; no difference.

  From Kevin McBrearty

• I agree with CesarB, try to declare it with stdcall directive as:

  function DecryptStr(s: PChar): PChar; stdcall; external 'strlib.dll';
  

  if it doesn't work, post the VB declaration here.

  CesarB : He already posted the VB declaration, it's two paragraphs above the Delphi one.

  Mohammed Nasman : opps sorry, you are right CesarB, I didn't noticed it because he didn't mark it as code.
  From Mohammed Nasman

• p2 isn't initialized. StrPCopy copies the string to a random memory location. And most likely the calling convention is stdcall.

  From Jozz

• 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.

  From andrius

• The best way in these kind of situation is to debug your program and check the stack before and after executing the callback. How knows, it might even be a bug in the external DLL?

  This way you will see pretty easy how to correct this.

  From botismarius

• Was the dll written in Borland C or C++Builder by any chance with the intention of being used with Delphi? In which case it could have been compiled using a pascal directive.

  From Cruachan

• As Jozz says, p2 (where you copy your string to) is never initialized in your example.

  Try this instead.

  var
    p1, p2 : pchar;
  begin
    GetMem( p2, 255 ); // allocate memory for 'some string...'
    StrPCopy( p2, 'some string to decrypt' );
    p1 := DecryptStr( p2 );
  end;
  

  Also, the memory you allocated by calling Getmem(p1,...) would have been leaked, because p1 was overwritten by the function return from DecryptStr.

  However, I'd be a bit concerned about exactly what DecryptStr is returning, and who owns the memory pointed to by p1. If it's returning a pointer to memory allocated by the DLL you will need to be careful how that memory is freed.

  From Roddy

• The suggestions that the strings must be "initialised" seem to be correct. This is because C will require that the string being passed in is null-terminated. Check that the character in the bufffer just after the end of the text is a null (#0).

  Why do you assume that the string passed in is exactly 255 chars long? You need to allocate Length(p1) + 1 bytes - for the chars in p1, and a #0 char at the end.

  Also, your code sample seems confused as to the use of p1 and p2. It looks like p1 is the buffer passed to the C DLL, which you allocate, and p2 is the returned string that the DLL allocates. But then the code would be (note use of p1 and p2)

  var
    p1, p2 : pchar;
  begin
    GetMem( p1, 255 );
    StrPCopy( p1, 'some string to decrypt' );
    p2 := DecryptStr( p1 );
  end;
  

  Better variable names would help you make this clearer.

  From Anthony