Question

I’ve searched around Stackoverflow for the issue I was having, but they were all very specific for that particular access violation.

The piece of code I believe I am having trouble with is a function called MpqExtractFile, stored in a DLL.

Function:

function MpqExtractFile(var hMPQ: Integer; szToExtract, szExtracted: AnsiString): Boolean; stdcall; external 'MpqLib.dll' name 'B2';

Call:

if MpqExtractFile(hMPQ, 'war3map.j', AnsiString(tempDir+'\war3map.j')) = True
then ShowMessage('Success.')
else ShowMessage('Failed.');

Upon execution of this code my application throws the access violation error so the ShowMessage is not displayed at all.

I am unsure if the above is adequate to even estimate what the problem could be, but if there is anything else I should please tell me.

Edit

This is an extract of the VB.NET source code that I have been interpreting:

Declare Function MpqExtractFile Lib "MpqLib.dll" Alias "B2" (ByVal hMPQ As Integer, ByVal szToExtract As String, ByVal szExtracted As String) As Boolean

I am obviously not familiar with declarations in other languages, but I have found the below function in the same VB file where the VB functions were declared.

BOOL WINAPI MpqExtractFile(HANDLE hMpq, const char * szToExtract, const char * szExtracted);

Thank you in advanced!

Answer 1

AnsiString is the completely wrong type to use, unless the DLL was writting in Delphi/C++Builder and actually used AnsiString in its parameters. If that were the case, you would need to know which version of Delphi/C++Builder the DLL was written in, because the memory layout of AnsiString was changed in 2009.

In any case, it is very dangerous to pass non-POD data across DLL boundaries, so most DLLs do not do that. The parameters in question are most likely PAnsiChar instead, eg:

function MpqExtractFile(var hMPQ: Integer; szToExtract, szExtracted: PAnsiChar): Boolean; stdcall; external 'MpqLib.dll' name 'B2'; 

.

if MpqExtractFile(hMPQ, 'war3map.j', PAnsiChar(AnsiString(tempDir+'\war3map.j'))) then
  ShowMessage('Success.') 
else
  ShowMessage('Failed.'); 

Other points to consider:

1) not all DLLs use the stdcall calling convention. It is not uncommon for DLLs written in C, like many open-source libraries are, to use the cdecl calling convention instead:

function MpqExtractFile(var hMPQ: Integer; szToExtract, szExtracted: PAnsiChar): Boolean; cdecl; external 'MpqLib.dll' name 'B2'; 

2) C does not have a true Boolean data type like Delphi and C++ do. It is not uncommon for C code to use Byte or even Integer to mimic a Boolean.

In order to use a DLL in Delphi, you really need to know the actual proper declaration of its exported functions. This is less of an issue in C/C++ because most DLLs have an accompanied .h file that provides the declarations. Do you have such a .h file? If so, post it here so someone can verify your translation to Delphi.

Update:

Based on new information, the correct Delphi declaration is this:

function MpqExtractFile(hMpq: THandle; const szToExtract, szExtracted: PAnsiChar): BOOL; stdcall; external 'MpqLib.dll' name 'B2';