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Calling C-DLL from 2.0

rene041697
rene041697 asked
on
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Last Modified: 2010-08-05
Help !!

I trie to call a function from a DLL called scom32. The DLL is written in MSVC++ 4.00 and i only have the headerfile. I have converted the headerfile to delphi and try to call the funtion. But something goes wrong with i think passing the function arguments. When the function i call only has one argument then the call works, but when i have two or more
arguments the call actualy changes arguments wich normally would't change.

Here is what i did:

-------------- C-header:

typedef struct tagPCDCONN
{
      int       Port;
      int       Protocol;
      int       Baud;
      BYTE         Sbusmode;
      BOOL        bEPROM;
      BYTE        TxMsg[270];
} PCDCONN;

typedef PCDCONN FAR *LPCONN;

int WINAPI PcdComOpen(LPCONN);
int WINAPI PcdRdVersion(LPCONN,LPSTR);

-------------- This is my DELPHI equivalent:

type
 PCDCONN = Record

      Port      :Integer;
      Protocol:Integer;
      Baud      :Integer;
      Sbusmode:Byte;
      bEPROM      :Longbool;
      TxMsg      :Array[0..269] of Byte;
 end;

var

 LPCONN      : ^PCDCONN;
 Version: Array [0..127] of AnsiChar;
 Status : Integer;

function PcdComOpen(LPCONNEC:PCDCONN):Integer;
 far; pascal; external 'SCOM32';
function PcdRdVersion(LPCONNEC:PCDCONN;Version:PChar):Integer;
 far; pascal; external 'SCOM32';
        
-------------- And so i call the functions --------
begin
 getmem(LPCONN,SizeOf(PCDCONN);
 PCDCONN^.Port:=0;
 etc.etc.      
 status:=PcdComOpen(PCDCONN^);           {1}
 status:=PcdRdVersion(PCDCONN^,Version); {2}
 freemem(LPCONN,SizeOf(PCDCONN);
end;

When i check what is returned in the status then i can only say it works correct (in statement {1} as well in {2}). But in statement {2} the result in 'version' is wrong and 'PDCONN' value's are destroyed. Only the first two fields are not correct anymore, so i suspect it has something to do with memory allocation or with passing the function arguments. I have searched for two days but can't seem to find out what the problem is.

Is there anyone who can help me !!!

Kind Regards,

Rene Reuscher, ANTONIUS Vessel heads        
Comment
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I think that you should first change your calling convention in your declaration of the routines PcdComOpen and PcdRdVersion. If you are trying to use a Win32 DLL the calling convention should stdcall so your routines should look like :
function PcdComOpen(LPCONNEC:PCDCONN):Integer;
                     stdcall; external 'SCOM32';
                    function PcdRdVersion(LPCONNEC:PCDCONN;Version:PChar):Integer;
                     stdcall; external 'SCOM32';

The problems that you describe could be caused by a wrong calling convention. You must test it to make sure that I have given you the correct answer.
You defined the functions as follows:

function PcdComOpen(LPCONNEC:PCDCONN):Integer;
                   far; pascal; external 'SCOM32';
                  function
                  PcdRdVersion(LPCONNEC:PCDCONN;Version:PChar):Integer;
                   far; pascal; external 'SCOM32';

You have used the "pascal" keyword which sets the calling convention to pascal-style. The DLL was written in C so you need to use "C-style" calling convention i.e.  "stdcall".

Fixed declarations:

function PcdComOpen(LPCONNEC:PCDCONN):Integer;
                   far; stdcall; external 'SCOM32';
                  function
                  PcdRdVersion(LPCONNEC:PCDCONN;Version:PChar):Integer;
                   far; stdcall; external 'SCOM32';

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Author

Commented:
I had tried 'stdcall' already because C++ says that 'WINAPI is equivalent with far_pascal, pascal. But anyway i tried it again, and still got the same result: I get an error message -->

 'Raised exception class EaccessViolation with message 'Access     violation at adress 0000009. Read of address 709F9147'

I tried this also with the other calling conventions like 'register','cdecl'. But 'pascal' was the only one were i didn't got this error message.
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