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Calling C-DLL from 2.0

Posted on 1997-04-17
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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        
0
Comment
Question by:rene041697
3 Comments
 

Expert Comment

by:Tonetheman041797
ID: 1335426
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.
0
 
LVL 2

Accepted Solution

by:
alona041797 earned 50 total points
ID: 1335427
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';

0
 

Author Comment

by:rene041697
ID: 1335428
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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