Calling a PowerBASIC DLL from C

[Guest]

I am attempting to call a PB/DLL created DLL from C. I have no
problems with Asciiz strings or numerics when the function parameter uses BYVAL.

The following is the "test" function I am using. Some functions in the DLL use PB
dynamic (variable length) strings and numerics passed by reference. The DLL is not
mine so I can't change its functions.
When i run the C program the first parameter gets logged to the file just fine, but
the 2nd and 3rd parameters do not make it. A protection fault is raised.

Here's what currently gets logged by the PB Dll:
lDBN: 1 lDBName: ! lPrompt:
lDBN: 1 lDBName: ! lPrompt:
lDBN: 1 lDBName: ! lPrompt:

'[b]RP_TEST.DLL code[/b]
Function RP_TestDLL Alias "RP_TestDLL"(ByVal lDBNumber As Long, _
                    ByVal lDBname As String, _
                    lPrompt As Long) Export As Long
  Open "D:\LCC\Sqltest\RP_TestDLL.Log" For Append As #1
  Print #1, $CrLf;"lDBN:"; lDBNumber, "  lDBName: "; lDBName;"!","  lPrompt:";lPrompt
  Close 1
  Function = 199
End Function

The C code I am using to call the function in the PB Dll is:

//[b]TestPBDll.C[/b]
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <win.h>
 
typedef int __stdcall RP_TestDLL( int lDBNumber,
                                  CHAR* lDBname,
                                  int &Prompt );
 
int main(int argc,char *argv[])
{
  DWORD rc=0;
  CHAR dbn[128];
 
  RP_TestDLL* pRP_TestDLL;
  HMODULE hRPDll;
  
  pRP_TestDLL = (RP_TestDLL* ) GetProcAddress(hRPDll,"RP_TestDLL");
  hRPDll = LoadLibrary("RP_Test.dll");
 
  printf("Loaded DLL: %d\r\n",hRPDll );
 
  if (hRPDll == NULL) printf( "hRPDll = Null\r\n");
  else
  {
    sprintf( dbn, "DRIVER=SQL Server;server=DevTestServer;database=DB1;uid=dbuser;pwd=pwrd\0"); 
    printf ("Before Calling RP_TestDLL - pRP_TestDLL: %d\r\n", pRP_TestDLL );
    rc = pRP_TestDLL(1,
                     dbn,
                     123 );
 
    FreeLibrary(hRPDll);
  }
   
  printf("Done!\r\n");
  return 0;
{

Any help very much appreciated.
------------------
Ron

[This message has been edited by Ron Pierce (edited August 19, 2000).]

[Florent Heyworth]

hi ron

the reason you are getting gpfs is because c does not natively
use bstrs (pb dynamic strings). since you can't change the dll
to use asciiz you'll need to handle bstrs from c as in the
following example. there was a long discussion on that subject
at http://www.powerbasic.com/support/pb...ead.php?t=2191

here's some example code that shows how to handle bstrs from
c and pass them to pbdll

// c code
#include <windows.h>
#include <stdio.h>
#include <oleauto.h>

typedef int __stdcall lpfncalldll( int dbnumber, bstr dbname, int &prompt  );

int __cdecl main( int argc, char *argv[], char *envp[] ) 
{
	hmodule hlib;
	lpfncalldll* lpcalldll;
	bstr bstring;
	char *chdsn;
	int prompt;
	int lreturn;

	hlib = loadlibrary( "mydll.dll" );
	if ( hlib == null )
	{
		printf( "could not load mydll.dll" );
		return false;
	}
	lpcalldll = (lpfncalldll*)getprocaddress( hlib, "myfunction" );
	if ( lpcalldll == null )
	{
		freelibrary( hlib );
		return false;
	}
	chdsn = "driver=sql server;server=devtestserver;database=db1;uid=dbuser;pwd=pwrd";
	/* use ole strings */
	bstring = sysallocstringbytelen( chdsn, strlen(chdsn) ); 
	if ( bstring==null ) 
	{
		printf( "could not allocate string" );
		return false;
	}
	prompt = 123;
	lreturn = lpcalldll( 1, bstring, prompt );
	if ( lreturn != 0 )
		printf( "function returned %d", lreturn );

	/* ole string must be freed afer use */
	sysfreestring( bstring );
	freelibrary( hlib );
	return true;
}


-------------
'basic code
#compile dll
 
#include "win32api.inc"
 
function myfunction alias "myfunction" ( byval ldbnumber as long, byval sdbname as string, byref lprompt as long ) export as long
 
  open "c:\temp\rp_testdll.log" for append as #1
  print #1, $crlf;"ldbn:"; ldbnumber, "  dbname: "; sdbname;"!","  lprompt:";lprompt
  close 1
  function = 199
  
end function
 
function libmain(byval hinstance   as long, _
                 byval fwdreason   as long, _
                 byval lpvreserved as long) export as long
 
  select case fwdreason
 
    case %dll_process_attach
       libmain = 1   'success!
      exit function
 
    case %dll_process_detach
       libmain = 1   'success!
      exit function
 
    case %dll_thread_attach
       libmain = 1   'success!
       exit function
 
    case %dll_thread_detach
       libmain = 1   'success!
 
      exit function
 
  end select
 
end function

note that if the dll declare byref sdbname as string instead of
byval sdbname as string you'd need to declare the function
prototype in c as:
typedef int __stdcall lpfncalldll( int dbnumber, bstr &dbname, int &prompt );



[this message has been edited by florent heyworth (edited august 19, 2000).]

[Guest]

Thanks for the superb help Florent. The BSTR and other oleaut functions were the cure!

------------------
Ron