DLL calling procedure

[mikef]

I have a DLL that is written in C. I am trying to use it from VB. I can use most commands, but the most important one is giving me trouble. The problem is in my declaration I am sure. It is causing a GPF... which does not occur in its Visual C++ counterpart calling program. Can anyone help me figure out what is wrong with my VB declare from this????

Here is my VB Declaration:

Private Declare Function mnCheckDevices Lib "mnw.dll" (ByRef mn_DevFlg() As Long, ByRef mn_Port As Long, ByRef mn_DevCnt As Long, ByRef mn_ErrMsg() As Byte, ByRef mn_BuffSize As Long) As Long

Here is the C procedure:

//Inputs:
//dev_flag => pointer to array for active devices
// port => pointer to selected com port
//dev_cnt => pointer to variable for number of devices found
//error_mess => string that holds return error message
//buff_size => pointer to variable for buffer size
//Outputs:
//None
/////////////////////////////////////////////////////////////////////////////
extern "C"
int __declspec(dllexport) WINAPI mnCheckDevices(int *dev_flg, int *port, int *dev_cnt, char *error_mess, int *buff_size)
{
static long start, end;
static int port_num;
static int i, rec_addr[2];
static int errflag[MAX_BOARDS];
static int ret_code;
static int baud_rate;
static bool BaudRatesTried[5];
static char address;

DeviceDialog = new TDeviceDialog(NULL);
DeviceDialog->Show();
DeviceDialog->Update();

port_num = *port;

for (i = 0; i <= MAX_BOARDS; i++)
errflag[i] = dev_flg[i] = 0;

for (i = 0; i < 5; i++)
BaudRatesTried[i] = false;

baud_rate = BaudRates[0];
BaudRatesTried[0] = true;

CloseComm(&port_num); // close comm port
InitComm(&port_num, &baud_rate, buff_size); //reinitialize comm port

while (1)
{
// detect number of devices present on line and their addresses
address = 0x00;
ret_code = XmitPacket("*", &address, &port_num);
if (ret_code < 0)
return ret_code;

ret_code = DeviceDialog->GetActiveDevices(&port_num, rec_addr, error_mess,
dev_flg, dev_cnt);
if (ret_code == 0)
{
chk_devices = true;
break;
}
else if (ret_code < 0)
{
Application->MessageBox("CheckDevices failed.\n\rCommunication error.", NULL, MB_OK);
break;
}
// Select a different baud rate
// BaudRatesTried[port_num-5] = true;
for (i = 0; i < 5; i++)
{
if (!BaudRatesTried[i])
{
baud_rate = BaudRates[i];
BaudRatesTried[i] = true;
break;
}
}
if (i == 5)
{
// DeviceDialog->CloseDialog();
DeviceDialog->Close();
Application->MessageBox("CheckDevices failed. No active devices found.",
NULL, MB_OK);
return -1;
// baud_rate = -1;
// break;
}
else
{
CloseComm(&port_num); // close comm port
InitComm(&port_num, &baud_rate, buff_size); // reinitialize comm port
}
}

if (baud_rate > 0)
{
start = GetTime();
do
{
end = GetTime();
if (end - start < 0)
end = end + 8640000;
} while (end - start < 50); // delay 0.50 seconds
// DeviceDialog->CloseDialog();
DeviceDialog->Close();
}
return baud_rate;
} // mnCheckDevices() ends

[jim mcnamara]

Ok.

For the arrays:

Dim arr(10) as Long

Not this: Dim arr() as Long

before you pass it to your dll? Same for the byte array. They have to be allocated from process (VB) memory before you make the call. Plus with the byte array, you can use a string like this

Dim str as String
str = string$(100, chr(0) ) ' this allocates space in the string

instead of your byte array.

the most important thing: all of the things that are pointers in your C code have to get passed ByRef from VB must be existing , already Dim - ed(is this a word?) variables.

GPF's are caused when you have a pointer aimed at nothing.

[parksie]

Originally posted by jim mcnamara
GPF's are caused when you have a pointer aimed at nothing.

I thought that gave an Access Violation?