INFO: GetCommProperties Returns Error 122 If Called from TAPI Ap

• Microsoft Win32 Application Programming Interface (API) included with: - Microsoft Windows NT version 4.0

SUMMARY

Under Windows NT 4.0, GetCommProperties API call fails when called using a communication handle obtained from the TAPI lineGetID API if COMMPROP structure is not properly initialized.

Under Windows 95, UNIMODEM expects the structure to be zero initialized.

This behavior is specific to UNIMODEM as a TAPI Service Provider.

MORE INFORMATION

Under Windows 95's UNIMODEM, GetCommProperties succeeds when COMMPROP is allocated and zero initialized as follows:

   DWORD dwError;
   COMMPROP commprop;

   memset(&commprop, 0, sizeof(COMMPROP));
   if(!GetCommProperties(hCommHandle, &commprop))
   {
     dwError = GetLastError();
   }

This occurs when UNIMODEM tries to append provider specific information to the end of COMMPROP in a form of MODEMDEVCAPS. To work around this, you need to allocate space for MODEMDEVCAPS structure after COMMPROP. You also need to set the appropriate member variable in COMMPROP so UNIMODEM knows that the structure has been allocated to the proper size. The following code sample demonstrates this process:

   DWORD dwSize;
   COMMPROP *commprop;
   DWORD dwError;

   dwSize = sizeof(COMMPROP) + sizeof(MODEMDEVCAPS) ;
   commprop = (COMMPROP *)malloc(dwSize);
   memset(commprop, 0, dwSize);

   commprop->wPacketLength = dwSize;
   commprop->dwProvSubType = PST_MODEM;
   commprop->dwProvSpec1 = COMMPROP_INITIALIZED;

   if(!GetCommProperties(hNewCommFile, commprop))
   {
     dwError = GetLastError();
   }

   COMMTIMEOUTS commtimeouts;
   DCB dcb;
   DWORD dwSize;
   COMMPROP *commprop;
   DWORD fdwEvtMask;
   DWORD dwError;

   dwSize = sizeof(COMMPROP);
   commprop = (COMMPROP *)malloc(dwSize);
   memset(commprop, 0, dwSize);

   GetCommState(hComm, &dcb);
   if(!GetCommProperties(hComm, commprop))
   {
     if(GetLastError()==122)
     {
        free(commprop);
        dwSize = sizeof(COMMPROP) + sizeof(MODEMDEVCAPS);
        commprop = (COMMPROP *)malloc(dwSize);
        memset(commprop, 0, dwSize);

        commprop->wPacketLength = dwSize;
        commprop->dwProvSubType = PST_MODEM;
        commprop->dwProvSpec1 = COMMPROP_INITIALIZED;
        if(!GetCommProperties(hComm, commprop))
        {
          dwError = GetLastError();
        }
     }
   }

   free(commprop);