BUG: DDESPY GP Faults Upon Return of CBR_BLOCK

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SYMPTOMS

DDESPY GP faults when a DDEML server application's dynamic data exchange (DDE) callback function returns CBR_BLOCK from a transaction of XCLASS_DATA class.

CAUSE

Expecting to always obtain a valid data handle from transactions of XCLASS_DATA class, DDESPY calls DdeAccessData() on the hData it receives in an attempt to dump the data to its output window. Internally, DDEML's DdeAccessData() translates to a GlobalLock() call. Consequently, a return value of CBR_BLOCK (defined in WINDOWS.H as -1) would mean calling GlobalLock() on an invalid hData == -1, and thus results in a general protection (GP) fault.

STATUS

Microsoft has confirmed this to be a bug in the version of DDESPY that comes with the Windows 3.1 SDK. With the improved parameter validation in Windows NT, this problem should be corrected in the Windows NT version.

MORE INFORMATION

In DDEML, transactions of XCLASS_DATA class are typically expected to return a data handle. (Among the DDEML transactions of this class are XTYP_REQUEST, XTYP_ADVREQ, and XTYP_WILDCONNECT.) However, in situations where such transactions require lengthy processing (such as a server gathering data from a network), thereby making it impossible to return a data handle immediately, an application may choose to return CBR_BLOCK from its DDE callback function. This allows the server application to gather data "asynchronously," as DDEML queues up further transactions on the current conversation.

Using DDEPSY to monitor this type of activity in the system results in a GP fault, because DDESPY calls DdeAccessData() on an invalid hData (== -1), as described in the CAUSE section above.

More information on DDEML transaction classes can be found in Section 5.8.7 of the Windows 3.1 SDK "Programmer's Reference, Volume 1: Overview." More information on returning CBR_BLOCK to suspend DDEML transactions may be found in Section 5.8.6 of the Windows 3.1 SDK "Programmer's Reference, Volume 1: Overview," or by searching on the following words in the Microsoft Knowledge Base:

   DDEML and CBR_BLOCK