The DdeEnableCallback function enables or disables transactions for a specific conversation or for all conversations currently established by the calling application.
After disabling transactions for a conversation, the operating system places the transactions for that conversation in a transaction queue associated with the application. The application should reenable the conversation as soon as possible to avoid losing queued transactions.
BOOL DdeEnableCallback(
| DWORD idInst, | // instance identifier |
| HCONV hConv, | // handle to conversation |
| UINT wCmd | // enable or disable function code |
| ); |
Parameters
idInst
Specifies the application-instance identifier obtained by a previous call to the DdeInitialize function.
hConv
Identifies the conversation to enable or disable. If this parameter is NULL, the function affects all conversations.
wCmd
Specifies the function code. This parameter can be one of the following values:
| Value | Meaning |
| EC_ENABLEALL | Enables all transactions for the specified conversation. |
| EC_ENABLEONE | Enables one transaction for the specified conversation. |
| EC_DISABLE | Disables all blockable transactions for the specified conversation.
A server application can disable the following transactions: XTYP_ADVSTART A client application can disable the following transactions: XTYP_ADVDATA |
| EC_QUERYWAITING | Determines whether any transactions are in the queue for the specified conversation. |
Return Values
If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero.
If the wCmd parameter is EC_QUERYWAITING, and the application transaction queue contains one or more unprocessed transactions that are not being processed, the return value is TRUE; otherwise, it is FALSE.
Errors
Use the DdeGetLastError function to retrieve the error code, which may be one of the following:
DMLERR_DLL_NOT_INITIALIZED
DMLERR_NO_ERROR
DMLERR_INVALIDPARAMETER
Remarks
An application can disable transactions for a specific conversation by returning the CBR_BLOCK return code from its dynamic data exchange (DDE) callback function. When you reenable the conversation by using the DdeEnableCallback function, the operating system generates the same transaction that was in process when the conversation was disabled.
Using the EC_QUERYWAITING flag does not change the enable state of the conversation and does not cause transactions to be issued within the context of the call to DdeEnableCallback.
If DdeEnableCallback is called with EC_QUERYWAITING and the return value is TRUE, an application should try to quickly allow message processing, return from its callback, or enable callbacks. Such a result does not guarantee that subsequent callbacks will be made. Calling DdeEnableCallback with EC_QUERYWAITING lets an application with blocked callbacks determine whether there are any transactions pending on the blocked conversation. Of course, even if such a call returns false, an application should always process messages in a timely manner.
See Also