An application can suspend transactions to its DDE callback function
either those transactions associated with a specific conversation handle or
all transactions regardless of the conversation handle. This capability is
useful when an application receives a transaction that requires lengthy processing.
In such a case, the application can return the CBR_BLOCK return code to suspend
future transactions associated with the transaction's conversation handle, so
that the application is free to process other conversations.
When processing has been completed, the application calls the DdeEnableCallback
function to resume transactions associated with the suspended conversation.
causes the DDEML to resend the transaction that resulted in the application
suspending the conversation. Therefore, the application should store the result
of the transaction in such a way that it can obtain and return the result
without reprocessing the transaction.
An application can suspend all transactions associated with a specific
conversation handle by specifying the handle and the EC_DISABLE flag in a call to DdeEnableCallback
. By specifying a NULL handle, an application can suspend all transactions for
When a conversation has been suspended, the DDEML saves transactions for the
conversation in a transaction queue. When the application reenables the
conversation, the DDEML removes the saved transactions from the queue and passes each
transaction to the appropriate callback function. The capacity of the
transaction queue is large, but an application should reenable a suspended conversation
as soon as possible to avoid losing transactions.
An application can resume usual transaction processing by specifying the
EC_ENABLEALL flag in DdeEnableCallback
. For a more controlled resumption of transaction processing, the application
can specify the EC_ENABLEONE flag. This flag removes one transaction from the
transaction queue and passes it to the appropriate callback function; after that
transaction has been processed, any conversations are again disabled.
If the EC_ENABLEONE flag and a conversation handle are specified in the call
, only that conversation is blocked after the transaction has been processed.
If a NULL conversation handle is specified, all conversations are blocked after
a transaction has been processed in any conversation.
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