Is DdePostAdvise Synchronous?

The server's callback function will have received XTYP_ADVREQ for each existing advise loop that was started without a XTYPF_ACKREQ flag, by the time DdePostAdvise returns. So DdePostAdvise is synchronous except for advise loops started with the XTYPF_ACKREQ flag.

The DdePostAdvise function causes the Dynamic Data Exchange Management Library (DDEML) to send a XTYP_ADVREQ transaction to the calling (server) application's DDEML callback function for each client that has an advise loop active on the specified topic or item name pair. A server application calls this function whenever the data associated with the topic or item name pair changes.

DDEML will have sent XTYP_ADVREQ for each advise loop that was started without a XTYPF_ACKREQ when DdePostAdvise returns.

If an advise loop was started with the XTYPF_ACKREQ flag, the server will have to wait until the client acknowledges that it received the previous data item. Consequently, if the client did not acknowledge the previous data item, DdePostAdvise will set a flag for the advise loop indicating the data change and XTYP_ADVREQ will be sent by DDEML only after the client acknowledges the previous data item. The cAdvReq parameter of XTYP_ADVREQ for the slow client will contain CADV_LATEACK instead of the remaining advise loops to be processed.

This means that DdePostAdvise can return before the server callback receives the XTYP_ADVREQ corresponding to a XTYPF_ACKREQ advise loop. This is not a problem because when the client used XTYPF_ACKREQ, it was willing to miss some of the data transitions in order to prevent the server from sending it data faster than could be processed. The server can send the slow client the latest data when it finally receives the XTYPF_ADVREQ with cAdvReq set to CADV_LATEACK.