A client application can use DDE to establish a link to an item in a server application. When such a link is established, the server sends periodic updates about the linked item to the client (typically, whenever the data associated with the item in the server application has changed). A permanent data stream is established between the two applications and remains in place until it is explicitly disconnected.

The client sends the server a WM_DDE_ADVISE message to set up the data link. The advise message contains a shared-memory pointer containing a DDESTRUCT data structure with the item name, format information, and status information.

If the server has access to the requested item and can render it in the desired format, the server records the new link, then sends the client a positive WM_DDE_ACK message. Until the client issues a WM_DDE_UNADVISE message, the server sends data messages to the client every time a change occurs in the source data associated with the item in the server application.

If the server is unable to satisfy the request, it sends the client a negative WM_DDE_ACK message.

When a link is established with the DDE_FNODATA status bit cleared, the client is sent the data each time the data changes. In such cases, the server renders the new version of the item in the previously specified format and posts a WM_DDE_DATA message to the client.

When the client receives a WM_DDE_DATA message, it extracts data from the shared-memory object by using the DDESTRUCT data structure at the beginning of the object. If the DDE_FACKREQ status bit in the status word of the DDESTRUCT data structure is set, the client must post a positive WM_DDE_ACK message to the server.

When a link is established with the DDE_FNODATA status flag set, a notification, not the data itself, is posted to the client each time the data changes. In this case, the server does not render the new version of the item when the source data changes, but simply posts a WM_DDE_DATA message with 0 bytes of data and the DDE_FNODATA status flag set.

The client can request the latest version of the data by performing a regular one-time WM_DDE_REQUEST transaction, or it can simply ignore the data-change notice from the server. In either case, if the DDE_FACKREQ status bit is set, the client should send a positive WM_DDE_ACK message to the server.

When a client sends a WM_DDE_ADVISE message on a topic/item pair that is already engaged in an advise loop but has a different format specified, the server interprets this as a request to add an advise loop with the given format requested. Therefore, several advise loops can exist for a given topic/item pair. If a server does not support this extent of advise loops, it rejects the advise request.

Correspondingly, when a server receives a WM_DDE_UNADVISE message, the server must compare the format field with the current format of the advise loop. Only if the specified format is 0, meaning all advise loops, or matches an active advise loop does the server stop the advise loop and return a positive acknowledgment.

To terminate a specific item link, the client posts a WM_DDE_UNADVISE message to the server. The server ensures that the client currently has a link to the specified item in this exchange. If the link exists, the server sends a positive WM_DDE_ACK message to the client and no longer sends updates on the item in this exchange. If the server has no such link, it sends a negative WM_DDE_ACK message.

To terminate all links for a particular exchange, the client application posts a WM_DDE_UNADVISE message with a zero-length item name to the server. The server ensures that the exchange has at least one link currently established. If so, the server posts a positive WM_DDE_ACK message to the client, and no longer sends any updates in the exchange. If the server has no links in the exchange, it posts a negative WM_DDE_ACK message.