A client application initiates a DDE conversation by calling WinDdeInitiate, specifying the server application-name string and the topic-name string. WinDdeInitiate fills a DDEINIT data structure with the specified strings, then sends a WM_DDE_INITIATE message to all frame windows that have HWND_DESKTOP as their parent. The message contains the handle of the client application and a pointer to the DDEINIT data structure. The following figure illustrates the DDEINIT data structure:

     typedef struct _DDEINIT
     {
       ULONG   cb;
       PSZ     pszAppName;
       PSZ     pszTopic;
       USHORT  usConvContext;
      } DDEINIT;

Because the message is sent rather than posted, WinDdeInitiate requires a response from all recipients of the message before it returns control to the client application.

Any potential server must contain a server window, a top-level frame window that has been subclassed to receive and process WM_DDE_INITIATE messages. When a server window receives WM_DDE_INITIATE, it examines the application-name and topic-name strings in the DDEINIT data structure. If the application-name string matches and the server supports the requested topic, the server acknowledges the client's request.

Either the application-name or topic-name string can be zero-length. If the application-name string is zero-length, all servers check the topic-name string. Each server that supports the topic sends a separate acknowledgment to the client. If the topic-name string is zero-length, the server sends an acknowledgment for each supported topic. Using zero-length strings, a client can obtain the names of all the active servers in the system or the names of all the topics a server supports.

The following pseudocode shows how servers respond to WM_DDE_INITIATE messages:

    If (specific app requested and server is instance of app) or
       (specific app not requested)
    {

         If (specific topic requested)
              If (server can support topic)

                   acknowledge the requested topic

         else
              acknowledge each supported topic
    }

A server acknowledges its support of a specific topic by calling WinDdeRespond, specifying the handle of its server window, its application name, and the name of the supported topic. WinDdeRespond fills a DDEINIT data structure with the specified strings, then sends a WM_DDE_INITIATEACK message to the client. The message contains the handle of the server window and a pointer to the DDEINIT data structure. The client examines the topic-name string in the DDEINIT data structure and decides whether to begin a transaction on the topic.

If two applications agree on some unspecified protocol and can exchange window handles by some means, they can use DDE messages on those window handles without going through an initiate sequence.

An application does not need to fill in a DDEINIT data structure; WinDdeInitiate and WinDdeRespond automatically fill the data structure. However, applications must extract the application name and topic name from the DDEINIT data structure when receiving a WM_DDE_INITIATE or WM_DDE_INITIATEACK message.