The System topic provides a context for information of general interest to any DDE client. It is recommended that server applications support the System topic at all times. The System topic is defined in the DDEML.H header file as SZDDESYS_TOPIC.
To determine which servers are present and the kinds of information they can provide, a client application can request a conversation on the System topic upon starting, setting the device name to NULL. Such wildcard conversations are costly in terms of system performance, so they should be kept to a minimum. For more information about initiating DDE conversations, see Conversation Management.
A server must support the following item names within the System topic and any other item names that are useful to a client.
| Item | Description |
|---|---|
| SZDDE_ITEM_ITEMLIST | A list of the items supported under a non-System topic. (This list may vary from moment to moment and from topic to topic.) |
| SZDDESYS_ITEM_FORMATS | A tab-delimited list of strings representing all clipboard formats potentially supported by the service application. Strings that represent predefined clipboard formats are equivalent to the CF_ values with the "CF_" prefix removed. For example, the CF_TEXT format is represented by the string "TEXT". These strings must be uppercase to further identify them as predefined formats. The list of formats must appear in the order of most rich in content to least rich in content. |
| For more information about clipboard formats and rendering data, see Clipboard. | |
| SZDDESYS_ITEM_HELP | User-readable information of general interest. This item must contain, at a minimum, information on how to use the server application's DDE features. This information may include, but is not limited to, how to specify items within topics, what execute strings the server can perform, what poke transactions are allowed, and how to find help on other System topic items. |
| SZDDESYS_ITEM_RTNMSG | Supporting detail for the most recently used WM_DDE_ACK message. This item is useful when more than 8 bits of application-specific return data are required. |
| SZDDESYS_ITEM_STATUS | An indication of the current status of the server. Typically, this item supports only the CF_TEXT format and contains the Ready or Busy string. |
| SZDDESYS_ITEM_SYSITEMS | A list of the items supported under the System topic by this server. |
| SZDDESYS_ITEM_TOPICS | A list of the topics supported by the server at the current time. (This list may vary from moment to moment.) |
These item names are values defined in the DDEML.H header file. To obtain string handles for these strings, an application must use the DDEML string-management functions, just as it would for any other string in a DDEML application. For more information about managing strings, see String Management.