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## *service-topics*

The default value of the topics argument to open-server, which determines the DDE topic names that DDE clients may specify to connect with DDE servers running in Lisp. The initial value is the list `(nil :system :eval)`, meaning that the client should specify the topic as one of the strings "nil", "system", or "eval".

The default value `:system` follows a DDE convention where a DDE client can send a request with the "System" topic and the "sysitems" item to find out what items are available under the standard "System" topic, or with the "topics" item to find out what other topics are available. A built-in answer-request method will reply to these requests, and return the sysitems or service-topics for the Lisp process that the server is running in.

The default value `:eval` provides some built-in behavior unique to an Allegro DDE server. If sent by a client in a request with the name of a Lisp variable as the item, a built-in answer-request method returns the value of the variable (as a string). If the item is "command-result", a built-in answer-request method returns the value that was returned by the most recent command that the client asked this Lisp server to perform (see execute-command). (When Lisp is the client, it sends a command by calling send-command.) When the item is "help", a string that explains these points is returned.

The default value `nil` accepts a topic of "nil" and passes the symbol `nil` to overridable DDE server generic functions such as answer-request and execute-command, but the default methods for topic `nil` do nothing.

Compatibility note: Starting with release 6.2, this variable is no longer automatically modified to match the service topics that were most recently passed to open-server. You may still modify the value of this variable to change the global default for service topics, but to retrieve the service topics that are actually being used by a particular Lisp process, you should call the function service-topics instead.

See dde.htm for information about DDE support in Common Graphics.

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This page was not revised from the 8.2 page.
Created 2012.5.30.

|  | ToC | DocOverview | CGDoc | RelNotes | FAQ | Index | PermutedIndex
| Allegro CL version 9.0 Unrevised from 8.2 to 9.0. 8.2 version