The predicates in this section deal with MS-Windows‘Dynamic
Data Exchange’or DDE protocol.170This
interface is contributed by Don Dwiggins. A Windows DDE
conversation is a form of interprocess communication based on sending
reserved window events between the communicating processes.
Failing DDE operations raise an error of the structure below, where
Operation is the name of the (partial) operation that failed
and
Message is a translation of the operator error code. For some
errors, Context provides additional comments.
error(dde_error(Operation, Message), Context)
The DDE client interface allows Prolog to talk to DDE server
programs. We will demonstrate the use of the DDE interface using the
Windows PROGMAN (Program Manager) application:
1 ?- open_dde_conversation(progman, progman, C).
C = 0
2 ?- dde_request(0, groups, X)
--> Unifies X with description of groups
3 ?- dde_execute(0, '[CreateGroup("DDE Demo")]').
true.
4 ?- close_dde_conversation(0).
true.
For details on interacting with progman, use the SDK online
manual section on the Shell DDE interface. See also the Prolog
library(progman), which may be used to write simple Windows
setup scripts in Prolog.
- open_dde_conversation(+Service,
+Topic, -Handle)
- Open a conversation with a server supporting the given service name and
topic (atoms). If successful, Handle may be used to send
transactions to the server. If no willing server is found this predicate
fails silently.
- close_dde_conversation(+Handle)
- Close the conversation associated with Handle. All opened
conversations should be closed when they're no longer needed, although
the system will close any that remain open on process termination.
- dde_request(+Handle,
+Item, -Value)
- Request a value from the server. Item is an atom that
identifies the requested data, and Value will be a string (
CF_TEXT
data in DDE parlance) representing that data, if the request is
successful.
- dde_execute(+Handle,
+Command)
- Request the DDE server to execute the given command string. Succeeds if
the command could be executed and fails with an error message otherwise.
- dde_poke(+Handle,
+Item, +Command)
- Issue a
POKE command to the server on the specified Item.
command is passed as data of type CF_TEXT.
The library(dde) defines primitives to realise simple
DDE server applications in SWI-Prolog. These features are provided as of
version 2.0.6 and should be regarded as prototypes. The C part of the
DDE server can handle some more primitives, so if you need features not
provided by this interface, please study library(dde).
- dde_register_service(+Template,
+Goal)
- Register a server to handle DDE request or DDE
execute
requests from other applications. To register a service for a DDE
request, Template is of the form:
+Service(+Topic, +Item, +Value)
Service is the name of the DDE service provided (like
progman in the client example above). Topic is either
an atom, indicating Goal only handles requests on this topic,
or a variable that also appears in Goal. Item and Value
are variables that also appear in Goal. Item
represents the request data as a Prolog atom.171Up
to version 3.4.5 this was a list of character codes. As recent versions
have atom garbage collection there is no need for this anymore.
The example below registers the Prolog current_prolog_flag/2
predicate to be accessible from other applications. The request may be
given from the same Prolog as well as from another application.
?- dde_register_service(prolog(current_prolog_flag, F, V),
current_prolog_flag(F, V)).
?- open_dde_conversation(prolog, current_prolog_flag, Handle),
dde_request(Handle, home, Home),
close_dde_conversation(Handle).
Home = '/usr/local/lib/pl-2.0.6/'
Handling DDE execute requests is very similar. In this
case the template is of the form:
+Service(+Topic, +Item)
Passing a Value argument is not needed as execute
requests either succeed or fail. If Goal fails, a‘not
processed’is passed back to the caller of the DDE request.
- dde_unregister_service(+Service)
- Stop responding to Service. If Prolog is halted, it will
automatically call this on all open services.
- dde_current_service(-Service,
-Topic)
- Find currently registered services and the topics served on them.
- dde_current_connection(-Service,
-Topic)
- Find currently open conversations.
