DDEInitiate() from a Client May Require a Pause

• Microsoft FoxPro for Windows, versions 2.5 and 2.5a

SUMMARY

When a dynamic data exchange (DDE) client application starts FoxPro for Windows and runs a program that sets up FoxPro as a DDE server application, the client application may need to pause while FoxPro's server program is loading. Not waiting until FoxPro is properly configured can result in error messages stating that the DDE server is unobtainable.

Having to wait, and the length of the pause while a FoxPro program sets up FoxPro as a server application, may be influenced by the speed of the hardware and software being used. The length of the required pause or wait may vary. For example, some applications and some machines may require little or no pause to initiate a channel, while others may need to wait several seconds.

For example, Microsoft Excel version 4.0 may return the error "Remote data not accessible. Start application MYSERVER.EXE?" This error may be generated even if Microsoft Excel had just launched FoxPro and run a .PRG file to publish MYSERVER as a DDE server application (example below). Consequently, when FoxPro is being used as a DDE client application, it may also have to pause before issuing a DDEInitiate() command to an application it just started. Any program that multitasks in the Microsoft Windows environment may exhibit these symptoms.

MORE INFORMATION

The following Microsoft Excel macro requires a pause. ("A1" indicates cell A1 on a Microsoft Excel macro sheet.).

   A1:   =EXEC("foxprow.exe c:\foxprow\tutorial\ddeserve.prg",3)
   A2:   =APP.ACTIVATE()
   A3:   m.initial.chan=INITIATE("myserver","DO")
   A4:   =EXECUTE(m.initial.chan,"SET DEFAULT TO c:\foxprow\tutorial")
   A5:   =EXECUTE(m.initial.chan,"USE customer")
   A6:   =TERMINATE(m.initial.chan)
   A7:   =RETURN()

However, on machines that have a fast CPU and hard disk, this macro results in a "Remote data not accessible. Start application MYSERVER.EXE?" error message in Microsoft Excel. This error occurs because Microsoft Excel, as a multitasking program for Windows, executes its DDEInitiate() command before FoxPro has time to configure itself as a DDE server application. In other words, Microsoft Excel and FoxPro are running their respective programs at the same time, resulting in Microsoft Excel trying to establish a DDE channel to a server application that is not yet running.

To correct this problem, add a pause or wait statement in the DDE client application, letting FoxPro (or any server application) finish configuring itself.

In Microsoft Excel, a WAIT() statement will pause a macro long enough for FoxPro to finish executing its server program. The revised Microsoft Excel macro would then be:

   A1:   =EXEC("foxprow.exe c:\foxprow\tutorial\ddeserve.prg",3)
   A2:   =APP.ACTIVATE()
   A3:   =WAIT(NOW()+"00:00:03")
   A4:   m.initial.chan=INITIATE("myserver","DO")
   A5:   =EXECUTE(m.initial.chan,"SET DEFAULT TO c:\foxprow\tutorial")
   A6:   =EXECUTE(m.initial.chan,"USE customer")
   A7:   =TERMINATE(m.initial.chan)
   A8:   =RETURN()

The DDESERVE.PRG Program

   *** Save as DDESERVE.PRG in the \FOXPROW\TUTORIAL directory ***

   *** Set FoxPro up as a DDE server ***
   = DDESetService('myserver', 'DEFINE')
   = DDESetService('myserver', 'EXECUTE', .T.)
   = DDESetTopic('myserver', 'DO', 'DOTOPIC')

   PROCEDURE dotopic
   PARAMETERS channel, action, item, data, format, advise
   mresult = .F.

   *** It's necessary to return .T. from an     ***
   *** INITIATE action or no connection is made ***

   IF action = 'INITIATE'
        mresult = .T.
   ENDIF

   IF action = 'EXECUTE'
        &data
        mresult = .T.
   ENDIF

   IF action = 'TERMINATE'
        WAIT WINDOW 'Goodbye ... ' NOWAIT
        mresult = .T.
   ENDIF
   RETURN mresult

REFERENCES

Microsoft Excel "Function Reference"

FoxPro for Windows "Language Reference"