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Well, a blast from a past 🙂 I'm getting this error trying to connect to FS2004 - FSX/P3D work fine. Just extending some support of my app to FS2004 by request.

{"FSUIPC Error #12: FSUIPC_ERR_SENDMSG. Error sending message to FSUIPC."}
   at FSUIPC.FSUIPCConnection.Process(Byte ClassInstance, IEnumerable`1 GroupNames)
   at FSUIPC.FSUIPCConnection.Open(Byte ClassInstance, Int32 RequiredFlightSimVersion)
   at Acars.SimClient.FsuipcClient.FsuipcClient.Connect(String address, Boolean enable_retry) in C:\...\Client.cs:line 89

I'm using this to connect:


I'm not sure if I'm missing something - versions are as follows:

FSUIPC DLL: 3.1.25
FSUIPC (non-registered): 3.999 (latest one available on the site)
FS2004: Latest with the 9.1 patch
Windows 10, with FS9 running in Windows 7 compat mode, as admin (both apps)

I'm not sure where to look next. Is a registered version of FSUIPC required? There doesn't seem to be anything in the logs; I've enabled IPC read/writes in the logs to see if something pop up.

********* FSUIPC, Version 3.999z9b by Pete Dowson *********
Running on Windows Version 5.1 Build 2600 Service Pack 2
Running inside FS2004 (FS9.1 CONTROLS.DLL, FS9.1 WEATHER.DLL)
User Name=""
User Addr=""
FSUIPC not user registered
WIDEFS not user registered, or expired
Module base=61000000
     1031 System time = 12/01/2021 11:46:50
     1047 E:\Program Files (x86)\Microsoft Games\Flight Simulator 9\
     1047 System time = 12/01/2021 11:46:50, FS2004 time = 12:00:00 (00:00Z)
     1422 C:\Users\xxx\Documents\Flight Simulator Files\test flight.flt
     1438 AIRCRAFT\c172\Cessna172SP.air
    10563 C:\Users\xxx\Documents\Flight Simulator Files\UI generated flight.flt
    10672 Clear All Weather requested: external weather discarded
    10922 Advanced Weather Interface Enabled
    14250 Traffic File #18 = "scenery\world\scenery\traffic030528"
   830594 WeatherOptions set, now 0000B027 (timer=0)
   830594 LogOptions changed, now 0000001D (LogExtras=1)
   830656 Ready Flags: Ready-To-Fly=Y, In Menu=Y, In Dlg=Y
   830656 Ready Flags: Ready-To-Fly=Y, In Menu=N, In Dlg=N
   830656 AP READOUTS: Flags1=00000000, Flags2=00000000
   830859 *VIS* SM: Min=0.00, MaxRny=0.00, MaxOvc=0.00, MaxCld=0.00, MaxClr=0.00, Upper=60.00, FT: LwrAlt=0, UppAlt=25000



Edited by Nabeel
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I use FS2004 here for testing sometimes so it does work.

You shouldn't need a registered FSUIPC for IPC activity, but Pete has made FSUIPC3 registration free. See at the bottom of this post for the free reg key.


The only cause of FSUIPC_ERR_SENDMSG I've seen is a mismatch between the admin permissions for the app and the sim. Double check that both are 'as admin' or both are not.

If that's definitely not the problem then the log files might tell us something. Paste here if you can't see anything obvious. The log should be in the modules folder (FSUIPC.log).


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Thanks that seemed to work. There was a mismatch - was launching FS9 as admin but Visual Studio was not. I could have sworn I ran Prepar3d as admin but I guess not. Thanks for your help!

Edited by Nabeel
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