Can't connect to jsbsim server

[achodoglugil]

I set the input parameters like this in c1723.xm

l
i run the command .\JSBSim --script=scripts/c1723.xml --realtime --suspend and then from another terminal i run the command "telnet localhost 1137" and doesnt get the message "connected to jsbsim server". I just get a blank terminal and cant type anything

[seanmcleod]

As a quick test what happens if you don't include --suspend when you run JSBSim?

[achodoglugil]

nothing changes

[seanmcleod]

Ah, I've just looked at your screenshot again. You've placed the <input> element in the wrong file, you've placed it in the script file whereas it needs to be placed in the aircraft configuration file.

JSBSim has the capability to output data at runtime via a socket. The capability also exists to connect to JSBSim via socket for data input. The capability is more suited for interaction and debugging or investigations more than for interprocess communications. The aircraft configuration file may contain the optional element that defines how an outside process can connect to a running instance of JSBSim:

[achodoglugil]

what is the aircraft configuration file for c1723.xml can you help me with this

[seanmcleod]

It's listed 1 line below in your screenshot, i.e.

<use aircraft="c172x".... />

[bcoconni]

Works for me when the --realtime option is removed. It seems that the --realtime option is not compatible with the socket interface, most likely because it makes JSBSim spend most of its time calling sleep() which prevents it from responding to the socket (and telnet).

The steps I followed:

1. Uncommented the lines 1193-1195 in aircraft/c172x/c172x.xml
   

   jsbsim/aircraft/c172x/c172x.xml

   Lines 1193 to 1195 in 3f939fa

   	<!--
   	<input port="1137"/>
   	-->

2. Run JSBSim with the --suspend option

> ./JSBSim scripts/c1723.xml --suspend

3. Launch telnet in another terminal:

> telnet localhost 1137

[achodoglugil]

i followed all the steps you said and still getting a blank terminal. I don't know what i am doing wrong everything seems fine but can't establish the connection, even checked the firewall permissions and they seem fine too.

[bcoconni]

What do you mean by "getting a blank terminal" ? Because, on my side, when the connection fails, I get the following message:

> telnet localhost 1137
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused

[achodoglugil]

let me show you with screenshots, when i enter the telnet localhost 1137 command i get a message like this

and then the terminal becomes like this and cant type or do anything

and this is the screenshot of the report

[achodoglugil]

bağlanıyor means connecting, sorry about that i use windows in turkish

[bcoconni]

Also, could you check that JSBSim reports the following output in the terminal where you ran the command ./JSBSim scripts/c1723.xml --suspend ?

Creating input TCP socket on port 1137
Successfully bound to TCP input socket on port 1137

[seanmcleod]

Hmm, so I can reproduce the issue with the same symptoms on Windows 11. Telnet connects but there is no output shown.

Used netsat -a to confirm the connection.

(base) C:\Users\Sean>netstat -a

Active Connections

  Proto  Local Address          Foreign Address        State
  TCP    0.0.0.0:135            ThinkCentre:0          LISTENING
  TCP    0.0.0.0:445            ThinkCentre:0          LISTENING
  TCP    0.0.0.0:1137           ThinkCentre:0          LISTENING
  TCP    0.0.0.0:2179           ThinkCentre:0          LISTENING
  TCP    0.0.0.0:2869           ThinkCentre:0          LISTENING
  TCP    0.0.0.0:3389           ThinkCentre:0          LISTENING
  TCP    0.0.0.0:5040           ThinkCentre:0          LISTENING
  TCP    0.0.0.0:49664          ThinkCentre:0          LISTENING
  TCP    0.0.0.0:49665          ThinkCentre:0          LISTENING
  TCP    0.0.0.0:49666          ThinkCentre:0          LISTENING
  TCP    0.0.0.0:49667          ThinkCentre:0          LISTENING
  TCP    0.0.0.0:49669          ThinkCentre:0          LISTENING
  TCP    0.0.0.0:49671          ThinkCentre:0          LISTENING
  TCP    0.0.0.0:49683          ThinkCentre:0          LISTENING
  TCP    127.0.0.1:1137         ThinkCentre:58408      ESTABLISHED

I'll run JSBSim in the debugger to try and see what's happening. Is it sending the text connected etc... and if so is there some terminal issue which isn't displaying the text sent?

[seanmcleod]

Hmm, so the first thing I noticed is that in order to get to send("Connected to JSBSim server..") that means sckt_in needs to be <= 0. However sckt_in as seen in the Locals list is an unsigned 64bit int.

From FGfdmSocket.h

#if defined(_MSC_VER) || defined(__MINGW32__)
  SOCKET sckt;
  SOCKET sckt_in;
#else
  int sckt;
  int sckt_in;
#endif

From WinSock2.h

/*
 * The new type to be used in all
 * instances which refer to sockets.
 */
typedef UINT_PTR        SOCKET;

And as mentioned here - https://docs.microsoft.com/en-us/windows/win32/winsock/socket-data-type-2

This means, for example, that checking for errors when the socket and accept functions return should not be done by comparing the return value with –1, or seeing if the value is negative (both common and legal approaches in UNIX). Instead, an application should use the manifest constant INVALID_SOCKET as defined in the Winsock2.h header file. For example:

Which makes me wonder how this code ever worked on Windows?

Taking a quick glance at the git history for FGfdmSocket.h it looks like the type for sckt and sckt_in has been changed/toggled between int and SOCKET a couple of times.

The most recent change was on 1/18/2020 which changed them back to SOCKET for Windows - 41bc659

[bcoconni]

Good catch @seanmcleod 👍

Which makes me wonder how this code ever worked on Windows?

Well, it seems it never did since the commit 41bc659 has been pushed. Since the test TestInputSocket is broken on Windows the regression went unnoticed.

Taking a quick glance at the git history for FGfdmSocket.h it looks like the type for sckt and sckt_in has been changed/toggled between int and SOCKET a couple of times.

The problem is that the function accept() returns a SOCKET under the Windows while it returns an int on BSD/Unix

The most recent change was on 1/18/2020 which changed them back to SOCKET for Windows - 41bc659

Oops, I did push this commit. Apparently, the change was motivated by the removal of warnings issued by MSVC. However as you pointed out, on Windows the failure of accept() is not reported by a negative value but by the special value INVALID_SOCKET. So on Windows we should check that sckt_in is not equal to INVALID_SOCKET rather than checking it is positive. Actually there are a lot of places in FGfdmSocket.cpp where the wrong comparison is made on sckt_in and sckt.

[seanmcleod]

As a quick test I changed the type from SOCKET to int and recompiled, bunch of warnings but it compiled.

1>FGfdmSocket.cpp
1>C:\source\jsbsim\src\input_output\FGfdmSocket.cpp(124,16): warning C4244: '=': conversion from 'SOCKET' to 'int', possible loss of data
1>C:\source\jsbsim\src\input_output\FGfdmSocket.cpp(168,18): warning C4244: '=': conversion from 'SOCKET' to 'int', possible loss of data
1>C:\source\jsbsim\src\input_output\FGfdmSocket.cpp(178,18): warning C4244: '=': conversion from 'SOCKET' to 'int', possible loss of data
1>C:\source\jsbsim\src\input_output\FGfdmSocket.cpp(204,27): warning C4244: '=': conversion from 'SOCKET' to 'int', possible loss of data
1>C:\source\jsbsim\src\input_output\FGfdmSocket.cpp(246,23): warning C4244: '=': conversion from 'SOCKET' to 'int', possible loss of data
1>FGInput.cpp

And sure enough now I can telnet to JSBSim successfully.

[bcoconni]

Unfortunately, changing the type of sckt and sckt_in to int did not make any difference on my side: JSBSim does not respond to telnet in both cases. It only works on Linux.

I finally managed to get the socket connection working when setting sckt and sckt_in as int. I'm currently preparing a fix to keep sckt and sckt_in being of the SOCKET type and having a working socket connection.

[achodoglugil]

Thanks for the replies, i will try the fixes as soon as possible and let you know if it worked for me.

[achodoglugil]

I changed the sckt and sckt_in as int but still cant get the socket connection working. I made the changes on FGfdmSockect.h , do i need to change anything else

[seanmcleod]

That's all I changed to get it to work.

[achodoglugil]

After rebuilding i get to work it, thanks for all the help.

[bcoconni]

FYI, I have created issue #698 upon the problems reported in this discussion.