iOS App Crash when calling UDPSocket.Write

I’ve come across a rather show stopping bug with API2.0 and UDPSockets. I’ve detailed it in this feedback report:

feedback://showreport?report_id=62875

Basically if you have a UDPSocket, it works great until the device is locked. If you attempt to use the UDPSocket after the device is unlocked, the app will crash hard. Exception handling doesn’t catch it. Calling UDPSocket.Write is what triggers the crash.

And this has to be on an actual device. The simulator doesn’t show this behavior.

Did you add NSLocalNetworkUsageDescription to your plist file ?

I’ve never added anything to any plist file. I would assume Xojo should be doing all that. The app requests access to connect to devices on my network and I give it permission. So that should not be the problem. And the socket works fine when the app is open and the screen unlocked. It’s after locking and then unlocking that the problem occurs.

And I don’t think it’s entitlements either. When I tried to access photos without the entitlement enabled, the app would hard crash. But iOS would generate a crash report with the message that I was trying to access photos. I’m not seeing any crash reports from iOS here.

Xojo is not doing every plist entry. If you want you can add a feedback request to add all required plist entries for the classes.

I’m sure apple closes the socket when the screen locks. They do all kinds of stuff to keep the battery level up as much as they can. Keeping sockets open is a thing that you must be careful with. You can use the App.Deactivated event to close sockets. Then reopen/connect or bind on App.Activate

Problem is, the socket is not closed - at least not what Xojo sees. Here’s some of my code in a write method of a subclass:

If Not MyUDPSocket.IsConnected Then
  System.DebugLog "UDPSocket not connected - Connecting."
  
  MyUDPSocket.Connect
  If Not MyUDPSocket.JoinMulticastGroup(UDPGroupIP) Then
    System.DebugLog "Could not join the UDP group - Leaving."
    MyUDPSocket.Close
    Return
  End If
  
End

UDPSent = True

system.debuglog("UDP - Writing to UDP Socket: "+txt)

If MyUDPSocket = Nil Then
  System.DebugLog "UDPSocket is NIL"
End If

If Not MyUDPSocket.IsConnected Then
  System.DebugLog "UDPSocket is not connected."
End If

System.DebugLog "UDP Group: "+UDPGroupIP
System.DebugLog txt

Try
  MyUDPSocket.Write(UDPGroupIP,txt+EndOfLine)
Catch
  Try
    System.DebugLog "Exception - Trying again"
    MyUDPSocket.Connect
    Call MyUDPSocket.JoinMulticastGroup(UDPGroupIP)
    System.DebugLog "Trying the write again."
    MyUDPSocket.Write(UDPGroupIP,txt+EndOfLine)
  Catch
    System.DebugLog "Second Exception."
  End Try
End Try

UDPSent = False
End

The DebugLog statements and several IF/Then blocks are all in there trying to figure out where things go awry. Xojo never sees the socket as closed. And the Try/Catch blocks don’t catch the exception/crash. As soon as the write method is called the app crashes.

So I agree that something is being shut down. I can see it happening in the console. Problem is I don’t want it shut down. I still want the app to be able to receive UDP packets when the screen is locked. Minimizing the app by switching back to Springboard and opening another app and then going back into the app doesn’t cause the same problem. It’s a lock that does.

You cannot just connect and expect it to work. Apple can stop port bindings if the port is already used or inaccessible (even if the batery is low etc). It’s best to check if the isconnected returns true and if the port is actually set only if these conditions are valid, you can join the group and/or write.

Perhaps:

If upd.isconnected and udp.port > 0 then
// join the group and/or write
End if

Uh, I think you missed a lot of my code. See below. There’s also code I didn’t show where I set the port, etc. I’ve used this code without issues for years on the desktop It does work and works fine UNTIL THE SCREEN IS LOCKED AND THEN UNLOCKED. Xojo thinks the socket is still connected. So it does me no good to check if the socket is connected as it is connected! And the port has been set back when the socket is first setup.

To continue running while the phone is locked I think you need to enable background tasks for your app. I’m not sure if that will work in Xojo.

The best approach is likely to destroy and rebuild the connection in the deactivate and activate events so you definitely have a working connection

Can you open the device console in Xcode and cause the crash? There may be messages in the console explaining what is happening/going wrong

Here is the console log from the test app I built and included with the bug report. This is basically everything reported when the screen locks.

debug  16:27:12.984848-0600  TestApp   *|machport|* <BSMachPortTransferableSendRight: 0x2823a32a0; usable: YES; (6113:400b:send_mv xpcCode) from (62:82fab:send_mv make) from (62:82fab:receive init)> -> (<NSThread: 0x2836b7380>{number = 12, name = (null)}) (
  0   BaseBoard                           0x00000001a2c0b494 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 25748
  1   BaseBoard                           0x00000001a2c3f3c4 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 238532
  2   BaseBoard                           0x00000001a2c16c6c BSCreateDeserializedBSXPCEncodableObjectFromXPCDictionary + 164
  3   BaseBoard                           0x00000001a2c16ba8 BSCreateDeserializedBSXPCEncodableObjectFromXPCDictionaryWithKey + 40
  4   BaseBoard                           0x00000001a2c08314 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 13076
  5   FrontBoardServices                  0x00000001ad9ef144 E22FDD4D-3F20-3D60-A568-E82AE2B4151C + 377156
  6   BaseBoard                           0x00000001a2c16c6c BSCreateDeserializedBSXPCEncodableObjectFromXPCDictionary + 164
  7
debug  16:27:12.984945-0600  TestApp   *|machport|* <BSMachPortTransferableSendRight: 0x282385040; usable: YES; (6113:400b:send_mv copy) from (6113:400b:send_mv xpcCode) from (62:82fab:send_mv make) from (62:82fab:receive init)> -> (<NSThread: 0x2836b7380>{number = 12, name = (null)}) (
  0   BaseBoard                           0x00000001a2c0b494 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 25748
  1   BaseBoard                           0x00000001a2c089ec 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 14828
  2   BaseBoard                           0x00000001a2c09330 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 17200
  3   BaseBoard                           0x00000001a2c08b40 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 15168
  4   BaseBoard                           0x00000001a2c0833c 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 13116
  5   FrontBoardServices                  0x00000001ad9ef144 E22FDD4D-3F20-3D60-A568-E82AE2B4151C + 377156
  6   BaseBoard                           0x00000001a2c16c6c BSCreateDeserializedBSXPCEncodableObjectFromXPCDictionary + 164
  7   BaseBoard
info  16:27:12.985184-0600  TestApp  sceneOfRecord: sceneID: sceneID:com.justaddsoftware.udptest-default  persistentID: F08AA976-B8C5-4814-8F65-AD142CCDB261
info  16:27:12.989787-0600  TestApp  Deactivation reason added: 12; deactivation reasons: 0 -> 4096; animating application lifecycle event: 0
info  16:27:12.990068-0600  TestApp  Deactivation reason added: 11; deactivation reasons: 4096 -> 6144; animating application lifecycle event: 0
info  16:27:12.991461-0600  TestApp  <private>: Will add backgroundTask with taskName: <private>, expirationHandler: (null)
info  16:27:12.991506-0600  TestApp  <private>: Created background task <private>. Assertion will be assigned soon
info  16:27:12.991552-0600  TestApp  <private>: Creating new assertion since assertion is nil
info  16:27:12.991595-0600  TestApp  <private>: Created new assertion <BKSProcessAssertion: 0x2800b9040>
info  16:27:12.991779-0600  TestApp  Acquiring assertion: <RBSAssertionDescriptor| "Shared Background Assertion 2 for com.justaddsoftware.udptest" ID:(null) target:6113>
debug  16:27:12.992082-0600  TestApp  Adding assertion 32-6113-42014 to dictionary
info  16:27:12.999089-0600  TestApp  <private>: Will add backgroundTask with taskName: <private>, expirationHandler: <__NSStackBlock__: 0x16dafdd70>
info  16:27:13.005327-0600  TestApp  <private>: Created background task <private>. Assertion will be assigned soon
info  16:27:13.005376-0600  TestApp  <private>: Reusing assertion <BKSProcessAssertion: 0x2800b9040>
debug  16:27:13.018888-0600  TestApp   *|machport|* <BSMachPortReceiveRight: 0x2823a21a0; usable: YES; (6113:630b:receive init)> -> (<NSThread: 0x2836b41c0>{number = 1, name = main}) (
  0   BaseBoard                           0x00000001a2c0b494 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 25748
  1   BaseBoard                           0x00000001a2c3f8f0 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 239856
  2   BaseBoard                           0x00000001a2c0c0a8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 28840
  3   BaseBoard                           0x00000001a2c61cc8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 380104
  4   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  5   libdispatch.dylib                   0x000000019e297428 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 74792
  6   BaseBoard                           0x00000001a2c095d8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 17880
  7   BaseBoard                           0x00000001a2c16a48 BSSerializeBSXPCEncodableObjectToXPCDictionary + 72
  8   BaseBoard
debug  16:27:13.019126-0600  TestApp   *|machport|* <BSMachPortTransferableSendRight: 0x2823985a0; usable: YES; (6113:630b:send_mv make) from (6113:630b:receive init)> -> (<NSThread: 0x2836b41c0>{number = 1, name = main}) (
  0   BaseBoard                           0x00000001a2c0b494 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 25748
  1   BaseBoard                           0x00000001a2c0cd54 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 32084
  2   BaseBoard                           0x00000001a2c0c0c8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 28872
  3   BaseBoard                           0x00000001a2c61cc8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 380104
  4   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  5   libdispatch.dylib                   0x000000019e297428 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 74792
  6   BaseBoard                           0x00000001a2c095d8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 17880
  7   BaseBoard                           0x00000001a2c16a48 BSSerializeBSXPCEncodableObjectToXPCDict
debug  16:27:13.019356-0600  TestApp   *|machport|* invalidate <BSMachPortTransferableSendRight:0x2823985a0 usable=YES (6113:630b:send_mv make) from (6113:630b:receive init)> -> (<NSThread: 0x2836b41c0>{number = 1, name = main}) (
  0   BaseBoard                           0x00000001a2c3f16c 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 237932
  1   BaseBoard                           0x00000001a2c40834 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 243764
  2   BaseBoard                           0x00000001a2c3f444 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 238660
  3   BaseBoard                           0x00000001a2c16a48 BSSerializeBSXPCEncodableObjectToXPCDictionary + 72
  4   BaseBoard                           0x00000001a2c169e4 BSCreateSerializedBSXPCEncodableObject + 64
  5   BaseBoard                           0x00000001a2c16b34 BSSerializeBSXPCEncodableObjectToXPCDictionaryWithKey + 68
  6   BaseBoard                           0x00000001a2c61cdc 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 380124
  7   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9
debug  16:27:13.300539-0600  TestApp   *|machport|* invalidate <BSMachPortReceiveRight:0x2823a21a0 usable=YES (6113:630b:receive init)> -> (<NSThread: 0x2836bdd40>{number = 13, name = (null)}) (
  0   BaseBoard                           0x00000001a2c3f16c 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 237932
  1   BaseBoard                           0x00000001a2c09df0 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 19952
  2   BaseBoard                           0x00000001a2c09734 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 18228
  3   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  4   libdispatch.dylib                   0x000000019e297428 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 74792
  5   BaseBoard                           0x00000001a2c630b0 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 385200
  6   libdispatch.dylib                   0x000000019e28724c 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 8780
  7   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  8   libdispatch.dylib
debug  16:27:13.304554-0600  TestApp   *|machport|* <BSMachPortReceiveRight: 0x2823bd000; usable: YES; (6113:a00f:receive init)> -> (<NSThread: 0x2836b41c0>{number = 1, name = main}) (
  0   BaseBoard                           0x00000001a2c0b494 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 25748
  1   BaseBoard                           0x00000001a2c3f8f0 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 239856
  2   BaseBoard                           0x00000001a2c0c0a8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 28840
  3   BaseBoard                           0x00000001a2c61cc8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 380104
  4   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  5   libdispatch.dylib                   0x000000019e297428 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 74792
  6   BaseBoard                           0x00000001a2c095d8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 17880
  7   BaseBoard                           0x00000001a2c16a48 BSSerializeBSXPCEncodableObjectToXPCDictionary + 72
  8   BaseBoard
debug  16:27:13.307179-0600  TestApp   *|machport|* <BSMachPortTransferableSendRight: 0x2823bcf60; usable: YES; (6113:a00f:send_mv make) from (6113:a00f:receive init)> -> (<NSThread: 0x2836b41c0>{number = 1, name = main}) (
  0   BaseBoard                           0x00000001a2c0b494 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 25748
  1   BaseBoard                           0x00000001a2c0cd54 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 32084
  2   BaseBoard                           0x00000001a2c0c0c8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 28872
  3   BaseBoard                           0x00000001a2c61cc8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 380104
  4   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  5   libdispatch.dylib                   0x000000019e297428 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 74792
  6   BaseBoard                           0x00000001a2c095d8 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 17880
  7   BaseBoard                           0x00000001a2c16a48 BSSerializeBSXPCEncodableObjectToXPCDict
debug  16:27:13.311619-0600  TestApp   *|machport|* invalidate <BSMachPortTransferableSendRight:0x2823bcf60 usable=YES (6113:a00f:send_mv make) from (6113:a00f:receive init)> -> (<NSThread: 0x2836b41c0>{number = 1, name = main}) (
  0   BaseBoard                           0x00000001a2c3f16c 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 237932
  1   BaseBoard                           0x00000001a2c40834 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 243764
  2   BaseBoard                           0x00000001a2c3f444 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 238660
  3   BaseBoard                           0x00000001a2c16a48 BSSerializeBSXPCEncodableObjectToXPCDictionary + 72
  4   BaseBoard                           0x00000001a2c169e4 BSCreateSerializedBSXPCEncodableObject + 64
  5   BaseBoard                           0x00000001a2c16b34 BSSerializeBSXPCEncodableObjectToXPCDictionaryWithKey + 68
  6   BaseBoard                           0x00000001a2c61cdc 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 380124
  7   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9
debug  16:27:13.339870-0600  TestApp  Received message from server: async_didChangeInheritances:completion:
debug  16:27:13.340151-0600  TestApp  didChangeInheritances: <RBSInheritanceChangeSet: 0x282384e40>
debug  16:27:13.353099-0600  TestApp   *|machport|* invalidate <BSMachPortReceiveRight:0x2823bd000 usable=YES (6113:a00f:receive init)> -> (<NSThread: 0x2836b9c80>{number = 14, name = (null)}) (
  0   BaseBoard                           0x00000001a2c3f16c 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 237932
  1   BaseBoard                           0x00000001a2c09df0 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 19952
  2   BaseBoard                           0x00000001a2c09734 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 18228
  3   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  4   libdispatch.dylib                   0x000000019e297428 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 74792
  5   BaseBoard                           0x00000001a2c630b0 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 385200
  6   libdispatch.dylib                   0x000000019e28724c 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 8780
  7   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  8   libdispatch.dylib
info  16:27:13.353739-0600  TestApp  <private>: Ending background task with UIBackgroundTaskIdentifier: 5
info  16:27:13.353794-0600  TestApp  <private>: Ending task with identifier 5 and description: <private>, _expireHandler: <__NSMallocBlock__: 0x282db2c40>
info  16:27:13.353842-0600  TestApp  <private>: Decrementing reference count for assertion used by background task with identifier 5: <private>
debug  16:27:13.357014-0600  TestApp   *|machport|* invalidate <BSMachPortTransferableSendRight:0x2823a32a0 usable=YES (6113:400b:send_mv xpcCode) from (62:82fab:send_mv make) from (62:82fab:receive init)> -> (<NSThread: 0x2836b41c0>{number = 1, name = main}) (
  0   BaseBoard                           0x00000001a2c3f16c 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 237932
  1   BaseBoard                           0x00000001a2c09df0 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 19952
  2   BaseBoard                           0x00000001a2c09704 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 18180
  3   BaseBoard                           0x00000001a2c626a0 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 382624
  4   libdispatch.dylib                   0x000000019e288db0 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 15792
  5   libdispatch.dylib                   0x000000019e297428 3277BF1E-B994-3609-9B30-E0186BBF3C25 + 74792
  6   BaseBoard                           0x00000001a2c085a4 196F8707-8DAF-3D1D-9AD7-D4BED13A52E4 + 13732
  7   FrontBoardServices                  0x00000001ad9eed90 E22FDD4D-3F
info  16:27:13.411159-0600  TestApp  <private>: Ending background task with UIBackgroundTaskIdentifier: 4
info  16:27:13.411207-0600  TestApp  <private>: Ending task with identifier 4 and description: <private>, _expireHandler: (null)
info  16:27:13.411257-0600  TestApp  <private>: Decrementing reference count for assertion used by background task with identifier 4: <private>
info  16:27:13.411351-0600  TestApp  <private>: Will invalidate assertion: <BKSProcessAssertion: 0x2800b9040> for task identifier: 4
info  16:27:21.504542-0600  TestApp  Deactivation reason removed: 11; deactivation reasons: 6176 -> 4128; animating application lifecycle event: 1
debug  16:27:21.504740-0600  TestApp  Received message from server: async_didChangeInheritances:completion:
info  16:27:21.982947-0600  TestApp  sceneOfRecord: sceneID: sceneID:com.justaddsoftware.udptest-default  persistentID: F08AA976-B8C5-4814-8F65-AD142CCDB261
info  16:27:21.985078-0600  TestApp  Deactivation reason removed: 12; deactivation reasons: 4128 -> 32; animating application lifecycle event: 1
info  16:27:21.992199-0600  TestApp  Deactivation reason removed: 5; deactivation reasons: 32 -> 0; animating application lifecycle event: 0
info  16:27:23.270450-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 1; ignoreInteractionEvents: 0, systemGestureStateChange: 0
info  16:27:23.270498-0600  TestApp  Sending UIEvent type: 0; subtype: 0; to windows: 1
info  16:27:23.278923-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 1; ignoreInteractionEvents: 0, systemGestureStateChange: 0
info  16:27:23.278983-0600  TestApp  Sending UIEvent type: 0; subtype: 0; to windows: 1
info  16:27:23.279072-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 0; ignoreInteractionEvents: 0, systemGestureStateChange: 1
info  16:27:23.279120-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 1; ignoreInteractionEvents: 0, systemGestureStateChange: 0
info  16:27:23.279170-0600  TestApp  Sending UIEvent type: 0; subtype: 0; to windows: 1
info  16:27:23.311505-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 1; ignoreInteractionEvents: 0, systemGestureStateChange: 0
info  16:27:23.311636-0600  TestApp  Sending UIEvent type: 0; subtype: 0; to windows: 1
info  16:27:23.332627-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 1; ignoreInteractionEvents: 0, systemGestureStateChange: 0
info  16:27:23.332866-0600  TestApp  Sending UIEvent type: 0; subtype: 0; to windows: 1
info  16:27:23.351952-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 1; ignoreInteractionEvents: 0, systemGestureStateChange: 0
info  16:27:23.353277-0600  TestApp  Sending UIEvent type: 0; subtype: 0; to windows: 1
info  16:27:23.374257-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 1; ignoreInteractionEvents: 0, systemGestureStateChange: 0
info  16:27:23.374321-0600  TestApp  Sending UIEvent type: 0; subtype: 0; to windows: 1
info  16:27:23.375319-0600  TestApp  Dispatching UIEvent type: 0; subtype: 0; backingtype: 11, shouldSend: 1; ignoreInteractionEvents: 0, systemGestureStateChange: 0
info  16:27:23.375489-0600  TestApp  Sending UIEvent type: 0; subtype: 0; to windows: 1

Hmm… nothing there screams crash. There are typically explicit messages about the crash or termination reason. I there more in the log?

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The socket definitely seems to be torn down when the phone is locked. Xojo just doesn’t know it. I was exchanging my UDP data between the iOS and desktop app. Then after locking the phone, I tried sending data from the desktop app to the iOS app. After unlocking the phone, nothing was updated.

I’m kinda disappointed as that was part of the whole reason I was excited about having a UDP socket. I do have a protocol where I can request updates. Maybe I’ll have to do as you say and destroy and rebuild the connection.

Xojo can enable background tasks but there’s no entitlement for UDP sockets or networking in general. Just things like VoIP, Notifications, etc.

Now, Xojo should still not crash with this. The odd part is I use TCPSockets a lot as well and those don’t have this problem. They seem to stay connected or at least Xojo can detect that the TCPSocket is disconnected. With UDP, there’s problems.

Nope. No crash log. Nothing. That’s the problem.

Are you doing anything that could use a lot of memory? Some limited googling seems to say that if you run out of memory while in the background the app can crash when it comes back to the foreground and leave no crash log. Maybe you have an old iOS device with less RAM?

It’s an iPhone Xs Max. I’m not doing anything that uses a lot of memory at all. So it can’t be that. It’s an issue with the socket and how Xojo is doing it.

When your app is active but off screen, your socket is downed by apple to keep the battery alive. The only way to keep it active is register for mDns (bonjour) use and add a backgroundTask but that’s not possible in xojo. You can however add background pushnotification. That works in xojo now. So if you use a http or tcpsocket you could send data after the background push notification is received. (You’ll have 30 sec before the app shuts the background push notification again).

As far as i know streaming tcp/udp is not possible when the app is in the background normally.

Some more looking says that an unhandled SIGPIPE will cause the app to crash without leaving a crash report. There is your issue:

I bet this is not xojo but apple. Is this iOS 14?
Since apple locked down networking and has some udp bugs.