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Multiplayer replication crash

When running a multiplayer game, we have a random crash in what appears to be the engine's replication logic. Our setup uses a packaged (non-editor) build on two different machines over wired ethernet. At the point of the crash in UObjectPropertyBase::Identical line 45, ObjectA is not null but appears to be garbage. ObjectB looks fine. This happens when running the exact same build copied to both machines. So far it seems to happen on the game running as the server. Do you know what would cause this or is there a workaround?

So far we have seen this in shipping and development builds but haven't reproduced it in a debug build. We are running 4.4.0 built from source. Here is the crash stack:

UObjectPropertyBase::Identical() 0x3f39653f + O bytes [File= c:\gamedev\Game\ue4\engine\source\runtime\coreuobject\private\uobject\propertybaseobject.cpp:45] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
FRepLayout::CompareProperties() 0x3f95745e + 311 bytes [File= c:\gamedev\Game\ue4\engine\source\runtime\engine\private\replayout.cpp:327] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
FRepLayout::ReplicateProperties() 0x3f96fca8 + 46 bytes [File= c:\gamedev\Game\ue4\engine\source\runtime\engine\private\replayout.cpp:531] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe] }
FObjectReplicator::ReplicateProperties() 0x3f841adf + O bytes [File= c:\gamedev\Game\ue4\engine\source\runtime\engine\private\datareplication.cpp:983] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
UActorChannel::ReplicateActor() 0x3f840b49 + O bytes [File= c:\gamedev\Game\ue4\engine\source\runtime\engine\private\datachannel.cpp:1809] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
UNetDriver::ServerReplicateActors() 0x3f92190f + 8 bytes [File= c:\gamedev\Game\ue4\engine\source\runtime\engine\private\networkdriver.cpp:2422] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
UNetDriver::TickFlush() 0x3f9243a1 + 15 bytes [File=c:\gamedev\Game\ue4\engine\source\runtime\engine\private\networkdriver.cpp:153] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
TBaseUObjectMethodDelegateInstance_OneParam::ExecuteIfSafe() 0x3f93965c + O bytes [File=c:\gamedev\Game\ue4\engine\source\runtime\core\public\delegates\delegateinstancesimpl.inl:532] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
TBaseMulticastDelegate_OneParam::Broadcast() 0x3f8e7975 + 14 bytes [File=c:\gamedev\Game\ue4\engine\source\runtime\core\public\delegates\delegatesignatureimpl.inl:1852] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
UWorld::Tick() 0x3f8fbddf + O bytes [File= c:\gamedev\Game\ue4\engine\source\runtime\engine\private\leveltick.cpp:1233] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
UGameEngine::Tick() 0x3f87e23c + O bytes [File=c:\gamedev\Game\ue4\engine\source\runtime\engine\private\gameengine.cpp:873] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
FEngineLoop::Tick() 0x3f25b13d + O bytes [File=c:\gamedev\Game\ue4\engine\source\runtime\launch\private\launchengineloop.cpp:2098] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
GuardedMain() 0x3f25725c + O bytes [File=c:\gamedev\Game\ue4\engine\source\runtime\launch\private\launch.cpp:133] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
GuardedMainWrapper() 0x3f2572aa + 5 bytes [File=c:\gamedev\Game\ue4\engine\source\runtime\launch\private\windows\launchwindows.cpp:125] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
WinMain() 0x3f25ba20 + 17 bytes [File=c:\gamedev\Game\ue4\engine\source\runtime\launch\private\windows\launchwindows.cpp:201] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
_tmainCRTStartup() 0x4047cd2d + 21 bytes [File=f:\dd\vctools\crt\crtw32\dllstuff\crtexe.c:618] [in C:\gamedev\Game\GameRelease\WindowsNoEditor\Game\Binaries\Win64\Game-Win64-Shipping.exe]
Address = 0x76d459ed (filename not found) [in C:\Windows\system32\kernel32.dll]
Address = 0x76e7c541 (filename not found) [in C:\Wind0ws\SYSTEM32\ntdll.dll]
Address = 0x76e7c541 (filename not found) [in C:\Wind0ws\SYSTEM32\ntdll.dll]

Product Version: Not Selected
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asked Sep 10 '14 at 09:39 PM in Bug Reports

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avatar image Ben Halliday STAFF Sep 11 '14 at 09:18 PM

Hi Marc,

Thank you for your report. We have assigned this issue to a member of our support staff, and they will begin investigation as soon as possible. If we are unable to reproduce the issue, or we need additional information, our staff member will comment here with additional questions. Otherwise, we will post an ‘Answer’ once we have logged the issue in our bug database or we have a solution for it.

In the meantime, please be sure to review our suggestions for how to report a bug, and feel free to edit your post if you have additional information to provide:


Please be sure to include crash logs and dump files for your project after it crashes.


avatar image Jonathan Dorman STAFF Sep 12 '14 at 03:02 PM

Hey Marc,

I can internally report the crash if you give me your crash logs and dmp files, but if you want me to further investigate, I'm going to need more information about your project and the objects that aren't replicating properly. It sounds like you're working on getting a repro, which would be extremely helpful as well.

avatar image Marc.Hall Sep 12 '14 at 07:52 PM

Here is the crash dump and logs. ObjectA is always garbage and ObjectB seems to be consistently the name of a particular StaticMeshActor that is the ground mesh for a part of the level. I wouldn't expect that mesh to be doing any replication. The map has many sublevels, but that mesh is in a sublevel that is streamed in on game start and never streamed out. The crash usually happens around 1-2 minutes into the game. If ObjectB is a red herring, there are other parts of the level that stream in closer the time the crash happens.

dump-486254736.zip (43.1 kB)
avatar image Jonathan Dorman STAFF Sep 12 '14 at 09:17 PM

As a side question, have you tried running a copy of your project in 4.4.2?

Edit: Also, have you submitted a crash report after this crash happened?

avatar image [Epic] John Pollard STAFF Dec 12 '14 at 10:29 PM

Some time around 4.5/6 we added this which should fix this issue if it's related to GC and shadow state:

 static FORCEINLINE bool CompareObject( const FRepLayoutCmd& Cmd, const void* A, const void* B )
 #if 1
     // Until UObjectPropertyBase::Identical is made safe for GC'd objects, we need to do it manually
     // This saves us from having to add referenced objects during GC
     UObjectPropertyBase * ObjProperty = CastChecked< UObjectPropertyBase>( Cmd.Property );
     UObject* ObjectA = ObjProperty->GetObjectPropertyValue( A );
     UObject* ObjectB = ObjProperty->GetObjectPropertyValue( B );
     return ObjectA == ObjectB;
     return Cmd.Property->Identical( A, B );

avatar image Rudy Q ♦♦ STAFF Apr 22 '15 at 05:36 PM

Hello Marc.Hall,

I just wanted to check in and see if you were able to reproduce this issue in the latest version of the engine with a clean project? If you can, could you please provide a detailed set of steps to reproduce this issue on our end?

avatar image Marc.Hall Apr 22 '15 at 06:01 PM

We decided to remove the game modes using unreal's multiplayer logic and write custom network synchronization logic, so we no longer have a test case for this.

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Hi Marc,

We have not heard back from you in a few days, so we are marking this post as Resolved for tracking purposes. If you are still experiencing the issue you reported, please respond to this message with additional information and we will offer further assistance.

Thank you.

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answered Oct 03 '14 at 08:44 PM

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