UnrealBuildTool Unable to open log file for writing because it is being used by another process

Development Programming & Scripting C++
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1

I keep getting

UnrealBuildTool : error : Unhandled exception: System.Exception: Unable to open log file for writing (UnrealEngine-dev-rendering\Engine\Programs\UnrealBuildTool\Log.txt) —> System.IO.IOException: The process cannot access the file ‘\UnrealEngine-dev-rendering\Engine\Programs\UnrealBuildTool\Log.txt’ because it is being used by another process.

But there is no other process using it besides the build itself ??

2

So I was able to get passed this by changing Maximum number of parallel project builds to 1 in Visual Studio (Menu> Tools > Options > Projects and solutions > Build and Run. )

Of course this is not the fastest, but it seems to resolve error which I assume is caused by race condition and simultaneous building

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3

I hit this too. IMO the log file should include the VS project name in its name so that it’s unique.

Disabling parallel builds allowed the real error to come through:


[245/2741] Module.ContentBrowser.2_of_3.cpp
d:\epic games\source\dev-rendering\engine\source\editor\contentbrowser\private\sassetview.cpp(2031): fatal error C1001: An internal error has occurred in the compiler.
  (compiler file 'd:\agent\_work\1\s\src\vctools\compiler\utc\src\p2\main.c', line 187)
   To work around this problem, try simplifying or changing the program near the locations listed above.
  Please choose the Technical Support command on the Visual C++
   Help menu, or open the Technical Support help file for more information
    cl!InvokeCompilerPassW()+0x3e53c
    cl!InvokeCompilerPassW()+0x3d72c
    cl!InvokeCompilerPassW()+0x3c59f
    cl!InvokeCompilerPassW()+0x3b991
    cl!InvokeCompilerPassW()+0x12c3ae
    cl!InvokeCompilerPassW()+0x50db7
    cl!InvokeCompilerPassW()+0x50a2e
    cl!CloseTypeServerPDB()+0x494af
    cl!o_exp()+0x5a
    cl!BaseThreadInitThunk()+0x14
    cl!RtlUserThreadStart()+0x21

The rest of them are basically the same thing.

4

I’m also running into this when building the entire UE4 solution from the latest github sources master branch. Has anyone here reported the issue to Epic yet?

5

I found the answer and had a moment of deja vu. You need to turn parallel builds down to 1 in VS. You can possibly get away with more but most likely not.

This will cause it to build slower, but it will actually work. Seems like a crazy thing to not have fixed in 22 versions of the engine though.

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6

Oh, I had incorrectly skim read @OptimisticMonkey’s solution. When I read “Maximum number of parallel project builds” I stopped reading and assumed it was going to be setting MaxProcessorCount to 1 in BuildConfiguration.xml, which if was true would have pointed to a UE4 build bug. But this parallel build option in VS is a global setting, not per project. I didn’t even know such an option existed.

Although I can understand why this would mess up Unreal Build Tool, it would be nice if you got a warning about this (perhaps during VS project file generation), or if Epic said to set the value to 1 in the UE4 Visual Studio Setup Guide or github README. I’ll try this solution out and if it works for me, I’ll make a pull request on the README.

7

The weird thing is VS has nothing to do with the UE build, it just kicks off too many instances of UBT at once.