Another Microsoft Unicode I/O Problem

I encountered an annoying bug in Visual C++ 2017 recently. It started when I found my favourite logging library, Easylogging++, output Chinese as garbage characters on Windows. Checking the documentation carefully, I noted that I should have used the macro START_EASYLOGGINGPP. It turned out to be worse: all output starting from the Chinese character was gone. Puzzled but busy, I put it down and worked on something else.

I spend another hour of detective work on this issue today. The result was quite surprising.

  • First, it is not an issue with Easylogging++. The problem can occur if I purely use std::wcout.
  • Second, the magical thing about START_EASYLOGGINGPP is that it will invoke std::locale::global(std::locale("")). This is the switch that leads to the different behaviour.
  • Myteriously, with the correct locale setting, I can get the correct result with both std::wcout and Easylogging++ in a test program. I was not able to get it working in my real project.
  • Finally, it turns out that the difference above is caused by /MT vs. /MD! The former (default if neither is specified on the command line) tells the Visual C++ compiler to use the static multi-threading library, and the latter (set by default in Visual Studio projects) tells the compiler to use the dynamic multi-threading library.

People may remember that I wrote about MSVCRT.DLL Console I/O Bug. While Visual C++ 2013 shows consistent behaviour between /MT and /MD, Visual C++ 2015 and 2017 exhibit the same woeful bug when /MD is specified on the command line. This is something perplexingly weird: it seems someone at Microsoft messed up with the MSVCRT.DLL shipped with Windows first (around 2006), and then the problem spread to the Visual C++ runtime DLL after nearly a decade!

I am using many modern C++ features, so I definitely do not want to go back to Visual C++ 2013 for new projects. It seems I have to tolerate garbage characters in the log for now. Meanwhile, I submitted a bug to Microsoft. Given that I have a bug report that is deferred for four years, I am not very hopeful. But let us wait and see.

Update (20 December 2017)

A few more tests show that the debug versions (/MTd and /MDd) both work well. So only the default release build (using the dynamic C runtime) exhibits this problem, where the executable depends on DLLs like api-ms-win-crt-stdio-l1-1-0.dll. It seems this issue is related to the Universal C Runtime introduced in Visual Studio 2015 and Windows 10. . . .

Update (25 March 2018)

The bug was closed, and a Microsoft developer indicated that the issue had already been fixed since the Windows 10 Anniversary Update SDK (build 10.0.14393). Actually I had had build 10.0.15063 installed. The reason why I still saw the problem was that the Universal C Runtime on Windows 7 had not been updated (‘the issue will be fixed in a future update to the Universal C Runtime on Windows 7’), and I should not have seen the problem on a Windows 10 box. The current workaround is either use static linking (as I did), or copy the redistributable DLLs under C:\Program Files (x86)\Windows Kits\10\Redist\ucrt\DLLs\x86 (or x64 etc.) to the app directory (so called ‘app-local deployment’; which should not be used on Windows 10, as the system version is always preferred). My test showed that copying ucrtbase.dll was enough to fix my test case.


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