Mysterious crash in cppwinrt example - c++

Question

I am using Visual Studio 17 v15.0 and Win 10 Anniversary Update SDK.
I build the following code (basically sample in github repo) with cl /EHsc /O2 /DUNICODE /bigobj /await /std:c++latest, with /MT or MD. It compiles without error.
If I run when `"message.png" is not present in current directory, exception will be thrown, caught and reported with printf, then exit without crashing.
If I run when `"message.png" is present in current directory, "Hello World!" will be printed, then crash for no reason.
Weird thing is If I run it inside GDB debugger, GDB always say the program exits normally (and indeed no crash happen).
GDB output:
[New Thread 1364.0x2324]
[New Thread 1364.0x624]
[New Thread 1364.0x12cc]
[New Thread 1364.0x58c]
[New Thread 1364.0x1134]
[New Thread 1364.0x10d8]
[New Thread 1364.0x18a8]
[New Thread 1364.0x1794]
[New Thread 1364.0x20e8]
[New Thread 1364.0x2204]
[New Thread 1364.0x1030]
[New Thread 1364.0x1474]
Hello world!
[Thread 1364.0x10d8 exited with code 0]
[Thread 1364.0x624 exited with code 0]
[Thread 1364.0x20e8 exited with code 0]
[Thread 1364.0x1794 exited with code 0]
[Thread 1364.0x18a8 exited with code 0]
[Thread 1364.0x58c exited with code 0]
[Thread 1364.0x1134 exited with code 0]
[Thread 1364.0x12cc exited with code 0]
[Thread 1364.0x8d0 exited with code 0]
[Thread 1364.0x2324 exited with code 0]
[Thread 1364.0x1b38 exited with code 0]
[Thread 1364.0x2204 exited with code 0]
[Thread 1364.0x1030 exited with code 0]
[Thread 1364.0x1474 exited with code 0]
[Inferior 1 (process 1364) exited normally]
Code:
#pragma comment(lib, "windowsapp")
#pragma comment(lib, "pathcch")
#include <winrt/Windows.Storage.Streams.h>
#include <winrt/Windows.Graphics.Imaging.h>
#include <winrt/Windows.Media.Ocr.h>
#include <winrt/Windows.Networking.Sockets.h>
#include <pathcch.h>
using namespace winrt;
using namespace std::chrono;
using namespace Windows::Foundation;
using namespace Windows::Storage;
using namespace Windows::Storage::Streams;
using namespace Windows::Graphics::Imaging;
using namespace Windows::Media::Ocr;
hstring MessagePath()
{
wchar_t buffer[1024]{};
GetCurrentDirectory(_countof(buffer), buffer);
check_hresult(PathCchAppendEx(buffer, _countof(buffer), L"message.png", PATHCCH_ALLOW_LONG_PATHS));
return buffer;
}
IAsyncOperation<hstring> AsyncSample()
{
StorageFile file = co_await StorageFile::GetFileFromPathAsync(MessagePath());
IRandomAccessStream stream = co_await file.OpenAsync(FileAccessMode::Read);
BitmapDecoder decoder = co_await BitmapDecoder::CreateAsync(stream);
SoftwareBitmap bitmap = co_await decoder.GetSoftwareBitmapAsync();
OcrEngine engine = OcrEngine::TryCreateFromUserProfileLanguages();
OcrResult result = co_await engine.RecognizeAsync(bitmap);
return result.Text();
}
int main()
{
init_apartment();
try
{
printf("%ls\n", AsyncSample().get().c_str());
}
catch (hresult_error const & e)
{
printf("hresult_error: (0x%8X) %ls\n", e.code(), e.message().c_str());
}
return 0;
}

Answer 1

Turns out hstring returned by AsyncSample().get() is not null terminated, so printf crashes.
try
{
auto ans = AsyncSample().get();
printf("[%u]: ", ans.size());
auto s = ans.c_str();
for (uint32_t i = 0; i < ans.size(); i++) {
printf("%lc", s[i]);
}
putchar('\n');
}