为什么WP8.1运行时组件在对boost :: filesystem :: file_size（）的调用上引发C ++异常？

Question

Windows Phone 8.x developers porting C++ with Boost code to WP8.x are aware that several Boost binary libraries contain banned Win32 calls. Thus, these Boost libraries need to be ported to WP8.x.

Steven Gates, a Microsoft engineer, ported several Boost libraries (ie, system, chrono, date_time, smart_ptr, signals2, and thread) and wrote an excellent blog describing how he did it.

One critical Boost library he did not port is filesystem. Although I am new to WinRT and runtime components, I want to attempt to port this library to Windows Phone 8.1 (WP8.1 has less banned functionality than WP8.0, and does not require RPAL ["restricted platform allow list"] for many functions).

I began by commenting out all filesystem source code that does not compile under Windows Phone 8.1, reaching a state where I could build the filesystem library for WP8.1 ARM using the following Boost b2 command:

b2 toolset=msvc-12.0 windows-api=phone link=static architecture=arm filesystem

My overall plan is to implement the commented-out functions one at a time, testing the ported code through a minimal WP 8.1 app.

My next step was to write a minimal Windows Phone 8.1 app together with a Windows Phone Runtime Component 8.1 project, both bundled in a single solution (the runtime component will interface with the Boost library). To get them working I:

• added the runtime component as a reference to the main app project
• linked the Boost filesystem static library to the runtime component
• added an API function within the runtime component called GetFileSize() that the app can invoke. The purpose is to exercise the boost::filesystem::file_size() function within the ported Boost library.
• linked code to a UI button that, when pressed, calls into the runtime component to invoke the GetFileSize() function

The problem I have is that the call to boost::filesystem::file_size() throws an exception, and it is not obvious what the problem is.

Here is the relevant code I've written.

namespace MinimalWindowsRuntimeApp
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();

            this.NavigationCacheMode = NavigationCacheMode.Required;
        }

        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
        }

        private void GetFileSize_Click(object sender, RoutedEventArgs e)
        {
            String filepath = "\\Phone\\Pictures\\Camera Roll\\WP_20140106_001.jpg";
            var newObject = new WindowsRuntimeComponent1.Class1();
            newObject.GetFileSize(filepath);
        }
    }
}

When I press the dedicated button on the Windows Phone screen it invokes the GetFileSize_Click() function. This code creates an instance of the WindowsRuntimeComponent1.Class1() activatable class and calls its GetFileSize () function:

// Class1.cpp
#include "pch.h"
#include "Class1.h"
#include "boost/filesystem.hpp"

using namespace WindowsRuntimeComponent1;
using namespace Platform;

Class1::Class1()
{
}

int64 Class1::GetFileSize(Platform::String ^filepath)
{
    boost::filesystem::path p (filepath->Data ());
    __int64 filesize = (__int64) boost::filesystem::file_size (p);

    return filesize;
}

The intention is for the GetFileSize() function in the runtime component to instantiate a boost::filesystem::path object (which takes a wide-char string of the path) and invoke the boost::filesystem::file_size() function.

The problem is that the call to file_size() throws an exception, as shown in the output window:

First-chance exception at 0x774E59A3 in MinimalWindowsRuntimeApp.exe: Microsoft C++ exception: boost::filesystem::filesystem_error at memory location 0x0315E498.

Tracing into Boost with the native debugger shows the code in Boost's operations.cpp that fails:

  BOOST_FILESYSTEM_DECL
  boost::uintmax_t file_size(const path& p, error_code* ec)
  {
    . . .
#   else  // Windows

    // assume uintmax_t is 64-bits on all Windows compilers

    WIN32_FILE_ATTRIBUTE_DATA fad;

    if (error(::GetFileAttributesExW(p.c_str(), ::GetFileExInfoStandard, &fad)== 0,
        p, ec, "boost::filesystem::file_size"))
        return static_cast<boost::uintmax_t>(-1);
    . . .    
#   endif
  }

The failure occurs while calling the Win32 API GetFileAttributesExW() function. At the point of invocation, p.c_str() equals the expected "\\Phone\\Pictures\\Camera Roll\\WP_20140106_001.jpg". The function, which is supported for Windows Phone 8.1 , fails and returns 0, which causes Boost's error() function to throw an exception.

My two questions are:

1. Why is the Win32 API GetFileAttributesExW function failing?
2. Is the Windows Phone file path I pass to GetFileSize (ie, "\\Phone\\Pictures\\Camera Roll\\WP_20140106_001.jpg") the valid way to specify a file on the Windows Phone?

Many thanks in advance for any help you can give.

Answer 1

You need to include the drive letter ("C:\\\\path\\\\to\\\\file") when creating paths on Windows. Note that in a real Windows Phone app you should never hard-code the path because the user can move stuff to the SD card. You should also never store the absolute path to a file for the same reason. It's OK for your test though.

Also, you're trying to read from the Camera Roll, which you can't do directly (you can only read protected files like that through WinRT APIs). But here is a sample showing it working (replace App::App from a new, blank C++ Windows Phone project):

App::App()
{
  InitializeComponent();
  Suspending += ref new SuspendingEventHandler(this, &App::OnSuspending);

  auto path = std::wstring(Windows::ApplicationModel::Package::Current->InstalledLocation->Path->Data()) + L"\\Assets\\SplashScreen.scale-240.png";
  std::wstringstream ss;
  ss << L"Path is " << path << std::endl;
  WIN32_FILE_ATTRIBUTE_DATA data{};
  if (GetFileAttributesExW(path.c_str(), GET_FILEEX_INFO_LEVELS::GetFileExInfoStandard, &data))
  {
    ss << L"Size is " << (((LONG)data.nFileSizeHigh << sizeof(DWORD)) + (LONG)data.nFileSizeLow) << std::endl;
  }
  else
  {
    ss << L"Can't get size info: " << GetLastError() << std::endl;
  }
  OutputDebugString(ss.str().c_str());
}