Porting Open Asset Import Library (Assimp) to WinRT (4)

So, I changed my mind about the whole zlib vs System.IO.Compression, partly because the latter is not already a Windows Runtime accessible type (afaik), and secondly because the code would not be at all portable, and I’d like to be able to contribute my work back to the community if I can. I’ve run into a couple of other problems though, one to do with Microsoft’s war on Buffer Overrun exploits (fair enough, Windows XP was a real problem that way), and the other to do with the security constraints that come with Windows Store apps.

Buffer Overrun / Overflow

First, the whole buffer overrun thing: it seems that Microsoft have deprecated large portions of the C Runtime on windows (fopen, strcpy, pretty much anything that takes a potentially unsanitary char* as an argument, and writes the data contained therein to another unsanitary char* argument), in favour of the ‘safe’ variants of these functions (which only exist on windows). The gist here is that you need to tell it how long the destination buffer is, so that the function will not overrun the buffer. I can understand why they did this, after the whole security shake-up they had during the Vista development cycle (rumoured to be the reason it took so long). Incidentally, we might see Apple making similar moves after this little hiring.

Apparently these warnings have existed since Visual Studio 2005, but with Windows Store apps, the warnings about these functions have become errors. You can turn them off by defining _CRT_SECURE_NO_WARNINGS, but I’m not sure if I want to. On the one hand, it does concretely close a potential attack vector in any code I write. On the other hand, Microsoft’s series of _s functions (fopen_s, strcpy_s, wcstombs_s, etc) are not part of Standard C, and are thus not portable either. Some people have suggested that if one is re-writing C code to use these functions, they should just re-write it in C++ and use the iostream classes instead, which is great, except most OSS newbies like me don’t have the clout to get a C++ re-write of zlib accepted. So, I’m left with two options: write code that Microsoft have deemed they don’t want running on their OS, or write ugly, macro-heavy wrappers so that the code uses fopen_s on windows, and fopen elsewhere. I’m going to try the second option for now, and see how far that gets me. Don’t be surprised if I blog in the future about how ssimp core team won’t let me commit my code.

Windows Store App Security Model

The next issue that I’ve encountered is more interesting, and it has risen out of the unique space that Windows RT fills in the market, as more than a tablet, but a little bit less than a full-blown Windows PC. If we look at the iPad/iPhone, and Windows Phones as well (I’m, not even going to discuss Android, it’s just wrong to put Linux on a phone), each app is a completely walled garden, except for a few API’s which give access to things like, Pictures, Emails, Contacts – your personal data that apps can add value to. The thing about accessing this data is, as I mentioned before, it is done through and API, you don’t just go poking around the file system opening files as you please. Furthermore, when these apps do try to access the file system using direct, fairly low-level calls, like fopen, they can only see within their own little sandbox. This is fine on a device whose paradigm is that Apps only work on their own data, but Windows is a bit different. Windows is traditionally a desktop OS, I have a Documents Library, and a Pictures Library, and a Downloads Library, and my files are a thing in themselves which transcend my current set of Apps. So, you can access the file system, but it has to be through an API (StoreageFile) if it isn’t within your sandbox, and you can’t even ask for files outside your sandbox unless the paths have come from a FilePicker. I think this model is sound – an app can’t touch anything it didn’t create without your very explicit permission. My goal, however, is to write a Windows Store app version of Assimp Viewer, and so I have a need to use fopen on files outside my sandbox, which was a bit of a conundrum. Luckily though, I’m not blazing a trail here, and I found this post which details a neat strategy which lets you use the StoreageFile API to bring the file into (and out of I presume) your sandbox, so that you can use your low-level C code on it. This shouldn’t be too hard for me to get into the app, because AssimpView is already a Windows only application.

So, my current status is that I’m testing out my build of zlib for a Windows Store App, and it seems to work thus far. I think building the assimp library itself might not be so complicated, because, today’s points aside, any ANSI C code should “just work” when compiled as a Windows Store App static (or dynamic) library.

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