Using new Windows features with fallback

Question

I've been using dynamic libraries and GetProcAddress stuff for quite some time, but it always seems tedious, intellisense hostile, and ugly way to do things.

Does anyone know a clean way to import new features while staying compatible with older OSes.

Say I want to use a XML library which is a part of Vista. I call LoadLibraryW and then I can use the functions if HANDLE is non-null.

But I really don't want to go the #typedef (void*)(PFNFOOOBAR)(int, int, int) and PFNFOOOBAR foo = reinterpret_cast<PFNFOOOBAR>(GetProcAddress(GetModuleHandle(), "somecoolfunction"));, all that times 50, way.

Is there a non-hackish solution with which I could avoid this mess?

I was thinking of adding coolxml.lib in project settings, then including coolxml.dll in delayload dll list, and, maybe, copying the few function signatures I will use in the needed file. Then checking the LoadLibraryW return with non null, and if it's non-null then branching to Vista branch like in a regular program flow.

But I'm not sure if LoadLibrary and delay-load can work together and if some branch prediction will not mess things up in some cases.

Also, not sure if this approach will work, and if it wont cause problems after upgrading to the next SDK.

Answer 1

Delay loading is the best way to avoid using LoadLibrary() and GetProcAddress() directly. Regarding the security issues mentioned, about the only thing you can do about that is use the delay load hooks to make sure (and optionally force) the desired DLL is being loaded during the dliNotePreLoadLibrary notification using the correct system path, and not relative to your app folder. Using the callbacks will also allow you to substitute your own fallback implementations in the dliFailLoadLib/dliFailGetProc notifications when the desired API function(s) are not available. That way, the rest of your code does not have to worry about platform differences (or very little).

Answer 2

IMO, LoadLibrary and GetProcAddress are the best way to do it.

(Make some wrapper objects which take care of that for you, so you don't pollute your main code with that logic and ugliness.)

1. DelayLoad brings with it security problems (see this OldNewThing post) (edit: though not if you ensure you never call those APIs on older versions of windows).

   DelayLoad also makes it too easy to accidentally depend on an API which won't be available on all targets. Yes, you can use tools to check which APIs you call at runtime but it's better to deal with these things at compile time, IMO, and those tools can only check the code you actually exercise when running under them.

2. Also, avoid compiling some parts of your code with different Windows header versions, unless you are very careful to segregate code and the objects that are passed to/from it.

   It's not absolutely wrong -- and it's completely normal with things like plug-in DLLs where two entirely different teams probably worked on the two modules without knowing what SDK version each other targeted -- but it can lead to difficult problems if you aren't careful, so it's best avoided in general.

   If you mix header versions you can get very strange errors. For example, we had a static object which contained an OS structure which changed size in Vista. Most of our project was compiled for XP, but we added a new .cpp file whose name happened to start with A and which was set to use the Vista headers. That new file then (arbitrarily) became the one which triggered the static object to be allocated, using the Vista structure sizes, but the actual code for that object was build using the XP structures. The constructor thought the object's members were in different places to the code which allocated the object. Strange things resulted!

   Once we got to the bottom of that we banned the practise entirely; everything in our project uses the XP headers and if we need anything from the newer headers we manually copy it out, renaming the structures if needed.

It is very tedious to write all the typedef and GetProcAddress stuff, and to copy structures and defines out of headers (which seems wrong, but they're a binary interface so not going to change) (don't forget to check for #pragma pack stuff, too :(), but IMO that is the best way if you want the best compile-time notification of issues.

I'm sure others will disagree!

PS: Somewhere I've got a little template I made to make the GetProcAddress stuff slightly less tedious... Trying to find it; will update this when/if I do. Found it, but it wasn't actually that useful. In fact, none of my code even used it. :)

Answer 3

Yes, use delay loading. That leaves the ugliness to the compiler. Of course you'll still have to ensure that you're not calling a Vista function on XP.