How do I build a runtime version agnostic DLL in C

Question

My product is a C++ library, which, on Windows, is distributed as a dll. It makes very little use of the c-runtime (basic iostream and that's it), so I'm sure that all recent versions of the CRT will be fine.

Since my client is supposed to build his application using my dll, I don't want to impose upon him any specific runtime version. I'd like my dll to bind to whatever runtime library version my client's app is using (and I can assume that he'll use dynamic linking for his CRT). After all, isn't that what dynamic linking is all about? Is that possible?

EDIT: linking the dll against the static runtime libs won't work either, because then the static runtime (from the dll) and the dynamic runtime (from the client's application) will be mixed, which is bad.

EDIT: What I'm mainly asking is how do I tell the runtime loader to link my dll against whatever CRT the application is linked with? Something with the manifest, perhaps? More generally, my question is how to build a nicely-behaving dll, that's to be used by clients building they're own applications?

EDIT: Thanks to the advice in the answers, I've transferred all references to std classes into inlined functions in my headers, and linked my dll with the static runtime libraries. It now seems to work even in applications linked with different CRT versions.

Answer 1

You can achieve this by using WinAPI calls for I/O and anything else that possibly relies on the runtime.

The most painful part is that you may have to override the global new and delete to use WinAPI functions exclusively because they are likely to use malloc/free internally. There are many other painful aspects to this, and my opinions is that it isn't worth the trouble. Here is an article that covers this topic.

Answer 2

Well, there is a huge difference between the C-runtime and the C++-runtime. If you where to use msvcrt.dll, which in recent years became "knighted" as a true system DLL, you could rely on its existence on XP onwards (though for Windows 2000 you would need some redistributable for the version 6 of msvcrt.dll). You can make use of msvcrt.dll by compiling your code with the compiler from the latest WDKs (Windows Driver Kits). Even though this is user-mode code, this is a viable and good method to compiler it.

IOStreams, however, require the C++-runtime. And that complicates things a lot.

EDIT: linking the dll against the static runtime libs won't work either, because then the static runtime (from the dll) and the dynamic runtime (from the client's application) will be mixed, which is bad.

Well, if you mix code in such a way, you have something wrong in your design. You would have similar problems when running a debug build of your DLL with a release build of the other code or vice versa.

I can only recommend that you either make direct use of COM, or - if that's too big - try to emulate some ideas of COM. The most important one would be that you have a factory function and that there is an (class) interface declared (and gets never changed) between those two pieces of code (i.e. the DLL and its caller). The factory function would return an instance of the class and the class would manage its lifetime itself (which implies that all code for allocation and deallocation would reside in the same entity, i.e. your DLL). The lifetime management would then be exposed via addref and release member functions. IUnknown could be the basis for this interface of yours without relying on other parts of the actual COM.

EDIT: What I'm mainly asking is how do I tell the runtime loader to link my dll against whatever CRT the application is linked with? Something with the manifest, perhaps? More generally, my question is how to build a nicely-behaving dll, that's to be used by clients building they're own applications?

Not easy at all. Even if you had all the versions of VS installed, you'd have to script your way out of this dilemma to pick the right version.