In effect it is yet another CRT and the idea is sound. But many times what I found was you may have one that works on say linux and windows and bsd. All the same 'code' but you dig under the covers a bit and it is a maze of ifdefs so each platform has its own quirks. For example threading between fork and createthread is on the surface not too different and you can wrap createthread with it (several libs did). But you dig into it a bit and you find portions that just do not map at all between the systems (usually with IPC and locks). At best they do not compile, slightly worse they return error codes, at worst they act like they work.

A real good example of what I am talking about is the pthread library. It works up to a point but it is a very linux/bsd orientated library. There are some gaps in there from windows that just do not map and the other way around. What is worse is the docs on some of these do not talk about cross platform issues. Luckily you can see the source code of most of them and can tell what is going on. Annoying but one of the things I learned moving code between platforms is that each one has its own way of doing things. You can try to work against it or sit down and unwind what is going on, which takes time. I have even seen this sort of issue in python and java. Where you get down to some low level thing and it just is different on different platforms.

The moment you start doing things like threads or shared libraries, yeah, it all breaks down very quickly. But that isn't standard C.

But I can't think of any implementation that doesn't conform to C90 in that regard. So long as you don't venture into implementation defined / UB category...