You couldn't have given a better example. C as a language doesn't have a standard library, with the standard library being Win32, or whatever comes on POSIX/Unix systems these days. You even get a different libc on Android, which isn't glibc, so with its own quirks and bugs. Win32 doesn't do fork and I/O is very different. And on embedded systems you're going to work with a C subset. And each compiler has special extensions.
In other words what you call C is actually very different from platform to platform, people solving that with a crap load of ifdef statements.
No, what I call C is the same on every platform. Otherwise, all those "crap load of ifdef statements" wouldn't work. What is called the C standard library on each platform is different, because as you said C itself does not define a standard library. So yes, I did pick my example very appropriately.
What if the platforms are the JVM and Javascript, and you are chosing those platforms because of their extensive infrastructure and libraries?
It is possible to implement C faithfully on these platforms, including all of C's weirdness like pointer-arithmetic and unsafe casting. But you'd end up with something that departs far from the platform's runtime model and calling conventions.
> It is possible to implement C faithfully on these platforms, including all of C's weirdness like pointer-arithmetic and unsafe casting.
Of course it can. The two platforms you listed can both faithfully emulate an entire computer, upon which you can build your C runtime.
http://bellard.org/jslinux/tech.html
Now, if what you really meant was more along the lines of "can I faithfully cross-compile C to Java bytecode", the answer would be "not without a layer of abstraction".
If you can't implement C on a platform, I think you're in some pretty strange territory and you're probably having to invent your own language to match the platform.