It will almost certainly be the standardization of the run-time library that has the most effect on the portability of C programs. Prospective users of C really should read through this chapter carefully and familiarize themselves with its contents. The lack of a widely implemented, portable library was historically the biggest single barrier to portability.
If you are writing programs for embedded systems, bad luck! The library is not defined for stand-alone applications, but in practice we can expect suppliers to produce a stand-alone library package too. It will probably come without the file handling, but there is no reason why, say, the string-handling functions should not work just as well in hosted and unhosted environments.