>>21
I hold the dim view that "portability" is largely a myth bolstered by the dominance of certain machine architectures and language implementations over alternatives. In that respect Scheme doesn't seem appreciably worse than C in being under-specified, but without any dominant implementations or even consensus among its users the ecosystem stayed fragmented while C's mostly coalesced around a handful of flagship implementations on x86.
More often than not, truly portable C results from contortions by the programmer to write for the lowest common denominator. For non-trivial programs, this is not usually something to aspire to.