I think that calling C "portable assembly" is really a bit untrue. One of the core features of most any machine language is that flags are part of the result of many computing operations. Yet C completely removes access to it.
Suppose you have a code that adds two things, then jumps on overflow. On most machines that's two instructions if the operands have the right size. You look at it, and the intent is obvious: we add, then jump on overflow.
Things are seriously wrong (IMHO) if a higher level language completely obfuscates this and requires code where it's not obvious at all what you mean! Heck, what's worse, each compiler likely requires slightly different code so that the meaning is extracted by the optimizer and the correct assembly output is produced without paying both code size and performance penalties! In C, the best you can do on a good compiler is to have an inlineable function that returns the numerical part of the result, uses comparisons to "recreate" the detection of overflow, and returns the overflow condition in a char* out-parameter. If the optimizer is good, it'll recognize that the out parameter accesses an automatic variable in the caller, and that your comparison is just checking for overflow. This code, while portable C, will perform horribly as soon as you compile it without state-of-the-art optimization capabilities. I'd think that means that if your compiler wasn't released in the last year or two, and isn't a mainstream decently optimizing one (like gcc, llvm, visual studio), you're out of luck. On many platforms a saturating increment/decrement is also two or three assembly instructions at most, without jumps -- but good luck getting a compiler to actually emit such code.
I think that providing no way to access the flags part of arithmetic operation results is one of the biggest oversights in C. I'd think that every platform C runs on provides such flags.