The alternative is to use a separate variable:

    y = realloc(x, ...);

Then if it fails, you can still free(x) before continuing. Of course, if you're going to exit() due to the failure it doesn't matter and perhaps that's why the original code looks that way. I still think it's worth pointing out, though.

About sizeof (char), there can be no "portability concerns", and that very thing is what I think gives rise to a lot of (in my opinion, bad) cargo cult programming. The value of "sizeof (char)" in C is 1. Note that I didn't say "using GCC x.y.z on x86". It's true for all conforming compilers, on all platforms, and on all architectures. It's defined by the language.

Regarding realloc:

    y = realloc(x, ...)
    if (y == null) {
       free(x);
       // Handle OOM situation
    }
    x = y;

> What would the alternative be here? Allocate a new region, copy everything over and free the other region? Or is there something better?

No.

If realloc fails it returns NULL. In which case the original `x` is still valid. ==> However with `x = realloc(x,...)` you don't don't have a reference to `x` any more!