The fix wasn't easy, or C precedence bites

There's another bug, where you don't update psrc in those error return cases. The parser will be stuck at the malformed % forever. Or maybe that is desired; it's hard to tell.

The precedence stuff here is pretty basic. When in doubt, using parentheses to make order explicit is never wrong. Or consult https://en.cppreference.com/w/c/language/operator_precedence... .

Even as someone who doesn't mind writing in C, I would absolutely flag that function as way, way, way, way too terse for no good reason.

The code gets ridiculously easier to read if you write src[0] and src[1] instead of (*src) and (*(src+1)). And, as a bonus, the precedence problem disappears.

I really don't understand people why write C code like the original code. It's just asking for a bug.