[tz] [PROPOSED 1/7] Avoid undefined behavior if no Link lines
Ian Abbott
abbotti at mev.co.uk
Wed Oct 26 14:16:39 UTC 2022
On 26/10/2022 11:03, Paul Eggert wrote:
> The core dump occurred because GCC translates this:
>
> qsort(links, nlinks, sizeof *links, qsort_linkcmp);
>
> as if it were this:
>
> if (nlinks == 0)
> __builtin_trap();
> qsort(links, nlinks, sizeof *links, qsort_linkcmp);
>
> That is, if qsort's second argument is zero, the code generated by GCC
> doesn't call the qsort library function. Instead, it directly executes
> the ud2 instruction <https://www.felixcloutier.com/x86/ud>, which raises
> the invalid opcode exception. Presumably this is because the GCC
> maintainers are in the faction that says a null pointer cannot be used
> to pass a size-zero object to a library function. This is likely the
> same faction that says "char *p = NULL; return p + 0;" has undefined
> behavior.
Does it also call __builtin_trap() if nlinks is 0 and links is a valid
pointer to modifiable storage? Because it should allow nlinks to be 0
in that case, according to the C standard.
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