[tz] [PROPOSED 1/7] Avoid undefined behavior if no Link lines

[Paul Eggert]

On 10/26/22 01:58, Ian Abbott via tz wrote:
> 
> Also in 7.22.5 Searching and sorting utilities, paragraph 1:
> 
> [...] Pointer arguments on such a call shall still have valid values, as 
> described in 7.1.4.

Yes, that's why portable C code can't call qsort(NULL, 0, ...).

It's not just an academic point. Without the patch, 'zic /dev/null' 
dumps core on Fedora 36 x86-64, when tzcode is built via "make 
CFLAGS='$(GCC_DEBUG_FLAGS)'". That's how I discovered the need for the 
patch.

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.