[PATCH] Clarify English strsignal() messages.

[Florian Weimer]

* Zack Weinberg:

> Several of the strsignal() messages for the English locale are
> cryptic, inconsistent among a subgroup, or reflect what the signal
> was *originally* used for, rather than what it is used for today.
> Improve this somewhat.
>
> 95% of the practical value of this patch is in the change for the
> SIGFPE message, but if we’re gonna edit these messages at all,
> I think we should edit all of them at once.  I'm working on a follow-up
> patch that tackles the strerror() messages and the manual's explanation
> of each error code, but this requires quite a bit more research.
>
> “Floating point exception” -> “Arithmetic exception”:
>   Nowadays SIGFPE is almost always caused by *integer* divide by zero,
>   since ISO C says the floating point environment at program startup
>   has trapping disabled for all exceptions (C2011: Annex F subsection 8.3).
>   When I was teaching CS I regularly got questions from students about
>   why they got this error message when their program didn’t use any
>   floating point at all.  “Arithmetic exception” is the phrase used by
>   musl libc, and also by the entire Sun clade for many years.

But is “exception” really the right term here?  Language-level
exceptions don't result in signals.

> “Illegal instruction” -> “Invalid CPU instruction”
>   I vaguely recall something in the GNU coding standards about
>   preferring “invalid” to “illegal” unless something is actually
>   against the law.  Indicate what kind of instructions are meant.

It's not necessarily an invalid instruction.  Several architectures have
an instruction that is defined to produce this signal.

Thanks,
Florian