bug#12446: port better to POSIX hosts lacking _setjmp

[Eli Zaretskii]

> Date: Fri, 14 Sep 2012 14:28:23 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: 12446@debbugs.gnu.org, lekktu@gmail.com
> 
> On 09/14/2012 01:30 PM, Eli Zaretskii wrote:
> > Why not simply switch to using
> > ANSI- and Posix-standard setjmp/longjmp?
> 
> Because longjmp might alter the signal mask,
> and it might not.  POSIX doesn't say either way,
> platforms differ in this respect, and the
> differences can mess things up and slow things down.

Can you elaborate?  On which platforms does longjmp change the signal
mask and how, and in what ways can this "mess things up and slow
things down"?