ctrl+c and destructors

[H. S. Teoh]

On Wed, Oct 02, 2013 at 10:36:08AM -0700, Sean Kelly wrote:
> On Oct 1, 2013, at 10:12 PM, H. S. Teoh <hsteoh at quickfur.ath.cx> wrote:
> > 
> > Now, having *druntime* throw an exception upon ctrl-C is something
> > totally different, and yes it will require some kind of scaffolding/hack
> > to make it work, 'cos the exception must propagate outside of signal
> > handler context.
> 
> I believe the way this currently works is [...]

Wait, this *currently* works?!


> [...] by dynamically modifying code at the point the SEGV occurred,
> which first patches up the code that was modified and then throws.

I thought the hack posted in the thread deadalnix linked to was to have
the signal handler "corrupt" the stack -- i.e., modify it so that the
return address is redirected to a segfault handler. Thus, as soon as the
signal handler returns (and leaves signal handler context), instead of
returning to its original caller it "returns" to the segfault handler
instead, which can then set things up properly for stack unwinding and
throw an exception (or Error as the case may be).

Though, granted, since segfaults can happen anywhere, including inside
nothrow functions, there is no guarantee dtors will run or things will
properly cleanup, etc.. All the usual caveat emptors apply.


> But if the SEGV was inside a system call, you could be in serious
> trouble, since that code isn't exception safe.

Well, SEGV itself is already serious trouble. :P  I don't think there's
any safe way to "recover" from a SEGV, since it indicates that something
has gone horribly wrong in a way the code can't handle.

I think the OP's original point was how to ensure certain cleanups
always ran (e.g. switch terminal back to normal teletype mode when the
program has been running in ncurses mode). Well, there is no guarantee
that it will run, due to the nature of segfaults, but assuming that the
dtors concerned are high enough up the call stack to not be affected by
Errors bypassing dtors due to being inside nothrow functions, it
*should* work in the general case. Obviously, the program should never
try to catch a SegfaultError and attempt to "recover" from it, since the
program is already horribly screwed up by that point.


T

-- 
It said to install Windows 2000 or better, so I installed Linux instead.