Making alloca more safe

[BCS]

Hello Walter,

> Tomas Lindquist Olsen wrote:
> 
>> You spent quite a bit of effort explaining that segfaults never cause
>> memory corruption, so it seems fairly reasonable to assume that some
>> parts of the application state could still be valid and useful not to
>> throw away.
>> 
> When a seg fault occurs, it is because your program is in a state that
> you, the programmer, never anticipated. Therefore, you cannot know
> what state your data is in. Therefore, your data is unreliable. While
> it may not be in a bad state from memory corruption, it could very
> well be in a bad state from your program's logic being wrong.
> 
> Do you want to bet your life on assuming your program and its data is
> still valid?

No, at that point I wouldn't count on the program doing any thing correctly. 
But that is a long way from trying to get it to do something useful on the 
way down, like try and save off what data it can and generate a crash log 
with whatever it can salvage. If either of these fail, I'm, at worst, in 
exactly the same position I was in before I attempted them and, at best, 
they work.

And before you say it, if the system is truly critical, I'd have the crash 
handler in ROM, a hardware lock out to stop the system from mucking with 
any thing external and a watchdog timer to reset it if the crash handler 
hangs.