Mail Archives: cygwin/2006/03/16/15:33:18
Eric Blake wrote:
> Top-posting reformatted - cygwin.com/acronyms/#TOFU
>
>
>>>>>The "efault.faulted()" two lines above your change is supposed to catch
>>>>>NULL dereferences.
>>>>
>>> Take a /look/ at the source for myfault::faulted in cygtls.h, it calls out
>>>to _cygtls::setup_fault, which calls _sjfault, which appears to be a q'n'd
>>>hacked-up version of setjmp in a context where it's going to get called back
>>>by an SEH handler. So IIUIC, calling 'efault.faulted' will catch any
>>>exception that happens from the point of the call until the point where the
>>>efault object goes out of scope and gets destructed and will cause execution
>>>to jump back to the if... clause.
>
>
>>Ah, got it--it behaves like exception handling, but it
>>doesn't *look* like exception handling. Seems like a
>>good place to add some comments! ;-) (Offer to submit
>>a patch, but seeing as I had to ask, I doubt I'm the
>>right person to do so.) Thanks for clearing this up
>>for me!
>
>
> The only logical place for such a comment would be at the source
> for myfault::faulted, as the idiom of efault.faulted() appears
> throughout cygwin.
Agreed.
> One more thing to be aware of - the reason cygwin uses
> this (IMHO very slick) feature of C++ is that it is more efficient
> to assume that code will not fault, and blindly deference
> pointers with the minimal overhead of setting up the
> setjmp buffer with a pre-installed exception handler already
> prepared for this usage, than it is to use a syscall to Window's
> routines to validate every pointer before dereferencing it. On
> the exceptional case that the code actually did get passed a
> bad pointer, the overhead of the exception handling and longjmp
> are slower, but that is okay since it is the exception.
>
> So maybe it looks weird. C++ is like that!
I would argue that this isn't a feature of C++ in that it cannot be
implemented within the language but must use assembler for a
specific architecture, but I agree that the paradigm of not needing
to check every case is very cool.
-Norton
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