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Date: Thu, 16 Mar 2006 15:33:05 -0500
From: Norton Allen <allen AT huarp DOT harvard DOT edu>
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To: Eric Blake <ericblake AT comcast DOT net>
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Subject: Re: Bug in dlopen() (or following) code in Cygwin1.dll v 1.5.19-4
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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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