[c++-pthreads] Restating the Jason model

[Alexander Terekhov]

Dave Butenhof wrote:
[...]
> >Anything else?
> >
> >
> Yes; but I don't yet know what it is. ;-)

2-phase EH (with fixed throw spec semantics) and "intelligent" cancel
request delivery. I thought you were in favor of it...

http://groups.google.com/groups?selm=3ec0f83b%40usenet01.boi.hp.com

<quote>

OK, fine. Let's standardize on a low-level exception support environment 
with DETACHED 2-phase PC-mapped exceptions. On the first "search" phase, 
the handler (if any) associated with each PC region that's active on the 
call stack is presented (in order from most recent frame) with the cause of 
the exception; say, a status code and arbitrary counted argument list. At 
any point, a handler may determine that an UNWIND is necessary, targeting a 
particular active stack frame. All handlers from the "inmost" out to that 
frame's will be activated a second time with a status that indicates an 
UNWIND is in progress. If the search phase reaches the base frame with no 
unwind, then a core dump is taken. Only on the UNWIND phase should any 
handler make actual changes in any program state, consistent with the 
unwinding of that frame. Thus, cleanup occurs only when an UNWIND has been 
ordered -- and of course it may also be conditionalized on the identity of 
the exception that's unwinding.

This provides everything you need for your conditional "cleanup only if 
unwind will occur" syntax, in C++ and/or C. It still provides sufficient 
support for existing exception models like Ada and Modula-2+ that already 
have 'finally' and depend on a traditional interpretation. (Having any of 
these frames active will cause your conditional cleanup for any "inner 
frame" to fire because there WILL be an unwind, but that's life.)

(What I've described, by the way, is essentially how the OpenVMS condition 
handling facility has worked since 1977, though many users have always 
ignored the unenforced recommendation that cleanup be done only on unwind.)

</quote>

regards,
alexander.