[c++-pthreads] Re: pthread_cancel and EH: let's try this again

[Peter Dimov]

Nathan Myers wrote:
> On Mon, Jul 18, 2005 at 02:15:35AM +0300, Peter Dimov wrote:
>> Nathan Myers wrote:
>>
>>> 2. Existing thread-safe and exception-safe C and C++ libraries that
>>> do *not* take cancellation into account should still work.
>>
>> Even when cancelled?
>
> Of course.  Otherwise what's the point?

I just wanted to highlight that this is a more ambitious goal than the rest. 
Code that was never supposed to be cancel-safe and has never been tested 
under such circumstances is dragged kicking and screaming into the new 
millenium.

This comes at a cost.

First, the behavior of the standard C library (and the various Posix 
functions) changes depending on whether 'main' is C++. As a consequence, the 
behavior of C libraries changes when called from C++, so their test suites 
that call them from C (and cancel them from C) now don't test their C++ 
behavior.

Second, new C++ code is forced to use an outdated (and rejected) model. The 
natural way to signal cancellation is with an exception, not an error code 
and a multitude of ifs. The exception model has obviously been considered 
superior by the Posix folks, so superior that they even defined their own EH 
mechanism in the C binding.

Third, you seem to propose a further deviation from Posix, starting the 
thread in "cancellation disabled" mode.

All this in an attempt to make existing cancellation-unaware C++ code 
cancellation-safe.

Making C functions not throw under C++ and throw under C is almost surreal. 
I can follow your trail of arguments in its entirety, but the end result is 
still hard to swallow. The language that supports exceptions doesn't use 
them and vice versa.