Alexander Terekhov <terekhov@web.de> writes:
Anthony Williams wrote: [...]
N2420 (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2420.pdf) covers some of the relevant ground --- it's the minutes of the POSIX/C++ liaison committee.
"Gnu gcc and Solaris pthreads: The Gnu gcc and Solaris pthreads implementations are two known implementations that attempt to map POSIX pthread cancellation onto C++ exception handling, but both do so at the cost of breaking the exception model (i.e., they no longer conform to ISO C++) because the alternative appears to be that C++ destructors and catch blocks would not be invoked for cancellation which would mean that resources would be leaked."
Uhmm. Don't know about (modern) Solaris, but glibc (presumably that is meant by "Gnu gcc pthreads implementation") does invoke C++ destructors and catch blocks. But instead of using ordinary exceptions, it uses "forced unwinding" for thread cancel and exit. For some unknown reason (other than "I say it must be so"), glibc maintainer Drepper of Red Hat simply refuses to even contemplate changing it to use ordinary exceptions instead of "forced unwinding"... and I suspect that is the reason why WG21 was asked to drop standardization of thread cancel (and exit) as exceptions.
That may be a contributory factor, but it's certainly not the only reason. At the WG21 meeting in Oxford last April, Nick Stoughton made it quite clear that POSIX cancellation is sticky --- you cannot catch it and resume processing (as Dave Abrahams was keen to point out, you could write a cleanup handler that didn't return, and then proceeded to do what the main function would have done, but you couldn't then cancel the thread again). Exceptions are not --- you can catch everything in catch(...), and you don't have to rethrow. Also, Howard pointed out that the C ABI for one of the Apple compilers doesn't handle C++ exceptions in a way that would allow cancellation implemented as a C++ exception to call pthread cleanup handlers in a C stack frame. Breaking the C ABI to support the C++ thread library isn't nice. I still like the exception-based cancel. Though interoperability with pthread_cancel would be nice, it's not a must-have from where I'm sitting. Anthony -- Anthony Williams Just Software Solutions Ltd - http://www.justsoftwaresolutions.co.uk Registered in England, Company Number 5478976. Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL