concrete library-code example (was: C++ and posix threads)

[Nathan Myers]

On Wed, Dec 24, 2003 at 08:09:57AM -0500, Jason Merrill wrote:
> On Tue, 23 Dec 2003 11:03:14 -0500, Ted Baker <baker at cs.fsu.edu> wrote:
> 
> > How do you propose to modify read() to throw an exception and
> > still have backwards compatability with applications that expect
> > read() to always return (more specifically, to return -1 if it
> > fails)?
> 
> read() already doesn't return if it's acting on a cancellation request.
> Throwing an exception is just a different way of not returning.

Enlarging on this question...

Here is a more-or-less concrete example, for discussion purposes.
It's meant as a generic example of code written according to the 
existing contract offered by C libraries.

  int affect_world(struct state* s)
  {
    int result;
    violate_invariants_or_claim_resources(s);
    result = c_function_or_system_call(s->member);
    if (result < 0) {
      clean_up(s, result);
      return result;
    }
    act_on_result(s, result);
    restore_invariants_and_release_resources(s);
    return 0;
  }

This pattern is extremely common in both C and C++ libraries.  If 
read() were to throw (or to "just ... not return"), the program state 
would be corrupted.  A redefinition of c_function_or_system_call 
semantics that breaks this code breaks many thousands of existing 
thread-safe C and C++ libraries.

(The cancellation model described in
  http://www.codesourcery.com/archives/c++-pthreads/msg00021.html
is designed to preserve libraries that contain code that follows 
this pattern.)

Jason, do you not consider those libraries worth preserving?

Nathan Myers
ncm at cantrip.org