[c++-pthreads] cancellation points report failure

[Alexander Terekhov]

Nathan Myers wrote:
[...]
> > int peekc(FILE * f) {
> >   int c = ungetc(getc_unlocked(f), f);

/*

> >   if (c == EOF && !feof(f) && errno == ECANCELED) {
> >     /*... ?re-inject cancel request? ... */
> >     /*... ?re-enable cancel state? ... */
> >     pthread_testcancel(); /* hurrah! */
> >   }

*/
      if (c == EOF && !feof(f) && errno == ECANCELED)
        pthread_exit(PTHREAD_CANCELED);

> >   return c;
> > }
> 
> I don't understand where you get that.  I thought I had made clear
> that I didn't expect any ordinary (correct!) C library code to need
> changes.  Since getc_unlocked(f) may always return EOF, so you had
> better be prepared for it.  I.e. your original function is of
> questionable correctness. 

Huh?

http://www.opengroup.org/onlinepubs/007904975/functions/ungetc.html

"If the value of c equals that of the macro EOF, the operation 
 shall fail and the input stream shall be left unchanged.

 [...]

 Upon successful completion, ungetc() shall return the byte 
 pushed back after conversion. Otherwise, it shall return EOF."

> 
> > What if someone[1] has it written as
> >
> > int peekc(FILE * f) {
> >   int c = getc(f);
> >   if (c != EOF)
> >     ungetc(c, f);
> >   return c;
> > }
> >
> > and cancelation hits at "unchecked" ungetc(), in your model?
> >
> > [1] google.com/groups?selm=slrn8cvkde.mf3.kaz at ashi.FootPrints.net
> 
> Buggy code is buggy.  

Except that it isn't buggy under the POSIX model where ungetc() 
does throw thread cancel exception.

regards,
alexander.