Re: Question for ISO C standards gurus

[Ian Lance Taylor <ian at wasabisystems dot com>]

Diego Novillo <dnovillo@redhat.com> writes:

> On Thu, 2004-03-25 at 13:08, Dave Korn wrote:
> 
> > "Here is the complete list of all sequence points in C++: 
> > 
> > at the end of a full expression 
> > 
> > after the evaluation of all function arguments in a function call and before
> > execution of any expressions in the function body 
> > 
> > after copying of a returned value and before execution of any expressions
> > outside the function 
> > 
> > after evaluation of the first expression in a&&b,  a||b,  a?b:c,  or  a,b 
> > 
> > after the initialization of each base and member in the constructor
> > initialization list "
> > 
> So, the program below should print 5?  It certainly doesn't with gcc 3.2
> nor mainline:
> 
> int foo (int x)
> {
>   return x;
> }
> 
> main ()
> {
>   int *p;
>   int a[2];
>   a[0] = 4;
>   a[1] = 5;
>   p = a;
>   printf ("%d\n", foo (*p++));
> }
> 
> It will print 5 if I use pre-increment on 'p', which is what I would've
> expected (I'm no language lawyer, though).

I can't see any reason why that would print 5.  Based on the above, p
should be incremented before the function call to foo().  But that
doesn't change the value of *p++, which means "get the value at *p,
then increment p".  The above just means that this should not happen:
    t1 = *p;
    foo (t1);
    p = p + 1;
(at least, it should not be possible to observe that that happened; in
the above program, p, as a local variable whose address is not taken,
probably could be incremented after the function call, since there is
no way for anything to detect when the increment occurred.)

Ian