Re: [Qemu-devel] [PATCH] target_posix_types.h

[Thayne Harbaugh <thayne@c2.net>]

On Wed, 2007-11-14 at 20:39 +0000, Paul Brook wrote:
> > This means that time_t had to be tracked down on varying architectures
> > to find the size and there was an assumption made that time_t is 32 bits
> > - which isn't true for all targets.  The next problem is that if the
> > target is 32 bits but the host is 64 bits then there's a sign extension
> > problem because (time_t)-1 is used for an error condition.  If you don't
> > correctly assign assign the 32-bit -1 to a 64-bit type then, rather than
> > -1, you get 4294967295.
> 
> Is there any guarantee that time_t is a signed type? The fact that you said 
> (time_t)-1 suggests it could be an unsigned type. If time_t is an unsigned 
> type, then casting to a wider value is still wrong. You have to special-case 
> the error condition.
> 
> In the case of time_t this only becomes relevant after 32-bit time_t wrap in 
> approx. 99 years time, but I'd expect there are cases where it matters.

time_t is only one example.  There are similar problems with the
handling of struct target_iovec.  There are still other places with
similar problems.

Yes, special casing can work.  There's the possible problem of value
truncation when moving between 32 and 64 bits.