Re: [Qemu-devel] [PATCH] osdep: Deal with TIME_MAX and OpenBSD 64-bit time_t

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/11/2017 12:26, Daniel P. Berrange wrote:
> On Mon, Nov 06, 2017 at 10:51:16AM +0000, Peter Maydell wrote:
>> On 6 November 2017 at 00:56, Brad Smith <brad@comstyle.com> wrote:
>>> Define TIME_MAX to LLONG_MAX for OpenBSD since OpenBSD uses 64-bit time_t.
>>>
>>> Signed-off-by: Brad Smith <brad@comstyle.com>
>>>
>>>
>>> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
>>> index 6855b94bbf..824714049b 100644
>>> --- a/include/qemu/osdep.h
>>> +++ b/include/qemu/osdep.h
>>> @@ -132,8 +132,12 @@ extern int daemon(int, int);
>>>  #define ESHUTDOWN 4099
>>>  #endif
>>>  #ifndef TIME_MAX
>>> +#ifdef __OpenBSD__
>>> +#define TIME_MAX LLONG_MAX
>>> +#else
>>>  #define TIME_MAX LONG_MAX
>>>  #endif
>>> +#endif
>>
>> I'm not really a fan of adding new OS-specific #ifdefs --
>> what if one of the other BSDs uses or switches to 64-bit
>> time_t for 32-bit platforms? Is there some way we can detect
>> this generically at compile time (possibly in configure) ?
> 
> You could use a pair of compile time asserts to figure it out.
> 
> Would need one compile test to check 32 vs 64 bit:
> 
>   #include <time.h>
>   char time_t_64bit[sizeof(time_t) == 8 ? 1 : -1];
> 
> and a second to check signed vs unsigned:
> 
>   #include <time.h>
>   char time_t_signed[(time_t) -1 < 0 ? 1 : -1];
> 
> 
> Save each of these programs to the file $TMPC, and then run 'compile_object'
> from configure. You then have a decision matrix for 4 different TIME_MAX
> values to write into config-host.h.

What about

#define type_max(t)                   \
    ((t) -1 > 0                       \
     ? (t)~0                          \
     : (((t)1) <<                     \
         (sizeof(t) * 8 - 1)) - 1)    \

#define TIME_MAX type_max(time_t)

?  We don't need it to be a cpp constant, do we?  Or if we did, we could
assume it to be signed, it's enough for the three users.

Thanks,

Paolo