Re: Time not representable

[Carsten Dominik <carsten.dominik@gmail.com>]

Paul Eggert <eggert <at> cs.ucla.edu> writes:

> 
> On 03/11/2011 05:01 AM, Carsten Dominik wrote:
> > Why is it that these things are different on different systems?  Is
> > this under the control of Emacs, or does this depend on system
> > libraries which are being used?
> 
> Both.
> 
> Time stamps on most modern systems have nanosecond resolution,
> but Emacs's internal time stamps only use microsecond resolution.
> This means (for example) that Emacs cannot determine that
> one file is newer than another, even when it is newer.
> This bug really should get fixed at some point.
> 
> In addition to resolution problems, there are also the
> range problems that you alluded to.  time_t is usually
> a signed 32-bit or 64-bit integer; a few systems use unsigned
> integers, and several use signed integers but do not allow
> negative values.  (Emacs in theory could use 32-bit EMACS_INT
> internally on a system with 64-bit time_t, but I expect that
> combination is rare.)
> 
> You have to be careful about inferring ranges from behavior,
> though, because when Emacs converts times, it sometimes doesn't check
> for overflow.  This means you can get undefined behavior if you use
> time stamps that cannot be represented internally.
> For example, (encode-time 0 0 0 1 1 1152921504606846976)
> returns the obviously-bogus value (-948597 62170)
> on my RHEL 5.5 x86-64 host, whereas it correctly reports
> a "Specified time is not representable" error on
> my Ubuntu 10.10 x86 host.  This is a bug that should get
> fixed at some point too, though I expect it's less important
> in practice than the nanosecond-resolution bug.


Hi Paul (and Andreas)

thanks for the information.  So if a system uses a signed
64 bit integer, then the representation problems will be
fixed automatically, without further changes in Emacs?
This does sound hopeful!  Is there anyone here who has
a 64 bit system and con confirm that encode-time can
handle times both before 1970 and after 2040 without
any problems?

- Carsten