struct tm problem

[Corinna Vinschen]

On Mar  5 12:35, Corinna Vinschen wrote:
> On Mar  5 11:45, Irfan Adilovic wrote:
> > On Tue, Mar 4, 2014 at 9:19 AM, Corinna Vinschen wrote:
> > > On Mar  3 23:28, Irfan Adilovic wrote:
> > >> > tm_gmtoff is not required by the standard:
> > >> > http://www.opengroup.org/onlinepubs/009695399/basedefs/time.h.html
> > >> >
> > >> > However, it is an extension available in the Olson tzcode, which
> > >> > cygwin seems to use.  Enabling it would seem to be a matter of
> > >> > setting -DTM_GMTOFF=tm_gmtoff and adding it to time.h.
> > >
> > > And same for TM_ZONE.
> > >
> > >> I have successfully done this and use the tm_gmtoff in my code
> > >> actively. Is there a reason this isn't enabled? Is there any interest
> > >> in the community to make tm_gmtoff available by default (It's so easy,
> > >> it's a shame it's not :-))?
> > >
> > > It's easy to change the struct, but changing the size of a structure
> > > is an incompatible change to existing applications which leads to
> > > overwriting memory.
> > >
> > > A change to Cygwin involves an extra check if the application has been
> > > build against an older or a newer version of Cygwin, and to fill the
> > > tm_gmtoff/tm_zone structure members dependent on that.  So it's not just
> > > done by defining TM_GMTOFF and TM_ZONE.
> > 
> > Now that you mention the incompatibility, it seems logical -- the size
> > of the struct is hard-coded when allocation frames in the executable
> > (or when mallocing), and passing that to the library will write
> > tm_gmtoff past the allocated area -- but how can one even try to
> > recognize and act upon that at runtime, without recompiling the
> > executable?
> 
> The Cygwin DLL can check under which version of the Cygwin DLL an
> executable has been compiled.  We're using this knowledge already in
> other circumstances.  If the DLL finds it has been compiled under an old
> Cygwin version which doesn't know these new members, it will not try to
> read or write these members, if it's an executabled compiled under a
> newer DLL which knows these members, it will utilize them.  If you're
> running a new executable under an old version of the DLL, the members
> will probably contain garbage, but that's an unsupported scenario
> anyway.

I created new snapshots on http://cygwin.com/snapshots/ which provide
tm_gmtoff and tm_zone for executables linked against that version of
the DLL.  Please note that it's *not* sufficient to install the new DLL
for this functionality to get.  You *must* install the full snapshot,
including libs and header files.

Please give this a thorough test.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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