TIme zones revisited

[F.A.Akeroyd at rl.ac.uk]

Ray, 

Both "gmtime" and "localtime" return pointers to "static" data and so if you
called "gmtime" twice with a different time argument, it would be expected
to
change the results of a previous call. From what you have said, it would 
appear that your system has gmtime() and localtime() implemented together
using 
the same storage; you can check this by using

printf("%p\n", (void*)value_from_localtime_or_gmtime);

To see if they are pointing at the same memory location. I'd say the
behaviour
was slightly odd, but not unimaginable. The solution would be to either (1)
make a copy of the result before calling another function or (2) use
"reentrant"
versions e.g. localtime_r(), gmtime_r()

Regarding the change to timezone calculation, i was not sure whether
we should be using

    gmt_offset = difftime(mktime(localtime(&timer)),
mktime(gmtime(&timer)));

Is  "mktime(localtime(&timer))"   always the same as   "timer"? Some help
pages i see say that "timer" is related to "Coordinated Universal Time"
and so might it have daylight/timezone information hidden in it?

Freddie

--
Freddie Akeroyd                              Tel:  (01235) 445457
ISIS Facility                                    Fax: (01235) 445720
Rutherford Appleton Laboratory
Chilton, DIDCOT, OX11 OQX, GB      http://www.isis.rl.ac.uk/


-----Original Message-----
From: Ray Osborn [mailto:ROsborn at anl.gov]
Sent: 13 March 2000 20:31
To: NeXus
Subject: TIme zones revisited


Eric says that the modifications suggested in my last post worked OK.
However, I discovered a strange side effect when using gcc on Linux.  It
appears that running "gmtime" then changes the value of the tm struct
earlier set by "localtime" i.e. the hour is set to the GMT hour.  This seems
to be a "feature" of gcc.  It doesn't happen with other compilers I've used.

What this means is that you may have to add a second call to localtime after
calculating gmt_offset.  If you edit napi.c, use :

#else
   gmt_offset = difftime (timer, mktime(gmtime(&timer)));
   time_info = localtime (&timer);
#endif

Following this message is a simple program to decide if your compiler has
strange behaviour. 

The problem is not particularly serious.  It just means that you will have
the wrong file creation time.  We will try and sort this out before the next
release of the API.

Ray Osborn
-- 
Dr Ray Osborn                Tel: +1 (630) 252-9011
Materials Science Division   Fax: +1 (630) 252-7777
Argonne National Laboratory  E-mail: ROsborn at anl.gov
Argonne, IL 60439-4845

 

--
#include <time.h>
#include <stdio.h>

main ()
{
    time_t timer;
    struct tm *time_info, *gmt_info;

    time(&timer);
    time_info = localtime(&timer);
    printf ("After calling localtime, the local hour is %d\n",
time_info->tm_hour);
    gmt_info = gmtime(&timer);
    printf ("After calling gmtime, the GMT hour is %d\n",
gmt_info->tm_hour);
    printf ("After calling gmtime, the local hour is %d\n",
time_info->tm_hour);
    time_info = localtime(&timer);
    printf ("Ater calling localtime again, the local hour is %d\n",
time_info->tm_hour);
}