localtime and TZ

Ken Brown kbrown@cornell.edu
Fri Oct 29 23:39:00 GMT 2010

On 10/29/2010 6:16 PM, Eric Blake wrote:
> On 10/29/2010 04:11 PM, Ken Brown wrote:
>> Thanks, Eric.  I didn't know about any of this.  (I was using a modification of a configure test from the emacs sources.)
> Probably worth pointing it out to the emacs upstream, then :)
>>   But I get the same behavior with the following revised test case:
>> #include<time.h>
>> #include<stdio.h>
>> int
>> main (void)
>> {
>>    time_t now = time ((time_t *) 0);
>>    printf ("TZ is initially unset; hour = %d\n", localtime (&now)->tm_hour);
>>    putenv ("TZ=GMT0");
>>    printf ("TZ=GMT0; hour = %d\n", localtime (&now)->tm_hour);
>>    unsetenv("TZ");
>>    printf ("TZ unset; hour = %d\n", localtime (&now)->tm_hour);
>>    putenv ("TZ=PST8");
>>    printf ("TZ=PST8; hour = %d\n", localtime (&now)->tm_hour);
>>    unsetenv("TZ");
>>    printf ("TZ unset again; hour = %d\n", localtime (&now)->tm_hour);
>> }
>> So the question remains whether this difference between Cygwin and Linux is a bug or by design.
> Apparently by design.  POSIX requires:
> http://www.opengroup.org/onlinepubs/9699919799/functions/localtime.html
> Local timezone information is used as though localtime() calls tzset().
> http://www.opengroup.org/onlinepubs/9699919799/functions/tzset.html
> The tzset() function shall use the value of the environment variable TZ
> to set time conversion information used by ctime , localtime , mktime ,
> and strftime . If TZ is absent from the environment,
> implementation-defined default timezone information shall be used.

Wouldn't you interpret this as meaning that the implementation-defined 
default timezone information should be the same every time localtime is 
called with TZ unset?  If not, what should a program do to get the 
"standard" default timezone information that it would get if TZ had 
never been set in the first place?


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