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In article <4fovvr$f1d@blackice.winternet.com> jdege@winternet.com (Jeff
Dege) writes:

>     An off-the-wall question:
> 
> time() returns a time_t, localtime() converts this to a struct tm with
> local time values, gmtime() converts this to a struct tm with GMT values,
> mktime() converts this back to a time_t, but _which_ struct tm does
> it convert back?

According to ISO 9899 (included by reference in the C++ standard): "The
mktime function converts the broken-down time, expressed as
_local_time_..."  (Emphesis added.)

> i.e.,
> 
> time_t t0 = time(NULL);
> 
> struct tm T1 = *localtime(&t0);
> struct tm T2 = *gmtime(&t0);
> 
> time_t t1 = mktime(&T1);
> time_t t2 = mktime(&T2);
> 
> Either t0 == t1 or t0 == t2, but which is true?

t0 == t1.

> Does the standard specify?
> If so, how many compilers get it right?

Given that this is a standard C function, and has been around for a
while, I would imagine that all of them do.  Particularly as the
function author has to make a choice in the matter, and there is no
particular motivation for a wrong choice.

More problematic is using mktime to correct the time, which it also
supports.  For example:

    time_t          t0( time( NULL ) ) ;
    struct tm       t = *localtime( &t0 ) ;
    t.tm_min += 60 ;
    mktime( &t ) ;

The last call to mktime should `correct' t so that the struct contains
the correct entry for the time one hour after that returned by time.  I
often wonder how well this has been tested, say when the original time
was 10 minutes before midnight Dec. 31, or before a change in time due
to switching between summer time and winter time.
-- 
James Kanze           (+33) 88 14 49 00          email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
Conseils, itudes et rialisations en logiciel orienti objet --
              -- A la recherche d'une activiti dans une region francophone
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