NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/10941

Internet Message Format | 1996-08-05 | 2.6 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: Fun with <time.h>
Date: Wed, 20 Mar 96 21:45:10 GMT
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In article <[e-mail]>
           [e-mail] "The Amorphous Mass" writes:

>
>  I'm writing a little routine that takes a date, a format to parse the
>date with, and a format to convert the date to, and so I've been sniffing
>around in <time.h>.  One minor annoyance has been that if I'm given a
>date like 5/6/95 and a format that wants the day (Monday, Tuesday, etc)
>I couldn't find any functionality to do that.  But someone noted in
>comp.lang.c.moderated that it was possible to pass an incomplete struct
>tm to mktime() and mktime() would "fill in the blanks" if there was
>enough information.  Is this true, or am I going to have to join the
>vast and lonely horde of people looking for perpetual calendar
>algorithms? :-)

mktime() will do what you want in this case. It ignores the original
values of tm_wday and tm_yday and fills them in calculated from the
other members of the structure.

>  On a side note, what does happen if you pass an incompletely filled
>struct tm (ie, some fields set to 0) to mktime()?  I got some pretty
>colorful results from DEC C...

You *must* set all of the (standard) struct tm members other than tm_wday
and tm_yday appropriately before calling mktime(). The values need not
be within any specific bounds in which case mktime() will adjust the values
of all of the members to bring them within appropriate ranges, while
maintaining the same point in time. So, for example, if you want to determine
what the date is tomorrow you can:

...
    time_t timeval;
    struct tm *tmptr;

    timeval = time(NULL);
    tmptr = localtime(&timeval);
    tmptr->tm_mday++;
    tmptr->is_dst = -1;  /* This allows it to choose an appropriate
                            daylight-saving-time state and preserve the
                            hour value, if possible */
    mktime(tmptr);

And now all of the struct tm fields are set up appropriately for this time
tomorrow.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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