Re: [PATCH] ctime.3: mktime() may modify tm_hour due to tm_isdst

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

Hi Michael,

Please don't apply this patch.  I'll resend with the same contents but 
with a different commit message.

On 10/10/21 12:52 PM, Alejandro Colomar wrote:
> If the input DST value is the opposite of the one that mktime()
> uses, which comes from the current system timezone (see tzset(3)),
> mktime() will modify the hour (and if it's in a day limit, it may
> carry up to modify other fields) to normalize the time to the
> correct DST.
> 
> If a user wants to avoid this, it should use UTC time, which never
> has DST.  For that, setenv("TZ", "", 1); sets UTC time for the
> program.  After that, the program should specify that DST is not
> in effect, by setting tm_isdst to 0 (or let the system guess it
> with -1), since setting tm_isdst to a positive value will result
> in an error, (probably) due to mktime() considering that it is
> invalid to have DST enabled for UTC times.

Today I found timegm(3), which does the same as mktime(3) with TZ set to 
UTC, but doesn't require the whole program to be using UTC times, so 
I'll add this to the commit message.

BTW, timegm(3) says that you should "avoid their use" because timegm(3) 
is a Linux and BSD extension, but its use can NOT be avoided (well, it 
can, but if not done very carefully, you are likely to introduce bugs 
due to setenv(3) not being thread-safe), so I'd remove that sentence 
from timegm(3).  I think it should be in POSIX.  As FreeBSD timegm(3) 
says, there's no (safe and easy) way to roll your own timegm() based on 
standard functions:

[
      The timegm() function is not specified by any standard; its 
function can-
      not be completely emulated	using the standard functions described 
above.
]

Thanks,

Alex


> 
> Cc: Paul Eggert <eggert@cs.ucla.edu>
> Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
> ---
>   man3/ctime.3 | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/man3/ctime.3 b/man3/ctime.3
> index 0e2068a09..7a5714be8 100644
> --- a/man3/ctime.3
> +++ b/man3/ctime.3
> @@ -260,6 +260,13 @@ normalized (so that, for example, 40 October is changed into 9 November);
>   is set (regardless of its initial value)
>   to a positive value or to 0, respectively,
>   to indicate whether DST is or is not in effect at the specified time.
> +If the initial value of
> +.I tm_isdst
> +is inconsistent with the one set by
> +.BR mktime (),
> +.I tm_hour
> +(and possibly other fields)
> +will be modified to normalize the time to the correct DST.
>   Calling
>   .BR mktime ()
>   also sets the external variable \fItzname\fP with
> 


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/