[TUHS] Re: Off topic: BSD timezone function vs. POSIX timezone variable

On Thu, Dec 14, 2023 at 6:23 AM Aharon Robbins <arnold(a)skeeve.com> wrote: tl;dr: FreeBSD retains the old timezone() function because the 4BSD series of releases had it. Other, more modern interfaces are available, but it can be a PITA. It's an XSI-only variable, but one of the few that FreeBSD does not provide. Yes. These were compile-time constants of some kind 4*60, "AST", "ADT", /* Atlantic */ 5*60, "EST", "EDT", /* Eastern */ 6*60, "CST", "CDT", /* Central */ 7*60, "MST", "MDT", /* Mountain */ 8*60, "PST", "PDT", /* Pacific */ 0, "GMT", 0, /* Greenwich */ with the comment * Sorry, I don't know all the names. which is (a) not helpful and (b) wrong in the case of GMT (even at the time V7 was released, the official name was UTC for the non-shifting version). This function is only slightly changed to include a couple of more timezones and has the warning in the man page: This interface is for compatibility only; it is impossible to reliably map timezone's arguments to a time zone abbreviation. See ctime(3). This function should never be used. BSD has had a timezone argument to gettimeofday for a long time. It used to return the compiled-in offset from GMT(sic), but that option has been eliminated. The more reliable tzset() function has replaced all of that. It describes how the TZ variable, if set, is used and where to find information about the timezone files. To get the local offset, you need to compare gmtime() to localtime(), though most of the time you want to use asctime to format times, or better, strftime(3). There's extern char *tzname[2] that exports the timezone name(s) from the tzcode that's the companion to what's now the IANA TZ database (aka the olson database). It' is an optional part of POSIX, though, it's part of XSI_C_LANG_SUPPORT: XSI General C Library a64l( ), daylight, drand48( ), erand48( ), ffs( ), ffsl( ), ffsll( ), getdate( ), hcreate( ), hdestroy( ), hsearch( ), initstate( ), insque( ), jrand48( ), l64a( ), lcong48( ), lfind( ), lrand48( ), lsearch( ), memccpy( ), mrand48( ), nrand48( ), random( ), remque( ), seed48( ), setstate( ), signgam, srand48( ), srandom( ), strptime( ), swab( ), tdelete( ), tfind( ), timezone, tsearch( ), twalk( ) So properly, should only be visible when XSI has been requested (or when everything has been requested). The man pages for all of {Net,Free,Open}BSD seem to indicate that both timezone, the variable, isn't available on FreeBSD. It's not declared anywhere. Only timezone, the massively obsolete compatibility function is defined in time.h. I believe neither is the 'daylight' variable. Generally, the programmer should choose to use other, more modern interfaces. :) timezone, the function, is about useless. To get the offset, you need to get the current time (or the time you are interested in) as a time_t. You'd then call localtime with that time_t to get a struct tm and them timegm with that tm to get localtime as a time_t. Subtract the original time from the result to get seconds west of UTC. Arguably BSDs should just support the timezone variable, but that's hard when there's also a function. You'd likely want it more often than the function, but you also want old code that's referenced it to work too. The code is there, butdisabled in tzconfig.h due to the conflict.