Issue 28108: Python configure fails to detect tzname on platforms that have it. (original) (raw)
Created on 2016-09-12 19:33 by belopolsky, last changed 2022-04-11 14:58 by admin.
Messages (9)
Author: Alexander Belopolsky (belopolsky) * 
Date: 2016-09-12 19:33
After running ./configure on Linux or MacOS X, check the generated pyconfig.h header:
$ grep TZNAME pyconfig.h /* #undef HAVE_DECL_TZNAME / / #undef HAVE_TZNAME */
However, tzname exists and is declared in time.h on Linux and MacOS X as can be seen by compiling and running the following simple program:
$ cat tzname.c #include <time.h> #include <stdio.h>
int main() { tzset(); printf("%s/%s\n", tzname[0], tzname[1]); } $ clang tzname.c -o tzname $ ./tzname EST/EDT
Note that tzname is mandated by the recent editions of POSIX <http://pubs.opengroup.org/onlinepubs/009695399/functions/tzset.html>, so I am not sure whether we need to check for it in configure.
Author: Marc-Andre Lemburg (lemburg) *
Date: 2016-09-12 20:20
Looking at the autoconf documentation, HAVE_TZNAME is only set iff struct tm does not have a tm_zone member and the external array tzname is found:
https://www.gnu.org/software/autoconf/manual/autoconf-2.64/html_node/Particular-Structures.html
If the struct tm does have a tm_zone member (which it does on Linux), the check for tzname is not even run.
Author: Alexander Belopolsky (belopolsky) *
Date: 2016-09-12 20:24
HAVE_TZNAME is only set iff struct tm does not have a tm_zone member ...
I've figured that much from reading the generated configure script, but thanks for the pointer to the documentation.
Author: Marc-Andre Lemburg (lemburg) *
Date: 2016-09-12 20:55
If you want to separately check for the definition of tzname, I guess you have to add a AC_DEFINE() section specifically for tzname.
Author: Alexander Belopolsky (belopolsky) *
Date: 2016-09-12 21:04
The real issue is that when setting the tzname tuple in the time module, we use a guess based on the value of tm_zone probed in June and January. I am not sure whether this is wise. Shouldn't we just use C tzname is it is available?
Author: Marc-Andre Lemburg (lemburg) *
Date: 2016-09-12 21:16
The real issue is that when setting the tzname tuple in the time module, we use a guess based on the value of tm_zone probed in June and January. I am not sure whether this is wise. Shouldn't we just use C tzname is it is available?
I don't think tzname is really all that useful. In mxDateTime, I use strftime() with "%Z" to obtain the timezone string for a given local time.
tzname tries to identify non-DST vs. DST of the local time zone, but this may fail for cases where a country switches DST settings in a particular year as it happened in Russia:
https://www.timeanddate.com/time/zone/russia/moscow
Author: Alexander Belopolsky (belopolsky) *
Date: 2016-09-12 21:31
I don't think tzname is really all that useful.
I agree. More so after issue 25283 (Make tm_gmtoff and tm_zone available on all platforms). That's why I don't see why the time module need to set tzname to anything other than what the C library does.
Interestingly, glibc may even change tzname[0] as a side-effect of calling localtime:
(on Linux) $ cat lt.c #include <time.h> #include <stdio.h>
int main() { struct tm tm = {0, 0, 0, 1, 1, -100}; time_t t; t = mktime(&tm); localtime(&t); printf("%s/%s %d %d\n", tzname[0], tzname[1], timezone, daylight); t = 0; localtime(&t); printf("%s/%s %d %d\n", tzname[0], tzname[1], timezone, daylight); }
$ gcc lt.c -o lt $ ./lt LMT/EDT 18000 1 EST/EDT 18000 1
I think that's a bug in glibc because it makes no sense to change tzname[0] while not updating timezone.
Author: Alexey Izbyshev (izbyshev) * 
Date: 2018-12-23 19:23
The resolution of this [1] glibc bug report effectively says that the use of global variables tzname, timezone and daylight is not supported by glibc unless a POSIX-style TZ setting is used (which is probably never in real world).
[1] https://sourceware.org/bugzilla/show_bug.cgi?id=23859
Author: STINNER Victor (vstinner) *
Date: 2019-01-04 23:50
See also bpo-35385.
History
Date
User
Action
Args
2022-04-11 14:58:36
admin
set
github: 72295
2019-01-04 23:50:18
vstinner
set
messages: +
2018-12-23 19:23:00
izbyshev
set
nosy: + izbyshev
messages: +
2016-09-12 21:31:58
belopolsky
set
messages: +
2016-09-12 21:16:01
lemburg
set
messages: +
2016-09-12 21:04:00
belopolsky
set
messages: +
2016-09-12 20:55:14
lemburg
set
messages: +
2016-09-12 20:24:22
belopolsky
set
messages: +
2016-09-12 20:20:33
lemburg
set
messages: +
2016-09-12 19:34:49
belopolsky
set
2016-09-12 19:33:55
belopolsky
create