[issue28108] Python configure fails to detect tzname on platforms that have it.

[STINNER Victor]

STINNER Victor  added the comment:

See also bpo-35385.

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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Alexey Izbyshev]

Alexey Izbyshev  added the comment:

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

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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

> 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 
#include 


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.

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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Marc-Andre Lemburg]

Marc-Andre Lemburg added the comment:

> 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

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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

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?

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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Marc-Andre Lemburg]

Marc-Andre Lemburg added the comment:

If you want to separately check for the definition of tzname,
I guess you have to add a AC_DEFINE() section specifically
for tzname.

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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

> 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.

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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Marc-Andre Lemburg]

Marc-Andre Lemburg added the comment:

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.

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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Alexander Belopolsky]

Changes by Alexander Belopolsky :


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[issue28108] Python configure fails to detect tzname on platforms that have it.

[Alexander Belopolsky]

New submission from Alexander Belopolsky:

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 
#include 

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 
, so I am 
not sure whether we need to check for it in configure.

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messages: 276098
nosy: belopolsky
priority: normal
severity: normal
status: open
title: Python configure fails to detect tzname on platforms that have it.
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7

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