MT-unsafe time modules

MT-unsafe time modules

[Bruno Haible]

When a module is under LGPL, it is implicitly advertised as suitable for
libraries. And libraries should be MT-safe (for 20 years already). So,
such modules should be MT-safe.

Looking at the LGPLed time-related modules:

* This one is already MT-safe:

timespec

* These need to be made MT-safe:

strftime-fixes
wcsftime
ctime
localtime

mktime, mktime-internal
timegm

time_r

strptime

time_rz
c-nstrftime, nstrftime
parse-datetime, parse-datetime2


I will try to tackle them, in roughly the given order.

Bruno

Re: MT-unsafe time modules

[Bruno Haible]

This patch makes ctime, localtime, tzset, wcsftime multithread-safe.


2024-02-09  Bruno Haible  <bruno@clisp.org>

	ctime, localtime, tzset, wcsftime: Make multithread-safe.
	* lib/ctime.c: Include <wchar.h>.
	(rpl_ctime): Modify _environ and _wenviron without using _putenv.
	* lib/localtime.c: Include <wchar.h>.
	(rpl_localtime): Modify _environ and _wenviron without using _putenv.
	* lib/tzset.c: Include <wchar.h>.
	(rpl_tzset): Modify _environ and _wenviron without using _putenv.
	* lib/wcsftime.c (rpl_wcsftime): Likewise.

diff --git a/lib/ctime.c b/lib/ctime.c
index a11adc5c74..8c54ef463c 100644
--- a/lib/ctime.c
+++ b/lib/ctime.c
@@ -21,6 +21,9 @@
 
 #include <stdlib.h>
 #include <string.h>
+#if defined _WIN32 && ! defined __CYGWIN__
+# include <wchar.h>
+#endif
 
 #undef ctime
 
@@ -52,7 +55,22 @@ rpl_ctime (const time_t *tp)
      responsibility.  */
   const char *tz = getenv ("TZ");
   if (tz != NULL && strchr (tz, '/') != NULL)
-    _putenv ("TZ=");
+    {
+      /* Neutralize it, in a way that is multithread-safe.
+         (If we were to use _putenv ("TZ="), it would free the memory allocated
+         for the environment variable "TZ", and thus other threads that are
+         using the previously fetched value of getenv ("TZ") could crash.)  */
+      char **env = _environ;
+      wchar_t **wenv = _wenviron;
+      if (env != NULL)
+        for (char *s = env; *s != NULL; s++)
+          if (s[0] == 'T' && s[1] == 'Z' && s[2] == '=')
+            s[0] = '$';
+      if (wenv != NULL)
+        for (wchar_t *ws = wenv; *ws != NULL; ws++)
+          if (ws[0] == L'T' && ws[1] == L'Z' && ws[2] == L'=')
+            ws[0] = L'$';
+    }
 #endif
 
   return ctime (tp);
diff --git a/lib/localtime.c b/lib/localtime.c
index bdea1cab10..f0e91ac647 100644
--- a/lib/localtime.c
+++ b/lib/localtime.c
@@ -21,6 +21,9 @@
 
 #include <stdlib.h>
 #include <string.h>
+#if defined _WIN32 && ! defined __CYGWIN__
+# include <wchar.h>
+#endif
 
 #undef localtime
 
@@ -52,7 +55,22 @@ rpl_localtime (const time_t *tp)
      responsibility.  */
   const char *tz = getenv ("TZ");
   if (tz != NULL && strchr (tz, '/') != NULL)
-    _putenv ("TZ=");
+    {
+      /* Neutralize it, in a way that is multithread-safe.
+         (If we were to use _putenv ("TZ="), it would free the memory allocated
+         for the environment variable "TZ", and thus other threads that are
+         using the previously fetched value of getenv ("TZ") could crash.)  */
+      char **env = _environ;
+      wchar_t **wenv = _wenviron;
+      if (env != NULL)
+        for (char *s = env; *s != NULL; s++)
+          if (s[0] == 'T' && s[1] == 'Z' && s[2] == '=')
+            s[0] = '$';
+      if (wenv != NULL)
+        for (wchar_t *ws = wenv; *ws != NULL; ws++)
+          if (ws[0] == L'T' && ws[1] == L'Z' && ws[2] == L'=')
+            ws[0] = L'$';
+    }
 #endif
 
   return localtime (tp);
diff --git a/lib/tzset.c b/lib/tzset.c
index 0eb8c161f4..f307f0c3d1 100644
--- a/lib/tzset.c
+++ b/lib/tzset.c
@@ -24,6 +24,9 @@
 
 #include <stdlib.h>
 #include <string.h>
+#if defined _WIN32 && ! defined __CYGWIN__
+# include <wchar.h>
+#endif
 
 void
 rpl_tzset (void)
@@ -54,7 +57,22 @@ rpl_tzset (void)
      responsibility.  */
   const char *tz = getenv ("TZ");
   if (tz != NULL && strchr (tz, '/') != NULL)
-    _putenv ("TZ=");
+    {
+      /* Neutralize it, in a way that is multithread-safe.
+         (If we were to use _putenv ("TZ="), it would free the memory allocated
+         for the environment variable "TZ", and thus other threads that are
+         using the previously fetched value of getenv ("TZ") could crash.)  */
+      char **env = _environ;
+      wchar_t **wenv = _wenviron;
+      if (env != NULL)
+        for (char *s = env; *s != NULL; s++)
+          if (s[0] == 'T' && s[1] == 'Z' && s[2] == '=')
+            s[0] = '$';
+      if (wenv != NULL)
+        for (wchar_t *ws = wenv; *ws != NULL; ws++)
+          if (ws[0] == L'T' && ws[1] == L'Z' && ws[2] == L'=')
+            ws[0] = L'$';
+    }
 
   /* On native Windows, tzset() is deprecated.  Use _tzset() instead.  See
      <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/posix-tzset>
diff --git a/lib/wcsftime.c b/lib/wcsftime.c
index 93e0470dd3..d8b471ab57 100644
--- a/lib/wcsftime.c
+++ b/lib/wcsftime.c
@@ -53,7 +53,22 @@ rpl_wcsftime (wchar_t *buf, size_t bufsize, const wchar_t *format, const struct
      responsibility.  */
   const char *tz = getenv ("TZ");
   if (tz != NULL && strchr (tz, '/') != NULL)
-    _putenv ("TZ=");
+    {
+      /* Neutralize it, in a way that is multithread-safe.
+         (If we were to use _putenv ("TZ="), it would free the memory allocated
+         for the environment variable "TZ", and thus other threads that are
+         using the previously fetched value of getenv ("TZ") could crash.)  */
+      char **env = _environ;
+      wchar_t **wenv = _wenviron;
+      if (env != NULL)
+        for (char *s = env; *s != NULL; s++)
+          if (s[0] == 'T' && s[1] == 'Z' && s[2] == '=')
+            s[0] = '$';
+      if (wenv != NULL)
+        for (wchar_t *ws = wenv; *ws != NULL; ws++)
+          if (ws[0] == L'T' && ws[1] == L'Z' && ws[2] == L'=')
+            ws[0] = L'$';
+    }
 #endif
 
   return wcsftime (buf, bufsize, format, tp);

Re: MT-unsafe time modules

[Bruno Haible]

> * These need to be made MT-safe:
> 
> mktime, mktime-internal
> timegm
> 
> time_r
> 
> time_rz
> c-nstrftime, nstrftime
> parse-datetime, parse-datetime2

For these modules, the next function to provide in an MT-safe way is
localtime_r. On native Windows, when the 'localtime_s' function [1][2]
is not available, such as on the older Windows versions that Emacs cares
about, the solution is to use GetTimeZoneInformation [3].

The next function, then, is a localtime_r variant with time zone argument.
It's needed
  * for implementing 'localtime_rz', in the 'time_rz' module.
  * as a 'convert' subroutine for the 'mktime_z' function, in the 'time_rz'
    module.
  * indirectly by nstrftime, c-nstrftime, fprintftime, parse-datetime,
    that all rely on 'time_rz'.

Such a localtime_r variant with time zone argument needs to use the
time zone database that the system has access to. But the only public
API that the system offers (localtime{,_r}) relies on the environment "TZ"
variable "TZ" and some global variables (tzname, timezone, daylight)
derived from it, and changing an environment variable is not MT-safe.

So, there are only two approaches that I can see:

  (a) Read the time zone database information into memory from a file
      shipped together with Gnulib. This is how the libstdc++ implementation
      of the C++ time zone functionality [4] does it [5][6].

  (b) Read the time zone database information into memory from the
      system locations, in a platform dependent way.

The code for doing this exists in glibc and libstdc++ and would need to be
adapted.

BUT this is a major project: it would take several weeks.

And it is overkill for
  - the proposed safer_ctime function,
  - most practical uses of nstrftime, c_nstrftime.
Namely, for these cases, a general timezone_t is not needed, only a boolean
that covers the case of local time and GMT/UTC. And these cases *can* be
done in a multithread-safe way:
  - The value of "TZ" does not need to be changed on the fly.
  - The values of the global variables (tzname, timezone, daylight) may
    be set through tzset(), in a thread, but these values are OK for the
    other threads as well, since getenv ("TZ") is the same in all threads.
  - The GMT/UTC case can be implemented by not looking at "TZ" nor the
    global variables.

I therefore intend to do this latter part:
  1) Make localtime_r on native Windows MT-safe.
  2) Document which LGPLed interfaces are not MT-safe.
  3) Provide an MT-safe and bug-fixed 'strftime' function (that assumes
     local time, no time zone argument).
  4) Provide an analogous 'c_strftime' function that does not even access
     "TZ".
  5) The proposed safer_ctime, based on localtime_r and c_strftime.

Bruno

[1] https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s
[2] https://learn.microsoft.com/en-us/previous-versions/a442x3ye
[3] https://learn.microsoft.com/en-us/windows/win32/api/timezoneapi/nf-timezoneapi-gettimezoneinformation
[4] https://en.cppreference.com/w/cpp/chrono
[5] https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/include/std/chrono
[6] https://gcc.gnu.org/git/?p=gcc.git;a=tree;f=libstdc%2B%2B-v3/src/c%2B%2B20

localtime on native Windows

[Bruno Haible]

[-- Attachment #1: Type: text/plain, Size: 1802 bytes --]

I wrote:
> For these modules, the next function to provide in an MT-safe way is
> localtime_r.

Our gmtime_r and localtime_r are MT-safe on native Windows. I ran the
test-gmtime_r-mt and test-localtime_r-mt tests for 2000 seconds each, and
they did not crash.

But the problem is that localtime() and localtime_r() on native Windows
produce nonsensical results:
  - They pretend that in France, in 2007, DST began on 2007-03-11. When in
    fact, it started on 2007-03-25.
  - The hour is wrong.
Witness: The attached program loc.c.

> On native Windows, when the 'localtime_s' function [1][2]
> is not available, such as on the older Windows versions that Emacs cares
> about, the solution is to use GetTimeZoneInformation [3].

None of the GetTimeZoneInformation APIs from the Windows DLLs works either.
They pretend that in German and French time zones, DST starts on March 5,
in all years. Witness: The attached program tzi.c.

So, there is no way around implementing a correct localtime_r, based on
tzdata, in Gnulib.

It will be useful
  - for localtime_r on native Windows,
  - for nstrftime, c_nstrftime, parse-datetime, which all take a timezone_t
    argument.

For reading tzdata: The first question is how to include tzdata in gnulib.
  - AFAICS, the main data file (without comments) is tzdata.zi and is about
    100 KB large. It can be upgraded simply by copying the newest tzdata.zi
    from a newer tzdata distribution. Including such a file in gnulib would
    be OK (re copyright, number of files, total size), right?
  - Whereas including all files from /usr/share/zoneinfo is probably not
    acceptable (> 1300 files, ca. 6 MB total size).
  - Access pattern: In a running program, very few among the time zones will
    be used. Therefore, caching in memory is essential.

Bruno

[-- Attachment #2: loc.c --]
[-- Type: text/x-csrc, Size: 1519 bytes --]

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>


/* Some common time zone name.  */

#if defined _WIN32 && !defined __CYGWIN__
/* Cf. <https://ss64.com/timezones.html>  */
# define FRENCH_TZ  "Romance Standard Time"
#else
/* Cf. <https://en.wikipedia.org/wiki/List_of_tz_database_time_zones>  */
//# define FRENCH_TZ  "Europe/Paris"
# define FRENCH_TZ  "Europe/Berlin"
#endif


static void
show_localtime_result (time_t t)
{
  struct tm *result = localtime (&t);
  printf ("%ld -> %04d-%02d-%02d %02d:%02d:%02d DST=%d\n",
          (long) t,
          result->tm_year + 1900, result->tm_mon + 1, result->tm_mday,
          result->tm_hour, result->tm_min, result->tm_sec,
          result->tm_isdst);
}

int
main (int argc, char *argv[])
{
#if defined _WIN32 && !defined __CYGWIN__
  _putenv ("TZ" "=" FRENCH_TZ);
#else
  setenv ("TZ", FRENCH_TZ, 1);
#endif

  show_localtime_result (1173578399); /* 2007-03-11 02:59:59 */
  show_localtime_result (1173578401); /* 2007-03-11 03:00:01 */

  show_localtime_result (1174784399); /* 2007-03-25 01:59:59 */
  show_localtime_result (1174784401); /* 2007-03-25 03:00:01 */

  return 0;
}
/*
glibc, Cygwin:
1173578399 -> 2007-03-11 02:59:59 DST=0
1173578401 -> 2007-03-11 03:00:01 DST=0
1174784399 -> 2007-03-25 01:59:59 DST=0
1174784401 -> 2007-03-25 03:00:01 DST=1

Native Windows:
1173578399 -> 2007-03-11 01:59:59 DST=0
1173578401 -> 2007-03-11 03:00:01 DST=1
1174784399 -> 2007-03-25 01:59:59 DST=1
1174784401 -> 2007-03-25 02:00:01 DST=1
*/

[-- Attachment #3: tzi.c --]
[-- Type: text/x-csrc, Size: 4372 bytes --]

#include <stdio.h>
#include <windows.h>
#include <timezoneapi.h>
#include <wchar.h>

/* Cf. <https://ss64.com/timezones.html>  */
# define FRENCH_TZ  "Romance Standard Time"

int main ()
{
  _putenv ("TZ=" FRENCH_TZ);

  DWORD ret;

  TIME_ZONE_INFORMATION info1;
  ret = GetTimeZoneInformation (&info1);
  printf ("GetTimeZoneInformation:\n"
          "ret = %lu  info1 =\n"
          "standard: |%ls| bias=%ld date=%4d-%02d-%02d %02d:%02d:%02d.%03d\n"
          "daylight: |%ls| bias=%ld date=%4d-%02d-%02d %02d:%02d:%02d.%03d\n",
          ret,
          info1.StandardName,
          info1.StandardBias,
          info1.StandardDate.wYear,
          info1.StandardDate.wMonth,
          info1.StandardDate.wDay,
          info1.StandardDate.wHour,
          info1.StandardDate.wMinute,
          info1.StandardDate.wSecond,
          info1.StandardDate.wMilliseconds,
          info1.DaylightName,
          info1.DaylightBias,
          info1.DaylightDate.wYear,
          info1.DaylightDate.wMonth,
          info1.DaylightDate.wDay,
          info1.DaylightDate.wHour,
          info1.DaylightDate.wMinute,
          info1.DaylightDate.wSecond,
          info1.DaylightDate.wMilliseconds);

  DYNAMIC_TIME_ZONE_INFORMATION info2;
  ret = GetDynamicTimeZoneInformation (&info2);
  printf ("GetDynamicTimeZoneInformation:\n"
          "ret = %lu  info2 = bias=%ld timezonekey=%ls dynamicdisabled=%d\n"
          "standard: |%ls| bias=%ld date=%4d-%02d-%02d %02d:%02d:%02d.%03d\n"
          "daylight: |%ls| bias=%ld date=%4d-%02d-%02d %02d:%02d:%02d.%03d\n",
          ret,
          info2.Bias,
          info2.TimeZoneKeyName,
          info2.DynamicDaylightTimeDisabled,
          info2.StandardName,
          info2.StandardBias,
          info2.StandardDate.wYear,
          info2.StandardDate.wMonth,
          info2.StandardDate.wDay,
          info2.StandardDate.wHour,
          info2.StandardDate.wMinute,
          info2.StandardDate.wSecond,
          info2.StandardDate.wMilliseconds,
          info2.DaylightName,
          info2.DaylightBias,
          info2.DaylightDate.wYear,
          info2.DaylightDate.wMonth,
          info2.DaylightDate.wDay,
          info2.DaylightDate.wHour,
          info2.DaylightDate.wMinute,
          info2.DaylightDate.wSecond,
          info2.DaylightDate.wMilliseconds);

  DYNAMIC_TIME_ZONE_INFORMATION info3i;
  TIME_ZONE_INFORMATION info3;
  {
    DWORD i;
    for (i = 0; ; i++)
      {
        if (EnumDynamicTimeZoneInformation (i, &info3i) == ERROR_SUCCESS // Link error in mingw, OK in MSVC.
            && wcscmp (info3i.TimeZoneKeyName, L"Romance Standard Time") == 0)
          break;
      }
  }
  ret = GetTimeZoneInformationForYear (2007, &info3i, &info3);
  printf ("GetTimeZoneInformationForYear(2007):\n"
          "ret = %lu  info3 =\n"
          "standard: |%ls| bias=%ld date=%4d-%02d-%02d %02d:%02d:%02d.%03d\n"
          "daylight: |%ls| bias=%ld date=%4d-%02d-%02d %02d:%02d:%02d.%03d\n",
          ret,
          info3.StandardName,
          info3.StandardBias,
          info3.StandardDate.wYear,
          info3.StandardDate.wMonth,
          info3.StandardDate.wDay,
          info3.StandardDate.wHour,
          info3.StandardDate.wMinute,
          info3.StandardDate.wSecond,
          info3.StandardDate.wMilliseconds,
          info3.DaylightName,
          info3.DaylightBias,
          info3.DaylightDate.wYear,
          info3.DaylightDate.wMonth,
          info3.DaylightDate.wDay,
          info3.DaylightDate.wHour,
          info3.DaylightDate.wMinute,
          info3.DaylightDate.wSecond,
          info3.DaylightDate.wMilliseconds);

}
/* Compile:
   $CC tzi.c -Wall -D_WIN32_WINNT=_WIN32_WINNT_WIN8 -ladvapi32
 */
/* Results:

GetTimeZoneInformation:
ret = 1  info1 =
standard: |W. Europe Standard Time| bias=0 date=   0-10-05 03:00:00.000
daylight: |W. Europe Daylight Time| bias=-60 date=   0-03-05 02:00:00.000
GetDynamicTimeZoneInformation:
ret = 1  info2 = bias=-60 timezonekey=W. Europe Standard Time dynamicdisabled=0
standard: |W. Europe Standard Time| bias=0 date=   0-10-05 03:00:00.000
daylight: |W. Europe Daylight Time| bias=-60 date=   0-03-05 02:00:00.000
GetTimeZoneInformationForYear(2007):
ret = 1  info3 =
standard: |Romance Standard Time| bias=0 date=   0-10-05 03:00:00.000
daylight: |Romance Daylight Time| bias=-60 date=   0-03-05 02:00:00.000

*/

Re: localtime on native Windows

[Paul Eggert]

On 2024-02-11 04:46, Bruno Haible wrote:
> The first question is how to include tzdata in gnulib.
>    - AFAICS, the main data file (without comments) is tzdata.zi and is about
>      100 KB large. It can be upgraded simply by copying the newest tzdata.zi
>      from a newer tzdata distribution. Including such a file in gnulib would
>      be OK (re copyright, number of files, total size), right?

No problem with copyright; the file is in the public domain.

>   - Whereas including all files from /usr/share/zoneinfo is probably not
>     acceptable (> 1300 files, ca. 6 MB total size).

That can be automatically generated from tzdata.zi, by using zic, the 
source code of which is also in the public domain. This is common in 
GNU/Linux systems. If memory serves Android uses another approach: a 
single binary file indexed by timezone name.

Re: localtime on native Windows

[Bruno Haible]

Paul Eggert wrote:
> > The first question is how to include tzdata in gnulib.
> >    - AFAICS, the main data file (without comments) is tzdata.zi and is about
> >      100 KB large. It can be upgraded simply by copying the newest tzdata.zi
> >      from a newer tzdata distribution. Including such a file in gnulib would
> >      be OK (re copyright, number of files, total size), right?
> 
> No problem with copyright; the file is in the public domain.
> 
> >   - Whereas including all files from /usr/share/zoneinfo is probably not
> >     acceptable (> 1300 files, ca. 6 MB total size).
> 
> That can be automatically generated from tzdata.zi, by using zic, the 
> source code of which is also in the public domain.

Thanks for the confirmation.

My current plan, in order to cover the two goals
  - localtime_r on native Windows,
  - nstrftime, c_nstrftime, parse-datetime, which all take a timezone_t
    argument,
is to write four pieces of code:
  1) code that represents the data for a single time zone in memory,
     (based on glibc's tzfile.c),
  2) code that reads a binary time zone file, for use on Unix,
     (based on glibc's tzfile.c), returning it in the format 1).
  3) code that reads a tzdata.zi, for use on Windows,
     (based on glibc's zic.c and tzfile.c), returning it in the format 1).
  4) an implementation of localtime_r for a given time zone
     (based on glibc's tzfile.c:__tzfile_compute, tzset.c:__tz_convert, and
     localtime_r).

This should fulfil Gnulib's needs (code under LGPLv2+, small number
of source files, small number and size of data files to include).

Note that this will have to wait for a few weeks. I have more urgent work on
gettext to do first.

Note also: The above approach will need locking, in order to protect caches
(in 1) the data for a single time zone, in 3) the contents of tzdata.zi).

Bruno

Re: localtime on native Windows

[Paul Eggert]

On 2024-02-17 18:38, Bruno Haible wrote:
> Note also: The above approach will need locking, in order to protect caches
> (in 1) the data for a single time zone, in 3) the contents of tzdata.zi).

The idea I had was to go more in the NetBSD / TZDB direction. These 
implementations don't need either locks or lock-free caching algorithms, 
because it's the caller's responsibility to cache.

Re: localtime on native Windows

[Bruno Haible]

Paul Eggert wrote:
> > Note also: The above approach will need locking, in order to protect caches
> > (in 1) the data for a single time zone, in 3) the contents of tzdata.zi).
> 
> The idea I had was to go more in the NetBSD / TZDB direction. These 
> implementations don't need either locks or lock-free caching algorithms, 
> because it's the caller's responsibility to cache.

I don't understand what you mean:
  - How can the caller do the caching for something internal to the callee?
    I've never seen callbacks being used for that purpose. I've once seen
    a calling convention where the caller allocates memory for a cache and
    passes it to the callee; but that was for a case where the cache's
    lifetime ends when the callee returns.
  - The NetBSD code [1][2] does locking in the top-level functions (tzset,
    mktime, strftime). I guess that it is because NetBSD uses global variables
    (tzname, timezone, daylight, altzone).

For Gnulib, I intend to avoid global variables of this kind — they belong
in data allocated for each time zone — and therefore move the locking to
lower levels. In the frequent case that a timezone is used for the second
time or later, no locking will be needed at all; this will thus be faster
than what NetBSD does.

Bruno

[1] http://cvsweb.netbsd.org/bsdweb.cgi/~checkout~/src/lib/libc/time/localtime.c?rev=1.141&only_with_tag=MAIN
[2] http://cvsweb.netbsd.org/bsdweb.cgi/~checkout~/src/lib/libc/time/strftime.c?rev=1.55

Re: localtime on native Windows

[Paul Eggert]

On 2024-02-18 06:38, Bruno Haible wrote:
>    - How can the caller do the caching for something internal to the callee?

The caller calls tzalloc, gets a timezone object, and calls tzfree when 
it no longer needs that timezone object.

>   - The NetBSD code [1][2] does locking in the top-level functions (tzset,
>     mktime, strftime). I guess that it is because NetBSD uses global variables
>     (tzname, timezone, daylight, altzone).

This is for backward compatibility, and is required for POSIX 
compatibility. It has obvious problems with threads. New code should not 
use these vestigial interfaces. They should use localtime_z and 
mktime_z. (There should be no need for strftime_z.)

https://data.iana.org/time-zones/theory.html#vestigial

> For Gnulib, I intend to avoid global variables of this kind — they belong
> in data allocated for each time zone — and therefore move the locking to
> lower levels. In the frequent case that a timezone is used for the second
> time or later, no locking will be needed at all; this will thus be faster
> than what NetBSD does.

You're right to avoid the global variables, but I don't see why tzalloc 
needs to lock, even in MS-Windows. Perhaps there could be an "I'll do 
the caching" layer above tzalloc, but the lower-level code need not lock 
at all.

Re: MT-unsafe time modules

[Bruno Haible]

[-- Attachment #1: Type: text/plain, Size: 452 bytes --]

This patch adds a unit test that verifies that gmtime_r and localtime_r
are MT-safe.


2024-02-11  Bruno Haible  <bruno@clisp.org>

	time_r: Add tests.
	* lib/time_r.c: Add comment.
	* tests/test-gmtime_r.c: New file.
	* tests/test-gmtime_r-mt.c: New file, based on
	tests/test-nl_langinfo-mt.c.
	* tests/test-localtime_r.c: New file.
	* tests/test-localtime_r-mt.c: New file, based on
	tests/test-nl_langinfo-mt.c.
	* modules/time_r-tests: New file.


[-- Attachment #2: 0001-time_r-Add-tests.patch --]
[-- Type: text/x-patch, Size: 18997 bytes --]

From 10321db3cd2d0883f085f10eb25f99aa80a97075 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sun, 11 Feb 2024 11:23:14 +0100
Subject: [PATCH] time_r: Add tests.

* lib/time_r.c: Add comment.
* tests/test-gmtime_r.c: New file.
* tests/test-gmtime_r-mt.c: New file, based on
tests/test-nl_langinfo-mt.c.
* tests/test-localtime_r.c: New file.
* tests/test-localtime_r-mt.c: New file, based on
tests/test-nl_langinfo-mt.c.
* modules/time_r-tests: New file.
---
 ChangeLog                   |  12 +++
 lib/time_r.c                |   5 ++
 modules/time_r-tests        |  27 ++++++
 tests/test-gmtime_r-mt.c    | 123 ++++++++++++++++++++++++++
 tests/test-gmtime_r.c       |  56 ++++++++++++
 tests/test-localtime_r-mt.c | 139 ++++++++++++++++++++++++++++++
 tests/test-localtime_r.c    | 167 ++++++++++++++++++++++++++++++++++++
 7 files changed, 529 insertions(+)
 create mode 100644 modules/time_r-tests
 create mode 100644 tests/test-gmtime_r-mt.c
 create mode 100644 tests/test-gmtime_r.c
 create mode 100644 tests/test-localtime_r-mt.c
 create mode 100644 tests/test-localtime_r.c

diff --git a/ChangeLog b/ChangeLog
index 8b6c726087..a25e9f8706 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,15 @@
+2024-02-11  Bruno Haible  <bruno@clisp.org>
+
+	time_r: Add tests.
+	* lib/time_r.c: Add comment.
+	* tests/test-gmtime_r.c: New file.
+	* tests/test-gmtime_r-mt.c: New file, based on
+	tests/test-nl_langinfo-mt.c.
+	* tests/test-localtime_r.c: New file.
+	* tests/test-localtime_r-mt.c: New file, based on
+	tests/test-nl_langinfo-mt.c.
+	* modules/time_r-tests: New file.
+
 2024-02-10  Paul Eggert  <eggert@cs.ucla.edu>
 
 	doc: improve warnings about ctime etc.
diff --git a/lib/time_r.c b/lib/time_r.c
index 3ef0b36802..b724f3b38d 100644
--- a/lib/time_r.c
+++ b/lib/time_r.c
@@ -21,6 +21,11 @@
 
 #include <time.h>
 
+/* The replacement functions in this file are only used on native Windows.
+   They are multithread-safe, because the gmtime() and localtime() functions
+   on native Windows — both in the ucrt and in the older MSVCRT — return a
+   pointer to a 'struct tm' in thread-local memory.  */
+
 static struct tm *
 copy_tm_result (struct tm *dest, struct tm const *src)
 {
diff --git a/modules/time_r-tests b/modules/time_r-tests
new file mode 100644
index 0000000000..56346c0f7c
--- /dev/null
+++ b/modules/time_r-tests
@@ -0,0 +1,27 @@
+Files:
+tests/test-gmtime_r.c
+tests/test-gmtime_r-mt.c
+tests/test-localtime_r.c
+tests/test-localtime_r-mt.c
+tests/macros.h
+
+Depends-on:
+setenv
+thread
+nanosleep
+
+configure.ac:
+dnl Possibly define HAVE_STRUCT_TM_TM_ZONE.
+AC_REQUIRE([AC_STRUCT_TIMEZONE])
+dnl Possibly define HAVE_STRUCT_TM_TM_GMTOFF.
+AC_CHECK_MEMBERS([struct tm.tm_gmtoff],,,[[#include <time.h>]])
+
+Makefile.am:
+TESTS += \
+  test-gmtime_r test-gmtime_r-mt \
+  test-localtime_r test-localtime_r-mt
+check_PROGRAMS += \
+  test-gmtime_r test-gmtime_r-mt \
+  test-localtime_r test-localtime_r-mt
+test_gmtime_r_mt_LDADD = $(LDADD) $(LIBMULTITHREAD) $(NANOSLEEP_LIB)
+test_localtime_r_mt_LDADD = $(LDADD) $(LIBMULTITHREAD) $(NANOSLEEP_LIB)
diff --git a/tests/test-gmtime_r-mt.c b/tests/test-gmtime_r-mt.c
new file mode 100644
index 0000000000..4d38bad1a3
--- /dev/null
+++ b/tests/test-gmtime_r-mt.c
@@ -0,0 +1,123 @@
+/* Multithread-safety test for gmtime_r().
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2024.  */
+
+#include <config.h>
+
+/* Work around GCC bug 44511.  */
+#if 4 < __GNUC__ + (3 <= __GNUC_MINOR__)
+# pragma GCC diagnostic ignored "-Wreturn-type"
+#endif
+
+#if USE_ISOC_THREADS || USE_POSIX_THREADS || USE_ISOC_AND_POSIX_THREADS || USE_WINDOWS_THREADS
+
+/* Specification.  */
+#include <time.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "glthread/thread.h"
+#include "macros.h"
+
+static void *
+thread1_func (void *arg)
+{
+  for (;;)
+    {
+      time_t t = 1509000003; /* 2017-10-26 06:40:03 */
+      struct tm tm;
+      struct tm *result = gmtime_r (&t, &tm);
+      ASSERT (result == &tm);
+      if (!(result->tm_sec == 3
+            && result->tm_min == 40
+            && result->tm_hour == 6
+            && result->tm_mday == 26
+            && result->tm_mon == 10 - 1
+            && result->tm_year == 2017 - 1900
+            && result->tm_wday == 4
+            && result->tm_yday == 298
+            && result->tm_isdst == 0))
+        {
+          fprintf (stderr, "thread1 disturbed by thread2!\n"); fflush (stderr);
+          abort ();
+        }
+    }
+
+  /*NOTREACHED*/
+}
+
+static void *
+thread2_func (void *arg)
+{
+  for (;;)
+    {
+      time_t t = 2000050005; /* 2033-05-18 17:26:45 */
+      struct tm tm;
+      struct tm *result = gmtime_r (&t, &tm);
+      ASSERT (result == &tm);
+      if (!(result->tm_sec == 45
+            && result->tm_min == 26
+            && result->tm_hour == 17
+            && result->tm_mday == 18
+            && result->tm_mon == 5 - 1
+            && result->tm_year == 2033 - 1900
+            && result->tm_wday == 3
+            && result->tm_yday == 137
+            && result->tm_isdst == 0))
+        {
+          fprintf (stderr, "thread2 disturbed by thread1!\n"); fflush (stderr);
+          abort ();
+        }
+    }
+
+  /*NOTREACHED*/
+}
+
+int
+main (int argc, char *argv[])
+{
+  /* Create the threads.  */
+  gl_thread_create (thread1_func, NULL);
+  gl_thread_create (thread2_func, NULL);
+
+  /* Let them run for 1 second.  */
+  {
+    struct timespec duration;
+    duration.tv_sec = (argc > 1 ? atoi (argv[1]) : 1);
+    duration.tv_nsec = 0;
+
+    nanosleep (&duration, NULL);
+  }
+
+  return 0;
+}
+
+#else
+
+/* No multithreading available.  */
+
+#include <stdio.h>
+
+int
+main ()
+{
+  fputs ("Skipping test: multithreading not enabled\n", stderr);
+  return 77;
+}
+
+#endif
diff --git a/tests/test-gmtime_r.c b/tests/test-gmtime_r.c
new file mode 100644
index 0000000000..01eebf3c29
--- /dev/null
+++ b/tests/test-gmtime_r.c
@@ -0,0 +1,56 @@
+/* Test gmtime_r().
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2024.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <time.h>
+
+#include <string.h>
+
+#include "macros.h"
+
+int
+main (void)
+{
+  {
+    time_t t = 1509000003; /* 2017-10-26 06:40:03 */
+    struct tm tm;
+    struct tm *result = gmtime_r (&t, &tm);
+    ASSERT (result == &tm);
+    ASSERT (result->tm_sec == 3);
+    ASSERT (result->tm_min == 40);
+    ASSERT (result->tm_hour == 6);
+    ASSERT (result->tm_mday == 26);
+    ASSERT (result->tm_mon == 10 - 1);
+    ASSERT (result->tm_year == 2017 - 1900);
+    ASSERT (result->tm_wday == 4);
+    ASSERT (result->tm_yday == 298);
+    ASSERT (result->tm_isdst == 0);
+#if HAVE_STRUCT_TM_TM_GMTOFF /* glibc, musl, macOS, FreeBSD, NetBSD, OpenBSD, Minix, Cygwin, Android */
+    ASSERT (result->tm_gmtoff == 0);
+#endif
+#if HAVE_STRUCT_TM_TM_ZONE /* glibc, musl, macOS, FreeBSD, NetBSD, OpenBSD, Minix, Cygwin, Android */
+    printf ("tm_zone = %s\n", result->tm_zone == NULL ? "(null)" : result->tm_zone);
+    ASSERT (strcmp (result->tm_zone, "GMT") == 0 /* glibc, NetBSD, OpenBSD, Minix, Cygwin, Android */
+            || strcmp (result->tm_zone, "UTC") == 0 /* musl, macOS, FreeBSD */);
+#endif
+  }
+
+  return 0;
+}
diff --git a/tests/test-localtime_r-mt.c b/tests/test-localtime_r-mt.c
new file mode 100644
index 0000000000..ae371a8946
--- /dev/null
+++ b/tests/test-localtime_r-mt.c
@@ -0,0 +1,139 @@
+/* Multithread-safety test for localtime_r().
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2024.  */
+
+#include <config.h>
+
+/* Work around GCC bug 44511.  */
+#if 4 < __GNUC__ + (3 <= __GNUC_MINOR__)
+# pragma GCC diagnostic ignored "-Wreturn-type"
+#endif
+
+#if USE_ISOC_THREADS || USE_POSIX_THREADS || USE_ISOC_AND_POSIX_THREADS || USE_WINDOWS_THREADS
+
+/* Specification.  */
+#include <time.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "glthread/thread.h"
+#include "macros.h"
+
+
+/* Some common time zone name.  */
+
+#if defined _WIN32 && !defined __CYGWIN__
+/* Cf. <https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/default-time-zones>
+   or <https://ss64.com/timezones.html>  */
+# define FRENCH_TZ  "Romance Standard Time"
+#else
+/* Cf. <https://en.wikipedia.org/wiki/List_of_tz_database_time_zones>  */
+# define FRENCH_TZ  "Europe/Paris"
+#endif
+
+
+static void *
+thread1_func (void *arg)
+{
+  for (;;)
+    {
+
+      time_t t = 1178467200; /* 2007-05-06 18:00:00 */
+      struct tm tm;
+      struct tm *result = localtime_r (&t, &tm);
+      ASSERT (result == &tm);
+      if (!(result->tm_sec == 0
+            && result->tm_min == 0
+            && result->tm_hour == 18
+            && result->tm_mday == 6
+            && result->tm_mon == 5 - 1
+            && result->tm_year == 2007 - 1900
+            && result->tm_wday == 0
+            && result->tm_yday == 125
+            && result->tm_isdst == 1))
+        {
+          fprintf (stderr, "thread1 disturbed by thread2!\n"); fflush (stderr);
+          abort ();
+        }
+    }
+
+  /*NOTREACHED*/
+}
+
+static void *
+thread2_func (void *arg)
+{
+  for (;;)
+    {
+      time_t t = 1336320000; /* 2012-05-06 18:00:00 */
+      struct tm tm;
+      struct tm *result = localtime_r (&t, &tm);
+      ASSERT (result == &tm);
+      if (!(result->tm_sec == 0
+            && result->tm_min == 0
+            && result->tm_hour == 18
+            && result->tm_mday == 6
+            && result->tm_mon == 5 - 1
+            && result->tm_year == 2012 - 1900
+            && result->tm_wday == 0
+            && result->tm_yday == 126
+            && result->tm_isdst == 1))
+        {
+          fprintf (stderr, "thread2 disturbed by thread1!\n"); fflush (stderr);
+          abort ();
+        }
+    }
+
+  /*NOTREACHED*/
+}
+
+int
+main (int argc, char *argv[])
+{
+  setenv ("TZ", FRENCH_TZ, 1);
+
+  /* Create the threads.  */
+  gl_thread_create (thread1_func, NULL);
+  gl_thread_create (thread2_func, NULL);
+
+  /* Let them run for 1 second.  */
+  {
+    struct timespec duration;
+    duration.tv_sec = (argc > 1 ? atoi (argv[1]) : 1);
+    duration.tv_nsec = 0;
+
+    nanosleep (&duration, NULL);
+  }
+
+  return 0;
+}
+
+#else
+
+/* No multithreading available.  */
+
+#include <stdio.h>
+
+int
+main ()
+{
+  fputs ("Skipping test: multithreading not enabled\n", stderr);
+  return 77;
+}
+
+#endif
diff --git a/tests/test-localtime_r.c b/tests/test-localtime_r.c
new file mode 100644
index 0000000000..70ec3b5d4f
--- /dev/null
+++ b/tests/test-localtime_r.c
@@ -0,0 +1,167 @@
+/* Test localtime_r().
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2024.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <time.h>
+
+#include <string.h>
+
+#include "macros.h"
+
+
+/* Some common time zone name.  */
+
+#if defined _WIN32 && !defined __CYGWIN__
+/* Cf. <https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/default-time-zones>
+   or <https://ss64.com/timezones.html>  */
+# define FRENCH_TZ  "Romance Standard Time"
+#else
+/* Cf. <https://en.wikipedia.org/wiki/List_of_tz_database_time_zones>  */
+# define FRENCH_TZ  "Europe/Paris"
+#endif
+
+
+int
+main (void)
+{
+  setenv ("TZ", FRENCH_TZ, 1);
+
+  /* Note: The result->tm_gmtoff values and the result->tm_zone values are the
+     same (3600, "CET" or 7200, "CEST") across all tested platforms:
+     glibc, musl, macOS, FreeBSD, NetBSD, OpenBSD, Minix, Cygwin, Android.  */
+
+  /* A time point when DST was in effect.  */
+  {
+    time_t t = 1178467200; /* 2007-05-06 18:00:00 */
+    struct tm tm;
+    struct tm *result = localtime_r (&t, &tm);
+    ASSERT (result == &tm);
+    ASSERT (result->tm_sec == 0);
+    ASSERT (result->tm_min == 0);
+    ASSERT (result->tm_hour == 18);
+    ASSERT (result->tm_mday == 6);
+    ASSERT (result->tm_mon == 5 - 1);
+    ASSERT (result->tm_year == 2007 - 1900);
+    ASSERT (result->tm_wday == 0);
+    ASSERT (result->tm_yday == 125);
+    ASSERT (result->tm_isdst == 1);
+#if HAVE_STRUCT_TM_TM_GMTOFF
+    ASSERT (result->tm_gmtoff == (1 + result->tm_isdst) * 3600);
+#endif
+#if HAVE_STRUCT_TM_TM_ZONE
+    printf ("tm_zone = %s\n", result->tm_zone == NULL ? "(null)" : result->tm_zone);
+    ASSERT (strcmp (result->tm_zone, "CEST") == 0);
+#endif
+  }
+
+  /* 1 second before and 1 second after the DST interval started.  */
+  {
+    time_t t = 1174784399; /* 2007-03-25 01:59:59 */
+    struct tm tm;
+    struct tm *result = localtime_r (&t, &tm);
+    ASSERT (result == &tm);
+    ASSERT (result->tm_sec == 59);
+    ASSERT (result->tm_min == 59);
+    ASSERT (result->tm_hour == 1);
+    ASSERT (result->tm_mday == 25);
+    ASSERT (result->tm_mon == 3 - 1);
+    ASSERT (result->tm_year == 2007 - 1900);
+    ASSERT (result->tm_wday == 0);
+    ASSERT (result->tm_yday == 83);
+    ASSERT (result->tm_isdst == 0);
+#if HAVE_STRUCT_TM_TM_GMTOFF
+    ASSERT (result->tm_gmtoff == (1 + result->tm_isdst) * 3600);
+#endif
+#if HAVE_STRUCT_TM_TM_ZONE
+    printf ("tm_zone = %s\n", result->tm_zone == NULL ? "(null)" : result->tm_zone);
+    ASSERT (strcmp (result->tm_zone, "CET") == 0);
+#endif
+  }
+  {
+    time_t t = 1174784401; /* 2007-03-25 03:00:01 */
+    struct tm tm;
+    struct tm *result = localtime_r (&t, &tm);
+    ASSERT (result == &tm);
+    ASSERT (result->tm_sec == 1);
+    ASSERT (result->tm_min == 0);
+    ASSERT (result->tm_hour == 3);
+    ASSERT (result->tm_mday == 25);
+    ASSERT (result->tm_mon == 3 - 1);
+    ASSERT (result->tm_year == 2007 - 1900);
+    ASSERT (result->tm_wday == 0);
+    ASSERT (result->tm_yday == 83);
+    ASSERT (result->tm_isdst == 1);
+#if HAVE_STRUCT_TM_TM_GMTOFF
+    ASSERT (result->tm_gmtoff == (1 + result->tm_isdst) * 3600);
+#endif
+#if HAVE_STRUCT_TM_TM_ZONE
+    printf ("tm_zone = %s\n", result->tm_zone == NULL ? "(null)" : result->tm_zone);
+    ASSERT (strcmp (result->tm_zone, "CEST") == 0);
+#endif
+  }
+
+  /* 1 second before and 1 second after the DST interval ended.  */
+  {
+    time_t t = 1193533199; /* 2007-10-28 02:59:59 */
+    struct tm tm;
+    struct tm *result = localtime_r (&t, &tm);
+    ASSERT (result == &tm);
+    ASSERT (result->tm_sec == 59);
+    ASSERT (result->tm_min == 59);
+    ASSERT (result->tm_hour == 2);
+    ASSERT (result->tm_mday == 28);
+    ASSERT (result->tm_mon == 10 - 1);
+    ASSERT (result->tm_year == 2007 - 1900);
+    ASSERT (result->tm_wday == 0);
+    ASSERT (result->tm_yday == 300);
+    ASSERT (result->tm_isdst == 1);
+#if HAVE_STRUCT_TM_TM_GMTOFF
+    ASSERT (result->tm_gmtoff == (1 + result->tm_isdst) * 3600);
+#endif
+#if HAVE_STRUCT_TM_TM_ZONE
+    printf ("tm_zone = %s\n", result->tm_zone == NULL ? "(null)" : result->tm_zone);
+    ASSERT (strcmp (result->tm_zone, "CEST") == 0);
+#endif
+  }
+  {
+    time_t t = 1193533201; /* 2007-10-28 02:00:01 */
+    struct tm tm;
+    struct tm *result = localtime_r (&t, &tm);
+    ASSERT (result == &tm);
+    ASSERT (result->tm_sec == 1);
+    ASSERT (result->tm_min == 0);
+    ASSERT (result->tm_hour == 2);
+    ASSERT (result->tm_mday == 28);
+    ASSERT (result->tm_mon == 10 - 1);
+    ASSERT (result->tm_year == 2007 - 1900);
+    ASSERT (result->tm_wday == 0);
+    ASSERT (result->tm_yday == 300);
+    ASSERT (result->tm_isdst == 0);
+#if HAVE_STRUCT_TM_TM_GMTOFF
+    ASSERT (result->tm_gmtoff == (1 + result->tm_isdst) * 3600);
+#endif
+#if HAVE_STRUCT_TM_TM_ZONE
+    printf ("tm_zone = %s\n", result->tm_zone == NULL ? "(null)" : result->tm_zone);
+    ASSERT (strcmp (result->tm_zone, "CET") == 0);
+#endif
+  }
+
+  return 0;
+}
-- 
2.34.1

Re: localtime on native Windows

[Brian Inglis]

On Sun, 11 Feb 2024 13:46:50 +0100, Bruno Haible wrote on bug-gnulib:
> I wrote:
>> For these modules, the next function to provide in an MT-safe way is
>> localtime_r.
> 
> Our gmtime_r and localtime_r are MT-safe on native Windows. I ran the
> test-gmtime_r-mt and test-localtime_r-mt tests for 2000 seconds each, and
> they did not crash.
> 
> But the problem is that localtime() and localtime_r() on native Windows
> produce nonsensical results:
>   - They pretend that in France, in 2007, DST began on 2007-03-11. When in
>     fact, it started on 2007-03-25.
>   - The hour is wrong.
> Witness: The attached program loc.c.
> 
>> On native Windows, when the 'localtime_s' function [1][2]
>> is not available, such as on the older Windows versions that Emacs cares
>> about, the solution is to use GetTimeZoneInformation [3].
> 
> None of the GetTimeZoneInformation APIs from the Windows DLLs works either.
> They pretend that in German and French time zones, DST starts on March 5,
> in all years. Witness: The attached program tzi.c.
> 
> So, there is no way around implementing a correct localtime_r, based on
> tzdata, in Gnulib.
> 
> It will be useful
>   - for localtime_r on native Windows,
>   - for nstrftime, c_nstrftime, parse-datetime, which all take a timezone_t
>     argument.
> 
> For reading tzdata: The first question is how to include tzdata in gnulib.
>   - AFAICS, the main data file (without comments) is tzdata.zi and is about
>     100 KB large. It can be upgraded simply by copying the newest tzdata.zi
>     from a newer tzdata distribution. Including such a file in gnulib would
>     be OK (re copyright, number of files, total size), right?
>   - Whereas including all files from /usr/share/zoneinfo is probably not
>     acceptable (> 1300 files, ca. 6 MB total size).
>   - Access pattern: In a running program, very few among the time zones will
>     be used. Therefore, caching in memory is essential.

I was looking around to see if there was some way to leverage Windows UWP/.NET 
tzdata provided in:

	https://github.com/microsoft/icu

https://github.com/search?q=repo%3Amicrosoft%2Ficu%20tzdata&type=code

and noticed that some comments under:

https://learn.microsoft.com/en-ca/dotnet/core/extensions/globalization-icu#icu-on-webassembly

suggest that required data can be shrunk to ~300KB using Brotli!?

Perhaps gnulib could leverage some of the interfaces in:

	https://github.com/sillsdev/icu-dotnet

as an alternative interface to access tzdata on native Windows or .NET 5+ Core 
platforms, without building in tzdata and requiring updates?

Other options would be for some selection of generated data such as using 
zonenow.tab, supporting only the current time onward, for the 449 time zones 
currently in:

https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml

perhaps using CBOR format and/or Brotli compression to improve on using a zip or 
tar container format:

	470KB -> 221KB zip, 134KB tar.gz, 69KB tar.lz, 68KB tar.xz;

or using the usual built-in POSIX TZ parser and the POSIX TZ rules output by:

	tail -1 /usr/share/zoneinfo/**/*

for the selected time zones, including time zone ids and rules:

	12.6KB, 4KB zip, 3.8KB gz, 3.5KB xz/lz/bz2.

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer     but when there is no more to cut
                                  -- Antoine de Saint-Exupéry

Re: localtime on native Windows

[Bruno Haible]

Brian Inglis wrote:
> I was looking around ... that required data can be shrunk to ~300KB using Brotli!?

Whereas the entire tzdata.zi (without comments, and with abbreviations) is
only around 100 KB. I'll definitely prefer the latter.

> Other options would be for some selection of generated data such as using 
> zonenow.tab, supporting only the current time onward

This is a simplification that one may do when working on an embedded system
or such. But not in gnulib. When the entire data file is 100 KB, it is not
a good compromise to give up correctness for dates in the past, in exchange
for further data size reduction.

> https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml

This file is interesting for its mapping table from Windows time zones.

Bruno

Re: localtime on native Windows

[Brian.Inglis]

On 2024-02-17 19:14, Bruno Haible wrote:
> Brian Inglis wrote:
>> I was looking around ... that required data can be shrunk to ~300KB using Brotli!?
> 
> Whereas the entire tzdata.zi (without comments, and with abbreviations) is
> only around 100 KB. I'll definitely prefer the latter.
> 
>> Other options would be for some selection of generated data such as using
>> zonenow.tab, supporting only the current time onward
> 
> This is a simplification that one may do when working on an embedded system
> or such. But not in gnulib. When the entire data file is 100 KB, it is not
> a good compromise to give up correctness for dates in the past, in exchange
> for further data size reduction.
> 
>> https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml
> 
> This file is interesting for its mapping table from Windows time zones.

My point was that this data is already built into Windows, and accessible via 
its ICU API, so gnulib does not need to include, update, and compile tzdata to 
support Windows.

It just has to adapt its localization, as does Cygwin, which uses the Windows 
zones to look up tz ids, which it uses to load tzdata to provide POSIX 
interfaces, using otherwise mostly BSD code adapted from tzcode, required by 
newlib to allow commercial and other uses by RedHat and other embedded vendors.

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer     but when there is no more to cut
                                 -- Antoine de Saint-Exupéry