I have a setup working using localtime() to get a tm with the local times represented in it. And that is all good.
However, if I change timezone while the application is running, it does not notice that I have changed timezones.
Is there some way to tell it to 'go look again' to refresh to the system timezone?
I know this is probably not a common case, but it is what test are doing to test this feature, so they want it supported!
Take a look at tzset (this is posix only). This might give you what you need. If your TZ environment variable is unset, it should reinitialize from the OS.
From the man page:
DESCRIPTION
The tzset() function initializes the tzname variable from the TZ environment variable. This function is automatically called by the
other time conversion functions that depend on the time zone. In a
SysV-like environment it will also set the variables timezone
(seconds West of GMT) and daylight (0 if this time zone does not
have any daylight savings time rules, non-zero if there is a time
during the year when daylight savings time applies).
If the TZ variable does not appear in the environment, the tzname variable is initialized with the best approximation of local
wall clock time, as specified by the tzfile(5)-format file localtime
found in the system timezone directory (see below). (One also often
sees /etc/localtime used here, a symlink to the right file in the
system timezone directory.)
A simple test:
#include <iostream>
#include <time.h>
#include <stdlib.h>
int main()
{
tzset();
time_t t;
time(&t);
std::cout << "tz: " << tzname[0] << " - " << tzname[1] << " " << ctime(&t) << std::endl;
setenv("TZ", "EST5EDT", 1);
tzset();
std::cout << "tz: " << tzname[0] << " - " << tzname[1] << " " << ctime(&t) << std::endl;
return 0;
}
Gives me output:
tz: CST - CDT Wed Jan 11 12:35:02 2012
tz: EST - EDT Wed Jan 11 13:35:02 2012
There's nothing in the standard library to do this. Unless your platform offers some extension to the library for updating the time zone, your program's calls to localtime() will probably always use the time zone that was active at program start up.
You could probably work around that by putting the localtime stuff in a separate process that your main program can startup and shutdown at will, thus re-initializing that process's time zone.
Or instead your platform may offer some other API for getting the local time that will reflect changes in the system time zone.
Related
In Python or Java you can get the UTC offset (at present time) given the IANA name of the timezone ("America/Los_Angeles" for instance). See Get UTC offset from time zone name in python for example.
How can you do the same using C/C++ on Ubuntu 14.04?
EDIT: Preferably in a thread-safe way (no environment variables).
You alluded to this fact, but it's important to note that the offset between UTC and the time in a time zone is not necessarily constant. If the time zone performs daylight saving (summer) time adjustments, the offset will vary depending on the time of year.
One way to find the offset is to take the time you're interested in, hand it to the localtime() function, then look at the tm_gmtoff field. Do this with the TZ environment variable set to the zone name you're interested in. Here's an example that does so for the current time:
#include <time.h>
#include <stdio.h>
int main()
{
setenv("TZ", "America/Los_Angeles", 1);
time_t t = time(NULL);
struct tm *tmp = localtime(&t);
printf("%ld\n", tmp->tm_gmtoff);
}
At the moment this prints -25200, indicating that Los Angeles is 25200 seconds, or 420 minutes, or 7 hours west of Greenwich. But next week (actually tomorrow) the U.S goes off of DST, at which point this code will start printing -28800.
This isn't guaranteed to work, since the tm_gmtoff field is not portable. But I believe all versions of Linux will have it. (You might have to compile with -D_BSD_SOURCE or something, or refer to the field as __tm_gmtoff. But in my experience it tends to work by default, as plain tm_gmtoff.)
The other way is to go back and forth with gmtime and mktime, as described in Sam Varshavchik's answer.
Addendum: You asked about not using environment variables. There is a way, but unfortunately it's even less standard. There are BSD functions tzalloc and localtime_rz which do the job, but they do not exist on Linux. Here's how the code looks:
timezone_t tz = tzalloc("America/Los_Angeles");
if(tz == NULL) return 1;
time_t t = time(NULL);
struct tm tm, *tmp = localtime_rz(tz, &t, &tm);
printf("%ld\n", tmp->tm_gmtoff);
For me this prints -28800 (because PDT fell back to PST just a few minutes ago).
If you had it, you could also use localtime_rz along with mktime in Sam Varshavchik's answer. And of course Howard Hinnant's library is pretty obviously thread-safe, not requiring mucking with TZ at all.
EDIT (OP): The code for localtime_rz and tzalloc can be downloaded from https://www.iana.org/time-zones and works on Ubuntu 14.04.
You could use this free open source C++11/14 library to do it like this:
#include "chrono_io.h"
#include "tz.h"
#include <iostream>
int
main()
{
using namespace date;
using namespace std::chrono;
auto zt = make_zoned("America/Los_Angeles", system_clock::now());
std::cout << zt.get_info().offset << '\n';
}
This currently outputs:
-25200s
Or you could format it differently like this:
std::cout << make_time(zt.get_info().offset) << '\n';
which currently outputs:
-07:00:00
The factory function make_zoned creates a zoned_time using the IANA name "America/Los_Angeles" and the current time from std::chrono::system_clock. A zoned_time has a member getter to get the information about the timezone at that time. The information is a type called sys_info which contains all kinds of useful information, including the current UTC offset.
The UTC offset is stored as a std::chrono::seconds. The header "chrono_io.h" will format durations for streaming. Or the make_time utility can be used to format the duration into hh:mm:ss.
The program above is thread-safe. You don't have to worry about some other process changing TZ out from under you, or changing the current time zone of the computer in any other way. If you want information about the current time zone, that is available too, just use current_zone() in place of "America/Los_Angeles".
If you wanted to explore other times, that is just as easy. For example beginning at Nov/6/2016 at 2am local time:
auto zt = make_zoned("America/Los_Angeles", local_days{nov/6/2016} + 2h);
The output changes to:
-28800s
-08:00:00
More information about this library was presented at Cppcon 2016 and can be viewed here:
https://www.youtube.com/watch?v=Vwd3pduVGKY
Use gmtime(), first, to convert the current epoch time into UTC time, then use mktime() to recalculate the epoch time, then compare the result to the real epoch time.
gmtime() calculates the struct tm in UTC, while mktime() assumes that the struct tm represents the current local calendar time. So, by making this round-about calculation, you indirectly figure out the current timezone offset.
Note that mktime() can return an error if struct tm cannot be convert to epoch time, which will happen during certain transitions between standard time and alternate time. It's up to you to figure out what that means, in your case.
The recipe looks something like this:
time_t t = time(NULL);
struct tm *tmp = gmtime(&t);
time_t t2 = mktime(tmp);
int offset = t - t2;
See the documentation of these library functions for more information.
To use a specific time zone, either set the TZ environment variable, or you can try using localtime_rz as in Steve Summit's answer. As mentioned, beware that mktime can sometimes return -1 for unconvertible times.
Have a Unix timestamp and I need equivalents of localtime_s, but where I can pass in the timezone. Alternately I'm looking for a way to change the timezone programatically for the app (not the whole system), so that localtime_s returns the correct values.
Strangely enough, if I set TZ environment variable in a shell to GMT, and then launch my application, localtime_s returns values in GMT.
I've tried:
::SetEnvironmentVariable("TZ", "GMT");
tzset();
But it does not change the results of later calls to localtime_s
Note that I'm running the application on Windows 7, using VS 2013.
If you are willing to use a free, open-source 3rd party library, this works on VS-2013. There are many examples on how to use it here:
https://github.com/HowardHinnant/date/wiki/Examples-and-Recipes
For example here is how to output the current time in New York and Chicago:
#include "tz.h"
#include <iostream>
int
main()
{
auto now = std::chrono::system_clock::now();
std::cout << "Current time in New York: "
<< date::make_zoned("America/New_York", now) << '\n';
std::cout << "Current time in Chicago : "
<< date::make_zoned("America/Chicago", now) << '\n';
}
This just output for me:
Current time in New York: 2016-08-12 10:36:24.670966 EDT
Current time in Chicago : 2016-08-12 09:36:24.670966 CDT
I have been wracking my head crazy trying to figure this out with this API
My original implementation was something like:
// TimezonePtr is just a share_ptr to the timezone
std::tm getGMT(const std::tm& rawtime, TimezonePtr tz)
{
std::tm result = rawtime;
const auto loct = mktime_z(tz.get(), &result);
gmtime_r(&loct, &result);
return result;
}
However, this does not take into account DST. For example, if I feed it a date of Sep 28 2012 15:54:24 I get back Sep 28 2012 20:54:24, which is incorrect. It looks like I want to use localtime_rz, except that takes an epoch, which is driving me nuts because if I could get the epoch then I'd already have my answer. :(
How can I accomplish this?
mktime_z takes a struct tm as one of its arguments. If you don't know whether DST is in effect for the input date, you want to set the tm_isdst member of that tm to -1 to signify that the system should figure out whether DST is in effect for that date/time/timezone when you call mktime.
At least for me, this seems to work correctly (i.e., it correctly concludes that at least in my time zone, DST was in effect in September of 2012).
In addition to Jerry Coffin's correct (and up-voted) answer, I wanted to show how this computation could be done with a modern C++11/14 library (free and open source).
I've kept the API the same in the interest of making the code easy to compare:
template <class Duration>
auto
getGMT(date::local_time<Duration> rawtime, const date::time_zone* tz)
{
return tz->to_sys(rawtime);
}
This returns a std::chrono::time_point<system_clock, Duration> where Duration is the finer of the input Duration and seconds. If the ragtime doesn't have a unique mapping to UTC according to the indicated time zone, an exception will be thrown. Such an event can occur (for example) if rawtime is during a daylight saving transition and occurs twice, or not at all. If desired, there exists API for avoiding the exception if you want to "pre-decide" how you would like to map ambiguous and non-existent local times into UTC.
This function can be exercised like this:
#include "tz.h"
#include <iostream>
int
main()
{
using namespace date;
using namespace std::chrono_literals;
std::cout << getGMT(local_days{sep/28/2012} + 15h + 54min + 24s,
current_zone()) << " UTC\n";
std::cout << getGMT(local_days{sep/28/2012} + 15h + 54min + 24s,
locate_zone("America/New_York")) << " UTC\n";
}
This exercises the code twice:
With whatever the current time zone is for this computer.
With the time zone "America/New_York"
For me these are both the same time zone, resulting in the following output:
2012-09-28 19:54:24 UTC
2012-09-28 19:54:24 UTC
Not only is it simpler to use this library than the BSD ctime API,
but this API has type safety. For example the UTC time point and local time point are represented by different types, allowing the compiler to tell you if you accidentally use the wrong one. In contrast the BSD ctime API uses the same type (tm) for both local time and UTC.
I am writing a Linux (Ubuntu and Debian Lenny) application in C++.
Now I need to know the distance/offset between UTC and the currently set system time at a given day in the past. Since I need to convert recorded data, I the distance need to be calculated with respect to a date in the past (which may have a different DST setting than the current day).
Anyone know how to do this?
Edit: Reading the first answer I think I was misunderstood: I do not want to compare dates/times. I have date/time values which I want to convert from UTC to local time.
Prepare the tm structure with date:
struct tm date;
memset(&date,0,sizeof(date));
date.tm_year = year - 1900;
date.tm_mon = month - 1;
date.tm_mday = day;
date.tm_hour = hour;
date.tm_min = minute;
date.tm_sec = second;
date.tm_isdst = -1; // VERY IMPORTANT
mktime(&date); /// fill rest of fields
And then take a look on tm_gmtoff
printf("%d\n",date.tm_gmtoff);
This is distance from UTC.
Now this is Linux and BSD specific, it would not work on other stystems, and this works
with respect to DST.
Read man mktime for more information. And filling struct tm with correct values
P.S.: Converting from UTC to Local and back?
time_t posix_time = timegm(&UTC_Filled_struct_tm); // Linux specific line
localtime_r(&posix_time,&local_Filled_struct_tm);
Local to UTC
time_t posix_time = mktime(&local_Filled_struct_tm);
gmtime_r(&posix_time,&UTC_Filled_struct_tm);
I think you may benefit from using Boost.DateTime or ICU.
As for Boost.DateTime tt might be like this:
1) You prepare a database with timezone information Boost.Datetime and create a timezone. Timezones are important since they hold information about DST
tz_database tz_db;
tz_db.load_from_file("./date_time_zonespec.csv");
time_zone_ptr nyc = tz_db.time_zone_from_region("America/New_York"); // or other timezone
Or just create a timezone like this.
std::string kaliningrad_string = "EET+02:00:00EEST+01:00:00,M3.5.0/02:00:00,M10.5.0/03:00:00";
boost::local_time::time_zone_ptr kaliningrad_tzone_posix(new boost::local_time::posix_time_zone(kaliningrad_string));
std::string vladivostok_string = "VLAT+10:00:00VLAST+01:00:00,M3.5.0/02:00:00,M10.5.0/03:00:00";
boost::local_time::time_zone_ptr vladivostok_tzone_posix(new boost::local_time::posix_time_zone(vladivostok_string));
Creating timezones using a string specification of timezones looks more difficult but you can make use of it if you can't find a particular timezone in date_time_zonespec.csv.
For example Samara used to be in UTC+4 before March 2010 and now it is in UTC+3. date_time_zonespec.csv doesn't have history of changes, so in this situation it is necessary to create a timezone out of the string specification. However I recall that ICU seems to have timezones with this sort of history ICU TimeZone Classes:
Time zone data changes often in
response to governments around the
world changing their local rules and
the areas where they apply. The ICU
time zone data is updated for each
release, and the easiest way to stay
up to date may be to upgrade to the
latest ICU release, which also
provides bug fixes, code improvements
and additional features.
3) Make a localtime you need, for example
local_date_time tmp(boost::gregorian::date(2010, 3, 28), boost::posix_time::time_duration(1,59,0),nyc, boost::local_time::local_date_time::EXCEPTION_ON_ERROR);
4) And then calculate difference using functions utc_time and local_time
There is an example on that page:
ptime pt(date(2004,Nov,5),
hours(10));
time_zone_ptr zone(new posix_time_zone("MST-07"));
local_date_time az(pt, zone);
az.utc_time(); // 10am 2004-Nov-5
az.local_time(); // 3am 2004-Nov-5
5) Another example. The local time is the same but UTC is different
local_date_time tmp(boost::gregorian::date(2010, 3, 28), boost::posix_time::time_duration(1,59,0),kaliningrad_tzone_posix, boost::local_time::local_date_time::EXCEPTION_ON_ERROR);
std::cout << "As is: " << tmp << ", UTC: " << tmp.utc_time() << std::endl;
local_date_time tmp(boost::gregorian::date(2010, 3, 28), boost::posix_time::time_duration(1,59,0),vladivostok_tzone_posix, boost::local_time::local_date_time::EXCEPTION_ON_ERROR);
std::cout << "As is: " << tmp << ", UTC: " << tmp.utc_time() << std::endl;
Make sure all the times are converted to UTC (see mktime() for example). You then can use either difftime() or the timeval_subtract example function from the same link, depending on what structure the times are stored in.
i m new to c.I want to FTP to a system and get the timezone of that system.in c++
There is no way in the FTP standard to do this. You could try uploading a new (small) file and check its date-time as reported by FTP, compared to current local time.
New answer for old question.
Rationale for revisiting this: We have better tools now.
This free, open-source, portable library will get the current time zone setting in terms of the IANA time zone database. On Windows it will convert the Windows timezone into an IANA time zone. Fully documented.
Here's how you print out the name of the current time zone:
#include "tz.h"
#include <iostream>
int
main()
{
std::cout << date::current_zone()->name() << '\n';
}
For me this currently outputs:
America/New_York
You can convert a std::chrono::system_clock to a local time and print it out:
std::cout << date::make_zoned(date::current_zone(),
std::chrono::system_clock::now()) << '\n';
which just output for me:
2016-07-16 19:24:00.447015 EDT
See the github wiki for many more examples.