get current time in specific timezone using c++ - c++

My code should work on linux & windlows as well.
I want to get current time in YYYY-MM-DD HH24:MI:SS. Default timezone is UTC+08, where as my system can be located in any timezone.
It would be great help if you can help me with c++ code (I do not have c++11 , 14 compiler)
I saw one solution - use time to get the current time in UTC and then manipulate the TZ environment variable to your destination timezone. Then use localtime_r to convert to that timezone's local time.
But not sure how to achieve this with c++ that will work for Windows and linux.

I suggest looking into the boost library boost/date_time/posix_time/posix_time.hpp.
From there you can then simply get the current local time like so:
boost::posix_time::ptime curr_time = boost::posix_time::microsec_clock::local_time();
And it has methods to turn it into a string as required:
std::string curr_time_str = to_simple_string(curr_time);
And back to the ptime object:
curr_time = boost::posix_time::time_from_string(curr_time_str);
etc.
http://www.boost.org/doc/libs/1_61_0/doc/html/date_time/posix_time.html

Should work on most platforms:
int main(int argc, const char * argv[])
{
time_t ts = 0;
struct tm t;
char buf[16];
::localtime_r(&ts, &t);
::strftime(buf, sizeof(buf), "%z", &t);
std::cout << "Current timezone: " << buf << std::endl;
::strftime(buf, sizeof(buf), "%Z", &t);
std::cout << "Current timezone: " << buf << std::end;
...
}

Related

Microseconds epoch to a Date string using chrono

I have been looking around to get what I want but I couldn't find anything hence my question (hopefully not a duplicate!)
I am looking to get a microsecond resolution epoch time (to be converted to a Date string) of the clock perhaps using chrono.
Following is what works for me for seconds resolution:
auto secondsEpochTime = std::chrono::duration_cast<std::chrono::seconds>(std::chrono::system_clock::now().time_since_epoch()).count();
std::cout << "Date string = " << ctime(&secondsEpochTime);
However when I change seconds to microseconds, ctime doesn't seem to reflect the correct date.
auto microSecondsEpochTime = std::chrono::duration_cast<std::chrono::microseconds>(std::chrono::system_clock::now().time_since_epoch()).count();
std::cout << "Date string = " << ctime(&microSecondsEpochTime); // incorrect Date
Unfortunately std::chrono is not complete to provide a full answer to your question. You will have to use parts of the C library until C++23 at least otherwise you might end up with a race-prone implementation.
The idea is to get the timestamp and convert it to an integer as microseconds since epoch (1970-01-01).
Then use localtime_r to get the local time broken down in year/month/day/hour/minute/seconds and print it to string.
Finally append the milliseconds as an int padded to 3 digits and return the entire result as an std::string.
constexpr static int64_t ONEMICROSECOND = 1000000;
static std::string nowstr() {
auto now = std::chrono::system_clock::now();
auto onems = std::chrono::microseconds(1);
int64_t epochus = now.time_since_epoch()/onems;
time_t epoch = epochus/ONEMICROSECOND;
struct tm tms{};
localtime_r( &epoch, &tms );
char buf[128];
size_t nb = strftime( buf, sizeof(buf), "%Y-%m-%d %H:%M:%S", &tms );
nb += ::sprintf( &buf[nb], ".%06d", int(epochus%ONEMICROSECOND) );
return std::string( buf, nb );
}
If you run this as-is it will likely return the timestamp in GMT. You will heave to set your timezone programatically if not set in the environment (as it happens with compiler explorer/Godbolt.
int main() {
setenv("TZ", "/usr/share/zoneinfo/America/New_York", 1);
std::cout << nowstr() << std::endl;
}
Results in
Program stdout
2022-10-01 22:51:03.988759
Compiler explorer link: https://godbolt.org/z/h88zhrr73
UPDATE: if you prefer to use boost::format (std::format is still incomplete on most compilers unfortunately) then you can do
static std::string nowstr() {
auto now = std::chrono::system_clock::now();
auto onems = std::chrono::microseconds(1);
int64_t epochus = now.time_since_epoch()/onems;
time_t epoch = epochus/ONEMICROSECOND;
struct tm tms{};
localtime_r( &epoch, &tms );
std::ostringstream ss;
ss << boost::format( "%04d-%02d-%02d %02d:%02d:%02d.%06d" )
% (tms.tm_year+1900) % (tms.tm_mon+1) % tms.tm_mday
% tms.tm_hour % tms.tm_min % tms.tm_sec
% (epochus%ONEMICROSECOND);
return ss.str();
}
You will have to use parts of the C library until C++23 at least
Umm... If your platform supports the full C++20 spec (at least with regards to format and chrono):
#include <chrono>
#include <format>
#include <iostream>
int
main()
{
auto tp = std::chrono::system_clock::now();
std::chrono::zoned_time zt{std::chrono::current_zone(),
std::chrono::time_point_cast<std::chrono::microseconds>(tp)};
std::cout << "Date string = " << std::format("{:%a %b %e %T %Y}", zt) << '\n';
}
Sample output:
Date string = Sat Oct 1 23:32:24.843844 2022

How to check that timezone exists with boost locale

I need to parse time from string (%Y-%M-%d %H:%m:%s) according to some timezone.
My first idea was to try boost::date_time, however it looks like its database is outdated and timezone detection algorithm is wrong in general. So I decided to try boost::locale. It has ICU backend, so timezone support should be good. I use the following code:
namespace as = boost::locale::as;
void foo(std::string time, std::string timezone) {
auto glob = boost::locale::localization_backend_manager::global();
glob.select("icu"); // select icu backend
boost::locale::generator gen{glob};
auto loc = gen.generate(""); // generate locale with boost facets
auto cal = boost::locale::calendar{loc, timezone};
boost::locale::date_time dt{cal};
std::stringstream ss{time};
ss.imbue(loc);
std::cout.imbue(loc);
ss >> as::ftime("%Y-%m-%d %T") >> as::time_zone(timezone) >> dt;
std::cout << as::time_zone("UTC") << dt << std::endl;
std::cout << as::time_zone(timezone) << dt << std::endl;
}
This works well, however if I pass some invalid timezone name ("foo"), the library accepts it, no exception is thrown, the time is parsed as if it is UTC time. That's not good for me, I want to detect this case somehow, so that I can notify user that the result will not be what he/she expects.
My first idea was to check cal.get_time_zone(), but it always returns the string that was passed to constructor ("foo" in my case), no matter if it's valid or not.
Next, I tried to extract calendar_facet from the generated locale, like so:
const auto &icu_cal = std::use_facet<boost::locale::calendar_facet>(loc);
so that I can access an internal abstract_calendar class. Unfortunately, this line doesn't compile. The reason is that boost/locale/generator.hpp has a static constant with the same name (calendar_facet) in the same boost::locale namespace. The compiler reports that it can not instantiate std::use_facet. Maybe I can move it to a separate compilation unit and avoid including generator.hpp header there, but it looks like a hack for me. Is it a bug or I'm missing something here?
Is there a straightforward way how to validate timezone name with boost::locale? Do you recommend it in general? Thanks for your help.
Edit: here is a minimal example of code that doesn't compile for me
#include <boost/locale.hpp>
int main() {
auto my = boost::locale::localization_backend_manager::global();
my.select("icu");
boost::locale::generator gen{my};
std::use_facet<boost::locale::calendar_facet>(gen.generate(""));
return 0;
}
I compile it like so (on ubuntu 16.04, gcc 5.4):
g++ -std=c++14 -L/usr/lib/x86_64-linux-gnu/ test.cpp -lboost_locale -lboost_date_time
Edit 2: With Sehe's help I managed to get calendar facet from locale and now can I check timezone like this:
int main(int argc, char **argv) {
auto my = boost::locale::localization_backend_manager::global();
my.select("icu");
boost::locale::generator gen{my};
auto ptr = std::unique_ptr<boost::locale::abstract_calendar>(std::use_facet<class boost::locale::calendar_facet>(gen.generate("")).create_calendar());
ptr->set_timezone(argv[1]);
// if ICU backend does not recognize timezone, it sets it to Etc/Unknown
if (ptr->get_timezone() != argv[1]) {
std::cout << "bad timezone " << ptr->get_timezone() << std::endl;
} else {
std::cout << "good timezone " << ptr->get_timezone() << std::endl;
}
return 0;
}
Update: while I managed to make boost locale do what I want on linux, I later faced some weird errors when I ported my code to OS X (it looks like mac doesn't have ICU backend by default...). So, I decided to switch to Howard Hinnant's date library instead. This library is of a high quality, works well on both linux and mac, author is helpful and responsive, so highly recommended.
The fix to the non-compiling sample:
Live On Coliru
#include <boost/locale.hpp>
int main() {
auto my = boost::locale::localization_backend_manager::global();
my.select("icu");
boost::locale::generator gen{my};
std::use_facet<class boost::locale::calendar_facet>(gen.generate(""));
}
Here is an alternative timezone library that may be easier to use:
#include "tz.h"
#include <iostream>
#include <sstream>
int
main(int argc, char **argv)
{
try
{
auto tz = date::locate_zone(argv[1]);
std::cout << "good timezone " << tz->name() << std::endl;
date::local_seconds tp;
std::istringstream in{"2017-09-08 11:30:15"};
in >> date::parse("%Y-%m-%d %H:%M:%S", tp);
auto zt = date::make_zoned(tz, tp);
std::cout << date::format("%Y-%m-%d %T %Z which is ", zt);
std::cout << date::format("%Y-%m-%d %T %Z\n", zt.get_sys_time());
}
catch (std::exception const& e)
{
std::cout << "bad timezone " << e.what() << std::endl;
}
}
Sample output 1:
good timezone America/New_York
2017-09-08 11:30:15 EDT which is 2017-09-08 15:30:15 UTC
Sample output 2:
bad timezone America/New_Yor not found in timezone database

How to get date and time string accurate to milliseconds in C++ in Linux?

I want to be able to put into a string the local time and date with millisecond resolution like so:
YYYY-MM-DD hh:mm:ss.sss
Seems like a simple thing to do, but I haven't found a simple answer for how to do this. I am writing in C++ and do have access to 11 compiler but am fine using a C solution if it's cleaner. I found a post here with a solution Get both date and time in milliseconds but surely it can't be that difficult given use of standard libraries. I'm probably going to move forward with that type of solution but was hoping to add to the knowledge base by asking the question here on SO.
I know this will work but again, seems unnecessarily difficult:
#include <sys/time.h>
#include <stdio.h>
int main(void)
{
string sTimestamp;
char acTimestamp[256];
struct timeval tv;
struct tm *tm;
gettimeofday(&tv, NULL);
tm = localtime(&tv.tv_sec);
sprintf(acTimestamp, "%04d-%02d-%02d %02d:%02d:%02d.%03d\n",
tm->tm_year + 1900,
tm->tm_mon + 1,
tm->tm_mday,
tm->tm_hour,
tm->tm_min,
tm->tm_sec,
(int) (tv.tv_usec / 1000)
);
sTimestamp = acTimestamp;
cout << sTimestamp << endl;
return 0;
}
Tried looking at put_time for C++ and strftime for the old C way. Both only allow me to get to second resolution best I can tell. You can see the two approaches I've gotten so far below. I would like to put it into a string
auto t = std::time(nullptr);
auto tm = *std::localtime(&t);
std::cout << std::put_time(&tm, "%Y-%m-%d %H:%M:%S") << std::endl;
time_t rawtime;
struct tm * timeinfo;
char buffer[80];
time (&rawtime);
timeinfo = localtime(&rawtime);
strftime(buffer,sizeof(buffer),"%Y-%m-%d %I:%M:%S",timeinfo);
std::string str(buffer);
std::cout << str;
Only thing I can figure out is to use gettimeofday and get rid of all the data except the last second and append it to the timestamp, still wish there was a cleaner approach.
Anyone find a solution that works better?
I would recommend looking at Howard Hinnant's date library. One of the examples given in the wiki shows how to get the current local time, up to the given precision of your std::chrono::system_clock implementation (nanoseconds on Linux, from memory?):
EDIT: As Howard points out in the comments, you can use date::floor() to obtain the desired precision. So to generate a string as requested in the question, you could do something like this:
#include "tz.h"
#include <iostream>
#include <string>
#include <sstream>
std::string current_time()
{
const auto now_ms = date::floor<std::chrono::milliseconds>(std::chrono::system_clock::now());
std::stringstream ss;
ss << date::make_zoned(date::current_zone(), now_ms);
return ss.str();
}
int main()
{
std::cout << current_time() << '\n';
}

gmtime is giving me some times localtime?

I'm printing events in a multi-thread environment the console is a static mutex shared by all the threads.
The problem is randomly every hundred or thousand events the time I get is local time and not UTC.
I have seen this error just in Linux machines (build with g++) but not in Windows (build VC++). I have no clue where I could start, any idea?
void Publish(std::string source, std::string topic, std::string msg) NOEXCEPT {
console.lock();
std::time_t now = std::chrono::high_resolution_clock::to_time_t(*localTime);
char buf[50];
strftime(buf, sizeof (buf), "%Y-%m-%dT%H:%M:%S+00:00", gmtime(&now));
cout << buf << " " << source << " " << topic << " " << msg << endl;
cout.flush();
console.unlock();
}
struct tm * gmtime (const time_t * timer);
Converts time_t to tm as UTC time. This call uses the value pointed by timer to fill a tm structure with the values that represent the corresponding time, expressed as a UTC time (i.e., the time at the GMT timezone).
This must be working correctly.
However, not sure about this:
std::time_t now = std::chrono::high_resolution_clock::to_time_t(*localTime);
Is your *localtime giving the current time accurately under multithreaded situation? Is it a shared variable?
Instead could you use the following:
std::time_t now = time(0);

Capturing a time in milliseconds

The following piece of code is used to print the time in the logs:
#define PRINTTIME() struct tm * tmptime;
time_t tmpGetTime;
time(&tmpGetTime);
tmptime = localtime(&tmpGetTime);
cout << tmptime->tm_mday << "/" <<tmptime->tm_mon+1 << "/" << 1900+tmptime->tm_year << " " << tmptime->tm_hour << ":" << tmptime->tm_min << ":" << tmptime->tm_sec<<">>";
Is there any way to add milliseconds to this?
To have millisecond precision you have to use system calls specific to your OS.
In Linux you can use
#include <sys/time.h>
timeval tv;
gettimeofday(&tv, 0);
// then convert struct tv to your needed ms precision
timeval has microsecond precision.
In Windows you can use:
#include <Windows.h>
SYSTEMTIME st;
GetSystemTime(&st);
// then convert st to your precision needs
Of course you can use Boost to do that for you :)
//C++11 Style:
cout << "Time in Milliseconds =" <<
chrono::duration_cast<chrono::milliseconds>(chrono::steady_clock::now().time_since_epoch()).count()
<< std::endl;
cout << "Time in MicroSeconds=" <<
chrono::duration_cast<chrono::microseconds>(chrono::steady_clock::now().time_since_epoch()).count()
<< std::endl;
You need a timer with a higher resolution in order to capture milliseconds. Try this:
int cloc = clock();
//do something that takes a few milliseconds
cout << (clock() - cloc) << endl;
This is of course dependent on your OS.
The high resolution timers are usually gettimeofday on Linux style platforms and QueryPerformanceCounter on Windows.
You should be aware that timing the duration of a single operation (even with a high resolution timer) will not yield accurate results. There are too many random factors at play. To get reliable timing information, you should run the task to be timed in a loop and compute the average task time. For this type of timing, the clock() function should be sufficient.
If you don't want to use any OS-specific code, you can use the ACE package which supplies the ACE_OS::gettimeofday function for most standard operating systems.
For example:
ACE_Time_Value startTime = ACE_OS::gettimeofday();
do_something();
ACE_Time_Value endTime = ACE_OS::gettimeofday();
cout << "Elapsed time: " << (endTime.sec() - startTime.sec()) << " seconds and " << double(endTime.usec() - startTime.usec()) / 1000 << " milliseconds." << endl;
This code will work regardless of your OS (as long as ACE supports this OS).
In Ubuntu 16.04 this worked for me...
const std::string currentDateTime() {
char fmt[64], buf[64];
struct timeval tv;
struct tm *tm;
gettimeofday(&tv, NULL);
tm = localtime(&tv.tv_sec);
strftime(fmt, sizeof fmt, "%Y-%m-%d %H:%M:%S.%%06u", tm);
snprintf(buf, sizeof buf, fmt, tv.tv_usec);
return buf;
}
Then, with...
std::cout << currentDateTime();
I get...
2016-12-29 11:09:55.331008
New answer for old question using C++11 or C++14 and this free, open-source library:
#include "tz.h"
#include <iostream>
int
main()
{
using namespace date;
using namespace std;
using namespace std::chrono;
auto now = make_zoned(current_zone(), floor<milliseconds>(system_clock::now()));
cout << format("%e/%m/%Y %T", now) << '\n';
}
This just output for me:
16/01/2017 15:34:32.167
which is my current local date and time to millisecond precision. By eliminating the floor<milliseconds>() you will automatically get whatever precision your system_clock has.
If you wanted the result as a UTC timestamp instead of a local timestamp, it is even easier:
auto now = floor<milliseconds>(system_clock::now());
cout << format("%e/%m/%Y %T", now) << '\n';
And if you want a UTC timestamp and you aren't picky about the precision or the format, you can just:
cout << system_clock::now() << '\n';
which just output for me:
2017-01-16 20:42:11.267245