The constructors QQmlApplicationEngine and QSettings spoil the time zone (daylight)

[xintrea]

I came across the strange behavior of the QQmlApplicationEngine constructor. The minimum code is main.cpp:

#include <QApplication>
#include <QQmlApplicationEngine>
#include <QDebug>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    qDebug() << "1ST timezone=" << timezone << " daylight=" << daylight;
    tzset();
    qDebug() << "2ND timezone=" << timezone << " daylight=" << daylight;

    QQmlApplicationEngine engine;

    qDebug() << "3ND timezone=" << timezone << " daylight=" << daylight;

    return app.exec();
}

Result of work:

1ST timezone= 0  daylight= 0
2ND timezone= -10800  daylight= 1
3ND timezone= -10800  daylight= 0

That is, after the QQmlApplicationEngine constructor is triggered, daylight spoils (switch to zero).

I was surprised, but it's necessary to work and therefore I tried to get around this case like this:

long saveTimezone=timezone;
int saveDaylight=daylight;

QQmlApplicationEngine engine;

timezone=saveTimezone;
daylight=saveDaylight;

It worked, but in the project I still found another one place in which the same behavior, after the QSettings constructor (but already in the heap). Looks like that:

qDebug() << "CURRENT timezone=" << timezone << " daylight=" << daylight;
QSettings *config=new QSettings(fileName, QSettings::IniFormat, this);
qDebug() << "CURRENT timezone=" << timezone << " daylight=" << daylight;

CURRENT timezone= -10800  daylight= 1
CURRENT timezone= -10800  daylight= 0

But in a minimal code, I could not repeat this.

Where else the variable daylight can spoil, I do not know, but I do not rule out such probabilities. The problem is that my project uses a legacy library that works with daylight, and because of these problems it gives the wrong time.

Questions: what kind of problem is this, why do some Qt classes spoil daylight, how is this thing guaranteed to work around?

PS: The project file that would quickly compile a minimal example (only the project file and main.cpp will be required):

time.pro

CONFIG+=c++11
CONFIG+=qml_debug
QT+=gui
QT+=core
QT+=quick
QT+=widgets
SOURCES += main.cpp
DEFINES += QT_DEPRECATED_WARNINGS

[SGaist]

Hi,

Intriguing !

What version of Qt are you using ?
On what platform ?

[xintrea]

@SGaist
Debian Linux Stable 64 bit, Qt 5.10.1 by official site.

[kshegunov]

@xintrea said in The constructors QQmlApplicationEngine and QSettings spoil the time zone (daylight):

Qt 5.10.1 by official site

You mean downloaded from qt.io, correct? Because debian doesn't provide that in stable, but the current Qt version for the testing flavor is exactly 5.10.1.

[xintrea]

@kshegunov
Yes. I use Debian Stable 9 (64-bit) with all updates:

> lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 9.3 (stretch)
Release:        9.3
Codename:       stretch

I get Qt from qt.io as file qt-opensource-linux-x64-5.10.1.run.

[SGaist]

Do you get the same behaviour if you use Debian's provided version of Qt ?

[xintrea]

@SGaist

I not experiment with Qt version.

But i debug this situation. I set watchpoint to system global variable daylight. This is stack if run constructor QQmlApplicationEngine:

(gdb) where
#0  update_vars () at tzset.c:156
#1  0x00007ffff3c8a569 in __tzset_parse_tz (tz=<optimized out>) at tzset.c:391
#2  0x00007ffff3c8c27b in __tzfile_compute (timer=<optimized out>, use_localtime=use_localtime@entry=1, 
    leap_correct=leap_correct@entry=0x7fffffffd938, leap_hit=leap_hit@entry=0x7fffffffd934, tp=tp@entry=0x7fffffffd980)
    at tzfile.c:691
#3  0x00007ffff3c8ac44 in __tz_convert (timer=0x7fffffffd978, use_localtime=1, tp=0x7fffffffd980) at tzset.c:624
#4  0x00007ffff608230a in ?? () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Qml.so.5
#5  0x00007ffff608afab in ?? () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Qml.so.5
#6  0x00007ffff60692db in QV4::ExecutionEngine::ExecutionEngine(QV4::EvalISelFactory*) ()
   from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Qml.so.5
#7  0x00007ffff61c949a in QV8Engine::QV8Engine(QJSEngine*) () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Qml.so.5
#8  0x00007ffff5fffb14 in QJSEngine::QJSEngine(QJSEnginePrivate&, QObject*) ()
   from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Qml.so.5
#9  0x00007ffff6127969 in QQmlEngine::QQmlEngine(QQmlEnginePrivate&, QObject*) ()
   from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Qml.so.5
#10 0x00007ffff61b44e0 in QQmlApplicationEngine::QQmlApplicationEngine(QObject*) ()
   from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Qml.so.5

And this is stack if run constructor QSettings:

(gdb) where
#0  update_vars () at tzset.c:156
#1  0x00007ffff3c8a569 in __tzset_parse_tz (tz=<optimized out>) at tzset.c:391
#2  0x00007ffff3c8c27b in __tzfile_compute (timer=<optimized out>, use_localtime=use_localtime@entry=1, 
    leap_correct=leap_correct@entry=0x7fffffffd3f8, leap_hit=leap_hit@entry=0x7fffffffd3f4, tp=tp@entry=0x7fffffffd4a0)
    at tzfile.c:691
#3  0x00007ffff3c8ac44 in __tz_convert (timer=0x7fffffffd498, use_localtime=1, tp=0x7fffffffd4a0) at tzset.c:624
#4  0x00007ffff4dc8709 in QDateTime::setMSecsSinceEpoch(long long) () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Core.so.5
#5  0x00007ffff4dcab4d in QDateTime::fromMSecsSinceEpoch(long long, Qt::TimeSpec, int) ()
   from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Core.so.5
#6  0x00007ffff4dcb00d in QDateTime::fromMSecsSinceEpoch(long long) () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Core.so.5
#7  0x00007ffff4e87226 in QFileInfo::fileTime(QFileDevice::FileTime) const ()
   from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Core.so.5
#8  0x00007ffff4e8732e in QFileInfo::lastModified() const () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Core.so.5
#9  0x00007ffff4ec90e1 in ?? () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Core.so.5
#10 0x00007ffff4ec98cd in ?? () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Core.so.5
#11 0x00007ffff4eca4b4 in ?? () from /opt/qt_5_10_1/5.10.1/gcc_64/lib/libQt5Core.so.5
#12 0x00007ffff4eca57f in QSettings::QSettings(QString const&, QSettings::Format, QObject*)

Really, QQmlApplicationEngine and QSettings is change timezone and, therefore, daylight.

[xintrea]

@SGaist

And function update_vars() is:

151 static void                                                                                                              
152 internal_function                                                                                                        
153 update_vars (void)                                                                                                       
154 {                                                                                                                        
155   __daylight = tz_rules[0].offset != tz_rules[1].offset;                                                                 
156   __timezone = -tz_rules[0].offset;                                                                                      
157   __tzname[0] = (char *) tz_rules[0].name;                                                                               
158   __tzname[1] = (char *) tz_rules[1].name;                                                                               
159                                                                                                                          
160   /* Keep __tzname_cur_max up to date.  */                                                                               
161   size_t len0 = strlen (__tzname[0]);                                                                                    
162   size_t len1 = strlen (__tzname[1]);                                                                                    
163   if (len0 > __tzname_cur_max)                                                                                           
164     __tzname_cur_max = len0;                                                                                             
165   if (len1 > __tzname_cur_max)                                                                                           
166     __tzname_cur_max = len1;                                                                                             
167 }