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/****************************************************************************
**
** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
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** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Digia gives you certain additional
** rights. These rights are described in the Digia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
#include "qelapsedtimer.h"
QT_BEGIN_NAMESPACE
/*!
\class QElapsedTimer
\inmodule QtCore
\brief The QElapsedTimer class provides a fast way to calculate elapsed times.
\since 4.7
\reentrant
\ingroup tools
The QElapsedTimer class is usually used to quickly calculate how much
time has elapsed between two events. Its API is similar to that of QTime,
so code that was using that can be ported quickly to the new class.
However, unlike QTime, QElapsedTimer tries to use monotonic clocks if
possible. This means it's not possible to convert QElapsedTimer objects
to a human-readable time.
The typical use-case for the class is to determine how much time was
spent in a slow operation. The simplest example of such a case is for
debugging purposes, as in the following example:
\snippet qelapsedtimer/main.cpp 0
In this example, the timer is started by a call to start() and the
elapsed timer is calculated by the elapsed() function.
The time elapsed can also be used to recalculate the time available for
another operation, after the first one is complete. This is useful when
the execution must complete within a certain time period, but several
steps are needed. The \tt{waitFor}-type functions in QIODevice and its
subclasses are good examples of such need. In that case, the code could
be as follows:
\snippet qelapsedtimer/main.cpp 1
Another use-case is to execute a certain operation for a specific
timeslice. For this, QElapsedTimer provides the hasExpired() convenience
function, which can be used to determine if a certain number of
milliseconds has already elapsed:
\snippet qelapsedtimer/main.cpp 2
\section1 Reference clocks
QElapsedTimer will use the platform's monotonic reference clock in all
platforms that support it (see QElapsedTimer::isMonotonic()). This has
the added benefit that QElapsedTimer is immune to time adjustments, such
as the user correcting the time. Also unlike QTime, QElapsedTimer is
immune to changes in the timezone settings, such as daylight saving
periods.
On the other hand, this means QElapsedTimer values can only be compared
with other values that use the same reference. This is especially true if
the time since the reference is extracted from the QElapsedTimer object
(QElapsedTimer::msecsSinceReference()) and serialised. These values
should never be exchanged across the network or saved to disk, since
there's no telling whether the computer node receiving the data is the
same as the one originating it or if it has rebooted since.
It is, however, possible to exchange the value with other processes
running on the same machine, provided that they also use the same
reference clock. QElapsedTimer will always use the same clock, so it's
safe to compare with the value coming from another process in the same
machine. If comparing to values produced by other APIs, you should check
that the clock used is the same as QElapsedTimer (see
QElapsedTimer::clockType()).
\section2 32-bit overflows
Some of the clocks used by QElapsedTimer have a limited range and may
overflow after hitting the upper limit (usually 32-bit). QElapsedTimer
deals with this overflow issue and presents a consistent timing. However,
when extracting the time since reference from QElapsedTimer, two
different processes in the same machine may have different understanding
of how much time has actually elapsed.
The information on which clocks types may overflow and how to remedy that
issue is documented along with the clock types.
\sa QTime, QTimer
*/
/*!
\enum QElapsedTimer::ClockType
This enum contains the different clock types that QElapsedTimer may use.
QElapsedTimer will always use the same clock type in a particular
machine, so this value will not change during the lifetime of a program.
It is provided so that QElapsedTimer can be used with other non-Qt
implementations, to guarantee that the same reference clock is being
used.
\value SystemTime The human-readable system time. This clock is not monotonic.
\value MonotonicClock The system's monotonic clock, usually found in Unix systems. This clock is monotonic and does not overflow.
\value TickCounter The system's tick counter, used on Windows systems. This clock may overflow.
\value MachAbsoluteTime The Mach kernel's absolute time (Mac OS X). This clock is monotonic and does not overflow.
\value PerformanceCounter The high-resolution performance counter provided by Windows. This clock is monotonic and does not overflow.
\section2 SystemTime
The system time clock is purely the real time, expressed in milliseconds
since Jan 1, 1970 at 0:00 UTC. It's equivalent to the value returned by
the C and POSIX \tt{time} function, with the milliseconds added. This
clock type is currently only used on Unix systems that do not support
monotonic clocks (see below).
This is the only non-monotonic clock that QElapsedTimer may use.
\section2 MonotonicClock
This is the system's monotonic clock, expressed in milliseconds since an
arbitrary point in the past. This clock type is used on Unix systems
which support POSIX monotonic clocks (\tt{_POSIX_MONOTONIC_CLOCK}).
This clock does not overflow.
\section2 TickCounter
The tick counter clock type is based on the system's or the processor's
tick counter, multiplied by the duration of a tick. This clock type is
used on Windows platforms. If the high-precision performance
counter is available on Windows, the \tt{PerformanceCounter} clock type
is used instead.
The TickCounter clock type is the only clock type that may overflow.
Windows Vista and Windows Server 2008 support the extended 64-bit tick
counter, which allows avoiding the overflow.
On Windows systems, the clock overflows after 2^32 milliseconds, which
corresponds to roughly 49.7 days. This means two processes' reckoning of
the time since the reference may be different by multiples of 2^32
milliseconds. When comparing such values, it's recommended that the high
32 bits of the millisecond count be masked off.
\section2 MachAbsoluteTime
This clock type is based on the absolute time presented by Mach kernels,
such as that found on Mac OS X. This clock type is presented separately
from MonotonicClock since Mac OS X is also a Unix system and may support
a POSIX monotonic clock with values differing from the Mach absolute
time.
This clock is monotonic and does not overflow.
\section2 PerformanceCounter
This clock uses the Windows functions \tt{QueryPerformanceCounter} and
\tt{QueryPerformanceFrequency} to access the system's high-precision
performance counter. Since this counter may not be available on all
systems, QElapsedTimer will fall back to the \tt{TickCounter} clock
automatically, if this clock cannot be used.
This clock is monotonic and does not overflow.
\sa clockType(), isMonotonic()
*/
/*!
\fn bool QElapsedTimer::operator ==(const QElapsedTimer &other) const
Returns \c true if this object and \a other contain the same time.
*/
/*!
\fn bool QElapsedTimer::operator !=(const QElapsedTimer &other) const
Returns \c true if this object and \a other contain different times.
*/
static const qint64 invalidData = Q_INT64_C(0x8000000000000000);
/*!
Marks this QElapsedTimer object as invalid.
An invalid object can be checked with isValid(). Calculations of timer
elapsed since invalid data are undefined and will likely produce bizarre
results.
\sa isValid(), start(), restart()
*/
void QElapsedTimer::invalidate() Q_DECL_NOTHROW
{
t1 = t2 = invalidData;
}
/*!
Returns \c false if this object was invalidated by a call to invalidate() and
has not been restarted since.
\sa invalidate(), start(), restart()
*/
bool QElapsedTimer::isValid() const Q_DECL_NOTHROW
{
return t1 != invalidData && t2 != invalidData;
}
/*!
Returns \c true if this QElapsedTimer has already expired by \a timeout
milliseconds (that is, more than \a timeout milliseconds have elapsed).
The value of \a timeout can be -1 to indicate that this timer does not
expire, in which case this function will always return false.
\sa elapsed()
*/
bool QElapsedTimer::hasExpired(qint64 timeout) const Q_DECL_NOTHROW
{
// if timeout is -1, quint64(timeout) is LLINT_MAX, so this will be
// considered as never expired
return quint64(elapsed()) > quint64(timeout);
}
QT_END_NAMESPACE
|
