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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 true if this object and \a other contain the same time. */ /*! \fn bool QElapsedTimer::operator !=(const QElapsedTimer &other) const Returns 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 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 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