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Doc: Update documentation for QElapsedTimer's in Windows

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We only support Windows versions that feature the PerformanceCounter
API (see also 48fd845c61).

Pick-to: 6.3 6.2 5.15
Task-number: QTBUG-84433
Change-Id: I477f9015ee8c87d31f859f529631f3a6b0215ae6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Kai Köhne 2022-03-15 10:52:46 +01:00
parent 38c7c2ea30
commit aaa3184f8d
1 changed files with 3 additions and 24 deletions
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27
src/corelib/kernel/qelapsedtimer.cpp
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@ -139,9 +139,9 @@ QT_BEGIN_NAMESPACE
\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 TickCounter Not used anymore.
\value MachAbsoluteTime The Mach kernel's absolute time (\macos and iOS). 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.
\value PerformanceCounter The performance counter provided by Windows. This clock is monotonic and does not overflow.
\section2 SystemTime
@ -161,24 +161,6 @@ QT_BEGIN_NAMESPACE
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,
@ -192,10 +174,7 @@ QT_BEGIN_NAMESPACE
\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.
\tt{QueryPerformanceFrequency} to access the system's performance counter.
This clock is monotonic and does not overflow.