GetTickCount使用的注意事项

最新推荐文章于 2023-08-16 16:57:22 发布

oushengfen

最新推荐文章于 2023-08-16 16:57:22 发布

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文章标签： performance wince system timer delay function

本文链接：https://blog.youkuaiyun.com/oushengfen/article/details/5671424

在Windows CE（Wince）系统下，使用GetTickCount()进行延时操作时可能会遇到49.7天后计数回零的问题。本文介绍了该问题的原因及两种改进方法：一是直接计算未来时间点进行比较，二是通过TickDiff函数处理溢出情况，结合Sleep和Application.ProcessMessages实现更精确的延时。

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最近在做一个项目,wince下面的短信接收模块.

其中我使用GetTickCount()来延时等待.如下

            uint dwStart = 0, dwStop = 0;

            dwStart = GetTickCount();
            dwStop = dwStart;
            while (dwStop - ReadTotalTimeout < dwStart)
            {
                //处理过程....
                dwStop = GetTickCount();
            }

经调试程序,如果readtotaltimeout值又点大,再加wince系统刚开机时,则会出现dwstop-readtotaltimeout由于负数在无符号下表示会出现最高为1,则表示非常大的一个数,查MSDN得知.

The resolution is limited to the resolution of the system timer. This value is also affected by adjustments made by the GetSystemTimeAdjustment function.

The elapsed time is stored as a DWORD value. Therefore, the time will wrap around to zero if the system is run continuously for 49.7 days. To avoid this problem, use GetTickCount64. Otherwise, check for an overflow condition when comparing times.

If you need a higher resolution timer, use a multimedia timer or a high-resolution timer.

To obtain the time elapsed since the computer was started, retrieve the System Up Time counter in the performance data in the registry key HKEY_PERFORMANCE_DATA. The value returned is an 8-byte value. For more information, see Performance Counters.

发现Gettickcount的函数的原理.只能正常使用49.7 days,因此,需要对其进行改进.

改进方法一,如果不考虑程序连续运行超过49.7,则对以下程序进行了改进.

            uint dwStart = 0, dwStop = 0;

            dwStart = GetTickCount();

            dwStop = GetTickCount() + ReadTotalTimeout;
            while (dwStop > GetTickCount())
            {

//程序处理
}

当然,如果以上方法处理还是存在问题.因此有更进一步的改进延时方案

function TickDiff(NewTick, OldTick: LongWord): LongWord;
begin
   if NewTick >= OldTick then
      Result := NewTick - OldTick
   else
      Result := (MAXDWORD - OldTick) + NewTick;
end;

procedure Delay(MilliSeconds: LongWord); // 2009.3.12更新
var
   TheTime: LongWord;
begin
   TheTime := GetTickCount;
   repeat
      Application.ProcessMessages;
      Sleep(1);
   until (TickDiff(GetTickCount, TheTime) > MilliSeconds) or Application.Terminated;
end;