使用 c++ 在 windows 上穩定定時執行一個函數

#include <iostream>
#include <Windows.h>
#include <thread>
#pragma comment( lib, "Winmm" )

static int counter = 0;
static int64_t ticks_per_second;
void __stdcall on_timer(HWND h, UINT ui, UINT_PTR up, DWORD dw)
{
    std::cout << "time out, counter=" << counter << std::endl;
    counter = 0;
}
void get_message_trd_func()
{
    SetTimer(NULL, 0, 1000, on_timer);
    MSG msg;
    while (GetMessageA(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessageA(&msg);
    }
}
int main()
{
    std::cout << "go!" << std::endl;
    timeBeginPeriod(1);
    QueryPerformanceFrequency((LARGE_INTEGER*)&ticks_per_second);
    const double expected = 1.0 / 60.0;
    const int64_t expected_ticks = (int64_t)(expected * ticks_per_second);

    std::thread thr(get_message_trd_func);
    for (;;)
    {
        int64_t before_ticks = 0;
        QueryPerformanceCounter((LARGE_INTEGER*)&before_ticks);

        // do something...
        for (int i = 0; i < 10000; i++)
        {
            float a = i * i + i + sin(i) + sqrt(i);
        }

        int64_t after_ticks = 0;
        QueryPerformanceCounter((LARGE_INTEGER*)&after_ticks);
        counter++;

        int64_t ticks_need_sleep = expected_ticks - (after_ticks - before_ticks);

        double ms_need_sleep = (double)ticks_need_sleep / ticks_per_second * 1000.0;
        if (ms_need_sleep >= 1.0)
            Sleep((DWORD)ms_need_sleep);
        else
            continue;
    }
}

這裡主要用到的幾個 win32api 為

• MMRESULT timeBeginPeriod
  使用該 api 需要鏈接 Winmm, 所以我們在文件頂部加入#pragma comment( lib, "Winmm" ).
  它的作用是請求提高一些計時器的精度比如這裡的Sleep, 預設 windows 似乎只會給我們提供 10ms 左右很粗糙的精度, 所以這裡我們直接請求儘可能的高的精度, 即 1ms.

• QueryPerformanceFrequency
  該 api 用於獲取"性能計時器"的精度, 單位是 ticks每秒. 在我的機子上它的值是 10000000, 可以看到精度還是很令人滿意的. 在這裡我們將其與expected(也就是期望每次調用的間隔,單位s)進行相乘, 得到一個以ticks為單位的間隔.

• QueryPerformanceCounter
  該 api 會檢測"性能計時器"的值, 單位為 ticks, 在 msdn 中其精度的描述為 <1us.

那麼結合上述幾個 api 以及幾個簡單的數學運算, 這樣就可以相對穩定的定時調用函數了(在這裡是 1s 60 次):

time out, counter=59
time out, counter=59
time out, counter=60
time out, counter=59
time out, counter=58
time out, counter=60

當你註釋掉timeBeginPeriod的調用後你會發現結果不是很樂觀(即使我們期望 1s 調用 60 次):

time out, counter=33
time out, counter=31
time out, counter=32
time out, counter=31

最後, 這個可能常見於游戲的幀率控制, 實際上我就是從這裡知道的這些東西(x