c - Posix Timer 周期性地向前跳过其周期的一半

Question

我有一个任务链接到每 20 毫秒/50 赫兹执行一次的 Posix 定时器 (timer_create())。大多数情况下工作正常，除了每 334.5 秒(大约)定时器提前 10 毫秒执行一个周期。在接下来的 334.5 秒左右，间隔再次全部为 20 毫秒。

我已经包含了配置定时器的相关代码。该应用程序在 Gumstix 的默认 Linux 版本的 Gumstix Verdex Pro XL6P 上运行。我还使用 FIFO 调度算法对其进行了调度。

我的直觉告诉我这是一个整数溢出问题。也许还有其他东西使用相同的信号？我一直能够在董事会的执行和 session 中重现跳过。

这个问题不是问题，但我真的很想了解为什么会这样。

下面是配置定时器的代码:

//------------------------------------------------------------------------------
// Create a timer which fires at the specified time and calls a timer event
// handler.
//
// handler : function to be called when the timer expires
// us      : number of microseconds to add to timer
// ms      : number of milliseconds to add to timer
// sec     : number of seconds to add to timer
//------------------------------------------------------------------------------
void createTimer(void (*handler)(void), uint32 us, uint32 ms, uint32 sec)
{
    struct sigaction sigact;
    struct sigevent sigev;
    timer_t timerid;
    struct itimerspec itval;
    struct itimerspec oitval;
    timer_info_t* newTimer = NULL;

    // Initialize signalNum.
    if (timers == NULL)
        signalNum = SIGRTMAX;

    if (signalNum < SIGRTMIN)
        exitWithError("no avaiable signals, unable to create timers");

    sigemptyset(&sigact.sa_mask);
    sigact.sa_flags = SA_SIGINFO;
    sigact.sa_sigaction = signalHandler;

    // Set up sigaction to catch signal
    if (sigaction(signalNum, &sigact, NULL) == -1)
        exitWithError("sigaction() failed, unabled to creat timers");

    // Create the POSIX timer to generate signo
    sigev.sigev_notify = SIGEV_SIGNAL;
    sigev.sigev_signo = signalNum;
    sigev.sigev_value.sival_ptr = &timerid;        

    long ret = timer_create(CLOCK_REALTIME, &sigev, &timerid);

    if (ret == 0) 
    {
        // Prevent overflow in calculation of nsec below.
        if (ms >= 1000)
        {
            sec += (ms / 1000);
            ms = ms % 1000;
        }

        itval.it_value.tv_sec = sec;
        itval.it_value.tv_nsec = (long)(us * 1000L) + (long)(ms * 1000L * 1000L);

        // Configure it as a repeat timer.
        itval.it_interval.tv_sec = itval.it_value.tv_sec;
        itval.it_interval.tv_nsec = itval.it_value.tv_nsec;

        if (timer_settime(timerid, 0, &itval, &oitval) != 0)
            exitWithError("time_settime() error!");
    }
    else
        exitWithError("timer_create() error!");

    newTimer = (timer_info_t*)malloc(sizeof(timer_info_t));
    newTimer->timer = timerid;
    newTimer->handler = handler;
    newTimer->sigNum = signalNum;
    newTimer->next = NULL;

    // Check to see if this is the first time through.
    if (timers == NULL)
    {
        timers = newTimer;
        atexit(deleteTimers);
    }
    else
        lastTimer->next = newTimer;

    lastTimer = newTimer;

    signalNum--;
}

提前致谢。

Answer 1

我的猜测是您正在使用 ntp，并且它会以这些间隔调整时间。您可以尝试改用 CLOCK_MONOTONIC，但根据 http://juliusdavies.ca/posix_clocks/clock_realtime_linux_faq.html它也可能受到影响。