一個調度線程suspend的nanosleep函數介紹

函數代碼如下：

/*

*  Suspend 當前線程的執行

 */

int ThreadSleep (time_t sec, long nsec) 

{

    struct  timespec req, rem;

    //設定要suspend的時間長度

    req.tv_sec   =  (time_t) sec;

    req.tv_nsec =  (long) nsec;

    //注意，若線程suspend，則函數nanosleep不會返回，即線程進入suspend狀態

   // 若返回0，則說明已經經過了req指定的時間了，否則返回-1

    while (nanosleep(&req, &rem) != 0)

    {       

        //此時返回但不是0，說明是錯誤或者被訊號中斷

        //若被訊號中斷，則此時errno會被設定為EINTR，而輸出參數rem會儲存剩餘的時間，這時，繼續用剩餘的時間設定為req 迴圈調用nanosleep

        if (errno == EINTR)

        {

            req = rem;

        }

       //其他錯誤，則返回-1，不繼續進入迴圈了

       else

       {

           return -1;

       }  

  }  //end of while

    return 0;

}

關於nanosleep函數的使用說明如下：

NAME
       nanosleep - high resolution sleep (REALTIME)
SYNOPSIS
       #include <time.h>
       int nanosleep(const struct timespec *rqtp, struct timespec *rmtp);
DESCRIPTION
       The  nanosleep()  function  shall  cause the current thread to be suspended from execution
until either the time interval
       specified by the rqtp argument has elapsed or a signal is delivered to the calling thread, and its action is to invoke  a
       signal-catching  function or to terminate the process. The suspension time may be longer than requested because the argu-
       ment value is rounded up to an integer multiple of the sleep resolution or because of the scheduling of other activity by
       the  system.  But,  except  for the case of being interrupted by a signal, the suspension time shall not be less than the
       time specified by rqtp, as measured by the system clock CLOCK_REALTIME.

       The use of the nanosleep() function has no effect on the action or blockage of any signal.

RETURN VALUE
       If the nanosleep() function returns because the requested time has elapsed, its return value shall be zero.

       If the nanosleep() function returns because it has been interrupted by a signal, it shall return a value of  -1  and  set
       errno  to indicate the interruption. If the rmtp argument is non-NULL, the timespec structure referenced by it is updated
       to contain the amount of time remaining in the interval (the requested time minus the time actually slept). If  the  rmtp
       argument is NULL, the remaining time is not returned.

       If nanosleep() fails, it shall return a value of -1 and set errno to indicate the error.

       否則不會返回，即意味著線程進入suspend狀態。
       
ERRORS
       The nanosleep() function shall fail if:
       EINTR  The nanosleep() function was interrupted by a signal.
       EINVAL The rqtp argument specified a nanosecond value less than zero or greater than or equal to 1000 million.
       The following sections are informative.

RATIONALE
       It  is  common  to suspend execution of a process for an interval in order to poll the status of a non-interrupting func-
       tion. A large number of actual needs can be met with a simple extension to sleep() that provides finer resolution.

       In the POSIX.1-1990 standard and SVR4, it is possible to implement such a routine, but the frequency of wakeup is limited
       by  the  resolution  of  the  alarm()  and sleep() functions. In 4.3 BSD, it is possible to write such a routine using no
       static storage and reserving no system facilities. Although it is possible to write a function with similar functionality
       to  sleep()  using the remainder of the timer_*() functions, such a function requires the use of signals and the reserva-
       tion of some signal number. This volume of IEEE Std 1003.1-2001 requires that nanosleep() be non-intrusive of the signals
       function.

       The  nanosleep()  function  shall return a value of 0 on success and -1 on failure or if interrupted. This latter case is
       different from sleep(). This was done because the remaining time is returned via an  argument  structure  pointer,  rmtp,
       instead of as the return value.