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NANOSLEEP(2) Linux Programmer's Manual NANOSLEEP(2)

NAME

     nanosleep - high-resolution sleep

SYNOPSIS

     #include <time.h>

     int nanosleep(const struct timespec *req, struct timespec *rem);

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     nanosleep(): _POSIX_C_SOURCE >= 199309L

DESCRIPTION

     nanosleep()  suspends  the execution of the calling thread until either
     at least the time specified in *req has elapsed, or the delivery  of  a
     signal  that triggers the invocation of a handler in the calling thread
     or that terminates the process.

     If the call is interrupted by a signal handler, nanosleep() returns -1,
     sets  errno  to EINTR, and writes the remaining time into the structure
     pointed to by rem unless rem is NULL.  The value of *rem  can  then  be
     used  to  call  nanosleep() again and complete the specified pause (but
     see NOTES).

     The structure timespec is  used  to  specify  intervals  of  time  with
     nanosecond precision.  It is defined as follows:

         struct timespec {
             time_t tv_sec;        /* seconds */
             long   tv_nsec;       /* nanoseconds */ };

     The value of the nanoseconds field must be in the range 0 to 999999999.

     Compared to sleep(3)  and  usleep(3),  nanosleep()  has  the  following
     advantages:  it  provides  a higher resolution for specifying the sleep
     interval; POSIX.1 explicitly specifies that it does not  interact  with
     signals; and it makes the task of resuming a sleep that has been inter-
     rupted by a signal handler easier.

RETURN VALUE

     On  successfully  sleeping  for  the  requested  interval,  nanosleep()
     returns  0.   If the call is interrupted by a signal handler or encoun-
     ters an error, then it returns -1,  with  errno  set  to  indicate  the
     error.

ERRORS

     EFAULT Problem with copying information from user space.

     EINTR  The pause has been interrupted by a signal that was delivered to
            the thread (see signal(7)).  The remaining sleep time  has  been
            written into *rem so that the thread can easily call nanosleep()
            again and continue with the pause.

     EINVAL The value in the tv_nsec  field  was  not  in  the  range  0  to
            999999999 or tv_sec was negative.

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008.

NOTES

     If the interval specified in req is not an exact multiple of the granu-
     larity underlying clock  (see  time(7)),  then  the  interval  will  be
     rounded  up  to  the  next multiple.  Furthermore, after the sleep com-
     pletes, there may still be a delay before the CPU becomes free to  once
     again execute the calling thread.

     The  fact  that nanosleep() sleeps for a relative interval can be prob-
     lematic if the call is repeatedly restarted after being interrupted  by
     signals,  since  the time between the interruptions and restarts of the
     call will lead to drift in the time when the sleep  finally  completes.
     This  problem  can be avoided by using clock_nanosleep(2) with an abso-
     lute time value.

     POSIX.1 specifies that nanosleep()  should  measure  time  against  the
     CLOCK_REALTIME  clock.   However,  Linux  measures  the  time using the
     CLOCK_MONOTONIC clock.   This  probably  does  not  matter,  since  the
     POSIX.1  specification  for  clock_settime(2)  says  that discontinuous
     changes in CLOCK_REALTIME should not affect nanosleep():

            Setting the value of the  CLOCK_REALTIME  clock  via  clock_set-
            time(2) shall have no effect on threads that are blocked waiting
            for a relative time service based upon this clock, including the
            nanosleep()  function;  ...   Consequently,  these time services
            shall expire when the requested relative interval elapses, inde-
            pendently of the new or old value of the clock.

 Old behavior
     In  order  to  support  applications requiring much more precise pauses
     (e.g., in order to control some  time-critical  hardware),  nanosleep()
     would  handle  pauses  of  up  to  2  milliseconds by busy waiting with
     microsecond precision when called from a thread scheduled under a real-
     time  policy  like  SCHED_FIFO or SCHED_RR.  This special extension was
     removed in kernel 2.5.39, and is thus not available in Linux 2.6.0  and
     later kernels.

BUGS

     If a program that catches signals and uses nanosleep() receives signals
     at a very high rate, then scheduling delays and rounding errors in  the
     kernel's  calculation  of  the  sleep  interval and the returned remain
     value mean that the remain value may steadily  increase  on  successive
     restarts  of  the  nanosleep()  call.   To  avoid  such  problems,  use
     clock_nanosleep(2) with the TIMER_ABSTIME flag to sleep to an  absolute
     deadline.

     In  Linux  2.4,  if nanosleep() is stopped by a signal (e.g., SIGTSTP),
     then the call fails with the error EINTR after the thread is resumed by
     a  SIGCONT  signal.  If the system call is subsequently restarted, then
     the time that the thread spent in the  stopped  state  is  not  counted
     against  the  sleep interval.  This problem is fixed in Linux 2.6.0 and
     later kernels.