Manpage of NANOSLEEP

NANOSLEEP

Section: Linux Programmer's Manual (2)
Updated: 2016-03-15
Index
 

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 *reqhas 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 errnoto EINTR, and writes the remaining time into the structure pointed to by remunless remis NULL. The value of *remcan then be used to call nanosleep() again and complete the specified pause (but see NOTES).

The structure timespecis 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 interrupted 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 encounters an error, then it returns -1, with errnoset 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 *remso that the thread can easily call nanosleep() again and continue with the pause.
EINVAL
The value in the tv_nsecfield was not in the range 0 to 999999999 or tv_secwas negative.
 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.  

NOTES

If the interval specified in reqis not an exact multiple of the granularity underlying clock (see time(7)), then the interval will be rounded up to the next multiple. Furthermore, after the sleep completes, 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 problematic 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 absolute time value.

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

Setting the value of the CLOCK_REALTIMEclock via clock_settime(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, independently 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 ms by busy waiting with microsecond precision when called from a thread scheduled under a real-time policy like SCHED_FIFOor SCHED_RR. This special extension was removed in kernel 2.5.39, hence is still present in current 2.4 kernels, but not in 2.6 kernels.  

BUGS

In Linux 2.4, if nanosleep() is stopped by a signal (e.g., SIGTSTP), then the call fails with the error EINTRafter the thread is resumed by a SIGCONTsignal. If the system call is subsequently restarted, then the time that the thread spent in the stopped state is notcounted against the sleep interval.  

SEE ALSO

clock_nanosleep(2), restart_syscall(2), sched_setscheduler(2), timer_create(2), sleep(3), usleep(3), time(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
Old behavior
BUGS
SEE ALSO

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