NAME
nanosleep - high-resolution sleep SYNOPSIS
#include
int nanosleep(const struct timespec *req, struct timespec *rem); Feature Test Macro Requirements for glibc (see featuretestmacros(7)): nanosleep(): POSIXCSOURCE >= 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 { timet tvsec; /* seconds */ long tvnsec; /* 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. The remaining sleep time has been written into *rem so that the thread can easily call nanosleep() again and con‐ tinue with the pause. EINVAL The value in the tvnsec field was not in the range 0 to 999999999 or tvsec was negative. CONFORMING TO
POSIX.1-2001. 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 clocknanosleep(2) with an abso‐ lute time value. POSIX.1 specifies that nanosleep() should measure time against the CLOCKREALTIME clock. However, Linux measures the time using the CLOCKMONOTONIC clock. This probably does not matter, since the POSIX.1 specification for clocksettime(2) says that discontinuous changes in CLOCKREALTIME should not affect nanosleep(): Setting the value of the CLOCKREALTIME clock via clockset‐ 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 ms by busy waiting with microsecond pre‐
cision when called from a thread scheduled under a real-time policy like SCHEDFIFO or SCHEDRR. 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 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. SEE ALSO clocknanosleep(2), restartsyscall(2), schedsetscheduler(2), timercreate(2), sleep(3), usleep(3), time(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
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Linux 2013-07-30 NANOSLEEP(2)