nanosleep — pause execution for a specified time
#define _POSIX_C_SOURCE 199309 #include <time.h>
int
nanosleep( |
const struct timespec * | req, |
struct timespec * | rem) ; |
nanosleep
() delays the
execution of the program for at least the time specified in
*req
. The function
can return earlier if a signal has been delivered to the
process. In this case, it 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.
The structure timespec
is used to specify
intervals of time with nanosecond precision. It is specified
in <time.h>
and has the form
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 advantage of not
affecting any signals, it is standardized by POSIX, it
provides higher timing resolution, and it allows to continue
a sleep that has been interrupted by a signal more
easily.
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 errno
set to indicate the error.
Problem with copying information from user space.
The pause has been interrupted by a non-blocked
signal that was delivered to the process. The remaining
sleep time has been written into *rem
so that the process
can easily call nanosleep
() again and continue with
the pause.
The value in the tv_nsec
field was not
in the range 0 to 999999999 or tv_sec
was
negative.
The current implementation of nanosleep
() is based on the normal kernel
timer mechanism, which has a resolution of 1/HZ
s (see time(7)). Therefore,
nanosleep
() pauses always for
at least the specified time, however it can take up to 10 ms
longer than specified until the process becomes runnable
again. For the same reason, the value returned in case of a
delivered signal in *rem
is usually rounded to the
next larger multiple of 1/HZ
s.
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 process scheduled under a real-time policy like
SCHED_FIFO
or 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.
In Linux 2.4, if nanosleep
() is stopped by a signal (e.g.,
SIGTSTP), then the call fails with the error EINTR after the process is resumed by a
SIGCONT signal. If the system call is subsequently
restarted, then the time that the process spent in the
stopped state is not
counted against the
sleep interval.
sched_setscheduler(2), timer_create(2), sleep(3), usleep(3), feature_test_macros(7), time(7)
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