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
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.
In case of an error or exception, the nanosleep system call returns -1
instead of 0 and sets errno to one of the following values:
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 (i.e,
10 ms on Linux/i386 and 1 ms on Linux/Alpha). 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/
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