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

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.

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/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.

POSIX.1b (formerly POSIX.4).

sched_setscheduler(2), timer_create(2), sleep(3), usleep(3)

2004-10-24 Linux 2.6.9

Different Versions of this Page: