nanosleep - pause execution for a specified time


#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


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.

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 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), sleep(3), timer_create(3), usleep(3), time(7)