сон высокого разрешения (high-resolution sleep)
Имя (Name)
nanosleep - high-resolution sleep
Синопсис (Synopsis)
#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
Описание (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 {
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 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 interrupted by a signal handler easier.
Возвращаемое значение (Return value)
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.
Ошибки (Error)
EFAULT Problem with copying information from user space.
EINTR The pause has been interrupted by a signal that was
delivered to the thread (see signal(7)). The remaining
sleep time has been written into *rem so that the thread
can easily call nanosleep() again and continue with the
pause.
EINVAL The value in the tv_nsec field was not in the range 0 to
999999999 or tv_sec was negative.
Стандарты (Conforming to)
POSIX.1-2001, POSIX.1-2008.
Примечание (Note)
If the interval specified in req is not an exact multiple of the
granularity underlying clock (see time(7)), then the interval
will be rounded up to the next multiple. Furthermore, after the
sleep completes, 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
problematic 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
clock_nanosleep(2) with an absolute time value.
POSIX.1 specifies that nanosleep() should measure time against
the CLOCK_REALTIME clock. However, Linux measures the time using
the CLOCK_MONOTONIC clock. This probably does not matter, since
the POSIX.1 specification for clock_settime(2) says that
discontinuous changes in CLOCK_REALTIME should not affect
nanosleep():
Setting the value of the CLOCK_REALTIME clock via
clock_settime(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, independently 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 milliseconds by busy
waiting with microsecond precision when called from a thread
scheduled under a real-time policy like SCHED_FIFO or SCHED_RR.
This special extension was removed in kernel 2.5.39, and is thus
not available in Linux 2.6.0 and later kernels.
Ошибки (баги) (Bugs)
If a program that catches signals and uses nanosleep() receives
signals at a very high rate, then scheduling delays and rounding
errors in the kernel's calculation of the sleep interval and the
returned remain value mean that the remain value may steadily
increase on successive restarts of the nanosleep() call. To
avoid such problems, use clock_nanosleep(2) with the
TIMER_ABSTIME flag to sleep to an absolute deadline.
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.
This problem is fixed in Linux 2.6.0 and later kernels.
Смотри также (See also)
clock_nanosleep(2), restart_syscall(2), sched_setscheduler(2),
timer_create(2), sleep(3), usleep(3), time(7)
