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man pages section 3: Basic Library Functions

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Updated: Thursday, June 13, 2019
 
 

clock_gettime(3C)

Name

clock_settime, clock_gettime, clock_getres, timespec_get - high-resolution clock operations

Synopsis

#include <time.h>

int clock_settime(clockid_t clock_id, const struct timespec *tp);
int clock_gettime(clockid_t clock_id, struct timespec *tp);
int clock_getres(clockid_t clock_id, struct timespec *res);
int timespec_get(struct timespec *ts, int base);

Description

The clock_settime() function sets the specified clock, clock_id, to the value specified by tp. Time values that are between two consecutive non-negative integer multiples of the resolution of the specified clock are truncated down to the smaller multiple of the resolution.

The clock_gettime() function returns the current value tp for the specified clock, clock_id.

The resolution of any clock can be obtained by calling clock_getres(). Clock resolutions are system-dependent and cannot be set by a process. If the argument res is not NULL, the resolution of the specified clock is stored in the location pointed to by res. If res is NULL, the clock resolution is not returned. If the time argument of clock_settime() is not a multiple of res, then the value is truncated to a multiple of res.

A clock may be systemwide (that is, visible to all processes) or per-process (measuring time that is meaningful only within a process).

A clock_id of CLOCK_REALTIME is defined in <time.h>. This clock represents the realtime clock for the system. For this clock, the values returned by clock_gettime() and specified by clock_settime() represent the amount of time (in seconds and nanoseconds) since the Epoch. Additional clocks may also be supported. The interpretation of time values for these clocks is unspecified.

A clock_id of CLOCK_HIGHRES represents the non-adjustable, high-resolution clock for the system. For this clock, the value returned by clock_gettime(3C) represents the amount of time (in seconds and nanoseconds) since some arbitrary time in the past; it is not correlated in any way to the time of day, and thus is not subject to resetting or drifting by way of adjtime(2), ntp_adjtime(2), settimeofday(3C), or clock_settime(). The time source for this clock is the same as that for gethrtime(3C).

A clock_id of CLOCK_PROCESS_CPUTIME_ID represents the CPU-time clock of the calling process when invoking one of the clock_*() or timer_*() functions.

A clock_id of CLOCK_THREAD_CPUTIME_ID represents the CPU-time clock associated with the calling thread when invoking one of the clock_*() or timer_*() functions.

The timespec_get() function modifies the timespec object pointed to by ts to hold the current calendar time, based on the defined time base. If base is TIME_UTC, tv_sec is set to the number of seconds since epoch, truncated to a whole value. The implementation defines epoch, in this case, standard Unix epoch January 1, 1970. tv_nsec is set to the integral number of nanoseconds, rounded to the resolution of the system clock. The timespec_get() function is specified in the C11 standard (INCITS/ISO/IEC 9899:2011).

Additional clocks may also be supported. The interpretation of time values for these clocks is unspecified.

Return Values

Upon successful completion, the clock_settime(), clock_gettime(), and clock_getres() functions return 0. Otherwise, -1 is returned and errno is set to indicate the error.

Upon successful completion, the timespec_get() function returns the nonzero value of base, otherwise, 0 is returned.

Errors

The clock_settime(), clock_gettime(), clock_getres(), and timespec_get() functions will fail if:

EINVAL

The clock_id argument does not specify a known clock, or the base argument does not specify a known time base.

ENOSYS

The functions clock_settime(), clock_gettime(), and clock_getres() are not supported by this implementation.

The clock_settime() function will fail if:

EINVAL

The tp argument to clock_settime() is outside the range for the given clock ID; or the tp argument specified a nanosecond value less than zero or greater than or equal to 1000 million.

The clock_settime() function may fail if:

EPERM

The requesting process does not have the appropriate privilege to set the specified clock.

EINTR

The calling process belongs to a non-global zone and receive a signal while setting the time.

ECANCELED

The calling process belongs to a non-global zone and there exists an internal I/O error.

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
clock_gettime() is Async-Signal-Safe
Standard

See Also

time(2), ctime(3C), gethrtime(3C), timer_gettime(3C), timespeccmp(3C), clock_getcpuclockid(3C), pthread_getcpuclockid(3C), time.h(3HEAD), attributes(7), standards(7)