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Manual Pages  — CLOCK_GETTIME

NAME

clock_gettime, clock_settime, clock_getres – get/set/calibrate date and time

CONTENTS

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <time.h>

int
clock_gettime(clockid_t clock_id, struct timespec *tp);

int
clock_settime(clockid_t clock_id, const struct timespec *tp);

int
clock_getres(clockid_t clock_id, struct timespec *tp);

DESCRIPTION

The clock_gettime() and clock_settime() system calls allow the calling process to retrieve or set the value used by a clock which is specified by clock_id.

The clock_id argument can be a value obtained from clock_getcpuclockid(3) or pthread_getcpuclockid(3) as well as the following values:

CLOCK_REALTIME
CLOCK_REALTIME_PRECISE
CLOCK_REALTIME_FAST
  Increments as a wall clock should.
CLOCK_MONOTONIC
CLOCK_MONOTONIC_PRECISE
CLOCK_MONOTONIC_FAST
  Increments in SI seconds.
CLOCK_UPTIME
CLOCK_UPTIME_PRECISE
CLOCK_UPTIME_FAST
  Starts at zero when the kernel boots and increments monotonically in SI seconds while the machine is running.
CLOCK_VIRTUAL
  Increments only when the CPU is running in user mode on behalf of the calling process.
CLOCK_PROF
  Increments when the CPU is running in user or kernel mode.
CLOCK_SECOND
  Returns the current second without performing a full time counter query, using an in-kernel cached value of the current second.
CLOCK_PROCESS_CPUTIME_ID
  Returns the execution time of the calling process.
CLOCK_THREAD_CPUTIME_ID
  Returns the execution time of the calling thread.

The clock IDs CLOCK_REALTIME_FAST, CLOCK_MONOTONIC_FAST, CLOCK_UPTIME_FAST are analogs of corresponding IDs without _FAST suffix but do not perform a full time counter query, so their accuracy is one timer tick. Similarly, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_PRECISE, CLOCK_UPTIME_PRECISE are used to get the most exact value as possible, at the expense of execution time.

The structure pointed to by tp is defined in <sys/timespec.h> as:

struct timespec {
        time_t  tv_sec;         /* seconds */
        long    tv_nsec;        /* and nanoseconds */
};

Only the super-user may set the time of day, using only CLOCK_REALTIME. If the system securelevel is greater than 1 (see init(8)), the time may only be advanced. This limitation is imposed to prevent a malicious super-user from setting arbitrary time stamps on files. The system time can still be adjusted backwards using the adjtime(2) system call even when the system is secure.

The resolution (granularity) of a clock is returned by the clock_getres() system call. This value is placed in a (non-NULL) *tp.

RETURN VALUES

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

ERRORS

The following error codes may be set in errno:
[EINVAL]
  The clock_id or timespec argument was not a valid value.
[EPERM]
  A user other than the super-user attempted to set the time.

SEE ALSO

date(1), adjtime(2), clock_getcpuclockid(3), ctime(3), pthread_getcpuclockid(3), timed(8)

STANDARDS

The clock_gettime(), clock_settime(), and clock_getres() system calls conform to IEEE Std 1003.1b-93 ("POSIX.1"). The clock IDs CLOCK_REALTIME_FAST, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_FAST, CLOCK_MONOTONIC_PRECISE, CLOCK_UPTIME, CLOCK_UPTIME_FAST, CLOCK_UPTIME_PRECISE, CLOCK_SECOND are FreeBSD extensions to the POSIX interface.

CLOCK_GETTIME (2) March 20, 2017

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Please direct any comments about this manual page service to Ben Bullock.

… one of the main causes of the fall of the Roman Empire was that, lacking zero, they had no way to indicate successful termination of their C programs.
— Robert Firth