CLOCK_GETRES

NAME

clock_getres, clock_gettime, clock_settime - clock and time functions

SYNOPSIS

#include <time.h>

int clock_getres(clockid_t clk_id, struct timespec *res);
int clock_gettime(clockid_t clk_id, struct timespec *tp);
int clock_settime(clockid_t clk_id, const struct timespec *tp);

DESCRIPTION

The function clock_getres() finds the resolution (precision) of the specified clock clk_id, and, if res is non-NULL, stores it in the struct timespec pointed to by res. The resolution of clocks depends on the implementation and cannot be configured by a particular process. If the time value pointed to by the argument tp of clock_settime() is not a multiple of res, then it is truncated to a multiple of res.

The functions clock_gettime() and clock_settime() retrieve and set the time of the specified clock clk_id.

The res and tp arguments are timespec structs, as specified in <time.h>:

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

The clk_id argument is the identifier of the particular clock on which to act. A clock may be system-wide and hence visible for all processes, or per-process if it measures time only within a single process.

All implementations support the system-wide realtime clock, which is identified by CLOCK_REALTIME. Its time represents seconds and nanoseconds since the Epoch. When its time is changed, timers for a relative interval are unaffected, but timers for an absolute point in time are affected.

More clocks may be implemented. The interpretation of the corresponding time values and the effect on timers is unspecified.

Sufficiently recent versions of GNU libc and the Linux kernel support the following clocks:

CLOCK_REALTIME: System-wide realtime clock. Setting this clock requires appropriate privileges.
CLOCK_MONOTONIC: Clock that cannot be set and represents monotonic time since some unspecified starting point.
CLOCK_PROCESS_CPUTIME_ID: High-resolution per-process timer from the CPU.
CLOCK_THREAD_CPUTIME_ID: Thread-specific CPU-time clock.

RETURN VALUE

clock_gettime(), clock_settime() and clock_getres() return 0 for success, or -1 for failure (in which case errno is set appropriately).

ERRORS

EPERM: clock_settime() does not have permission to set the clock indicated.
EINVAL: The clk_id specified is not supported on this system.
EFAULT: tp points outside the accessible address space.

NOTE

Most systems require the program be linked with the librt library to use these functions.

AVAILABILITY

On POSIX systems on which these functions are available, the symbol _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID, CLOCK_THREAD_CPUTIME_ID are available. (See also .BR sysconf (3).)

CONFORMING TO

SUSv2, POSIX 1003.1-2001.