CLOCK_GETRES(3)

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);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

clock_getres(), clock_gettime(), clock_settime(): _POSIX_C_SOURCE >= 199309L

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

EFAULT
    tp points outside the accessible address space.

EINVAL
    The clk_id specified is not supported on this system.

EPERM
    clock_settime() does not have permission to set the clock indicated.

CONFORMING TO
SUSv2, POSIX.1-2001.

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 sysconf(3).)

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

  Note for SMP systems
The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks are realized on many platforms using timers from the CPUs (TSC on i386, AR.ITC on Itanium). These registers may differ between CPUs and as a consequence these clocks may return bogus results if a process is migrated to another CPU.

If the CPUs in an SMP system have different clock sources then there is no way to maintain a correlation between the timer registers since each CPU will run at a slightly different frequency. If that is the case then clock_getcpuclockid(0) will return ENOENT to signify this condition. The two clocks will then only be useful if it can be ensured that a process stays on a certain CPU.

The processors in an SMP system do not start all at exactly the same time and therefore the timer registers are typically running at an offset. Some architectures include code that attempts to limit these offsets on bootup. However, the code cannot guarantee to accurately tune the offsets. Glibc contains no provisions to deal with these offsets (unlike the Linux Kernel). Typically these offsets are small and therefore the effects may be negligible in most cases.

SEE ALSO
date(1), adjtimex(2), gettimeofday(2), settimeofday(2), time(2), ctime(3), ftime(3), sysconf(3)