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home | helpx minix x x minixx CLOCK_GETRES(2) Linux Programmer's Manual CLOCK_GETRES(2) 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); Link with -lrt (only for glibc versions before 2.17). 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 structures, 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 pro- cesses, or per-process if it measures time only within a single process. All implementations support the system-wide real-time clock, which is identified by CLOCK_REALTIME. Its time represents seconds and nanosec- onds 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 correspond- ing time values and the effect on timers is unspecified. Sufficiently recent versions of glibc and the Linux kernel support the following clocks: CLOCK_REALTIME System-wide clock that measures real (i.e., wall-clock) time. Setting this clock requires appropriate privileges. This clock is affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), and by the incremental adjustments performed by adjtime(3) and NTP. CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific) A faster but less precise version of CLOCK_REALTIME. Use when you need very fast, but not fine-grained timestamps. Requires per-architecture support, and probably also architecture support for this flag in the vdso(7). CLOCK_MONOTONIC Clock that cannot be set and represents monotonic time since--as described by POSIX--"some unspecified point in the past". On Linux, that point corresponds to the number of seconds that the system has been running since it was booted. The CLOCK_MONOTONIC clock is not affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), but is affected by the incremental adjust- ments performed by adjtime(3) and NTP. This clock does not count time that the system is suspended. CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific) A faster but less precise version of CLOCK_MONOTONIC. Use when you need very fast, but not fine-grained timestamps. Requires per-architecture support, and probably also architecture support for this flag in the vdso(7). CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific) Similar to CLOCK_MONOTONIC, but provides access to a raw hard- ware-based time that is not subject to NTP adjustments or the incremental adjustments performed by adjtime(3). This clock does not count time that the system is suspended. CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific) Identical to CLOCK_MONOTONIC, except it also includes any time that the system is suspended. This allows applications to get a suspend-aware monotonic clock without having to deal with the complications of CLOCK_REALTIME, which may have discontinuities if the time is changed using settimeofday(2) or similar. CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12) Per-process CPU-time clock (measures CPU time consumed by all threads in the process). CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12) Thread-specific CPU-time clock. RETURN VALUE clock_gettime(), clock_settime(), and clock_getres() return 0 for suc- cess, 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. EINVAL (clock_settime()): tp.tv_sec is negative or tp.tv_nsec is out- side the range [0..999,999,999]. EINVAL (since Linux 4.3) A call to clock_settime() with a clk_id of CLOCK_REALTIME at- tempted to set the time to a value less than the current value of the CLOCK_MONOTONIC clock. EPERM clock_settime() does not have permission to set the clock indi- cated. VERSIONS These system calls first appeared in Linux 2.6. ATTRIBUTES For an explanation of the terms used in this section, see at- tributes(7). +---------------------------------+---------------+---------+ |Interface | Attribute | Value | +---------------------------------+---------------+---------+ |clock_getres(), clock_gettime(), | Thread safety | MT-Safe | |clock_settime() | | | +---------------------------------+---------------+---------+ CONFORMING TO POSIX.1-2001, POSIX.1-2008, SUSv2. 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 POSIX.1 specifies the following: Setting the value of the CLOCK_REALTIME clock via clock_set- time() shall have no effect on threads that are blocked waiting for a relative time service based upon this clock, including the nanosleep() function; nor on the expiration of relative timers based upon this clock. Consequently, these time services shall expire when the requested relative interval elapses, indepen- dently of the new or old value of the clock. C library/kernel differences On some architectures, an implementation of clock_gettime() is provided in the vdso(7). Historical note for SMP systems Before Linux added kernel support for CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID, glibc implemented these clocks on many plat- forms using timer registers from the CPUs (TSC on i386, AR.ITC on Ita- nium). These registers may differ between CPUs and as a consequence these clocks may return bogus results if a process is migrated to an- other 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 be useful only if it can be en- sured 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 off- set. Some architectures include code that attempts to limit these off- sets 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. Since glibc 2.4, the wrapper functions for the system calls described in this page avoid the abovementioned problems by employing the kernel implementation of CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID, on systems that provide such an implementation (i.e., Linux 2.6.12 and later). BUGS According to POSIX.1-2001, a process with "appropriate privileges" may set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks us- ing clock_settime(). On Linux, these clocks are not settable (i.e., no process has "appropriate privileges"). SEE ALSO date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3), clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3), sysconf(3), time(7), vdso(7), hwclock(8) COLOPHON This page is part of release 5.05 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. 2019-03-06 CLOCK_GETRES(2)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | AVAILABILITY | NOTES | BUGS | SEE ALSO | COLOPHON