| clock_getres(2) | System Calls Manual | clock_getres(2) |
clock_getres, clock_gettime, clock_settime - clock and time functions
Standard C library (libc, -lc), since glibc 2.17
Before glibc 2.17, Real-time library (librt, -lrt)
#include <time.h>
int clock_getres(clockid_t clockid, struct timespec *_Nullable res);
int clock_gettime(clockid_t clockid, struct timespec *tp); int clock_settime(clockid_t clockid, const struct timespec *tp);
clock_getres(), clock_gettime(), clock_settime():
_POSIX_C_SOURCE >= 199309L
The function clock_getres() finds the resolution (precision) of the specified clock clockid, 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 clockid.
The res and tp arguments are timespec(3) structures.
The clockid 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 real-time 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 glibc and the Linux kernel support the following clocks:
Linux also implements dynamic clock instances as described below.
In addition to the hard-coded System-V style clock IDs described above, Linux also supports POSIX clock operations on certain character devices. Such devices are called "dynamic" clocks, and are supported since Linux 2.6.39.
Using the appropriate macros, open file descriptors may be converted into clock IDs and passed to clock_gettime(), clock_settime(), and clock_adjtime(2). The following example shows how to convert a file descriptor into a dynamic clock ID.
#define CLOCKFD 3
#define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
#define CLOCKID_TO_FD(clk) ((unsigned int) ~((clk) >> 3))
struct timespec ts;
clockid_t clkid;
int fd;
fd = open("/dev/ptp0", O_RDWR);
clkid = FD_TO_CLOCKID(fd);
clock_gettime(clkid, &ts);
clock_gettime(), clock_settime(), and clock_getres() return 0 for success. On error, -1 is returned and errno is set to indicate the error.
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
| clock_getres (), clock_gettime (), clock_settime () | Thread safety | MT-Safe |
POSIX.1 specifies the following:
Setting the value of the CLOCK_REALTIME clock via clock_settime() 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, independently of the new or old value of the clock.
According to POSIX.1-2001, a process with "appropriate privileges" may set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks using clock_settime(). On Linux, these clocks are not settable (i.e., no process has "appropriate privileges").
On some architectures, an implementation of clock_gettime() is provided in the vdso(7).
POSIX.1-2008.
POSIX.1-2001, SUSv2. Linux 2.6.
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).) POSIX.1-2008 makes these APIs mandatory.
Before Linux added kernel support for CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID, glibc implemented these clocks on many platforms using timer registers 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 be useful only 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.
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).
The program below demonstrates the use of clock_gettime() and clock_getres() with various clocks. This is an example of what we might see when running the program:
$ ./clock_times x CLOCK_REALTIME : 1585985459.446 (18356 days + 7h 30m 59s)
resolution: 0.000000001 CLOCK_TAI : 1585985496.447 (18356 days + 7h 31m 36s)
resolution: 0.000000001 CLOCK_MONOTONIC: 52395.722 (14h 33m 15s)
resolution: 0.000000001 CLOCK_BOOTTIME : 72691.019 (20h 11m 31s)
resolution: 0.000000001
/* clock_times.c
Licensed under GNU General Public License v2 or later. */ #define _XOPEN_SOURCE 600 #include <stdbool.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <time.h> #define SECS_IN_DAY (24 * 60 * 60) static void displayClock(clockid_t clock, const char *name, bool showRes) {
long days;
struct timespec ts;
if (clock_gettime(clock, &ts) == -1) {
perror("clock_gettime");
exit(EXIT_FAILURE);
}
printf("%-15s: %10jd.%03ld (", name,
(intmax_t) ts.tv_sec, ts.tv_nsec / 1000000);
days = ts.tv_sec / SECS_IN_DAY;
if (days > 0)
printf("%ld days + ", days);
printf("%2dh %2dm %2ds",
(int) (ts.tv_sec % SECS_IN_DAY) / 3600,
(int) (ts.tv_sec % 3600) / 60,
(int) ts.tv_sec % 60);
printf(")\n");
if (clock_getres(clock, &ts) == -1) {
perror("clock_getres");
exit(EXIT_FAILURE);
}
if (showRes)
printf(" resolution: %10jd.%09ld\n",
(intmax_t) ts.tv_sec, ts.tv_nsec); } int main(int argc, char *argv[]) {
bool showRes = argc > 1;
displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes); #ifdef CLOCK_TAI
displayClock(CLOCK_TAI, "CLOCK_TAI", showRes); #endif
displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes); #ifdef CLOCK_BOOTTIME
displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes); #endif
exit(EXIT_SUCCESS); }
date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3), clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3), sysconf(3), timespec(3), time(7), time_namespaces(7), vdso(7), hwclock(8)
| 2023-07-20 | Linux man-pages 6.05.01 |