CLOCK_GETTIME(2)

💖 CLOCK_GETTIME(2) 💖

okay bestie, welcome to the System Calls Manual section, page 2, and today we're talking about time itself — getting it, setting it, and finding out how precise it is. clock energy fr fr ⏰✨

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✨ NAME ✨

clock_gettime, clock_settime, clock_getres — get/set/calibrate date and time 🕰️

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📚 LIBRARY 📚

Standard C Library (libc, -lc) 🫶

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🌸 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);

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🦋 DESCRIPTION 🦋

so the clock_gettime() and clock_settime() system calls let the calling process retrieve or set the value used by a clock, and you pick which clock by handing over clock_id. iconic behaviour honestly.

The clock_id argument can be a value you got from clock_getcpuclockid(3) or pthread_getcpuclockid(3), or one of these lil guys:

• 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 the corresponding IDs without the _FAST suffix, but they 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 💀 (no free lunch, periodt).

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, and only using 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. so like... security said no but adjtime(2) said bet 🔥

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

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🎀 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. classic. 🙈

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💅 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. bestie you are NOT him 🚫

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🌈 SEE ALSO 🌈

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

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⭐ STANDARDS ⭐

The clock_gettime(), clock_settime(), and clock_getres() system calls conform to IEEE Std 1003.1b-1993 (“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. slay 💖

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👑 HISTORY 👑

The clock_gettime(), clock_settime(), and clock_getres() system calls first appeared in FreeBSD 3.0. they've been iconic since day one, no cap.

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and that's the whole page bestie!! this one shipped from Debian on December 7, 2019 and lives at the bottom of page 2 of the CLOCK_GETTIME manual. go forth and measure time responsibly 🫶💖✨🦋