clock_getcpuclockid(3)

🌸 clock_getcpuclockid — the manual page, bestie edition 🌸

okay so you found yourself staring at a clock_getcpuclockid(3) page — that's a Library Functions Manual entry, section 3, aka the "library calls" section, and honestly? she's a little icon. let's get into it 💅

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✨ NAME ✨

clock_getcpuclockid — obtain ID of a process CPU-time clock 👑

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🎀 LIBRARY 🎀

Standard C library (libc, -lc), and she's been there since glibc 2.17 🫶

But listen — before glibc 2.17 you had to link the Real-time library: (librt, -lrt). throwback era fr.

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💖 SYNOPSIS 💖

#include <time.h>

int clock_getcpuclockid(pid_t pid, clockid_t *clockid);

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

clock_getcpuclockid():

_POSIX_C_SOURCE >= 200112L

like yes you have to ask for her. she doesn't just show up. 🙈

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🌈 DESCRIPTION 🌈

The clock_getcpuclockid() function goes and gets the ID of the CPU-time clock of the process whose ID is pid, and drops it into the location pointed to by clockid. that's it, that's the whole job, and she does it flawlessly ✨

And here's the cute part: if pid is zero, then you get back the clock ID of the CPU-time clock of the calling process — i.e. you. main character energy, literally. 💅

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⭐ RETURN VALUE ⭐

On success, clock_getcpuclockid() returns 0. slay. 🎉

On error, it returns one of the positive error numbers listed down in ERRORS. not so slay. 😭

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💀 ERRORS 💀

• ENOSYS — the kernel does not support obtaining the per-process CPU-time clock of another process, and pid does not specify the calling process. so like, the kernel said "no" and you asked about somebody who isn't you. awkward.
• EPERM — the caller does not have permission to access the CPU-time clock of the process specified by pid. (Specified in POSIX.1-2001; does not occur on Linux unless the kernel does not support obtaining the per-process CPU-time clock of another process.) so on Linux you're usually fine, but the standard still says it could happen 🫠
• ESRCH — there is no process with the ID pid. bestie doesn't exist. ghosted, literally. 💀

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🧚 ATTRIBUTES 🧚

For an explanation of the terms used in this section, see attributes(7).

• Interface: clock_getcpuclockid()
• Attribute: Thread safety
• Value: MT-Safe

so yes she's MT-Safe — go off, take her to every thread, she will not betray you 🫶✨

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👑 STANDARDS 👑

POSIX.1-2008. certified. iconic. periodt.

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🌸 HISTORY 🌸

glibc 2.2. POSIX.1-2001. she's been around, she's seen things 🎀

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💜 NOTES 💜

Calling clock_gettime(2) with the clock ID you got from a call to clock_getcpuclockid() where the pid was 0, is the same as using the clock ID CLOCK_PROCESS_CPUTIME_ID.

...so you can do it the long way or the short way, they're literally the same thing. no cap. 🔥

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🍓 EXAMPLES 🍓

The example program below obtains the CPU-time clock ID of the process whose ID is given on the command line, and then uses clock_gettime(2) to obtain the time on that clock. An example run is the following:

$ ./a.out 1                 # Show CPU clock of init process
CPU-time clock for PID 1 is 2.213466748 seconds

🦋 Program source 🦋

#define _XOPEN_SOURCE 600
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    clockid_t clockid;
    struct timespec ts;

    if (argc != 2) {
        fprintf(stderr, "%s <process-ID>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (clock_getcpuclockid(atoi(argv[1]), &clockid) != 0) {
        perror("clock_getcpuclockid");
        exit(EXIT_FAILURE);
    }

    if (clock_gettime(clockid, &ts) == -1) {
        perror("clock_gettime");
        exit(EXIT_FAILURE);
    }

    printf("CPU-time clock for PID %s is %jd.%09ld seconds\n",
           argv[1], (intmax_t) ts.tv_sec, ts.tv_nsec);
    exit(EXIT_SUCCESS);
}

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🫶 SEE ALSO 🫶

clock_getres(2), timer_create(2), pthread_getcpuclockid(3), time(7)

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okay that's the whole page bestie, now go clock somebody's CPU time 💖

and hey — this one came out of Linux man-pages 6.20, dated 2025-05-17. ty for reading, mwah 💅✨🌈