Tuesday, February 22, 2022

[SOLVED] Getting uptime on a kernel module?

Issue

I've got this code where I'm trying to make a kernel module print the uptime of the system, exactly on the simple_init.

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <sys/sysinfo.h>

/* This function is called when the module is loaded. */

int simple_init(void)
{
    struct sysinfo info;
    sysinfo(&info);
    printk("This pc has been on for %ld seconds\n", info.uptime);;
    printk(KERN_INFO "Loading Module\n");
    return 0;
}

This is what I would do if this wasn't a kernel module, I found that there's a similar linux library for sysinfo which is linux/sysinfo, but even when I use that one, it only has a Struct sysinfo and not a function I can call sysinfo(), and when I try to do so I get

error: implicit declaration of function ‘sysinfo’ [-Werror=implicit-function-declaration]
     sysinfo(&info);

Does anyone knows any other effective approach?

Thanks


Solution

Since the information you seek is provided by the kernel pseudofile /proc/uptime, we can look at fs/proc/uptime.c:uptime_proc_show() in the kernel sources to see how the information is gathered.

Currently, the relevant code is

#include <linux/ktime.h>

    struct timespec  uptime;
    get_monotonic_boottime(&uptime);

where uptime.tv_sec is the number of seconds, and uptime.tv_nsec nanoseconds (0 to 999,999,999 inclusive).

However, since the kernel is moving to 64-bit time, you'd better use

#include <linux/ktime.h>

    s64  uptime_ms;
    uptime_ms = ktime_to_ms(ktime_get_boottime());

to get the uptime in milliseconds. If you only need the full seconds, use

#include <linux/ktime.h>

    s64  uptime;
    uptime = ktime_divns(ktime_get_coarse_boottime(), NSEC_PER_SEC);

("coarse" meaning only the full seconds part are read.)



Answered By - Nominal Animal
Answer Checked By - Terry (WPSolving Volunteer)
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