Thorsten Kukuk wrote:
> > What API do you suggest we use instead?
>
> For me clock_gettime works very well:
> https://github.com/thkukuk/utmpx/blob/main/utmp-to-logind.md#determine-boot-time
Indeed, this provides the boot time with a resolution of 1 µsec. Whereas the
/proc/uptime approach only has a resolution of 10000 µsec. Thanks for the
suggestion.
2023-08-08 Bruno Haible <br...@clisp.org>
readutmp: Get the boot time with higher precision.
Suggested by Thorsten Kukuk <ku...@suse.com> in
<https://github.com/thkukuk/utmpx/blob/main/utmp-to-logind.md#determine-boot-time>.
* lib/readutmp.c (get_boot_time_uncached): Try clock_gettime first.
diff --git a/lib/readutmp.c b/lib/readutmp.c
index 7ef5bfe84c..f7e43eb4a6 100644
--- a/lib/readutmp.c
+++ b/lib/readutmp.c
@@ -284,6 +284,31 @@ finish_utmp (struct utmp_alloc a)
static struct timespec
get_boot_time_uncached (void)
{
+ /* The clock_gettime facility returns the uptime with a resolution of 1 µsec.
+ It is available with glibc >= 2.14. In glibc < 2.17 it required linking
+ with librt. */
+# if __GLIBC__ + (__GLIBC_MINOR__ >= 17) > 2
+ struct timespec up;
+ if (clock_gettime (CLOCK_BOOTTIME, &up) >= 0)
+ {
+ struct timespec result;
+ /* equivalent to:
+ if (clock_gettime (CLOCK_REALTIME, &result) >= 0)
+ */
+ if (timespec_get (&result, TIME_UTC) >= 0)
+ {
+ if (result.tv_nsec < up.tv_nsec)
+ {
+ result.tv_nsec += 1000000000;
+ result.tv_sec -= 1;
+ }
+ result.tv_sec -= up.tv_sec;
+ result.tv_nsec -= up.tv_nsec;
+ return result;
+ }
+ }
+# endif
+
/* /proc/uptime contains the uptime with a resolution of 0.01 sec. */
FILE *fp = fopen ("/proc/uptime", "re");
if (fp != NULL)
