Using the test program found at the bottom of the Linux [[ https://man7.org/linux/man-pages/man2/timerfd_create.2.html | timerfd man page ]]:
./timerfd 3 1 100
0.000: timer started
3.001: read: 1; total=1
4.011: read: 1; total=2 # random variability in response
5.001: read: 1; total=3
6.001: read: 1; total=4
7.001: read: 1; total=5
^Z[1] + Suspended ./timerfd 3 1 100
root@freebsd:~/tests # fg
./timerfd 3 1 100
12.893: read: 5; total=10
13.001: read: 1; total=11
14.001: read: 1; total=12
15.001: read: 1; total=13