Surprised by the behavior of pressing Ctrl+T during sleep(1)

TL;DR

On BSD-based systems, pressing Ctrl+T displays the status of the foreground process group.

% uname
FreeBSD
% sleep 3
load: 0.25  cmd: sleep 4097 [nanslp] 1.77r 0.00u 0.00s 0% 2060k
mi_switch+0xc2 sleepq_catch_signals+0x2e6 sleepq_timedwait_sig+0x12 _sleep+0x1d1 kern_clock_nanosleep+0x1c1 sys_nanosleep+0x3b amd64_syscall+0x10c fast_syscall_common+0xf8 
sleep: about 1 second(s) left out of the original 3
% 

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FreeBSD's sleep(1)

It seems that sending SIGINFO to FreeBSD's sleep(1) results in some interesting behavior.

If the sleep command receives a signal, it takes the standard action. When the SIGINFO signal is received, the estimate of the amount of seconds left to sleep is printed on the standard output.

Let's try it out.

Trying to send SIGINFO to sleep(1)

% uname -mrsv
FreeBSD 13.1-RELEASE-p1 FreeBSD 13.1-RELEASE-p1 GENERIC amd64
% sleep 20 &
[1] 3800
% kill -INFO 3800
sleep: about 13 second(s) left out of the original 20
% kill -INFO 3800
sleep: about 10 second(s) left out of the original 20
% kill -INFO 3800
sleep: about 6 second(s) left out of the original 20
% kill -INFO 3800
sleep: about 3 second(s) left out of the original 20
% kill -INFO 3800
sleep: about 2 second(s) left out of the original 20
% kill -INFO 3800
sleep: about 1 second(s) left out of the original 20
% kill -INFO 3800
sleep: about 0 second(s) left out of the original 20
% kill -INFO 3800
3800: No such process
[1]    Done                          sleep 20
% 

Yes, it certainly seems to report it.

I also tried it with NetBSD and OpenBSD implementations, and NetBSD showed similar behavior.

What is SIGINFO

The SIGINFO signal is one of the BSD extension signals, representing a status request from the keyboard. The default action when it is triggered is to ignore it. This signal does not seem to exist in Linux^[1].

The SIGINFO signal can be triggered (unless the key mapping has been changed by stty(1) or similar) by pressing Ctrl+T. The signal triggered here is delivered to the foreground process group of that terminal. Furthermore, it displays the status of the foreground process group.

Trying it out

Let's try inputting uname, cat, ^T, ^D, sleep 3, and ^T on various BSDs.

FreeBSD

% uname 
FreeBSD
% cat
load: 0.15  cmd: cat 4233 [ttyin] 0.65r 0.00u 0.00s 0% 2096k
mi_switch+0xc2 sleepq_catch_signals+0x2e6 sleepq_wait_sig+0x9 _cv_wait_sig+0xec tty_wait+0x1c ttydisc_read+0x2cc ttydev_read+0x56 devfs_read_f+0xd5 dofileread+0x81 sys_read+0xbc amd64_syscall+0x10c fast_syscall_common+0xf8 
% sleep 3
load: 0.13  cmd: sleep 4243 [nanslp] 0.37r 0.00u 0.00s 0% 2060k
mi_switch+0xc2 sleepq_catch_signals+0x2e6 sleepq_timedwait_sig+0x12 _sleep+0x1d1 kern_clock_nanosleep+0x1c1 sys_nanosleep+0x3b amd64_syscall+0x10c fast_syscall_common+0xf8 
sleep: about 2 second(s) left out of the original 3
% 

OpenBSD

$ uname
OpenBSD
$ cat
load: 0.57  cmd: cat 58626 [ttyin] 0.00u 0.00s 0% 55k
$ sleep 3
load: 0.53  cmd: sleep 94438 [nanoslp] 0.00u 0.00s 0% 24k
$ 

NetBSD

$ uname
NetBSD
$ cat
[ 218.3727855] load: 0.03  cmd: cat 492 [ttyraw] 0.00u 0.00s 0% 1092k
$ sleep 3
[ 225.5874919] load: 0.02  cmd: sleep 45 [nanoslp] 0.00u 0.00s 0% 1208k
sleep: Between 2 and 3 seconds left out of the original 3
$ 

Conclusion

That's it.

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References

• FreeBSD Project. sleep(1). FreeBSD Manual Pages. 2020-12-31. (accessed 2022-08-21).
• JM Project. Man page of SIGNAL. JM Project. 2020-12-21. (accessed 2022-08-21).
• W. Richard Stevens; Stephen A. Rago. “10 Signals”; “18 Terminal I/O”. Advanced Programming in the UNIX Environment (Japanese title: 詳解 UNIX プログラミング). Translated by Atsuo Ohki. 3rd Edition, Shoeisha, 2014, p.295–358; p.625–662, ISBN978-4-7981-3488-8.

脚注

1. It seems to be a synonym for SIGPWR on some architectures.