They are signals that application developers use. The kernel shouldn't ever send these to a process. You can send them using kill(2)
or using the utility kill(1)
.
If you intend to use signals for synchronization you might want to check real-time signals (there's more of them, they are queued, their delivery order is guaranteed etc).
terminal 1
dd if=/dev/sda of=debian.img
terminal 2
killall -SIGUSR1 dd
go back to terminal 1
34292201+0 records in
34292200+0 records out
17557606400 bytes (18 GB) copied, 1034.7 s, 17.0 MB/s
Source: Stackoverflow.com