Many Unix programs accept signals like USR1
and USR2
. For example, to upgrade the executable for Nginx on the fly, you send kill -USR2
.
I understand that USR1
is a "user defined" signal, meaning that whoever created the program can use it to mean "shut down" or "dump your logs" or "print foo a thousand times" or whatever. But I don't understand why they must use this arbitrary name. Why not kill -UPGRADE
, or kill -GRACEFUL_SHUTDOWN
? Does Unix only allow specific signals?
While we're at it, Nginx also uses the following signals (see documentation):
HUP? WINCH? What's the reason for these names? Where can I learn more about this?
The signals available on an OS are defined by the OS (usually following POSIX) - they're not "strings" but rather integer constants with standard names. USR1
and USR2
are the two signals that have no attached specific meaning - intended for whatever arbitrary use the developer wants.
On your linux machine, read man 7 signal
for an overview of signal handling and signals.
You can redefine the meaning of other signals if you're prepared to deal with the OS issuing those signals in response to events. You can e.g. make HUP
mean "reload configuration" - as long as you're either certain that the process will never get a hangup (terminal loss), or you're prepared to handle cases where the OS and not a user sends the HUP signal.
HUP
is short for "hang-up". This signal is sent to a process if its controlling terminal reaches end-of-file. In the old days, controlling terminals were typically attached to serial ports, possibly via a modem link over a telephone line. If the telephone connection was hung up, the local modem would lower the Carrier Detect line, which would result in the kernel reporting end-of-file and a SIGHUP
signal being sent.
WINCH
is short for "window change". It is sent to a process if its controlling terminal changes size. For obvious reasons, terminals that can change size are usually pseudo-terminals ultimately represented by a terminal emulator running in a windowing environment (like xterm
).
Try kill -l
and find the answer yourself:
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3
38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8
43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7
58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2
63) SIGRTMAX-1 64) SIGRTMAX
Because the names of the signals are standardised (by POSIX). You can write your own kill-type executable to take -UPGRADE
if you want and have it deliver the USR1
signal, but the standard kill
that comes with UNIX will not recognise it.
Alternatively, you could create an alias, function or shell script to do the translation for you, such as with the bash
alias:
alias upgrade='kill -USR1'
The signal.h
header file maps the signal names to their actual values, which are implementation-dependent.
In terms of WINCH
, I consider this a bit of an abomination. This is the signal that is delivered to applications when their window size changes (specifically when the window of their controlling terminal changes).
Using it to gracefully shut down worker threads is not a good idea unless you can guarantee the process will never run in a terminal. I know I'd be pretty miffed if I was running an application and it decided to cancel all work in flight just because I maximised the window :-)
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