Process – How to Check Signals a Process is Listening To

processsignals

How can I verify whether a running process will catch a signal, or ignore it, or block it? Ideally I'd like to see a list of signals, or at least not have to actually send the signal to check.

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Under Linux, you can find the PID of your process, then look at /proc/$PID/status. It contains lines describing which signals are blocked (SigBlk), ignored (SigIgn), or caught (SigCgt).

# cat /proc/1/status
...
SigBlk: 0000000000000000
SigIgn: fffffffe57f0d8fc
SigCgt: 00000000280b2603
...

The number to the right is a bitmask. If you convert it from hex to binary, each 1-bit represents a caught signal, counting from right to left starting with 1. So by interpreting the SigCgt line, we can see that my init process is catching the following signals:

00000000280b2603 ==> 101000000010110010011000000011
                     | |       | ||  |  ||       |`->  1 = SIGHUP
                     | |       | ||  |  ||       `-->  2 = SIGINT
                     | |       | ||  |  |`----------> 10 = SIGUSR1
                     | |       | ||  |  `-----------> 11 = SIGSEGV
                     | |       | ||  `--------------> 14 = SIGALRM
                     | |       | |`-----------------> 17 = SIGCHLD
                     | |       | `------------------> 18 = SIGCONT
                     | |       `--------------------> 20 = SIGTSTP
                     | `----------------------------> 28 = SIGWINCH
                     `------------------------------> 30 = SIGPWR

(I found the number-to-name mapping by running kill -l from bash.)

EDIT: And by popular demand, a script, in POSIX sh.

sigparse () {
    i=0
    # bits="$(printf "16i 2o %X p" "0x$1" | dc)" # variant for busybox
    bits="$(printf "ibase=16; obase=2; %X\n" "0x$1" | bc)"
    while [ -n "$bits" ] ; do
        i="$(expr "$i" + 1)"
        case "$bits" in
            *1) printf " %s(%s)" "$(kill -l "$i")" "$i" ;;
        esac
        bits="${bits%?}"
    done
}

grep "^Sig...:" "/proc/$1/status" | while read a b ; do
        printf "%s%s\n" "$a" "$(sigparse "$b")"
    done # | fmt -t  # uncomment for pretty-printing

Related Solutions

Process Signals – Causes of Various Signals Sent to Processes

In addition to processes calling kill(2), some signals are sent by the kernel (or sometimes by the process itself) in various circumstances:

Terminal drivers send signals corresponding to various events:
- Key press notifications: SIGINT (please go back to the main loop) on Ctrl+C, SIGQUIT (please quit immediately) on Ctrl+\, SIGTSTP (please suspend) on Ctrl+Z. The keys can be changed with the stty command.
- SIGTTIN and SIGTTOU are sent when a background process tries to read or write to its controlling terminal.
- SIGWINCH is sent to signal that the size of the terminal window has changed.
- SIGHUP is sent to signal that the terminal has disappeared (historically because your modem had hung up, nowadays usually because you've closed the terminal emulator window).
Some processor traps can generate a signal. The details are architecture and system dependent; here are typical examples:
- SIGBUS for an unaligned access memory;
- SIGSEGV for an access to an unmapped page;
- SIGILL for an illegal instruction (bad opcode);
- SIGFPE for a floating-point instruction with bad arguments (e.g. sqrt(-1)).
A number of signals notify the target process that some system event has occured:
- SIGALRM notifies that a timer set by the process has expired. Timers can be set with alarm, setitimer and others.
- SIGCHLD notifies a process that one of its children has died.
- SIGPIPE is generated when a process tries to write to a pipe when the reading end has been closed (the idea is that if you run foo | bar and bar exits, foo gets killed by a SIGPIPE).
- SIGPOLL (also called SIGIO) notifies the process that a pollable event has occured. POSIX specifies pollable events registered through the I_SETSIG ioctl. Many systems allow pollable events on any file descriptor, set via the O_ASYNC fcntl flag. A related signal is SIGURG, which notifies of urgent data on a device (registered via the I_SETSIG ioctl) or socket.
- On some systems, SIGPWR is sent to all processes when the UPS signals that a power failure is imminent.

These lists are not exhaustive. Standard signals are defined in signal.h.

Most signals can be caught and handled (or ignored) by the application. The only two portable signals that cannot be caught are SIGKILL (just die) and STOP (stop execution).

SIGSEGV (segmentation fault) and its cousin SIGBUS (bus error) can be caught, but it's a bad idea unless you really know what you're doing. A common application for catching them is printing a stack trace or other debug information. A more advanced application is to implement some kind of in-process memory management, or to trap bad instructions in virtual machine engines.

Finally, let me mention something that isn't a signal. When you press Ctrl+D at the beginning of a line in a program that's reading input from the terminal, this tells the program that the end of the input file is reached. This isn't a signal: it's transmitted via the input/output API. Like Ctrl+C and friends, the key can be configured with stty.

Processes do not respond to the signals

Recent versions of the nVidia proprietary drivers (possibly combined with other recent versions of libraries) have a bug which causes them to corrupt the signal mask.

You can look at signal masks like this:

anthony@Zia:~$ ps -eo blocked,pid,cmd | egrep -v '^0+ '
         BLOCKED   PID CMD
fffffffe7ffbfeff   605 udevd --daemon
0000000000000002  4052 /usr/lib/policykit-1/polkitd --no-debug
0000000000087007  4646 /usr/sbin/mysqld --basedir=/usr […]
0000000000010000 15508 bash

That's about what it should look like. If you run that on a system with the proprietary nVidia drivers, you'll see all kinds of crazy values for BLOCKED, for many of your programs—including, likely, all the misbehaving ones.

Note that signal masks are passed from parent to child through fork/exec, so once a parent process has a corrupt one, all the children it spawns from that point forward will, too.

See also my question After upgrade, X button in titlebar no longer closes xterm and various distro bugs you'll be able to find now, knowing which package to look at. You can modify the code in my answer to that question to reset the signal mask to none blocked (Elide sigaddset and change SIG_UNBLOCK to SIG_SETMASK).

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Related Solutions

Process Signals – Causes of Various Signals Sent to Processes

Processes do not respond to the signals

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