Why can't I kill a `Sl` process?

Question

On Lubuntu 18.04, I open pcmanfm

$ pcmanfm . 

and after looking at the thumbnails of the image files under the current directory in pcmanfm, I closed the window of pcmanfm by Alt-F4, but it is still hanging on the foreground in the terminal emulator.

I move it to background by Ctrl-Z and bg 2, and kill it, but doesn't work.

$ jobs -l 
[2]+ 31124 Running                 pcmanfm . &
$ kill %2
$ jobs -l
[2]+ 31124 Running                 pcmanfm . &
$ sudo kill 31124
$ jobs -l
[2]+ 31124 Running                 pcmanfm . &

Its state is Sl, S means "interruptible sleep (waiting for an event to complete)" and l means "is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)". So I wonder why I can't kill the process? How would you kill it? Thanks.

$ ps aux | grep [3]1124
t        31124  0.8  0.7 693952 57064 pts/9    Sl   06:34   0:47 pcmanfm 

.

Answer 1

By default, kill only sends a TERM signal, for some reason pcmanfm is ignoring this signal. If you pass option -KILL to kill, then it will send the signal to the scheduler, and the process will be removed with no chance to clean-up, or appeal.

You do not need extra privileges (sudo), to kill processes that you own. sudo can be dangerous, don't just use out of frustration.

Answer 2

kill by default sends SIGTERM. This gets handled by the signal handler of the process and the process can:

• install a signal handler that simply does nothing
• have a signal ignored
• mask the signal (and have it delivered once it's unmasked)

I guess that pcmanfm does something like that. You can find the latter two by looking at /proc/PID/status, at SigBlk and SigIgn

SIGKILL (9) on the other hand is not handled by the process itself and cannot have its signal handler changed, be ignored, or be masked.

Try running this python3 program against the pid of pcmanfn to see what exactly it ignores or blocks (needs python 3.5):

#!/usr/bin/python3

import os
import sys
import time
import signal

def show(label, value):
    ivalue = int(value, 16)
    print("%s: %s:"% (label, value.strip()), end=' ')
    cnt=1
    while ivalue:
        if ivalue & 1:
            print("%s(%s)" % (signal.Signals(cnt).name, cnt), end=' ')
        ivalue>>=1
        cnt+=1
    print()

if len(sys.argv)==1:
    pid=os.getpid()
else:
    pid=int(sys.argv[1])

status=open('/proc/%d/status' % (pid,)).readlines()
print("Pid: %d" % (pid,))
for line in status:
    what, value = line.split(':', 1)
    if what=='SigBlk':
        show('Blocked', value)
    elif what=='SigIgn':
        show('Ignored', value)

You should be able to see if SIGTERM is in there.