CPython Signal Handler Check for SIGKILL

[Thomas Jollans]

On 07/19/2010 07:28 PM, Scott McCarty wrote:
> All,  I just want to understand the C/Python piece better because I am
> writing a tutorial on signals and I am using python to demonstrate. I
> thought it would be fun to show that the SIGKILL is never processed, but
> instead python errors out. There is something in Python checking the
> SIGKILL signal handler, while not checking SIGTERM and I can't find the
> Python C code that handles it. When I am writing something like this, I
> like to point out the C code, not just cite the documentation (I did
> find this change in behaviour noted in the Python change log).

You cannot handle SIGKILL. Nothing to do with Python. Let me demonstrate:

0:pts/8:/tmp% cat sig.c
#include <signal.h>
#include <stdio.h>

void handle_sig(int sig)
{
    printf("SIGNAL: %d\n", sig);
}

int main(int argc, char **argv)
{
    if (signal(SIGUSR1, handle_sig) == SIG_ERR)
        printf("failed to set for USR1\n");
    if (signal(SIGTERM, handle_sig) == SIG_ERR)
        printf("failed to set for TERM\n");
    if (signal(SIGKILL, handle_sig) == SIG_ERR)
        printf("failed to set for KILL\n");

    for(;;);
}

0:pts/8:/tmp% cc -osig sig.c
0:pts/8:/tmp% ./sig
failed to set for KILL
^C
130:pts/8:/tmp%

> 
> I have searched everywhere (mostly the code and a little google) and I
> cannot understand where the SIGKILL signal gets checked when it is set
> as a handler. I have scoured the Modules/signalmodule.c only to find two
> instances of the RuntimeError exception, but I cannot understand how
> python knows when a handler is set for SIGKILL. I understand that this
> changed in 2.4 and I am not trying to change it, I just really want to
> understand where this happens. I used grep to find SIGKILL and SIGTERM
> to see if I could determine where the critical difference is, but I
> cannot figure it out.
> 
> I have about 2 hours of searching around and I can't figure it out, I
> assume it has to rely on some default behaviour in Unix, but I have no
> idea. I don't see a difference between SIGKILL and SIGTERM in the python
> code, but obviously there is some difference. I understand what the
> difference is in Unix/Linux, I just want to see it in the python code.
> Since python is checking at run time to see what signals handlers are
> added, I know there must be a difference. I am not asking about the
> signals, I understand them, I am asking about the registration of the
> SIGNAL handler and how it knows that you are trying to register SIGKILL,
> you get an error like this.
> 
>  ./signal-catcher.py
> Traceback (most recent call last):
>   File "./signal-catcher.py", line 22, in <module>
>     signal.signal(signal.SIGKILL, signal_handler_kill)
> RuntimeError: (22, 'Invalid argument')
> 
> And the code is very simple, this attempts to register a handler for
> SIGKILL, but python knows and won't let you.
> 
> signal.signal(signal.SIGKILL, signal_handler_kill)
> 
> Please  can someone just point me in the right direction.
> 
> Thank You
> Scott M
>