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Perl: trap signal 'exit': why I am not able to have it work??


 
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# 1  
Old 10-30-2017
Perl: trap signal 'exit': why I am not able to have it work??

First time trying to work with signals in Perl.
Reviewing example I try it, but not able to get it work for 'exit'.
I hope, I am correct, assuming, that the ending any code by
Code:
exit $return_code;

the $SIG{EXIT} should be de-referenced and processed?!
So, I have such code, that, I assume, should, but does not process signal handling:
Code:
#!/bin/perl
sub trp_h{
  print "\nIn 'trp_h()'. ".
       "\nReceived parameters \@_: ".(join ", ",@_).
       "\n\$! as a number: ".($!+0).", as a string: ".(" ".$!).
       "\n now 'sleep 5' before return.";
  sleep 5;
}
$SIG{EXIT}='trp_h';
print "\nSet handler: $SIG{EXIT}\n";
exit(5);
print "after first exit";

$SIG{EXIT}=\&trp_h;
print "\nSet handler to code: $SIG{EXIT}\n";
exit(3);
print "after second exit\n";

print "Restoring to default\n";
$SIG{EXIT}='DEFAULT';
exit(2);
exit 1;

Please, help me understand what is wrong or,
if I mistaken on assumption that the 'exit()' is processed by the $SIG{EXIT}, how it could be handled to process activity, such as in UNIX 'trap "..." EXIT' command?

Thanks!
# 2  
Old 10-30-2017
There is no SIGEXIT on UNIX systems. Standards conforming UNIX shells can set a trap on exit such as:
Code:
trap 'rm -f" $tempfile"' EXIT

which will cause the shell to execute the commands in that trap just before exiting. Note that the exit utility in the shell command language looks to see if a trap on exit has been installed (and if so invokes it before issuing the UNIX exit(exit_code); system call). There is no return from that trap handler -- once the commands specified in the exit condition trap handler complete, the shell exits.

I make no claim to any knowledge about how perl handles $SIG{EXIT}=handler;, but if perl handles it similarly to the way shells handle trap 'commands' EXIT, that trap will only be executed once. If perl's $SIG{EXIT}=handler; is similar to setting a signal handler in the C programming language; there is no SIGEXIT so setting a signal handler for that signal should fail.
This User Gave Thanks to Don Cragun For This Post:
# 3  
Old 10-31-2017
Thanks, Don Cragun, for the answer and information.
Yes, I have realized that there is no EXIT signal neither in shell (by kill -l), nor in Perl %SIG hash initially. Also, the $Config{sig_name} does not have it (available after 'use Config;'.)
But, some how the perl should process the 'exit()'.
As you mentioned and I have seen, in shell the 'trap EXIT' works, at least, once.
OK, I could accept, the EXIT in form of signal (say, just defined) is uninterruptible, but, even having the $SEG{EXIT} defined, it is not processed!
I my example it would, at least print out some info! No, not happening!
I would be completely fine correcting it to have the EXIT-trap be processed one time! That what I am looking for, actually!
# 4  
Old 10-31-2017
It looks like the $SIG(signal_number) function in perl sets a signal catching function for the signal specified by signal_number. Since no such signal is every received by perl, that signal handler is never invoked.

As I said, I'm not fluent in perl, but it is obvious that the mechanism you're using is not designed to work that way in perl. You need to either find another way to do it in perl or extend perl to do what you want. Just making up a new signal name isn't going to magically make perl guess that a should send itself a signal (that the underlying operating system doesn't provide) before it is terminated by some other signal or by the user exiting the code perl is running.

Or, maybe you could convert your perl script to a shell script and use the shell command language's trap command to do what you want?
# 5  
Old 11-01-2017
OK; Thanks, Don, for discussing all that with me!
Appreciate it!
I have found answer for my situation and that is simple!
Just I've been following the shell script activity and did not guess simplest way:
- the END{..} block!
That is any activities to be processed by any Perl script regular exit!
Thus; no any need to catch EXIT at all!
This User Gave Thanks to alex_5161 For This Post:
# 6  
Old 11-03-2017
See this link. When Perl encounters the exit keyword it will exit, but not before executing any END blocks defined by your code or imported packages.
Example:
Code:
#!/bin/perl
END { printf qq(Bye bye!\n); }
exit;

Andrew
This User Gave Thanks to apmcd47 For This Post:

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