Visit Our UNIX and Linux User Community


Signal handling in Perl


 
Thread Tools Search this Thread
Top Forums Shell Programming and Scripting Signal handling in Perl
# 1  
Old 12-19-2007
PHP Signal handling in Perl

Guys,

I'm doing signal handling in Perl. I'm trying to catch ^C signal inside the script.

There two scripts : one shell script and one perl script.

The shell script calls the perl script.

For e.g. shell script a.sh and perl scipt sig.pl.

Shell script a.sh looks something like this :

#!/usr/bin/sh

# some code

sig.pl # Call perl script.

# some more code.

Perl script sig.pl looks something like this :

#!/usr/bin/perl

#some code

#signal handling

$SIG{'INT'} = 'Handler';

sub Handler {
print "Caught ^C \n";
exit (0);
}

# Some more code.

The point is : The signal gets caught in sig.pl, but it doesn't return to
a.sh, but instead returns to the command prompt.

I need to do more processing in shell script a.sh.

I've used die() as well but the result is the same.

Does anyone have any idea how to make the perl script return back to the parent shell script after catching the signal.

Thanks.
# 2  
Old 12-19-2007
It's nothing to do with Perl, remember the shell script will get the ^C as well.

Add a trap handler to the shell script.
# 3  
Old 12-19-2007
Hi.

This worked for me. Here's the simple driver script:
Code:
#!/bin/sh -

# @(#) s1       Demonstrate catching signal in perl, return to shell.

echo
echo " Calling perl script."

./user1

echo
echo " Returned from perl script."

exit 0

and the perl script it calls is:
Code:
#!/usr/bin/perl

$SIG{'INT'} = 'Handler';

#some code
print "\n Will wait for you to enter ^C ";
my ($junk) + <>;

#signal handling

sub Handler {
  print "\n Caught ^C \n";
  exit(0);
}

I moved the hash setting above the print and read. Executed twice, once with a simple RETURN and once with ^C yields:
Code:
user-problem/228 % ./s1

 Calling perl script.

 Will wait for you to enter ^C

 Returned from perl script.
user-problem/228 % ./s1

 Calling perl script.

 Will wait for you to enter ^C
 Caught ^C

 Returned from perl script.

So in both cases, control returns to the shell. My understanding is that signals are caught by the perl process, but not seen by the shell process, like most things for parent-child processes.

If you replaced the shell process with the perl script, say with exec, then the behavior described by the OP would be seen. I verified that with a separate script.

The Programming Perl 3rd, p 413, advises not doing much in the handler beyond setting a global variable, q.v. So I suppose it's possible that if you had a lot of code in the handler, you might run into trouble; PP suggests that a memory fault could occur, even for print statements.

Do this help or confuse the issue? ... cheers, drl

Last edited by drl; 12-19-2007 at 04:52 PM.. Reason: Add PP reference.
# 4  
Old 12-19-2007
On Solaris all processes got the control C.

Code:
$ cat a.sh
#!/bin/sh

SCRIPT=$0

echo $0 started

mysig()
{
   echo $SCRIPT SIGINT
}

trap mysig 2

./b.sh

echo $0 exited

Code:
$ cat b.sh
#!/bin/sh

SCRIPT=$0

echo $0 started

mysig()
{
   echo $SCRIPT SIGINT
}

trap mysig 2

sleep 1000

echo $0 exited

results...

Code:
$ ./a.sh
./a.sh started
./b.sh started
^C./b.sh SIGINT
./b.sh exited
./a.sh SIGINT
./a.sh exited

# 5  
Old 12-19-2007
Hi, porter.

Interesting. Thanks for the example.

So in this Solais situation the OP needs to add a trap to his shell script to ignore INT (SIGINT) signals.

I don't immediately see the advantage to this way of doing things, but the elegance of design often escapes me ... cheers, drl
# 6  
Old 12-19-2007
Quote:
Originally Posted by drl
I don't immediately see the advantage to this way of doing things, but the elegance of design often escapes me ... cheers, drl
The SIGINT goes to all processes associated with the same controlling terminal (ie the one you type control-C on).
# 7  
Old 12-20-2007
Hi.

The results in post # 3 are from GNU/Linux Debian, kernel 2.6.11.

Another data point is for FreeBSD 4.11. It works the same way as it did on GNU/Linux ... cheers, drl

Previous Thread | Next Thread
Test Your Knowledge in Computers #864
Difficulty: Medium
In 2019, the UNIX.com UserCP was rewritten in Vue.js by Neo.
True or False?

10 More Discussions You Might Find Interesting

1. Programming

problem in reforking and signal handling

hi friends i have a problem in signal handling ... let me explain my problem clearly.. i have four process .. main process forks two child process and each child process again forks another new process respectively... the problem is whenever i kill the child process it is reforking and the... (2 Replies)
Discussion started by: senvenugopal
2 Replies

2. UNIX and Linux Applications

SIGSEGV Signal handling

Hello, Can anybody tell me how can i handle segmentation fault signal, in C code? (2 Replies)
Discussion started by: mustus
2 Replies

3. Programming

problem in SIGSEGV signal handling

i wrote handler for sigsegv such that i can allocate memory for a variable to which sigsegv generated for illlegal acces of memory. my code is #include <signal.h> #include<stdio.h> #include<stdlib.h> #include<string.h> char *j; void segv_handler(int dummy) { j=(char *)malloc(10); ... (4 Replies)
Discussion started by: pavan6754
4 Replies

4. Programming

Signal handling

I am trying to write a small program where I can send signals and then ask for an action to be triggered if that signal is received. For example, here is an example where I am trying to write a programme that will say you pressed ctrl*c when someone presses ctrl+c. My questions are what you would... (1 Reply)
Discussion started by: #moveon
1 Replies

5. Programming

Signal Handling and Context Switches

Hi guys, this is my first posting, so at first hi to everyone! ;) I have a problem with ucontext_t in connection with signal handling. I want to simulate a preemptive scheduler. I am using the iTimer with ITIMER_PROF, to schedule the interrupts. You find the code below: #include <stdio.h>... (18 Replies)
Discussion started by: XComp
18 Replies

6. Programming

signal handling while in a function other than main

Hi, I have a main loop which calls a sub loop, which finally returns to the main loop itself. The main loop runs when a flag is set. Now, I have a signal handler for SIGINT, which resets the flag and thus stops the main loop. Suppose I send SIGINT while the program is in subloop, I get an error... (1 Reply)
Discussion started by: Theju
1 Replies

7. Programming

signal handling question

Hello all, I am starting to learn signal handling in Linux and have been trying out some simple codes to deal with SIGALRM. The code shown below sets a timer to count down. When the timer is finished a SIGALRM is produced. The handler for the signal just increments a variable called count. This... (7 Replies)
Discussion started by: fox_hound_33
7 Replies

8. Programming

Signal Handling

Hi folks I'm trying to write a signal handler (in c on HPUX) that will catch the child process launched by execl when it's finished so that I can check a compliance file. The signal handler appears to catch the child process terminating however when the signal handler completes the parent... (3 Replies)
Discussion started by: themezzaman
3 Replies

9. UNIX for Advanced & Expert Users

signal handling in shell script

Hi can any please tell me is it possible to catch the signal in a shell script like we do in C. if yes please give me some idea or a link. (4 Replies)
Discussion started by: Raom
4 Replies

10. UNIX for Advanced & Expert Users

Handling SIGUSR2 signal

HI, I need to handle SIGUSR2 signal in my application to change the state of the application dynamically. I have implemented the signal handler. However the application is able to catch only one SIGUSR2 signal. The second SIGUSR2 signal causes the application to crash. This is happning only with... (3 Replies)
Discussion started by: diganta
3 Replies

Featured Tech Videos