NAME
Sys::SigAction - Perl extension for Consistent Signal Handling
SYNOPSYS
#do something non-interupt able
use Sys::SigAction qw( set_sig_handler );
{
my $h = set_sig_handler( 'INT' ,'mysubname' ,{ flags => SA_RESTART } );
... do stuff non-interupt able
} #signal handler is reset when $h goes out of scopeor
#timeout a system call:
use Sys::SigAction qw( set_sig_handler );
eval {
my $h = set_sig_handler( 'ALRM' ,\&mysubname ,{ mask=>'ALRM' ,safe=>1 } );
alarm(2)
... do something you want to timeout
alarm(0);
}; #signal handler is reset when $h goes out of scope
alarm(0);
if ( $@ ) ...or
use Sys::SigAction;
my $alarm = 0;
eval {
my $h = Sys::SigAction::set_sig_handler( 'ALRM' ,sub { $alarm = 1; } );
alarm(2)
... do something you want to timeout
alarm(0);
};
alarm(0);
if ( $@ or $alarm ) ...or
use Sys::SigAction;
my $alarm = 0;
Sys::SigAction::set_sig_handler( 'TERM' ,sub { "DUMMY" } );
#code from here on uses new handler.... (old handler is forgotten)or
use Sys::SigAction qw( timeout_call );
if ( timeout_call( 5 ,sub { $retval = DoSomething( @args ); } )
{
print "DoSomething() timed out\n" ;
}ABSTRACT
Implements sig_sethandler, which sets up a signal handler and (optionally) returns an object which causes the signal handler to be reset to the previous value, when it goes out of scope.
DESCRIPTION
Perl has been changing the way unix signals are implemented in an attempt to make them safer. Changes were made between perl 5.6 and perl 5.8, and more changes have been made in versions after perl 5.8. The 5.8 changes broke this author's database connection timeouts. Prior to version 5.8 a signal handlers would interupt system calls like connect() amd perl would immediately call the signal handler.
From the perl 5.8.2 perlvar man page:
The default delivery policy of signals changed in Perl 5.8.0
from immediate (also known as "unsafe") to deferred, also
known as "safe signals". Infortunately this 'deferred signal' approach causes system calls to be retried prior to the signal handler being called. The result is that might could never return. This is the case with the DBD-Oracle connect call, when the host on which a database resides is not available. This makes it impossible to implement open timeouts, at least with code looks like this:
eval {
local $SIG{ALRM} = sub { die "timeout" };
alarm 2;
$sth = DBI->connect(...);
alarm 0;
};
alarm 0;
die if $@;The workaround, if your system has sigaction(), is to use POSIX::sigaction() to install the signal handler. With sigaction(), one gets control over both the signal mask, and the flags that are used to install the handler. Further, with perl 5.8.2 and later a 'safe' switch is provided which can be used to ask for 'safe' signal handling. Using sigaction() does ensure that the system call is interupted, if one calls die within the signal handler. This is not longer the case when one uses $SIG{name} to set signal handlers in perls >= 5.8.0.
The usage of sigaction() however is not well documented however and in perls < 5.8.0 it does not work at all. (fortunately thats OK becase just setting $SIG does work for this purpose in that case.) Using sigaction() requires approximately 4 or 5 lines of code where previously one only had to set a code reference into the %SIG array.
This module wraps up the POSIX:: routines and objects necessary to call sigaction() in a way that is as efficient from coding perspective as just setting a localized $SIG{SIGNAL} with a code reference, with the advantange that the user has control over the flags passed to sigaction(). By default no additional args are passed to sigaction(), and the signal handler will be called when a signal (such as SIGALRM) is delivered.
While sigaction() is not fully functional in perl versions less than 5.8, this module has been tested with perls going back to 5.005 (solaris). With perls < 5.8 this modules just sets $SIG; the flags, mask and safe keys in the attributes hash are silently ignored.
It is hoped that with the use of this module your signal handling behavior can be coded in a way that does not change from one perl version to the next, and that it makes using sigaction() a little easier.
FUNCTIONS
set_sig_handler
$sig ,$handler ,$attrs Install a new signal handler and (if not called in a void context) returning a Sys::SigAction object containing the old signal handler, which will be restored on object destruction.
$sig is a signal name (without the 'SIG') or number.
$handler is either the name (string) of a signal handler
function or a subroutine CODE reference.
$attrs if defined is a hash reference containing the
following keys:
flags => the flags the passed sigaction
ex: SA_RESTART (defined in your signal.h)
mask => the array reference: signals you
do not want delivered while the signal
handler is executing
ex: [ SIGINT SIGUSR1 ] or
ex: [ qw( INT USR1 ]
safe => A bolean value requesting 'safe' signal
handling (usd in 5.8.2 and greater)timeout_call
$timeout ,$coderef Given a code reference, and a timeout value (in seconds), timeout() will (in an eval) setup a signal handler for SIGALRM which will die, set an alarm clock, and execute the code reference.
If the alarm goes off the code will be interupted. The alarm is canceled if the code returns before the alarm is fired. The routine returns true if the code being executed timed out. (was interrupted). Exceptions thrown by the code executed are propagated out.
The original signal handler is restored, prior to returning to the caller.
sig_name
Return the signal name (string) from a signal number.
ex:
sig_name( SIGINT ) returns 'INT'sig_name
Return the signal number (integer) from a signal name (minus the SIG part).
ex:
sig_number( 'INT' ) returns the integer values of SIGINT;AUTHOR
Lincoln A. Baxter <lab@lincolnbaxter.com.make.me.VALID>SEE ALSO
perldoc perlvar
perldoc POSIX