Sys::SigAction(3pm)					User Contributed Perl Documentation				       Sys::SigAction(3pm)

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 scope

       or

	  #timeout a system call:
	  use Sys::SigAction qw( set_sig_handler );
	  eval {
	     my $h = set_sig_handler( 'ALRM' ,&mysubname ,{ mask=>[ 'ALRM' ] ,safe=>1 } );
	     eval {
		alarm(2)
		... do something you want to timeout
		alarm(0);
	     };
	     alarm(0);
	     die $@ if $@;
	  }; #signal handler is reset when $h goes out of scope
	  if ( $@ ) ...

       or

	  use Sys::SigAction;
	  my $alarm = 0;
	  eval {
	     my $h = Sys::SigAction::set_sig_handler( 'ALRM' ,sub { $alarm = 1; } );
	     eval {
		alarm(2)
		... do something you want to timeout
		alarm(0);
	     };
	     alarm(0);
	     die $@ if $@;
	  }; #signal handler is reset when $h goes out of scope
	  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
" ;
	  }

       or

	  #use a floating point (fractional seconds) in timeout_call
	  use Sys::SigAction qw( timeout_call );
	  if ( timeout_call( 0.1 ,sub { $retval = DoSomething( @args ); } )
	  {
	     print "DoSomething() timed out
" ;
	  }

ABSTRACT

       This module implements "set_sig_handler()", 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.

       Also implemented is "timeout_call()" which takes a timeout value and a code reference, and executes the code reference wrapped with an
       alarm timeout. timeout_call accepts seconds in floating point format, so you can time out call with a resolution of 0.000001 seconds
       (assumes Time::HiRes is loadable.

       Finally, two convenience routines are defined which allow one to get the signal name from the number -- "sig_name()", and get the signal
       number from the name -- "sig_number()".

DESCRIPTION

       Prior to version 5.8.0 perl implemented 'unsafe' signal handling.  The reason it is consider unsafe, is that there is a risk that a signal
       will arrive, and be handled while perl is changing internal data structures.  This can result in all kinds of subtle and not so subtle
       problems.  For this reason it has always been recommended that one do as little as possible in a signal handler, and only variables that
       already exist be manipulated.

       Perl 5.8.0 and later versions implements 'safe' signal handling on platforms which support the POSIX sigaction() function.  This is
       accomplished by having perl note that a signal has arrived, but deferring the execution of the signal handler until such time as it is safe
       to do so.  Unfortunately these changes can break some existing scripts, if they depended on a system routine being interrupted by the
       signal's arrival.  The perl 5.8.0 implementation was modified further in version 5.8.2.

       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".

       The implementation of this changed the "sa_flags" with which the signal handler is installed by perl, and it causes some system routines
       (like connect()) to return EINTR, instead of another error when the signal arrives.  The problem comes when the code that made the system
       call sees the EINTR code and decides it's going to call it again before returning. Perl doesn't do this but some libraries do, including
       for instance, the Oracle OCI library.

       Thus the 'deferred signal' approach (as implemented by default in perl 5.8 and later) results in some system calls being retried prior to
       the signal handler being called by perl.  This breaks timeout logic for DBD-Oracle which works with earlier versions of perl.  This can be
       particularly vexing, when, for instance, the host on which a database resides is not available:	"DBI->connect()" hangs for minutes before
       returning an error (and cannot even be interrupted with control-C, even when the intended timeout is only seconds).  This is because SIGINT
       appears to be deferred as well.	The result is that it is impossible to implement open timeouts with code that looks like this in perl
       5.8.0 and later:

	  eval {
	     local $SIG{ALRM} = sub { die "timeout" };
	     alarm 2;
	     $sth = DBI->connect(...);
	     alarm 0;
	  };
	  alarm 0;
	  die if $@;

       Or as the author of bug #50628 pointed out, might probably better be written as:

	  eval {
	     local $SIG{ALRM} = sub { die "timeout" };
	     eval {
		alarm 2;
		$sth = DBI->connect(...);
		alarm 0;
	     };
	  }
	  alarm 0;
	  die if $@;

       The solution, if your system has the POSIX sigaction() function, is to use perl's "POSIX::sigaction()" to install the signal handler.  With
       "sigaction()", one gets control over both the signal mask, and the "sa_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(r) signal handling.

       Using sigaction() ensures that the system call won't be resumed after it's interrupted, so long as die is called within the signal handler.
       This is no longer the case when one uses $SIG{name} to set signal handlers in perls >= 5.8.0.

       The usage of sigaction() is not well documented however, and in perl versions less than 5.8.0, it does not work at all. (But that's OK,
       because just setting $SIG does work 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 hash.

       Unfortunately, at least with perl 5.8.0, the result is that doing this effectively reverts to the 'unsafe' signals behavior.  It is not
       clear whether this would be the case in perl 5.8.2, since the safe flag can be used to ask for safe signal handling.  I suspect this
       separates the logic which uses the "sa_flags" to install the handler, and whether deferred signal handling is used.

       The reader should also note, that the behavior of the 'safe' attribute is not consistent with what this author expected.  Specifically, it
       appears to disable signal masking. This can be examined further in the t/safe.t and the t/mask.t regression tests.  Never-the-less,
       Sys::SigAction provides an easy mechanism for the user to recover the pre-5.8.0 behavior for signal handling, and the mask attribute
       clearly works. (see t/mask.t) If one is looking for specific safe signal handling behavior that is considered broken, and the breakage can
       be demonstrated, then a patch to t/safe.t would be most welcome.

       This module wraps up the POSIX:: routines and objects necessary to call sigaction() in a way that is as efficient from a coding perspective
       as just setting a localized $SIG{SIGNAL} with a code reference.	Further, the user has control over the "sa_flags" passed to sigaction().
       By default, if no additional args are passed to sigaction(), then the signal handler will be called when a signal (such as SIGALRM) is
       delivered.

       Since sigaction() is not fully functional in perl versions less than 5.8, this module implements equivalent behavior using the standard
       %SIG array.  The version checking and implementation of the 'right' code is handled by this module, so the user does not have to write perl
       version dependent code.	The attrs hashref argument to set_sig_handler() is silently ignored, in perl versions less than 5.8.  This module
       has been tested with perls as old as 5.005 on solaris.

       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 sigaction() will be easier for you to use.

Note on ";Double evals"
       CPAN bug #50628 which was filed against Sys::SigAction-0.11 noting that the sample code was "buggy" because the evals that wrapped the code
       we wanted to timeout might die for an unanticipated reason, before the alarm could be cleared.  In that case, as the bug writer noted, if
       the alarm expires before the final alarm(0) can be called, either the code will completely die because there is no SIGALRM handler in place
       to catch the signal, or the wrong handler (not the local handler) will be called.

       All the code samples in this module have been modified to account for this.  Additionally we have made the same change in timeout_call()
       which could have exhibited this behavior, though the AUTHOR never knowing experienced it.

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 boolean value requesting 'safe' signal
			    handling (only in 5.8.2 and greater)
			    earlier versions will issue a warning if
			    you use this

			    NOTE: This breaks the signal masking

   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. $time (seconds) may be expressed as a floating point number.

       If Time::HiRes is present and useable, timeout_call() can be used with a timer resolution of 0.000001 seconds.  If Time:HiRes is not
       available then factional second values less than 1.0 are tranparently converted to 1.

       If the alarm goes off the code will be interrupted.  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.

       If HiRes is not loadable, Sys::SigAction will do the right thing and convert

   sig_alarm()
	  $seconds

       sig_alarm() is a drop in replacment for the standard alarm() function.  $seconds may be expressed as a floating point number.

       If Time::HiRes is present and useable, the alarm timers will be set to the floating point value with a resolution of 0.000001 seconds.  If
       Time::HiRes is not available then $seconds with values less than 1.0 will be converted to 1 second.

   sig_name()
       Return the signal name (string) from a signal number.

       ex:

	  sig_name( SIGINT ) returns 'INT'

   sig_number()
       Return the signal number (integer) from a signal name (minus the SIG part).

       ex:

	  sig_number( 'INT' ) returns the integer value of SIGINT;

AUTHOR

	  Lincoln A. Baxter <lab-at-lincolnbaxter-dot-com>

COPYRIGHT

	  Copyright (c) 2004-2009 Lincoln A. Baxter
	  All rights reserved.

	  You may distribute under the terms of either the GNU General Public
	  License or the Artistic License, as specified in the Perl README file,

SEE ALSO

	  perldoc perlvar
	  perldoc POSIX

       The dbd-oracle-timeout.pod file included with this module. This includes a DBD-Oracle test script, which illustrates the use of this module
       with the DBI with the DBD-Oracle driver.

perl v5.12.3							    2011-06-23						       Sys::SigAction(3pm)