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Lincoln A Baxter


Sys::SigAction - Perl extension for Consistent Signal Handling


   #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


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


   use Sys::SigAction;
   my $alarm = 0;
   eval {
      my $h = Sys::SigAction::set_sig_handler( 'ALRM' ,sub { $alarm = 1; } );
      ... do something you want to timeout
   if ( $@ or $alarm ) ...


   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)


   use Sys::SigAction qw( timeout_call );
   if ( timeout_call( 5 ,sub { $retval = DoSomething( @args ); } )
      print "DoSomething() timed out\n" ;


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.

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().


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 interupted 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, the host on which a database resides is not available: DBI->connect() hangs for minutes before returning an error (and cannot even be interupted 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 $@;

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.



   $sig ,$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


   $timeout ,$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.


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


   sig_name( SIGINT ) returns 'INT'


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


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


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


   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,


   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.