Search

Documents

AtExit (B<AtExit> -- perform exit processing for a program or object)

• NAME
• SYNOPSIS
• DESCRIPTION
• Adding and removing callbacks during exit/destruction time.
• EXPORTS
• CAVEATS
• Program-termination and Object-destruction
• END{} block processing order
• SEE ALSO
• AUTHOR

NAME

atexit, AtExit -- perform exit processing for a program or object

SYNOPSIS

 use AtExit;

 sub cleanup {
     my @args = @_;
     print "cleanup() executing: args = @args\n";
 }
 
 ## Register subroutines to be called when this program exits

 $_ = atexit(\&cleanup, "This call was registered first");
 print "first call to atexit() returned $_\n";

 $_ = atexit("cleanup", "This call was registered second");
 print "second call to atexit() returned $_\n";

 $_ = atexit("cleanup", "This call should've been unregistered by rmexit");
 rmexit($_)  or  warn "couldnt' unregister exit-sub $_!";

 if (@ARGV == 0) {
    ## Register subroutines to be called when this lexical scope is exited
    my $scope1 = AtExit->new( \&cleanup, "Scope 1, Callback 1" );
    {
       ## Do the same for this nested scope
       my $scope2 = AtExit->new;
       $_ = $scope2->atexit( \&cleanup, "Scope 2, Callback 1" );
       $scope1->atexit( \&cleanup, "Scope 1, Callback 2");
       $scope2->atexit( \&cleanup, "Scope 2, Callback 2" );
       $scope2->rmexit($_) or warn "couldn't unregister exit-sub $_!";

       print "*** Leaving Scope 2 ***\n";
     }
     print "*** Finished Scope 2 ***\n";
     print "*** Leaving Scope 1 ***\n";
 }
 print "*** Finished Scope 1 ***\n"  if (@ARGV == 0);

 END {
     print "*** Now performing program-exit processing ***\n";
 }

DESCRIPTION

The AtExit module provides ANSI-C style exit processing modeled after the atexit function in the standard C library (see atexit(3C)). Various exit processing routines may be registered by calling atexit and passing it the desired subroutine along with any desired arguments. Then, at program-exit time, the subroutines registered with atexit are invoked with their given arguments in the reverse order of registration (last one registered is invoked first). Registering the same subroutine more than once will cause that subroutine to be invoked once for each registration.

An AtExit object can be created in any scope. When invoked as a function, atexit registers callbacks to be executed at program-exit time. But when invoked as an object-method (using the $object->method_name syntax), callbacks registered with an AtExit object are executed at object-destruction time! The rules for order of execution of the registered subroutines are the same for objects during object-destruction, as for the program during program-termination.

The atexit function/method should be passed a subroutine name or reference, optionally followed by the list of arguments with which to invoke it at program/object exit time. Anonymous subroutine references passed to atexit act as ``closures'' (which are described in perlref). If a subroutine name is specified (as opposed to a subroutine reference) then, unless the subroutine name has an explicit package prefix, it is assumed to be the name of a subroutine in the caller's current package. A reference to the specified subroutine is obtained, and, if invocation arguments were specified, it is ``wrapped up'' in a closure which invokes the subroutine with the specified arguments. The resulting subroutine reference is added to the front of the list of exit-handling subroutines for the program (atexit) or the AtExit object ($exitObject->atexit) and the reference is then returned to the caller (just in case you might want to unregister it later using rmexit. If the given subroutine could not be registered, then the value zero is returned.

The rmexit function/method should be passed one or more subroutine references, each of which was returned by a previous call to atexit. For each argument given, rmexit will look in the list of exit-handling subroutines for the program (rmexit) or the AtExit object ($exitObject->rmexit) and remove the first matching entry from the list. If no arguments are given, then all program or object exit-handlers are unregistered! The value returned will be the number of subroutines that were successfully unregistered.

At object destruction time, the DESTROY{} subroutine in the AtExit module iterates over the subroutine references in the AtExit object and invokes each one in turn (each subroutine is removed from the front of the queue immediately before it is invoked). At program-exit time, the END{} block in the AtExit module iterates over the subroutines in the array returned by the exit_subs method and invokes each one in turn (each subroutine is removed from the front of the queue immediately before it is invoked). Note that in both cases (program-exit, and object-destruction) the subroutines in this queue are invoked in first-to-last order (the reverse order in which they were registered with atexit).

Adding and removing callbacks during exit/destruction time.

The method ignore_when_exiting specifies how exit-callback registration and unregistration will be handled during program-exit or object-destruction time, while exit-callbacks are in process of being invoked.

When invoked as a class method (e.g., AtExit->ignore_when_exiting), ignore_when_exiting corresponds to the handling of calls to atexit and rmexit during program-termination. But when invoked as an object method (e.g., $exitObject->ignore_when_exiting), then ignore_when_exiting corresponds to the handling of calls to atexit and rmexit during object-destruction for the particular object.

By default, ignore_when_exiting returns a non-zero value, which causes atexit to ignore any calls made to it during this time (a value of zero will be returned). This behavior is consistent with that of the standard C library function of the same name. If desired however, the user may enable the registration of subroutines by atexit during this time by invoking ignore_when_exiting and passing it an argument of 0, "", or undef (for example, AtExit->ignore_when_exiting(0) or $exitObject->ignore_when_exiting(0), Just remember that any subroutines registered with atexit be placed at the front of the queue of yet-to-be-invoked exit-processing subroutines for the program (atexit) or the AtExit object ($exitObject->atexit).

Regardless of when it is invoked, rmexit will always attempt to unregister the given subroutines (even when called during program/object exit processing). Keep in mind however that if it is invoked during program/object exit then it will fail to unregister any subroutines that have already been invoked (since those subroutine calls have already been removed from the corresponding list of exit-handling subroutines).

The method is_exiting may consulted examined to determine if routines registered using atexit are currently in the process of being invoked. It will be non-zero if they are and zero otherwise. When invoked as a class method (e.g., AtExit->is_exiting), the return value will correspond to program-exit processing; but when invoked as an object method (e.g., $exitObject->is_exiting) the return value will correspond to object-destruction processing for the given object.

If, for any reason, the list of registered callback needs to be directly accessed or manipulated, the exit_subs function will return a reference to the list of program-exit callbacks. When invoked as a method, exit_subs will return a reference to the list of object-destruction callbacks for the corresponding object.

EXPORTS

For backward compatibility, atexit and rmexit are exported by default. Note however that exit_subs, is_exiting, and ignore_when_exiting are not exported by default, and should be invoked as class methods (e.g. AtExit->is_exiting) if they are to manipulate program-exit information (rather than object-destruction) and not explicitly imported.

CAVEATS

Program-termination and Object-destruction

The usual Perl way of doing program/module-exit processing is through the use of END{} blocks (see perlmod/``Package Constructors and Destructors''). The AtExit module implements its program-exit processing with with an END{} block that invokes all the subroutines registered by atexit in the array whose referenced is returned by exit_subs.

For an object, object-destruction processing is implemented by having the DESTROY method for the object invoke all the subroutines registered by $exitObject->atexit. This occurs when the object loses it's last reference, which is not necessarily at program end time.

For objects defined in the global context, if any other END{} block processing is specified in the user's code or in any other packages it uses, then the order in which the exit processing takes place is subject to Perl's rules for the order in which objects loose their last references and END{} blocks are processed. This may affect when subroutines registered with atexit are invoked with respect to other exit processing that is to be performed. In particular, if rmexit is invoked from within an END{} block that executes after the AtExit object was destroyed, then the corresponding subroutine will not be registered and will never be invoked by the AtExit module's destructor code.

END{} block processing order

END{} blocks, including those in other packages, get called in the reverse order in which they appear in the code. (atexit subroutines get called in the reverse order in which they are registered.) If a package gets read via ``use'', it will act as if the END{} block was defined at that particular part of the ``main'' code. Packages read via ``require'' will be executed after the code of ``main'' has been parsed and will be seen last so will execute first (they get executed in the context of the package in which they exist).

It is important to note that END{} blocks and object destruction only get called on normal termination (which includes calls to die or Carp::croak). They do not get called when the program terminates abnormally (due to a signal for example) unless special arrangements have been made by the programmer (e.g. using a signal handler -- see perlvar/``%SIG{expr}'').

SEE ALSO

atexit(3C) describes the atexit function for the standard C library (the actual Unix manual section in which it appears may differ from platform to platform - try sections 3C, 3, 2C, and 2). Further information on anonymous subroutines (``closures'') may be found in perlref. For more information on END{} blocks, see perlmod/``Package Constructors and Destructors''. See perlvar/``%SIG{expr}'' for handling abnormal program termination.

AUTHOR

Andrew Langmead <aml@world.std.com> (initial draft).

Brad Appleton <bradapp@enteract.com> (Version 1.02 and 2.00).

Michael A. Chase <mchase@ix.netcom.com> (Version 2.00).

Information