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Guard(3)	      User Contributed Perl Documentation	      Guard(3)

       Guard - safe cleanup blocks

	  use Guard;

	  # temporarily	chdir to "/etc"	directory, but make sure
	  # to go back to "/" no matter	how myfun exits:
	  sub myfun {
	     scope_guard { chdir "/" };
	     chdir "/etc";


	  # create an object that, when	the last reference to it is gone,
	  # invokes the	given codeblock:
	  my $guard = guard { print "destroyed!\n" };
	  undef	$guard;	# probably destroyed here

       This module implements so-called	"guards". A guard is something
       (usually	an object) that	"guards" a resource, ensuring that it is
       cleaned up when expected.

       Specifically, this module supports two different	types of guards: guard
       objects,	which execute a	given code block when destroyed, and scoped
       guards, which are tied to the scope exit.

       This module currently exports the "scope_guard" and "guard" functions
       by default.

       scope_guard BLOCK
       scope_guard ($coderef)
	   Registers a block that is executed when the current scope (block,
	   function, method, eval etc.)	is exited.

	   See the EXCEPTIONS section for an explanation of how	exceptions
	   (i.e. "die")	are handled inside guard blocks.

	   The description below sounds	a bit complicated, but that's just
	   because "scope_guard" tries to get even corner cases	"right": the
	   goal	is to provide you with a rock solid clean up tool.

	   The behaviour is similar to this code fragment:

	      eval ... code following scope_guard ...
		 local $@;
		 eval BLOCK;
		 eval {	$Guard::DIED->() } if $@;
	      die if $@;

	   Except it is	much faster, and the whole thing gets executed even
	   when	the BLOCK calls	"exit",	"goto",	"last" or escapes via other

	   If multiple BLOCKs are registered to	the same scope,	they will be
	   executed in reverse order. Other scope-related things such as
	   "local" are managed via the same mechanism, so variables
	   "local"ised after calling "scope_guard" will	be restored when the
	   guard runs.

	   Example: temporarily	change the timezone for	the current process,
	   ensuring it will be reset when the "if" scope is exited:

	      use Guard;
	      use POSIX	();

	      if ($need_to_switch_tz) {
		 # make	sure we	call tzset after $ENV{TZ} has been restored
		 scope_guard { POSIX::tzset };

		 # localise after the scope_guard, so it gets undone in	time
		 local $ENV{TZ}	= "Europe/London";

		 # do something	with the new timezone

       my $guard = guard BLOCK
       my $guard = guard ($coderef)
	   Behaves the same as "scope_guard", except that instead of executing
	   the block on	scope exit, it returns an object whose lifetime
	   determines when the BLOCK gets executed: when the last reference to
	   the object gets destroyed, the BLOCK	gets executed as with

	   See the EXCEPTIONS section for an explanation of how	exceptions
	   (i.e. "die")	are handled inside guard blocks.

	   Example: acquire a Coro::Semaphore for a second by registering a
	   timer. The timer callback references	the guard used to unlock it
	   again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
	   method that does this already):

	      use Guard;
	      use Coro::AnyEvent;
	      use Coro::Semaphore;

	      my $sem =	new Coro::Semaphore;

	      sub lock_for_a_second {
		 my $guard = guard { $sem->up };

		 Coro::AnyEvent::sleep 1;

		 # $sem->up gets executed when returning

	   The advantage of doing this with a guard instead of simply calling
	   "$sem->down"	in the callback	is that	you can	opt not	to create the
	   timer, or your code can throw an exception before it	can create the
	   timer (or the thread	gets canceled),	or you can create multiple
	   timers or other event watchers and only when	the last one gets
	   executed will the lock be unlocked. Using the "guard", you do not
	   have	to worry about catching	all the	places where you have to
	   unlock the semaphore.

	   Calling this	function will "disable"	the guard object returned by
	   the "guard" function, i.e. it will free the BLOCK originally	passed
	   to "guard "and will arrange for the BLOCK not to be executed.

	   This	can be useful when you use "guard" to create a cleanup handler
	   to be called	under fatal conditions and later decide	it is no
	   longer needed.

       Guard blocks should not normally	throw exceptions (that is, "die").
       After all, they are usually used	to clean up after such exceptions.
       However,	if something truly exceptional is happening, a guard block
       should of course	be allowed to die. Also, programming errors are	a
       large source of exceptions, and the programmer certainly	wants to know
       about those.

       Since in	most cases, the	block executing	when the guard gets executed
       does not	know or	does not care about the	guard blocks, it makes little
       sense to	let containing code handle the exception.

       Therefore, whenever a guard block throws	an exception, it will be
       caught by Guard,	followed by calling the	code reference stored in
       $Guard::DIED (with $@ set to the	actual exception), which is similar to
       how most	event loops handle this	case.

       The default for $Guard::DIED is to call "warn "$@"", i.e. the error is
       printed as a warning and	the program continues.

       The $@ variable will be restored	to its value before the	guard call in
       all cases, so guards will not disturb $@	in any way.

       The code	reference stored in $Guard::DIED should	not die	(behaviour is
       not guaranteed, but right now, the exception will simply	be ignored).

	Marc Lehmann <>

       Thanks to Marco Maisenhelder, who reminded me of	the $Guard::DIED
       solution	to the problem of exceptions.

       Scope::Guard and	Sub::ScopeFinalizer, which actually implement
       dynamically scoped guards only, not the lexically scoped	guards that
       their documentation promises, and have a	lot higher CPU,	memory and
       typing overhead.

       Hook::Scope, which has apparently never been finished and can corrupt
       memory when used.

       Scope::Guard seems to have a big	SEE ALSO section for even more modules
       like it.

perl v5.32.1			  2014-11-20			      Guard(3)


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