Guard(3) User Contributed Perl Documentation Guard(3)NAMEGuard - safe cleanup blocks
SYNOPSIS
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";
code_that_might_die_or_does_other_fun_stuff;
}
# create an object that, when the last reference to it is gone,
# invokes the given codeblock:
my $guard = guard { print "destroyed!\n" };
undef $guard; # probably destroyed here
DESCRIPTION
This module implements so-called "guards". A guard is something
(usually an object) that "guards" a resource, ensuring that it is
cleaned up when expected.
Specifically, this module supports two different types of guards: guard
objects, which execute a given code block when destroyed, and scoped
guards, which are tied to the scope exit.
FUNCTIONS
This module currently exports the "scope_guard" and "guard" functions
by default.
scope_guard BLOCK
scope_guard ($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 $@;
Except it is much faster, and the whole thing gets executed even
when the BLOCK calls "exit", "goto", "last" or escapes via other
means.
If multiple BLOCKs are registered to the same scope, they will be
executed in reverse order. Other scope-related things such as
"local" are managed via the same mechanism, so variables
"local"ised after calling "scope_guard" will be restored when the
guard runs.
Example: temporarily change the timezone for the current process,
ensuring it will be reset when the "if" scope is exited:
use Guard;
use POSIX ();
if ($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";
POSIX::tzset;
# 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
"scope_guard".
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 {
$sem->down;
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.
$guard->cancel
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.
EXCEPTIONSGuard 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).
AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/
THANKS
Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
solution to the problem of exceptions.
SEE ALSO
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 v5.14.2 2011-07-02 Guard(3)
