Perl

This is really a quick reminder about a perl function which does a little more than you’d perhaps expect. Need to convert a binary (or hex or octal) string to an integer? The perl documentation for the oct(EXPR) function starts out with: Interprets EXPR as an octal string and returns the corresponding value. Then includes the comment: (If EXPR happens to start off with 0x, interprets it as a hex string. If EXPR starts off with 0b, it is interpreted as a binary string. Leading whitespace is ignored in all three cases.) ...

Scenario For a certain code-base it’s decided it would be useful to be able to trigger an exception on-demand (mid-way through a process working). Why? Perhaps we have a process which loops over a repeated task which takes a reasonable amount of time, and we know that from time to time the input data to this process might need to be quickly updated and we’d like to abort whatever the current loop is doing to read in the latest data. Meanwhile for another process we’d simply like some diagnostic information (which might be slightly expensive to generate) to be available on demand. We might also have a standard application design whereby all otherwise unhandled exceptions will be captured at the last moment (before causing a process to fail) and signal sysadmins with emails or management systems with failure states (rather than simply aborting a process with a kill and having to communicate that this has been done). Signals are helpful here, and we even have SIGUSR1 and SIGUSR2 (see “Standard Signals” in your handy signal(7) man page) allocated for user-defined purposes. Rather than having to build some other signalling method with our application, we can use one of those to trigger an exception by setting up a signal handler. ...

This article starts with a conversation about adjusting compiled perl regular expressions and a sudden realisation - which went a little along the lines of: OtherPerson> I can force a regular expression to change, for example: OtherPerson> DB<1> $a = qr{Abc} OtherPerson> DB<2> $a =~ s/A/a/ OtherPerson> DB<3> x $a OtherPerson> 0 '(?^:abc)' Me> No that doesn't work does it, oh wait <strong>it does</strong>, why? It took me a few moments of poking this myself in the debugger to notice the key change - which was that the compiled regular expression had become a string. See in the below example that $a starts as a RegExp and then becomes a scalar (at which point the ref function returns the empty string): ...

If using perl Cache::FileCache, some comments offering a helpful starting point to get a cache that works in a relatively well behaved and unsurprising way: use Cache::FileCache; use File::Spec; my $cache = Cache::FileCache->new({ # "The namespace associated with this # cache." namespace => 'BJD-TESTING-app_id_cache_id', # "The default expiration time for # objects place in the cache." # This is in seconds. default_expires_in => 1, # "Sets the auto purge interval. If this # option is set to a particular time ( in # the same format as the expires_in ), # then the purge( ) routine will be # called during the first set after the # interval expires. The interval will # then be reset." # # XXX To work needs at least one of # auto_purge_on_set or auto_purge_on_get # to be set to true auto_purge_interval => 5, auto_purge_on_set => 1, # restrict access to the cache to just this # user (data security) # # NOTE that if you set directory_umask # but no cache_root you can end up clashing # with other users who will also be trying # to use (and maybe set the umask) on # /tmp/FileCache # # If we care about data security set # cache_root as well to put the files in # out own private directory: directory_umask => 0077, cache_root => File::Spec->tmpdir() . "/bjdean-perl-Cache-FileCache", });

Perl has a logical xor operator but it happens to be at the very bottom of the precedence priority list , importantly below the humble comma. So this innocent looking piece of code: foobar( $a xor $b, $c ); Is interpreted as: foobar( $a xor <strong>($b, $c)</strong> ); And note unlike the not, and and or low-precedence logical operators, there is no high-precedence xor operator. So it’s a good idea to wrap xors in parentheses to guarantee intended behaviour: ...