Tsunami relief - how Sun employees can help

I've noticed a few BSC folks blogging (1) (2) (3) about how to contribute to the various relief funds that are being set up by aid agencies. If I might make a suggestion - if we can channel this money throught the Sun Foundation, I think Sun will match all the contributions via the Employee Matched Giving Program. I've mailed HR here in the UK but at the moment people are still on holiday, so I've not had a response yet.

All you folks in other GEOs might like to ask your HR departments about this as well. I've mailed Scott and Crawford to ask if there is going to be any sort of coordinated way we can all donate, when I get a reply I'll post the details to my blog.

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Categories : Work

Making a perl filehandle into an object

I'm working on a project that uses both perl and MySQL, and I needed an easy way of piping stuff from perl into mysqlimport so that I could do fast bulk uploads to the database. MySQL is a bit odd in how it does this - it insists that the data has to come from a file who's filename prefix is the same as the table into which you are loading. This precludes a vanilla perl piped open and requires the use of a named pipe - the easiest way being to create a named pipe under /tmp of the form <table name>.<pid>.

I needed several loaders open simultaneously, so the obvious thing was to abstract the functionality into a class. I still wanted to be able to use normal print statements to output the table rows to the loader, so I wanted the filehandle to simultaneously be an object as well. Creating objects in perl is done with the bless operator, which requires a reference to bless - you can't bless a normal scalar value into an object. Fortunately open() in later versions of perl actually gives you a reference to a filehandle:

$ perl -d -e 1

Loading DB routines from perl5db.pl version 1.25
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):	1
  DB<1> my $a;

  DB<2> open($a, '>', '/dev/null');

  DB<3> x $a
0  GLOB(0x3a40e0)
   -> *main::$a
         FileHandle({*main::$a}) => fileno(3)
  DB<4> bless($a, 'MyClass');

  DB<5> x $a
0  MyClass=GLOB(0x3a40e0)
   -> *main::$a
         FileHandle({*main::$a}) => fileno(3)

Having blessed the filehandle into the appropriate class we can then invoke methods on it (e.g. $a->do_stuff()), as well as print to it directly (e.g. print($a "hello\n");). However, having an object without any associated properties isn't really much use - in my case I needed to be able to store the name of the named pipe I was using to communicate with mysqlimport so that I could remove it when the filehandle was closed. It's possible to use the perl tie and overloading mechanisms to do this, but as this is perl and tmtowtdi always applies, and there is in fact a simpler although less obvious way.

To follow this it's first necessary to understand a little about how perl actually stores values internally. Variables come in different types, the common ones being scalars, hashes and arrays, repectively denoted by the leading $, % and @ characters on variables. $a, %a and @a look like they are entirely different variables, but in fact they aren't - they are all slots in a single perl symbol table entry called "a". These symbol table entries are called typeglobs or globs for short and are accessible by using the "*" prefix on a variable, so *a refers to the typeglob where $a, %a and @a all live.

The "a" filehandle that we opened in the example above also has a hash slot, so if we want to store additional attributes on "a" we need some way of getting the associated hash slot in its typeglob. This is actually very easy, although the syntax is a little abstruse:

my $fh;
open($fh, '>', $fifo);
my $self = \%{*$fh};
$self->{fifo} = $fifo;
bless($fh, $class);

Let's pick apart the line that assigns to $self. $fh is actually a reference to a filehandle, so we dereference it with *$ to the entire "fh" glob. The %{...} says we want to access the hash slot of the "fh" glob, and the \ gets us a reference to that, so $self ends up being a reference to the hash slot associated with "fh". Phew. We can then assign to it as a normal hash reference. When we subsequently call a method on the blessed $fh filehandle, we can use exactly the same chant to get back the hash reference and access the data that we put in it. This trick is used by the standard perl IO::Socket class to squirrel away socket attributes, but it's often useful to be able to associate properties with a filehandle yourself so I think this particular technique deserves to be more widely known. If you want further information on how all this stuff hangs together, you should check out the perlref manpage.

Tags : , ,
Categories : Tech, Perl

Snow, Hare

A couple of weeks ago we had the first snow of the year - according to some pundits we are in for a cold winter this year. I was out on patrol that weekend, trying to finish my survey of the Bleaklow fence. Unfortunately the snow made it such slow going that by the time I'd reached Grinah it was time to turn around and come back. I finally got the survey finished yesterday, it took me seven days in total to cover the 32Km and record the 153 stiles and fences - much of the time being taken up by walking to and from the fenceline rather than along it.

Anyway, I got some photos of Bleaklow in the snow which I've posted below. The first one is of the 'mushroom rock' at Bleaklow Stones - a well-known example of the sometimes wierd shapes that the gritstone weathers into. I've jazzed the picture up a little, but the sun really was directly behind it ;-)

Mushroom rock

The footprints in the snow in from of the rock are those of the mountain hares that live up here - you can see the prints even more clearly in the photo below. This was taken from Bleaklow Stones, the western end of the Kinder plateau can be seen to the left of the photo.

Hare prints

On the same day I was out blundering around in the snow and falling through it into what seemed like just about every rank peat bog in the Peak District, Bob and John were over by Lightside, about 2Km from where I live. John spotted this hare up in the rocks - they often hide up in areas like this as it gives them protection from both the weather and predators - not that they actually have that many predators in the Peak District!

Mountain hare

They are normally fairly skittish (well, they are Lepus timidus after all ;-), so Bob and John spent some time carefully approaching it and as well as the superb photo below, John got some excellent video footage. It is just in the process of shedding its brown summer coat and switching into its white winter camouflage - not a particularly sucessful ruse bearing in mind the limeted amounts of snow we get - all the snow that was on the ground when these pictures were taken has long since gone.

Mountain hare

If you are interested in more information on Mountain Hares, I can recommend a visit to both The Mammal Society's page on them, and the image collection at ARKive.