perlpacktut.1

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.\" ========================================================================
.\"
.IX Title "PERLPACKTUT 1"
.TH PERLPACKTUT 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
perlpacktut \- tutorial on \f(CW\*(C`pack\*(C'\fR and \f(CW\*(C`unpack\*(C'\fR
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`pack\*(C'\fR and \f(CW\*(C`unpack\*(C'\fR are two functions for transforming data according
to a user-defined template, between the guarded way Perl stores values
and some well-defined representation as might be required in the 
environment of a Perl program. Unfortunately, they're also two of 
the most misunderstood and most often overlooked functions that Perl
provides. This tutorial will demystify them for you.
.SH "The Basic Principle"
.IX Header "The Basic Principle"
Most programming languages don't shelter the memory where variables are
stored. In C, for instance, you can take the address of some variable,
and the \f(CW\*(C`sizeof\*(C'\fR operator tells you how many bytes are allocated to
the variable. Using the address and the size, you may access the storage
to your heart's content.
.PP
In Perl, you just can't access memory at random, but the structural and
representational conversion provided by \f(CW\*(C`pack\*(C'\fR and \f(CW\*(C`unpack\*(C'\fR is an
excellent alternative. The \f(CW\*(C`pack\*(C'\fR function converts values to a byte
sequence containing representations according to a given specification,
the so-called \*(L"template\*(R" argument. \f(CW\*(C`unpack\*(C'\fR is the reverse process,
deriving some values from the contents of a string of bytes. (Be cautioned,
however, that not all that has been packed together can be neatly unpacked \- 
a very common experience as seasoned travellers are likely to confirm.)
.PP
Why, you may ask, would you need a chunk of memory containing some values
in binary representation? One good reason is input and output accessing
some file, a device, or a network connection, whereby this binary
representation is either forced on you or will give you some benefit
in processing. Another cause is passing data to some system call that
is not available as a Perl function: \f(CW\*(C`syscall\*(C'\fR requires you to provide
parameters stored in the way it happens in a C program. Even text processing 
(as shown in the next section) may be simplified with judicious usage 
of these two functions.
.PP
To see how (un)packing works, we'll start with a simple template
code where the conversion is in low gear: between the contents of a byte
sequence and a string of hexadecimal digits. Let's use \f(CW\*(C`unpack\*(C'\fR, since
this is likely to remind you of a dump program, or some desperate last
message unfortunate programs are wont to throw at you before they expire
into the wild blue yonder. Assuming that the variable \f(CW$mem\fR holds a 
sequence of bytes that we'd like to inspect without assuming anything 
about its meaning, we can write
.PP
.Vb 2
\&   my( $hex ) = unpack( \*(AqH*\*(Aq, $mem );
\&   print "$hex\en";
.Ve
.PP
whereupon we might see something like this, with each pair of hex digits
corresponding to a byte:
.PP
.Vb 1
\&   41204d414e204120504c414e20412043414e414c2050414e414d41
.Ve
.PP
What was in this chunk of memory? Numbers, characters, or a mixture of
both? Assuming that we're on a computer where \s-1ASCII\s0 (or some similar)
encoding is used: hexadecimal values in the range \f(CW0x40\fR \- \f(CW0x5A\fR
indicate an uppercase letter, and \f(CW0x20\fR encodes a space. So we might
assume it is a piece of text, which some are able to read like a tabloid;
but others will have to get hold of an \s-1ASCII\s0 table and relive that
firstgrader feeling. Not caring too much about which way to read this,
we note that \f(CW\*(C`unpack\*(C'\fR with the template code \f(CW\*(C`H\*(C'\fR converts the contents
of a sequence of bytes into the customary hexadecimal notation. Since
\&\*(L"a sequence of\*(R" is a pretty vague indication of quantity, \f(CW\*(C`H\*(C'\fR has been
defined to convert just a single hexadecimal digit unless it is followed
by a repeat count. An asterisk for the repeat count means to use whatever
remains.
.PP
The inverse operation \- packing byte contents from a string of hexadecimal
digits \- is just as easily written. For instance:
.PP
.Vb 2
\&   my $s = pack( \*(AqH2\*(Aq x 10, map { "3$_" } ( 0..9 ) );
\&   print "$s\en";
.Ve
.PP
Since we feed a list of ten 2\-digit hexadecimal strings to \f(CW\*(C`pack\*(C'\fR, the
pack template should contain ten pack codes. If this is run on a computer
with \s-1ASCII\s0 character coding, it will print \f(CW0123456789\fR.
.SH "Packing Text"
.IX Header "Packing Text"
Let's suppose you've got to read in a data file like this:
.PP
.Vb 4
\&    Date      |Description                | Income|Expenditure
\&    01/24/2001 Ahmed\*(Aqs Camel Emporium                  1147.99
\&    01/28/2001 Flea spray                                24.99
\&    01/29/2001 Camel rides to tourists      235.00
.Ve
.PP
How do we do it? You might think first to use \f(CW\*(C`split\*(C'\fR; however, since
\&\f(CW\*(C`split\*(C'\fR collapses blank fields, you'll never know whether a record was
income or expenditure. Oops. Well, you could always use \f(CW\*(C`substr\*(C'\fR:
.PP
.Vb 7
\&    while (<>) { 
\&        my $date   = substr($_,  0, 11);
\&        my $desc   = substr($_, 12, 27);
\&        my $income = substr($_, 40,  7);
\&        my $expend = substr($_, 52,  7);
\&        ...
\&    }
.Ve
.PP
It's not really a barrel of laughs, is it? In fact, it's worse than it
may seem; the eagle-eyed may notice that the first field should only be
10 characters wide, and the error has propagated right through the other
numbers \- which we've had to count by hand. So it's error-prone as well
as horribly unfriendly.
.PP
Or maybe we could use regular expressions:
.PP
.Vb 5
\&    while (<>) { 
\&        my($date, $desc, $income, $expend) = 
\&            m|(\ed\ed/\ed\ed/\ed{4}) (.{27}) (.{7})(.*)|;
\&        ...
\&    }
.Ve
.PP
Urgh. Well, it's a bit better, but \- well, would you want to maintain
that?
.PP
Hey, isn't Perl supposed to make this sort of thing easy? Well, it does,
if you use the right tools. \f(CW\*(C`pack\*(C'\fR and \f(CW\*(C`unpack\*(C'\fR are designed to help
you out when dealing with fixed-width data like the above. Let's have a
look at a solution with \f(CW\*(C`unpack\*(C'\fR:
.PP
.Vb 4
\&    while (<>) { 
\&        my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
\&        ...
\&    }
.Ve
.PP
That looks a bit nicer; but we've got to take apart that weird template.
Where did I pull that out of?
.PP
\&\s-1OK\s0, let's have a look at some of our data again; in fact, we'll include
the headers, and a handy ruler so we can keep track of where we are.
.PP
.Vb 5
\&             1         2         3         4         5        
\&    1234567890123456789012345678901234567890123456789012345678
\&    Date      |Description                | Income|Expenditure
\&    01/28/2001 Flea spray                                24.99
\&    01/29/2001 Camel rides to tourists      235.00
.Ve
.PP
From this, we can see that the date column stretches from column 1 to
column 10 \- ten characters wide. The \f(CW\*(C`pack\*(C'\fR\-ese for \*(L"character\*(R" is
\&\f(CW\*(C`A\*(C'\fR, and ten of them are \f(CW\*(C`A10\*(C'\fR. So if we just wanted to extract the
dates, we could say this:
.PP
.Vb 1
\&    my($date) = unpack("A10", $_);
.Ve
.PP
\&\s-1OK\s0, what's next? Between the date and the description is a blank column;
we want to skip over that. The \f(CW\*(C`x\*(C'\fR template means \*(L"skip forward\*(R", so we
want one of those. Next, we have another batch of characters, from 12 to
38. That's 27 more characters, hence \f(CW\*(C`A27\*(C'\fR. (Don't make the fencepost
error \- there are 27 characters between 12 and 38, not 26. Count 'em!)
.PP
Now we skip another character and pick up the next 7 characters:
.PP
.Vb 1
\&    my($date,$description,$income) = unpack("A10xA27xA7", $_);
.Ve
.PP
Now comes the clever bit. Lines in our ledger which are just income and
not expenditure might end at column 46. Hence, we don't want to tell our
\&\f(CW\*(C`unpack\*(C'\fR pattern that we \fBneed\fR to find another 12 characters; we'll
just say \*(L"if there's anything left, take it\*(R". As you might guess from
regular expressions, that's what the \f(CW\*(C`*\*(C'\fR means: \*(L"use everything
remaining\*(R".
.IP "\(bu" 3
Be warned, though, that unlike regular expressions, if the \f(CW\*(C`unpack\*(C'\fR
template doesn't match the incoming data, Perl will scream and die.
.PP
Hence, putting it all together:
.PP
.Vb 1
\&    my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_);
.Ve
.PP
Now, that's our data parsed. I suppose what we might want to do now is
total up our income and expenditure, and add another line to the end of
our ledger \- in the same format \- saying how much we've brought in and
how much we've spent:
.PP
.Vb 5
\&    while (<>) {
\&        my($date, $desc, $income, $expend) = unpack("A10xA27xA7xA*", $_);
\&        $tot_income += $income;
\&        $tot_expend += $expend;
\&    }
\&
\&    $tot_income = sprintf("%.2f", $tot_income); # Get them into 
\&    $tot_expend = sprintf("%.2f", $tot_expend); # "financial" format
\&
\&    $date = POSIX::strftime("%m/%d/%Y", localtime); 
\&
\&    # OK, let\*(Aqs go:
\&
\&    print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend);
.Ve
.PP
Oh, hmm. That didn't quite work. Let's see what happened:
.PP
.Vb 4
\&    01/24/2001 Ahmed\*(Aqs Camel Emporium                   1147.99
\&    01/28/2001 Flea spray                                 24.99
\&    01/29/2001 Camel rides to tourists     1235.00
\&    03/23/2001Totals                     1235.001172.98