source: trunk/essentials/dev-lang/perl/pod/perlpacktut.pod@ 3310

=head1 NAME

perlpacktut - tutorial on C<pack> and C<unpack>

=head1 DESCRIPTION

C<pack> and C<unpack> 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.


=head1 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 C<sizeof> 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.

In Perl, you just can't access memory at random, but the structural and
representational conversion provided by C<pack> and C<unpack> is an
excellent alternative. The C<pack> function converts values to a byte
sequence containing representations according to a given specification,
the so-called "template" argument. C<unpack> 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.)

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: C<syscall> 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.

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 C<unpack>, 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 C<$mem> holds a 
sequence of bytes that we'd like to inspect without assuming anything 
about its meaning, we can write

   my( $hex ) = unpack( 'H*', $mem );
   print "$hex\n";

whereupon we might see something like this, with each pair of hex digits
corresponding to a byte:

   41204d414e204120504c414e20412043414e414c2050414e414d41

What was in this chunk of memory? Numbers, characters, or a mixture of
both? Assuming that we're on a computer where ASCII (or some similar)
encoding is used: hexadecimal values in the range C<0x40> - C<0x5A>
indicate an uppercase letter, and C<0x20> 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 ASCII table and relive that
firstgrader feeling. Not caring too much about which way to read this,
we note that C<unpack> with the template code C<H> converts the contents
of a sequence of bytes into the customary hexadecimal notation. Since
"a sequence of" is a pretty vague indication of quantity, C<H> 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.

The inverse operation - packing byte contents from a string of hexadecimal
digits - is just as easily written. For instance:

   my $s = pack( 'H2' x 10, map { "3$_" } ( 0..9 ) );
   print "$s\n";

Since we feed a list of ten 2-digit hexadecimal strings to C<pack>, the
pack template should contain ten pack codes. If this is run on a computer
with ASCII character coding, it will print C<0123456789>.


=head1 Packing Text

Let's suppose you've got to read in a data file like this:

    Date      |Description                | Income|Expenditure
    01/24/2001 Ahmed's Camel Emporium                  1147.99
    01/28/2001 Flea spray                                24.99
    01/29/2001 Camel rides to tourists      235.00

How do we do it? You might think first to use C<split>; however, since
C<split> collapses blank fields, you'll never know whether a record was
income or expenditure. Oops. Well, you could always use C<substr>:

    while (<>) { 
        my $date   = substr($_,  0, 11);
        my $desc   = substr($_, 12, 27);
        my $income = substr($_, 40,  7);
        my $expend = substr($_, 52,  7);
        ...
    }

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.

Or maybe we could use regular expressions:

    while (<>) { 
        my($date, $desc, $income, $expend) = 
            m|(\d\d/\d\d/\d{4}) (.{27}) (.{7})(.*)|;
        ...
    }

Urgh. Well, it's a bit better, but - well, would you want to maintain
that?

Hey, isn't Perl supposed to make this sort of thing easy? Well, it does,
if you use the right tools. C<pack> and C<unpack> are designed to help
you out when dealing with fixed-width data like the above. Let's have a
look at a solution with C<unpack>:

    while (<>) { 
        my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
        ...
    }

That looks a bit nicer; but we've got to take apart that weird template.
Where did I pull that out of? 

OK, 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.

             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

From this, we can see that the date column stretches from column 1 to
column 10 - ten characters wide. The C<pack>-ese for "character" is
C<A>, and ten of them are C<A10>. So if we just wanted to extract the
dates, we could say this:

    my($date) = unpack("A10", $_);

OK, what's next? Between the date and the description is a blank column;
we want to skip over that. The C<x> template means "skip forward", so we
want one of those. Next, we have another batch of characters, from 12 to
38. That's 27 more characters, hence C<A27>. (Don't make the fencepost
error - there are 27 characters between 12 and 38, not 26. Count 'em!)

Now we skip another character and pick up the next 7 characters:

    my($date,$description,$income) = unpack("A10xA27xA7", $_);

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
C<unpack> pattern that we B<need> to find another 12 characters; we'll
just say "if there's anything left, take it". As you might guess from
regular expressions, that's what the C<*> means: "use everything
remaining".

=over 3

=item *

Be warned, though, that unlike regular expressions, if the C<unpack>
template doesn't match the incoming data, Perl will scream and die.

=back


Hence, putting it all together:

    my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_);

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:

    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's go:

    print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend);

Oh, hmm. That didn't quite work. Let's see what happened:

    01/24/2001 Ahmed's 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

OK, it's a start, but what happened to the spaces? We put C<x>, didn't
we? Shouldn't it skip forward? Let's look at what L<perlfunc/pack> says:

    x   A null byte.

Urgh. No wonder. There's a big difference between "a null byte",
character zero, and "a space", character 32. Perl's put something
between the date and the description - but unfortunately, we can't see
it! 

What we actually need to do is expand the width of the fields. The C<A>
format pads any non-existent characters with spaces, so we can use the
additional spaces to line up our fields, like this:

    print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);

(Note that you can put spaces in the template to make it more readable,
but they don't translate to spaces in the output.) Here's what we got
this time:

    01/24/2001 Ahmed's Camel Emporium                   1147.99
    01/28/2001 Flea spray                                 24.99
    01/29/2001 Camel rides to tourists     1235.00
    03/23/2001 Totals                      1235.00 1172.98

That's a bit better, but we still have that last column which needs to
be moved further over. There's an easy way to fix this up:
unfortunately, we can't get C<pack> to right-justify our fields, but we
can get C<sprintf> to do it:

    $tot_income = sprintf("%.2f", $tot_income); 
    $tot_expend = sprintf("%12.2f", $tot_expend);
    $date = POSIX::strftime("%m/%d/%Y", localtime); 
    print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);

This time we get the right answer:

    01/28/2001 Flea spray                                 24.99
    01/29/2001 Camel rides to tourists     1235.00
    03/23/2001 Totals                      1235.00      1172.98

So that's how we consume and produce fixed-width data. Let's recap what
we've seen of C<pack> and C<unpack> so far:

=over 3

=item *

Use C<pack> to go from several pieces of data to one fixed-width
version; use C<unpack> to turn a fixed-width-format string into several
pieces of data. 

=item *

The pack format C<A> means "any character"; if you're C<pack>ing and
you've run out of things to pack, C<pack> will fill the rest up with
spaces.

=item *

C<x> means "skip a byte" when C<unpack>ing; when C<pack>ing, it means
"introduce a null byte" - that's probably not what you mean if you're
dealing with plain text.

=item *

You can follow the formats with numbers to say how many characters
should be affected by that format: C<A12> means "take 12 characters";
C<x6> means "skip 6 bytes" or "character 0, 6 times".

=item *

Instead of a number, you can use C<*> to mean "consume everything else
left". 

B<Warning>: when packing multiple pieces of data, C<*> only means
"consume all of the current piece of data". That's to say

    pack("A*A*", $one, $two)

packs all of C<$one> into the first C<A*> and then all of C<$two> into
the second. This is a general principle: each format character
corresponds to one piece of data to be C<pack>ed.

=back



=head1 Packing Numbers

So much for textual data. Let's get onto the meaty stuff that C<pack>
and C<unpack> are best at: handling binary formats for numbers. There is,
of course, not just one binary format  - life would be too simple - but
Perl will do all the finicky labor for you.


=head2 Integers

Packing and unpacking numbers implies conversion to and from some
I<specific> binary representation. Leaving floating point numbers
aside for the moment, the salient properties of any such representation
are:

=over 4

=item *

the number of bytes used for storing the integer,

=item *

whether the contents are interpreted as a signed or unsigned number,

=item *

the byte ordering: whether the first byte is the least or most
significant byte (or: little-endian or big-endian, respectively).

=back

So, for instance, to pack 20302 to a signed 16 bit integer in your
computer's representation you write

   my $ps = pack( 's', 20302 );

Again, the result is a string, now containing 2 bytes. If you print 
this string (which is, generally, not recommended) you might see
C<ON> or C<NO> (depending on your system's byte ordering) - or something
entirely different if your computer doesn't use ASCII character encoding.
Unpacking C<$ps> with the same template returns the original integer value:

   my( $s ) = unpack( 's', $ps );

This is true for all numeric template codes. But don't expect miracles:
if the packed value exceeds the allotted byte capacity, high order bits
are silently discarded, and unpack certainly won't be able to pull them
back out of some magic hat. And, when you pack using a signed template
code such as C<s>, an excess value may result in the sign bit
getting set, and unpacking this will smartly return a negative value.

16 bits won't get you too far with integers, but there is C<l> and C<L>
for signed and unsigned 32-bit integers. And if this is not enough and
your system supports 64 bit integers you can push the limits much closer
to infinity with pack codes C<q> and C<Q>. A notable exception is provided
by pack codes C<i> and C<I> for signed and unsigned integers of the 
"local custom" variety: Such an integer will take up as many bytes as
a local C compiler returns for C<sizeof(int)>, but it'll use I<at least>
32 bits.

Each of the integer pack codes C<sSlLqQ> results in a fixed number of bytes,
no matter where you execute your program. This may be useful for some 
applications, but it does not provide for a portable way to pass data 
structures between Perl and C programs (bound to happen when you call 
XS extensions or the Perl function C<syscall>), or when you read or
write binary files. What you'll need in this case are template codes that
depend on what your local C compiler compiles when you code C<short> or
C<unsigned long>, for instance. These codes and their corresponding
byte lengths are shown in the table below.  Since the C standard leaves
much leeway with respect to the relative sizes of these data types, actual
values may vary, and that's why the values are given as expressions in
C and Perl. (If you'd like to use values from C<%Config> in your program
you have to import it with C<use Config>.)

   signed unsigned  byte length in C   byte length in Perl       
     s!     S!      sizeof(short)      $Config{shortsize}
     i!     I!      sizeof(int)        $Config{intsize}
     l!     L!      sizeof(long)       $Config{longsize}
     q!     Q!      sizeof(long long)  $Config{longlongsize}

The C<i!> and C<I!> codes aren't different from C<i> and C<I>; they are
tolerated for completeness' sake.


=head2 Unpacking a Stack Frame

Requesting a particular byte ordering may be necessary when you work with
binary data coming from some specific architecture whereas your program could
run on a totally different system. As an example, assume you have 24 bytes
containing a stack frame as it happens on an Intel 8086:

      +---------+        +----+----+               +---------+
 TOS: |   IP    |  TOS+4:| FL | FH | FLAGS  TOS+14:|   SI    |
      +---------+        +----+----+               +---------+
      |   CS    |        | AL | AH | AX            |   DI    |
      +---------+        +----+----+               +---------+
                         | BL | BH | BX            |   BP    |
                         +----+----+               +---------+
                         | CL | CH | CX            |   DS    |
                         +----+----+               +---------+
                         | DL | DH | DX            |   ES    |
                         +----+----+               +---------+

First, we note that this time-honored 16-bit CPU uses little-endian order,
and that's why the low order byte is stored at the lower address. To
unpack such a (signed) short we'll have to use code C<v>. A repeat
count unpacks all 12 shorts:

   my( $ip, $cs, $flags, $ax, $bx, $cd, $dx, $si, $di, $bp, $ds, $es ) =
     unpack( 'v12', $frame );

Alternatively, we could have used C<C> to unpack the individually
accessible byte registers FL, FH, AL, AH, etc.:

   my( $fl, $fh, $al, $ah, $bl, $bh, $cl, $ch, $dl, $dh ) =
     unpack( 'C10', substr( $frame, 4, 10 ) );

It would be nice if we could do this in one fell swoop: unpack a short,
back up a little, and then unpack 2 bytes. Since Perl I<is> nice, it
proffers the template code C<X> to back up one byte. Putting this all
together, we may now write:

   my( $ip, $cs,
       $flags,$fl,$fh,
       $ax,$al,$ah, $bx,$bl,$bh, $cx,$cl,$ch, $dx,$dl,$dh, 
       $si, $di, $bp, $ds, $es ) =
   unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame );

(The clumsy construction of the template can be avoided - just read on!)  

We've taken some pains to construct the template so that it matches
the contents of our frame buffer. Otherwise we'd either get undefined values,
or C<unpack> could not unpack all. If C<pack> runs out of items, it will
supply null strings (which are coerced into zeroes whenever the pack code
says so).


=head2 How to Eat an Egg on a Net

The pack code for big-endian (high order byte at the lowest address) is
C<n> for 16 bit and C<N> for 32 bit integers. You use these codes
if you know that your data comes from a compliant architecture, but,
surprisingly enough, you should also use these pack codes if you
exchange binary data, across the network, with some system that you
know next to nothing about. The simple reason is that this
order has been chosen as the I<network order>, and all standard-fearing
programs ought to follow this convention. (This is, of course, a stern
backing for one of the Lilliputian parties and may well influence the
political development there.) So, if the protocol expects you to send
a message by sending the length first, followed by just so many bytes,
you could write:

   my $buf = pack( 'N', length( $msg ) ) . $msg;

or even:

   my $buf = pack( 'NA*', length( $msg ), $msg );