Hence, putting it all together:
- my($date,$description,$income,$expend) = unpack("A10xA27xA7A*", $_);
+ my($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
$si, $di, $bp, $ds, $es ) =
unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame );
+(The clumsy construction of the template can be avoided - just read on!)
+
We've taken some pains to construct the template so that it matches
the contents of our frame buffer. Otherwise we'd either get undefined values,
or C<unpack> could not unpack all. If C<pack> runs out of items, it will
simply assigned to C<undef>, a convenient notation for "I don't care where
this goes".
- ($carry, undef, $parity, undef, $auxcarry, undef, $sign,
+ ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
$trace, $interrupt, $direction, $overflow) =
split( //, unpack( 'b16', $status ) );
128, C<unpack> knows where to stop.
+=head1 Template Grouping
+
+Prior to Perl 5.8, repetitions of templates had to be made by
+C<x>-multiplication of template strings. Now there is a better way as
+we may use the pack codes C<(> and C<)> combined with a repeat count.
+The C<unpack> template from the Stack Frame example can simply
+be written like this:
+
+ unpack( 'v2 (vXXCC)5 v5', $frame )
+
+Let's explore this feature a little more. We'll begin with the equivalent of
+
+ join( '', map( substr( $_, 0, 1 ), @str ) )
+
+which returns a string consisting of the first character from each string.
+Using pack, we can write
+
+ pack( '(A)'.@str, @str )
+
+or, because a repeat count C<*> means "repeat as often as required",
+simply
+
+ pack( '(A)*', @str )
+
+(Note that the template C<A*> would only have packed C<$str[0]> in full
+length.)
+
+To pack dates stored as triplets ( day, month, year ) in an array C<@dates>
+into a sequence of byte, byte, short integer we can write
+
+ $pd = pack( '(CCS)*', map( @$_, @dates ) );
+
+To swap pairs of characters in a string (with even length) one could use
+several techniques. First, let's use C<x> and C<X> to skip forward and back:
+
+ $s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) );
+
+We can also use C<@> to jump to an offset, with 0 being the position where
+we were when the last C<(> was encountered:
+
+ $s = pack( '(A)*', unpack( '(@1A @0A @2)*', $s ) );
+
+Finally, there is also an entirely different approach by unpacking big
+endian shorts and packing them in the reverse byte order:
+
+ $s = pack( '(v)*', unpack( '(n)*', $s );
+
+
=head1 Lengths and Widths
=head2 String Lengths
So far, we've seen literals used as templates. If the list of pack
items doesn't have fixed length, an expression constructing the
-template has to be used. Here's an example:
-To store named string values in a way that can be conveniently parsed
-by a C program, we create a sequence of names and null terminated ASCII
-strings, with C<=> between the name and the value, followed by an
-additional delimiting null byte. Here's how:
+template is required (whenever, for some reason, C<()*> cannot be used).
+Here's an example: To store named string values in a way that can be
+conveniently parsed by a C program, we create a sequence of names and
+null terminated ASCII strings, with C<=> between the name and the value,
+followed by an additional delimiting null byte. Here's how:
- my $env = pack( 'A*A*Z*' x keys( %Env ) . 'C',
+ my $env = pack( '(A*A*Z*)' . keys( %Env ) . 'C',
map( { ( $_, '=', $Env{$_} ) } keys( %Env ) ), 0 );
Let's examine the cogs of this byte mill, one by one. There's the C<map>
call, creating the items we intend to stuff into the C<$env> buffer:
to each key (in C<$_>) it adds the C<=> separator and the hash entry value.
Each triplet is packed with the template code sequence C<A*A*Z*> that
-is multiplied with the number of keys. (Yes, that's what the C<keys>
+is repeated according to the number of keys. (Yes, that's what the C<keys>
function returns in scalar context.) To get the very last null byte,
we add a C<0> at the end of the C<pack> list, to be packed with C<C>.
(Attentive readers may have noticed that we could have omitted the 0.)
in the buffer before we can let C<unpack> rip it apart:
my $n = $env =~ tr/\0// - 1;
- my %env = map( split( /=/, $_ ), unpack( 'Z*' x $n, $env ) );
+ my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) );
The C<tr> counts the null bytes. The C<unpack> call returns a list of
name-value pairs each of which is taken apart in the C<map> block.
+=head2 Counting Repetitions
+
+Rather than storing a sentinel at the end of a data item (or a list of items),
+we could precede the data with a count. Again, we pack keys and values of
+a hash, preceding each with an unsigned short length count, and up front
+we store the number of pairs:
+
+ my $env = pack( 'S(S/A* S/A*)*', scalar keys( %Env ), %Env );
+
+This simplifies the reverse operation as the number of repetitions can be
+unpacked with the C</> code:
+
+ my %env = unpack( 'S/(S/A* S/A*)', $env );
+
+Note that this is one of the rare cases where you cannot use the same
+template for C<pack> and C<unpack> because C<pack> can't determine
+a repeat count for a C<()>-group.
+
+
=head1 Packing and Unpacking C Structures
In previous sections we have seen how to pack numbers and character
given a C<struct> type and one of its field names ("member-designator" in
C standardese).
+Neither using offsets nor adding C<x>'s to bridge the gaps is satisfactory.
+(Just imagine what happens if the structure changes.) What we really need
+is a way of saying "skip as many bytes as required to the next multiple of N".
+In fluent Templatese, you say this with C<x!N> where N is replaced by the
+appropriate value. Here's the next version of our struct packaging:
+
+ my $gappy = pack( 'c x!2 s c x!4 l!', $c1, $s, $c2, $l );
+
+That's certainly better, but we still have to know how long all the
+integers are, and portability is far away. Rather than C<2>,
+for instance, we want to say "however long a short is". But this can be
+done by enclosing the appropriate pack code in brackets: C<[s]>. So, here's
+the very best we can do:
+
+ my $gappy = pack( 'c x![s] s c x![l!] l!', $c1, $s, $c2, $l );
+
=head2 Alignment, Take 2
spacing - 16 bytes to a line:
my $i;
- print map { ++$i % 16 ? "$_ " : "$_\n" }
- unpack( 'H2' x length( $mem ), $mem ),
+ print map( ++$i % 16 ? "$_ " : "$_\n",
+ unpack( 'H2' x length( $mem ), $mem ) ),
length( $mem ) % 16 ? "\n" : '';
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