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-=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 $s = pack( 'H2' x 10, 30..39 );
- print "$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 (unsigned) 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 ), $msg );
-
-and pass C<$buf> to your send routine. Some protocols demand that the
-count should include the length of the count itself: then just add 4
-to the data length. (But make sure to read L<"Lengths and Widths"> before
-you really code this!)
-
-
-=head2 Byte-order modifiers
-
-In the previous sections we've learned how to use C<n>, C<N>, C<v> and
-C<V> to pack and unpack integers with big- or little-endian byte-order.
-While this is nice, it's still rather limited because it leaves out all
-kinds of signed integers as well as 64-bit integers. For example, if you
-wanted to unpack a sequence of signed big-endian 16-bit integers in a
-platform-independent way, you would have to write:
-
- my @data = unpack 's*', pack 'S*', unpack 'n*', $buf;
-
-This is ugly. As of Perl 5.9.2, there's a much nicer way to express your
-desire for a certain byte-order: the C<E<gt>> and C<E<lt>> modifiers.
-C<E<gt>> is the big-endian modifier, while C<E<lt>> is the little-endian
-modifier. Using them, we could rewrite the above code as:
-
- my @data = unpack 's>*', $buf;
-
-As you can see, the "big end" of the arrow touches the C<s>, which is a
-nice way to remember that C<E<gt>> is the big-endian modifier. The same
-obviously works for C<E<lt>>, where the "little end" touches the code.
-
-You will probably find these modifiers even more useful if you have
-to deal with big- or little-endian C structures. Be sure to read
-L<"Packing and Unpacking C Structures"> for more on that.
-
-
-=head2 Floating point Numbers
-
-For packing floating point numbers you have the choice between the
-pack codes C<f>, C<d>, C<F> and C<D>. C<f> and C<d> pack into (or unpack
-from) single-precision or double-precision representation as it is provided
-by your system. If your systems supports it, C<D> can be used to pack and
-unpack extended-precision floating point values (C<long double>), which
-can offer even more resolution than C<f> or C<d>. C<F> packs an C<NV>,
-which is the floating point type used by Perl internally. (There
-is no such thing as a network representation for reals, so if you want
-to send your real numbers across computer boundaries, you'd better stick
-to ASCII representation, unless you're absolutely sure what's on the other
-end of the line. For the even more adventuresome, you can use the byte-order
-modifiers from the previous section also on floating point codes.)
-
-
-
-=head1 Exotic Templates
-
-
-=head2 Bit Strings
-
-Bits are the atoms in the memory world. Access to individual bits may
-have to be used either as a last resort or because it is the most
-convenient way to handle your data. Bit string (un)packing converts
-between strings containing a series of C<0> and C<1> characters and
-a sequence of bytes each containing a group of 8 bits. This is almost
-as simple as it sounds, except that there are two ways the contents of
-a byte may be written as a bit string. Let's have a look at an annotated
-byte:
-
- 7 6 5 4 3 2 1 0
- +-----------------+
- | 1 0 0 0 1 1 0 0 |
- +-----------------+
- MSB LSB
-
-It's egg-eating all over again: Some think that as a bit string this should
-be written "10001100" i.e. beginning with the most significant bit, others
-insist on "00110001". Well, Perl isn't biased, so that's why we have two bit
-string codes:
-
- $byte = pack( 'B8', '10001100' ); # start with MSB
- $byte = pack( 'b8', '00110001' ); # start with LSB
-
-It is not possible to pack or unpack bit fields - just integral bytes.
-C<pack> always starts at the next byte boundary and "rounds up" to the
-next multiple of 8 by adding zero bits as required. (If you do want bit
-fields, there is L<perlfunc/vec>. Or you could implement bit field
-handling at the character string level, using split, substr, and
-concatenation on unpacked bit strings.)
-
-To illustrate unpacking for bit strings, we'll decompose a simple
-status register (a "-" stands for a "reserved" bit):
-
- +-----------------+-----------------+
- | S Z - A - P - C | - - - - O D I T |
- +-----------------+-----------------+
- MSB LSB MSB LSB
-
-Converting these two bytes to a string can be done with the unpack
-template C<'b16'>. To obtain the individual bit values from the bit
-string we use C<split> with the "empty" separator pattern which dissects
-into individual characters. Bit values from the "reserved" positions are
-simply assigned to C<undef>, a convenient notation for "I don't care where
-this goes".
-
- ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
- $trace, $interrupt, $direction, $overflow) =
- split( //, unpack( 'b16', $status ) );
-
-We could have used an unpack template C<'b12'> just as well, since the
-last 4 bits can be ignored anyway.
-
-
-=head2 Uuencoding
-
-Another odd-man-out in the template alphabet is C<u>, which packs an
-"uuencoded string". ("uu" is short for Unix-to-Unix.) Chances are that
-you won't ever need this encoding technique which was invented to overcome
-the shortcomings of old-fashioned transmission mediums that do not support
-other than simple ASCII data. The essential recipe is simple: Take three
-bytes, or 24 bits. Split them into 4 six-packs, adding a space (0x20) to
-each. Repeat until all of the data is blended. Fold groups of 4 bytes into
-lines no longer than 60 and garnish them in front with the original byte count
-(incremented by 0x20) and a C<"\n"> at the end. - The C<pack> chef will
-prepare this for you, a la minute, when you select pack code C<u> on the menu:
-
- my $uubuf = pack( 'u', $bindat );
-
-A repeat count after C<u> sets the number of bytes to put into an
-uuencoded line, which is the maximum of 45 by default, but could be
-set to some (smaller) integer multiple of three. C<unpack> simply ignores
-the repeat count.
-
-
-=head2 Doing Sums
-
-An even stranger template code is C<%>E<lt>I<number>E<gt>. First, because
-it's used as a prefix to some other template code. Second, because it
-cannot be used in C<pack> at all, and third, in C<unpack>, doesn't return the
-data as defined by the template code it precedes. Instead it'll give you an
-integer of I<number> bits that is computed from the data value by
-doing sums. For numeric unpack codes, no big feat is achieved:
-
- my $buf = pack( 'iii', 100, 20, 3 );
- print unpack( '%32i3', $buf ), "\n"; # prints 123
-
-For string values, C<%> returns the sum of the byte values saving
-you the trouble of a sum loop with C<substr> and C<ord>:
-
- print unpack( '%32A*', "\x01\x10" ), "\n"; # prints 17
-
-Although the C<%> code is documented as returning a "checksum":
-don't put your trust in such values! Even when applied to a small number
-of bytes, they won't guarantee a noticeable Hamming distance.
-
-In connection with C<b> or C<B>, C<%> simply adds bits, and this can be put
-to good use to count set bits efficiently:
-
- my $bitcount = unpack( '%32b*', $mask );
-
-And an even parity bit can be determined like this:
-
- my $evenparity = unpack( '%1b*', $mask );
-
-
-=head2 Unicode
-
-Unicode is a character set that can represent most characters in most of
-the world's languages, providing room for over one million different
-characters. Unicode 3.1 specifies 94,140 characters: The Basic Latin
-characters are assigned to the numbers 0 - 127. The Latin-1 Supplement with
-characters that are used in several European languages is in the next
-range, up to 255. After some more Latin extensions we find the character
-sets from languages using non-Roman alphabets, interspersed with a
-variety of symbol sets such as currency symbols, Zapf Dingbats or Braille.
-(You might want to visit L<http://www.unicode.org/> for a look at some of
-them - my personal favourites are Telugu and Kannada.)
-
-The Unicode character sets associates characters with integers. Encoding
-these numbers in an equal number of bytes would more than double the
-requirements for storing texts written in Latin alphabets.
-The UTF-8 encoding avoids this by storing the most common (from a western
-point of view) characters in a single byte while encoding the rarer
-ones in three or more bytes.
-
-Perl uses UTF-8, internally, for most Unicode strings.
-
-So what has this got to do with C<pack>? Well, if you want to compose a
-Unicode string (that is internally encoded as UTF-8), you can do so by
-using template code C<U>. As an example, let's produce the Euro currency
-symbol (code number 0x20AC):
-
- $UTF8{Euro} = pack( 'U', 0x20AC );
- # Equivalent to: $UTF8{Euro} = "\x{20ac}";
-
-Inspecting C<$UTF8{Euro}> shows that it contains 3 bytes:
-"\xe2\x82\xac". However, it contains only 1 character, number 0x20AC.
-The round trip can be completed with C<unpack>:
-
- $Unicode{Euro} = unpack( 'U', $UTF8{Euro} );
-
-Unpacking using the C<U> template code also works on UTF-8 encoded byte
-strings.
-
-Usually you'll want to pack or unpack UTF-8 strings:
-
- # pack and unpack the Hebrew alphabet
- my $alefbet = pack( 'U*', 0x05d0..0x05ea );
- my @hebrew = unpack( 'U*', $utf );
-
-Please note: in the general case, you're better off using
-Encode::decode_utf8 to decode a UTF-8 encoded byte string to a Perl
-Unicode string, and Encode::encode_utf8 to encode a Perl Unicode string
-to UTF-8 bytes. These functions provide means of handling invalid byte
-sequences and generally have a friendlier interface.
-
-=head2 Another Portable Binary Encoding
-
-The pack code C<w> has been added to support a portable binary data
-encoding scheme that goes way beyond simple integers. (Details can
-be found at L<http://Casbah.org/>, the Scarab project.) A BER (Binary Encoded
-Representation) compressed unsigned integer stores base 128
-digits, most significant digit first, with as few digits as possible.
-Bit eight (the high bit) is set on each byte except the last. There
-is no size limit to BER encoding, but Perl won't go to extremes.
-
- my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 );
-
-A hex dump of C<$berbuf>, with spaces inserted at the right places,
-shows 01 8100 8101 81807F. Since the last byte is always less than
-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
-
-In the previous section we've seen a network message that was constructed
-by prefixing the binary message length to the actual message. You'll find
-that packing a length followed by so many bytes of data is a
-frequently used recipe since appending a null byte won't work
-if a null byte may be part of the data. Here is an example where both
-techniques are used: after two null terminated strings with source and
-destination address, a Short Message (to a mobile phone) is sent after
-a length byte:
-
- my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm );
-
-Unpacking this message can be done with the same template:
-
- ( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg );
-
-There's a subtle trap lurking in the offing: Adding another field after
-the Short Message (in variable C<$sm>) is all right when packing, but this
-cannot be unpacked naively:
-
- # pack a message
- my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio );
-
- # unpack fails - $prio remains undefined!
- ( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg );
-
-The pack code C<A*> gobbles up all remaining bytes, and C<$prio> remains
-undefined! Before we let disappointment dampen the morale: Perl's got
-the trump card to make this trick too, just a little further up the sleeve.
-Watch this:
-
- # pack a message: ASCIIZ, ASCIIZ, length/string, byte
- my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio );
-
- # unpack
- ( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg );
-
-Combining two pack codes with a slash (C</>) associates them with a single
-value from the argument list. In C<pack>, the length of the argument is
-taken and packed according to the first code while the argument itself
-is added after being converted with the template code after the slash.
-This saves us the trouble of inserting the C<length> call, but it is
-in C<unpack> where we really score: The value of the length byte marks the
-end of the string to be taken from the buffer. Since this combination
-doesn't make sense except when the second pack code isn't C<a*>, C<A*>
-or C<Z*>, Perl won't let you.
-
-The pack code preceding C</> may be anything that's fit to represent a
-number: All the numeric binary pack codes, and even text codes such as
-C<A4> or C<Z*>:
-
- # pack/unpack a string preceded by its length in ASCII
- my $buf = pack( 'A4/A*', "Humpty-Dumpty" );
- # unpack $buf: '13 Humpty-Dumpty'
- my $txt = unpack( 'A4/A*', $buf );
-
-C</> is not implemented in Perls before 5.6, so if your code is required to
-work on older Perls you'll need to C<unpack( 'Z* Z* C')> to get the length,
-then use it to make a new unpack string. For example
-
- # pack a message: ASCIIZ, ASCIIZ, length, string, byte (5.005 compatible)
- my $msg = pack( 'Z* Z* C A* C', $src, $dst, length $sm, $sm, $prio );
-
- # unpack
- ( undef, undef, $len) = unpack( 'Z* Z* C', $msg );
- ($src, $dst, $sm, $prio) = unpack ( "Z* Z* x A$len C", $msg );
-
-But that second C<unpack> is rushing ahead. It isn't using a simple literal
-string for the template. So maybe we should introduce...
-
-=head2 Dynamic Templates
-
-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 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*)' . 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 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.)
-
-For the reverse operation, we'll have to determine the number of items
-in the buffer before we can let C<unpack> rip it apart:
-
- my $n = $env =~ tr/\0// - 1;
- 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.
-
-
-=head2 Intel HEX
-
-Intel HEX is a file format for representing binary data, mostly for
-programming various chips, as a text file. (See
-L<http://en.wikipedia.org/wiki/.hex> for a detailed description, and
-L<http://en.wikipedia.org/wiki/SREC_(file_format)> for the Motorola
-S-record format, which can be unravelled using the same technique.)
-Each line begins with a colon (':') and is followed by a sequence of
-hexadecimal characters, specifying a byte count I<n> (8 bit),
-an address (16 bit, big endian), a record type (8 bit), I<n> data bytes
-and a checksum (8 bit) computed as the least significant byte of the two's
-complement sum of the preceding bytes. Example: C<:0300300002337A1E>.
-
-The first step of processing such a line is the conversion, to binary,
-of the hexadecimal data, to obtain the four fields, while checking the
-checksum. No surprise here: we'll start with a simple C<pack> call to
-convert everything to binary:
-
- my $binrec = pack( 'H*', substr( $hexrec, 1 ) );
-
-The resulting byte sequence is most convenient for checking the checksum.
-Don't slow your program down with a for loop adding the C<ord> values
-of this string's bytes - the C<unpack> code C<%> is the thing to use
-for computing the 8-bit sum of all bytes, which must be equal to zero:
-
- die unless unpack( "%8C*", $binrec ) == 0;
-
-Finally, let's get those four fields. By now, you shouldn't have any
-problems with the first three fields - but how can we use the byte count
-of the data in the first field as a length for the data field? Here
-the codes C<x> and C<X> come to the rescue, as they permit jumping
-back and forth in the string to unpack.
-
- my( $addr, $type, $data ) = unpack( "x n C X4 C x3 /a", $bin );
-
-Code C<x> skips a byte, since we don't need the count yet. Code C<n> takes
-care of the 16-bit big-endian integer address, and C<C> unpacks the
-record type. Being at offset 4, where the data begins, we need the count.
-C<X4> brings us back to square one, which is the byte at offset 0.
-Now we pick up the count, and zoom forth to offset 4, where we are
-now fully furnished to extract the exact number of data bytes, leaving
-the trailing checksum byte alone.
-
-
-
-=head1 Packing and Unpacking C Structures
-
-In previous sections we have seen how to pack numbers and character
-strings. If it were not for a couple of snags we could conclude this
-section right away with the terse remark that C structures don't
-contain anything else, and therefore you already know all there is to it.
-Sorry, no: read on, please.
-
-If you have to deal with a lot of C structures, and don't want to
-hack all your template strings manually, you'll probably want to have
-a look at the CPAN module C<Convert::Binary::C>. Not only can it parse
-your C source directly, but it also has built-in support for all the
-odds and ends described further on in this section.
-
-=head2 The Alignment Pit
-
-In the consideration of speed against memory requirements the balance
-has been tilted in favor of faster execution. This has influenced the
-way C compilers allocate memory for structures: On architectures
-where a 16-bit or 32-bit operand can be moved faster between places in
-memory, or to or from a CPU register, if it is aligned at an even or
-multiple-of-four or even at a multiple-of eight address, a C compiler
-will give you this speed benefit by stuffing extra bytes into structures.
-If you don't cross the C shoreline this is not likely to cause you any
-grief (although you should care when you design large data structures,
-or you want your code to be portable between architectures (you do want
-that, don't you?)).
-
-To see how this affects C<pack> and C<unpack>, we'll compare these two
-C structures:
-
- typedef struct {
- char c1;
- short s;
- char c2;
- long l;
- } gappy_t;
-
- typedef struct {
- long l;
- short s;
- char c1;
- char c2;
- } dense_t;
-
-Typically, a C compiler allocates 12 bytes to a C<gappy_t> variable, but
-requires only 8 bytes for a C<dense_t>. After investigating this further,
-we can draw memory maps, showing where the extra 4 bytes are hidden:
-
- 0 +4 +8 +12
- +--+--+--+--+--+--+--+--+--+--+--+--+
- |c1|xx| s |c2|xx|xx|xx| l | xx = fill byte
- +--+--+--+--+--+--+--+--+--+--+--+--+
- gappy_t
-
- 0 +4 +8
- +--+--+--+--+--+--+--+--+
- | l | h |c1|c2|
- +--+--+--+--+--+--+--+--+
- dense_t
-
-And that's where the first quirk strikes: C<pack> and C<unpack>
-templates have to be stuffed with C<x> codes to get those extra fill bytes.
-
-The natural question: "Why can't Perl compensate for the gaps?" warrants
-an answer. One good reason is that C compilers might provide (non-ANSI)
-extensions permitting all sorts of fancy control over the way structures
-are aligned, even at the level of an individual structure field. And, if
-this were not enough, there is an insidious thing called C<union> where
-the amount of fill bytes cannot be derived from the alignment of the next
-item alone.
-
-OK, so let's bite the bullet. Here's one way to get the alignment right
-by inserting template codes C<x>, which don't take a corresponding item
-from the list:
-
- my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );
-
-Note the C<!> after C<l>: We want to make sure that we pack a long
-integer as it is compiled by our C compiler. And even now, it will only
-work for the platforms where the compiler aligns things as above.
-And somebody somewhere has a platform where it doesn't.
-[Probably a Cray, where C<short>s, C<int>s and C<long>s are all 8 bytes. :-)]
-
-Counting bytes and watching alignments in lengthy structures is bound to
-be a drag. Isn't there a way we can create the template with a simple
-program? Here's a C program that does the trick:
-
- #include <stdio.h>
- #include <stddef.h>
-
- typedef struct {
- char fc1;
- short fs;
- char fc2;
- long fl;
- } gappy_t;
-
- #define Pt(struct,field,tchar) \
- printf( "@%d%s ", offsetof(struct,field), # tchar );
-
- int main() {
- Pt( gappy_t, fc1, c );
- Pt( gappy_t, fs, s! );
- Pt( gappy_t, fc2, c );
- Pt( gappy_t, fl, l! );
- printf( "\n" );
- }
-
-The output line can be used as a template in a C<pack> or C<unpack> call:
-
- my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l );
-
-Gee, yet another template code - as if we hadn't plenty. But
-C<@> saves our day by enabling us to specify the offset from the beginning
-of the pack buffer to the next item: This is just the value
-the C<offsetof> macro (defined in C<E<lt>stddef.hE<gt>>) returns when
-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 Dealing with Endian-ness
-
-Now, imagine that we want to pack the data for a machine with a
-different byte-order. First, we'll have to figure out how big the data
-types on the target machine really are. Let's assume that the longs are
-32 bits wide and the shorts are 16 bits wide. You can then rewrite the
-template as:
-
- my $gappy = pack( 'c x![s] s c x![l] l', $c1, $s, $c2, $l );
-
-If the target machine is little-endian, we could write:
-
- my $gappy = pack( 'c x![s] s< c x![l] l<', $c1, $s, $c2, $l );
-
-This forces the short and the long members to be little-endian, and is
-just fine if you don't have too many struct members. But we could also
-use the byte-order modifier on a group and write the following:
-
- my $gappy = pack( '( c x![s] s c x![l] l )<', $c1, $s, $c2, $l );
-
-This is not as short as before, but it makes it more obvious that we
-intend to have little-endian byte-order for a whole group, not only
-for individual template codes. It can also be more readable and easier
-to maintain.
-
-
-=head2 Alignment, Take 2
-
-I'm afraid that we're not quite through with the alignment catch yet. The
-hydra raises another ugly head when you pack arrays of structures:
-
- typedef struct {
- short count;
- char glyph;
- } cell_t;
-
- typedef cell_t buffer_t[BUFLEN];
-
-Where's the catch? Padding is neither required before the first field C<count>,
-nor between this and the next field C<glyph>, so why can't we simply pack
-like this:
-
- # something goes wrong here:
- pack( 's!a' x @buffer,
- map{ ( $_->{count}, $_->{glyph} ) } @buffer );
-
-This packs C<3*@buffer> bytes, but it turns out that the size of
-C<buffer_t> is four times C<BUFLEN>! The moral of the story is that
-the required alignment of a structure or array is propagated to the
-next higher level where we have to consider padding I<at the end>
-of each component as well. Thus the correct template is:
-
- pack( 's!ax' x @buffer,
- map{ ( $_->{count}, $_->{glyph} ) } @buffer );
-
-=head2 Alignment, Take 3
-
-And even if you take all the above into account, ANSI still lets this:
-
- typedef struct {
- char foo[2];
- } foo_t;
-
-vary in size. The alignment constraint of the structure can be greater than
-any of its elements. [And if you think that this doesn't affect anything
-common, dismember the next cellphone that you see. Many have ARM cores, and
-the ARM structure rules make C<sizeof (foo_t)> == 4]
-
-=head2 Pointers for How to Use Them
-
-The title of this section indicates the second problem you may run into
-sooner or later when you pack C structures. If the function you intend
-to call expects a, say, C<void *> value, you I<cannot> simply take
-a reference to a Perl variable. (Although that value certainly is a
-memory address, it's not the address where the variable's contents are
-stored.)
-
-Template code C<P> promises to pack a "pointer to a fixed length string".
-Isn't this what we want? Let's try:
-
- # allocate some storage and pack a pointer to it
- my $memory = "\x00" x $size;
- my $memptr = pack( 'P', $memory );
-
-But wait: doesn't C<pack> just return a sequence of bytes? How can we pass this
-string of bytes to some C code expecting a pointer which is, after all,
-nothing but a number? The answer is simple: We have to obtain the numeric
-address from the bytes returned by C<pack>.
-
- my $ptr = unpack( 'L!', $memptr );
-
-Obviously this assumes that it is possible to typecast a pointer
-to an unsigned long and vice versa, which frequently works but should not
-be taken as a universal law. - Now that we have this pointer the next question
-is: How can we put it to good use? We need a call to some C function
-where a pointer is expected. The read(2) system call comes to mind:
-
- ssize_t read(int fd, void *buf, size_t count);
-
-After reading L<perlfunc> explaining how to use C<syscall> we can write
-this Perl function copying a file to standard output:
-
- require 'syscall.ph';
- sub cat($){
- my $path = shift();
- my $size = -s $path;
- my $memory = "\x00" x $size; # allocate some memory
- my $ptr = unpack( 'L', pack( 'P', $memory ) );
- open( F, $path ) || die( "$path: cannot open ($!)\n" );
- my $fd = fileno(F);
- my $res = syscall( &SYS_read, fileno(F), $ptr, $size );
- print $memory;
- close( F );
- }
-
-This is neither a specimen of simplicity nor a paragon of portability but
-it illustrates the point: We are able to sneak behind the scenes and
-access Perl's otherwise well-guarded memory! (Important note: Perl's
-C<syscall> does I<not> require you to construct pointers in this roundabout
-way. You simply pass a string variable, and Perl forwards the address.)
-
-How does C<unpack> with C<P> work? Imagine some pointer in the buffer
-about to be unpacked: If it isn't the null pointer (which will smartly
-produce the C<undef> value) we have a start address - but then what?
-Perl has no way of knowing how long this "fixed length string" is, so
-it's up to you to specify the actual size as an explicit length after C<P>.
-
- my $mem = "abcdefghijklmn";
- print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"
-
-As a consequence, C<pack> ignores any number or C<*> after C<P>.
-
-
-Now that we have seen C<P> at work, we might as well give C<p> a whirl.
-Why do we need a second template code for packing pointers at all? The
-answer lies behind the simple fact that an C<unpack> with C<p> promises
-a null-terminated string starting at the address taken from the buffer,
-and that implies a length for the data item to be returned:
-
- my $buf = pack( 'p', "abc\x00efhijklmn" );
- print unpack( 'p', $buf ); # prints "abc"
-
-
-
-Albeit this is apt to be confusing: As a consequence of the length being
-implied by the string's length, a number after pack code C<p> is a repeat
-count, not a length as after C<P>.
-
-
-Using C<pack(..., $x)> with C<P> or C<p> to get the address where C<$x> is
-actually stored must be used with circumspection. Perl's internal machinery
-considers the relation between a variable and that address as its very own
-private matter and doesn't really care that we have obtained a copy. Therefore:
-
-=over 4
-
-=item *
-
-Do not use C<pack> with C<p> or C<P> to obtain the address of variable
-that's bound to go out of scope (and thereby freeing its memory) before you
-are done with using the memory at that address.
-
-=item *
-
-Be very careful with Perl operations that change the value of the
-variable. Appending something to the variable, for instance, might require
-reallocation of its storage, leaving you with a pointer into no-man's land.
-
-=item *
-
-Don't think that you can get the address of a Perl variable
-when it is stored as an integer or double number! C<pack('P', $x)> will
-force the variable's internal representation to string, just as if you
-had written something like C<$x .= ''>.
-
-=back
-
-It's safe, however, to P- or p-pack a string literal, because Perl simply
-allocates an anonymous variable.
-
-
-
-=head1 Pack Recipes
-
-Here are a collection of (possibly) useful canned recipes for C<pack>
-and C<unpack>:
-
- # Convert IP address for socket functions
- pack( "C4", split /\./, "123.4.5.6" );
-
- # Count the bits in a chunk of memory (e.g. a select vector)
- unpack( '%32b*', $mask );
-
- # Determine the endianness of your system
- $is_little_endian = unpack( 'c', pack( 's', 1 ) );
- $is_big_endian = unpack( 'xc', pack( 's', 1 ) );
-
- # Determine the number of bits in a native integer
- $bits = unpack( '%32I!', ~0 );
-
- # Prepare argument for the nanosleep system call
- my $timespec = pack( 'L!L!', $secs, $nanosecs );
-
-For a simple memory dump we unpack some bytes into just as
-many pairs of hex digits, and use C<map> to handle the traditional
-spacing - 16 bytes to a line:
-
- my $i;
- print map( ++$i % 16 ? "$_ " : "$_\n",
- unpack( 'H2' x length( $mem ), $mem ) ),
- length( $mem ) % 16 ? "\n" : '';
-
-
-=head1 Funnies Section
-
- # Pulling digits out of nowhere...
- print unpack( 'C', pack( 'x' ) ),
- unpack( '%B*', pack( 'A' ) ),
- unpack( 'H', pack( 'A' ) ),
- unpack( 'A', unpack( 'C', pack( 'A' ) ) ), "\n";
-
- # One for the road ;-)
- my $advice = pack( 'all u can in a van' );
-
-
-=head1 Authors
-
-Simon Cozens and Wolfgang Laun.
-