MirOS Manual: perlpacktut(1)


PERLPACKTUT(1)  Perl Programmers Reference Guide   PERLPACKTUT(1)

NAME

     perlpacktut - tutorial on "pack" and "unpack"

DESCRIPTION

     "pack" and "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.

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 "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
     "pack" and "unpack" is an excellent alternative. The "pack"
     function converts values to a byte sequence containing
     representations according to a given specification, the so-
     called "template" argument. "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 contain-
     ing 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: "syscall" requires you to
     provide parameters stored in the way it happens in a C pro-
     gram. Even text processing (as shown in the next section)
     may be simplified with judicious usage of these two func-
     tions.

     To see how (un)packing works, we'll start with a simple tem-
     plate code where the conversion is in low gear: between the
     contents of a byte sequence and a string of hexadecimal
     digits. Let's use "unpack", since this is likely to remind
     you of a dump program, or some desperate last message unfor-
     tunate programs are wont to throw at you before they expire
     into the wild blue yonder. Assuming that the variable $mem
     holds a sequence of bytes that we'd like to inspect without
     assuming anything about its meaning, we can write

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        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 0x40 - 0x5A indicate an uppercase letter, and
     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 "unpack" with the template code "H"
     converts the contents of a sequence of bytes into the cus-
     tomary hexadecimal notation. Since "a sequence of" is a
     pretty vague indication of quantity, "H" has been defined to
     convert just a single hexadecimal digit unless it is fol-
     lowed 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
     "pack", the pack template should contain ten pack codes. If
     this is run on a computer with ASCII character coding, it
     will print 0123456789.

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 "split"; how-
     ever, since "split" collapses blank fields, you'll never
     know whether a record was income or expenditure. Oops. Well,
     you could always use "substr":

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         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 horri-
     bly 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. "pack" and
     "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 "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 "pack"-ese

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     for "character" is "A", and ten of them are "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 "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 "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 charac-
     ters:

         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 "unpack" pattern that we 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 "*" means: "use everything remaining".

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

     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:

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         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
     "x", didn't we? Shouldn't it skip forward? Let's look at
     what "pack" in perlfunc 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 "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 "pack" to right-
     justify our fields, but we can get "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

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

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

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

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

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

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

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

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

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

Packing Numbers

     So much for textual data. Let's get onto the meaty stuff
     that "pack" and "unpack" are best at: handling binary for-
     mats 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.

     Integers

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

     +   the number of bytes used for storing the integer,

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

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

     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 "ON" or "NO" (depending on your system's byte
     ordering) - or something entirely different if your computer
     doesn't use ASCII character encoding. Unpacking $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 "s", an excess value may result in the sign bit get-
     ting set, and unpacking this will smartly return a negative
     value.

     16 bits won't get you too far with integers, but there is
     "l" and "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 "q" and "Q". A notable exception is provided by pack
     codes "i" and "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 "sizeof(int)", but
     it'll use at least 32 bits.

     Each of the integer pack codes "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 exten-
     sions or the Perl function "syscall"), or when you read or
     write binary files. What you'll need in this case are tem-
     plate codes that depend on what your local C compiler com-
     piles when you code "short" or "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 %Config in your program you have to import it

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     with "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 "i!" and "I!" codes aren't different from "i" and "I";
     they are tolerated for completeness' sake.

     Unpacking a Stack Frame

     Requesting a particular byte ordering may be necessary when
     you work with binary data coming from some specific archi-
     tecture whereas your program could run on a totally dif-
     ferent system. As an example, assume you have 24 bytes con-
     taining 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 "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" to unpack the indivi-
     dually 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 is nice, it proffers the template code "X" to
     back up one byte. Putting this all together, we may now
     write:

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        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 "unpack" could not unpack
     all. If "pack" runs out of items, it will supply null
     strings (which are coerced into zeroes whenever the pack
     code says so).

     How to Eat an Egg on a Net

     The pack code for big-endian (high order byte at the lowest
     address) is "n" for 16 bit and "N" for 32 bit integers. You
     use these codes if you know that your data comes from a com-
     pliant 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 network order, and all standard-fearing pro-
     grams 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 $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 "Lengths and Widths" before you really code this!)

     Floating point Numbers

     For packing floating point numbers you have the choice
     between the pack codes "f" and "d" which pack into (or
     unpack from) single-precision or double-precision represen-
     tation as it is provided by your system. (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

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     absolutely sure what's on the other end of the line.)

Exotic Templates

     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 0 and 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. "pack" always starts at the next byte boun-
     dary and "rounds up" to the next multiple of 8 by adding
     zero bits as required. (If you do want bit fields, there is
     "vec" in perlfunc. Or you could implement bit field handling
     at the character string level, using split, substr, and con-
     catenation 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 'b16'. To obtain the individual bit values
     from the bit string we use "split" with the "empty" separa-
     tor pattern which dissects into individual characters. Bit
     values from the "reserved" positions are simply assigned to
     "undef", a convenient notation for "I don't care where this

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     goes".

        ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
         $trace, $interrupt, $direction, $overflow) =
           split( //, unpack( 'b16', $status ) );

     We could have used an unpack template 'b12' just as well,
     since the last 4 bits can be ignored anyway.

     Uuencoding

     Another odd-man-out in the template alphabet is "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 "\n" at
     the end. - The "pack" chef will prepare this for you, a la
     minute, when you select pack code "u" on the menu:

        my $uubuf = pack( 'u', $bindat );

     A repeat count after "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. "unpack" simply ignores the repeat count.

     Doing Sums

     An even stranger template code is "%"<number>. First,
     because it's used as a prefix to some other template code.
     Second, because it cannot be used in "pack" at all, and
     third, in "unpack", doesn't return the data as defined by
     the template code it precedes. Instead it'll give you an
     integer of 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, "%" returns the sum of the byte values
     saving you the trouble of a sum loop with "substr" and
     "ord":

         print unpack( '%32A*', "\x01\x10" ), "\n";  # prints 17

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     Although the "%" code is documented as returning a "check-
     sum": don't put your trust in such values! Even when applied
     to a small number of bytes, they won't guarantee a notice-
     able Hamming distance.

     In connection with "b" or "B", "%" 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 );

     Unicode

     Unicode is a character set that can represent most charac-
     ters 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 charac-
     ters 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 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.

     So what has this got to do with "pack"? Well, if you want to
     convert between a Unicode number and its UTF-8 representa-
     tion you can do so by using template code "U". As an exam-
     ple, let's produce the UTF-8 representation of the Euro
     currency symbol (code number 0x20AC):

        $UTF8{Euro} = pack( 'U', 0x20AC );

     Inspecting $UTF8{Euro} shows that it contains 3 bytes:
     "\xe2\x82\xac". The round trip can be completed with
     "unpack":

        $Unicode{Euro} = unpack( 'U', $UTF8{Euro} );

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     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 );

     Another Portable Binary Encoding

     The pack code "w" has been added to support a portable
     binary data encoding scheme that goes way beyond simple
     integers. (Details can be found at 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 $berbuf, with spaces inserted at the right
     places, shows 01 8100 8101 81807F. Since the last byte is
     always less than 128, "unpack" knows where to stop.

Template Grouping

     Prior to Perl 5.8, repetitions of templates had to be made
     by "x"-multiplication of template strings. Now there is a
     better way as we may use the pack codes "(" and ")" combined
     with a repeat count. The "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 "*" means "repeat as often as
     required", simply

        pack( '(A)*', @str )

     (Note that the template "A*" would only have packed $str[0]
     in full length.)

     To pack dates stored as triplets ( day, month, year ) in an
     array @dates into a sequence of byte, byte, short integer we

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     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 "x" and
     "X" to skip forward and back:

        $s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) );

     We can also use "@" to jump to an offset, with 0 being the
     position where we were when the last "(" 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 );

Lengths and Widths

     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 fol-
     lowed 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 $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 "A*" gobbles up all remaining bytes, and $prio
     remains undefined! Before we let disappointment dampen the

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     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 ("/") associates them
     with a single value from the argument list. In "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 "length" call, but it is in
     "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 "a*", "A*" or "Z*", Perl won't let
     you.

     The pack code preceding "/" may be anything that's fit to
     represent a number: All the numeric binary pack codes, and
     even text codes such as "A4" or "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 );

     "/" is not implemented in Perls before 5.6, so if your code
     is required to work on older Perls you'll need to "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 "unpack" is rushing ahead. It isn't using a
     simple literal string for the template. So maybe we should
     introduce...

     Dynamic Templates

     So far, we've seen literals used as templates. If the list
     of pack items doesn't have fixed length, an expression con-
     structing the template is required (whenever, for some rea-
     son, "()*" cannot be used). Here's an example: To store

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     named string values in a way that can be conveniently parsed
     by a C program, we create a sequence of names and null ter-
     minated ASCII strings, with "=" 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 "map" call, creating the items we intend to
     stuff into the $env buffer: to each key (in $_) it adds the
     "=" separator and the hash entry value. Each triplet is
     packed with the template code sequence "A*A*Z*" that is
     repeated according to the number of keys. (Yes, that's what
     the "keys" function returns in scalar context.) To get the
     very last null byte, we add a 0 at the end of the "pack"
     list, to be packed with "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 "unpack" rip
     it apart:

        my $n = $env =~ tr/\0// - 1;
        my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) );

     The "tr" counts the null bytes. The "unpack" call returns a
     list of name-value pairs each of which is taken apart in the
     "map" block.

     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 "/" 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 "pack" and "unpack" because "pack"
     can't determine a repeat count for a "()"-group.

Packing and Unpacking C Structures

     In previous sections we have seen how to pack numbers and

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     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.

     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 "pack" and "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 "gappy_t"
     variable, but requires only 8 bytes for a "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

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        0           +4          +8
        +--+--+--+--+--+--+--+--+
        |     l     |  h  |c1|c2|
        +--+--+--+--+--+--+--+--+
        dense_t

     And that's where the first quirk strikes: "pack" and
     "unpack" templates have to be stuffed with "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 com-
     pilers 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
     "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 "x", which don't
     take a corresponding item from the list:

       my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );

     Note the "!" after "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 plat-
     form where it doesn't. [Probably a Cray, where "short"s,
     "int"s and "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 );

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        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 "pack" or
     "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
     "@" 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 "offsetof" macro (defined in
     "<stddef.h>") returns when given a "struct" type and one of
     its field names ("member-designator" in C standardese).

     Neither using offsets nor adding "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 "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 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: "[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 );

     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];

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     Where's the catch? Padding is neither required before the
     first field "count", nor between this and the next field
     "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 "3*@buffer" bytes, but it turns out that the size
     of "buffer_t" is four times "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 con-
     sider padding at the end of each component as well. Thus the
     correct template is:

        pack( 's!ax' x @buffer,
              map{ ( $_->{count}, $_->{glyph} ) } @buffer );

     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 "sizeof (foo_t)" == 4]

     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, "void *"
     value, you cannot simply take a reference to a Perl vari-
     able. (Although that value certainly is a memory address,
     it's not the address where the variable's contents are
     stored.)

     Template code "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 "pack" just return a sequence of bytes?
     How can we pass this string of bytes to some C code

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     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 "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 perlfunc explaining how to use "syscall" we
     can write this Perl function copying a file to standard out-
     put:

         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 "syscall" does not
     require you to construct pointers in this roundabout way.
     You simply pass a string variable, and Perl forwards the
     address.)

     How does "unpack" with "P" work? Imagine some pointer in the
     buffer about to be unpacked: If it isn't the null pointer
     (which will smartly produce the "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 "P".

        my $mem = "abcdefghijklmn";
        print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"

     As a consequence, "pack" ignores any number or "*" after
     "P".

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     Now that we have seen "P" at work, we might as well give "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 "unpack" with "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 "p" is a repeat count, not a length as after "P".

     Using "pack(..., $x)" with "P" or "p" to get the address
     where $x is actually stored must be used with circumspec-
     tion. 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:

     +   Do not use "pack" with "p" or "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.

     +   Be very careful with Perl operations that change the
         value of the variable. Appending something to the vari-
         able, for instance, might require reallocation of its
         storage, leaving you with a pointer into no-man's land.

     +   Don't think that you can get the address of a Perl vari-
         able when it is stored as an integer or double number!
         "pack('P', $x)" will force the variable's internal
         representation to string, just as if you had written
         something like "$x .= ''".

     It's safe, however, to P- or p-pack a string literal,
     because Perl simply allocates an anonymous variable.

Pack Recipes

     Here are a collection of (possibly) useful canned recipes
     for "pack" and "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 );

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         # 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 "map" to handle the tradi-
     tional spacing - 16 bytes to a line:

         my $i;
         print map( ++$i % 16 ? "$_ " : "$_\n",
                    unpack( 'H2' x length( $mem ), $mem ) ),
               length( $mem ) % 16 ? "\n" : '';

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' );

Authors

     Simon Cozens and Wolfgang Laun.

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