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perlpacktut



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 rep­
       resentation 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 con­
       taining representations according to a given specifica­
       tion, 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 con­
       taining some values in binary representation? One good
       reason is input and output accessing some file, a device,
       or a network connection, whereby this binary representa­
       tion is either forced on you or will give you some benefit
       in processing. Another cause is passing data to some sys­
       tem call that is not available as a Perl function:
       "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 judi­
       cious 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 hexadeci­
       mal digits. Let's use "unpack", since this is likely to
       remind you of a dump program, or some desperate last mes­
       sage unfortunate 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

       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 customary 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 followed by a repeat count. An asterisk for the
       repeat count means to use whatever remains.

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

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

       Since we feed a list of ten 2-digit hexadecimal strings to
       "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";
       however, since "split" collapses blank fields, you'll
       never know whether a record was income or expenditure.
       Oops. Well, you could always use "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) = 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 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 char­
       acters:

           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.

           }

           $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
       "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);

       ·  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 "character 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,

       ·   the byte ordering: whether the first byte is the least
       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 getting 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 pro­
       vided 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
       extensions or the Perl function "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 com­
       piler compiles 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 stan­
       dard 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 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
                                | CL | CH | CX            |   DS    |
                                +----+----+               +---------+
                                | DL | DH | DX            |   ES    |
                                +----+----+               +---------+

       First, we note that this time-honored 16-bit CPU uses lit­
       tle-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 indi­
       vidually 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:

          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 low­
       est address) is "n" for 16 bit and "N" for 32 bit inte­
       gers. 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 network order, and all stan­
       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 repre­
       sentation 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 bound­
       aries, you'd better stick to ASCII representation, unless
       you're 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 individ­
       ual 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 sim­
       ple as it sounds, except that there are two ways the con­
       tents 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
       boundary 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 concatenation on unpacked bit strings.)

       values from the bit string we use "split" with the "empty"
       separator pattern which dissects into individual charac­
       ters. Bit values from the "reserved" positions are simply
       assigned to "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 '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 multi­
       ple 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

       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 speci­
       fies 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 lan­
       guages 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 repre­
       sentation you can do so by using template code "U". As an
       example, 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} );

       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


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 ")" com­
       bined 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 inte­
       ger 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 "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

       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 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
       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, "()*" cannot be used).  Here's an example: To
       store named string values in a way that can be conve­
       niently parsed by a C program, we create a sequence of
       names and null terminated ASCII strings, with "=" between
       the name and the value, followed by an additional delimit­
       ing 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
       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, show­
       ing 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: "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
       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
       "short"s, "int"s and "long"s are all 8 bytes. :-)]

       Counting bytes and watching alignments in lengthy struc­
       tures is bound to be a drag. Isn't there a way we can cre­
       ate 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 "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 struc­
       ture 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 ver­
       sion of our struct packaging:

       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 "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 consider 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
       variable. (Although that value certainly is a memory
       address, it's not the address where the variable's con­
       tents are stored.)
           my $ptr = unpack( 'L!', $memptr );

       Obviously this assumes that it is possible to typecast a
       pointer to an unsigned long and vice versa, which fre­
       quently 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 func­
       tion 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
       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 "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".

       Now that we have seen "P" at work, we might as well give

       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 pri­
       vate 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
           variable, 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
           variable 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 );

           # 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 tra­
       ditional spacing - 16 bytes to a line:

           my $advice = pack( 'all u can in a van' );


Authors

       Simon Cozens and Wolfgang Laun.

perl v5.8.1                 2003-09-02             PERLPACKTUT(1)
  
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