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

       perlpacktut - tutorial on "pack" and "unpack"

       "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" func-
       tion converts values to a byte sequence containing representations according to a given
       specification, the so-called "template" argument. "unpack" is the reverse process, deriv-
       ing 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 repre-
       sentation? 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 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 dig-
       its. Let's use "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 $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:


       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" con-
       verts 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) =
		   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
	   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 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 "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 remain-

       o  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("A10xA27xA7A*", $_);

       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 - say-
       ing 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 "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-jus-
       tify 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

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

       o  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.

       o  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.

       o  "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.

       o  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 "charac-
	  ter 0, 6 times".

       o  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 cur-
	  rent 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

Packing Numbers
       So much for textual data. Let's get onto the meaty stuff that "pack" and "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.


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

       o   the number of bytes used for storing the integer,

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

       o   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 representa-
       tion 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 inte-
       gers 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 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 compiler 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 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 with "use

	  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 com-
       pleteness' sake.

       Unpacking a Stack Frame

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

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

       First, we note that this time-honored 16-bit CPU uses little-endian order, and that's why
       the low order byte is stored at the lower address. To unpack such a (signed) short we'll
       have to use code "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 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 is nice, it proffers the template code "X" to back up
       one byte. Putting this all together, we may now write:

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

       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 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 develop-
       ment 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 representation 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 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 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 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" sepa-
       rator 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 goes".

	  ($carry, undef, $parity, undef, $auxcarry, undef, $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.


       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 tech-
       nique which was invented to overcome the shortcomings of old-fashioned transmission medi-
       ums 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

       Although the "%" 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 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 is a character set that can represent most characters in most of the world's lan-
       guages, 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 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 representation 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

	  $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 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.

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 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 $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

	  # 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 constructing the template has to be used. Here's an example:
       To store named string values in a way that can be conveniently parsed by a C program, we
       create a sequence of names and null terminated ASCII strings, with "=" between the name
       and the value, followed by an additional delimiting null byte. Here's how:

	  my $env = pack( 'A*A*Z*' x keys( %Env ) . 'C',
			  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 "=" separa-
       tor and the hash entry value.  Each triplet is packed with the template code sequence
       "A*A*Z*" that is multiplied with 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*' x $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.

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.

       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 ben-
       efit 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

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

       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 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 "union"
       where the amount of fill bytes cannot be derived from the alignment of the next item

       OK, so let's bite the bullet. Here's one way to get the alignment right by inserting tem-
       plate 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 platform where it doesn't.  [Proba-
       bly 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 );

	  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).

       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 "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 cer-
       tainly is a memory address, it's not the address where the variable's contents are

       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 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 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 "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

       Using "pack(..., $x)" with "P" or "p" to get the address where $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:

       o   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 mem-
	   ory at that address.

       o   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.

       o   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', $x)" will force the variable's internal represen-
	   tation to string, just as if you had written something like "$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 /\./, "" );

	   # 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 traditional 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' );

       Simon Cozens and Wolfgang Laun.

perl v5.8.0				    2003-02-18				   PERLPACKTUT(1)
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