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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" 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 containing some
       values in binary	representation?	One good reason	is input and output
       accessing some file, a device, or a network connection, whereby this
       binary representation is	either forced on you or	will give you some
       benefit in processing. Another cause is passing data to some system
       call that is not	available as a Perl function: "syscall"	requires you
       to provide parameters stored in the way it happens in a C program. Even
       text processing (as shown in the	next section) may be simplified	with
       judicious usage of these	two functions.

       To see how (un)packing works, we'll start with a	simple template	code
       where the conversion is in low gear: between the	contents of a byte
       sequence	and a string of	hexadecimal digits. Let's use "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" 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

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

	  my $s	= pack(	'H2' x 10, 30..39 );
	  print	"$s\n";

       Since we	feed a list of ten 2-digit hexadecimal strings to "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 Zed'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

       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

       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("A10xA27xA7xA*", $_);

       Now, that's our data parsed. I suppose what we might want to do now is
       total up	our income and expenditure, and	add another line to the	end of
       our ledger - in the same	format - saying	how much we've brought in and
       how much	we've spent:

	   while (<>) {
	       my ($date, $desc, $income, $expend) =
		   unpack("A10xA27xA7xA*", $_);
	       $tot_income += $income;
	       $tot_expend += $expend;

	   $tot_income = sprintf("%.2f", $tot_income); # Get them into
	   $tot_expend = sprintf("%.2f", $tot_expend); # "financial" format

	   $date = POSIX::strftime("%m/%d/%Y", localtime);

	   # OK, let's go:

	   print pack("A10xA27xA7xA*", $date, "Totals",
	       $tot_income, $tot_expend);

       Oh, hmm.	That didn't quite work.	Let's see what happened:

	   01/24/2001 Zed'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

	   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

       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 Zed'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

       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

       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 "character 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 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 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 representation. 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

       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 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 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
       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 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 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

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

       First, we note that this	time-honored 16-bit CPU	uses little-endian
       order, and that's why the low order byte	is stored at the lower
       address.	To unpack such a (unsigned) short we'll	have to	use code "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 );

       (The clumsy construction	of the template	can be avoided - just read

       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	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 standard-fearing programs
       ought to	follow this convention.	(This is, of course, a stern backing
       for one of the Lilliputian parties and may well influence the political
       development there.) So, if the protocol expects you to send a message
       by sending the length first, followed by	just so	many bytes, you	could

	  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!)

   Byte-order modifiers
       In the previous sections	we've learned how to use "n", "N", "v" and "V"
       to pack and unpack integers with	big- or	little-endian byte-order.
       While this is nice, it's	still rather limited because it	leaves out all
       kinds of	signed integers	as well	as 64-bit integers. For	example, if
       you wanted to unpack a sequence of signed big-endian 16-bit integers in
       a platform-independent way, you would have to write:

	  my @data = unpack 's*', pack 'S*', unpack 'n*', $buf;

       This is ugly. As	of Perl	5.9.2, there's a much nicer way	to express
       your desire for a certain byte-order: the ">" and "<" modifiers.	 ">"
       is the big-endian modifier, while "<" is	the little-endian modifier.
       Using them, we could rewrite the	above code as:

	  my @data = unpack 's>*', $buf;

       As you can see, the "big	end" of	the arrow touches the "s", which is a
       nice way	to remember that ">" is	the big-endian modifier. The same
       obviously works for "<",	where the "little end" touches the code.

       You will	probably find these modifiers even more	useful if you have to
       deal with big- or little-endian C structures. Be	sure to	read "Packing
       and Unpacking C Structures" for more on that.

   Floating point Numbers
       For packing floating point numbers you have the choice between the pack
       codes "f", "d", "F" and "D". "f"	and "d"	pack into (or unpack from)
       single-precision	or double-precision representation as it is provided
       by your system. If your systems supports	it, "D"	can be used to pack
       and unpack ("long double") values, which	can offer even more resolution
       than "f"	or "d".	 Note that there are different long double formats.

       "F" packs an "NV", which	is the floating	point type used	by Perl

       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	text representation, possibly using the	hexadecimal float
       format (avoiding	the decimal conversion loss), unless you're absolutely
       sure what's on the other	end of the line. For the even more
       adventuresome, you can use the byte-order modifiers from	the previous
       section also on floating	point codes.

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

	    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.)

       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" separator 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, $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.

       Another odd-man-out in the template alphabet is "u", which packs	a
       "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

       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 noticeable	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 languages, providing room	for over one million different
       characters. Unicode 3.1 specifies 94,140	characters: The	Basic Latin
       characters are assigned to the numbers 0	- 127. The Latin-1 Supplement
       with characters that are	used in	several	European languages is in the
       next range, up to 255. After some more Latin extensions we find the
       character sets from languages using non-Roman alphabets,	interspersed
       with a variety of symbol	sets such as currency symbols, Zapf Dingbats
       or Braille.  (You might want to visit <> for a
       look at some of them - my personal favourites are Telugu	and Kannada.)

       The Unicode character sets associates characters	with integers.
       Encoding	these numbers in an equal number of bytes would	more than
       double the requirements for storing texts written in Latin alphabets.
       The UTF-8 encoding avoids this by storing the most common (from a
       western point of	view) characters in a single byte while	encoding the
       rarer ones in three or more bytes.

       Perl uses UTF-8,	internally, for	most Unicode strings.

       So what has this	got to do with "pack"? Well, if	you want to compose a
       Unicode string (that is internally encoded as UTF-8), you can do	so by
       using template code "U".	As an example, let's produce the Euro currency
       symbol (code number 0x20AC):

	  $UTF8{Euro} =	pack( 'U', 0x20AC );
	  # Equivalent to: $UTF8{Euro} = "\x{20ac}";

       Inspecting $UTF8{Euro} shows that it contains 3 bytes: "\xe2\x82\xac".
       However,	it contains only 1 character, number 0x20AC.  The round	trip
       can be completed	with "unpack":

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

       Unpacking using the "U" template	code also works	on UTF-8 encoded byte

       Usually you'll want to pack or unpack UTF-8 strings:

	  # pack and unpack the	Hebrew alphabet
	  my $alefbet =	pack( 'U*', 0x05d0..0x05ea );
	  my @hebrew = unpack( 'U*', $utf );

       Please note: in the general case, you're	better off using
       "Encode::decode('UTF-8',	$utf)" to decode a UTF-8 encoded byte string
       to a Perl Unicode string, and "Encode::encode('UTF-8', $str)" to	encode
       a Perl Unicode string to	UTF-8 bytes. These functions provide means of
       handling	invalid	byte sequences and generally have a friendlier

   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	<>, 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",

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

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

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

	  $pd =	pack( '(CCS)*',	map( @$_, @dates ) );

       To swap pairs of	characters in a	string (with even length) one could
       use several techniques. First, let's use	"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 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 ancient 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	is required (whenever, for some	reason,	"()*" cannot be	used).
       Here's an example: To store named string	values in a way	that can be
       conveniently parsed by a	C program, we create a sequence	of names and
       null terminated ASCII strings, with "=" 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.

   Intel HEX
       Intel HEX is a file format for representing binary data,	mostly for
       programming various chips, as a text file. (See
       <> for a detailed description, and
       <> for the Motorola
       S-record	format,	which can be unravelled	using the same technique.)
       Each line begins	with a colon (':') and is followed by a	sequence of
       hexadecimal characters, specifying a byte count n (8 bit), an address
       (16 bit,	big endian), a record type (8 bit), n data bytes and a
       checksum	(8 bit)	computed as the	least significant byte of the two's
       complement sum of the preceding bytes. Example: ":0300300002337A1E".

       The first step of processing such a line	is the conversion, to binary,
       of the hexadecimal data,	to obtain the four fields, while checking the
       checksum. No surprise here: we'll start with a simple "pack" call to
       convert everything to binary:

	  my $binrec = pack( 'H*', substr( $hexrec, 1 )	);

       The resulting byte sequence is most convenient for checking the
       checksum.  Don't	slow your program down with a for loop adding the
       "ord" values of this string's bytes - the "unpack" code "%" is the
       thing to	use for	computing the 8-bit sum	of all bytes, which must be
       equal to	zero:

	  die unless unpack( "%8C*", $binrec ) == 0;

       Finally,	let's get those	four fields. By	now, you shouldn't have	any
       problems	with the first three fields - but how can we use the byte
       count of	the data in the	first field as a length	for the	data field?
       Here the	codes "x" and "X" come to the rescue, as they permit jumping
       back and	forth in the string to unpack.

	  my( $addr, $type, $data ) = unpack( "x n C X4	C x3 /a", $bin );

       Code "x"	skips a	byte, since we don't need the count yet. Code "n"
       takes care of the 16-bit	big-endian integer address, and	"C" unpacks
       the record type.	Being at offset	4, where the data begins, we need the
       count.  "X4" brings us back to square one, which	is the byte at offset
       0.  Now we pick up the count, and zoom forth to offset 4, where we are
       now fully furnished to extract the exact	number of data bytes, leaving
       the trailing checksum byte alone.

Packing	and Unpacking C	Structures
       In previous sections we have seen how to	pack numbers and character
       strings.	If it were not for a couple of snags we	could conclude this
       section right away with the terse remark	that C structures don't
       contain anything	else, and therefore you	already	know all there is to
       it.  Sorry, no: read on,	please.

       If you have to deal with	a lot of C structures, and don't want to hack
       all your	template strings manually, you'll probably want	to have	a look
       at the CPAN module "Convert::Binary::C".	Not only can it	parse your C
       source directly,	but it also has	built-in support for all the odds and
       ends described further on in this section.

   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

	  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

	  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 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	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 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).

       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 );

   Dealing with	Endian-ness
       Now, imagine that we want to pack the data for a	machine	with a
       different byte-order. First, we'll have to figure out how big the data
       types on	the target machine really are. Let's assume that the longs are
       32 bits wide and	the shorts are 16 bits wide. You can then rewrite the
       template	as:

	 my $gappy = pack( 'c x![s] s c	x![l] l', $c1, $s, $c2,	$l );

       If the target machine is	little-endian, we could	write:

	 my $gappy = pack( 'c x![s] s< c x![l] l<', $c1, $s, $c2, $l );

       This forces the short and the long members to be	little-endian, and is
       just fine if you	don't have too many struct members. But	we could also
       use the byte-order modifier on a	group and write	the following:

	 my $gappy = pack( '( c	x![s] s	c x![l]	l )<', $c1, $s,	$c2, $l	);

       This is not as short as before, but it makes it more obvious that we
       intend to have little-endian byte-order for a whole group, not only for
       individual template codes. It can also be more readable and easier to

   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 certainly 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

	   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 ''; # run h2ph to generate	this file
	   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

	  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 "P".

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

	   # 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 v5.28.3			  2020-05-14			PERLPACKTUT(1)

NAME | DESCRIPTION | The Basic Principle | Packing Text | Packing Numbers | Exotic Templates | Template Grouping | Lengths and Widths | Packing and Unpacking C Structures | Pack Recipes | Funnies Section | Authors

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