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

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

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

NNAAMMEE #

 perlpacktut - tutorial on "pack" and "unpack"

DDEESSCCRRIIPPTTIIOONN #

 "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.

TThhee BBaassiicc PPrriinncciippllee 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:

    41204d414e204120504c414e20412043414e414c2050414e414d41

 What was in this chunk of memory? Numbers, characters, or a mixture of
 both? Assuming that we're on a computer where ASCII (or some similar)
 encoding is used: hexadecimal values in the range 0x40 - 0x5A indicate an
 uppercase letter, and 0x20 encodes a space. So we might assume it is a
 piece of text, which some are able to read like a tabloid; but others
 will have to get hold of an ASCII table and relive that firstgrader
 feeling. Not caring too much about which way to read this, we note that
 "unpack" with the template code "H" converts the contents of a sequence
 of bytes into the 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 $s = pack( 'H2' x 10, 30..39 );
    print "$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.

PPaacckkiinngg TTeexxtt 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
 that?

 Hey, isn't Perl supposed to make this sort of thing easy? Well, it does,
 if you use the right tools. "pack" and "unpack" are designed to help you
 out when dealing with fixed-width data like the above. Let's have a look
 at a solution with "unpack":

     while (<>) {
         my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
         ...
     }

 That looks a bit nicer; but we've got to take apart that weird template.
 Where did I pull that out of?

 OK, let's have a look at some of our data again; in fact, we'll include
 the headers, and a handy ruler so we can keep track of where we are.

              1         2         3         4         5
     1234567890123456789012345678901234567890123456789012345678
     Date      |Description                | Income|Expenditure
     01/28/2001 Flea spray                                24.99
     01/29/2001 Camel rides to tourists      235.00

 From this, we can see that the date column stretches from column 1 to
 column 10 - ten characters wide. The "pack"-ese 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 nneeeedd to find another 12 characters; we'll just
 say "if there's anything left, take it". As you might guess from regular
 expressions, that's what the "*" means: "use everything remaining".

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

 Hence, putting it all together:

     my ($date, $description, $income, $expend) =
         unpack("A10xA27xA7xA*", $_);

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

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

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

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

     # OK, let's go:

     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 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 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:

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

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

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

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

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

    WWaarrnniinngg: 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.

PPaacckkiinngg NNuummbbeerrss 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.

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

 •   the number of bytes used for storing the integer,

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

 •   the byte ordering: whether the first byte is the least 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 _a_t _l_e_a_s_t 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.

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

       +---------+        +----+----+               +---------+

TOS: | IP | TOS+4:| FL | FH | FLAGS TOS+14:| SI | #

       +---------+        +----+----+               +---------+

| CS | | AL | AH | AX | DI | #

       +---------+        +----+----+               +---------+

| BL | BH | BX | BP | #

                          +----+----+               +---------+

| CL | CH | CX | DS | #

                          +----+----+               +---------+

| DL | DH | DX | ES | #

                          +----+----+               +---------+

 First, we note that this time-honored 16-bit CPU uses little-endian
 order, and that's why the low order byte is stored at the lower address.
 To unpack such a (unsigned) short we'll have to use code "v". A repeat
 count unpacks all 12 shorts:

    my( $ip, $cs, $flags, $ax, $bx, $cx, $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 _i_s nice, it
 proffers the template code "X" to back up one byte. Putting this all
 together, we may now write:

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

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

 We've taken some pains to construct the template so that it matches the
 contents of our frame buffer. Otherwise we'd either get undefined values,
 or "unpack" could not unpack all. If "pack" runs out of items, it will
 supply null strings (which are coerced into zeroes whenever the pack code
 says so).

HHooww ttoo EEaatt aann EEgggg oonn aa NNeett 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 _n_e_t_w_o_r_k _o_r_d_e_r, and all standard-fearing programs ought to follow this convention. (This is, of course, a stern backing for one of the Lilliputian parties and may well influence the political development there.) So, if the protocol expects you to send a message by sending the length first, followed by just so many bytes, you could write:

    my $buf = pack( 'N', length( $msg ) ) . $msg;

 or even:

    my $buf = pack( 'NA*', length( $msg ), $msg );

 and pass $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!)

BByyttee--oorrddeerr mmooddiiffiieerrss 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.

FFllooaattiinngg ppooiinntt NNuummbbeerrss 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”. NNoottee tthhaatt tthheerree aarree ddiiffffeerreenntt lloonngg ddoouubbllee ffoorrmmaattss..

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

 There is no such thing as a network representation for reals, so if you
 want to send your real numbers across computer boundaries, you'd better
 stick to 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.

EExxoottiicc TTeemmppllaatteess BBiitt SSttrriinnggss 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
 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.

UUuueennccooddiinngg 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.

DDooiinngg SSuummss An even stranger template code is “%"<_n_u_m_b_e_r>. 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 _n_u_m_b_e_r 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 );

UUnniiccooddee 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 https://www.unicode.org/ for a look at some of them - my personal favourites are Telugu and Kannada.)

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

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

 So what has this got to do with "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
 strings.

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

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

 Please note: in the general case, you're better off using
 "Encode::decode('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
 interface.

AAnnootthheerr PPoorrttaabbllee BBiinnaarryy EEnnccooddiinngg 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 https://github.com/mworks-project/mw_scarab/blob/master/Scarab-0.1.00d19/doc/binary-serialization.txt, 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.

TTeemmppllaattee GGrroouuppiinngg Prior to Perl 5.8, repetitions of templates had to be made by “x”-multiplication of template strings. Now there is a better way as we may use the pack codes “(” and “)” combined with a repeat count. The “unpack” template from the Stack Frame example can simply be written like this:

    unpack( 'v2 (vXXCC)5 v5', $frame )

 Let's explore this feature a little more. We'll begin with the equivalent
 of

    join( '', map( substr( $_, 0, 1 ), @str ) )

 which returns a string consisting of the first character from each
 string.  Using pack, we can write

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

 or, because a repeat count "*" means "repeat as often as required",
 simply

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

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

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

LLeennggtthhss aanndd WWiiddtthhss SSttrriinngg LLeennggtthhss 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...

DDyynnaammiicc TTeemmppllaatteess 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.

CCoouunnttiinngg RReeppeettiittiioonnss 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.

IInntteell HHEEXX Intel HEX is a file format for representing binary data, mostly for programming various chips, as a text file. (See https://en.wikipedia.org/wiki/.hex for a detailed description, and https://en.wikipedia.org/wiki/SREC_(file_format) for the Motorola S-record format, which can be unravelled using the same technique.) Each line begins with a colon (’:’) and is followed by a sequence of hexadecimal characters, specifying a byte count _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.

PPaacckkiinngg aanndd UUnnppaacckkiinngg CC SSttrruuccttuurreess 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.

TThhee AAlliiggnnmmeenntt PPiitt In the consideration of speed against memory requirements the balance has been tilted in favor of faster execution. This has influenced the way C compilers allocate memory for structures: On architectures where a 16-bit or 32-bit operand can be moved faster between places in memory, or to or from a CPU register, if it is aligned at an even or multiple-of-four or even at a multiple-of eight address, a C compiler will give you this speed benefit by stuffing extra bytes into structures. If you don’t cross the C shoreline this is not likely to cause you any grief (although you should care when you design large data structures, or you want your code to be portable between architectures (you do want that, don’t you?)).

 To see how this affects "pack" and "unpack", we'll compare these two C
 structures:

    typedef struct {
      char     c1;
      short    s;
      char     c2;
      long     l;
    } gappy_t;

    typedef struct {
      long     l;
      short    s;
      char     c1;
      char     c2;
    } dense_t;

 Typically, a C compiler allocates 12 bytes to a "gappy_t" variable, but
 requires only 8 bytes for a "dense_t". After investigating this further,
 we can draw memory maps, showing where the extra 4 bytes are hidden:

    0           +4          +8          +12
    +--+--+--+--+--+--+--+--+--+--+--+--+
    |c1|xx|  s  |c2|xx|xx|xx|     l     |    xx = fill byte
    +--+--+--+--+--+--+--+--+--+--+--+--+
    gappy_t

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

 And that's where the first quirk strikes: "pack" and "unpack" templates
 have to be stuffed with "x" codes to get those extra fill bytes.

 The natural question: "Why can't Perl compensate for the gaps?" warrants
 an answer. One good reason is that C 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 templates, 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 );

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

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

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

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

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

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

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

AAlliiggnnmmeenntt,, TTaakkee 22 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 _a_t _t_h_e _e_n_d of each
 component as well. Thus the correct template is:

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

AAlliiggnnmmeenntt,, TTaakkee 33 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]

PPooiinntteerrss ffoorr HHooww ttoo UUssee TThheemm 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 _c_a_n_n_o_t simply take a reference to a Perl variable. (Although that value certainly is a memory address, it’s not the address where the variable’s contents are stored.)

 Template code "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 rreeaadd(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'; # 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 _n_o_t 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 "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:

 •   Do not use "pack" with "p" or "P" to obtain the address of variable
     that's bound to go out of scope (and thereby freeing its memory)
     before you are done with using the memory at that address.

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

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

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

PPaacckk RReecciippeess Here are a collection of (possibly) useful canned recipes for “pack” and “unpack”:

     # Convert IP address for socket functions
     pack( "C4", split /\./, "123.4.5.6" );

     # Count the bits in a chunk of memory (e.g. a select vector)
     unpack( '%32b*', $mask );

     # Determine the endianness of your system
     $is_little_endian = unpack( 'c', pack( 's', 1 ) );
     $is_big_endian = unpack( 'xc', pack( 's', 1 ) );

     # Determine the number of bits in a native integer
     $bits = unpack( '%32I!', ~0 );

     # Prepare argument for the nanosleep system call
     my $timespec = pack( 'L!L!', $secs, $nanosecs );

 For a simple memory dump we unpack some bytes into just as many pairs of
 hex digits, and use "map" to handle the traditional spacing - 16 bytes to
 a line:

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

FFuunnnniieess SSeeccttiioonn # 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' );

AAuutthhoorrss Simon Cozens and Wolfgang Laun.

perl v5.36.3 2023-02-15 PERLPACKTUT(1)