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1=head1 NAME
2
3perlpacktut - tutorial on C<pack> and C<unpack>
4
5=head1 DESCRIPTION
6
7C<pack> and C<unpack> are two functions for transforming data according
8to a user-defined template, between the guarded way Perl stores values
9and some well-defined representation as might be required in the
10environment of a Perl program. Unfortunately, they're also two of
11the most misunderstood and most often overlooked functions that Perl
12provides. This tutorial will demystify them for you.
13
14
15=head1 The Basic Principle
16
17Most programming languages don't shelter the memory where variables are
18stored. In C, for instance, you can take the address of some variable,
19and the C<sizeof> operator tells you how many bytes are allocated to
20the variable. Using the address and the size, you may access the storage
21to your heart's content.
22
23In Perl, you just can't access memory at random, but the structural and
24representational conversion provided by C<pack> and C<unpack> is an
25excellent alternative. The C<pack> function converts values to a byte
26sequence containing representations according to a given specification,
27the so-called "template" argument. C<unpack> is the reverse process,
28deriving some values from the contents of a string of bytes. (Be cautioned,
29however, that not all that has been packed together can be neatly unpacked -
30a very common experience as seasoned travellers are likely to confirm.)
31
32Why, you may ask, would you need a chunk of memory containing some values
33in binary representation? One good reason is input and output accessing
34some file, a device, or a network connection, whereby this binary
35representation is either forced on you or will give you some benefit
36in processing. Another cause is passing data to some system call that
37is not available as a Perl function: C<syscall> requires you to provide
38parameters stored in the way it happens in a C program. Even text processing
39(as shown in the next section) may be simplified with judicious usage
40of these two functions.
41
42To see how (un)packing works, we'll start with a simple template
43code where the conversion is in low gear: between the contents of a byte
44sequence and a string of hexadecimal digits. Let's use C<unpack>, since
45this is likely to remind you of a dump program, or some desparate last
46message unfortunate programs are wont to throw at you before they expire
47into the wild blue yonder. Assuming that the variable C<$mem> holds a
48sequence of bytes that we'd like to inspect without assuming anything
49about its meaning, we can write
50
51 my( $hex ) = unpack( 'H*', $mem );
52 print "$hex\n";
53
54whereupon we might see something like this, with each pair of hex digits
55corresponding to a byte:
56
57 41204d414e204120504c414e20412043414e414c2050414e414d41
58
59What was in this chunk of memory? Numbers, charactes, or a mixture of
60both? Assuming that we're on a computer where ASCII (or some similar)
61encoding is used: hexadecimal values in the range C<0x40> - C<0x5A>
62indicate an uppercase letter, and <0x20> encodes a space. So we might
63assume it is a piece of text, which some are able to read like a tabloid;
64but others will have to get hold of an ASCII table and relive that
65firstgrader feeling. Not caring too much about which way to read this,
66we note that C<unpack> with the template code C<H> converts the contents
67of a sequence of bytes into the customary hexadecimal notation. Since
68"a sequence of" is a pretty vague indication of quantity, C<H> has been
69defined to convert just a single hexadecimal digit unless it is followed
70by a repeat count. An asterisk for the repeat count means to use whatever
71remains.
72
73The inverse operation - packing byte contents from a string of hexadecimal
74digits - is just as easily written. For instance:
75
76 my $s = pack( 'H2' x 10, map { "3$_" } ( 0..9 ) );
77 print "$s\n";
78
79Since we feed a list of ten 2-digit hexadecimal strings to C<pack>, the
80pack template should contain ten pack codes. If this is run on a computer
81with ASCII character coding, it will print C<0123456789>.
82
83
84=head1 Packing Text
85
86Let's suppose you've got to read in a data file like this:
87
88 Date |Description | Income|Expenditure
89 01/24/2001 Ahmed's Camel Emporium 1147.99
90 01/28/2001 Flea spray 24.99
91 01/29/2001 Camel rides to tourists 235.00
92
93How do we do it? You might think first to use C<split>; however, since
94C<split> collapses blank fields, you'll never know whether a record was
95income or expenditure. Oops. Well, you could always use C<substr>:
96
97 while (<>) {
98 my $date = substr($_, 0, 11);
99 my $desc = substr($_, 12, 27);
100 my $income = substr($_, 40, 7);
101 my $expend = substr($_, 52, 7);
102 ...
103 }
104
105It's not really a barrel of laughs, is it? In fact, it's worse than it
106may seem; the eagle-eyed may notice that the first field should only be
10710 characters wide, and the error has propagated right through the other
108numbers - which we've had to count by hand. So it's error-prone as well
109as horribly unfriendly.
110
111Or maybe we could use regular expressions:
112
113 while (<>) {
114 my($date, $desc, $income, $expend) =
115 m|(\d\d/\d\d/\d{4}) (.{27}) (.{7})(.*)|;
116 ...
117 }
118
119Urgh. Well, it's a bit better, but - well, would you want to maintain
120that?
121
122Hey, isn't Perl supposed to make this sort of thing easy? Well, it does,
123if you use the right tools. C<pack> and C<unpack> are designed to help
124you out when dealing with fixed-width data like the above. Let's have a
125look at a solution with C<unpack>:
126
127 while (<>) {
128 my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
129 ...
130 }
131
132That looks a bit nicer; but we've got to take apart that weird template.
133Where did I pull that out of?
134
135OK, let's have a look at some of our data again; in fact, we'll include
136the headers, and a handy ruler so we can keep track of where we are.
137
138 1 2 3 4 5
139 1234567890123456789012345678901234567890123456789012345678
140 Date |Description | Income|Expenditure
141 01/28/2001 Flea spray 24.99
142 01/29/2001 Camel rides to tourists 235.00
143
144>From this, we can see that the date column stretches from column 1 to
145column 10 - ten characters wide. The C<pack>-ese for "character" is
146C<A>, and ten of them are C<A10>. So if we just wanted to extract the
147dates, we could say this:
148
149 my($date) = unpack("A10", $_);
150
151OK, what's next? Between the date and the description is a blank column;
152we want to skip over that. The C<x> template means "skip forward", so we
153want one of those. Next, we have another batch of characters, from 12 to
15438. That's 27 more characters, hence C<A27>. (Don't make the fencepost
155error - there are 27 characters between 12 and 38, not 26. Count 'em!)
156
157Now we skip another character and pick up the next 7 characters:
158
159 my($date,$description,$income) = unpack("A10xA27xA7", $_);
160
161Now comes the clever bit. Lines in our ledger which are just income and
162not expenditure might end at column 46. Hence, we don't want to tell our
163C<unpack> pattern that we B<need> to find another 12 characters; we'll
164just say "if there's anything left, take it". As you might guess from
165regular expressions, that's what the C<*> means: "use everything
166remaining".
167
168=over 3
169
170=item *
171
172Be warned, though, that unlike regular expressions, if the C<unpack>
173template doesn't match the incoming data, Perl will scream and die.
174
175=back
176
177
178Hence, putting it all together:
179
180 my($date,$description,$income,$expend) = unpack("A10xA27xA7A*", $_);
181
182Now, that's our data parsed. I suppose what we might want to do now is
183total up our income and expenditure, and add another line to the end of
184our ledger - in the same format - saying how much we've brought in and
185how much we've spent:
186
187 while (<>) {
188 my($date, $desc, $income, $expend) = unpack("A10xA27xA7xA*", $_);
189 $tot_income += $income;
190 $tot_expend += $expend;
191 }
192
193 $tot_income = sprintf("%.2f", $tot_income); # Get them into
194 $tot_expend = sprintf("%.2f", $tot_expend); # "financial" format
195
196 $date = POSIX::strftime("%m/%d/%Y", localtime);
197
198 # OK, let's go:
199
200 print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend);
201
202Oh, hmm. That didn't quite work. Let's see what happened:
203
204 01/24/2001 Ahmed's Camel Emporium 1147.99
205 01/28/2001 Flea spray 24.99
206 01/29/2001 Camel rides to tourists 1235.00
207 03/23/2001Totals 1235.001172.98
208
209OK, it's a start, but what happened to the spaces? We put C<x>, didn't
210we? Shouldn't it skip forward? Let's look at what L<perlfunc/pack> says:
211
212 x A null byte.
213
214Urgh. No wonder. There's a big difference between "a null byte",
215character zero, and "a space", character 32. Perl's put something
216between the date and the description - but unfortunately, we can't see
217it!
218
219What we actually need to do is expand the width of the fields. The C<A>
220format pads any non-existent characters with spaces, so we can use the
221additional spaces to line up our fields, like this:
222
223 print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
224
225(Note that you can put spaces in the template to make it more readable,
226but they don't translate to spaces in the output.) Here's what we got
227this time:
228
229 01/24/2001 Ahmed's Camel Emporium 1147.99
230 01/28/2001 Flea spray 24.99
231 01/29/2001 Camel rides to tourists 1235.00
232 03/23/2001 Totals 1235.00 1172.98
233
234That's a bit better, but we still have that last column which needs to
235be moved further over. There's an easy way to fix this up:
236unfortunately, we can't get C<pack> to right-justify our fields, but we
237can get C<sprintf> to do it:
238
239 $tot_income = sprintf("%.2f", $tot_income);
240 $tot_expend = sprintf("%12.2f", $tot_expend);
241 $date = POSIX::strftime("%m/%d/%Y", localtime);
242 print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
243
244This time we get the right answer:
245
246 01/28/2001 Flea spray 24.99
247 01/29/2001 Camel rides to tourists 1235.00
248 03/23/2001 Totals 1235.00 1172.98
249
250So that's how we consume and produce fixed-width data. Let's recap what
251we've seen of C<pack> and C<unpack> so far:
252
253=over 3
254
255=item *
256
257Use C<pack> to go from several pieces of data to one fixed-width
258version; use C<unpack> to turn a fixed-width-format string into several
259pieces of data.
260
261=item *
262
263The pack format C<A> means "any character"; if you're C<pack>ing and
264you've run out of things to pack, C<pack> will fill the rest up with
265spaces.
266
267=item *
268
269C<x> means "skip a byte" when C<unpack>ing; when C<pack>ing, it means
270"introduce a null byte" - that's probably not what you mean if you're
271dealing with plain text.
272
273=item *
274
275You can follow the formats with numbers to say how many characters
276should be affected by that format: C<A12> means "take 12 characters";
277C<x6> means "skip 6 bytes" or "character 0, 6 times".
278
279=item *
280
281Instead of a number, you can use C<*> to mean "consume everything else
282left".
283
284B<Warning>: when packing multiple pieces of data, C<*> only means
285"consume all of the current piece of data". That's to say
286
287 pack("A*A*", $one, $two)
288
289packs all of C<$one> into the first C<A*> and then all of C<$two> into
290the second. This is a general principle: each format character
291corresponds to one piece of data to be C<pack>ed.
292
293=back
294
295
296
297=head1 Packing Numbers
298
299So much for textual data. Let's get onto the meaty stuff that C<pack>
300and C<unpack> are best at: handling binary formats for numbers. There is,
301of course, not just one binary format - life would be too simple - but
302Perl will do all the finicky labor for you.
303
304
305=head2 Integers
306
307Packing and unpacking numbers implies conversion to and from some
308I<specific> binary representation. Leaving floating point numbers
309aside for the moment, the salient properties of any such representation
310are:
311
312=over 4
313
314=item *
315
316the number of bytes used for storing the integer,
317
318=item *
319
320whether the contents are interpreted as a signed or unsigned number,
321
322=item *
323
324the byte ordering: whether the first byte is the least or most
325significant byte (or: little-endian or big-endian, respectively).
326
327=back
328
329So, for instance, to pack 20302 to a signed 16 bit integer in your
330computer's representation you write
331
332 my $ps = pack( 's', 20302 );
333
334Again, the result is a string, now containing 2 bytes. If you print
335this string (which is, generally, not recommended) you might see
336C<ON> or C<NO> (depending on your system's byte ordering) - or something
337entirely different if your computer doesn't use ASCII character encoding.
338Unpacking C<$ps> with the same template returns the original integer value:
339
340 my( $s ) = unpack( 's', $ps );
341
342This is true for all numeric template codes. But don't expect miracles:
343if the packed value exceeds the alotted byte capacity, high order bits
344are silently discarded, and unpack certainly won't be able to pull them
345back out of some magic hat. And, when you pack using a signed template
346code such as C<s>, an excess value may result in the sign bit
347getting set, and unpacking this will smartly return a negative value.
348
34916 bits won't get you too far with integers, but there is C<l> and C<L>
350for signed and unsigned 32-bit integers. And if this is not enough and
351your system supports 64 bit integers you can push the limits much closer
352to infinity with pack codes C<q> and C<Q>. A notable exception is provided
353by pack codes C<i> and C<I> for signed and unsigned integers of the
354"local custom" variety: Such an integer will take up as many bytes as
355a local C compiler returns for C<sizeof(int)>, but it'll use I<at least>
35632 bits.
357
358Each of the integer pack codes C<sSlLqQ> results in a fixed number of bytes,
359no matter where you execute your program. This may be useful for some
360applications, but it does not provide for a portable way to pass data
361structures between Perl and C programs (bound to happen when you call
362XS extensions or the Perl function C<syscall>), or when you read or
363write binary files. What you'll need in this case are template codes that
364depend on what your local C compiler compiles when you code C<short> or
365C<unsigned long>, for instance. These codes and their corresponding
366byte lengths are shown in the table below. Since the C standard leaves
367much leeway with respect to the relative sizes of these data types, actual
368values may vary, and that's why the values are given as expressions in
369C and Perl. (If you'd like to use values from C<%Config> in your program
370you have to import it with C<use Config>.)
371
372 signed unsigned byte length in C byte length in Perl
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373 s! S! sizeof(short) $Config{shortsize}
374 i! I! sizeof(int) $Config{intsize}
375 l! L! sizeof(long) $Config{longsize}
376 q! Q! sizeof(longlong) $Config{longlongsize}
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377
378The C<i!> and C<I!> codes aren't different from C<i> and C<I>; they are
379tolerated for completeness' sake.
380
381
382=head2 Unpacking a Stack Frame
383
384Requesting a particular byte ordering may be necessary when you work with
385binary data coming from some specific architecture while your program could
386run on a totally different system. As an example, assume you have 24 bytes
387containing a stack frame as it happens on an Intel 8086:
388
389 +---------+ +----+----+ +---------+
390 TOS: | IP | TOS+4:| FL | FH | FLAGS TOS+14:| SI |
391 +---------+ +----+----+ +---------+
392 | CS | | AL | AH | AX | DI |
393 +---------+ +----+----+ +---------+
394 | BL | BH | BX | BP |
395 +----+----+ +---------+
396 | CL | CH | CX | DS |
397 +----+----+ +---------+
398 | DL | DH | DX | ES |
399 +----+----+ +---------+
400
401First, we note that this time-honored 16-bit CPU uses little-endian order,
402and that's why the low order byte is stored at the lower address. To
403unpack such a (signed) short we'll have to use code C<v>. A repeat
404count unpacks all 12 shorts:
405
406 my( $ip, $cs, $flags, $ax, $bx, $cd, $dx, $si, $di, $bp, $ds, $es ) =
407 unpack( 'v12', $frame );
408
409Alternatively, we could have used C<C> to unpack the individually
410accessible byte registers FL, FH, AL, AH, etc.:
411
412 my( $fl, $fh, $al, $ah, $bl, $bh, $cl, $ch, $dl, $dh ) =
413 unpack( 'C10', substr( $frame, 4, 10 ) );
414
415It would be nice if we could do this in one fell swoop: unpack a short,
416back up a little, and then unpack 2 bytes. Since Perl I<is> nice, it
417proffers the template code C<X> to back up one byte. Putting this all
418together, we may now write:
419
420 my( $ip, $cs,
421 $flags,$fl,$fh,
422 $ax,$al,$ah, $bx,$bl,$bh, $cx,$cl,$ch, $dx,$dl,$dh,
423 $si, $di, $bp, $ds, $es ) =
424 unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame );
425
426We've taken some pains to get construct the template so that it matches
427the contents of our frame buffer. Otherwise we'd either get undefined values,
428or C<unpack> could not unpack all. If C<pack> runs out of items, it will
429supply null strings.
430
431
432=head2 How to Eat an Egg on a Net
433
434The pack code for big-endian (high order byte at the lowest address) is
435C<n> for 16 bit and C<N> for 32 bit integers. You use these codes
436if you know that your data comes from a compliant architecture, but,
437surprisingly enough, you should also use these pack codes if you
438exchange binary data, across the network, with some system that you
439know next to nothing about. The simple reason is that this
440order has been chosen as the I<network order>, and all standard-fearing
441programs ought to follow this convention. (This is, of course, a stern
442backing for one of the Lilliputian parties and may well influence the
443political development there.) So, if the protocol expects you to send
444a message by sending the length first, followed by just so many bytes,
445you could write:
446
447 my $buf = pack( 'N', length( $msg ) ) . $msg;
448
449or even:
450
451 my $buf = pack( 'NA*', length( $msg ), $msg );
452
453and pass C<$buf> to your send routine. Some protocols demand that the
454count should include the length of the count itself: then just add 4
455to the data length. (But make sure to read L<"Lengths and Widths"> before
456you really code this!)
457
458
459
460=head2 Floating point Numbers
461
462For packing floating point numbers you have the choice between the
463pack codes C<f> and C<d> which pack into (or unpack from) single-precision or
464double-precision representation as it is provided by your system. (There
465is no such thing as a network representation for reals, so if you want
466to send your real numbers across computer boundaries, you'd better stick
467to ASCII representation, unless you're absolutely sure what's on the other
468end of the line.)
469
470
471
472=head1 Exotic Templates
473
474
475=head2 Bit Strings
476
477Bits are the atoms in the memory world. Access to individual bits may
478have to be used either as a last resort or because it is the most
479convenient way to handle your data. Bit string (un)packing converts
480between strings containing a series of C<0> and C<1> characters and
481a sequence of bytes each containing a group of 8 bits. This is almost
482as simple as it sounds, except that there are two ways the contents of
483a byte may be written as a bit string. Let's have a look at an annotated
484byte:
485
486 7 6 5 4 3 2 1 0
487 +-----------------+
488 | 1 0 0 0 1 1 0 0 |
489 +-----------------+
490 MSB LSB
491
492It's egg-eating all over again: Some think that as a bit string this should
493be written "10001100" i.e. beginning with the most significant bit, others
494insist on "00110001". Well, Perl isn't biased, so that's why we have two bit
495string codes:
496
497 $byte = pack( 'B8', '10001100' ); # start with MSB
498 $byte = pack( 'b8', '00110001' ); # start with LSB
499
500It is not possible to pack or unpack bit fields - just integral bytes.
501C<pack> always starts at the next byte boundary and "rounds up" to the
502next multiple of 8 by adding zero bits as required. (If you do want bit
503fields, there is L<perlfunc/vec>. Or you could implement bit field
504handling at the character string level, using split, substr, and
505concatenation on unpacked bit strings.)
506
507To illustrate unpacking for bit strings, we'll decompose a simple
508status register (a "-" stands for a "reserved" bit):
509
510 +-----------------+-----------------+
511 | S Z - A - P - C | - - - - O D I T |
512 +-----------------+-----------------+
513 MSB LSB MSB LSB
514
515Converting these two bytes to a string can be done with the unpack
516template C<'b16'>. To obtain the individual bit values from the bit
517string we use C<split> with the "empty" separator pattern which splits
518into individual characters. Bit values from the "reserved" positions are
519simply assigned to C<undef>, a convenient notation for "I don't care where
520this goes".
521
522 ($carry, undef, $parity, undef, $auxcarry, undef, $sign,
523 $trace, $interrupt, $direction, $overflow) =
524 split( '', unpack( 'b16', $status ) );
525
526We could have used an unpack template C<'b12'> just as well, since the
527last 4 bits can be ignored anyway.
528
529
530=head2 Uuencoding
531
532Another odd-man-out in the template alphabet is C<u>, which packs an
533"uuencoded string". ("uu" is short for Unix-to-Unix.) Chances are that
534you won't ever need this encoding technique which was invented to overcome
535the shortcomings of old-fashioned transmission mediums that do not support
536other than simple ASCII data. The essential recipe is simple: Take three
537bytes, or 24 bits. Split them into 4 six-packs, adding a space (0x20) to
538each. Repeat until all of the data is blended. Fold groups of 4 bytes into
539lines no longer than 60 and garnish them in front with the original byte count
540(incremented by 0x20) and a C<"\n"> at the end. - The C<pack> chef will
541prepare this for you, a la minute, when you select pack code C<u> on the menu:
542
543 my $uubuf = pack( 'u', $bindat );
544
545A repeat count after C<u> sets the number of bytes to put into an
546uuencoded line, which is the maximum of 45 by default, but could be
547set to some (smaller) integer multiple of three. C<unpack> simply ignores
548the repeat count.
549
550
551=head2 Doing Sums
552
553An even stranger template code is C<%>E<lt>I<number>E<gt>. First, because
554it's used as a prefix to some other template code. Second, because it
555cannot be used in C<pack> at all, and third, in C<unpack>, doesn't return the
556data as defined by the template code it precedes. Instead it'll give you an
557integer of I<number> bits that is computed from the data value by
558doing sums. For numeric unpack codes, no big feat is achieved:
559
560 my $buf = pack( 'iii', 100, 20, 3 );
561 print unpack( '%32i3', $buf ), "\n"; # prints 123
562
563For string values, C<%> returns the sum of the byte values saving
564you the trouble of a sum loop with C<substr> and C<ord>:
565
566 print unpack( '%32A*', "\x01\x10" ), "\n"; # prints 17
567
568Although the C<%> code is documented as returning a "checksum":
569don't put your trust in such values! Even when applied to a small number
570of bytes, they won't guarantee a noticeable Hamming distance.
571
572In connection with C<b> or C<B>, C<%> simply adds bits, and this can be put
573to good use to count set bits efficiently:
574
575 my $bitcount = unpack( '%32b*', $mask );
576
577And an even parity bit can be determined like this:
578
579 my $evenparity = unpack( '%1b*', $mask );
580
581
582=head2 Unicode
583
584Unicode is a character set that can represent most characters in most of
585the world's languages, providing room for over one million different
586characters. Unicode 3.1 specifies 94,140 characters: The Basic Latin
587characters are assigned to the numbers 0 - 127. The Latin-1 Supplement with
588characters that are used in several European languages is in the next
589range, up to 255. After some more Latin extensions we find the character
590sets from languages using non-roman alphabets, interspersed with a
591variety of symbol sets such as currency symbols, Zapf Dingbats or Braille.
592(You might want to visit L<www.unicode.org> for a look at some of
593them - my personal favourites are Telugu and Kannada.)
594
595The Unicode character sets associates characters with integers. Encoding
596these numbers in an equal number of bytes would more than double the
597requirements for storing texts written in latin alphabets.
598The UTF-8 encoding avoids this by storing the most common (from a western
599point of view) characters in a single byte while encoding the rarer
600ones in three or more bytes.
601
602So what has this got to do with C<pack>? Well, if you want to convert
603between a Unicode number and its UTF-8 representation you can do so by
604using template code C<U>. As an example, let's produce the UTF-8
605representation of the Euro currency symbol (code number 0x20AC):
606
607 $UTF8{Euro} = pack( 'U', 0x20AC );
608
609Inspecting C<$UTF8{Euro}> shows that it contains 3 bytes: "\xe2\x82\xac". The
610round trip can be completed with C<unpack>:
611
612 $Unicode{Euro} = unpack( 'U', $UTF8{Euro} );
613
614Usually you'll want to pack or unpack UTF-8 strings:
615
616 # pack and unpack the Hebrew alphabet
617 my $alefbet = pack( 'U*', 0x05d0..0x05ea );
618 my @hebrew = unpack( 'U*', $utf );
619
620
621
622=head1 Lengths and Widths
623
624=head2 String Lengths
625
626In the previous section we've seen a network message that was constructed
627by prefixing the binary message length to the actual message. You'll find
628that packing a length followed by so many bytes of data is a
629frequently used recipe since appending a null byte won't work
630if a null byte may be part of the data. - Here is an example where both
631techniques are used: after two null terminated strings with source and
632destination address, a Short Message (to a mobile phone) is sent after
633a length byte:
634
635 my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm );
636
637Unpacking this message can be done with the same template:
638
639 ( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg );
640
641There's a subtle trap lurking in the offings: Adding another field after
642the Short Message (in variable C<$sm>) is all right when packing, but this
643cannot be unpacked naively:
644
645 # pack a message
646 my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio );
647
648 # unpack fails - $prio remains undefined!
649 ( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg );
650
651The pack code C<A*> gobbles up all remaining bytes, and C<$prio> remains
652undefined! Before we let disappointment dampen the morale: Perl's got
653the trump card to make this trick too, just a little further up the sleeve.
654Watch this:
655
656 # pack a message: ASCIIZ, ASCIIZ, length/string, byte
657 my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio );
658
659 # unpack
660 ( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg );
661
662Combining two pack codes with a slash (C</>) associates them with a single
663value from the argument list. In C<pack>, the length of the argument is
664taken and packed according to the first code while the argument itself
665is added after being converted with the template code after the slash.
666This saves us the trouble of inserting the C<length> call, but it is
667in C<unpack> where we really score: The value of the length byte marks the
668end of the string to be taken from the buffer. Since this combination
f8b4d74f 669doesn't make sense except when the second pack code isn't C<a*>, C<A*>
34babc16
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670or C<Z*>, Perl won't let you.
671
672The pack code preceding C</> may be anything that's fit to represent a
673number: All the numeric binary pack codes, and even text codes such as
674C<A4> or C<Z*>:
675
676 # pack/unpack a string preceded by its length in ASCII
677 my $buf = pack( 'A4/A*', "Humpty-Dumpty" );
678 # unpack $buf: '13 Humpty-Dumpty'
679 my $txt = unpack( 'A4/A*', $buf );
680
681
682=head2 Dynamic Templates
683
684So far, we've seen literals used as templates. If the list of pack
685items doesn't have fixed length, an expression constructing the
686template has to be used. Here's an example:
687To store named string values in a way that can be conveniently parsed
688by a C program, we create a sequence of names and null terminated ASCII
689strings, with C<=> between the name and the value, followed by an
690additional delimiting null byte. Here's how:
691
692 my $env = pack( 'A*A*Z*' x keys( %Env ) . 'C',
693 map{ ( $_, '=', $Env{$_} ) } keys( %Env ), 0 );
694
695For the reverse operation, we'll have to determine the number of items
696in the buffer before we can let C<unpack> rip it apart:
697
698 my $n = ( $env =~ s/\0/\0/g - 1 );
699 my %env = map { split( '=', $_ ) } unpack( 'Z*' x $n, $env );
700
701The substitution counts the null bytes. The C<unpack> call returns a
702list of name-value pairs each of which is taken apart in the C<map>
703block.
704
705
706=head2 Another Portable Binary Encoding
707
708The pack code C<w> has been added to support a portable binary data
709encoding scheme that goes way beyond simple integers. (Details can
710be found at L<Casbah.org>, the Scarab project.) A BER (Binary Encoded
711Representation) compressed unsigned integer stores base 128
712digits, most significant digit first, with as few digits as possible.
713Bit eight (the high bit) is set on each byte except the last. There
714is no size limit to BER encoding, but Perl won't go to extremes.
715
716 my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 );
717
718A hex dump of C<$berbuf>, with spaces inserted at the right places,
719shows 01 8100 8101 81807F. Since the last byte is always less than
720128, C<unpack> knows where to stop.
721
722
723=head1 Packing and Unpacking C Structures
724
725In previous sections we have seen how to pack numbers and character
726strings. If it were not for a couple of snags we could conclude this
727section right away with the terse remark that C structures don't
728contain anything else, and therefore you already know all there is to it.
729Sorry, no: read on, please.
730
731=head2 The Alignment Pit
732
733In the consideration of speed against memory requirements the balance
734has been tilted in favor of faster execution. This has influenced the
735way C compilers allocate memory for structures: On architectures
736where a 16-bit or 32-bit operand can be moved faster between places in
737memory, or to or from a CPU register, if it is aligned at an even or
738multiple-of-four or even at a multiple-of eight address, a C compiler
739will give you this speed benefit by stuffing extra bytes into structures.
740If you don't cross the C shoreline this is not likely to cause you any
741grief (although you should care when you design large data structures).
742
743To see how this affects C<pack> and C<unpack>, we'll compare these two
744C structures:
745
746 typedef struct {
747 char c1;
748 short s;
749 char c2;
750 long l;
751 } gappy_t;
752
753 typedef struct {
754 long l;
755 short s;
756 char c1;
757 char c2;
758 } dense_t;
759
760Typically, a C compiler allocates 12 bytes to a C<gappy_t> variable, but
761requires only 8 bytes for a C<dense_t>. After investigating this further,
762we can draw memory maps, showing where the extra 4 bytes are hidden:
763
764 0 +4 +8 +12
765 +--+--+--+--+--+--+--+--+--+--+--+--+
766 |c1|xx| s |c2|xx|xx|xx| l | xx = fill byte
767 +--+--+--+--+--+--+--+--+--+--+--+--+
768 gappy_t
769
770 0 +4 +8
771 +--+--+--+--+--+--+--+--+
772 | l | h |c1|c2|
773 +--+--+--+--+--+--+--+--+
774 dense_t
775
776And that's where the first quirk strikes: C<pack> and C<unpack>
777templates have to be stuffed with C<x> codes to get those extra fill bytes.
778
779The natural question: "Why can't Perl compensate for the gaps?" warrants
780an answer. One good reason is that C compilers might provide (non-ANSI)
781extensions permitting all sorts of fancy control over the way structures
782are aligned, even at the level of an individual structure field. And, if
783this were not enough, there is an insidious thing called C<union> where
784the amount of fill bytes cannot be derived from the alignment of the next
785item alone.
786
787OK, so let's bite the bullet. Here's one way to get the alignment right
788by inserting template codes C<x>, which don't take a corresponding item
789from the list:
790
791 my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );
792
793Note the C<!> after C<l>: We want to make sure that we pack a long
794integer as it is compiled by our C compiler.
795
796Counting bytes and watching alignments in lengthy structures is bound to
797be a drag. Isn't there a way we can create the template with a simple
798program? Here's a C program that does the trick:
799
800 #include <stdio.h>
801 #include <stddef.h>
802
803 typedef struct {
804 char fc1;
805 short fs;
806 char fc2;
807 long fl;
808 } gappy_t;
809
810 #define Pt(struct,field,tchar) \
811 printf( "@%d%s ", offsetof(struct,field), # tchar );
812
813 int main(){
814 Pt( gappy_t, fc1, c );
815 Pt( gappy_t, fs, s! );
816 Pt( gappy_t, fc2, c );
817 Pt( gappy_t, fl, l! );
818 printf( "\n" );
819 }
820
821The output line can be used as a template in a C<pack> or C<unpack> call:
822
823 my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l );
824
825Gee, yet another template code - as if we hadn't plenty. But
826C<@> saves our day by enabling us to specify the offset from the beginning
827of the pack buffer to the next item: This is just the value
828the C<offsetof> macro (defined in C<E<lt>stddef.hE<gt>>) returns when
829given a C<struct> type and one of its field names ("member-designator" in
830C standardese).
831
832
833=head2 Alignment, Take 2
834
835I'm afraid that we're not quite through with the alignment catch yet. The
836hydra raises another ugly head when you pack arrays of structures:
837
838 typedef struct {
839 short count;
840 char glyph;
841 } cell_t;
842
843 typedef cell_t buffer_t[BUFLEN];
844
845Where's the catch? Padding is neither required before the first field C<count>,
846nor between this and the next field C<glyph>, so why can't we simply pack
847like this:
848
849 # something goes wrong here:
850 pack( 's!a' x @buffer,
851 map{ ( $_->{count}, $_->{glyph} ) } @buffer );
852
853This packs C<3*@buffer> bytes, but it turns out that the size of
854C<buffer_t> is four times C<BUFLEN>! The moral of the story is that
855the required alignment of a structure or array is propagated to the
856next higher level where we have to consider padding I<at the end>
857of each component as well. Thus the correct template is:
858
859 pack( 's!ax' x @buffer,
860 map{ ( $_->{count}, $_->{glyph} ) } @buffer );
861
862
863
864=head2 Pointers for How to Use Them
865
866The title of this section indicates the second problem you may run into
867sooner or later when you pack C structures. If the function you intend
868to call expects a, say, C<void *> value, you I<cannot> simply take
869a reference to a Perl variable. (Although that value certainly is a
870memory address, it's not the address where the variable's contents are
871stored.)
872
873Template code C<P> promises to pack a "pointer to a fixed length string".
874Isn't this what we want? Let's try:
875
876 # allocate some storage and pack a pointer to it
877 my $memory = "\x00" x $size;
878 my $memptr = pack( 'P', $memory );
879
880But wait: doesn't C<pack> just return a sequence of bytes? How can we pass this
881string of bytes to some C code expecting a pointer which is, after all,
882nothing but a number? The answer is simple: We have to obtain the numeric
883address from the bytes returned by C<pack>.
884
885 my $ptr = unpack( 'L!', $memptr );
886
887Obviously this assumes that it is possible to typecast a pointer
888to an unsigned long and vice versa, which frequently works but should not
889be taken as a universal law. - Now that we have this pointer the next question
890is: How can we put it to good use? We need a call to some C function
891where a pointer is expected. The read(2) system call comes to mind:
892
893 ssize_t read(int fd, void *buf, size_t count);
894
895After reading L<perlfunc> explaining how to use C<syscall> we can write
896this Perl function copying a file to standard output:
897
898 require 'syscall.ph';
899 sub cat($){
900 my $path = shift();
901 my $size = -s $path;
902 my $memory = "\x00" x $size; # allocate some memory
903 my $ptr = unpack( 'L', pack( 'P', $memory ) );
904 open( F, $path ) || die( "$path: cannot open ($!)\n" );
905 my $fd = fileno(F);
906 my $res = syscall( &SYS_read, fileno(F), $ptr, $size );
907 print $memory;
908 close( F );
909 }
910
911This is neither a specimen of simplicity nor a paragon of portability but
912it illustrates the point: We are able to sneak behind the scenes and
913access Perl's otherwise well-guarded memory! (Important note: Perl's
914C<syscall> does I<not> require you to construct pointers in this roundabout
915way. You simply pass a string variable, and Perl forwards the address.)
916
917How does C<unpack> with C<P> work? Imagine some pointer in the buffer
918about to be unpacked: If it isn't the null pointer (which will smartly
919produce the C<undef> value) we have a start address - but then what?
920Perl has no way of knowing how long this "fixed length string" is, so
921it's up to you to specify the actual size as an explicit length after C<P>.
922
923 my $mem = "abcdefghijklmn";
924 print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"
925
926As a consequence, C<pack> ignores any number or C<*> after C<P>.
927
928
929Now that we have seen C<P> at work, we might as well give C<p> a whirl.
930Why do we need a second template code for packing pointers at all? The
931answer lies behind the simple fact that an C<unpack> with C<p> promises
932a null-terminated string starting at the address taken from the buffer,
933and that implies a length for the data item to be returned:
934
935 my $buf = pack( 'p', "abc\x00efhijklmn" );
936 print unpack( 'p', $buf ); # prints "abc"
937
938
939
940Albeit this is apt to be confusing: As a consequence of the length being
941implied by the string's length, a number after pack code C<p> is a repeat
942count, not a length as after C<P>.
943
944
945Using C<pack(..., $x)> with C<P> or C<p> to get the address where C<$x> is
946actually stored must be used with circumspection. Perl's internal machinery
947considers the relation between a variable and that address as its very own
948private matter and doesn't really care that we have obtained a copy. Therefore:
949
950=over 4
951
952=item *
953
954Do not use C<pack> with C<p> or C<P> to obtain the address of variable
955that's bound to go out of scope (and thereby freeing its memory) before you
956are done with using the memory at that address.
957
958=item *
959
960Be very careful with Perl operations that change the value of the
961variable. Appending something to the variable, for instance, might require
962reallocation of its storage, leaving you with a pointer into no-man's land.
963
964=item *
965
966Don't think that you can get the address of a Perl variable
967when it is stored as an integer or double number! C<pack('P', $x)> will
968force the variable's internal representation to string, just as if you
969had written something like C<$x .= ''>.
970
971=back
972
973It's safe, however, to P- or p-pack a string literal, because Perl simply
974allocates an anonymous variable.
975
976
977
978=head1 Pack Recipes
979
980Here are a collection of (possibly) useful canned recipes for C<pack>
981and C<unpack>:
982
983 # Convert IP address for socket functions
984 pack( "C4", split /\./, "123.4.5.6" );
985
986 # Count the bits in a chunk of memory (e.g. a select vector)
987 unpack( '%32b*', $mask );
988
989 # Determine the endianness of your system
990 $is_little_endian = unpack( 'c', pack( 's', 1 ) );
991 $is_big_endian = unpack( 'xc', pack( 's', 1 ) );
992
993 # Determine the number of bits in a native integer
994 $bits = unpack( '%32I!', ~0 );
995
996 # Prepare argument for the nanosleep system call
997 my $timespec = pack( 'L!L!', $secs, $nanosecs );
998
f8b4d74f
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999For a simple memory dump we unpack some bytes into just as
1000many pairs of hex digits, and use C<map> to handle the traditional
1001spacing - 16 bytes to a line:
1002
34babc16 1003 my $i;
f8b4d74f
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1004 print map { ++$i % 16 ? "$_ " : "$_\n" }
1005 unpack( 'H2' x length( $mem ), $mem ),
1006 length( $mem ) % 16 ? "\n" : '';
34babc16
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1007
1008
1009=head1 Authors
1010
1011Simon Cozens and Wolfgang Laun.
1012