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Add the byte-order modifiers '<' and '>' to the pack tutorial.
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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
47b6252e 9and some well-defined representation as might be required in the
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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
47b6252e 45this is likely to remind you of a dump program, or some desperate last
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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
47b6252e 59What was in this chunk of memory? Numbers, characters, or a mixture of
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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>
47f22e19 62indicate an uppercase letter, and C<0x20> encodes a space. So we might
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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:
7207e29d 112
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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*", $_);
129 ...
130 }
131
132That looks a bit nicer; but we've got to take apart that weird template.
133Where did I pull that out of?
134
135OK, let's have a look at some of our data again; in fact, we'll include
136the headers, and a handy ruler so we can keep track of where we are.
137
138 1 2 3 4 5
139 1234567890123456789012345678901234567890123456789012345678
140 Date |Description | Income|Expenditure
141 01/28/2001 Flea spray 24.99
142 01/29/2001 Camel rides to tourists 235.00
143
47b6252e 144From this, we can see that the date column stretches from column 1 to
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145column 10 - ten characters wide. The C<pack>-ese for "character" is
146C<A>, and ten of them are C<A10>. So if we just wanted to extract the
147dates, we could say this:
148
149 my($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", $_);
160
161Now comes the clever bit. Lines in our ledger which are just income and
162not expenditure might end at column 46. Hence, we don't want to tell our
163C<unpack> pattern that we B<need> to find another 12 characters; we'll
164just say "if there's anything left, take it". As you might guess from
165regular expressions, that's what the C<*> means: "use everything
166remaining".
167
168=over 3
169
170=item *
171
172Be warned, though, that unlike regular expressions, if the C<unpack>
173template doesn't match the incoming data, Perl will scream and die.
174
175=back
176
177
178Hence, putting it all together:
179
49704364 180 my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_);
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181
182Now, that's our data parsed. I suppose what we might want to do now is
183total up our income and expenditure, and add another line to the end of
184our ledger - in the same format - saying how much we've brought in and
185how much we've spent:
186
187 while (<>) {
188 my($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:
47b6252e 343if the packed value exceeds the allotted byte capacity, high order bits
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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}
d832b8f6 376 q! Q! sizeof(long long) $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
47f22e19 385binary data coming from some specific architecture whereas your program could
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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
9dc383df 403unpack such a (unsigned) short we'll have to use code C<v>. A repeat
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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
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426(The clumsy construction of the template can be avoided - just read on!)
427
47f22e19 428We've taken some pains to construct the template so that it matches
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429the contents of our frame buffer. Otherwise we'd either get undefined values,
430or C<unpack> could not unpack all. If C<pack> runs out of items, it will
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431supply null strings (which are coerced into zeroes whenever the pack code
432says so).
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433
434
435=head2 How to Eat an Egg on a Net
436
437The pack code for big-endian (high order byte at the lowest address) is
438C<n> for 16 bit and C<N> for 32 bit integers. You use these codes
439if you know that your data comes from a compliant architecture, but,
440surprisingly enough, you should also use these pack codes if you
441exchange binary data, across the network, with some system that you
442know next to nothing about. The simple reason is that this
443order has been chosen as the I<network order>, and all standard-fearing
444programs ought to follow this convention. (This is, of course, a stern
445backing for one of the Lilliputian parties and may well influence the
446political development there.) So, if the protocol expects you to send
447a message by sending the length first, followed by just so many bytes,
448you could write:
449
450 my $buf = pack( 'N', length( $msg ) ) . $msg;
451
452or even:
453
454 my $buf = pack( 'NA*', length( $msg ), $msg );
455
456and pass C<$buf> to your send routine. Some protocols demand that the
457count should include the length of the count itself: then just add 4
458to the data length. (But make sure to read L<"Lengths and Widths"> before
459you really code this!)
460
461
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462=head2 Byte-order modifiers
463
464In the previous sections we've learned how to use C<n>, C<N>, C<v> and
465C<V> to pack and unpack integers with big- or little-endian byte-order.
466While this is nice, it's still rather limited because it leaves out all
467kinds of signed integers as well as 64-bit integers. For example, if you
468wanted to unpack a sequence of signed big-endian 16-bit integers in a
469platform-independent way, you would have to write:
470
471 my @data = unpack 's*', pack 'S*', unpack 'n*', $buf;
472
473This is ugly. As of Perl 5.8.5, there's a much nicer way to express your
474desire for a certain byte-order: the C<E<gt>> and C<E<lt>> modifiers.
475C<E<gt>> is the big-endian modifier, while C<E<lt>> is the little-endian
476modifier. Using them, we could rewrite the above code as:
477
478 my @data = unpack 's>*', $buf;
479
480As you can see, the "big end" of the arrow touches the C<s>, which is a
481nice way to remember that C<E<gt>> is the big-endian modifier. The same
482obviously works for C<E<lt>>, where the "little end" touches the code.
483
484You will probably find these modifiers even more useful if you have
485to deal with big- or little-endian C structures. Be sure to read
486L<"Packing and Unpacking C Structures"> for more on that.
487
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488
489=head2 Floating point Numbers
490
491For packing floating point numbers you have the choice between the
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492pack codes C<f>, C<d>, C<F> and C<D>. C<f> and C<d> pack into (or unpack
493from) single-precision or double-precision representation as it is provided
494by your system. If your systems supports it, C<D> can be used to pack and
495unpack extended-precision floating point values (C<long double>), which
496can offer even more resolution than C<f> or C<d>. C<F> packs an C<NV>,
497which is the floating point type used by Perl internally. (There
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498is no such thing as a network representation for reals, so if you want
499to send your real numbers across computer boundaries, you'd better stick
500to ASCII representation, unless you're absolutely sure what's on the other
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501end of the line. For the even more adventuresome, you can use the byte-order
502modifiers from the previous section also on floating point codes.)
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503
504
505
506=head1 Exotic Templates
507
508
509=head2 Bit Strings
510
511Bits are the atoms in the memory world. Access to individual bits may
512have to be used either as a last resort or because it is the most
513convenient way to handle your data. Bit string (un)packing converts
514between strings containing a series of C<0> and C<1> characters and
515a sequence of bytes each containing a group of 8 bits. This is almost
516as simple as it sounds, except that there are two ways the contents of
517a byte may be written as a bit string. Let's have a look at an annotated
518byte:
519
520 7 6 5 4 3 2 1 0
521 +-----------------+
522 | 1 0 0 0 1 1 0 0 |
523 +-----------------+
524 MSB LSB
525
526It's egg-eating all over again: Some think that as a bit string this should
527be written "10001100" i.e. beginning with the most significant bit, others
528insist on "00110001". Well, Perl isn't biased, so that's why we have two bit
529string codes:
530
531 $byte = pack( 'B8', '10001100' ); # start with MSB
532 $byte = pack( 'b8', '00110001' ); # start with LSB
533
534It is not possible to pack or unpack bit fields - just integral bytes.
535C<pack> always starts at the next byte boundary and "rounds up" to the
536next multiple of 8 by adding zero bits as required. (If you do want bit
537fields, there is L<perlfunc/vec>. Or you could implement bit field
538handling at the character string level, using split, substr, and
539concatenation on unpacked bit strings.)
540
541To illustrate unpacking for bit strings, we'll decompose a simple
542status register (a "-" stands for a "reserved" bit):
543
544 +-----------------+-----------------+
545 | S Z - A - P - C | - - - - O D I T |
546 +-----------------+-----------------+
547 MSB LSB MSB LSB
548
549Converting these two bytes to a string can be done with the unpack
550template C<'b16'>. To obtain the individual bit values from the bit
47f22e19 551string we use C<split> with the "empty" separator pattern which dissects
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552into individual characters. Bit values from the "reserved" positions are
553simply assigned to C<undef>, a convenient notation for "I don't care where
554this goes".
555
49704364 556 ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
34babc16 557 $trace, $interrupt, $direction, $overflow) =
47f22e19 558 split( //, unpack( 'b16', $status ) );
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559
560We could have used an unpack template C<'b12'> just as well, since the
561last 4 bits can be ignored anyway.
562
563
564=head2 Uuencoding
565
566Another odd-man-out in the template alphabet is C<u>, which packs an
567"uuencoded string". ("uu" is short for Unix-to-Unix.) Chances are that
568you won't ever need this encoding technique which was invented to overcome
569the shortcomings of old-fashioned transmission mediums that do not support
570other than simple ASCII data. The essential recipe is simple: Take three
571bytes, or 24 bits. Split them into 4 six-packs, adding a space (0x20) to
572each. Repeat until all of the data is blended. Fold groups of 4 bytes into
573lines no longer than 60 and garnish them in front with the original byte count
574(incremented by 0x20) and a C<"\n"> at the end. - The C<pack> chef will
575prepare this for you, a la minute, when you select pack code C<u> on the menu:
576
577 my $uubuf = pack( 'u', $bindat );
578
579A repeat count after C<u> sets the number of bytes to put into an
580uuencoded line, which is the maximum of 45 by default, but could be
581set to some (smaller) integer multiple of three. C<unpack> simply ignores
582the repeat count.
583
584
585=head2 Doing Sums
586
587An even stranger template code is C<%>E<lt>I<number>E<gt>. First, because
588it's used as a prefix to some other template code. Second, because it
589cannot be used in C<pack> at all, and third, in C<unpack>, doesn't return the
590data as defined by the template code it precedes. Instead it'll give you an
591integer of I<number> bits that is computed from the data value by
592doing sums. For numeric unpack codes, no big feat is achieved:
593
594 my $buf = pack( 'iii', 100, 20, 3 );
595 print unpack( '%32i3', $buf ), "\n"; # prints 123
596
597For string values, C<%> returns the sum of the byte values saving
598you the trouble of a sum loop with C<substr> and C<ord>:
599
600 print unpack( '%32A*', "\x01\x10" ), "\n"; # prints 17
601
602Although the C<%> code is documented as returning a "checksum":
603don't put your trust in such values! Even when applied to a small number
604of bytes, they won't guarantee a noticeable Hamming distance.
605
606In connection with C<b> or C<B>, C<%> simply adds bits, and this can be put
607to good use to count set bits efficiently:
608
609 my $bitcount = unpack( '%32b*', $mask );
610
611And an even parity bit can be determined like this:
612
613 my $evenparity = unpack( '%1b*', $mask );
614
615
616=head2 Unicode
617
618Unicode is a character set that can represent most characters in most of
619the world's languages, providing room for over one million different
620characters. Unicode 3.1 specifies 94,140 characters: The Basic Latin
621characters are assigned to the numbers 0 - 127. The Latin-1 Supplement with
622characters that are used in several European languages is in the next
623range, up to 255. After some more Latin extensions we find the character
47b6252e 624sets from languages using non-Roman alphabets, interspersed with a
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625variety of symbol sets such as currency symbols, Zapf Dingbats or Braille.
626(You might want to visit L<www.unicode.org> for a look at some of
627them - my personal favourites are Telugu and Kannada.)
628
629The Unicode character sets associates characters with integers. Encoding
630these numbers in an equal number of bytes would more than double the
47b6252e 631requirements for storing texts written in Latin alphabets.
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632The UTF-8 encoding avoids this by storing the most common (from a western
633point of view) characters in a single byte while encoding the rarer
634ones in three or more bytes.
635
636So what has this got to do with C<pack>? Well, if you want to convert
637between a Unicode number and its UTF-8 representation you can do so by
638using template code C<U>. As an example, let's produce the UTF-8
639representation of the Euro currency symbol (code number 0x20AC):
640
641 $UTF8{Euro} = pack( 'U', 0x20AC );
642
643Inspecting C<$UTF8{Euro}> shows that it contains 3 bytes: "\xe2\x82\xac". The
644round trip can be completed with C<unpack>:
645
646 $Unicode{Euro} = unpack( 'U', $UTF8{Euro} );
647
648Usually you'll want to pack or unpack UTF-8 strings:
649
650 # pack and unpack the Hebrew alphabet
651 my $alefbet = pack( 'U*', 0x05d0..0x05ea );
652 my @hebrew = unpack( 'U*', $utf );
653
654
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655=head2 Another Portable Binary Encoding
656
657The pack code C<w> has been added to support a portable binary data
658encoding scheme that goes way beyond simple integers. (Details can
659be found at L<Casbah.org>, the Scarab project.) A BER (Binary Encoded
660Representation) compressed unsigned integer stores base 128
661digits, most significant digit first, with as few digits as possible.
662Bit eight (the high bit) is set on each byte except the last. There
663is no size limit to BER encoding, but Perl won't go to extremes.
664
665 my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 );
666
667A hex dump of C<$berbuf>, with spaces inserted at the right places,
668shows 01 8100 8101 81807F. Since the last byte is always less than
669128, C<unpack> knows where to stop.
670
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672=head1 Template Grouping
673
674Prior to Perl 5.8, repetitions of templates had to be made by
675C<x>-multiplication of template strings. Now there is a better way as
676we may use the pack codes C<(> and C<)> combined with a repeat count.
677The C<unpack> template from the Stack Frame example can simply
678be written like this:
679
680 unpack( 'v2 (vXXCC)5 v5', $frame )
681
682Let's explore this feature a little more. We'll begin with the equivalent of
683
684 join( '', map( substr( $_, 0, 1 ), @str ) )
685
686which returns a string consisting of the first character from each string.
687Using pack, we can write
688
689 pack( '(A)'.@str, @str )
690
691or, because a repeat count C<*> means "repeat as often as required",
692simply
693
694 pack( '(A)*', @str )
695
696(Note that the template C<A*> would only have packed C<$str[0]> in full
697length.)
ffc145e8 698
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699To pack dates stored as triplets ( day, month, year ) in an array C<@dates>
700into a sequence of byte, byte, short integer we can write
701
702 $pd = pack( '(CCS)*', map( @$_, @dates ) );
703
704To swap pairs of characters in a string (with even length) one could use
705several techniques. First, let's use C<x> and C<X> to skip forward and back:
706
707 $s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) );
708
709We can also use C<@> to jump to an offset, with 0 being the position where
710we were when the last C<(> was encountered:
711
712 $s = pack( '(A)*', unpack( '(@1A @0A @2)*', $s ) );
713
714Finally, there is also an entirely different approach by unpacking big
715endian shorts and packing them in the reverse byte order:
716
717 $s = pack( '(v)*', unpack( '(n)*', $s );
718
719
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720=head1 Lengths and Widths
721
722=head2 String Lengths
723
724In the previous section we've seen a network message that was constructed
725by prefixing the binary message length to the actual message. You'll find
726that packing a length followed by so many bytes of data is a
727frequently used recipe since appending a null byte won't work
47b6252e 728if a null byte may be part of the data. Here is an example where both
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729techniques are used: after two null terminated strings with source and
730destination address, a Short Message (to a mobile phone) is sent after
731a length byte:
732
733 my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm );
734
735Unpacking this message can be done with the same template:
736
737 ( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg );
738
47b6252e 739There's a subtle trap lurking in the offing: Adding another field after
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740the Short Message (in variable C<$sm>) is all right when packing, but this
741cannot be unpacked naively:
742
743 # pack a message
744 my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio );
7207e29d 745
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746 # unpack fails - $prio remains undefined!
747 ( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg );
748
749The pack code C<A*> gobbles up all remaining bytes, and C<$prio> remains
750undefined! Before we let disappointment dampen the morale: Perl's got
751the trump card to make this trick too, just a little further up the sleeve.
752Watch this:
753
754 # pack a message: ASCIIZ, ASCIIZ, length/string, byte
755 my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio );
756
757 # unpack
758 ( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg );
759
760Combining two pack codes with a slash (C</>) associates them with a single
761value from the argument list. In C<pack>, the length of the argument is
762taken and packed according to the first code while the argument itself
763is added after being converted with the template code after the slash.
764This saves us the trouble of inserting the C<length> call, but it is
765in C<unpack> where we really score: The value of the length byte marks the
766end of the string to be taken from the buffer. Since this combination
f8b4d74f 767doesn't make sense except when the second pack code isn't C<a*>, C<A*>
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768or C<Z*>, Perl won't let you.
769
770The pack code preceding C</> may be anything that's fit to represent a
771number: All the numeric binary pack codes, and even text codes such as
772C<A4> or C<Z*>:
773
774 # pack/unpack a string preceded by its length in ASCII
775 my $buf = pack( 'A4/A*', "Humpty-Dumpty" );
776 # unpack $buf: '13 Humpty-Dumpty'
777 my $txt = unpack( 'A4/A*', $buf );
778
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779C</> is not implemented in Perls before 5.6, so if your code is required to
780work on older Perls you'll need to C<unpack( 'Z* Z* C')> to get the length,
781then use it to make a new unpack string. For example
782
783 # pack a message: ASCIIZ, ASCIIZ, length, string, byte (5.005 compatible)
784 my $msg = pack( 'Z* Z* C A* C', $src, $dst, length $sm, $sm, $prio );
785
786 # unpack
787 ( undef, undef, $len) = unpack( 'Z* Z* C', $msg );
788 ($src, $dst, $sm, $prio) = unpack ( "Z* Z* x A$len C", $msg );
789
790But that second C<unpack> is rushing ahead. It isn't using a simple literal
791string for the template. So maybe we should introduce...
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792
793=head2 Dynamic Templates
794
795So far, we've seen literals used as templates. If the list of pack
796items doesn't have fixed length, an expression constructing the
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797template is required (whenever, for some reason, C<()*> cannot be used).
798Here's an example: To store named string values in a way that can be
799conveniently parsed by a C program, we create a sequence of names and
800null terminated ASCII strings, with C<=> between the name and the value,
801followed by an additional delimiting null byte. Here's how:
34babc16 802
49704364 803 my $env = pack( '(A*A*Z*)' . keys( %Env ) . 'C',
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804 map( { ( $_, '=', $Env{$_} ) } keys( %Env ) ), 0 );
805
806Let's examine the cogs of this byte mill, one by one. There's the C<map>
807call, creating the items we intend to stuff into the C<$env> buffer:
808to each key (in C<$_>) it adds the C<=> separator and the hash entry value.
809Each triplet is packed with the template code sequence C<A*A*Z*> that
49704364 810is repeated according to the number of keys. (Yes, that's what the C<keys>
fe854a6f 811function returns in scalar context.) To get the very last null byte,
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812we add a C<0> at the end of the C<pack> list, to be packed with C<C>.
813(Attentive readers may have noticed that we could have omitted the 0.)
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814
815For the reverse operation, we'll have to determine the number of items
816in the buffer before we can let C<unpack> rip it apart:
817
47b6252e 818 my $n = $env =~ tr/\0// - 1;
49704364 819 my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) );
34babc16 820
47b6252e 821The C<tr> counts the null bytes. The C<unpack> call returns a list of
47f22e19 822name-value pairs each of which is taken apart in the C<map> block.
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823
824
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825=head2 Counting Repetitions
826
827Rather than storing a sentinel at the end of a data item (or a list of items),
828we could precede the data with a count. Again, we pack keys and values of
829a hash, preceding each with an unsigned short length count, and up front
830we store the number of pairs:
831
832 my $env = pack( 'S(S/A* S/A*)*', scalar keys( %Env ), %Env );
833
834This simplifies the reverse operation as the number of repetitions can be
835unpacked with the C</> code:
836
837 my %env = unpack( 'S/(S/A* S/A*)', $env );
838
839Note that this is one of the rare cases where you cannot use the same
840template for C<pack> and C<unpack> because C<pack> can't determine
841a repeat count for a C<()>-group.
842
843
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844=head1 Packing and Unpacking C Structures
845
846In previous sections we have seen how to pack numbers and character
847strings. If it were not for a couple of snags we could conclude this
848section right away with the terse remark that C structures don't
849contain anything else, and therefore you already know all there is to it.
850Sorry, no: read on, please.
851
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852If you have to deal with a lot of C structures, and don't want to
853hack all your template strings manually, you'll probably want to have
854a look at the CPAN module C<Convert::Binary::C>. Not only can it parse
855your C source directly, but it also has built-in support for all the
856odds and ends described further on in this section.
857
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858=head2 The Alignment Pit
859
860In the consideration of speed against memory requirements the balance
861has been tilted in favor of faster execution. This has influenced the
862way C compilers allocate memory for structures: On architectures
863where a 16-bit or 32-bit operand can be moved faster between places in
864memory, or to or from a CPU register, if it is aligned at an even or
865multiple-of-four or even at a multiple-of eight address, a C compiler
866will give you this speed benefit by stuffing extra bytes into structures.
867If you don't cross the C shoreline this is not likely to cause you any
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868grief (although you should care when you design large data structures,
869or you want your code to be portable between architectures (you do want
870that, don't you?)).
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871
872To see how this affects C<pack> and C<unpack>, we'll compare these two
873C structures:
874
875 typedef struct {
876 char c1;
877 short s;
878 char c2;
879 long l;
880 } gappy_t;
881
882 typedef struct {
883 long l;
884 short s;
885 char c1;
886 char c2;
887 } dense_t;
888
889Typically, a C compiler allocates 12 bytes to a C<gappy_t> variable, but
890requires only 8 bytes for a C<dense_t>. After investigating this further,
891we can draw memory maps, showing where the extra 4 bytes are hidden:
892
893 0 +4 +8 +12
894 +--+--+--+--+--+--+--+--+--+--+--+--+
895 |c1|xx| s |c2|xx|xx|xx| l | xx = fill byte
896 +--+--+--+--+--+--+--+--+--+--+--+--+
897 gappy_t
898
899 0 +4 +8
900 +--+--+--+--+--+--+--+--+
901 | l | h |c1|c2|
902 +--+--+--+--+--+--+--+--+
903 dense_t
904
905And that's where the first quirk strikes: C<pack> and C<unpack>
906templates have to be stuffed with C<x> codes to get those extra fill bytes.
907
908The natural question: "Why can't Perl compensate for the gaps?" warrants
909an answer. One good reason is that C compilers might provide (non-ANSI)
910extensions permitting all sorts of fancy control over the way structures
911are aligned, even at the level of an individual structure field. And, if
912this were not enough, there is an insidious thing called C<union> where
913the amount of fill bytes cannot be derived from the alignment of the next
914item alone.
915
916OK, so let's bite the bullet. Here's one way to get the alignment right
917by inserting template codes C<x>, which don't take a corresponding item
918from the list:
919
920 my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );
921
922Note the C<!> after C<l>: We want to make sure that we pack a long
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923integer as it is compiled by our C compiler. And even now, it will only
924work for the platforms where the compiler aligns things as above.
925And somebody somewhere has a platform where it doesn't.
926[Probably a Cray, where C<short>s, C<int>s and C<long>s are all 8 bytes. :-)]
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927
928Counting bytes and watching alignments in lengthy structures is bound to
929be a drag. Isn't there a way we can create the template with a simple
930program? Here's a C program that does the trick:
931
932 #include <stdio.h>
933 #include <stddef.h>
934
935 typedef struct {
936 char fc1;
937 short fs;
938 char fc2;
939 long fl;
940 } gappy_t;
941
942 #define Pt(struct,field,tchar) \
943 printf( "@%d%s ", offsetof(struct,field), # tchar );
944
d832b8f6 945 int main() {
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946 Pt( gappy_t, fc1, c );
947 Pt( gappy_t, fs, s! );
948 Pt( gappy_t, fc2, c );
949 Pt( gappy_t, fl, l! );
950 printf( "\n" );
951 }
952
953The output line can be used as a template in a C<pack> or C<unpack> call:
954
955 my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l );
956
957Gee, yet another template code - as if we hadn't plenty. But
958C<@> saves our day by enabling us to specify the offset from the beginning
959of the pack buffer to the next item: This is just the value
960the C<offsetof> macro (defined in C<E<lt>stddef.hE<gt>>) returns when
961given a C<struct> type and one of its field names ("member-designator" in
962C standardese).
963
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964Neither using offsets nor adding C<x>'s to bridge the gaps is satisfactory.
965(Just imagine what happens if the structure changes.) What we really need
966is a way of saying "skip as many bytes as required to the next multiple of N".
967In fluent Templatese, you say this with C<x!N> where N is replaced by the
968appropriate value. Here's the next version of our struct packaging:
969
970 my $gappy = pack( 'c x!2 s c x!4 l!', $c1, $s, $c2, $l );
971
972That's certainly better, but we still have to know how long all the
973integers are, and portability is far away. Rather than C<2>,
974for instance, we want to say "however long a short is". But this can be
975done by enclosing the appropriate pack code in brackets: C<[s]>. So, here's
976the very best we can do:
977
978 my $gappy = pack( 'c x![s] s c x![l!] l!', $c1, $s, $c2, $l );
979
34babc16 980
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981=head2 Dealing with Endian-ness
982
983Now, imagine that we want to pack the data for a machine with a
984different byte-order. First, we'll have to figure out how big the data
985types on the target machine really are. Let's assume that the longs are
98632 bits wide and the shorts are 16 bits wide. You can then rewrite the
987template as:
988
989 my $gappy = pack( 'c x![s] s c x![l] l', $c1, $s, $c2, $l );
990
991If the target machine is little-endian, we could write:
992
993 my $gappy = pack( 'c x![s] s< c x![l] l<', $c1, $s, $c2, $l );
994
995This forces the short and the long members to be little-endian, and is
996just fine if you don't have too many struct members. But we could also
997use the byte-order modifier on a group and write the following:
998
999 my $gappy = pack( '( c x![s] s c x![l] l )<', $c1, $s, $c2, $l );
1000
1001This is not as short as before, but it makes it more obvious that we
1002intend to have little-endian byte-order for a whole group, not only
1003for individual template codes. It can also be more readable and easier
1004to maintain.
1005
1006
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1007=head2 Alignment, Take 2
1008
1009I'm afraid that we're not quite through with the alignment catch yet. The
1010hydra raises another ugly head when you pack arrays of structures:
1011
1012 typedef struct {
1013 short count;
1014 char glyph;
1015 } cell_t;
1016
1017 typedef cell_t buffer_t[BUFLEN];
1018
1019Where's the catch? Padding is neither required before the first field C<count>,
1020nor between this and the next field C<glyph>, so why can't we simply pack
1021like this:
1022
1023 # something goes wrong here:
1024 pack( 's!a' x @buffer,
1025 map{ ( $_->{count}, $_->{glyph} ) } @buffer );
1026
1027This packs C<3*@buffer> bytes, but it turns out that the size of
1028C<buffer_t> is four times C<BUFLEN>! The moral of the story is that
1029the required alignment of a structure or array is propagated to the
1030next higher level where we have to consider padding I<at the end>
1031of each component as well. Thus the correct template is:
1032
1033 pack( 's!ax' x @buffer,
1034 map{ ( $_->{count}, $_->{glyph} ) } @buffer );
1035
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1036=head2 Alignment, Take 3
1037
1038And even if you take all the above into account, ANSI still lets this:
1039
1040 typedef struct {
1041 char foo[2];
1042 } foo_t;
34babc16 1043
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1044vary in size. The alignment constraint of the structure can be greater than
1045any of its elements. [And if you think that this doesn't affect anything
1046common, dismember the next cellphone that you see. Many have ARM cores, and
1047the ARM structure rules make C<sizeof (foo_t)> == 4]
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1048
1049=head2 Pointers for How to Use Them
1050
1051The title of this section indicates the second problem you may run into
1052sooner or later when you pack C structures. If the function you intend
1053to call expects a, say, C<void *> value, you I<cannot> simply take
1054a reference to a Perl variable. (Although that value certainly is a
1055memory address, it's not the address where the variable's contents are
1056stored.)
1057
1058Template code C<P> promises to pack a "pointer to a fixed length string".
1059Isn't this what we want? Let's try:
1060
1061 # allocate some storage and pack a pointer to it
1062 my $memory = "\x00" x $size;
1063 my $memptr = pack( 'P', $memory );
1064
1065But wait: doesn't C<pack> just return a sequence of bytes? How can we pass this
1066string of bytes to some C code expecting a pointer which is, after all,
1067nothing but a number? The answer is simple: We have to obtain the numeric
1068address from the bytes returned by C<pack>.
1069
1070 my $ptr = unpack( 'L!', $memptr );
1071
1072Obviously this assumes that it is possible to typecast a pointer
1073to an unsigned long and vice versa, which frequently works but should not
1074be taken as a universal law. - Now that we have this pointer the next question
1075is: How can we put it to good use? We need a call to some C function
1076where a pointer is expected. The read(2) system call comes to mind:
1077
1078 ssize_t read(int fd, void *buf, size_t count);
1079
1080After reading L<perlfunc> explaining how to use C<syscall> we can write
1081this Perl function copying a file to standard output:
1082
1083 require 'syscall.ph';
1084 sub cat($){
1085 my $path = shift();
1086 my $size = -s $path;
1087 my $memory = "\x00" x $size; # allocate some memory
1088 my $ptr = unpack( 'L', pack( 'P', $memory ) );
1089 open( F, $path ) || die( "$path: cannot open ($!)\n" );
1090 my $fd = fileno(F);
1091 my $res = syscall( &SYS_read, fileno(F), $ptr, $size );
1092 print $memory;
1093 close( F );
1094 }
1095
1096This is neither a specimen of simplicity nor a paragon of portability but
1097it illustrates the point: We are able to sneak behind the scenes and
1098access Perl's otherwise well-guarded memory! (Important note: Perl's
1099C<syscall> does I<not> require you to construct pointers in this roundabout
1100way. You simply pass a string variable, and Perl forwards the address.)
1101
1102How does C<unpack> with C<P> work? Imagine some pointer in the buffer
1103about to be unpacked: If it isn't the null pointer (which will smartly
1104produce the C<undef> value) we have a start address - but then what?
1105Perl has no way of knowing how long this "fixed length string" is, so
1106it's up to you to specify the actual size as an explicit length after C<P>.
1107
1108 my $mem = "abcdefghijklmn";
1109 print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"
1110
1111As a consequence, C<pack> ignores any number or C<*> after C<P>.
1112
1113
1114Now that we have seen C<P> at work, we might as well give C<p> a whirl.
1115Why do we need a second template code for packing pointers at all? The
1116answer lies behind the simple fact that an C<unpack> with C<p> promises
1117a null-terminated string starting at the address taken from the buffer,
1118and that implies a length for the data item to be returned:
1119
1120 my $buf = pack( 'p', "abc\x00efhijklmn" );
1121 print unpack( 'p', $buf ); # prints "abc"
1122
1123
1124
1125Albeit this is apt to be confusing: As a consequence of the length being
1126implied by the string's length, a number after pack code C<p> is a repeat
1127count, not a length as after C<P>.
1128
1129
1130Using C<pack(..., $x)> with C<P> or C<p> to get the address where C<$x> is
1131actually stored must be used with circumspection. Perl's internal machinery
1132considers the relation between a variable and that address as its very own
1133private matter and doesn't really care that we have obtained a copy. Therefore:
1134
1135=over 4
1136
1137=item *
1138
1139Do not use C<pack> with C<p> or C<P> to obtain the address of variable
1140that's bound to go out of scope (and thereby freeing its memory) before you
1141are done with using the memory at that address.
1142
1143=item *
1144
1145Be very careful with Perl operations that change the value of the
1146variable. Appending something to the variable, for instance, might require
1147reallocation of its storage, leaving you with a pointer into no-man's land.
1148
1149=item *
1150
1151Don't think that you can get the address of a Perl variable
1152when it is stored as an integer or double number! C<pack('P', $x)> will
1153force the variable's internal representation to string, just as if you
1154had written something like C<$x .= ''>.
1155
1156=back
1157
1158It's safe, however, to P- or p-pack a string literal, because Perl simply
1159allocates an anonymous variable.
1160
1161
1162
1163=head1 Pack Recipes
1164
1165Here are a collection of (possibly) useful canned recipes for C<pack>
1166and C<unpack>:
1167
1168 # Convert IP address for socket functions
1169 pack( "C4", split /\./, "123.4.5.6" );
1170
1171 # Count the bits in a chunk of memory (e.g. a select vector)
1172 unpack( '%32b*', $mask );
1173
1174 # Determine the endianness of your system
1175 $is_little_endian = unpack( 'c', pack( 's', 1 ) );
1176 $is_big_endian = unpack( 'xc', pack( 's', 1 ) );
1177
1178 # Determine the number of bits in a native integer
1179 $bits = unpack( '%32I!', ~0 );
1180
1181 # Prepare argument for the nanosleep system call
1182 my $timespec = pack( 'L!L!', $secs, $nanosecs );
1183
f8b4d74f
WL
1184For a simple memory dump we unpack some bytes into just as
1185many pairs of hex digits, and use C<map> to handle the traditional
1186spacing - 16 bytes to a line:
1187
34babc16 1188 my $i;
49704364
LW
1189 print map( ++$i % 16 ? "$_ " : "$_\n",
1190 unpack( 'H2' x length( $mem ), $mem ) ),
f8b4d74f 1191 length( $mem ) % 16 ? "\n" : '';
34babc16
JH
1192
1193
47f22e19
WL
1194=head1 Funnies Section
1195
1196 # Pulling digits out of nowhere...
1197 print unpack( 'C', pack( 'x' ) ),
1198 unpack( '%B*', pack( 'A' ) ),
1199 unpack( 'H', pack( 'A' ) ),
1200 unpack( 'A', unpack( 'C', pack( 'A' ) ) ), "\n";
1201
1202 # One for the road ;-)
1203 my $advice = pack( 'all u can in a van' );
1204
1205
34babc16
JH
1206=head1 Authors
1207
1208Simon Cozens and Wolfgang Laun.
1209