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Simplify example pack code in perlpacktut
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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
4e848ca9 76 my $s = pack( 'H2' x 10, 30..39 );
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77 print "$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
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83=head1 Packing Text
84
85Let's suppose you've got to read in a data file like this:
86
87 Date |Description | Income|Expenditure
88 01/24/2001 Ahmed's Camel Emporium 1147.99
89 01/28/2001 Flea spray 24.99
90 01/29/2001 Camel rides to tourists 235.00
91
92How do we do it? You might think first to use C<split>; however, since
93C<split> collapses blank fields, you'll never know whether a record was
94income or expenditure. Oops. Well, you could always use C<substr>:
95
96 while (<>) {
97 my $date = substr($_, 0, 11);
98 my $desc = substr($_, 12, 27);
99 my $income = substr($_, 40, 7);
100 my $expend = substr($_, 52, 7);
101 ...
102 }
103
104It's not really a barrel of laughs, is it? In fact, it's worse than it
105may seem; the eagle-eyed may notice that the first field should only be
10610 characters wide, and the error has propagated right through the other
107numbers - which we've had to count by hand. So it's error-prone as well
108as horribly unfriendly.
109
110Or maybe we could use regular expressions:
7207e29d 111
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112 while (<>) {
113 my($date, $desc, $income, $expend) =
114 m|(\d\d/\d\d/\d{4}) (.{27}) (.{7})(.*)|;
115 ...
116 }
117
118Urgh. Well, it's a bit better, but - well, would you want to maintain
119that?
120
121Hey, isn't Perl supposed to make this sort of thing easy? Well, it does,
122if you use the right tools. C<pack> and C<unpack> are designed to help
123you out when dealing with fixed-width data like the above. Let's have a
124look at a solution with C<unpack>:
125
126 while (<>) {
127 my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
128 ...
129 }
130
131That looks a bit nicer; but we've got to take apart that weird template.
132Where did I pull that out of?
133
134OK, let's have a look at some of our data again; in fact, we'll include
135the headers, and a handy ruler so we can keep track of where we are.
136
137 1 2 3 4 5
138 1234567890123456789012345678901234567890123456789012345678
139 Date |Description | Income|Expenditure
140 01/28/2001 Flea spray 24.99
141 01/29/2001 Camel rides to tourists 235.00
142
47b6252e 143From this, we can see that the date column stretches from column 1 to
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144column 10 - ten characters wide. The C<pack>-ese for "character" is
145C<A>, and ten of them are C<A10>. So if we just wanted to extract the
146dates, we could say this:
147
148 my($date) = unpack("A10", $_);
149
150OK, what's next? Between the date and the description is a blank column;
151we want to skip over that. The C<x> template means "skip forward", so we
152want one of those. Next, we have another batch of characters, from 12 to
15338. That's 27 more characters, hence C<A27>. (Don't make the fencepost
154error - there are 27 characters between 12 and 38, not 26. Count 'em!)
155
156Now we skip another character and pick up the next 7 characters:
157
158 my($date,$description,$income) = unpack("A10xA27xA7", $_);
159
160Now comes the clever bit. Lines in our ledger which are just income and
161not expenditure might end at column 46. Hence, we don't want to tell our
162C<unpack> pattern that we B<need> to find another 12 characters; we'll
163just say "if there's anything left, take it". As you might guess from
164regular expressions, that's what the C<*> means: "use everything
165remaining".
166
167=over 3
168
169=item *
170
171Be warned, though, that unlike regular expressions, if the C<unpack>
172template doesn't match the incoming data, Perl will scream and die.
173
174=back
175
176
177Hence, putting it all together:
178
49704364 179 my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_);
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180
181Now, that's our data parsed. I suppose what we might want to do now is
182total up our income and expenditure, and add another line to the end of
183our ledger - in the same format - saying how much we've brought in and
184how much we've spent:
185
186 while (<>) {
187 my($date, $desc, $income, $expend) = unpack("A10xA27xA7xA*", $_);
188 $tot_income += $income;
189 $tot_expend += $expend;
190 }
191
192 $tot_income = sprintf("%.2f", $tot_income); # Get them into
193 $tot_expend = sprintf("%.2f", $tot_expend); # "financial" format
194
195 $date = POSIX::strftime("%m/%d/%Y", localtime);
196
197 # OK, let's go:
198
199 print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend);
200
201Oh, hmm. That didn't quite work. Let's see what happened:
202
203 01/24/2001 Ahmed's Camel Emporium 1147.99
204 01/28/2001 Flea spray 24.99
205 01/29/2001 Camel rides to tourists 1235.00
206 03/23/2001Totals 1235.001172.98
207
208OK, it's a start, but what happened to the spaces? We put C<x>, didn't
209we? Shouldn't it skip forward? Let's look at what L<perlfunc/pack> says:
210
211 x A null byte.
212
213Urgh. No wonder. There's a big difference between "a null byte",
214character zero, and "a space", character 32. Perl's put something
215between the date and the description - but unfortunately, we can't see
216it!
217
218What we actually need to do is expand the width of the fields. The C<A>
219format pads any non-existent characters with spaces, so we can use the
220additional spaces to line up our fields, like this:
221
222 print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
223
224(Note that you can put spaces in the template to make it more readable,
225but they don't translate to spaces in the output.) Here's what we got
226this time:
227
228 01/24/2001 Ahmed's Camel Emporium 1147.99
229 01/28/2001 Flea spray 24.99
230 01/29/2001 Camel rides to tourists 1235.00
231 03/23/2001 Totals 1235.00 1172.98
232
233That's a bit better, but we still have that last column which needs to
234be moved further over. There's an easy way to fix this up:
235unfortunately, we can't get C<pack> to right-justify our fields, but we
236can get C<sprintf> to do it:
237
238 $tot_income = sprintf("%.2f", $tot_income);
239 $tot_expend = sprintf("%12.2f", $tot_expend);
240 $date = POSIX::strftime("%m/%d/%Y", localtime);
241 print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
242
243This time we get the right answer:
244
245 01/28/2001 Flea spray 24.99
246 01/29/2001 Camel rides to tourists 1235.00
247 03/23/2001 Totals 1235.00 1172.98
248
249So that's how we consume and produce fixed-width data. Let's recap what
250we've seen of C<pack> and C<unpack> so far:
251
252=over 3
253
254=item *
255
256Use C<pack> to go from several pieces of data to one fixed-width
257version; use C<unpack> to turn a fixed-width-format string into several
258pieces of data.
259
260=item *
261
262The pack format C<A> means "any character"; if you're C<pack>ing and
263you've run out of things to pack, C<pack> will fill the rest up with
264spaces.
265
266=item *
267
268C<x> means "skip a byte" when C<unpack>ing; when C<pack>ing, it means
269"introduce a null byte" - that's probably not what you mean if you're
270dealing with plain text.
271
272=item *
273
274You can follow the formats with numbers to say how many characters
275should be affected by that format: C<A12> means "take 12 characters";
276C<x6> means "skip 6 bytes" or "character 0, 6 times".
277
278=item *
279
280Instead of a number, you can use C<*> to mean "consume everything else
281left".
282
283B<Warning>: when packing multiple pieces of data, C<*> only means
284"consume all of the current piece of data". That's to say
285
286 pack("A*A*", $one, $two)
287
288packs all of C<$one> into the first C<A*> and then all of C<$two> into
289the second. This is a general principle: each format character
290corresponds to one piece of data to be C<pack>ed.
291
292=back
293
294
295
296=head1 Packing Numbers
297
298So much for textual data. Let's get onto the meaty stuff that C<pack>
299and C<unpack> are best at: handling binary formats for numbers. There is,
300of course, not just one binary format - life would be too simple - but
301Perl will do all the finicky labor for you.
302
303
304=head2 Integers
305
306Packing and unpacking numbers implies conversion to and from some
307I<specific> binary representation. Leaving floating point numbers
308aside for the moment, the salient properties of any such representation
309are:
310
311=over 4
312
313=item *
314
315the number of bytes used for storing the integer,
316
317=item *
318
319whether the contents are interpreted as a signed or unsigned number,
320
321=item *
322
323the byte ordering: whether the first byte is the least or most
324significant byte (or: little-endian or big-endian, respectively).
325
326=back
327
328So, for instance, to pack 20302 to a signed 16 bit integer in your
329computer's representation you write
330
331 my $ps = pack( 's', 20302 );
332
333Again, the result is a string, now containing 2 bytes. If you print
334this string (which is, generally, not recommended) you might see
335C<ON> or C<NO> (depending on your system's byte ordering) - or something
336entirely different if your computer doesn't use ASCII character encoding.
337Unpacking C<$ps> with the same template returns the original integer value:
338
339 my( $s ) = unpack( 's', $ps );
340
341This is true for all numeric template codes. But don't expect miracles:
47b6252e 342if the packed value exceeds the allotted byte capacity, high order bits
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343are silently discarded, and unpack certainly won't be able to pull them
344back out of some magic hat. And, when you pack using a signed template
345code such as C<s>, an excess value may result in the sign bit
346getting set, and unpacking this will smartly return a negative value.
347
34816 bits won't get you too far with integers, but there is C<l> and C<L>
349for signed and unsigned 32-bit integers. And if this is not enough and
350your system supports 64 bit integers you can push the limits much closer
351to infinity with pack codes C<q> and C<Q>. A notable exception is provided
352by pack codes C<i> and C<I> for signed and unsigned integers of the
353"local custom" variety: Such an integer will take up as many bytes as
354a local C compiler returns for C<sizeof(int)>, but it'll use I<at least>
35532 bits.
356
357Each of the integer pack codes C<sSlLqQ> results in a fixed number of bytes,
358no matter where you execute your program. This may be useful for some
359applications, but it does not provide for a portable way to pass data
360structures between Perl and C programs (bound to happen when you call
361XS extensions or the Perl function C<syscall>), or when you read or
362write binary files. What you'll need in this case are template codes that
363depend on what your local C compiler compiles when you code C<short> or
364C<unsigned long>, for instance. These codes and their corresponding
365byte lengths are shown in the table below. Since the C standard leaves
366much leeway with respect to the relative sizes of these data types, actual
367values may vary, and that's why the values are given as expressions in
368C and Perl. (If you'd like to use values from C<%Config> in your program
369you have to import it with C<use Config>.)
370
371 signed unsigned byte length in C byte length in Perl
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372 s! S! sizeof(short) $Config{shortsize}
373 i! I! sizeof(int) $Config{intsize}
374 l! L! sizeof(long) $Config{longsize}
d832b8f6 375 q! Q! sizeof(long long) $Config{longlongsize}
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376
377The C<i!> and C<I!> codes aren't different from C<i> and C<I>; they are
378tolerated for completeness' sake.
379
380
381=head2 Unpacking a Stack Frame
382
383Requesting a particular byte ordering may be necessary when you work with
47f22e19 384binary data coming from some specific architecture whereas your program could
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385run on a totally different system. As an example, assume you have 24 bytes
386containing a stack frame as it happens on an Intel 8086:
387
388 +---------+ +----+----+ +---------+
389 TOS: | IP | TOS+4:| FL | FH | FLAGS TOS+14:| SI |
390 +---------+ +----+----+ +---------+
391 | CS | | AL | AH | AX | DI |
392 +---------+ +----+----+ +---------+
393 | BL | BH | BX | BP |
394 +----+----+ +---------+
395 | CL | CH | CX | DS |
396 +----+----+ +---------+
397 | DL | DH | DX | ES |
398 +----+----+ +---------+
399
400First, we note that this time-honored 16-bit CPU uses little-endian order,
401and that's why the low order byte is stored at the lower address. To
9dc383df 402unpack such a (unsigned) short we'll have to use code C<v>. A repeat
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403count unpacks all 12 shorts:
404
405 my( $ip, $cs, $flags, $ax, $bx, $cd, $dx, $si, $di, $bp, $ds, $es ) =
406 unpack( 'v12', $frame );
407
408Alternatively, we could have used C<C> to unpack the individually
409accessible byte registers FL, FH, AL, AH, etc.:
410
411 my( $fl, $fh, $al, $ah, $bl, $bh, $cl, $ch, $dl, $dh ) =
412 unpack( 'C10', substr( $frame, 4, 10 ) );
413
414It would be nice if we could do this in one fell swoop: unpack a short,
415back up a little, and then unpack 2 bytes. Since Perl I<is> nice, it
416proffers the template code C<X> to back up one byte. Putting this all
417together, we may now write:
418
419 my( $ip, $cs,
420 $flags,$fl,$fh,
421 $ax,$al,$ah, $bx,$bl,$bh, $cx,$cl,$ch, $dx,$dl,$dh,
422 $si, $di, $bp, $ds, $es ) =
423 unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame );
424
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425(The clumsy construction of the template can be avoided - just read on!)
426
47f22e19 427We've taken some pains to construct the template so that it matches
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428the contents of our frame buffer. Otherwise we'd either get undefined values,
429or C<unpack> could not unpack all. If C<pack> runs out of items, it will
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430supply null strings (which are coerced into zeroes whenever the pack code
431says so).
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432
433
434=head2 How to Eat an Egg on a Net
435
436The pack code for big-endian (high order byte at the lowest address) is
437C<n> for 16 bit and C<N> for 32 bit integers. You use these codes
438if you know that your data comes from a compliant architecture, but,
439surprisingly enough, you should also use these pack codes if you
440exchange binary data, across the network, with some system that you
441know next to nothing about. The simple reason is that this
442order has been chosen as the I<network order>, and all standard-fearing
443programs ought to follow this convention. (This is, of course, a stern
444backing for one of the Lilliputian parties and may well influence the
445political development there.) So, if the protocol expects you to send
446a message by sending the length first, followed by just so many bytes,
447you could write:
448
449 my $buf = pack( 'N', length( $msg ) ) . $msg;
450
451or even:
452
453 my $buf = pack( 'NA*', length( $msg ), $msg );
454
455and pass C<$buf> to your send routine. Some protocols demand that the
456count should include the length of the count itself: then just add 4
457to the data length. (But make sure to read L<"Lengths and Widths"> before
458you really code this!)
459
460
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461=head2 Byte-order modifiers
462
463In the previous sections we've learned how to use C<n>, C<N>, C<v> and
464C<V> to pack and unpack integers with big- or little-endian byte-order.
465While this is nice, it's still rather limited because it leaves out all
466kinds of signed integers as well as 64-bit integers. For example, if you
467wanted to unpack a sequence of signed big-endian 16-bit integers in a
468platform-independent way, you would have to write:
469
470 my @data = unpack 's*', pack 'S*', unpack 'n*', $buf;
471
c4ecfaf1 472This is ugly. As of Perl 5.9.2, there's a much nicer way to express your
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473desire for a certain byte-order: the C<E<gt>> and C<E<lt>> modifiers.
474C<E<gt>> is the big-endian modifier, while C<E<lt>> is the little-endian
475modifier. Using them, we could rewrite the above code as:
476
477 my @data = unpack 's>*', $buf;
478
479As you can see, the "big end" of the arrow touches the C<s>, which is a
480nice way to remember that C<E<gt>> is the big-endian modifier. The same
481obviously works for C<E<lt>>, where the "little end" touches the code.
482
483You will probably find these modifiers even more useful if you have
484to deal with big- or little-endian C structures. Be sure to read
485L<"Packing and Unpacking C Structures"> for more on that.
486
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487
488=head2 Floating point Numbers
489
490For packing floating point numbers you have the choice between the
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491pack codes C<f>, C<d>, C<F> and C<D>. C<f> and C<d> pack into (or unpack
492from) single-precision or double-precision representation as it is provided
493by your system. If your systems supports it, C<D> can be used to pack and
494unpack extended-precision floating point values (C<long double>), which
495can offer even more resolution than C<f> or C<d>. C<F> packs an C<NV>,
496which is the floating point type used by Perl internally. (There
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497is no such thing as a network representation for reals, so if you want
498to send your real numbers across computer boundaries, you'd better stick
499to ASCII representation, unless you're absolutely sure what's on the other
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500end of the line. For the even more adventuresome, you can use the byte-order
501modifiers from the previous section also on floating point codes.)
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502
503
504
505=head1 Exotic Templates
506
507
508=head2 Bit Strings
509
510Bits are the atoms in the memory world. Access to individual bits may
511have to be used either as a last resort or because it is the most
512convenient way to handle your data. Bit string (un)packing converts
513between strings containing a series of C<0> and C<1> characters and
514a sequence of bytes each containing a group of 8 bits. This is almost
515as simple as it sounds, except that there are two ways the contents of
516a byte may be written as a bit string. Let's have a look at an annotated
517byte:
518
519 7 6 5 4 3 2 1 0
520 +-----------------+
521 | 1 0 0 0 1 1 0 0 |
522 +-----------------+
523 MSB LSB
524
525It's egg-eating all over again: Some think that as a bit string this should
526be written "10001100" i.e. beginning with the most significant bit, others
527insist on "00110001". Well, Perl isn't biased, so that's why we have two bit
528string codes:
529
530 $byte = pack( 'B8', '10001100' ); # start with MSB
531 $byte = pack( 'b8', '00110001' ); # start with LSB
532
533It is not possible to pack or unpack bit fields - just integral bytes.
534C<pack> always starts at the next byte boundary and "rounds up" to the
535next multiple of 8 by adding zero bits as required. (If you do want bit
536fields, there is L<perlfunc/vec>. Or you could implement bit field
537handling at the character string level, using split, substr, and
538concatenation on unpacked bit strings.)
539
540To illustrate unpacking for bit strings, we'll decompose a simple
541status register (a "-" stands for a "reserved" bit):
542
543 +-----------------+-----------------+
544 | S Z - A - P - C | - - - - O D I T |
545 +-----------------+-----------------+
546 MSB LSB MSB LSB
547
548Converting these two bytes to a string can be done with the unpack
549template C<'b16'>. To obtain the individual bit values from the bit
47f22e19 550string we use C<split> with the "empty" separator pattern which dissects
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551into individual characters. Bit values from the "reserved" positions are
552simply assigned to C<undef>, a convenient notation for "I don't care where
553this goes".
554
49704364 555 ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
34babc16 556 $trace, $interrupt, $direction, $overflow) =
47f22e19 557 split( //, unpack( 'b16', $status ) );
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558
559We could have used an unpack template C<'b12'> just as well, since the
560last 4 bits can be ignored anyway.
561
562
563=head2 Uuencoding
564
565Another odd-man-out in the template alphabet is C<u>, which packs an
566"uuencoded string". ("uu" is short for Unix-to-Unix.) Chances are that
567you won't ever need this encoding technique which was invented to overcome
568the shortcomings of old-fashioned transmission mediums that do not support
569other than simple ASCII data. The essential recipe is simple: Take three
570bytes, or 24 bits. Split them into 4 six-packs, adding a space (0x20) to
571each. Repeat until all of the data is blended. Fold groups of 4 bytes into
572lines no longer than 60 and garnish them in front with the original byte count
573(incremented by 0x20) and a C<"\n"> at the end. - The C<pack> chef will
574prepare this for you, a la minute, when you select pack code C<u> on the menu:
575
576 my $uubuf = pack( 'u', $bindat );
577
578A repeat count after C<u> sets the number of bytes to put into an
579uuencoded line, which is the maximum of 45 by default, but could be
580set to some (smaller) integer multiple of three. C<unpack> simply ignores
581the repeat count.
582
583
584=head2 Doing Sums
585
586An even stranger template code is C<%>E<lt>I<number>E<gt>. First, because
587it's used as a prefix to some other template code. Second, because it
588cannot be used in C<pack> at all, and third, in C<unpack>, doesn't return the
589data as defined by the template code it precedes. Instead it'll give you an
590integer of I<number> bits that is computed from the data value by
591doing sums. For numeric unpack codes, no big feat is achieved:
592
593 my $buf = pack( 'iii', 100, 20, 3 );
594 print unpack( '%32i3', $buf ), "\n"; # prints 123
595
596For string values, C<%> returns the sum of the byte values saving
597you the trouble of a sum loop with C<substr> and C<ord>:
598
599 print unpack( '%32A*', "\x01\x10" ), "\n"; # prints 17
600
601Although the C<%> code is documented as returning a "checksum":
602don't put your trust in such values! Even when applied to a small number
603of bytes, they won't guarantee a noticeable Hamming distance.
604
605In connection with C<b> or C<B>, C<%> simply adds bits, and this can be put
606to good use to count set bits efficiently:
607
608 my $bitcount = unpack( '%32b*', $mask );
609
610And an even parity bit can be determined like this:
611
612 my $evenparity = unpack( '%1b*', $mask );
613
614
615=head2 Unicode
616
617Unicode is a character set that can represent most characters in most of
618the world's languages, providing room for over one million different
619characters. Unicode 3.1 specifies 94,140 characters: The Basic Latin
620characters are assigned to the numbers 0 - 127. The Latin-1 Supplement with
621characters that are used in several European languages is in the next
622range, up to 255. After some more Latin extensions we find the character
47b6252e 623sets from languages using non-Roman alphabets, interspersed with a
34babc16 624variety of symbol sets such as currency symbols, Zapf Dingbats or Braille.
f979aebc 625(You might want to visit L<http://www.unicode.org/> for a look at some of
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626them - my personal favourites are Telugu and Kannada.)
627
628The Unicode character sets associates characters with integers. Encoding
629these numbers in an equal number of bytes would more than double the
47b6252e 630requirements for storing texts written in Latin alphabets.
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631The UTF-8 encoding avoids this by storing the most common (from a western
632point of view) characters in a single byte while encoding the rarer
633ones in three or more bytes.
634
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635Perl uses UTF-8, internally, for most Unicode strings.
636
637So what has this got to do with C<pack>? Well, if you want to compose a
638Unicode string (that is internally encoded as UTF-8), you can do so by
639using template code C<U>. As an example, let's produce the Euro currency
640symbol (code number 0x20AC):
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641
642 $UTF8{Euro} = pack( 'U', 0x20AC );
2575c402 643 # Equivalent to: $UTF8{Euro} = "\x{20ac}";
34babc16 644
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645Inspecting C<$UTF8{Euro}> shows that it contains 3 bytes:
646"\xe2\x82\xac". However, it contains only 1 character, number 0x20AC.
647The round trip can be completed with C<unpack>:
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648
649 $Unicode{Euro} = unpack( 'U', $UTF8{Euro} );
650
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651Unpacking using the C<U> template code also works on UTF-8 encoded byte
652strings.
653
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654Usually you'll want to pack or unpack UTF-8 strings:
655
656 # pack and unpack the Hebrew alphabet
657 my $alefbet = pack( 'U*', 0x05d0..0x05ea );
658 my @hebrew = unpack( 'U*', $utf );
659
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660Please note: in the general case, you're better off using
661Encode::decode_utf8 to decode a UTF-8 encoded byte string to a Perl
38a44b82 662Unicode string, and Encode::encode_utf8 to encode a Perl Unicode string
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663to UTF-8 bytes. These functions provide means of handling invalid byte
664sequences and generally have a friendlier interface.
34babc16 665
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666=head2 Another Portable Binary Encoding
667
668The pack code C<w> has been added to support a portable binary data
669encoding scheme that goes way beyond simple integers. (Details can
f979aebc 670be found at L<http://Casbah.org/>, the Scarab project.) A BER (Binary Encoded
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671Representation) compressed unsigned integer stores base 128
672digits, most significant digit first, with as few digits as possible.
673Bit eight (the high bit) is set on each byte except the last. There
674is no size limit to BER encoding, but Perl won't go to extremes.
675
676 my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 );
677
678A hex dump of C<$berbuf>, with spaces inserted at the right places,
679shows 01 8100 8101 81807F. Since the last byte is always less than
680128, C<unpack> knows where to stop.
681
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683=head1 Template Grouping
684
685Prior to Perl 5.8, repetitions of templates had to be made by
686C<x>-multiplication of template strings. Now there is a better way as
687we may use the pack codes C<(> and C<)> combined with a repeat count.
688The C<unpack> template from the Stack Frame example can simply
689be written like this:
690
691 unpack( 'v2 (vXXCC)5 v5', $frame )
692
693Let's explore this feature a little more. We'll begin with the equivalent of
694
695 join( '', map( substr( $_, 0, 1 ), @str ) )
696
697which returns a string consisting of the first character from each string.
698Using pack, we can write
699
700 pack( '(A)'.@str, @str )
701
702or, because a repeat count C<*> means "repeat as often as required",
703simply
704
705 pack( '(A)*', @str )
706
707(Note that the template C<A*> would only have packed C<$str[0]> in full
708length.)
ffc145e8 709
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710To pack dates stored as triplets ( day, month, year ) in an array C<@dates>
711into a sequence of byte, byte, short integer we can write
712
713 $pd = pack( '(CCS)*', map( @$_, @dates ) );
714
715To swap pairs of characters in a string (with even length) one could use
716several techniques. First, let's use C<x> and C<X> to skip forward and back:
717
718 $s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) );
719
720We can also use C<@> to jump to an offset, with 0 being the position where
721we were when the last C<(> was encountered:
722
723 $s = pack( '(A)*', unpack( '(@1A @0A @2)*', $s ) );
724
725Finally, there is also an entirely different approach by unpacking big
726endian shorts and packing them in the reverse byte order:
727
728 $s = pack( '(v)*', unpack( '(n)*', $s );
729
730
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731=head1 Lengths and Widths
732
733=head2 String Lengths
734
735In the previous section we've seen a network message that was constructed
736by prefixing the binary message length to the actual message. You'll find
737that packing a length followed by so many bytes of data is a
738frequently used recipe since appending a null byte won't work
47b6252e 739if a null byte may be part of the data. Here is an example where both
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740techniques are used: after two null terminated strings with source and
741destination address, a Short Message (to a mobile phone) is sent after
742a length byte:
743
744 my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm );
745
746Unpacking this message can be done with the same template:
747
748 ( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg );
749
47b6252e 750There's a subtle trap lurking in the offing: Adding another field after
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751the Short Message (in variable C<$sm>) is all right when packing, but this
752cannot be unpacked naively:
753
754 # pack a message
755 my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio );
7207e29d 756
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757 # unpack fails - $prio remains undefined!
758 ( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg );
759
760The pack code C<A*> gobbles up all remaining bytes, and C<$prio> remains
761undefined! Before we let disappointment dampen the morale: Perl's got
762the trump card to make this trick too, just a little further up the sleeve.
763Watch this:
764
765 # pack a message: ASCIIZ, ASCIIZ, length/string, byte
766 my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio );
767
768 # unpack
769 ( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg );
770
771Combining two pack codes with a slash (C</>) associates them with a single
772value from the argument list. In C<pack>, the length of the argument is
773taken and packed according to the first code while the argument itself
774is added after being converted with the template code after the slash.
775This saves us the trouble of inserting the C<length> call, but it is
776in C<unpack> where we really score: The value of the length byte marks the
777end of the string to be taken from the buffer. Since this combination
f8b4d74f 778doesn't make sense except when the second pack code isn't C<a*>, C<A*>
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779or C<Z*>, Perl won't let you.
780
781The pack code preceding C</> may be anything that's fit to represent a
782number: All the numeric binary pack codes, and even text codes such as
783C<A4> or C<Z*>:
784
785 # pack/unpack a string preceded by its length in ASCII
786 my $buf = pack( 'A4/A*', "Humpty-Dumpty" );
787 # unpack $buf: '13 Humpty-Dumpty'
788 my $txt = unpack( 'A4/A*', $buf );
789
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790C</> is not implemented in Perls before 5.6, so if your code is required to
791work on older Perls you'll need to C<unpack( 'Z* Z* C')> to get the length,
792then use it to make a new unpack string. For example
793
794 # pack a message: ASCIIZ, ASCIIZ, length, string, byte (5.005 compatible)
795 my $msg = pack( 'Z* Z* C A* C', $src, $dst, length $sm, $sm, $prio );
796
797 # unpack
798 ( undef, undef, $len) = unpack( 'Z* Z* C', $msg );
799 ($src, $dst, $sm, $prio) = unpack ( "Z* Z* x A$len C", $msg );
800
801But that second C<unpack> is rushing ahead. It isn't using a simple literal
802string for the template. So maybe we should introduce...
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803
804=head2 Dynamic Templates
805
806So far, we've seen literals used as templates. If the list of pack
807items doesn't have fixed length, an expression constructing the
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808template is required (whenever, for some reason, C<()*> cannot be used).
809Here's an example: To store named string values in a way that can be
810conveniently parsed by a C program, we create a sequence of names and
811null terminated ASCII strings, with C<=> between the name and the value,
812followed by an additional delimiting null byte. Here's how:
34babc16 813
49704364 814 my $env = pack( '(A*A*Z*)' . keys( %Env ) . 'C',
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815 map( { ( $_, '=', $Env{$_} ) } keys( %Env ) ), 0 );
816
817Let's examine the cogs of this byte mill, one by one. There's the C<map>
818call, creating the items we intend to stuff into the C<$env> buffer:
819to each key (in C<$_>) it adds the C<=> separator and the hash entry value.
820Each triplet is packed with the template code sequence C<A*A*Z*> that
49704364 821is repeated according to the number of keys. (Yes, that's what the C<keys>
fe854a6f 822function returns in scalar context.) To get the very last null byte,
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823we add a C<0> at the end of the C<pack> list, to be packed with C<C>.
824(Attentive readers may have noticed that we could have omitted the 0.)
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825
826For the reverse operation, we'll have to determine the number of items
827in the buffer before we can let C<unpack> rip it apart:
828
47b6252e 829 my $n = $env =~ tr/\0// - 1;
49704364 830 my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) );
34babc16 831
47b6252e 832The C<tr> counts the null bytes. The C<unpack> call returns a list of
47f22e19 833name-value pairs each of which is taken apart in the C<map> block.
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834
835
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836=head2 Counting Repetitions
837
838Rather than storing a sentinel at the end of a data item (or a list of items),
839we could precede the data with a count. Again, we pack keys and values of
840a hash, preceding each with an unsigned short length count, and up front
841we store the number of pairs:
842
843 my $env = pack( 'S(S/A* S/A*)*', scalar keys( %Env ), %Env );
844
845This simplifies the reverse operation as the number of repetitions can be
846unpacked with the C</> code:
847
848 my %env = unpack( 'S/(S/A* S/A*)', $env );
849
850Note that this is one of the rare cases where you cannot use the same
851template for C<pack> and C<unpack> because C<pack> can't determine
852a repeat count for a C<()>-group.
853
854
aa51dd41
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855=head2 Intel HEX
856
857Intel HEX is a file format for representing binary data, mostly for
858programming various chips, as a text file. (See
859L<http://en.wikipedia.org/wiki/.hex> for a detailed description, and
860L<http://en.wikipedia.org/wiki/SREC_(file_format)> for the Motorola
861S-record format, which can be unravelled using the same technique.)
862Each line begins with a colon (':') and is followed by a sequence of
863hexadecimal characters, specifying a byte count I<n> (8 bit),
864an address (16 bit, big endian), a record type (8 bit), I<n> data bytes
865and a checksum (8 bit) computed as the least significant byte of the two's
866complement sum of the preceding bytes. Example: C<:0300300002337A1E>.
867
868The first step of processing such a line is the conversion, to binary,
869of the hexadecimal data, to obtain the four fields, while checking the
870checksum. No surprise here: we'll start with a simple C<pack> call to
871convert everything to binary:
872
873 my $binrec = pack( 'H*', substr( $hexrec, 1 ) );
874
875The resulting byte sequence is most convenient for checking the checksum.
876Don't slow your program down with a for loop adding the C<ord> values
877of this string's bytes - the C<unpack> code C<%> is the thing to use
878for computing the 8-bit sum of all bytes, which must be equal to zero:
879
880 die unless unpack( "%8C*", $binrec ) == 0;
881
882Finally, let's get those four fields. By now, you shouldn't have any
883problems with the first three fields - but how can we use the byte count
884of the data in the first field as a length for the data field? Here
885the codes C<x> and C<X> come to the rescue, as they permit jumping
886back and forth in the string to unpack.
887
888 my( $addr, $type, $data ) = unpack( "x n C X4 C x3 /a", $bin );
889
890Code C<x> skips a byte, since we don't need the count yet. Code C<n> takes
891care of the 16-bit big-endian integer address, and C<C> unpacks the
892record type. Being at offset 4, where the data begins, we need the count.
893C<X4> brings us back to square one, which is the byte at offset 0.
894Now we pick up the count, and zoom forth to offset 4, where we are
895now fully furnished to extract the exact number of data bytes, leaving
896the trailing checksum byte alone.
897
898
899
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900=head1 Packing and Unpacking C Structures
901
902In previous sections we have seen how to pack numbers and character
903strings. If it were not for a couple of snags we could conclude this
904section right away with the terse remark that C structures don't
905contain anything else, and therefore you already know all there is to it.
906Sorry, no: read on, please.
907
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908If you have to deal with a lot of C structures, and don't want to
909hack all your template strings manually, you'll probably want to have
910a look at the CPAN module C<Convert::Binary::C>. Not only can it parse
911your C source directly, but it also has built-in support for all the
912odds and ends described further on in this section.
913
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914=head2 The Alignment Pit
915
916In the consideration of speed against memory requirements the balance
917has been tilted in favor of faster execution. This has influenced the
918way C compilers allocate memory for structures: On architectures
919where a 16-bit or 32-bit operand can be moved faster between places in
920memory, or to or from a CPU register, if it is aligned at an even or
921multiple-of-four or even at a multiple-of eight address, a C compiler
922will give you this speed benefit by stuffing extra bytes into structures.
923If you don't cross the C shoreline this is not likely to cause you any
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924grief (although you should care when you design large data structures,
925or you want your code to be portable between architectures (you do want
926that, don't you?)).
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927
928To see how this affects C<pack> and C<unpack>, we'll compare these two
929C structures:
930
931 typedef struct {
932 char c1;
933 short s;
934 char c2;
935 long l;
936 } gappy_t;
937
938 typedef struct {
939 long l;
940 short s;
941 char c1;
942 char c2;
943 } dense_t;
944
945Typically, a C compiler allocates 12 bytes to a C<gappy_t> variable, but
946requires only 8 bytes for a C<dense_t>. After investigating this further,
947we can draw memory maps, showing where the extra 4 bytes are hidden:
948
949 0 +4 +8 +12
950 +--+--+--+--+--+--+--+--+--+--+--+--+
951 |c1|xx| s |c2|xx|xx|xx| l | xx = fill byte
952 +--+--+--+--+--+--+--+--+--+--+--+--+
953 gappy_t
954
955 0 +4 +8
956 +--+--+--+--+--+--+--+--+
957 | l | h |c1|c2|
958 +--+--+--+--+--+--+--+--+
959 dense_t
960
961And that's where the first quirk strikes: C<pack> and C<unpack>
962templates have to be stuffed with C<x> codes to get those extra fill bytes.
963
964The natural question: "Why can't Perl compensate for the gaps?" warrants
965an answer. One good reason is that C compilers might provide (non-ANSI)
966extensions permitting all sorts of fancy control over the way structures
967are aligned, even at the level of an individual structure field. And, if
968this were not enough, there is an insidious thing called C<union> where
969the amount of fill bytes cannot be derived from the alignment of the next
970item alone.
971
972OK, so let's bite the bullet. Here's one way to get the alignment right
973by inserting template codes C<x>, which don't take a corresponding item
974from the list:
975
976 my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );
977
978Note the C<!> after C<l>: We want to make sure that we pack a long
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979integer as it is compiled by our C compiler. And even now, it will only
980work for the platforms where the compiler aligns things as above.
981And somebody somewhere has a platform where it doesn't.
982[Probably a Cray, where C<short>s, C<int>s and C<long>s are all 8 bytes. :-)]
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983
984Counting bytes and watching alignments in lengthy structures is bound to
985be a drag. Isn't there a way we can create the template with a simple
986program? Here's a C program that does the trick:
987
988 #include <stdio.h>
989 #include <stddef.h>
990
991 typedef struct {
992 char fc1;
993 short fs;
994 char fc2;
995 long fl;
996 } gappy_t;
997
998 #define Pt(struct,field,tchar) \
999 printf( "@%d%s ", offsetof(struct,field), # tchar );
1000
d832b8f6 1001 int main() {
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1002 Pt( gappy_t, fc1, c );
1003 Pt( gappy_t, fs, s! );
1004 Pt( gappy_t, fc2, c );
1005 Pt( gappy_t, fl, l! );
1006 printf( "\n" );
1007 }
1008
1009The output line can be used as a template in a C<pack> or C<unpack> call:
1010
1011 my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l );
1012
1013Gee, yet another template code - as if we hadn't plenty. But
1014C<@> saves our day by enabling us to specify the offset from the beginning
1015of the pack buffer to the next item: This is just the value
1016the C<offsetof> macro (defined in C<E<lt>stddef.hE<gt>>) returns when
1017given a C<struct> type and one of its field names ("member-designator" in
1018C standardese).
1019
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1020Neither using offsets nor adding C<x>'s to bridge the gaps is satisfactory.
1021(Just imagine what happens if the structure changes.) What we really need
1022is a way of saying "skip as many bytes as required to the next multiple of N".
1023In fluent Templatese, you say this with C<x!N> where N is replaced by the
1024appropriate value. Here's the next version of our struct packaging:
1025
1026 my $gappy = pack( 'c x!2 s c x!4 l!', $c1, $s, $c2, $l );
1027
1028That's certainly better, but we still have to know how long all the
1029integers are, and portability is far away. Rather than C<2>,
1030for instance, we want to say "however long a short is". But this can be
1031done by enclosing the appropriate pack code in brackets: C<[s]>. So, here's
1032the very best we can do:
1033
1034 my $gappy = pack( 'c x![s] s c x![l!] l!', $c1, $s, $c2, $l );
1035
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1037=head2 Dealing with Endian-ness
1038
1039Now, imagine that we want to pack the data for a machine with a
1040different byte-order. First, we'll have to figure out how big the data
1041types on the target machine really are. Let's assume that the longs are
104232 bits wide and the shorts are 16 bits wide. You can then rewrite the
1043template as:
1044
1045 my $gappy = pack( 'c x![s] s c x![l] l', $c1, $s, $c2, $l );
1046
1047If the target machine is little-endian, we could write:
1048
1049 my $gappy = pack( 'c x![s] s< c x![l] l<', $c1, $s, $c2, $l );
1050
1051This forces the short and the long members to be little-endian, and is
1052just fine if you don't have too many struct members. But we could also
1053use the byte-order modifier on a group and write the following:
1054
1055 my $gappy = pack( '( c x![s] s c x![l] l )<', $c1, $s, $c2, $l );
1056
1057This is not as short as before, but it makes it more obvious that we
1058intend to have little-endian byte-order for a whole group, not only
1059for individual template codes. It can also be more readable and easier
1060to maintain.
1061
1062
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1063=head2 Alignment, Take 2
1064
1065I'm afraid that we're not quite through with the alignment catch yet. The
1066hydra raises another ugly head when you pack arrays of structures:
1067
1068 typedef struct {
1069 short count;
1070 char glyph;
1071 } cell_t;
1072
1073 typedef cell_t buffer_t[BUFLEN];
1074
1075Where's the catch? Padding is neither required before the first field C<count>,
1076nor between this and the next field C<glyph>, so why can't we simply pack
1077like this:
1078
1079 # something goes wrong here:
1080 pack( 's!a' x @buffer,
1081 map{ ( $_->{count}, $_->{glyph} ) } @buffer );
1082
1083This packs C<3*@buffer> bytes, but it turns out that the size of
1084C<buffer_t> is four times C<BUFLEN>! The moral of the story is that
1085the required alignment of a structure or array is propagated to the
1086next higher level where we have to consider padding I<at the end>
1087of each component as well. Thus the correct template is:
1088
1089 pack( 's!ax' x @buffer,
1090 map{ ( $_->{count}, $_->{glyph} ) } @buffer );
1091
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1092=head2 Alignment, Take 3
1093
1094And even if you take all the above into account, ANSI still lets this:
1095
1096 typedef struct {
1097 char foo[2];
1098 } foo_t;
34babc16 1099
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1100vary in size. The alignment constraint of the structure can be greater than
1101any of its elements. [And if you think that this doesn't affect anything
1102common, dismember the next cellphone that you see. Many have ARM cores, and
1103the ARM structure rules make C<sizeof (foo_t)> == 4]
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1104
1105=head2 Pointers for How to Use Them
1106
1107The title of this section indicates the second problem you may run into
1108sooner or later when you pack C structures. If the function you intend
1109to call expects a, say, C<void *> value, you I<cannot> simply take
1110a reference to a Perl variable. (Although that value certainly is a
1111memory address, it's not the address where the variable's contents are
1112stored.)
1113
1114Template code C<P> promises to pack a "pointer to a fixed length string".
1115Isn't this what we want? Let's try:
1116
1117 # allocate some storage and pack a pointer to it
1118 my $memory = "\x00" x $size;
1119 my $memptr = pack( 'P', $memory );
1120
1121But wait: doesn't C<pack> just return a sequence of bytes? How can we pass this
1122string of bytes to some C code expecting a pointer which is, after all,
1123nothing but a number? The answer is simple: We have to obtain the numeric
1124address from the bytes returned by C<pack>.
1125
1126 my $ptr = unpack( 'L!', $memptr );
1127
1128Obviously this assumes that it is possible to typecast a pointer
1129to an unsigned long and vice versa, which frequently works but should not
1130be taken as a universal law. - Now that we have this pointer the next question
1131is: How can we put it to good use? We need a call to some C function
1132where a pointer is expected. The read(2) system call comes to mind:
1133
1134 ssize_t read(int fd, void *buf, size_t count);
1135
1136After reading L<perlfunc> explaining how to use C<syscall> we can write
1137this Perl function copying a file to standard output:
1138
1139 require 'syscall.ph';
1140 sub cat($){
1141 my $path = shift();
1142 my $size = -s $path;
1143 my $memory = "\x00" x $size; # allocate some memory
1144 my $ptr = unpack( 'L', pack( 'P', $memory ) );
1145 open( F, $path ) || die( "$path: cannot open ($!)\n" );
1146 my $fd = fileno(F);
1147 my $res = syscall( &SYS_read, fileno(F), $ptr, $size );
1148 print $memory;
1149 close( F );
1150 }
1151
1152This is neither a specimen of simplicity nor a paragon of portability but
1153it illustrates the point: We are able to sneak behind the scenes and
1154access Perl's otherwise well-guarded memory! (Important note: Perl's
1155C<syscall> does I<not> require you to construct pointers in this roundabout
1156way. You simply pass a string variable, and Perl forwards the address.)
1157
1158How does C<unpack> with C<P> work? Imagine some pointer in the buffer
1159about to be unpacked: If it isn't the null pointer (which will smartly
1160produce the C<undef> value) we have a start address - but then what?
1161Perl has no way of knowing how long this "fixed length string" is, so
1162it's up to you to specify the actual size as an explicit length after C<P>.
1163
1164 my $mem = "abcdefghijklmn";
1165 print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"
1166
1167As a consequence, C<pack> ignores any number or C<*> after C<P>.
1168
1169
1170Now that we have seen C<P> at work, we might as well give C<p> a whirl.
1171Why do we need a second template code for packing pointers at all? The
1172answer lies behind the simple fact that an C<unpack> with C<p> promises
1173a null-terminated string starting at the address taken from the buffer,
1174and that implies a length for the data item to be returned:
1175
1176 my $buf = pack( 'p', "abc\x00efhijklmn" );
1177 print unpack( 'p', $buf ); # prints "abc"
1178
1179
1180
1181Albeit this is apt to be confusing: As a consequence of the length being
1182implied by the string's length, a number after pack code C<p> is a repeat
1183count, not a length as after C<P>.
1184
1185
1186Using C<pack(..., $x)> with C<P> or C<p> to get the address where C<$x> is
1187actually stored must be used with circumspection. Perl's internal machinery
1188considers the relation between a variable and that address as its very own
1189private matter and doesn't really care that we have obtained a copy. Therefore:
1190
1191=over 4
1192
1193=item *
1194
1195Do not use C<pack> with C<p> or C<P> to obtain the address of variable
1196that's bound to go out of scope (and thereby freeing its memory) before you
1197are done with using the memory at that address.
1198
1199=item *
1200
1201Be very careful with Perl operations that change the value of the
1202variable. Appending something to the variable, for instance, might require
1203reallocation of its storage, leaving you with a pointer into no-man's land.
1204
1205=item *
1206
1207Don't think that you can get the address of a Perl variable
1208when it is stored as an integer or double number! C<pack('P', $x)> will
1209force the variable's internal representation to string, just as if you
1210had written something like C<$x .= ''>.
1211
1212=back
1213
1214It's safe, however, to P- or p-pack a string literal, because Perl simply
1215allocates an anonymous variable.
1216
1217
1218
1219=head1 Pack Recipes
1220
1221Here are a collection of (possibly) useful canned recipes for C<pack>
1222and C<unpack>:
1223
1224 # Convert IP address for socket functions
1225 pack( "C4", split /\./, "123.4.5.6" );
1226
1227 # Count the bits in a chunk of memory (e.g. a select vector)
1228 unpack( '%32b*', $mask );
1229
1230 # Determine the endianness of your system
1231 $is_little_endian = unpack( 'c', pack( 's', 1 ) );
1232 $is_big_endian = unpack( 'xc', pack( 's', 1 ) );
1233
1234 # Determine the number of bits in a native integer
1235 $bits = unpack( '%32I!', ~0 );
1236
1237 # Prepare argument for the nanosleep system call
1238 my $timespec = pack( 'L!L!', $secs, $nanosecs );
1239
f8b4d74f
WL
1240For a simple memory dump we unpack some bytes into just as
1241many pairs of hex digits, and use C<map> to handle the traditional
1242spacing - 16 bytes to a line:
1243
34babc16 1244 my $i;
49704364
LW
1245 print map( ++$i % 16 ? "$_ " : "$_\n",
1246 unpack( 'H2' x length( $mem ), $mem ) ),
f8b4d74f 1247 length( $mem ) % 16 ? "\n" : '';
34babc16
JH
1248
1249
47f22e19
WL
1250=head1 Funnies Section
1251
1252 # Pulling digits out of nowhere...
1253 print unpack( 'C', pack( 'x' ) ),
1254 unpack( '%B*', pack( 'A' ) ),
1255 unpack( 'H', pack( 'A' ) ),
1256 unpack( 'A', unpack( 'C', pack( 'A' ) ) ), "\n";
1257
1258 # One for the road ;-)
1259 my $advice = pack( 'all u can in a van' );
1260
1261
34babc16
JH
1262=head1 Authors
1263
1264Simon Cozens and Wolfgang Laun.
1265