1PERLPACKTUT(1) Perl Programmers Reference Guide PERLPACKTUT(1)
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6 perlpacktut - tutorial on "pack" and "unpack"
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9 "pack" and "unpack" are two functions for transforming data according
10 to a user-defined template, between the guarded way Perl stores values
11 and some well-defined representation as might be required in the
12 environment of a Perl program. Unfortunately, they're also two of the
13 most misunderstood and most often overlooked functions that Perl
14 provides. This tutorial will demystify them for you.
15
17 Most programming languages don't shelter the memory where variables are
18 stored. In C, for instance, you can take the address of some variable,
19 and the "sizeof" operator tells you how many bytes are allocated to the
20 variable. Using the address and the size, you may access the storage to
21 your heart's content.
22 In Perl, you just can't access memory at random, but the structural and
24 representational conversion provided by "pack" and "unpack" is an
25 excellent alternative. The "pack" function converts values to a byte
26 sequence containing representations according to a given specification,
27 the so-called "template" argument. "unpack" is the reverse process,
28 deriving some values from the contents of a string of bytes. (Be
29 cautioned, however, that not all that has been packed together can be
30 neatly unpacked - a very common experience as seasoned travellers are
31 likely to confirm.)
32 Why, you may ask, would you need a chunk of memory containing some
34 values in binary representation? One good reason is input and output
35 accessing some file, a device, or a network connection, whereby this
36 binary representation is either forced on you or will give you some
37 benefit in processing. Another cause is passing data to some system
38 call that is not available as a Perl function: "syscall" requires you
39 to provide parameters stored in the way it happens in a C program. Even
40 text processing (as shown in the next section) may be simplified with
41 judicious usage of these two functions.
42 To see how (un)packing works, we'll start with a simple template code
44 where the conversion is in low gear: between the contents of a byte
45 sequence and a string of hexadecimal digits. Let's use "unpack", since
46 this is likely to remind you of a dump program, or some desperate last
47 message unfortunate programs are wont to throw at you before they
48 expire into the wild blue yonder. Assuming that the variable $mem holds
49 a sequence of bytes that we'd like to inspect without assuming anything
50 about its meaning, we can write
51 my( $hex ) = unpack( 'H*', $mem );
53 print "$hex\n";
54 whereupon we might see something like this, with each pair of hex
56 digits corresponding to a byte:
57 41204d414e204120504c414e20412043414e414c2050414e414d41
59 What was in this chunk of memory? Numbers, characters, or a mixture of
61 both? Assuming that we're on a computer where ASCII (or some similar)
62 encoding is used: hexadecimal values in the range 0x40 - 0x5A indicate
63 an uppercase letter, and 0x20 encodes a space. So we might assume it is
64 a piece of text, which some are able to read like a tabloid; but others
65 will have to get hold of an ASCII table and relive that firstgrader
66 feeling. Not caring too much about which way to read this, we note that
67 "unpack" with the template code "H" converts the contents of a sequence
68 of bytes into the customary hexadecimal notation. Since "a sequence of"
69 is a pretty vague indication of quantity, "H" has been defined to
70 convert just a single hexadecimal digit unless it is followed by a
71 repeat count. An asterisk for the repeat count means to use whatever
72 remains.
73 The inverse operation - packing byte contents from a string of
75 hexadecimal digits - is just as easily written. For instance:
76 my $s = pack( 'H2' x 10, 30..39 );
78 print "$s\n";
79 Since we feed a list of ten 2-digit hexadecimal strings to "pack", the
81 pack template should contain ten pack codes. If this is run on a
82 computer with ASCII character coding, it will print 0123456789.
83
85 Let's suppose you've got to read in a data file like this:
86 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 How do we do it? You might think first to use "split"; however, since
93 "split" collapses blank fields, you'll never know whether a record was
94 income or expenditure. Oops. Well, you could always use "substr":
95 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 It's not really a barrel of laughs, is it? In fact, it's worse than it
105 may seem; the eagle-eyed may notice that the first field should only be
106 10 characters wide, and the error has propagated right through the
107 other numbers - which we've had to count by hand. So it's error-prone
108 as well as horribly unfriendly.
109 Or maybe we could use regular expressions:
111 while (<>) {
113 my($date, $desc, $income, $expend) =
114 m|(\d\d/\d\d/\d{4}) (.{27}) (.{7})(.*)|;
115 ...
116 }
117 Urgh. Well, it's a bit better, but - well, would you want to maintain
119 that?
120 Hey, isn't Perl supposed to make this sort of thing easy? Well, it
122 does, if you use the right tools. "pack" and "unpack" are designed to
123 help you out when dealing with fixed-width data like the above. Let's
124 have a look at a solution with "unpack":
125 while (<>) {
127 my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
128 ...
129 }
130 That looks a bit nicer; but we've got to take apart that weird
132 template. Where did I pull that out of?
133 OK, let's have a look at some of our data again; in fact, we'll include
135 the headers, and a handy ruler so we can keep track of where we are.
136 1 2 3 4 5
138 1234567890123456789012345678901234567890123456789012345678
139 Date |Description | Income|Expenditure
140 01/28/2001 Flea spray 24.99
141 01/29/2001 Camel rides to tourists 235.00
142 From this, we can see that the date column stretches from column 1 to
144 column 10 - ten characters wide. The "pack"-ese for "character" is "A",
145 and ten of them are "A10". So if we just wanted to extract the dates,
146 we could say this:
147 my($date) = unpack("A10", $_);
149 OK, what's next? Between the date and the description is a blank
151 column; we want to skip over that. The "x" template means "skip
152 forward", so we want one of those. Next, we have another batch of
153 characters, from 12 to 38. That's 27 more characters, hence "A27".
154 (Don't make the fencepost error - there are 27 characters between 12
155 and 38, not 26. Count 'em!)
156 Now we skip another character and pick up the next 7 characters:
158 my($date,$description,$income) = unpack("A10xA27xA7", $_);
160 Now comes the clever bit. Lines in our ledger which are just income and
162 not expenditure might end at column 46. Hence, we don't want to tell
163 our "unpack" pattern that we need to find another 12 characters; we'll
164 just say "if there's anything left, take it". As you might guess from
165 regular expressions, that's what the "*" means: "use everything
166 remaining".
167 · Be warned, though, that unlike regular expressions, if the "unpack"
169 template doesn't match the incoming data, Perl will scream and die.
170 Hence, putting it all together:
172 my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_);
174 Now, that's our data parsed. I suppose what we might want to do now is
176 total up our income and expenditure, and add another line to the end of
177 our ledger - in the same format - saying how much we've brought in and
178 how much we've spent:
179 while (<>) {
181 my($date, $desc, $income, $expend) = unpack("A10xA27xA7xA*", $_);
182 $tot_income += $income;
183 $tot_expend += $expend;
184 }
185 $tot_income = sprintf("%.2f", $tot_income); # Get them into
187 $tot_expend = sprintf("%.2f", $tot_expend); # "financial" format
188 $date = POSIX::strftime("%m/%d/%Y", localtime);
190 # OK, let's go:
192 print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend);
194 Oh, hmm. That didn't quite work. Let's see what happened:
196 01/24/2001 Ahmed's Camel Emporium 1147.99
198 01/28/2001 Flea spray 24.99
199 01/29/2001 Camel rides to tourists 1235.00
200 03/23/2001Totals 1235.001172.98
201 OK, it's a start, but what happened to the spaces? We put "x", didn't
203 we? Shouldn't it skip forward? Let's look at what "pack" in perlfunc
204 says:
205 x A null byte.
207 Urgh. No wonder. There's a big difference between "a null byte",
209 character zero, and "a space", character 32. Perl's put something
210 between the date and the description - but unfortunately, we can't see
211 it!
212 What we actually need to do is expand the width of the fields. The "A"
214 format pads any non-existent characters with spaces, so we can use the
215 additional spaces to line up our fields, like this:
216 print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
218 (Note that you can put spaces in the template to make it more readable,
220 but they don't translate to spaces in the output.) Here's what we got
221 this time:
222 01/24/2001 Ahmed's Camel Emporium 1147.99
224 01/28/2001 Flea spray 24.99
225 01/29/2001 Camel rides to tourists 1235.00
226 03/23/2001 Totals 1235.00 1172.98
227 That's a bit better, but we still have that last column which needs to
229 be moved further over. There's an easy way to fix this up:
230 unfortunately, we can't get "pack" to right-justify our fields, but we
231 can get "sprintf" to do it:
232 $tot_income = sprintf("%.2f", $tot_income);
234 $tot_expend = sprintf("%12.2f", $tot_expend);
235 $date = POSIX::strftime("%m/%d/%Y", localtime);
236 print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
237 This time we get the right answer:
239 01/28/2001 Flea spray 24.99
241 01/29/2001 Camel rides to tourists 1235.00
242 03/23/2001 Totals 1235.00 1172.98
243 So that's how we consume and produce fixed-width data. Let's recap what
245 we've seen of "pack" and "unpack" so far:
246 · Use "pack" to go from several pieces of data to one fixed-width
248 version; use "unpack" to turn a fixed-width-format string into
249 several pieces of data.
250 · The pack format "A" means "any character"; if you're "pack"ing and
252 you've run out of things to pack, "pack" will fill the rest up with
253 spaces.
254 · "x" means "skip a byte" when "unpack"ing; when "pack"ing, it means
256 "introduce a null byte" - that's probably not what you mean if
257 you're dealing with plain text.
258 · You can follow the formats with numbers to say how many characters
260 should be affected by that format: "A12" means "take 12 characters";
261 "x6" means "skip 6 bytes" or "character 0, 6 times".
262 · Instead of a number, you can use "*" to mean "consume everything
264 else left".
265 Warning: when packing multiple pieces of data, "*" only means
267 "consume all of the current piece of data". That's to say
268 pack("A*A*", $one, $two)
270 packs all of $one into the first "A*" and then all of $two into the
272 second. This is a general principle: each format character
273 corresponds to one piece of data to be "pack"ed.
274
276 So much for textual data. Let's get onto the meaty stuff that "pack"
277 and "unpack" are best at: handling binary formats for numbers. There
278 is, of course, not just one binary format - life would be too simple -
279 but Perl will do all the finicky labor for you.
280 Integers
282 Packing and unpacking numbers implies conversion to and from some
283 specific binary representation. Leaving floating point numbers aside
284 for the moment, the salient properties of any such representation are:
285 · the number of bytes used for storing the integer,
287 · whether the contents are interpreted as a signed or unsigned
289 number,
290 · the byte ordering: whether the first byte is the least or most
292 significant byte (or: little-endian or big-endian, respectively).
293 So, for instance, to pack 20302 to a signed 16 bit integer in your
295 computer's representation you write
296 my $ps = pack( 's', 20302 );
298 Again, the result is a string, now containing 2 bytes. If you print
300 this string (which is, generally, not recommended) you might see "ON"
301 or "NO" (depending on your system's byte ordering) - or something
302 entirely different if your computer doesn't use ASCII character
303 encoding. Unpacking $ps with the same template returns the original
304 integer value:
305 my( $s ) = unpack( 's', $ps );
307 This is true for all numeric template codes. But don't expect miracles:
309 if the packed value exceeds the allotted byte capacity, high order bits
310 are silently discarded, and unpack certainly won't be able to pull them
311 back out of some magic hat. And, when you pack using a signed template
312 code such as "s", an excess value may result in the sign bit getting
313 set, and unpacking this will smartly return a negative value.
314 16 bits won't get you too far with integers, but there is "l" and "L"
316 for signed and unsigned 32-bit integers. And if this is not enough and
317 your system supports 64 bit integers you can push the limits much
318 closer to infinity with pack codes "q" and "Q". A notable exception is
319 provided by pack codes "i" and "I" for signed and unsigned integers of
320 the "local custom" variety: Such an integer will take up as many bytes
321 as a local C compiler returns for "sizeof(int)", but it'll use at least
322 32 bits.
323 Each of the integer pack codes "sSlLqQ" results in a fixed number of
325 bytes, no matter where you execute your program. This may be useful for
326 some applications, but it does not provide for a portable way to pass
327 data structures between Perl and C programs (bound to happen when you
328 call XS extensions or the Perl function "syscall"), or when you read or
329 write binary files. What you'll need in this case are template codes
330 that depend on what your local C compiler compiles when you code
331 "short" or "unsigned long", for instance. These codes and their
332 corresponding byte lengths are shown in the table below. Since the C
333 standard leaves much leeway with respect to the relative sizes of these
334 data types, actual values may vary, and that's why the values are given
335 as expressions in C and Perl. (If you'd like to use values from %Config
336 in your program you have to import it with "use Config".)
337 signed unsigned byte length in C byte length in Perl
339 s! S! sizeof(short) $Config{shortsize}
340 i! I! sizeof(int) $Config{intsize}
341 l! L! sizeof(long) $Config{longsize}
342 q! Q! sizeof(long long) $Config{longlongsize}
343 The "i!" and "I!" codes aren't different from "i" and "I"; they are
345 tolerated for completeness' sake.
346 Unpacking a Stack Frame
348 Requesting a particular byte ordering may be necessary when you work
349 with binary data coming from some specific architecture whereas your
350 program could run on a totally different system. As an example, assume
351 you have 24 bytes containing a stack frame as it happens on an Intel
352 8086:
353 +---------+ +----+----+ +---------+
355 TOS: | IP | TOS+4:| FL | FH | FLAGS TOS+14:| SI |
356 +---------+ +----+----+ +---------+
357 | CS | | AL | AH | AX | DI |
358 +---------+ +----+----+ +---------+
359 | BL | BH | BX | BP |
360 +----+----+ +---------+
361 | CL | CH | CX | DS |
362 +----+----+ +---------+
363 | DL | DH | DX | ES |
364 +----+----+ +---------+
365 First, we note that this time-honored 16-bit CPU uses little-endian
367 order, and that's why the low order byte is stored at the lower
368 address. To unpack such a (unsigned) short we'll have to use code "v".
369 A repeat count unpacks all 12 shorts:
370 my( $ip, $cs, $flags, $ax, $bx, $cd, $dx, $si, $di, $bp, $ds, $es ) =
372 unpack( 'v12', $frame );
373 Alternatively, we could have used "C" to unpack the individually
375 accessible byte registers FL, FH, AL, AH, etc.:
376 my( $fl, $fh, $al, $ah, $bl, $bh, $cl, $ch, $dl, $dh ) =
378 unpack( 'C10', substr( $frame, 4, 10 ) );
379 It would be nice if we could do this in one fell swoop: unpack a short,
381 back up a little, and then unpack 2 bytes. Since Perl is nice, it
382 proffers the template code "X" to back up one byte. Putting this all
383 together, we may now write:
384 my( $ip, $cs,
386 $flags,$fl,$fh,
387 $ax,$al,$ah, $bx,$bl,$bh, $cx,$cl,$ch, $dx,$dl,$dh,
388 $si, $di, $bp, $ds, $es ) =
389 unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame );
390 (The clumsy construction of the template can be avoided - just read
392 on!)
393 We've taken some pains to construct the template so that it matches the
395 contents of our frame buffer. Otherwise we'd either get undefined
396 values, or "unpack" could not unpack all. If "pack" runs out of items,
397 it will supply null strings (which are coerced into zeroes whenever the
398 pack code says so).
399 How to Eat an Egg on a Net
401 The pack code for big-endian (high order byte at the lowest address) is
402 "n" for 16 bit and "N" for 32 bit integers. You use these codes if you
403 know that your data comes from a compliant architecture, but,
404 surprisingly enough, you should also use these pack codes if you
405 exchange binary data, across the network, with some system that you
406 know next to nothing about. The simple reason is that this order has
407 been chosen as the network order, and all standard-fearing programs
408 ought to follow this convention. (This is, of course, a stern backing
409 for one of the Lilliputian parties and may well influence the political
410 development there.) So, if the protocol expects you to send a message
411 by sending the length first, followed by just so many bytes, you could
412 write:
413 my $buf = pack( 'N', length( $msg ) ) . $msg;
415 or even:
417 my $buf = pack( 'NA*', length( $msg ), $msg );
419 and pass $buf to your send routine. Some protocols demand that the
421 count should include the length of the count itself: then just add 4 to
422 the data length. (But make sure to read "Lengths and Widths" before you
423 really code this!)
424 Byte-order modifiers
426 In the previous sections we've learned how to use "n", "N", "v" and "V"
427 to pack and unpack integers with big- or little-endian byte-order.
428 While this is nice, it's still rather limited because it leaves out all
429 kinds of signed integers as well as 64-bit integers. For example, if
430 you wanted to unpack a sequence of signed big-endian 16-bit integers in
431 a platform-independent way, you would have to write:
432 my @data = unpack 's*', pack 'S*', unpack 'n*', $buf;
434 This is ugly. As of Perl 5.9.2, there's a much nicer way to express
436 your desire for a certain byte-order: the ">" and "<" modifiers. ">"
437 is the big-endian modifier, while "<" is the little-endian modifier.
438 Using them, we could rewrite the above code as:
439 my @data = unpack 's>*', $buf;
441 As you can see, the "big end" of the arrow touches the "s", which is a
443 nice way to remember that ">" is the big-endian modifier. The same
444 obviously works for "<", where the "little end" touches the code.
445 You will probably find these modifiers even more useful if you have to
447 deal with big- or little-endian C structures. Be sure to read "Packing
448 and Unpacking C Structures" for more on that.
449 Floating point Numbers
451 For packing floating point numbers you have the choice between the pack
452 codes "f", "d", "F" and "D". "f" and "d" pack into (or unpack from)
453 single-precision or double-precision representation as it is provided
454 by your system. If your systems supports it, "D" can be used to pack
455 and unpack extended-precision floating point values ("long double"),
456 which can offer even more resolution than "f" or "d". "F" packs an
457 "NV", which is the floating point type used by Perl internally. (There
458 is no such thing as a network representation for reals, so if you want
459 to send your real numbers across computer boundaries, you'd better
460 stick to ASCII representation, unless you're absolutely sure what's on
461 the other end of the line. For the even more adventuresome, you can use
462 the byte-order modifiers from the previous section also on floating
463 point codes.)
464
466 Bit Strings
467 Bits are the atoms in the memory world. Access to individual bits may
468 have to be used either as a last resort or because it is the most
469 convenient way to handle your data. Bit string (un)packing converts
470 between strings containing a series of 0 and 1 characters and a
471 sequence of bytes each containing a group of 8 bits. This is almost as
472 simple as it sounds, except that there are two ways the contents of a
473 byte may be written as a bit string. Let's have a look at an annotated
474 byte:
475 7 6 5 4 3 2 1 0
477 +-----------------+
478 | 1 0 0 0 1 1 0 0 |
479 +-----------------+
480 MSB LSB
481 It's egg-eating all over again: Some think that as a bit string this
483 should be written "10001100" i.e. beginning with the most significant
484 bit, others insist on "00110001". Well, Perl isn't biased, so that's
485 why we have two bit string codes:
486 $byte = pack( 'B8', '10001100' ); # start with MSB
488 $byte = pack( 'b8', '00110001' ); # start with LSB
489 It is not possible to pack or unpack bit fields - just integral bytes.
491 "pack" always starts at the next byte boundary and "rounds up" to the
492 next multiple of 8 by adding zero bits as required. (If you do want bit
493 fields, there is "vec" in perlfunc. Or you could implement bit field
494 handling at the character string level, using split, substr, and
495 concatenation on unpacked bit strings.)
496 To illustrate unpacking for bit strings, we'll decompose a simple
498 status register (a "-" stands for a "reserved" bit):
499 +-----------------+-----------------+
501 | S Z - A - P - C | - - - - O D I T |
502 +-----------------+-----------------+
503 MSB LSB MSB LSB
504 Converting these two bytes to a string can be done with the unpack
506 template 'b16'. To obtain the individual bit values from the bit string
507 we use "split" with the "empty" separator pattern which dissects into
508 individual characters. Bit values from the "reserved" positions are
509 simply assigned to "undef", a convenient notation for "I don't care
510 where this goes".
511 ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
513 $trace, $interrupt, $direction, $overflow) =
514 split( //, unpack( 'b16', $status ) );
515 We could have used an unpack template 'b12' just as well, since the
517 last 4 bits can be ignored anyway.
518 Uuencoding
520 Another odd-man-out in the template alphabet is "u", which packs an
521 "uuencoded string". ("uu" is short for Unix-to-Unix.) Chances are that
522 you won't ever need this encoding technique which was invented to
523 overcome the shortcomings of old-fashioned transmission mediums that do
524 not support other than simple ASCII data. The essential recipe is
525 simple: Take three bytes, or 24 bits. Split them into 4 six-packs,
526 adding a space (0x20) to each. Repeat until all of the data is blended.
527 Fold groups of 4 bytes into lines no longer than 60 and garnish them in
528 front with the original byte count (incremented by 0x20) and a "\n" at
529 the end. - The "pack" chef will prepare this for you, a la minute, when
530 you select pack code "u" on the menu:
531 my $uubuf = pack( 'u', $bindat );
533 A repeat count after "u" sets the number of bytes to put into an
535 uuencoded line, which is the maximum of 45 by default, but could be set
536 to some (smaller) integer multiple of three. "unpack" simply ignores
537 the repeat count.
538 Doing Sums
540 An even stranger template code is "%"<number>. First, because it's used
541 as a prefix to some other template code. Second, because it cannot be
542 used in "pack" at all, and third, in "unpack", doesn't return the data
543 as defined by the template code it precedes. Instead it'll give you an
544 integer of number bits that is computed from the data value by doing
545 sums. For numeric unpack codes, no big feat is achieved:
546 my $buf = pack( 'iii', 100, 20, 3 );
548 print unpack( '%32i3', $buf ), "\n"; # prints 123
549 For string values, "%" returns the sum of the byte values saving you
551 the trouble of a sum loop with "substr" and "ord":
552 print unpack( '%32A*', "\x01\x10" ), "\n"; # prints 17
554 Although the "%" code is documented as returning a "checksum": don't
556 put your trust in such values! Even when applied to a small number of
557 bytes, they won't guarantee a noticeable Hamming distance.
558 In connection with "b" or "B", "%" simply adds bits, and this can be
560 put to good use to count set bits efficiently:
561 my $bitcount = unpack( '%32b*', $mask );
563 And an even parity bit can be determined like this:
565 my $evenparity = unpack( '%1b*', $mask );
567 Unicode
569 Unicode is a character set that can represent most characters in most
570 of the world's languages, providing room for over one million different
571 characters. Unicode 3.1 specifies 94,140 characters: The Basic Latin
572 characters are assigned to the numbers 0 - 127. The Latin-1 Supplement
573 with characters that are used in several European languages is in the
574 next range, up to 255. After some more Latin extensions we find the
575 character sets from languages using non-Roman alphabets, interspersed
576 with a variety of symbol sets such as currency symbols, Zapf Dingbats
577 or Braille. (You might want to visit <http://www.unicode.org/> for a
578 look at some of them - my personal favourites are Telugu and Kannada.)
579 The Unicode character sets associates characters with integers.
581 Encoding these numbers in an equal number of bytes would more than
582 double the requirements for storing texts written in Latin alphabets.
583 The UTF-8 encoding avoids this by storing the most common (from a
584 western point of view) characters in a single byte while encoding the
585 rarer ones in three or more bytes.
586 Perl uses UTF-8, internally, for most Unicode strings.
588 So what has this got to do with "pack"? Well, if you want to compose a
590 Unicode string (that is internally encoded as UTF-8), you can do so by
591 using template code "U". As an example, let's produce the Euro currency
592 symbol (code number 0x20AC):
593 $UTF8{Euro} = pack( 'U', 0x20AC );
595 # Equivalent to: $UTF8{Euro} = "\x{20ac}";
596 Inspecting $UTF8{Euro} shows that it contains 3 bytes: "\xe2\x82\xac".
598 However, it contains only 1 character, number 0x20AC. The round trip
599 can be completed with "unpack":
600 $Unicode{Euro} = unpack( 'U', $UTF8{Euro} );
602 Unpacking using the "U" template code also works on UTF-8 encoded byte
604 strings.
605 Usually you'll want to pack or unpack UTF-8 strings:
607 # pack and unpack the Hebrew alphabet
609 my $alefbet = pack( 'U*', 0x05d0..0x05ea );
610 my @hebrew = unpack( 'U*', $utf );
611 Please note: in the general case, you're better off using
613 Encode::decode_utf8 to decode a UTF-8 encoded byte string to a Perl
614 Unicode string, and Encode::encode_utf8 to encode a Perl Unicode string
615 to UTF-8 bytes. These functions provide means of handling invalid byte
616 sequences and generally have a friendlier interface.
617 Another Portable Binary Encoding
619 The pack code "w" has been added to support a portable binary data
620 encoding scheme that goes way beyond simple integers. (Details can be
621 found at <http://Casbah.org/>, the Scarab project.) A BER (Binary
622 Encoded Representation) compressed unsigned integer stores base 128
623 digits, most significant digit first, with as few digits as possible.
624 Bit eight (the high bit) is set on each byte except the last. There is
625 no size limit to BER encoding, but Perl won't go to extremes.
626 my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 );
628 A hex dump of $berbuf, with spaces inserted at the right places, shows
630 01 8100 8101 81807F. Since the last byte is always less than 128,
631 "unpack" knows where to stop.
632
634 Prior to Perl 5.8, repetitions of templates had to be made by
635 "x"-multiplication of template strings. Now there is a better way as we
636 may use the pack codes "(" and ")" combined with a repeat count. The
637 "unpack" template from the Stack Frame example can simply be written
638 like this:
639 unpack( 'v2 (vXXCC)5 v5', $frame )
641 Let's explore this feature a little more. We'll begin with the
643 equivalent of
644 join( '', map( substr( $_, 0, 1 ), @str ) )
646 which returns a string consisting of the first character from each
648 string. Using pack, we can write
649 pack( '(A)'.@str, @str )
651 or, because a repeat count "*" means "repeat as often as required",
653 simply
654 pack( '(A)*', @str )
656 (Note that the template "A*" would only have packed $str[0] in full
658 length.)
659 To pack dates stored as triplets ( day, month, year ) in an array
661 @dates into a sequence of byte, byte, short integer we can write
662 $pd = pack( '(CCS)*', map( @$_, @dates ) );
664 To swap pairs of characters in a string (with even length) one could
666 use several techniques. First, let's use "x" and "X" to skip forward
667 and back:
668 $s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) );
670 We can also use "@" to jump to an offset, with 0 being the position
672 where we were when the last "(" was encountered:
673 $s = pack( '(A)*', unpack( '(@1A @0A @2)*', $s ) );
675 Finally, there is also an entirely different approach by unpacking big
677 endian shorts and packing them in the reverse byte order:
678 $s = pack( '(v)*', unpack( '(n)*', $s );
680
682 String Lengths
683 In the previous section we've seen a network message that was
684 constructed by prefixing the binary message length to the actual
685 message. You'll find that packing a length followed by so many bytes of
686 data is a frequently used recipe since appending a null byte won't work
687 if a null byte may be part of the data. Here is an example where both
688 techniques are used: after two null terminated strings with source and
689 destination address, a Short Message (to a mobile phone) is sent after
690 a length byte:
691 my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm );
693 Unpacking this message can be done with the same template:
695 ( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg );
697 There's a subtle trap lurking in the offing: Adding another field after
699 the Short Message (in variable $sm) is all right when packing, but this
700 cannot be unpacked naively:
701 # pack a message
703 my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio );
704 # unpack fails - $prio remains undefined!
706 ( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg );
707 The pack code "A*" gobbles up all remaining bytes, and $prio remains
709 undefined! Before we let disappointment dampen the morale: Perl's got
710 the trump card to make this trick too, just a little further up the
711 sleeve. Watch this:
712 # pack a message: ASCIIZ, ASCIIZ, length/string, byte
714 my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio );
715 # unpack
717 ( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg );
718 Combining two pack codes with a slash ("/") associates them with a
720 single value from the argument list. In "pack", the length of the
721 argument is taken and packed according to the first code while the
722 argument itself is added after being converted with the template code
723 after the slash. This saves us the trouble of inserting the "length"
724 call, but it is in "unpack" where we really score: The value of the
725 length byte marks the end of the string to be taken from the buffer.
726 Since this combination doesn't make sense except when the second pack
727 code isn't "a*", "A*" or "Z*", Perl won't let you.
728 The pack code preceding "/" may be anything that's fit to represent a
730 number: All the numeric binary pack codes, and even text codes such as
731 "A4" or "Z*":
732 # pack/unpack a string preceded by its length in ASCII
734 my $buf = pack( 'A4/A*', "Humpty-Dumpty" );
735 # unpack $buf: '13 Humpty-Dumpty'
736 my $txt = unpack( 'A4/A*', $buf );
737 "/" is not implemented in Perls before 5.6, so if your code is required
739 to work on older Perls you'll need to "unpack( 'Z* Z* C')" to get the
740 length, then use it to make a new unpack string. For example
741 # pack a message: ASCIIZ, ASCIIZ, length, string, byte (5.005 compatible)
743 my $msg = pack( 'Z* Z* C A* C', $src, $dst, length $sm, $sm, $prio );
744 # unpack
746 ( undef, undef, $len) = unpack( 'Z* Z* C', $msg );
747 ($src, $dst, $sm, $prio) = unpack ( "Z* Z* x A$len C", $msg );
748 But that second "unpack" is rushing ahead. It isn't using a simple
750 literal string for the template. So maybe we should introduce...
751 Dynamic Templates
753 So far, we've seen literals used as templates. If the list of pack
754 items doesn't have fixed length, an expression constructing the
755 template is required (whenever, for some reason, "()*" cannot be used).
756 Here's an example: To store named string values in a way that can be
757 conveniently parsed by a C program, we create a sequence of names and
758 null terminated ASCII strings, with "=" between the name and the value,
759 followed by an additional delimiting null byte. Here's how:
760 my $env = pack( '(A*A*Z*)' . keys( %Env ) . 'C',
762 map( { ( $_, '=', $Env{$_} ) } keys( %Env ) ), 0 );
763 Let's examine the cogs of this byte mill, one by one. There's the "map"
765 call, creating the items we intend to stuff into the $env buffer: to
766 each key (in $_) it adds the "=" separator and the hash entry value.
767 Each triplet is packed with the template code sequence "A*A*Z*" that is
768 repeated according to the number of keys. (Yes, that's what the "keys"
769 function returns in scalar context.) To get the very last null byte, we
770 add a 0 at the end of the "pack" list, to be packed with "C".
771 (Attentive readers may have noticed that we could have omitted the 0.)
772 For the reverse operation, we'll have to determine the number of items
774 in the buffer before we can let "unpack" rip it apart:
775 my $n = $env =~ tr/\0// - 1;
777 my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) );
778 The "tr" counts the null bytes. The "unpack" call returns a list of
780 name-value pairs each of which is taken apart in the "map" block.
781 Counting Repetitions
783 Rather than storing a sentinel at the end of a data item (or a list of
784 items), we could precede the data with a count. Again, we pack keys and
785 values of a hash, preceding each with an unsigned short length count,
786 and up front we store the number of pairs:
787 my $env = pack( 'S(S/A* S/A*)*', scalar keys( %Env ), %Env );
789 This simplifies the reverse operation as the number of repetitions can
791 be unpacked with the "/" code:
792 my %env = unpack( 'S/(S/A* S/A*)', $env );
794 Note that this is one of the rare cases where you cannot use the same
796 template for "pack" and "unpack" because "pack" can't determine a
797 repeat count for a "()"-group.
798 Intel HEX
800 Intel HEX is a file format for representing binary data, mostly for
801 programming various chips, as a text file. (See
802 <http://en.wikipedia.org/wiki/.hex> for a detailed description, and
803 <http://en.wikipedia.org/wiki/SREC_(file_format)> for the Motorola
804 S-record format, which can be unravelled using the same technique.)
805 Each line begins with a colon (':') and is followed by a sequence of
806 hexadecimal characters, specifying a byte count n (8 bit), an address
807 (16 bit, big endian), a record type (8 bit), n data bytes and a
808 checksum (8 bit) computed as the least significant byte of the two's
809 complement sum of the preceding bytes. Example: ":0300300002337A1E".
810 The first step of processing such a line is the conversion, to binary,
812 of the hexadecimal data, to obtain the four fields, while checking the
813 checksum. No surprise here: we'll start with a simple "pack" call to
814 convert everything to binary:
815 my $binrec = pack( 'H*', substr( $hexrec, 1 ) );
817 The resulting byte sequence is most convenient for checking the
819 checksum. Don't slow your program down with a for loop adding the
820 "ord" values of this string's bytes - the "unpack" code "%" is the
821 thing to use for computing the 8-bit sum of all bytes, which must be
822 equal to zero:
823 die unless unpack( "%8C*", $binrec ) == 0;
825 Finally, let's get those four fields. By now, you shouldn't have any
827 problems with the first three fields - but how can we use the byte
828 count of the data in the first field as a length for the data field?
829 Here the codes "x" and "X" come to the rescue, as they permit jumping
830 back and forth in the string to unpack.
831 my( $addr, $type, $data ) = unpack( "x n C X4 C x3 /a", $bin );
833 Code "x" skips a byte, since we don't need the count yet. Code "n"
835 takes care of the 16-bit big-endian integer address, and "C" unpacks
836 the record type. Being at offset 4, where the data begins, we need the
837 count. "X4" brings us back to square one, which is the byte at offset
838 0. Now we pick up the count, and zoom forth to offset 4, where we are
839 now fully furnished to extract the exact number of data bytes, leaving
840 the trailing checksum byte alone.
841
843 In previous sections we have seen how to pack numbers and character
844 strings. If it were not for a couple of snags we could conclude this
845 section right away with the terse remark that C structures don't
846 contain anything else, and therefore you already know all there is to
847 it. Sorry, no: read on, please.
848 If you have to deal with a lot of C structures, and don't want to hack
850 all your template strings manually, you'll probably want to have a look
851 at the CPAN module "Convert::Binary::C". Not only can it parse your C
852 source directly, but it also has built-in support for all the odds and
853 ends described further on in this section.
854 The Alignment Pit
856 In the consideration of speed against memory requirements the balance
857 has been tilted in favor of faster execution. This has influenced the
858 way C compilers allocate memory for structures: On architectures where
859 a 16-bit or 32-bit operand can be moved faster between places in
860 memory, or to or from a CPU register, if it is aligned at an even or
861 multiple-of-four or even at a multiple-of eight address, a C compiler
862 will give you this speed benefit by stuffing extra bytes into
863 structures. If you don't cross the C shoreline this is not likely to
864 cause you any grief (although you should care when you design large
865 data structures, or you want your code to be portable between
866 architectures (you do want that, don't you?)).
867 To see how this affects "pack" and "unpack", we'll compare these two C
869 structures:
870 typedef struct {
872 char c1;
873 short s;
874 char c2;
875 long l;
876 } gappy_t;
877 typedef struct {
879 long l;
880 short s;
881 char c1;
882 char c2;
883 } dense_t;
884 Typically, a C compiler allocates 12 bytes to a "gappy_t" variable, but
886 requires only 8 bytes for a "dense_t". After investigating this
887 further, we can draw memory maps, showing where the extra 4 bytes are
888 hidden:
889 0 +4 +8 +12
891 +--+--+--+--+--+--+--+--+--+--+--+--+
892 |c1|xx| s |c2|xx|xx|xx| l | xx = fill byte
893 +--+--+--+--+--+--+--+--+--+--+--+--+
894 gappy_t
895 0 +4 +8
897 +--+--+--+--+--+--+--+--+
898 | l | h |c1|c2|
899 +--+--+--+--+--+--+--+--+
900 dense_t
901 And that's where the first quirk strikes: "pack" and "unpack" templates
903 have to be stuffed with "x" codes to get those extra fill bytes.
904 The natural question: "Why can't Perl compensate for the gaps?"
906 warrants an answer. One good reason is that C compilers might provide
907 (non-ANSI) extensions permitting all sorts of fancy control over the
908 way structures are aligned, even at the level of an individual
909 structure field. And, if this were not enough, there is an insidious
910 thing called "union" where the amount of fill bytes cannot be derived
911 from the alignment of the next item alone.
912 OK, so let's bite the bullet. Here's one way to get the alignment right
914 by inserting template codes "x", which don't take a corresponding item
915 from the list:
916 my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );
918 Note the "!" after "l": We want to make sure that we pack a long
920 integer as it is compiled by our C compiler. And even now, it will only
921 work for the platforms where the compiler aligns things as above. And
922 somebody somewhere has a platform where it doesn't. [Probably a Cray,
923 where "short"s, "int"s and "long"s are all 8 bytes. :-)]
924 Counting bytes and watching alignments in lengthy structures is bound
926 to be a drag. Isn't there a way we can create the template with a
927 simple program? Here's a C program that does the trick:
928 #include <stdio.h>
930 #include <stddef.h>
931 typedef struct {
933 char fc1;
934 short fs;
935 char fc2;
936 long fl;
937 } gappy_t;
938 #define Pt(struct,field,tchar) \
940 printf( "@%d%s ", offsetof(struct,field), # tchar );
941 int main() {
943 Pt( gappy_t, fc1, c );
944 Pt( gappy_t, fs, s! );
945 Pt( gappy_t, fc2, c );
946 Pt( gappy_t, fl, l! );
947 printf( "\n" );
948 }
949 The output line can be used as a template in a "pack" or "unpack" call:
951 my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l );
953 Gee, yet another template code - as if we hadn't plenty. But "@" saves
955 our day by enabling us to specify the offset from the beginning of the
956 pack buffer to the next item: This is just the value the "offsetof"
957 macro (defined in "<stddef.h>") returns when given a "struct" type and
958 one of its field names ("member-designator" in C standardese).
959 Neither using offsets nor adding "x"'s to bridge the gaps is
961 satisfactory. (Just imagine what happens if the structure changes.)
962 What we really need is a way of saying "skip as many bytes as required
963 to the next multiple of N". In fluent Templatese, you say this with
964 "x!N" where N is replaced by the appropriate value. Here's the next
965 version of our struct packaging:
966 my $gappy = pack( 'c x!2 s c x!4 l!', $c1, $s, $c2, $l );
968 That's certainly better, but we still have to know how long all the
970 integers are, and portability is far away. Rather than 2, for instance,
971 we want to say "however long a short is". But this can be done by
972 enclosing the appropriate pack code in brackets: "[s]". So, here's the
973 very best we can do:
974 my $gappy = pack( 'c x![s] s c x![l!] l!', $c1, $s, $c2, $l );
976 Dealing with Endian-ness
978 Now, imagine that we want to pack the data for a machine with a
979 different byte-order. First, we'll have to figure out how big the data
980 types on the target machine really are. Let's assume that the longs are
981 32 bits wide and the shorts are 16 bits wide. You can then rewrite the
982 template as:
983 my $gappy = pack( 'c x![s] s c x![l] l', $c1, $s, $c2, $l );
985 If the target machine is little-endian, we could write:
987 my $gappy = pack( 'c x![s] s< c x![l] l<', $c1, $s, $c2, $l );
989 This forces the short and the long members to be little-endian, and is
991 just fine if you don't have too many struct members. But we could also
992 use the byte-order modifier on a group and write the following:
993 my $gappy = pack( '( c x![s] s c x![l] l )<', $c1, $s, $c2, $l );
995 This is not as short as before, but it makes it more obvious that we
997 intend to have little-endian byte-order for a whole group, not only for
998 individual template codes. It can also be more readable and easier to
999 maintain.
1000 Alignment, Take 2
1002 I'm afraid that we're not quite through with the alignment catch yet.
1003 The hydra raises another ugly head when you pack arrays of structures:
1004 typedef struct {
1006 short count;
1007 char glyph;
1008 } cell_t;
1009 typedef cell_t buffer_t[BUFLEN];
1011 Where's the catch? Padding is neither required before the first field
1013 "count", nor between this and the next field "glyph", so why can't we
1014 simply pack like this:
1015 # something goes wrong here:
1017 pack( 's!a' x @buffer,
1018 map{ ( $_->{count}, $_->{glyph} ) } @buffer );
1019 This packs "3*@buffer" bytes, but it turns out that the size of
1021 "buffer_t" is four times "BUFLEN"! The moral of the story is that the
1022 required alignment of a structure or array is propagated to the next
1023 higher level where we have to consider padding at the end of each
1024 component as well. Thus the correct template is:
1025 pack( 's!ax' x @buffer,
1027 map{ ( $_->{count}, $_->{glyph} ) } @buffer );
1028 Alignment, Take 3
1030 And even if you take all the above into account, ANSI still lets this:
1031 typedef struct {
1033 char foo[2];
1034 } foo_t;
1035 vary in size. The alignment constraint of the structure can be greater
1037 than any of its elements. [And if you think that this doesn't affect
1038 anything common, dismember the next cellphone that you see. Many have
1039 ARM cores, and the ARM structure rules make "sizeof (foo_t)" == 4]
1040 Pointers for How to Use Them
1042 The title of this section indicates the second problem you may run into
1043 sooner or later when you pack C structures. If the function you intend
1044 to call expects a, say, "void *" value, you cannot simply take a
1045 reference to a Perl variable. (Although that value certainly is a
1046 memory address, it's not the address where the variable's contents are
1047 stored.)
1048 Template code "P" promises to pack a "pointer to a fixed length
1050 string". Isn't this what we want? Let's try:
1051 # allocate some storage and pack a pointer to it
1053 my $memory = "\x00" x $size;
1054 my $memptr = pack( 'P', $memory );
1055 But wait: doesn't "pack" just return a sequence of bytes? How can we
1057 pass this string of bytes to some C code expecting a pointer which is,
1058 after all, nothing but a number? The answer is simple: We have to
1059 obtain the numeric address from the bytes returned by "pack".
1060 my $ptr = unpack( 'L!', $memptr );
1062 Obviously this assumes that it is possible to typecast a pointer to an
1064 unsigned long and vice versa, which frequently works but should not be
1065 taken as a universal law. - Now that we have this pointer the next
1066 question is: How can we put it to good use? We need a call to some C
1067 function where a pointer is expected. The read(2) system call comes to
1068 mind:
1069 ssize_t read(int fd, void *buf, size_t count);
1071 After reading perlfunc explaining how to use "syscall" we can write
1073 this Perl function copying a file to standard output:
1074 require 'syscall.ph';
1076 sub cat($){
1077 my $path = shift();
1078 my $size = -s $path;
1079 my $memory = "\x00" x $size; # allocate some memory
1080 my $ptr = unpack( 'L', pack( 'P', $memory ) );
1081 open( F, $path ) || die( "$path: cannot open ($!)\n" );
1082 my $fd = fileno(F);
1083 my $res = syscall( &SYS_read, fileno(F), $ptr, $size );
1084 print $memory;
1085 close( F );
1086 }
1087 This is neither a specimen of simplicity nor a paragon of portability
1089 but it illustrates the point: We are able to sneak behind the scenes
1090 and access Perl's otherwise well-guarded memory! (Important note:
1091 Perl's "syscall" does not require you to construct pointers in this
1092 roundabout way. You simply pass a string variable, and Perl forwards
1093 the address.)
1094 How does "unpack" with "P" work? Imagine some pointer in the buffer
1096 about to be unpacked: If it isn't the null pointer (which will smartly
1097 produce the "undef" value) we have a start address - but then what?
1098 Perl has no way of knowing how long this "fixed length string" is, so
1099 it's up to you to specify the actual size as an explicit length after
1100 "P".
1101 my $mem = "abcdefghijklmn";
1103 print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"
1104 As a consequence, "pack" ignores any number or "*" after "P".
1106 Now that we have seen "P" at work, we might as well give "p" a whirl.
1108 Why do we need a second template code for packing pointers at all? The
1109 answer lies behind the simple fact that an "unpack" with "p" promises a
1110 null-terminated string starting at the address taken from the buffer,
1111 and that implies a length for the data item to be returned:
1112 my $buf = pack( 'p', "abc\x00efhijklmn" );
1114 print unpack( 'p', $buf ); # prints "abc"
1115 Albeit this is apt to be confusing: As a consequence of the length
1117 being implied by the string's length, a number after pack code "p" is a
1118 repeat count, not a length as after "P".
1119 Using "pack(..., $x)" with "P" or "p" to get the address where $x is
1121 actually stored must be used with circumspection. Perl's internal
1122 machinery considers the relation between a variable and that address as
1123 its very own private matter and doesn't really care that we have
1124 obtained a copy. Therefore:
1125 · Do not use "pack" with "p" or "P" to obtain the address of variable
1127 that's bound to go out of scope (and thereby freeing its memory)
1128 before you are done with using the memory at that address.
1129 · Be very careful with Perl operations that change the value of the
1131 variable. Appending something to the variable, for instance, might
1132 require reallocation of its storage, leaving you with a pointer
1133 into no-man's land.
1134 · Don't think that you can get the address of a Perl variable when it
1136 is stored as an integer or double number! "pack('P', $x)" will
1137 force the variable's internal representation to string, just as if
1138 you had written something like "$x .= ''".
1139 It's safe, however, to P- or p-pack a string literal, because Perl
1141 simply allocates an anonymous variable.
1142
1144 Here are a collection of (possibly) useful canned recipes for "pack"
1145 and "unpack":
1146 # Convert IP address for socket functions
1148 pack( "C4", split /\./, "123.4.5.6" );
1149 # Count the bits in a chunk of memory (e.g. a select vector)
1151 unpack( '%32b*', $mask );
1152 # Determine the endianness of your system
1154 $is_little_endian = unpack( 'c', pack( 's', 1 ) );
1155 $is_big_endian = unpack( 'xc', pack( 's', 1 ) );
1156 # Determine the number of bits in a native integer
1158 $bits = unpack( '%32I!', ~0 );
1159 # Prepare argument for the nanosleep system call
1161 my $timespec = pack( 'L!L!', $secs, $nanosecs );
1162 For a simple memory dump we unpack some bytes into just as many pairs
1164 of hex digits, and use "map" to handle the traditional spacing - 16
1165 bytes to a line:
1166 my $i;
1168 print map( ++$i % 16 ? "$_ " : "$_\n",
1169 unpack( 'H2' x length( $mem ), $mem ) ),
1170 length( $mem ) % 16 ? "\n" : '';
1171
1173 # Pulling digits out of nowhere...
1174 print unpack( 'C', pack( 'x' ) ),
1175 unpack( '%B*', pack( 'A' ) ),
1176 unpack( 'H', pack( 'A' ) ),
1177 unpack( 'A', unpack( 'C', pack( 'A' ) ) ), "\n";
1178 # One for the road ;-)
1180 my $advice = pack( 'all u can in a van' );
1181
1183 Simon Cozens and Wolfgang Laun.
1184perl v5.16.3 2013-03-04 PERLPACKTUT(1)