Commit | Line | Data |
---|---|---|
34babc16 JH |
1 | =head1 NAME |
2 | ||
3 | perlpacktut - tutorial on C<pack> and C<unpack> | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | C<pack> and C<unpack> are two functions for transforming data according | |
8 | to a user-defined template, between the guarded way Perl stores values | |
47b6252e | 9 | and some well-defined representation as might be required in the |
34babc16 JH |
10 | environment of a Perl program. Unfortunately, they're also two of |
11 | the most misunderstood and most often overlooked functions that Perl | |
12 | provides. This tutorial will demystify them for you. | |
13 | ||
14 | ||
15 | =head1 The Basic Principle | |
16 | ||
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 C<sizeof> operator tells you how many bytes are allocated to | |
20 | the variable. Using the address and the size, you may access the storage | |
21 | to your heart's content. | |
22 | ||
23 | In Perl, you just can't access memory at random, but the structural and | |
24 | representational conversion provided by C<pack> and C<unpack> is an | |
25 | excellent alternative. The C<pack> function converts values to a byte | |
26 | sequence containing representations according to a given specification, | |
27 | the so-called "template" argument. C<unpack> is the reverse process, | |
28 | deriving some values from the contents of a string of bytes. (Be cautioned, | |
29 | however, that not all that has been packed together can be neatly unpacked - | |
30 | a very common experience as seasoned travellers are likely to confirm.) | |
31 | ||
32 | Why, you may ask, would you need a chunk of memory containing some values | |
33 | in binary representation? One good reason is input and output accessing | |
34 | some file, a device, or a network connection, whereby this binary | |
35 | representation is either forced on you or will give you some benefit | |
36 | in processing. Another cause is passing data to some system call that | |
37 | is not available as a Perl function: C<syscall> requires you to provide | |
38 | parameters 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 | |
40 | of these two functions. | |
41 | ||
42 | To see how (un)packing works, we'll start with a simple template | |
43 | code where the conversion is in low gear: between the contents of a byte | |
44 | sequence and a string of hexadecimal digits. Let's use C<unpack>, since | |
47b6252e | 45 | this is likely to remind you of a dump program, or some desperate last |
34babc16 JH |
46 | message unfortunate programs are wont to throw at you before they expire |
47 | into the wild blue yonder. Assuming that the variable C<$mem> holds a | |
48 | sequence of bytes that we'd like to inspect without assuming anything | |
49 | about its meaning, we can write | |
50 | ||
51 | my( $hex ) = unpack( 'H*', $mem ); | |
52 | print "$hex\n"; | |
53 | ||
54 | whereupon we might see something like this, with each pair of hex digits | |
55 | corresponding to a byte: | |
56 | ||
57 | 41204d414e204120504c414e20412043414e414c2050414e414d41 | |
58 | ||
47b6252e | 59 | What was in this chunk of memory? Numbers, characters, or a mixture of |
34babc16 JH |
60 | both? Assuming that we're on a computer where ASCII (or some similar) |
61 | encoding is used: hexadecimal values in the range C<0x40> - C<0x5A> | |
47f22e19 | 62 | indicate an uppercase letter, and C<0x20> encodes a space. So we might |
34babc16 JH |
63 | assume it is a piece of text, which some are able to read like a tabloid; |
64 | but others will have to get hold of an ASCII table and relive that | |
65 | firstgrader feeling. Not caring too much about which way to read this, | |
66 | we note that C<unpack> with the template code C<H> converts the contents | |
67 | of 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 | |
69 | defined to convert just a single hexadecimal digit unless it is followed | |
70 | by a repeat count. An asterisk for the repeat count means to use whatever | |
71 | remains. | |
72 | ||
73 | The inverse operation - packing byte contents from a string of hexadecimal | |
74 | digits - is just as easily written. For instance: | |
75 | ||
76 | my $s = pack( 'H2' x 10, map { "3$_" } ( 0..9 ) ); | |
77 | print "$s\n"; | |
78 | ||
79 | Since we feed a list of ten 2-digit hexadecimal strings to C<pack>, the | |
80 | pack template should contain ten pack codes. If this is run on a computer | |
81 | with ASCII character coding, it will print C<0123456789>. | |
82 | ||
83 | ||
84 | =head1 Packing Text | |
85 | ||
86 | Let's suppose you've got to read in a data file like this: | |
87 | ||
88 | Date |Description | Income|Expenditure | |
89 | 01/24/2001 Ahmed's Camel Emporium 1147.99 | |
90 | 01/28/2001 Flea spray 24.99 | |
91 | 01/29/2001 Camel rides to tourists 235.00 | |
92 | ||
93 | How do we do it? You might think first to use C<split>; however, since | |
94 | C<split> collapses blank fields, you'll never know whether a record was | |
95 | income or expenditure. Oops. Well, you could always use C<substr>: | |
96 | ||
97 | while (<>) { | |
98 | my $date = substr($_, 0, 11); | |
99 | my $desc = substr($_, 12, 27); | |
100 | my $income = substr($_, 40, 7); | |
101 | my $expend = substr($_, 52, 7); | |
102 | ... | |
103 | } | |
104 | ||
105 | It's not really a barrel of laughs, is it? In fact, it's worse than it | |
106 | may seem; the eagle-eyed may notice that the first field should only be | |
107 | 10 characters wide, and the error has propagated right through the other | |
108 | numbers - which we've had to count by hand. So it's error-prone as well | |
109 | as horribly unfriendly. | |
110 | ||
111 | Or maybe we could use regular expressions: | |
7207e29d | 112 | |
34babc16 JH |
113 | while (<>) { |
114 | my($date, $desc, $income, $expend) = | |
115 | m|(\d\d/\d\d/\d{4}) (.{27}) (.{7})(.*)|; | |
116 | ... | |
117 | } | |
118 | ||
119 | Urgh. Well, it's a bit better, but - well, would you want to maintain | |
120 | that? | |
121 | ||
122 | Hey, isn't Perl supposed to make this sort of thing easy? Well, it does, | |
123 | if you use the right tools. C<pack> and C<unpack> are designed to help | |
124 | you out when dealing with fixed-width data like the above. Let's have a | |
125 | look at a solution with C<unpack>: | |
126 | ||
127 | while (<>) { | |
128 | my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_); | |
129 | ... | |
130 | } | |
131 | ||
132 | That looks a bit nicer; but we've got to take apart that weird template. | |
133 | Where did I pull that out of? | |
134 | ||
135 | OK, let's have a look at some of our data again; in fact, we'll include | |
136 | the headers, and a handy ruler so we can keep track of where we are. | |
137 | ||
138 | 1 2 3 4 5 | |
139 | 1234567890123456789012345678901234567890123456789012345678 | |
140 | Date |Description | Income|Expenditure | |
141 | 01/28/2001 Flea spray 24.99 | |
142 | 01/29/2001 Camel rides to tourists 235.00 | |
143 | ||
47b6252e | 144 | From this, we can see that the date column stretches from column 1 to |
34babc16 JH |
145 | column 10 - ten characters wide. The C<pack>-ese for "character" is |
146 | C<A>, and ten of them are C<A10>. So if we just wanted to extract the | |
147 | dates, we could say this: | |
148 | ||
149 | my($date) = unpack("A10", $_); | |
150 | ||
151 | OK, what's next? Between the date and the description is a blank column; | |
152 | we want to skip over that. The C<x> template means "skip forward", so we | |
153 | want one of those. Next, we have another batch of characters, from 12 to | |
154 | 38. That's 27 more characters, hence C<A27>. (Don't make the fencepost | |
155 | error - there are 27 characters between 12 and 38, not 26. Count 'em!) | |
156 | ||
157 | Now we skip another character and pick up the next 7 characters: | |
158 | ||
159 | my($date,$description,$income) = unpack("A10xA27xA7", $_); | |
160 | ||
161 | Now comes the clever bit. Lines in our ledger which are just income and | |
162 | not expenditure might end at column 46. Hence, we don't want to tell our | |
163 | C<unpack> pattern that we B<need> to find another 12 characters; we'll | |
164 | just say "if there's anything left, take it". As you might guess from | |
165 | regular expressions, that's what the C<*> means: "use everything | |
166 | remaining". | |
167 | ||
168 | =over 3 | |
169 | ||
170 | =item * | |
171 | ||
172 | Be warned, though, that unlike regular expressions, if the C<unpack> | |
173 | template doesn't match the incoming data, Perl will scream and die. | |
174 | ||
175 | =back | |
176 | ||
177 | ||
178 | Hence, putting it all together: | |
179 | ||
49704364 | 180 | my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_); |
34babc16 JH |
181 | |
182 | Now, that's our data parsed. I suppose what we might want to do now is | |
183 | total up our income and expenditure, and add another line to the end of | |
184 | our ledger - in the same format - saying how much we've brought in and | |
185 | how much we've spent: | |
186 | ||
187 | while (<>) { | |
188 | my($date, $desc, $income, $expend) = unpack("A10xA27xA7xA*", $_); | |
189 | $tot_income += $income; | |
190 | $tot_expend += $expend; | |
191 | } | |
192 | ||
193 | $tot_income = sprintf("%.2f", $tot_income); # Get them into | |
194 | $tot_expend = sprintf("%.2f", $tot_expend); # "financial" format | |
195 | ||
196 | $date = POSIX::strftime("%m/%d/%Y", localtime); | |
197 | ||
198 | # OK, let's go: | |
199 | ||
200 | print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend); | |
201 | ||
202 | Oh, hmm. That didn't quite work. Let's see what happened: | |
203 | ||
204 | 01/24/2001 Ahmed's Camel Emporium 1147.99 | |
205 | 01/28/2001 Flea spray 24.99 | |
206 | 01/29/2001 Camel rides to tourists 1235.00 | |
207 | 03/23/2001Totals 1235.001172.98 | |
208 | ||
209 | OK, it's a start, but what happened to the spaces? We put C<x>, didn't | |
210 | we? Shouldn't it skip forward? Let's look at what L<perlfunc/pack> says: | |
211 | ||
212 | x A null byte. | |
213 | ||
214 | Urgh. No wonder. There's a big difference between "a null byte", | |
215 | character zero, and "a space", character 32. Perl's put something | |
216 | between the date and the description - but unfortunately, we can't see | |
217 | it! | |
218 | ||
219 | What we actually need to do is expand the width of the fields. The C<A> | |
220 | format pads any non-existent characters with spaces, so we can use the | |
221 | additional spaces to line up our fields, like this: | |
222 | ||
223 | print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend); | |
224 | ||
225 | (Note that you can put spaces in the template to make it more readable, | |
226 | but they don't translate to spaces in the output.) Here's what we got | |
227 | this time: | |
228 | ||
229 | 01/24/2001 Ahmed's Camel Emporium 1147.99 | |
230 | 01/28/2001 Flea spray 24.99 | |
231 | 01/29/2001 Camel rides to tourists 1235.00 | |
232 | 03/23/2001 Totals 1235.00 1172.98 | |
233 | ||
234 | That's a bit better, but we still have that last column which needs to | |
235 | be moved further over. There's an easy way to fix this up: | |
236 | unfortunately, we can't get C<pack> to right-justify our fields, but we | |
237 | can get C<sprintf> to do it: | |
238 | ||
239 | $tot_income = sprintf("%.2f", $tot_income); | |
240 | $tot_expend = sprintf("%12.2f", $tot_expend); | |
241 | $date = POSIX::strftime("%m/%d/%Y", localtime); | |
242 | print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend); | |
243 | ||
244 | This time we get the right answer: | |
245 | ||
246 | 01/28/2001 Flea spray 24.99 | |
247 | 01/29/2001 Camel rides to tourists 1235.00 | |
248 | 03/23/2001 Totals 1235.00 1172.98 | |
249 | ||
250 | So that's how we consume and produce fixed-width data. Let's recap what | |
251 | we've seen of C<pack> and C<unpack> so far: | |
252 | ||
253 | =over 3 | |
254 | ||
255 | =item * | |
256 | ||
257 | Use C<pack> to go from several pieces of data to one fixed-width | |
258 | version; use C<unpack> to turn a fixed-width-format string into several | |
259 | pieces of data. | |
260 | ||
261 | =item * | |
262 | ||
263 | The pack format C<A> means "any character"; if you're C<pack>ing and | |
264 | you've run out of things to pack, C<pack> will fill the rest up with | |
265 | spaces. | |
266 | ||
267 | =item * | |
268 | ||
269 | C<x> means "skip a byte" when C<unpack>ing; when C<pack>ing, it means | |
270 | "introduce a null byte" - that's probably not what you mean if you're | |
271 | dealing with plain text. | |
272 | ||
273 | =item * | |
274 | ||
275 | You can follow the formats with numbers to say how many characters | |
276 | should be affected by that format: C<A12> means "take 12 characters"; | |
277 | C<x6> means "skip 6 bytes" or "character 0, 6 times". | |
278 | ||
279 | =item * | |
280 | ||
281 | Instead of a number, you can use C<*> to mean "consume everything else | |
282 | left". | |
283 | ||
284 | B<Warning>: when packing multiple pieces of data, C<*> only means | |
285 | "consume all of the current piece of data". That's to say | |
286 | ||
287 | pack("A*A*", $one, $two) | |
288 | ||
289 | packs all of C<$one> into the first C<A*> and then all of C<$two> into | |
290 | the second. This is a general principle: each format character | |
291 | corresponds to one piece of data to be C<pack>ed. | |
292 | ||
293 | =back | |
294 | ||
295 | ||
296 | ||
297 | =head1 Packing Numbers | |
298 | ||
299 | So much for textual data. Let's get onto the meaty stuff that C<pack> | |
300 | and C<unpack> are best at: handling binary formats for numbers. There is, | |
301 | of course, not just one binary format - life would be too simple - but | |
302 | Perl will do all the finicky labor for you. | |
303 | ||
304 | ||
305 | =head2 Integers | |
306 | ||
307 | Packing and unpacking numbers implies conversion to and from some | |
308 | I<specific> binary representation. Leaving floating point numbers | |
309 | aside for the moment, the salient properties of any such representation | |
310 | are: | |
311 | ||
312 | =over 4 | |
313 | ||
314 | =item * | |
315 | ||
316 | the number of bytes used for storing the integer, | |
317 | ||
318 | =item * | |
319 | ||
320 | whether the contents are interpreted as a signed or unsigned number, | |
321 | ||
322 | =item * | |
323 | ||
324 | the byte ordering: whether the first byte is the least or most | |
325 | significant byte (or: little-endian or big-endian, respectively). | |
326 | ||
327 | =back | |
328 | ||
329 | So, for instance, to pack 20302 to a signed 16 bit integer in your | |
330 | computer's representation you write | |
331 | ||
332 | my $ps = pack( 's', 20302 ); | |
333 | ||
334 | Again, the result is a string, now containing 2 bytes. If you print | |
335 | this string (which is, generally, not recommended) you might see | |
336 | C<ON> or C<NO> (depending on your system's byte ordering) - or something | |
337 | entirely different if your computer doesn't use ASCII character encoding. | |
338 | Unpacking C<$ps> with the same template returns the original integer value: | |
339 | ||
340 | my( $s ) = unpack( 's', $ps ); | |
341 | ||
342 | This is true for all numeric template codes. But don't expect miracles: | |
47b6252e | 343 | if the packed value exceeds the allotted byte capacity, high order bits |
34babc16 JH |
344 | are silently discarded, and unpack certainly won't be able to pull them |
345 | back out of some magic hat. And, when you pack using a signed template | |
346 | code such as C<s>, an excess value may result in the sign bit | |
347 | getting set, and unpacking this will smartly return a negative value. | |
348 | ||
349 | 16 bits won't get you too far with integers, but there is C<l> and C<L> | |
350 | for signed and unsigned 32-bit integers. And if this is not enough and | |
351 | your system supports 64 bit integers you can push the limits much closer | |
352 | to infinity with pack codes C<q> and C<Q>. A notable exception is provided | |
353 | by pack codes C<i> and C<I> for signed and unsigned integers of the | |
354 | "local custom" variety: Such an integer will take up as many bytes as | |
355 | a local C compiler returns for C<sizeof(int)>, but it'll use I<at least> | |
356 | 32 bits. | |
357 | ||
358 | Each of the integer pack codes C<sSlLqQ> results in a fixed number of bytes, | |
359 | no matter where you execute your program. This may be useful for some | |
360 | applications, but it does not provide for a portable way to pass data | |
361 | structures between Perl and C programs (bound to happen when you call | |
362 | XS extensions or the Perl function C<syscall>), or when you read or | |
363 | write binary files. What you'll need in this case are template codes that | |
364 | depend on what your local C compiler compiles when you code C<short> or | |
365 | C<unsigned long>, for instance. These codes and their corresponding | |
366 | byte lengths are shown in the table below. Since the C standard leaves | |
367 | much leeway with respect to the relative sizes of these data types, actual | |
368 | values may vary, and that's why the values are given as expressions in | |
369 | C and Perl. (If you'd like to use values from C<%Config> in your program | |
370 | you have to import it with C<use Config>.) | |
371 | ||
372 | signed unsigned byte length in C byte length in Perl | |
f8b4d74f WL |
373 | s! S! sizeof(short) $Config{shortsize} |
374 | i! I! sizeof(int) $Config{intsize} | |
375 | l! L! sizeof(long) $Config{longsize} | |
d832b8f6 | 376 | q! Q! sizeof(long long) $Config{longlongsize} |
34babc16 JH |
377 | |
378 | The C<i!> and C<I!> codes aren't different from C<i> and C<I>; they are | |
379 | tolerated for completeness' sake. | |
380 | ||
381 | ||
382 | =head2 Unpacking a Stack Frame | |
383 | ||
384 | Requesting a particular byte ordering may be necessary when you work with | |
47f22e19 | 385 | binary data coming from some specific architecture whereas your program could |
34babc16 JH |
386 | run on a totally different system. As an example, assume you have 24 bytes |
387 | containing a stack frame as it happens on an Intel 8086: | |
388 | ||
389 | +---------+ +----+----+ +---------+ | |
390 | TOS: | IP | TOS+4:| FL | FH | FLAGS TOS+14:| SI | | |
391 | +---------+ +----+----+ +---------+ | |
392 | | CS | | AL | AH | AX | DI | | |
393 | +---------+ +----+----+ +---------+ | |
394 | | BL | BH | BX | BP | | |
395 | +----+----+ +---------+ | |
396 | | CL | CH | CX | DS | | |
397 | +----+----+ +---------+ | |
398 | | DL | DH | DX | ES | | |
399 | +----+----+ +---------+ | |
400 | ||
401 | First, we note that this time-honored 16-bit CPU uses little-endian order, | |
402 | and that's why the low order byte is stored at the lower address. To | |
9dc383df | 403 | unpack such a (unsigned) short we'll have to use code C<v>. A repeat |
34babc16 JH |
404 | count unpacks all 12 shorts: |
405 | ||
406 | my( $ip, $cs, $flags, $ax, $bx, $cd, $dx, $si, $di, $bp, $ds, $es ) = | |
407 | unpack( 'v12', $frame ); | |
408 | ||
409 | Alternatively, we could have used C<C> to unpack the individually | |
410 | accessible byte registers FL, FH, AL, AH, etc.: | |
411 | ||
412 | my( $fl, $fh, $al, $ah, $bl, $bh, $cl, $ch, $dl, $dh ) = | |
413 | unpack( 'C10', substr( $frame, 4, 10 ) ); | |
414 | ||
415 | It would be nice if we could do this in one fell swoop: unpack a short, | |
416 | back up a little, and then unpack 2 bytes. Since Perl I<is> nice, it | |
417 | proffers the template code C<X> to back up one byte. Putting this all | |
418 | together, we may now write: | |
419 | ||
420 | my( $ip, $cs, | |
421 | $flags,$fl,$fh, | |
422 | $ax,$al,$ah, $bx,$bl,$bh, $cx,$cl,$ch, $dx,$dl,$dh, | |
423 | $si, $di, $bp, $ds, $es ) = | |
424 | unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame ); | |
425 | ||
49704364 WL |
426 | (The clumsy construction of the template can be avoided - just read on!) |
427 | ||
47f22e19 | 428 | We've taken some pains to construct the template so that it matches |
34babc16 JH |
429 | the contents of our frame buffer. Otherwise we'd either get undefined values, |
430 | or C<unpack> could not unpack all. If C<pack> runs out of items, it will | |
47f22e19 WL |
431 | supply null strings (which are coerced into zeroes whenever the pack code |
432 | says so). | |
34babc16 JH |
433 | |
434 | ||
435 | =head2 How to Eat an Egg on a Net | |
436 | ||
437 | The pack code for big-endian (high order byte at the lowest address) is | |
438 | C<n> for 16 bit and C<N> for 32 bit integers. You use these codes | |
439 | if you know that your data comes from a compliant architecture, but, | |
440 | surprisingly enough, you should also use these pack codes if you | |
441 | exchange binary data, across the network, with some system that you | |
442 | know next to nothing about. The simple reason is that this | |
443 | order has been chosen as the I<network order>, and all standard-fearing | |
444 | programs ought to follow this convention. (This is, of course, a stern | |
445 | backing for one of the Lilliputian parties and may well influence the | |
446 | political development there.) So, if the protocol expects you to send | |
447 | a message by sending the length first, followed by just so many bytes, | |
448 | you could write: | |
449 | ||
450 | my $buf = pack( 'N', length( $msg ) ) . $msg; | |
451 | ||
452 | or even: | |
453 | ||
454 | my $buf = pack( 'NA*', length( $msg ), $msg ); | |
455 | ||
456 | and pass C<$buf> to your send routine. Some protocols demand that the | |
457 | count should include the length of the count itself: then just add 4 | |
458 | to the data length. (But make sure to read L<"Lengths and Widths"> before | |
459 | you really code this!) | |
460 | ||
461 | ||
59f20ca5 MHM |
462 | =head2 Byte-order modifiers |
463 | ||
464 | In the previous sections we've learned how to use C<n>, C<N>, C<v> and | |
465 | C<V> to pack and unpack integers with big- or little-endian byte-order. | |
466 | While this is nice, it's still rather limited because it leaves out all | |
467 | kinds of signed integers as well as 64-bit integers. For example, if you | |
468 | wanted to unpack a sequence of signed big-endian 16-bit integers in a | |
469 | platform-independent way, you would have to write: | |
470 | ||
471 | my @data = unpack 's*', pack 'S*', unpack 'n*', $buf; | |
472 | ||
c4ecfaf1 | 473 | This is ugly. As of Perl 5.9.2, there's a much nicer way to express your |
59f20ca5 MHM |
474 | desire for a certain byte-order: the C<E<gt>> and C<E<lt>> modifiers. |
475 | C<E<gt>> is the big-endian modifier, while C<E<lt>> is the little-endian | |
476 | modifier. Using them, we could rewrite the above code as: | |
477 | ||
478 | my @data = unpack 's>*', $buf; | |
479 | ||
480 | As you can see, the "big end" of the arrow touches the C<s>, which is a | |
481 | nice way to remember that C<E<gt>> is the big-endian modifier. The same | |
482 | obviously works for C<E<lt>>, where the "little end" touches the code. | |
483 | ||
484 | You will probably find these modifiers even more useful if you have | |
485 | to deal with big- or little-endian C structures. Be sure to read | |
486 | L<"Packing and Unpacking C Structures"> for more on that. | |
487 | ||
34babc16 JH |
488 | |
489 | =head2 Floating point Numbers | |
490 | ||
491 | For packing floating point numbers you have the choice between the | |
59f20ca5 MHM |
492 | pack codes C<f>, C<d>, C<F> and C<D>. C<f> and C<d> pack into (or unpack |
493 | from) single-precision or double-precision representation as it is provided | |
494 | by your system. If your systems supports it, C<D> can be used to pack and | |
495 | unpack extended-precision floating point values (C<long double>), which | |
496 | can offer even more resolution than C<f> or C<d>. C<F> packs an C<NV>, | |
497 | which is the floating point type used by Perl internally. (There | |
34babc16 JH |
498 | is no such thing as a network representation for reals, so if you want |
499 | to send your real numbers across computer boundaries, you'd better stick | |
500 | to ASCII representation, unless you're absolutely sure what's on the other | |
59f20ca5 MHM |
501 | end of the line. For the even more adventuresome, you can use the byte-order |
502 | modifiers from the previous section also on floating point codes.) | |
34babc16 JH |
503 | |
504 | ||
505 | ||
506 | =head1 Exotic Templates | |
507 | ||
508 | ||
509 | =head2 Bit Strings | |
510 | ||
511 | Bits are the atoms in the memory world. Access to individual bits may | |
512 | have to be used either as a last resort or because it is the most | |
513 | convenient way to handle your data. Bit string (un)packing converts | |
514 | between strings containing a series of C<0> and C<1> characters and | |
515 | a sequence of bytes each containing a group of 8 bits. This is almost | |
516 | as simple as it sounds, except that there are two ways the contents of | |
517 | a byte may be written as a bit string. Let's have a look at an annotated | |
518 | byte: | |
519 | ||
520 | 7 6 5 4 3 2 1 0 | |
521 | +-----------------+ | |
522 | | 1 0 0 0 1 1 0 0 | | |
523 | +-----------------+ | |
524 | MSB LSB | |
525 | ||
526 | It's egg-eating all over again: Some think that as a bit string this should | |
527 | be written "10001100" i.e. beginning with the most significant bit, others | |
528 | insist on "00110001". Well, Perl isn't biased, so that's why we have two bit | |
529 | string codes: | |
530 | ||
531 | $byte = pack( 'B8', '10001100' ); # start with MSB | |
532 | $byte = pack( 'b8', '00110001' ); # start with LSB | |
533 | ||
534 | It is not possible to pack or unpack bit fields - just integral bytes. | |
535 | C<pack> always starts at the next byte boundary and "rounds up" to the | |
536 | next multiple of 8 by adding zero bits as required. (If you do want bit | |
537 | fields, there is L<perlfunc/vec>. Or you could implement bit field | |
538 | handling at the character string level, using split, substr, and | |
539 | concatenation on unpacked bit strings.) | |
540 | ||
541 | To illustrate unpacking for bit strings, we'll decompose a simple | |
542 | status register (a "-" stands for a "reserved" bit): | |
543 | ||
544 | +-----------------+-----------------+ | |
545 | | S Z - A - P - C | - - - - O D I T | | |
546 | +-----------------+-----------------+ | |
547 | MSB LSB MSB LSB | |
548 | ||
549 | Converting these two bytes to a string can be done with the unpack | |
550 | template C<'b16'>. To obtain the individual bit values from the bit | |
47f22e19 | 551 | string we use C<split> with the "empty" separator pattern which dissects |
34babc16 JH |
552 | into individual characters. Bit values from the "reserved" positions are |
553 | simply assigned to C<undef>, a convenient notation for "I don't care where | |
554 | this goes". | |
555 | ||
49704364 | 556 | ($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign, |
34babc16 | 557 | $trace, $interrupt, $direction, $overflow) = |
47f22e19 | 558 | split( //, unpack( 'b16', $status ) ); |
34babc16 JH |
559 | |
560 | We could have used an unpack template C<'b12'> just as well, since the | |
561 | last 4 bits can be ignored anyway. | |
562 | ||
563 | ||
564 | =head2 Uuencoding | |
565 | ||
566 | Another odd-man-out in the template alphabet is C<u>, which packs an | |
567 | "uuencoded string". ("uu" is short for Unix-to-Unix.) Chances are that | |
568 | you won't ever need this encoding technique which was invented to overcome | |
569 | the shortcomings of old-fashioned transmission mediums that do not support | |
570 | other than simple ASCII data. The essential recipe is simple: Take three | |
571 | bytes, or 24 bits. Split them into 4 six-packs, adding a space (0x20) to | |
572 | each. Repeat until all of the data is blended. Fold groups of 4 bytes into | |
573 | lines no longer than 60 and garnish them in front with the original byte count | |
574 | (incremented by 0x20) and a C<"\n"> at the end. - The C<pack> chef will | |
575 | prepare this for you, a la minute, when you select pack code C<u> on the menu: | |
576 | ||
577 | my $uubuf = pack( 'u', $bindat ); | |
578 | ||
579 | A repeat count after C<u> sets the number of bytes to put into an | |
580 | uuencoded line, which is the maximum of 45 by default, but could be | |
581 | set to some (smaller) integer multiple of three. C<unpack> simply ignores | |
582 | the repeat count. | |
583 | ||
584 | ||
585 | =head2 Doing Sums | |
586 | ||
587 | An even stranger template code is C<%>E<lt>I<number>E<gt>. First, because | |
588 | it's used as a prefix to some other template code. Second, because it | |
589 | cannot be used in C<pack> at all, and third, in C<unpack>, doesn't return the | |
590 | data as defined by the template code it precedes. Instead it'll give you an | |
591 | integer of I<number> bits that is computed from the data value by | |
592 | doing sums. For numeric unpack codes, no big feat is achieved: | |
593 | ||
594 | my $buf = pack( 'iii', 100, 20, 3 ); | |
595 | print unpack( '%32i3', $buf ), "\n"; # prints 123 | |
596 | ||
597 | For string values, C<%> returns the sum of the byte values saving | |
598 | you the trouble of a sum loop with C<substr> and C<ord>: | |
599 | ||
600 | print unpack( '%32A*', "\x01\x10" ), "\n"; # prints 17 | |
601 | ||
602 | Although the C<%> code is documented as returning a "checksum": | |
603 | don't put your trust in such values! Even when applied to a small number | |
604 | of bytes, they won't guarantee a noticeable Hamming distance. | |
605 | ||
606 | In connection with C<b> or C<B>, C<%> simply adds bits, and this can be put | |
607 | to good use to count set bits efficiently: | |
608 | ||
609 | my $bitcount = unpack( '%32b*', $mask ); | |
610 | ||
611 | And an even parity bit can be determined like this: | |
612 | ||
613 | my $evenparity = unpack( '%1b*', $mask ); | |
614 | ||
615 | ||
616 | =head2 Unicode | |
617 | ||
618 | Unicode is a character set that can represent most characters in most of | |
619 | the world's languages, providing room for over one million different | |
620 | characters. Unicode 3.1 specifies 94,140 characters: The Basic Latin | |
621 | characters are assigned to the numbers 0 - 127. The Latin-1 Supplement with | |
622 | characters that are used in several European languages is in the next | |
623 | range, up to 255. After some more Latin extensions we find the character | |
47b6252e | 624 | sets from languages using non-Roman alphabets, interspersed with a |
34babc16 | 625 | variety of symbol sets such as currency symbols, Zapf Dingbats or Braille. |
f979aebc | 626 | (You might want to visit L<http://www.unicode.org/> for a look at some of |
34babc16 JH |
627 | them - my personal favourites are Telugu and Kannada.) |
628 | ||
629 | The Unicode character sets associates characters with integers. Encoding | |
630 | these numbers in an equal number of bytes would more than double the | |
47b6252e | 631 | requirements for storing texts written in Latin alphabets. |
34babc16 JH |
632 | The UTF-8 encoding avoids this by storing the most common (from a western |
633 | point of view) characters in a single byte while encoding the rarer | |
634 | ones in three or more bytes. | |
635 | ||
2575c402 JW |
636 | Perl uses UTF-8, internally, for most Unicode strings. |
637 | ||
638 | So what has this got to do with C<pack>? Well, if you want to compose a | |
639 | Unicode string (that is internally encoded as UTF-8), you can do so by | |
640 | using template code C<U>. As an example, let's produce the Euro currency | |
641 | symbol (code number 0x20AC): | |
34babc16 JH |
642 | |
643 | $UTF8{Euro} = pack( 'U', 0x20AC ); | |
2575c402 | 644 | # Equivalent to: $UTF8{Euro} = "\x{20ac}"; |
34babc16 | 645 | |
2575c402 JW |
646 | Inspecting C<$UTF8{Euro}> shows that it contains 3 bytes: |
647 | "\xe2\x82\xac". However, it contains only 1 character, number 0x20AC. | |
648 | The round trip can be completed with C<unpack>: | |
34babc16 JH |
649 | |
650 | $Unicode{Euro} = unpack( 'U', $UTF8{Euro} ); | |
651 | ||
2575c402 JW |
652 | Unpacking using the C<U> template code also works on UTF-8 encoded byte |
653 | strings. | |
654 | ||
34babc16 JH |
655 | Usually you'll want to pack or unpack UTF-8 strings: |
656 | ||
657 | # pack and unpack the Hebrew alphabet | |
658 | my $alefbet = pack( 'U*', 0x05d0..0x05ea ); | |
659 | my @hebrew = unpack( 'U*', $utf ); | |
660 | ||
2575c402 JW |
661 | Please note: in the general case, you're better off using |
662 | Encode::decode_utf8 to decode a UTF-8 encoded byte string to a Perl | |
38a44b82 | 663 | Unicode string, and Encode::encode_utf8 to encode a Perl Unicode string |
2575c402 JW |
664 | to UTF-8 bytes. These functions provide means of handling invalid byte |
665 | sequences and generally have a friendlier interface. | |
34babc16 | 666 | |
47f22e19 WL |
667 | =head2 Another Portable Binary Encoding |
668 | ||
669 | The pack code C<w> has been added to support a portable binary data | |
670 | encoding scheme that goes way beyond simple integers. (Details can | |
f979aebc | 671 | be found at L<http://Casbah.org/>, the Scarab project.) A BER (Binary Encoded |
47f22e19 WL |
672 | Representation) compressed unsigned integer stores base 128 |
673 | digits, most significant digit first, with as few digits as possible. | |
674 | Bit eight (the high bit) is set on each byte except the last. There | |
675 | is no size limit to BER encoding, but Perl won't go to extremes. | |
676 | ||
677 | my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 ); | |
678 | ||
679 | A hex dump of C<$berbuf>, with spaces inserted at the right places, | |
680 | shows 01 8100 8101 81807F. Since the last byte is always less than | |
681 | 128, C<unpack> knows where to stop. | |
682 | ||
34babc16 | 683 | |
49704364 WL |
684 | =head1 Template Grouping |
685 | ||
686 | Prior to Perl 5.8, repetitions of templates had to be made by | |
687 | C<x>-multiplication of template strings. Now there is a better way as | |
688 | we may use the pack codes C<(> and C<)> combined with a repeat count. | |
689 | The C<unpack> template from the Stack Frame example can simply | |
690 | be written like this: | |
691 | ||
692 | unpack( 'v2 (vXXCC)5 v5', $frame ) | |
693 | ||
694 | Let's explore this feature a little more. We'll begin with the equivalent of | |
695 | ||
696 | join( '', map( substr( $_, 0, 1 ), @str ) ) | |
697 | ||
698 | which returns a string consisting of the first character from each string. | |
699 | Using pack, we can write | |
700 | ||
701 | pack( '(A)'.@str, @str ) | |
702 | ||
703 | or, because a repeat count C<*> means "repeat as often as required", | |
704 | simply | |
705 | ||
706 | pack( '(A)*', @str ) | |
707 | ||
708 | (Note that the template C<A*> would only have packed C<$str[0]> in full | |
709 | length.) | |
ffc145e8 | 710 | |
49704364 WL |
711 | To pack dates stored as triplets ( day, month, year ) in an array C<@dates> |
712 | into a sequence of byte, byte, short integer we can write | |
713 | ||
714 | $pd = pack( '(CCS)*', map( @$_, @dates ) ); | |
715 | ||
716 | To swap pairs of characters in a string (with even length) one could use | |
717 | several techniques. First, let's use C<x> and C<X> to skip forward and back: | |
718 | ||
719 | $s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) ); | |
720 | ||
721 | We can also use C<@> to jump to an offset, with 0 being the position where | |
722 | we were when the last C<(> was encountered: | |
723 | ||
724 | $s = pack( '(A)*', unpack( '(@1A @0A @2)*', $s ) ); | |
725 | ||
726 | Finally, there is also an entirely different approach by unpacking big | |
727 | endian shorts and packing them in the reverse byte order: | |
728 | ||
729 | $s = pack( '(v)*', unpack( '(n)*', $s ); | |
730 | ||
731 | ||
34babc16 JH |
732 | =head1 Lengths and Widths |
733 | ||
734 | =head2 String Lengths | |
735 | ||
736 | In the previous section we've seen a network message that was constructed | |
737 | by prefixing the binary message length to the actual message. You'll find | |
738 | that packing a length followed by so many bytes of data is a | |
739 | frequently used recipe since appending a null byte won't work | |
47b6252e | 740 | if a null byte may be part of the data. Here is an example where both |
34babc16 JH |
741 | techniques are used: after two null terminated strings with source and |
742 | destination address, a Short Message (to a mobile phone) is sent after | |
743 | a length byte: | |
744 | ||
745 | my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm ); | |
746 | ||
747 | Unpacking this message can be done with the same template: | |
748 | ||
749 | ( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg ); | |
750 | ||
47b6252e | 751 | There's a subtle trap lurking in the offing: Adding another field after |
34babc16 JH |
752 | the Short Message (in variable C<$sm>) is all right when packing, but this |
753 | cannot be unpacked naively: | |
754 | ||
755 | # pack a message | |
756 | my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio ); | |
7207e29d | 757 | |
34babc16 JH |
758 | # unpack fails - $prio remains undefined! |
759 | ( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg ); | |
760 | ||
761 | The pack code C<A*> gobbles up all remaining bytes, and C<$prio> remains | |
762 | undefined! Before we let disappointment dampen the morale: Perl's got | |
763 | the trump card to make this trick too, just a little further up the sleeve. | |
764 | Watch this: | |
765 | ||
766 | # pack a message: ASCIIZ, ASCIIZ, length/string, byte | |
767 | my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio ); | |
768 | ||
769 | # unpack | |
770 | ( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg ); | |
771 | ||
772 | Combining two pack codes with a slash (C</>) associates them with a single | |
773 | value from the argument list. In C<pack>, the length of the argument is | |
774 | taken and packed according to the first code while the argument itself | |
775 | is added after being converted with the template code after the slash. | |
776 | This saves us the trouble of inserting the C<length> call, but it is | |
777 | in C<unpack> where we really score: The value of the length byte marks the | |
778 | end of the string to be taken from the buffer. Since this combination | |
f8b4d74f | 779 | doesn't make sense except when the second pack code isn't C<a*>, C<A*> |
34babc16 JH |
780 | or C<Z*>, Perl won't let you. |
781 | ||
782 | The pack code preceding C</> may be anything that's fit to represent a | |
783 | number: All the numeric binary pack codes, and even text codes such as | |
784 | C<A4> or C<Z*>: | |
785 | ||
786 | # pack/unpack a string preceded by its length in ASCII | |
787 | my $buf = pack( 'A4/A*', "Humpty-Dumpty" ); | |
788 | # unpack $buf: '13 Humpty-Dumpty' | |
789 | my $txt = unpack( 'A4/A*', $buf ); | |
790 | ||
47b6252e NC |
791 | C</> is not implemented in Perls before 5.6, so if your code is required to |
792 | work on older Perls you'll need to C<unpack( 'Z* Z* C')> to get the length, | |
793 | then use it to make a new unpack string. For example | |
794 | ||
795 | # pack a message: ASCIIZ, ASCIIZ, length, string, byte (5.005 compatible) | |
796 | my $msg = pack( 'Z* Z* C A* C', $src, $dst, length $sm, $sm, $prio ); | |
797 | ||
798 | # unpack | |
799 | ( undef, undef, $len) = unpack( 'Z* Z* C', $msg ); | |
800 | ($src, $dst, $sm, $prio) = unpack ( "Z* Z* x A$len C", $msg ); | |
801 | ||
802 | But that second C<unpack> is rushing ahead. It isn't using a simple literal | |
803 | string for the template. So maybe we should introduce... | |
34babc16 JH |
804 | |
805 | =head2 Dynamic Templates | |
806 | ||
807 | So far, we've seen literals used as templates. If the list of pack | |
808 | items doesn't have fixed length, an expression constructing the | |
49704364 WL |
809 | template is required (whenever, for some reason, C<()*> cannot be used). |
810 | Here's an example: To store named string values in a way that can be | |
811 | conveniently parsed by a C program, we create a sequence of names and | |
812 | null terminated ASCII strings, with C<=> between the name and the value, | |
813 | followed by an additional delimiting null byte. Here's how: | |
34babc16 | 814 | |
49704364 | 815 | my $env = pack( '(A*A*Z*)' . keys( %Env ) . 'C', |
47f22e19 WL |
816 | map( { ( $_, '=', $Env{$_} ) } keys( %Env ) ), 0 ); |
817 | ||
818 | Let's examine the cogs of this byte mill, one by one. There's the C<map> | |
819 | call, creating the items we intend to stuff into the C<$env> buffer: | |
820 | to each key (in C<$_>) it adds the C<=> separator and the hash entry value. | |
821 | Each triplet is packed with the template code sequence C<A*A*Z*> that | |
49704364 | 822 | is repeated according to the number of keys. (Yes, that's what the C<keys> |
fe854a6f | 823 | function returns in scalar context.) To get the very last null byte, |
47f22e19 WL |
824 | we add a C<0> at the end of the C<pack> list, to be packed with C<C>. |
825 | (Attentive readers may have noticed that we could have omitted the 0.) | |
34babc16 JH |
826 | |
827 | For the reverse operation, we'll have to determine the number of items | |
828 | in the buffer before we can let C<unpack> rip it apart: | |
829 | ||
47b6252e | 830 | my $n = $env =~ tr/\0// - 1; |
49704364 | 831 | my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) ); |
34babc16 | 832 | |
47b6252e | 833 | The C<tr> counts the null bytes. The C<unpack> call returns a list of |
47f22e19 | 834 | name-value pairs each of which is taken apart in the C<map> block. |
34babc16 JH |
835 | |
836 | ||
49704364 WL |
837 | =head2 Counting Repetitions |
838 | ||
839 | Rather than storing a sentinel at the end of a data item (or a list of items), | |
840 | we could precede the data with a count. Again, we pack keys and values of | |
841 | a hash, preceding each with an unsigned short length count, and up front | |
842 | we store the number of pairs: | |
843 | ||
844 | my $env = pack( 'S(S/A* S/A*)*', scalar keys( %Env ), %Env ); | |
845 | ||
846 | This simplifies the reverse operation as the number of repetitions can be | |
847 | unpacked with the C</> code: | |
848 | ||
849 | my %env = unpack( 'S/(S/A* S/A*)', $env ); | |
850 | ||
851 | Note that this is one of the rare cases where you cannot use the same | |
852 | template for C<pack> and C<unpack> because C<pack> can't determine | |
853 | a repeat count for a C<()>-group. | |
854 | ||
855 | ||
aa51dd41 MB |
856 | =head2 Intel HEX |
857 | ||
858 | Intel HEX is a file format for representing binary data, mostly for | |
859 | programming various chips, as a text file. (See | |
860 | L<http://en.wikipedia.org/wiki/.hex> for a detailed description, and | |
861 | L<http://en.wikipedia.org/wiki/SREC_(file_format)> for the Motorola | |
862 | S-record format, which can be unravelled using the same technique.) | |
863 | Each line begins with a colon (':') and is followed by a sequence of | |
864 | hexadecimal characters, specifying a byte count I<n> (8 bit), | |
865 | an address (16 bit, big endian), a record type (8 bit), I<n> data bytes | |
866 | and a checksum (8 bit) computed as the least significant byte of the two's | |
867 | complement sum of the preceding bytes. Example: C<:0300300002337A1E>. | |
868 | ||
869 | The first step of processing such a line is the conversion, to binary, | |
870 | of the hexadecimal data, to obtain the four fields, while checking the | |
871 | checksum. No surprise here: we'll start with a simple C<pack> call to | |
872 | convert everything to binary: | |
873 | ||
874 | my $binrec = pack( 'H*', substr( $hexrec, 1 ) ); | |
875 | ||
876 | The resulting byte sequence is most convenient for checking the checksum. | |
877 | Don't slow your program down with a for loop adding the C<ord> values | |
878 | of this string's bytes - the C<unpack> code C<%> is the thing to use | |
879 | for computing the 8-bit sum of all bytes, which must be equal to zero: | |
880 | ||
881 | die unless unpack( "%8C*", $binrec ) == 0; | |
882 | ||
883 | Finally, let's get those four fields. By now, you shouldn't have any | |
884 | problems with the first three fields - but how can we use the byte count | |
885 | of the data in the first field as a length for the data field? Here | |
886 | the codes C<x> and C<X> come to the rescue, as they permit jumping | |
887 | back and forth in the string to unpack. | |
888 | ||
889 | my( $addr, $type, $data ) = unpack( "x n C X4 C x3 /a", $bin ); | |
890 | ||
891 | Code C<x> skips a byte, since we don't need the count yet. Code C<n> takes | |
892 | care of the 16-bit big-endian integer address, and C<C> unpacks the | |
893 | record type. Being at offset 4, where the data begins, we need the count. | |
894 | C<X4> brings us back to square one, which is the byte at offset 0. | |
895 | Now we pick up the count, and zoom forth to offset 4, where we are | |
896 | now fully furnished to extract the exact number of data bytes, leaving | |
897 | the trailing checksum byte alone. | |
898 | ||
899 | ||
900 | ||
34babc16 JH |
901 | =head1 Packing and Unpacking C Structures |
902 | ||
903 | In previous sections we have seen how to pack numbers and character | |
904 | strings. If it were not for a couple of snags we could conclude this | |
905 | section right away with the terse remark that C structures don't | |
906 | contain anything else, and therefore you already know all there is to it. | |
907 | Sorry, no: read on, please. | |
908 | ||
59f20ca5 MHM |
909 | If you have to deal with a lot of C structures, and don't want to |
910 | hack all your template strings manually, you'll probably want to have | |
911 | a look at the CPAN module C<Convert::Binary::C>. Not only can it parse | |
912 | your C source directly, but it also has built-in support for all the | |
913 | odds and ends described further on in this section. | |
914 | ||
34babc16 JH |
915 | =head2 The Alignment Pit |
916 | ||
917 | In the consideration of speed against memory requirements the balance | |
918 | has been tilted in favor of faster execution. This has influenced the | |
919 | way C compilers allocate memory for structures: On architectures | |
920 | where a 16-bit or 32-bit operand can be moved faster between places in | |
921 | memory, or to or from a CPU register, if it is aligned at an even or | |
922 | multiple-of-four or even at a multiple-of eight address, a C compiler | |
923 | will give you this speed benefit by stuffing extra bytes into structures. | |
924 | If you don't cross the C shoreline this is not likely to cause you any | |
47b6252e NC |
925 | grief (although you should care when you design large data structures, |
926 | or you want your code to be portable between architectures (you do want | |
927 | that, don't you?)). | |
34babc16 JH |
928 | |
929 | To see how this affects C<pack> and C<unpack>, we'll compare these two | |
930 | C structures: | |
931 | ||
932 | typedef struct { | |
933 | char c1; | |
934 | short s; | |
935 | char c2; | |
936 | long l; | |
937 | } gappy_t; | |
938 | ||
939 | typedef struct { | |
940 | long l; | |
941 | short s; | |
942 | char c1; | |
943 | char c2; | |
944 | } dense_t; | |
945 | ||
946 | Typically, a C compiler allocates 12 bytes to a C<gappy_t> variable, but | |
947 | requires only 8 bytes for a C<dense_t>. After investigating this further, | |
948 | we can draw memory maps, showing where the extra 4 bytes are hidden: | |
949 | ||
950 | 0 +4 +8 +12 | |
951 | +--+--+--+--+--+--+--+--+--+--+--+--+ | |
952 | |c1|xx| s |c2|xx|xx|xx| l | xx = fill byte | |
953 | +--+--+--+--+--+--+--+--+--+--+--+--+ | |
954 | gappy_t | |
955 | ||
956 | 0 +4 +8 | |
957 | +--+--+--+--+--+--+--+--+ | |
958 | | l | h |c1|c2| | |
959 | +--+--+--+--+--+--+--+--+ | |
960 | dense_t | |
961 | ||
962 | And that's where the first quirk strikes: C<pack> and C<unpack> | |
963 | templates have to be stuffed with C<x> codes to get those extra fill bytes. | |
964 | ||
965 | The natural question: "Why can't Perl compensate for the gaps?" warrants | |
966 | an answer. One good reason is that C compilers might provide (non-ANSI) | |
967 | extensions permitting all sorts of fancy control over the way structures | |
968 | are aligned, even at the level of an individual structure field. And, if | |
969 | this were not enough, there is an insidious thing called C<union> where | |
970 | the amount of fill bytes cannot be derived from the alignment of the next | |
971 | item alone. | |
972 | ||
973 | OK, so let's bite the bullet. Here's one way to get the alignment right | |
974 | by inserting template codes C<x>, which don't take a corresponding item | |
975 | from the list: | |
976 | ||
977 | my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l ); | |
978 | ||
979 | Note the C<!> after C<l>: We want to make sure that we pack a long | |
47b6252e NC |
980 | integer as it is compiled by our C compiler. And even now, it will only |
981 | work for the platforms where the compiler aligns things as above. | |
982 | And somebody somewhere has a platform where it doesn't. | |
983 | [Probably a Cray, where C<short>s, C<int>s and C<long>s are all 8 bytes. :-)] | |
34babc16 JH |
984 | |
985 | Counting bytes and watching alignments in lengthy structures is bound to | |
986 | be a drag. Isn't there a way we can create the template with a simple | |
987 | program? Here's a C program that does the trick: | |
988 | ||
989 | #include <stdio.h> | |
990 | #include <stddef.h> | |
991 | ||
992 | typedef struct { | |
993 | char fc1; | |
994 | short fs; | |
995 | char fc2; | |
996 | long fl; | |
997 | } gappy_t; | |
998 | ||
999 | #define Pt(struct,field,tchar) \ | |
1000 | printf( "@%d%s ", offsetof(struct,field), # tchar ); | |
1001 | ||
d832b8f6 | 1002 | int main() { |
34babc16 JH |
1003 | Pt( gappy_t, fc1, c ); |
1004 | Pt( gappy_t, fs, s! ); | |
1005 | Pt( gappy_t, fc2, c ); | |
1006 | Pt( gappy_t, fl, l! ); | |
1007 | printf( "\n" ); | |
1008 | } | |
1009 | ||
1010 | The output line can be used as a template in a C<pack> or C<unpack> call: | |
1011 | ||
1012 | my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l ); | |
1013 | ||
1014 | Gee, yet another template code - as if we hadn't plenty. But | |
1015 | C<@> saves our day by enabling us to specify the offset from the beginning | |
1016 | of the pack buffer to the next item: This is just the value | |
1017 | the C<offsetof> macro (defined in C<E<lt>stddef.hE<gt>>) returns when | |
1018 | given a C<struct> type and one of its field names ("member-designator" in | |
1019 | C standardese). | |
1020 | ||
49704364 WL |
1021 | Neither using offsets nor adding C<x>'s to bridge the gaps is satisfactory. |
1022 | (Just imagine what happens if the structure changes.) What we really need | |
1023 | is a way of saying "skip as many bytes as required to the next multiple of N". | |
1024 | In fluent Templatese, you say this with C<x!N> where N is replaced by the | |
1025 | appropriate value. Here's the next version of our struct packaging: | |
1026 | ||
1027 | my $gappy = pack( 'c x!2 s c x!4 l!', $c1, $s, $c2, $l ); | |
1028 | ||
1029 | That's certainly better, but we still have to know how long all the | |
1030 | integers are, and portability is far away. Rather than C<2>, | |
1031 | for instance, we want to say "however long a short is". But this can be | |
1032 | done by enclosing the appropriate pack code in brackets: C<[s]>. So, here's | |
1033 | the very best we can do: | |
1034 | ||
1035 | my $gappy = pack( 'c x![s] s c x![l!] l!', $c1, $s, $c2, $l ); | |
1036 | ||
34babc16 | 1037 | |
59f20ca5 MHM |
1038 | =head2 Dealing with Endian-ness |
1039 | ||
1040 | Now, imagine that we want to pack the data for a machine with a | |
1041 | different byte-order. First, we'll have to figure out how big the data | |
1042 | types on the target machine really are. Let's assume that the longs are | |
1043 | 32 bits wide and the shorts are 16 bits wide. You can then rewrite the | |
1044 | template as: | |
1045 | ||
1046 | my $gappy = pack( 'c x![s] s c x![l] l', $c1, $s, $c2, $l ); | |
1047 | ||
1048 | If the target machine is little-endian, we could write: | |
1049 | ||
1050 | my $gappy = pack( 'c x![s] s< c x![l] l<', $c1, $s, $c2, $l ); | |
1051 | ||
1052 | This forces the short and the long members to be little-endian, and is | |
1053 | just fine if you don't have too many struct members. But we could also | |
1054 | use the byte-order modifier on a group and write the following: | |
1055 | ||
1056 | my $gappy = pack( '( c x![s] s c x![l] l )<', $c1, $s, $c2, $l ); | |
1057 | ||
1058 | This is not as short as before, but it makes it more obvious that we | |
1059 | intend to have little-endian byte-order for a whole group, not only | |
1060 | for individual template codes. It can also be more readable and easier | |
1061 | to maintain. | |
1062 | ||
1063 | ||
34babc16 JH |
1064 | =head2 Alignment, Take 2 |
1065 | ||
1066 | I'm afraid that we're not quite through with the alignment catch yet. The | |
1067 | hydra raises another ugly head when you pack arrays of structures: | |
1068 | ||
1069 | typedef struct { | |
1070 | short count; | |
1071 | char glyph; | |
1072 | } cell_t; | |
1073 | ||
1074 | typedef cell_t buffer_t[BUFLEN]; | |
1075 | ||
1076 | Where's the catch? Padding is neither required before the first field C<count>, | |
1077 | nor between this and the next field C<glyph>, so why can't we simply pack | |
1078 | like this: | |
1079 | ||
1080 | # something goes wrong here: | |
1081 | pack( 's!a' x @buffer, | |
1082 | map{ ( $_->{count}, $_->{glyph} ) } @buffer ); | |
1083 | ||
1084 | This packs C<3*@buffer> bytes, but it turns out that the size of | |
1085 | C<buffer_t> is four times C<BUFLEN>! The moral of the story is that | |
1086 | the required alignment of a structure or array is propagated to the | |
1087 | next higher level where we have to consider padding I<at the end> | |
1088 | of each component as well. Thus the correct template is: | |
1089 | ||
1090 | pack( 's!ax' x @buffer, | |
1091 | map{ ( $_->{count}, $_->{glyph} ) } @buffer ); | |
1092 | ||
47b6252e NC |
1093 | =head2 Alignment, Take 3 |
1094 | ||
1095 | And even if you take all the above into account, ANSI still lets this: | |
1096 | ||
1097 | typedef struct { | |
1098 | char foo[2]; | |
1099 | } foo_t; | |
34babc16 | 1100 | |
47b6252e NC |
1101 | vary in size. The alignment constraint of the structure can be greater than |
1102 | any of its elements. [And if you think that this doesn't affect anything | |
1103 | common, dismember the next cellphone that you see. Many have ARM cores, and | |
1104 | the ARM structure rules make C<sizeof (foo_t)> == 4] | |
34babc16 JH |
1105 | |
1106 | =head2 Pointers for How to Use Them | |
1107 | ||
1108 | The title of this section indicates the second problem you may run into | |
1109 | sooner or later when you pack C structures. If the function you intend | |
1110 | to call expects a, say, C<void *> value, you I<cannot> simply take | |
1111 | a reference to a Perl variable. (Although that value certainly is a | |
1112 | memory address, it's not the address where the variable's contents are | |
1113 | stored.) | |
1114 | ||
1115 | Template code C<P> promises to pack a "pointer to a fixed length string". | |
1116 | Isn't this what we want? Let's try: | |
1117 | ||
1118 | # allocate some storage and pack a pointer to it | |
1119 | my $memory = "\x00" x $size; | |
1120 | my $memptr = pack( 'P', $memory ); | |
1121 | ||
1122 | But wait: doesn't C<pack> just return a sequence of bytes? How can we pass this | |
1123 | string of bytes to some C code expecting a pointer which is, after all, | |
1124 | nothing but a number? The answer is simple: We have to obtain the numeric | |
1125 | address from the bytes returned by C<pack>. | |
1126 | ||
1127 | my $ptr = unpack( 'L!', $memptr ); | |
1128 | ||
1129 | Obviously this assumes that it is possible to typecast a pointer | |
1130 | to an unsigned long and vice versa, which frequently works but should not | |
1131 | be taken as a universal law. - Now that we have this pointer the next question | |
1132 | is: How can we put it to good use? We need a call to some C function | |
1133 | where a pointer is expected. The read(2) system call comes to mind: | |
1134 | ||
1135 | ssize_t read(int fd, void *buf, size_t count); | |
1136 | ||
1137 | After reading L<perlfunc> explaining how to use C<syscall> we can write | |
1138 | this Perl function copying a file to standard output: | |
1139 | ||
1140 | require 'syscall.ph'; | |
1141 | sub cat($){ | |
1142 | my $path = shift(); | |
1143 | my $size = -s $path; | |
1144 | my $memory = "\x00" x $size; # allocate some memory | |
1145 | my $ptr = unpack( 'L', pack( 'P', $memory ) ); | |
1146 | open( F, $path ) || die( "$path: cannot open ($!)\n" ); | |
1147 | my $fd = fileno(F); | |
1148 | my $res = syscall( &SYS_read, fileno(F), $ptr, $size ); | |
1149 | print $memory; | |
1150 | close( F ); | |
1151 | } | |
1152 | ||
1153 | This is neither a specimen of simplicity nor a paragon of portability but | |
1154 | it illustrates the point: We are able to sneak behind the scenes and | |
1155 | access Perl's otherwise well-guarded memory! (Important note: Perl's | |
1156 | C<syscall> does I<not> require you to construct pointers in this roundabout | |
1157 | way. You simply pass a string variable, and Perl forwards the address.) | |
1158 | ||
1159 | How does C<unpack> with C<P> work? Imagine some pointer in the buffer | |
1160 | about to be unpacked: If it isn't the null pointer (which will smartly | |
1161 | produce the C<undef> value) we have a start address - but then what? | |
1162 | Perl has no way of knowing how long this "fixed length string" is, so | |
1163 | it's up to you to specify the actual size as an explicit length after C<P>. | |
1164 | ||
1165 | my $mem = "abcdefghijklmn"; | |
1166 | print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde" | |
1167 | ||
1168 | As a consequence, C<pack> ignores any number or C<*> after C<P>. | |
1169 | ||
1170 | ||
1171 | Now that we have seen C<P> at work, we might as well give C<p> a whirl. | |
1172 | Why do we need a second template code for packing pointers at all? The | |
1173 | answer lies behind the simple fact that an C<unpack> with C<p> promises | |
1174 | a null-terminated string starting at the address taken from the buffer, | |
1175 | and that implies a length for the data item to be returned: | |
1176 | ||
1177 | my $buf = pack( 'p', "abc\x00efhijklmn" ); | |
1178 | print unpack( 'p', $buf ); # prints "abc" | |
1179 | ||
1180 | ||
1181 | ||
1182 | Albeit this is apt to be confusing: As a consequence of the length being | |
1183 | implied by the string's length, a number after pack code C<p> is a repeat | |
1184 | count, not a length as after C<P>. | |
1185 | ||
1186 | ||
1187 | Using C<pack(..., $x)> with C<P> or C<p> to get the address where C<$x> is | |
1188 | actually stored must be used with circumspection. Perl's internal machinery | |
1189 | considers the relation between a variable and that address as its very own | |
1190 | private matter and doesn't really care that we have obtained a copy. Therefore: | |
1191 | ||
1192 | =over 4 | |
1193 | ||
1194 | =item * | |
1195 | ||
1196 | Do not use C<pack> with C<p> or C<P> to obtain the address of variable | |
1197 | that's bound to go out of scope (and thereby freeing its memory) before you | |
1198 | are done with using the memory at that address. | |
1199 | ||
1200 | =item * | |
1201 | ||
1202 | Be very careful with Perl operations that change the value of the | |
1203 | variable. Appending something to the variable, for instance, might require | |
1204 | reallocation of its storage, leaving you with a pointer into no-man's land. | |
1205 | ||
1206 | =item * | |
1207 | ||
1208 | Don't think that you can get the address of a Perl variable | |
1209 | when it is stored as an integer or double number! C<pack('P', $x)> will | |
1210 | force the variable's internal representation to string, just as if you | |
1211 | had written something like C<$x .= ''>. | |
1212 | ||
1213 | =back | |
1214 | ||
1215 | It's safe, however, to P- or p-pack a string literal, because Perl simply | |
1216 | allocates an anonymous variable. | |
1217 | ||
1218 | ||
1219 | ||
1220 | =head1 Pack Recipes | |
1221 | ||
1222 | Here are a collection of (possibly) useful canned recipes for C<pack> | |
1223 | and C<unpack>: | |
1224 | ||
1225 | # Convert IP address for socket functions | |
1226 | pack( "C4", split /\./, "123.4.5.6" ); | |
1227 | ||
1228 | # Count the bits in a chunk of memory (e.g. a select vector) | |
1229 | unpack( '%32b*', $mask ); | |
1230 | ||
1231 | # Determine the endianness of your system | |
1232 | $is_little_endian = unpack( 'c', pack( 's', 1 ) ); | |
1233 | $is_big_endian = unpack( 'xc', pack( 's', 1 ) ); | |
1234 | ||
1235 | # Determine the number of bits in a native integer | |
1236 | $bits = unpack( '%32I!', ~0 ); | |
1237 | ||
1238 | # Prepare argument for the nanosleep system call | |
1239 | my $timespec = pack( 'L!L!', $secs, $nanosecs ); | |
1240 | ||
f8b4d74f WL |
1241 | For a simple memory dump we unpack some bytes into just as |
1242 | many pairs of hex digits, and use C<map> to handle the traditional | |
1243 | spacing - 16 bytes to a line: | |
1244 | ||
34babc16 | 1245 | my $i; |
49704364 WL |
1246 | print map( ++$i % 16 ? "$_ " : "$_\n", |
1247 | unpack( 'H2' x length( $mem ), $mem ) ), | |
f8b4d74f | 1248 | length( $mem ) % 16 ? "\n" : ''; |
34babc16 JH |
1249 | |
1250 | ||
47f22e19 WL |
1251 | =head1 Funnies Section |
1252 | ||
1253 | # Pulling digits out of nowhere... | |
1254 | print unpack( 'C', pack( 'x' ) ), | |
1255 | unpack( '%B*', pack( 'A' ) ), | |
1256 | unpack( 'H', pack( 'A' ) ), | |
1257 | unpack( 'A', unpack( 'C', pack( 'A' ) ) ), "\n"; | |
1258 | ||
1259 | # One for the road ;-) | |
1260 | my $advice = pack( 'all u can in a van' ); | |
1261 | ||
1262 | ||
34babc16 JH |
1263 | =head1 Authors |
1264 | ||
1265 | Simon Cozens and Wolfgang Laun. | |
1266 |