1 |
NAME |
2 |
CBOR::XS - Concise Binary Object Representation (CBOR, RFC7049) |
3 |
|
4 |
SYNOPSIS |
5 |
use CBOR::XS; |
6 |
|
7 |
$binary_cbor_data = encode_cbor $perl_value; |
8 |
$perl_value = decode_cbor $binary_cbor_data; |
9 |
|
10 |
# OO-interface |
11 |
|
12 |
$coder = CBOR::XS->new; |
13 |
$binary_cbor_data = $coder->encode ($perl_value); |
14 |
$perl_value = $coder->decode ($binary_cbor_data); |
15 |
|
16 |
# prefix decoding |
17 |
|
18 |
my $many_cbor_strings = ...; |
19 |
while (length $many_cbor_strings) { |
20 |
my ($data, $length) = $cbor->decode_prefix ($many_cbor_strings); |
21 |
# data was decoded |
22 |
substr $many_cbor_strings, 0, $length, ""; # remove decoded cbor string |
23 |
} |
24 |
|
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DESCRIPTION |
26 |
This module converts Perl data structures to the Concise Binary Object |
27 |
Representation (CBOR) and vice versa. CBOR is a fast binary |
28 |
serialisation format that aims to use an (almost) superset of the JSON |
29 |
data model, i.e. when you can represent something useful in JSON, you |
30 |
should be able to represent it in CBOR. |
31 |
|
32 |
In short, CBOR is a faster and quite compact binary alternative to JSON, |
33 |
with the added ability of supporting serialisation of Perl objects. |
34 |
(JSON often compresses better than CBOR though, so if you plan to |
35 |
compress the data later and speed is less important you might want to |
36 |
compare both formats first). |
37 |
|
38 |
To give you a general idea about speed, with texts in the megabyte |
39 |
range, "CBOR::XS" usually encodes roughly twice as fast as Storable or |
40 |
JSON::XS and decodes about 15%-30% faster than those. The shorter the |
41 |
data, the worse Storable performs in comparison. |
42 |
|
43 |
Regarding compactness, "CBOR::XS"-encoded data structures are usually |
44 |
about 20% smaller than the same data encoded as (compact) JSON or |
45 |
Storable. |
46 |
|
47 |
In addition to the core CBOR data format, this module implements a |
48 |
number of extensions, to support cyclic and shared data structures (see |
49 |
"allow_sharing" and "allow_cycles"), string deduplication (see |
50 |
"pack_strings") and scalar references (always enabled). |
51 |
|
52 |
The primary goal of this module is to be *correct* and the secondary |
53 |
goal is to be *fast*. To reach the latter goal it was written in C. |
54 |
|
55 |
See MAPPING, below, on how CBOR::XS maps perl values to CBOR values and |
56 |
vice versa. |
57 |
|
58 |
FUNCTIONAL INTERFACE |
59 |
The following convenience methods are provided by this module. They are |
60 |
exported by default: |
61 |
|
62 |
$cbor_data = encode_cbor $perl_scalar |
63 |
Converts the given Perl data structure to CBOR representation. |
64 |
Croaks on error. |
65 |
|
66 |
$perl_scalar = decode_cbor $cbor_data |
67 |
The opposite of "encode_cbor": expects a valid CBOR string to parse, |
68 |
returning the resulting perl scalar. Croaks on error. |
69 |
|
70 |
OBJECT-ORIENTED INTERFACE |
71 |
The object oriented interface lets you configure your own encoding or |
72 |
decoding style, within the limits of supported formats. |
73 |
|
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$cbor = new CBOR::XS |
75 |
Creates a new CBOR::XS object that can be used to de/encode CBOR |
76 |
strings. All boolean flags described below are by default |
77 |
*disabled*. |
78 |
|
79 |
The mutators for flags all return the CBOR object again and thus |
80 |
calls can be chained: |
81 |
|
82 |
my $cbor = CBOR::XS->new->encode ({a => [1,2]}); |
83 |
|
84 |
$cbor = $cbor->max_depth ([$maximum_nesting_depth]) |
85 |
$max_depth = $cbor->get_max_depth |
86 |
Sets the maximum nesting level (default 512) accepted while encoding |
87 |
or decoding. If a higher nesting level is detected in CBOR data or a |
88 |
Perl data structure, then the encoder and decoder will stop and |
89 |
croak at that point. |
90 |
|
91 |
Nesting level is defined by number of hash- or arrayrefs that the |
92 |
encoder needs to traverse to reach a given point or the number of |
93 |
"{" or "[" characters without their matching closing parenthesis |
94 |
crossed to reach a given character in a string. |
95 |
|
96 |
Setting the maximum depth to one disallows any nesting, so that |
97 |
ensures that the object is only a single hash/object or array. |
98 |
|
99 |
If no argument is given, the highest possible setting will be used, |
100 |
which is rarely useful. |
101 |
|
102 |
Note that nesting is implemented by recursion in C. The default |
103 |
value has been chosen to be as large as typical operating systems |
104 |
allow without crashing. |
105 |
|
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See SECURITY CONSIDERATIONS, below, for more info on why this is |
107 |
useful. |
108 |
|
109 |
$cbor = $cbor->max_size ([$maximum_string_size]) |
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$max_size = $cbor->get_max_size |
111 |
Set the maximum length a CBOR string may have (in bytes) where |
112 |
decoding is being attempted. The default is 0, meaning no limit. |
113 |
When "decode" is called on a string that is longer then this many |
114 |
bytes, it will not attempt to decode the string but throw an |
115 |
exception. This setting has no effect on "encode" (yet). |
116 |
|
117 |
If no argument is given, the limit check will be deactivated (same |
118 |
as when 0 is specified). |
119 |
|
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See SECURITY CONSIDERATIONS, below, for more info on why this is |
121 |
useful. |
122 |
|
123 |
$cbor = $cbor->allow_unknown ([$enable]) |
124 |
$enabled = $cbor->get_allow_unknown |
125 |
If $enable is true (or missing), then "encode" will *not* throw an |
126 |
exception when it encounters values it cannot represent in CBOR (for |
127 |
example, filehandles) but instead will encode a CBOR "error" value. |
128 |
|
129 |
If $enable is false (the default), then "encode" will throw an |
130 |
exception when it encounters anything it cannot encode as CBOR. |
131 |
|
132 |
This option does not affect "decode" in any way, and it is |
133 |
recommended to leave it off unless you know your communications |
134 |
partner. |
135 |
|
136 |
$cbor = $cbor->allow_sharing ([$enable]) |
137 |
$enabled = $cbor->get_allow_sharing |
138 |
If $enable is true (or missing), then "encode" will not |
139 |
double-encode values that have been referenced before (e.g. when the |
140 |
same object, such as an array, is referenced multiple times), but |
141 |
instead will emit a reference to the earlier value. |
142 |
|
143 |
This means that such values will only be encoded once, and will not |
144 |
result in a deep cloning of the value on decode, in decoders |
145 |
supporting the value sharing extension. This also makes it possible |
146 |
to encode cyclic data structures (which need "allow_cycles" to ne |
147 |
enabled to be decoded by this module). |
148 |
|
149 |
It is recommended to leave it off unless you know your communication |
150 |
partner supports the value sharing extensions to CBOR |
151 |
(<http://cbor.schmorp.de/value-sharing>), as without decoder |
152 |
support, the resulting data structure might be unusable. |
153 |
|
154 |
Detecting shared values incurs a runtime overhead when values are |
155 |
encoded that have a reference counter large than one, and might |
156 |
unnecessarily increase the encoded size, as potentially shared |
157 |
values are encode as shareable whether or not they are actually |
158 |
shared. |
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|
160 |
At the moment, only targets of references can be shared (e.g. |
161 |
scalars, arrays or hashes pointed to by a reference). Weirder |
162 |
constructs, such as an array with multiple "copies" of the *same* |
163 |
string, which are hard but not impossible to create in Perl, are not |
164 |
supported (this is the same as with Storable). |
165 |
|
166 |
If $enable is false (the default), then "encode" will encode shared |
167 |
data structures repeatedly, unsharing them in the process. Cyclic |
168 |
data structures cannot be encoded in this mode. |
169 |
|
170 |
This option does not affect "decode" in any way - shared values and |
171 |
references will always be decoded properly if present. |
172 |
|
173 |
$cbor = $cbor->allow_cycles ([$enable]) |
174 |
$enabled = $cbor->get_allow_cycles |
175 |
If $enable is true (or missing), then "decode" will happily decode |
176 |
self-referential (cyclic) data structures. By default these will not |
177 |
be decoded, as they need manual cleanup to avoid memory leaks, so |
178 |
code that isn't prepared for this will not leak memory. |
179 |
|
180 |
If $enable is false (the default), then "decode" will throw an error |
181 |
when it encounters a self-referential/cyclic data structure. |
182 |
|
183 |
This option does not affect "encode" in any way - shared values and |
184 |
references will always be decoded properly if present. |
185 |
|
186 |
$cbor = $cbor->pack_strings ([$enable]) |
187 |
$enabled = $cbor->get_pack_strings |
188 |
If $enable is true (or missing), then "encode" will try not to |
189 |
encode the same string twice, but will instead encode a reference to |
190 |
the string instead. Depending on your data format, this can save a |
191 |
lot of space, but also results in a very large runtime overhead |
192 |
(expect encoding times to be 2-4 times as high as without). |
193 |
|
194 |
It is recommended to leave it off unless you know your |
195 |
communications partner supports the stringref extension to CBOR |
196 |
(<http://cbor.schmorp.de/stringref>), as without decoder support, |
197 |
the resulting data structure might not be usable. |
198 |
|
199 |
If $enable is false (the default), then "encode" will encode strings |
200 |
the standard CBOR way. |
201 |
|
202 |
This option does not affect "decode" in any way - string references |
203 |
will always be decoded properly if present. |
204 |
|
205 |
$cbor = $cbor->validate_utf8 ([$enable]) |
206 |
$enabled = $cbor->get_validate_utf8 |
207 |
If $enable is true (or missing), then "decode" will validate that |
208 |
elements (text strings) containing UTF-8 data in fact contain valid |
209 |
UTF-8 data (instead of blindly accepting it). This validation |
210 |
obviously takes extra time during decoding. |
211 |
|
212 |
The concept of "valid UTF-8" used is perl's concept, which is a |
213 |
superset of the official UTF-8. |
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|
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If $enable is false (the default), then "decode" will blindly accept |
216 |
UTF-8 data, marking them as valid UTF-8 in the resulting data |
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structure regardless of whether thats true or not. |
218 |
|
219 |
Perl isn't too happy about corrupted UTF-8 in strings, but should |
220 |
generally not crash or do similarly evil things. Extensions might be |
221 |
not so forgiving, so it's recommended to turn on this setting if you |
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receive untrusted CBOR. |
223 |
|
224 |
This option does not affect "encode" in any way - strings that are |
225 |
supposedly valid UTF-8 will simply be dumped into the resulting CBOR |
226 |
string without checking whether that is, in fact, true or not. |
227 |
|
228 |
$cbor = $cbor->filter ([$cb->($tag, $value)]) |
229 |
$cb_or_undef = $cbor->get_filter |
230 |
Sets or replaces the tagged value decoding filter (when $cb is |
231 |
specified) or clears the filter (if no argument or "undef" is |
232 |
provided). |
233 |
|
234 |
The filter callback is called only during decoding, when a |
235 |
non-enforced tagged value has been decoded (see "TAG HANDLING AND |
236 |
EXTENSIONS" for a list of enforced tags). For specific tags, it's |
237 |
often better to provide a default converter using the |
238 |
%CBOR::XS::FILTER hash (see below). |
239 |
|
240 |
The first argument is the numerical tag, the second is the (decoded) |
241 |
value that has been tagged. |
242 |
|
243 |
The filter function should return either exactly one value, which |
244 |
will replace the tagged value in the decoded data structure, or no |
245 |
values, which will result in default handling, which currently means |
246 |
the decoder creates a "CBOR::XS::Tagged" object to hold the tag and |
247 |
the value. |
248 |
|
249 |
When the filter is cleared (the default state), the default filter |
250 |
function, "CBOR::XS::default_filter", is used. This function simply |
251 |
looks up the tag in the %CBOR::XS::FILTER hash. If an entry exists |
252 |
it must be a code reference that is called with tag and value, and |
253 |
is responsible for decoding the value. If no entry exists, it |
254 |
returns no values. |
255 |
|
256 |
Example: decode all tags not handled internally into |
257 |
"CBOR::XS::Tagged" objects, with no other special handling (useful |
258 |
when working with potentially "unsafe" CBOR data). |
259 |
|
260 |
CBOR::XS->new->filter (sub { })->decode ($cbor_data); |
261 |
|
262 |
Example: provide a global filter for tag 1347375694, converting the |
263 |
value into some string form. |
264 |
|
265 |
$CBOR::XS::FILTER{1347375694} = sub { |
266 |
my ($tag, $value); |
267 |
|
268 |
"tag 1347375694 value $value" |
269 |
}; |
270 |
|
271 |
$cbor_data = $cbor->encode ($perl_scalar) |
272 |
Converts the given Perl data structure (a scalar value) to its CBOR |
273 |
representation. |
274 |
|
275 |
$perl_scalar = $cbor->decode ($cbor_data) |
276 |
The opposite of "encode": expects CBOR data and tries to parse it, |
277 |
returning the resulting simple scalar or reference. Croaks on error. |
278 |
|
279 |
($perl_scalar, $octets) = $cbor->decode_prefix ($cbor_data) |
280 |
This works like the "decode" method, but instead of raising an |
281 |
exception when there is trailing garbage after the CBOR string, it |
282 |
will silently stop parsing there and return the number of characters |
283 |
consumed so far. |
284 |
|
285 |
This is useful if your CBOR texts are not delimited by an outer |
286 |
protocol and you need to know where the first CBOR string ends amd |
287 |
the next one starts. |
288 |
|
289 |
CBOR::XS->new->decode_prefix ("......") |
290 |
=> ("...", 3) |
291 |
|
292 |
MAPPING |
293 |
This section describes how CBOR::XS maps Perl values to CBOR values and |
294 |
vice versa. These mappings are designed to "do the right thing" in most |
295 |
circumstances automatically, preserving round-tripping characteristics |
296 |
(what you put in comes out as something equivalent). |
297 |
|
298 |
For the more enlightened: note that in the following descriptions, |
299 |
lowercase *perl* refers to the Perl interpreter, while uppercase *Perl* |
300 |
refers to the abstract Perl language itself. |
301 |
|
302 |
CBOR -> PERL |
303 |
integers |
304 |
CBOR integers become (numeric) perl scalars. On perls without 64 bit |
305 |
support, 64 bit integers will be truncated or otherwise corrupted. |
306 |
|
307 |
byte strings |
308 |
Byte strings will become octet strings in Perl (the Byte values |
309 |
0..255 will simply become characters of the same value in Perl). |
310 |
|
311 |
UTF-8 strings |
312 |
UTF-8 strings in CBOR will be decoded, i.e. the UTF-8 octets will be |
313 |
decoded into proper Unicode code points. At the moment, the validity |
314 |
of the UTF-8 octets will not be validated - corrupt input will |
315 |
result in corrupted Perl strings. |
316 |
|
317 |
arrays, maps |
318 |
CBOR arrays and CBOR maps will be converted into references to a |
319 |
Perl array or hash, respectively. The keys of the map will be |
320 |
stringified during this process. |
321 |
|
322 |
null |
323 |
CBOR null becomes "undef" in Perl. |
324 |
|
325 |
true, false, undefined |
326 |
These CBOR values become "Types:Serialiser::true", |
327 |
"Types:Serialiser::false" and "Types::Serialiser::error", |
328 |
respectively. They are overloaded to act almost exactly like the |
329 |
numbers 1 and 0 (for true and false) or to throw an exception on |
330 |
access (for error). See the Types::Serialiser manpage for details. |
331 |
|
332 |
tagged values |
333 |
Tagged items consists of a numeric tag and another CBOR value. |
334 |
|
335 |
See "TAG HANDLING AND EXTENSIONS" and the description of "->filter" |
336 |
for details on which tags are handled how. |
337 |
|
338 |
anything else |
339 |
Anything else (e.g. unsupported simple values) will raise a decoding |
340 |
error. |
341 |
|
342 |
PERL -> CBOR |
343 |
The mapping from Perl to CBOR is slightly more difficult, as Perl is a |
344 |
typeless language. That means this module can only guess which CBOR type |
345 |
is meant by a perl value. |
346 |
|
347 |
hash references |
348 |
Perl hash references become CBOR maps. As there is no inherent |
349 |
ordering in hash keys (or CBOR maps), they will usually be encoded |
350 |
in a pseudo-random order. This order can be different each time a |
351 |
hahs is encoded. |
352 |
|
353 |
Currently, tied hashes will use the indefinite-length format, while |
354 |
normal hashes will use the fixed-length format. |
355 |
|
356 |
array references |
357 |
Perl array references become fixed-length CBOR arrays. |
358 |
|
359 |
other references |
360 |
Other unblessed references will be represented using the indirection |
361 |
tag extension (tag value 22098, |
362 |
<http://cbor.schmorp.de/indirection>). CBOR decoders are guaranteed |
363 |
to be able to decode these values somehow, by either "doing the |
364 |
right thing", decoding into a generic tagged object, simply ignoring |
365 |
the tag, or something else. |
366 |
|
367 |
CBOR::XS::Tagged objects |
368 |
Objects of this type must be arrays consisting of a single "[tag, |
369 |
value]" pair. The (numerical) tag will be encoded as a CBOR tag, the |
370 |
value will be encoded as appropriate for the value. You must use |
371 |
"CBOR::XS::tag" to create such objects. |
372 |
|
373 |
Types::Serialiser::true, Types::Serialiser::false, |
374 |
Types::Serialiser::error |
375 |
These special values become CBOR true, CBOR false and CBOR undefined |
376 |
values, respectively. You can also use "\1", "\0" and "\undef" |
377 |
directly if you want. |
378 |
|
379 |
other blessed objects |
380 |
Other blessed objects are serialised via "TO_CBOR" or "FREEZE". See |
381 |
"TAG HANDLING AND EXTENSIONS" for specific classes handled by this |
382 |
module, and "OBJECT SERIALISATION" for generic object serialisation. |
383 |
|
384 |
simple scalars |
385 |
Simple Perl scalars (any scalar that is not a reference) are the |
386 |
most difficult objects to encode: CBOR::XS will encode undefined |
387 |
scalars as CBOR null values, scalars that have last been used in a |
388 |
string context before encoding as CBOR strings, and anything else as |
389 |
number value: |
390 |
|
391 |
# dump as number |
392 |
encode_cbor [2] # yields [2] |
393 |
encode_cbor [-3.0e17] # yields [-3e+17] |
394 |
my $value = 5; encode_cbor [$value] # yields [5] |
395 |
|
396 |
# used as string, so dump as string (either byte or text) |
397 |
print $value; |
398 |
encode_cbor [$value] # yields ["5"] |
399 |
|
400 |
# undef becomes null |
401 |
encode_cbor [undef] # yields [null] |
402 |
|
403 |
You can force the type to be a CBOR string by stringifying it: |
404 |
|
405 |
my $x = 3.1; # some variable containing a number |
406 |
"$x"; # stringified |
407 |
$x .= ""; # another, more awkward way to stringify |
408 |
print $x; # perl does it for you, too, quite often |
409 |
|
410 |
You can force whether a string ie encoded as byte or text string by |
411 |
using "utf8::upgrade" and "utf8::downgrade"): |
412 |
|
413 |
utf8::upgrade $x; # encode $x as text string |
414 |
utf8::downgrade $x; # encode $x as byte string |
415 |
|
416 |
Perl doesn't define what operations up- and downgrade strings, so if |
417 |
the difference between byte and text is important, you should up- or |
418 |
downgrade your string as late as possible before encoding. |
419 |
|
420 |
You can force the type to be a CBOR number by numifying it: |
421 |
|
422 |
my $x = "3"; # some variable containing a string |
423 |
$x += 0; # numify it, ensuring it will be dumped as a number |
424 |
$x *= 1; # same thing, the choice is yours. |
425 |
|
426 |
You can not currently force the type in other, less obscure, ways. |
427 |
Tell me if you need this capability (but don't forget to explain why |
428 |
it's needed :). |
429 |
|
430 |
Perl values that seem to be integers generally use the shortest |
431 |
possible representation. Floating-point values will use either the |
432 |
IEEE single format if possible without loss of precision, otherwise |
433 |
the IEEE double format will be used. Perls that use formats other |
434 |
than IEEE double to represent numerical values are supported, but |
435 |
might suffer loss of precision. |
436 |
|
437 |
OBJECT SERIALISATION |
438 |
This module implements both a CBOR-specific and the generic |
439 |
Types::Serialier object serialisation protocol. The following |
440 |
subsections explain both methods. |
441 |
|
442 |
ENCODING |
443 |
This module knows two way to serialise a Perl object: The CBOR-specific |
444 |
way, and the generic way. |
445 |
|
446 |
Whenever the encoder encounters a Perl object that it cannot serialise |
447 |
directly (most of them), it will first look up the "TO_CBOR" method on |
448 |
it. |
449 |
|
450 |
If it has a "TO_CBOR" method, it will call it with the object as only |
451 |
argument, and expects exactly one return value, which it will then |
452 |
substitute and encode it in the place of the object. |
453 |
|
454 |
Otherwise, it will look up the "FREEZE" method. If it exists, it will |
455 |
call it with the object as first argument, and the constant string |
456 |
"CBOR" as the second argument, to distinguish it from other serialisers. |
457 |
|
458 |
The "FREEZE" method can return any number of values (i.e. zero or more). |
459 |
These will be encoded as CBOR perl object, together with the classname. |
460 |
|
461 |
These methods *MUST NOT* change the data structure that is being |
462 |
serialised. Failure to comply to this can result in memory corruption - |
463 |
and worse. |
464 |
|
465 |
If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail |
466 |
with an error. |
467 |
|
468 |
DECODING |
469 |
Objects encoded via "TO_CBOR" cannot (normally) be automatically |
470 |
decoded, but objects encoded via "FREEZE" can be decoded using the |
471 |
following protocol: |
472 |
|
473 |
When an encoded CBOR perl object is encountered by the decoder, it will |
474 |
look up the "THAW" method, by using the stored classname, and will fail |
475 |
if the method cannot be found. |
476 |
|
477 |
After the lookup it will call the "THAW" method with the stored |
478 |
classname as first argument, the constant string "CBOR" as second |
479 |
argument, and all values returned by "FREEZE" as remaining arguments. |
480 |
|
481 |
EXAMPLES |
482 |
Here is an example "TO_CBOR" method: |
483 |
|
484 |
sub My::Object::TO_CBOR { |
485 |
my ($obj) = @_; |
486 |
|
487 |
["this is a serialised My::Object object", $obj->{id}] |
488 |
} |
489 |
|
490 |
When a "My::Object" is encoded to CBOR, it will instead encode a simple |
491 |
array with two members: a string, and the "object id". Decoding this |
492 |
CBOR string will yield a normal perl array reference in place of the |
493 |
object. |
494 |
|
495 |
A more useful and practical example would be a serialisation method for |
496 |
the URI module. CBOR has a custom tag value for URIs, namely 32: |
497 |
|
498 |
sub URI::TO_CBOR { |
499 |
my ($self) = @_; |
500 |
my $uri = "$self"; # stringify uri |
501 |
utf8::upgrade $uri; # make sure it will be encoded as UTF-8 string |
502 |
CBOR::XS::tag 32, "$_[0]" |
503 |
} |
504 |
|
505 |
This will encode URIs as a UTF-8 string with tag 32, which indicates an |
506 |
URI. |
507 |
|
508 |
Decoding such an URI will not (currently) give you an URI object, but |
509 |
instead a CBOR::XS::Tagged object with tag number 32 and the string - |
510 |
exactly what was returned by "TO_CBOR". |
511 |
|
512 |
To serialise an object so it can automatically be deserialised, you need |
513 |
to use "FREEZE" and "THAW". To take the URI module as example, this |
514 |
would be a possible implementation: |
515 |
|
516 |
sub URI::FREEZE { |
517 |
my ($self, $serialiser) = @_; |
518 |
"$self" # encode url string |
519 |
} |
520 |
|
521 |
sub URI::THAW { |
522 |
my ($class, $serialiser, $uri) = @_; |
523 |
|
524 |
$class->new ($uri) |
525 |
} |
526 |
|
527 |
Unlike "TO_CBOR", multiple values can be returned by "FREEZE". For |
528 |
example, a "FREEZE" method that returns "type", "id" and "variant" |
529 |
values would cause an invocation of "THAW" with 5 arguments: |
530 |
|
531 |
sub My::Object::FREEZE { |
532 |
my ($self, $serialiser) = @_; |
533 |
|
534 |
($self->{type}, $self->{id}, $self->{variant}) |
535 |
} |
536 |
|
537 |
sub My::Object::THAW { |
538 |
my ($class, $serialiser, $type, $id, $variant) = @_; |
539 |
|
540 |
$class-<new (type => $type, id => $id, variant => $variant) |
541 |
} |
542 |
|
543 |
MAGIC HEADER |
544 |
There is no way to distinguish CBOR from other formats programmatically. |
545 |
To make it easier to distinguish CBOR from other formats, the CBOR |
546 |
specification has a special "magic string" that can be prepended to any |
547 |
CBOR string without changing its meaning. |
548 |
|
549 |
This string is available as $CBOR::XS::MAGIC. This module does not |
550 |
prepend this string to the CBOR data it generates, but it will ignore it |
551 |
if present, so users can prepend this string as a "file type" indicator |
552 |
as required. |
553 |
|
554 |
THE CBOR::XS::Tagged CLASS |
555 |
CBOR has the concept of tagged values - any CBOR value can be tagged |
556 |
with a numeric 64 bit number, which are centrally administered. |
557 |
|
558 |
"CBOR::XS" handles a few tags internally when en- or decoding. You can |
559 |
also create tags yourself by encoding "CBOR::XS::Tagged" objects, and |
560 |
the decoder will create "CBOR::XS::Tagged" objects itself when it hits |
561 |
an unknown tag. |
562 |
|
563 |
These objects are simply blessed array references - the first member of |
564 |
the array being the numerical tag, the second being the value. |
565 |
|
566 |
You can interact with "CBOR::XS::Tagged" objects in the following ways: |
567 |
|
568 |
$tagged = CBOR::XS::tag $tag, $value |
569 |
This function(!) creates a new "CBOR::XS::Tagged" object using the |
570 |
given $tag (0..2**64-1) to tag the given $value (which can be any |
571 |
Perl value that can be encoded in CBOR, including serialisable Perl |
572 |
objects and "CBOR::XS::Tagged" objects). |
573 |
|
574 |
$tagged->[0] |
575 |
$tagged->[0] = $new_tag |
576 |
$tag = $tagged->tag |
577 |
$new_tag = $tagged->tag ($new_tag) |
578 |
Access/mutate the tag. |
579 |
|
580 |
$tagged->[1] |
581 |
$tagged->[1] = $new_value |
582 |
$value = $tagged->value |
583 |
$new_value = $tagged->value ($new_value) |
584 |
Access/mutate the tagged value. |
585 |
|
586 |
EXAMPLES |
587 |
Here are some examples of "CBOR::XS::Tagged" uses to tag objects. |
588 |
|
589 |
You can look up CBOR tag value and emanings in the IANA registry at |
590 |
<http://www.iana.org/assignments/cbor-tags/cbor-tags.xhtml>. |
591 |
|
592 |
Prepend a magic header ($CBOR::XS::MAGIC): |
593 |
|
594 |
my $cbor = encode_cbor CBOR::XS::tag 55799, $value; |
595 |
# same as: |
596 |
my $cbor = $CBOR::XS::MAGIC . encode_cbor $value; |
597 |
|
598 |
Serialise some URIs and a regex in an array: |
599 |
|
600 |
my $cbor = encode_cbor [ |
601 |
(CBOR::XS::tag 32, "http://www.nethype.de/"), |
602 |
(CBOR::XS::tag 32, "http://software.schmorp.de/"), |
603 |
(CBOR::XS::tag 35, "^[Pp][Ee][Rr][lL]\$"), |
604 |
]; |
605 |
|
606 |
Wrap CBOR data in CBOR: |
607 |
|
608 |
my $cbor_cbor = encode_cbor |
609 |
CBOR::XS::tag 24, |
610 |
encode_cbor [1, 2, 3]; |
611 |
|
612 |
TAG HANDLING AND EXTENSIONS |
613 |
This section describes how this module handles specific tagged values |
614 |
and extensions. If a tag is not mentioned here and no additional filters |
615 |
are provided for it, then the default handling applies (creating a |
616 |
CBOR::XS::Tagged object on decoding, and only encoding the tag when |
617 |
explicitly requested). |
618 |
|
619 |
Tags not handled specifically are currently converted into a |
620 |
CBOR::XS::Tagged object, which is simply a blessed array reference |
621 |
consisting of the numeric tag value followed by the (decoded) CBOR |
622 |
value. |
623 |
|
624 |
Future versions of this module reserve the right to special case |
625 |
additional tags (such as base64url). |
626 |
|
627 |
ENFORCED TAGS |
628 |
These tags are always handled when decoding, and their handling cannot |
629 |
be overriden by the user. |
630 |
|
631 |
26 (perl-object, <http://cbor.schmorp.de/perl-object>) |
632 |
These tags are automatically created (and decoded) for serialisable |
633 |
objects using the "FREEZE/THAW" methods (the Types::Serialier object |
634 |
serialisation protocol). See "OBJECT SERIALISATION" for details. |
635 |
|
636 |
28, 29 (shareable, sharedref, L <http://cbor.schmorp.de/value-sharing>) |
637 |
These tags are automatically decoded when encountered (and they do |
638 |
not result in a cyclic data structure, see "allow_cycles"), |
639 |
resulting in shared values in the decoded object. They are only |
640 |
encoded, however, when "allow_sharing" is enabled. |
641 |
|
642 |
Not all shared values can be successfully decoded: values that |
643 |
reference themselves will *currently* decode as "undef" (this is not |
644 |
the same as a reference pointing to itself, which will be |
645 |
represented as a value that contains an indirect reference to itself |
646 |
- these will be decoded properly). |
647 |
|
648 |
Note that considerably more shared value data structures can be |
649 |
decoded than will be encoded - currently, only values pointed to by |
650 |
references will be shared, others will not. While non-reference |
651 |
shared values can be generated in Perl with some effort, they were |
652 |
considered too unimportant to be supported in the encoder. The |
653 |
decoder, however, will decode these values as shared values. |
654 |
|
655 |
256, 25 (stringref-namespace, stringref, L |
656 |
<http://cbor.schmorp.de/stringref>) |
657 |
These tags are automatically decoded when encountered. They are only |
658 |
encoded, however, when "pack_strings" is enabled. |
659 |
|
660 |
22098 (indirection, <http://cbor.schmorp.de/indirection>) |
661 |
This tag is automatically generated when a reference are encountered |
662 |
(with the exception of hash and array refernces). It is converted to |
663 |
a reference when decoding. |
664 |
|
665 |
55799 (self-describe CBOR, RFC 7049) |
666 |
This value is not generated on encoding (unless explicitly requested |
667 |
by the user), and is simply ignored when decoding. |
668 |
|
669 |
NON-ENFORCED TAGS |
670 |
These tags have default filters provided when decoding. Their handling |
671 |
can be overriden by changing the %CBOR::XS::FILTER entry for the tag, or |
672 |
by providing a custom "filter" callback when decoding. |
673 |
|
674 |
When they result in decoding into a specific Perl class, the module |
675 |
usually provides a corresponding "TO_CBOR" method as well. |
676 |
|
677 |
When any of these need to load additional modules that are not part of |
678 |
the perl core distribution (e.g. URI), it is (currently) up to the user |
679 |
to provide these modules. The decoding usually fails with an exception |
680 |
if the required module cannot be loaded. |
681 |
|
682 |
0, 1 (date/time string, seconds since the epoch) |
683 |
These tags are decoded into Time::Piece objects. The corresponding |
684 |
"Time::Piece::TO_CBOR" method always encodes into tag 1 values |
685 |
currently. |
686 |
|
687 |
The Time::Piece API is generally surprisingly bad, and fractional |
688 |
seconds are only accidentally kept intact, so watch out. On the plus |
689 |
side, the module comes with perl since 5.10, which has to count for |
690 |
something. |
691 |
|
692 |
2, 3 (positive/negative bignum) |
693 |
These tags are decoded into Math::BigInt objects. The corresponding |
694 |
"Math::BigInt::TO_CBOR" method encodes "small" bigints into normal |
695 |
CBOR integers, and others into positive/negative CBOR bignums. |
696 |
|
697 |
4, 5 (decimal fraction/bigfloat) |
698 |
Both decimal fractions and bigfloats are decoded into Math::BigFloat |
699 |
objects. The corresponding "Math::BigFloat::TO_CBOR" method *always* |
700 |
encodes into a decimal fraction. |
701 |
|
702 |
CBOR cannot represent bigfloats with *very* large exponents - |
703 |
conversion of such big float objects is undefined. |
704 |
|
705 |
Also, NaN and infinities are not encoded properly. |
706 |
|
707 |
21, 22, 23 (expected later JSON conversion) |
708 |
CBOR::XS is not a CBOR-to-JSON converter, and will simply ignore |
709 |
these tags. |
710 |
|
711 |
32 (URI) |
712 |
These objects decode into URI objects. The corresponding |
713 |
"URI::TO_CBOR" method again results in a CBOR URI value. |
714 |
|
715 |
CBOR and JSON |
716 |
CBOR is supposed to implement a superset of the JSON data model, and is, |
717 |
with some coercion, able to represent all JSON texts (something that |
718 |
other "binary JSON" formats such as BSON generally do not support). |
719 |
|
720 |
CBOR implements some extra hints and support for JSON interoperability, |
721 |
and the spec offers further guidance for conversion between CBOR and |
722 |
JSON. None of this is currently implemented in CBOR, and the guidelines |
723 |
in the spec do not result in correct round-tripping of data. If JSON |
724 |
interoperability is improved in the future, then the goal will be to |
725 |
ensure that decoded JSON data will round-trip encoding and decoding to |
726 |
CBOR intact. |
727 |
|
728 |
SECURITY CONSIDERATIONS |
729 |
When you are using CBOR in a protocol, talking to untrusted potentially |
730 |
hostile creatures requires relatively few measures. |
731 |
|
732 |
First of all, your CBOR decoder should be secure, that is, should not |
733 |
have any buffer overflows. Obviously, this module should ensure that and |
734 |
I am trying hard on making that true, but you never know. |
735 |
|
736 |
Second, you need to avoid resource-starving attacks. That means you |
737 |
should limit the size of CBOR data you accept, or make sure then when |
738 |
your resources run out, that's just fine (e.g. by using a separate |
739 |
process that can crash safely). The size of a CBOR string in octets is |
740 |
usually a good indication of the size of the resources required to |
741 |
decode it into a Perl structure. While CBOR::XS can check the size of |
742 |
the CBOR text, it might be too late when you already have it in memory, |
743 |
so you might want to check the size before you accept the string. |
744 |
|
745 |
Third, CBOR::XS recurses using the C stack when decoding objects and |
746 |
arrays. The C stack is a limited resource: for instance, on my amd64 |
747 |
machine with 8MB of stack size I can decode around 180k nested arrays |
748 |
but only 14k nested CBOR objects (due to perl itself recursing deeply on |
749 |
croak to free the temporary). If that is exceeded, the program crashes. |
750 |
To be conservative, the default nesting limit is set to 512. If your |
751 |
process has a smaller stack, you should adjust this setting accordingly |
752 |
with the "max_depth" method. |
753 |
|
754 |
Something else could bomb you, too, that I forgot to think of. In that |
755 |
case, you get to keep the pieces. I am always open for hints, though... |
756 |
|
757 |
Also keep in mind that CBOR::XS might leak contents of your Perl data |
758 |
structures in its error messages, so when you serialise sensitive |
759 |
information you might want to make sure that exceptions thrown by |
760 |
CBOR::XS will not end up in front of untrusted eyes. |
761 |
|
762 |
CBOR IMPLEMENTATION NOTES |
763 |
This section contains some random implementation notes. They do not |
764 |
describe guaranteed behaviour, but merely behaviour as-is implemented |
765 |
right now. |
766 |
|
767 |
64 bit integers are only properly decoded when Perl was built with 64 |
768 |
bit support. |
769 |
|
770 |
Strings and arrays are encoded with a definite length. Hashes as well, |
771 |
unless they are tied (or otherwise magical). |
772 |
|
773 |
Only the double data type is supported for NV data types - when Perl |
774 |
uses long double to represent floating point values, they might not be |
775 |
encoded properly. Half precision types are accepted, but not encoded. |
776 |
|
777 |
Strict mode and canonical mode are not implemented. |
778 |
|
779 |
LIMITATIONS ON PERLS WITHOUT 64-BIT INTEGER SUPPORT |
780 |
On perls that were built without 64 bit integer support (these are rare |
781 |
nowadays, even on 32 bit architectures), support for any kind of 64 bit |
782 |
integer in CBOR is very limited - most likely, these 64 bit values will |
783 |
be truncated, corrupted, or otherwise not decoded correctly. This also |
784 |
includes string, array and map sizes that are stored as 64 bit integers. |
785 |
|
786 |
THREADS |
787 |
This module is *not* guaranteed to be thread safe and there are no plans |
788 |
to change this until Perl gets thread support (as opposed to the |
789 |
horribly slow so-called "threads" which are simply slow and bloated |
790 |
process simulations - use fork, it's *much* faster, cheaper, better). |
791 |
|
792 |
(It might actually work, but you have been warned). |
793 |
|
794 |
BUGS |
795 |
While the goal of this module is to be correct, that unfortunately does |
796 |
not mean it's bug-free, only that I think its design is bug-free. If you |
797 |
keep reporting bugs they will be fixed swiftly, though. |
798 |
|
799 |
Please refrain from using rt.cpan.org or any other bug reporting |
800 |
service. I put the contact address into my modules for a reason. |
801 |
|
802 |
SEE ALSO |
803 |
The JSON and JSON::XS modules that do similar, but human-readable, |
804 |
serialisation. |
805 |
|
806 |
The Types::Serialiser module provides the data model for true, false and |
807 |
error values. |
808 |
|
809 |
AUTHOR |
810 |
Marc Lehmann <schmorp@schmorp.de> |
811 |
http://home.schmorp.de/ |
812 |
|