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Revision 1.17 by root, Wed Apr 27 09:40:18 2016 UTC

… …
21 # data was decoded 21 # data was decoded
22 substr $many_cbor_strings, 0, $length, ""; # remove decoded cbor string 22 substr $many_cbor_strings, 0, $length, ""; # remove decoded cbor string
23 } 23 }
24 24
25DESCRIPTION 25DESCRIPTION
26 WARNING! This module is very new, and not very well tested (that's up to
27 you to do). Furthermore, details of the implementation might change
28 freely before version 1.0. And lastly, the object serialisation protocol
29 depends on a pending IANA assignment, and until that assignment is
30 official, this implementation is not interoperable with other
31 implementations (even future versions of this module) until the
32 assignment is done.
33
34 You are still invited to try out CBOR, and this module.
35
36 This module converts Perl data structures to the Concise Binary Object 26 This module converts Perl data structures to the Concise Binary Object
37 Representation (CBOR) and vice versa. CBOR is a fast binary 27 Representation (CBOR) and vice versa. CBOR is a fast binary
38 serialisation format that aims to use a superset of the JSON data model, 28 serialisation format that aims to use an (almost) superset of the JSON
39 i.e. when you can represent something in JSON, you should be able to 29 data model, i.e. when you can represent something useful in JSON, you
40 represent it in CBOR. 30 should be able to represent it in CBOR.
41 31
42 In short, CBOR is a faster and very compact binary alternative to JSON, 32 In short, CBOR is a faster and quite compact binary alternative to JSON,
43 with the added ability of supporting serialisation of Perl objects. 33 with the added ability of supporting serialisation of Perl objects.
44 (JSON often compresses better than CBOR though, so if you plan to 34 (JSON often compresses better than CBOR though, so if you plan to
45 compress the data later you might want to compare both formats first). 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).
46 51
47 The primary goal of this module is to be *correct* and the secondary 52 The primary goal of this module is to be *correct* and the secondary
48 goal is to be *fast*. To reach the latter goal it was written in C. 53 goal is to be *fast*. To reach the latter goal it was written in C.
49 54
50 See MAPPING, below, on how CBOR::XS maps perl values to CBOR values and 55 See MAPPING, below, on how CBOR::XS maps perl values to CBOR values and
… …
72 *disabled*. 77 *disabled*.
73 78
74 The mutators for flags all return the CBOR object again and thus 79 The mutators for flags all return the CBOR object again and thus
75 calls can be chained: 80 calls can be chained:
76 81
77 #TODO my $cbor = CBOR::XS->new->encode ({a => [1,2]}); 82 my $cbor = CBOR::XS->new->encode ({a => [1,2]});
78 83
79 $cbor = $cbor->max_depth ([$maximum_nesting_depth]) 84 $cbor = $cbor->max_depth ([$maximum_nesting_depth])
80 $max_depth = $cbor->get_max_depth 85 $max_depth = $cbor->get_max_depth
81 Sets the maximum nesting level (default 512) accepted while encoding 86 Sets the maximum nesting level (default 512) accepted while encoding
82 or decoding. If a higher nesting level is detected in CBOR data or a 87 or decoding. If a higher nesting level is detected in CBOR data or a
… …
113 as when 0 is specified). 118 as when 0 is specified).
114 119
115 See SECURITY CONSIDERATIONS, below, for more info on why this is 120 See SECURITY CONSIDERATIONS, below, for more info on why this is
116 useful. 121 useful.
117 122
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.
159
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 FUTURE DIRECTION: the motivation behind this option is to avoid
184 *real* cycles - future versions of this module might chose to decode
185 cyclic data structures using weak references when this option is
186 off, instead of throwing an error.
187
188 This option does not affect "encode" in any way - shared values and
189 references will always be encoded properly if present.
190
191 $cbor = $cbor->pack_strings ([$enable])
192 $enabled = $cbor->get_pack_strings
193 If $enable is true (or missing), then "encode" will try not to
194 encode the same string twice, but will instead encode a reference to
195 the string instead. Depending on your data format, this can save a
196 lot of space, but also results in a very large runtime overhead
197 (expect encoding times to be 2-4 times as high as without).
198
199 It is recommended to leave it off unless you know your
200 communications partner supports the stringref extension to CBOR
201 (<http://cbor.schmorp.de/stringref>), as without decoder support,
202 the resulting data structure might not be usable.
203
204 If $enable is false (the default), then "encode" will encode strings
205 the standard CBOR way.
206
207 This option does not affect "decode" in any way - string references
208 will always be decoded properly if present.
209
210 $cbor = $cbor->text_keys ([$enable])
211 $enabled = $cbor->get_text_keys
212 If $enabled is true (or missing), then "encode" will encode all perl
213 hash keys as CBOR text strings/UTF-8 string, upgrading them as
214 needed.
215
216 If $enable is false (the default), then "encode" will encode hash
217 keys normally - upgraded perl strings (strings internally encoded as
218 UTF-8) as CBOR text strings, and downgraded perl strings as CBOR
219 byte strings.
220
221 This option does not affect "decode" in any way.
222
223 This option is useful for interoperability with CBOR decoders that
224 don't treat byte strings as a form of text. It is especially useful
225 as Perl gives very little control over hash keys.
226
227 Enabling this option can be slow, as all downgraded hash keys that
228 are encoded need to be scanned and converted to UTF-8.
229
230 $cbor = $cbor->text_strings ([$enable])
231 $enabled = $cbor->get_text_strings
232 This option works similar to "text_keys", above, but works on all
233 strings (including hash keys), so "text_keys" has no further effect
234 after enabling "text_strings".
235
236 If $enabled is true (or missing), then "encode" will encode all perl
237 strings as CBOR text strings/UTF-8 strings, upgrading them as
238 needed.
239
240 If $enable is false (the default), then "encode" will encode strings
241 normally (but see "text_keys") - upgraded perl strings (strings
242 internally encoded as UTF-8) as CBOR text strings, and downgraded
243 perl strings as CBOR byte strings.
244
245 This option does not affect "decode" in any way.
246
247 This option has similar advantages and disadvantages as "text_keys".
248 In addition, this option effectively removes the ability to encode
249 byte strings, which might break some "FREEZE" and "TO_CBOR" methods
250 that rely on this, such as bignum encoding, so this option is mainly
251 useful for very simple data.
252
253 $cbor = $cbor->validate_utf8 ([$enable])
254 $enabled = $cbor->get_validate_utf8
255 If $enable is true (or missing), then "decode" will validate that
256 elements (text strings) containing UTF-8 data in fact contain valid
257 UTF-8 data (instead of blindly accepting it). This validation
258 obviously takes extra time during decoding.
259
260 The concept of "valid UTF-8" used is perl's concept, which is a
261 superset of the official UTF-8.
262
263 If $enable is false (the default), then "decode" will blindly accept
264 UTF-8 data, marking them as valid UTF-8 in the resulting data
265 structure regardless of whether that's true or not.
266
267 Perl isn't too happy about corrupted UTF-8 in strings, but should
268 generally not crash or do similarly evil things. Extensions might be
269 not so forgiving, so it's recommended to turn on this setting if you
270 receive untrusted CBOR.
271
272 This option does not affect "encode" in any way - strings that are
273 supposedly valid UTF-8 will simply be dumped into the resulting CBOR
274 string without checking whether that is, in fact, true or not.
275
276 $cbor = $cbor->filter ([$cb->($tag, $value)])
277 $cb_or_undef = $cbor->get_filter
278 Sets or replaces the tagged value decoding filter (when $cb is
279 specified) or clears the filter (if no argument or "undef" is
280 provided).
281
282 The filter callback is called only during decoding, when a
283 non-enforced tagged value has been decoded (see "TAG HANDLING AND
284 EXTENSIONS" for a list of enforced tags). For specific tags, it's
285 often better to provide a default converter using the
286 %CBOR::XS::FILTER hash (see below).
287
288 The first argument is the numerical tag, the second is the (decoded)
289 value that has been tagged.
290
291 The filter function should return either exactly one value, which
292 will replace the tagged value in the decoded data structure, or no
293 values, which will result in default handling, which currently means
294 the decoder creates a "CBOR::XS::Tagged" object to hold the tag and
295 the value.
296
297 When the filter is cleared (the default state), the default filter
298 function, "CBOR::XS::default_filter", is used. This function simply
299 looks up the tag in the %CBOR::XS::FILTER hash. If an entry exists
300 it must be a code reference that is called with tag and value, and
301 is responsible for decoding the value. If no entry exists, it
302 returns no values.
303
304 Example: decode all tags not handled internally into
305 "CBOR::XS::Tagged" objects, with no other special handling (useful
306 when working with potentially "unsafe" CBOR data).
307
308 CBOR::XS->new->filter (sub { })->decode ($cbor_data);
309
310 Example: provide a global filter for tag 1347375694, converting the
311 value into some string form.
312
313 $CBOR::XS::FILTER{1347375694} = sub {
314 my ($tag, $value);
315
316 "tag 1347375694 value $value"
317 };
318
118 $cbor_data = $cbor->encode ($perl_scalar) 319 $cbor_data = $cbor->encode ($perl_scalar)
119 Converts the given Perl data structure (a scalar value) to its CBOR 320 Converts the given Perl data structure (a scalar value) to its CBOR
120 representation. 321 representation.
121 322
122 $perl_scalar = $cbor->decode ($cbor_data) 323 $perl_scalar = $cbor->decode ($cbor_data)
… …
134 the next one starts. 335 the next one starts.
135 336
136 CBOR::XS->new->decode_prefix ("......") 337 CBOR::XS->new->decode_prefix ("......")
137 => ("...", 3) 338 => ("...", 3)
138 339
340 INCREMENTAL PARSING
341 In some cases, there is the need for incremental parsing of JSON texts.
342 While this module always has to keep both CBOR text and resulting Perl
343 data structure in memory at one time, it does allow you to parse a CBOR
344 stream incrementally, using a similar to using "decode_prefix" to see if
345 a full CBOR object is available, but is much more efficient.
346
347 It basically works by parsing as much of a CBOR string as possible - if
348 the CBOR data is not complete yet, the pasrer will remember where it
349 was, to be able to restart when more data has been accumulated. Once
350 enough data is available to either decode a complete CBOR value or raise
351 an error, a real decode will be attempted.
352
353 A typical use case would be a network protocol that consists of sending
354 and receiving CBOR-encoded messages. The solution that works with CBOR
355 and about anything else is by prepending a length to every CBOR value,
356 so the receiver knows how many octets to read. More compact (and
357 slightly slower) would be to just send CBOR values back-to-back, as
358 "CBOR::XS" knows where a CBOR value ends, and doesn't need an explicit
359 length.
360
361 The following methods help with this:
362
363 @decoded = $cbor->incr_parse ($buffer)
364 This method attempts to decode exactly one CBOR value from the
365 beginning of the given $buffer. The value is removed from the
366 $buffer on success. When $buffer doesn't contain a complete value
367 yet, it returns nothing. Finally, when the $buffer doesn't start
368 with something that could ever be a valid CBOR value, it raises an
369 exception, just as "decode" would. In the latter case the decoder
370 state is undefined and must be reset before being able to parse
371 further.
372
373 This method modifies the $buffer in place. When no CBOR value can be
374 decoded, the decoder stores the current string offset. On the next
375 call, continues decoding at the place where it stopped before. For
376 this to make sense, the $buffer must begin with the same octets as
377 on previous unsuccessful calls.
378
379 You can call this method in scalar context, in which case it either
380 returns a decoded value or "undef". This makes it impossible to
381 distinguish between CBOR null values (which decode to "undef") and
382 an unsuccessful decode, which is often acceptable.
383
384 @decoded = $cbor->incr_parse_multiple ($buffer)
385 Same as "incr_parse", but attempts to decode as many CBOR values as
386 possible in one go, instead of at most one. Calls to "incr_parse"
387 and "incr_parse_multiple" can be interleaved.
388
389 $cbor->incr_reset
390 Resets the incremental decoder. This throws away any saved state, so
391 that subsequent calls to "incr_parse" or "incr_parse_multiple" start
392 to parse a new CBOR value from the beginning of the $buffer again.
393
394 This method can be caled at any time, but it *must* be called if you
395 want to change your $buffer or there was a decoding error and you
396 want to reuse the $cbor object for future incremental parsings.
397
139MAPPING 398MAPPING
140 This section describes how CBOR::XS maps Perl values to CBOR values and 399 This section describes how CBOR::XS maps Perl values to CBOR values and
141 vice versa. These mappings are designed to "do the right thing" in most 400 vice versa. These mappings are designed to "do the right thing" in most
142 circumstances automatically, preserving round-tripping characteristics 401 circumstances automatically, preserving round-tripping characteristics
143 (what you put in comes out as something equivalent). 402 (what you put in comes out as something equivalent).
… …
150 integers 409 integers
151 CBOR integers become (numeric) perl scalars. On perls without 64 bit 410 CBOR integers become (numeric) perl scalars. On perls without 64 bit
152 support, 64 bit integers will be truncated or otherwise corrupted. 411 support, 64 bit integers will be truncated or otherwise corrupted.
153 412
154 byte strings 413 byte strings
155 Byte strings will become octet strings in Perl (the byte values 414 Byte strings will become octet strings in Perl (the Byte values
156 0..255 will simply become characters of the same value in Perl). 415 0..255 will simply become characters of the same value in Perl).
157 416
158 UTF-8 strings 417 UTF-8 strings
159 UTF-8 strings in CBOR will be decoded, i.e. the UTF-8 octets will be 418 UTF-8 strings in CBOR will be decoded, i.e. the UTF-8 octets will be
160 decoded into proper Unicode code points. At the moment, the validity 419 decoded into proper Unicode code points. At the moment, the validity
… …
174 "Types:Serialiser::false" and "Types::Serialiser::error", 433 "Types:Serialiser::false" and "Types::Serialiser::error",
175 respectively. They are overloaded to act almost exactly like the 434 respectively. They are overloaded to act almost exactly like the
176 numbers 1 and 0 (for true and false) or to throw an exception on 435 numbers 1 and 0 (for true and false) or to throw an exception on
177 access (for error). See the Types::Serialiser manpage for details. 436 access (for error). See the Types::Serialiser manpage for details.
178 437
179 CBOR tag 256 (perl object) 438 tagged values
180 The tag value 256 (TODO: pending iana registration) will be used to
181 deserialise a Perl object serialised with "FREEZE". See OBJECT
182 SERIALISATION, below, for details.
183
184 CBOR tag 55799 (magic header)
185 The tag 55799 is ignored (this tag implements the magic header).
186
187 other CBOR tags
188 Tagged items consists of a numeric tag and another CBOR value. Tags 439 Tagged items consists of a numeric tag and another CBOR value.
189 not handled internally are currently converted into a
190 CBOR::XS::Tagged object, which is simply a blessed array reference
191 consisting of the numeric tag value followed by the (decoded) CBOR
192 value.
193 440
194 In the future, support for user-supplied conversions might get 441 See "TAG HANDLING AND EXTENSIONS" and the description of "->filter"
195 added. 442 for details on which tags are handled how.
196 443
197 anything else 444 anything else
198 Anything else (e.g. unsupported simple values) will raise a decoding 445 Anything else (e.g. unsupported simple values) will raise a decoding
199 error. 446 error.
200 447
201 PERL -> CBOR 448 PERL -> CBOR
202 The mapping from Perl to CBOR is slightly more difficult, as Perl is a 449 The mapping from Perl to CBOR is slightly more difficult, as Perl is a
203 truly typeless language, so we can only guess which CBOR type is meant 450 typeless language. That means this module can only guess which CBOR type
204 by a Perl value. 451 is meant by a perl value.
205 452
206 hash references 453 hash references
207 Perl hash references become CBOR maps. As there is no inherent 454 Perl hash references become CBOR maps. As there is no inherent
208 ordering in hash keys (or CBOR maps), they will usually be encoded 455 ordering in hash keys (or CBOR maps), they will usually be encoded
209 in a pseudo-random order. 456 in a pseudo-random order. This order can be different each time a
457 hash is encoded.
210 458
211 Currently, tied hashes will use the indefinite-length format, while 459 Currently, tied hashes will use the indefinite-length format, while
212 normal hashes will use the fixed-length format. 460 normal hashes will use the fixed-length format.
213 461
214 array references 462 array references
215 Perl array references become fixed-length CBOR arrays. 463 Perl array references become fixed-length CBOR arrays.
216 464
217 other references 465 other references
218 Other unblessed references are generally not allowed and will cause 466 Other unblessed references will be represented using the indirection
219 an exception to be thrown, except for references to the integers 0 467 tag extension (tag value 22098,
220 and 1, which get turned into false and true in CBOR. 468 <http://cbor.schmorp.de/indirection>). CBOR decoders are guaranteed
469 to be able to decode these values somehow, by either "doing the
470 right thing", decoding into a generic tagged object, simply ignoring
471 the tag, or something else.
221 472
222 CBOR::XS::Tagged objects 473 CBOR::XS::Tagged objects
223 Objects of this type must be arrays consisting of a single "[tag, 474 Objects of this type must be arrays consisting of a single "[tag,
224 value]" pair. The (numerical) tag will be encoded as a CBOR tag, the 475 value]" pair. The (numerical) tag will be encoded as a CBOR tag, the
225 value will be encoded as appropriate for the value. You cna use 476 value will be encoded as appropriate for the value. You must use
226 "CBOR::XS::tag" to create such objects. 477 "CBOR::XS::tag" to create such objects.
227 478
228 Types::Serialiser::true, Types::Serialiser::false, 479 Types::Serialiser::true, Types::Serialiser::false,
229 Types::Serialiser::error 480 Types::Serialiser::error
230 These special values become CBOR true, CBOR false and CBOR undefined 481 These special values become CBOR true, CBOR false and CBOR undefined
231 values, respectively. You can also use "\1", "\0" and "\undef" 482 values, respectively. You can also use "\1", "\0" and "\undef"
232 directly if you want. 483 directly if you want.
233 484
234 other blessed objects 485 other blessed objects
235 Other blessed objects are serialised via "TO_CBOR" or "FREEZE". See 486 Other blessed objects are serialised via "TO_CBOR" or "FREEZE". See
236 "OBJECT SERIALISATION", below, for details. 487 "TAG HANDLING AND EXTENSIONS" for specific classes handled by this
488 module, and "OBJECT SERIALISATION" for generic object serialisation.
237 489
238 simple scalars 490 simple scalars
239 TODO Simple Perl scalars (any scalar that is not a reference) are 491 Simple Perl scalars (any scalar that is not a reference) are the
240 the most difficult objects to encode: CBOR::XS will encode undefined 492 most difficult objects to encode: CBOR::XS will encode undefined
241 scalars as CBOR null values, scalars that have last been used in a 493 scalars as CBOR null values, scalars that have last been used in a
242 string context before encoding as CBOR strings, and anything else as 494 string context before encoding as CBOR strings, and anything else as
243 number value: 495 number value:
244 496
245 # dump as number 497 # dump as number
246 encode_cbor [2] # yields [2] 498 encode_cbor [2] # yields [2]
247 encode_cbor [-3.0e17] # yields [-3e+17] 499 encode_cbor [-3.0e17] # yields [-3e+17]
248 my $value = 5; encode_cbor [$value] # yields [5] 500 my $value = 5; encode_cbor [$value] # yields [5]
249 501
250 # used as string, so dump as string 502 # used as string, so dump as string (either byte or text)
251 print $value; 503 print $value;
252 encode_cbor [$value] # yields ["5"] 504 encode_cbor [$value] # yields ["5"]
253 505
254 # undef becomes null 506 # undef becomes null
255 encode_cbor [undef] # yields [null] 507 encode_cbor [undef] # yields [null]
… …
258 510
259 my $x = 3.1; # some variable containing a number 511 my $x = 3.1; # some variable containing a number
260 "$x"; # stringified 512 "$x"; # stringified
261 $x .= ""; # another, more awkward way to stringify 513 $x .= ""; # another, more awkward way to stringify
262 print $x; # perl does it for you, too, quite often 514 print $x; # perl does it for you, too, quite often
515
516 You can force whether a string is encoded as byte or text string by
517 using "utf8::upgrade" and "utf8::downgrade" (if "text_strings" is
518 disabled):
519
520 utf8::upgrade $x; # encode $x as text string
521 utf8::downgrade $x; # encode $x as byte string
522
523 Perl doesn't define what operations up- and downgrade strings, so if
524 the difference between byte and text is important, you should up- or
525 downgrade your string as late as possible before encoding. You can
526 also force the use of CBOR text strings by using "text_keys" or
527 "text_strings".
263 528
264 You can force the type to be a CBOR number by numifying it: 529 You can force the type to be a CBOR number by numifying it:
265 530
266 my $x = "3"; # some variable containing a string 531 my $x = "3"; # some variable containing a string
267 $x += 0; # numify it, ensuring it will be dumped as a number 532 $x += 0; # numify it, ensuring it will be dumped as a number
… …
277 the IEEE double format will be used. Perls that use formats other 542 the IEEE double format will be used. Perls that use formats other
278 than IEEE double to represent numerical values are supported, but 543 than IEEE double to represent numerical values are supported, but
279 might suffer loss of precision. 544 might suffer loss of precision.
280 545
281 OBJECT SERIALISATION 546 OBJECT SERIALISATION
547 This module implements both a CBOR-specific and the generic
548 Types::Serialier object serialisation protocol. The following
549 subsections explain both methods.
550
551 ENCODING
282 This module knows two way to serialise a Perl object: The CBOR-specific 552 This module knows two way to serialise a Perl object: The CBOR-specific
283 way, and the generic way. 553 way, and the generic way.
284 554
285 Whenever the encoder encounters a Perl object that it cnanot serialise 555 Whenever the encoder encounters a Perl object that it cannot serialise
286 directly (most of them), it will first look up the "TO_CBOR" method on 556 directly (most of them), it will first look up the "TO_CBOR" method on
287 it. 557 it.
288 558
289 If it has a "TO_CBOR" method, it will call it with the object as only 559 If it has a "TO_CBOR" method, it will call it with the object as only
290 argument, and expects exactly one return value, which it will then 560 argument, and expects exactly one return value, which it will then
… …
295 "CBOR" as the second argument, to distinguish it from other serialisers. 565 "CBOR" as the second argument, to distinguish it from other serialisers.
296 566
297 The "FREEZE" method can return any number of values (i.e. zero or more). 567 The "FREEZE" method can return any number of values (i.e. zero or more).
298 These will be encoded as CBOR perl object, together with the classname. 568 These will be encoded as CBOR perl object, together with the classname.
299 569
570 These methods *MUST NOT* change the data structure that is being
571 serialised. Failure to comply to this can result in memory corruption -
572 and worse.
573
300 If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail 574 If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail
301 with an error. 575 with an error.
302 576
577 DECODING
303 Objects encoded via "TO_CBOR" cannot be automatically decoded, but 578 Objects encoded via "TO_CBOR" cannot (normally) be automatically
304 objects encoded via "FREEZE" can be decoded using the following 579 decoded, but objects encoded via "FREEZE" can be decoded using the
305 protocol: 580 following protocol:
306 581
307 When an encoded CBOR perl object is encountered by the decoder, it will 582 When an encoded CBOR perl object is encountered by the decoder, it will
308 look up the "THAW" method, by using the stored classname, and will fail 583 look up the "THAW" method, by using the stored classname, and will fail
309 if the method cannot be found. 584 if the method cannot be found.
310 585
… …
331 606
332 sub URI::TO_CBOR { 607 sub URI::TO_CBOR {
333 my ($self) = @_; 608 my ($self) = @_;
334 my $uri = "$self"; # stringify uri 609 my $uri = "$self"; # stringify uri
335 utf8::upgrade $uri; # make sure it will be encoded as UTF-8 string 610 utf8::upgrade $uri; # make sure it will be encoded as UTF-8 string
336 CBOR::XS::tagged 32, "$_[0]" 611 CBOR::XS::tag 32, "$_[0]"
337 } 612 }
338 613
339 This will encode URIs as a UTF-8 string with tag 32, which indicates an 614 This will encode URIs as a UTF-8 string with tag 32, which indicates an
340 URI. 615 URI.
341 616
… …
352 "$self" # encode url string 627 "$self" # encode url string
353 } 628 }
354 629
355 sub URI::THAW { 630 sub URI::THAW {
356 my ($class, $serialiser, $uri) = @_; 631 my ($class, $serialiser, $uri) = @_;
357
358 $class->new ($uri) 632 $class->new ($uri)
359 } 633 }
360 634
361 Unlike "TO_CBOR", multiple values can be returned by "FREEZE". For 635 Unlike "TO_CBOR", multiple values can be returned by "FREEZE". For
362 example, a "FREEZE" method that returns "type", "id" and "variant" 636 example, a "FREEZE" method that returns "type", "id" and "variant"
… …
376 650
377MAGIC HEADER 651MAGIC HEADER
378 There is no way to distinguish CBOR from other formats programmatically. 652 There is no way to distinguish CBOR from other formats programmatically.
379 To make it easier to distinguish CBOR from other formats, the CBOR 653 To make it easier to distinguish CBOR from other formats, the CBOR
380 specification has a special "magic string" that can be prepended to any 654 specification has a special "magic string" that can be prepended to any
381 CBOR string without changing it's meaning. 655 CBOR string without changing its meaning.
382 656
383 This string is available as $CBOR::XS::MAGIC. This module does not 657 This string is available as $CBOR::XS::MAGIC. This module does not
384 prepend this string tot he CBOR data it generates, but it will ignroe it 658 prepend this string to the CBOR data it generates, but it will ignore it
385 if present, so users can prepend this string as a "file type" indicator 659 if present, so users can prepend this string as a "file type" indicator
386 as required. 660 as required.
387 661
388THE CBOR::XS::Tagged CLASS 662THE CBOR::XS::Tagged CLASS
389 CBOR has the concept of tagged values - any CBOR value can be tagged 663 CBOR has the concept of tagged values - any CBOR value can be tagged
… …
440 Wrap CBOR data in CBOR: 714 Wrap CBOR data in CBOR:
441 715
442 my $cbor_cbor = encode_cbor 716 my $cbor_cbor = encode_cbor
443 CBOR::XS::tag 24, 717 CBOR::XS::tag 24,
444 encode_cbor [1, 2, 3]; 718 encode_cbor [1, 2, 3];
719
720TAG HANDLING AND EXTENSIONS
721 This section describes how this module handles specific tagged values
722 and extensions. If a tag is not mentioned here and no additional filters
723 are provided for it, then the default handling applies (creating a
724 CBOR::XS::Tagged object on decoding, and only encoding the tag when
725 explicitly requested).
726
727 Tags not handled specifically are currently converted into a
728 CBOR::XS::Tagged object, which is simply a blessed array reference
729 consisting of the numeric tag value followed by the (decoded) CBOR
730 value.
731
732 Future versions of this module reserve the right to special case
733 additional tags (such as base64url).
734
735 ENFORCED TAGS
736 These tags are always handled when decoding, and their handling cannot
737 be overridden by the user.
738
739 26 (perl-object, <http://cbor.schmorp.de/perl-object>)
740 These tags are automatically created (and decoded) for serialisable
741 objects using the "FREEZE/THAW" methods (the Types::Serialier object
742 serialisation protocol). See "OBJECT SERIALISATION" for details.
743
744 28, 29 (shareable, sharedref, <http://cbor.schmorp.de/value-sharing>)
745 These tags are automatically decoded when encountered (and they do
746 not result in a cyclic data structure, see "allow_cycles"),
747 resulting in shared values in the decoded object. They are only
748 encoded, however, when "allow_sharing" is enabled.
749
750 Not all shared values can be successfully decoded: values that
751 reference themselves will *currently* decode as "undef" (this is not
752 the same as a reference pointing to itself, which will be
753 represented as a value that contains an indirect reference to itself
754 - these will be decoded properly).
755
756 Note that considerably more shared value data structures can be
757 decoded than will be encoded - currently, only values pointed to by
758 references will be shared, others will not. While non-reference
759 shared values can be generated in Perl with some effort, they were
760 considered too unimportant to be supported in the encoder. The
761 decoder, however, will decode these values as shared values.
762
763 256, 25 (stringref-namespace, stringref,
764 <http://cbor.schmorp.de/stringref>)
765 These tags are automatically decoded when encountered. They are only
766 encoded, however, when "pack_strings" is enabled.
767
768 22098 (indirection, <http://cbor.schmorp.de/indirection>)
769 This tag is automatically generated when a reference are encountered
770 (with the exception of hash and array references). It is converted
771 to a reference when decoding.
772
773 55799 (self-describe CBOR, RFC 7049)
774 This value is not generated on encoding (unless explicitly requested
775 by the user), and is simply ignored when decoding.
776
777 NON-ENFORCED TAGS
778 These tags have default filters provided when decoding. Their handling
779 can be overridden by changing the %CBOR::XS::FILTER entry for the tag,
780 or by providing a custom "filter" callback when decoding.
781
782 When they result in decoding into a specific Perl class, the module
783 usually provides a corresponding "TO_CBOR" method as well.
784
785 When any of these need to load additional modules that are not part of
786 the perl core distribution (e.g. URI), it is (currently) up to the user
787 to provide these modules. The decoding usually fails with an exception
788 if the required module cannot be loaded.
789
790 0, 1 (date/time string, seconds since the epoch)
791 These tags are decoded into Time::Piece objects. The corresponding
792 "Time::Piece::TO_CBOR" method always encodes into tag 1 values
793 currently.
794
795 The Time::Piece API is generally surprisingly bad, and fractional
796 seconds are only accidentally kept intact, so watch out. On the plus
797 side, the module comes with perl since 5.10, which has to count for
798 something.
799
800 2, 3 (positive/negative bignum)
801 These tags are decoded into Math::BigInt objects. The corresponding
802 "Math::BigInt::TO_CBOR" method encodes "small" bigints into normal
803 CBOR integers, and others into positive/negative CBOR bignums.
804
805 4, 5, 264, 265 (decimal fraction/bigfloat)
806 Both decimal fractions and bigfloats are decoded into Math::BigFloat
807 objects. The corresponding "Math::BigFloat::TO_CBOR" method *always*
808 encodes into a decimal fraction (either tag 4 or 264).
809
810 NaN and infinities are not encoded properly, as they cannot be
811 represented in CBOR.
812
813 See "BIGNUM SECURITY CONSIDERATIONS" for more info.
814
815 30 (rational numbers)
816 These tags are decoded into Math::BigRat objects. The corresponding
817 "Math::BigRat::TO_CBOR" method encodes rational numbers with
818 denominator 1 via their numerator only, i.e., they become normal
819 integers or "bignums".
820
821 See "BIGNUM SECURITY CONSIDERATIONS" for more info.
822
823 21, 22, 23 (expected later JSON conversion)
824 CBOR::XS is not a CBOR-to-JSON converter, and will simply ignore
825 these tags.
826
827 32 (URI)
828 These objects decode into URI objects. The corresponding
829 "URI::TO_CBOR" method again results in a CBOR URI value.
445 830
446CBOR and JSON 831CBOR and JSON
447 CBOR is supposed to implement a superset of the JSON data model, and is, 832 CBOR is supposed to implement a superset of the JSON data model, and is,
448 with some coercion, able to represent all JSON texts (something that 833 with some coercion, able to represent all JSON texts (something that
449 other "binary JSON" formats such as BSON generally do not support). 834 other "binary JSON" formats such as BSON generally do not support).
… …
488 Also keep in mind that CBOR::XS might leak contents of your Perl data 873 Also keep in mind that CBOR::XS might leak contents of your Perl data
489 structures in its error messages, so when you serialise sensitive 874 structures in its error messages, so when you serialise sensitive
490 information you might want to make sure that exceptions thrown by 875 information you might want to make sure that exceptions thrown by
491 CBOR::XS will not end up in front of untrusted eyes. 876 CBOR::XS will not end up in front of untrusted eyes.
492 877
878BIGNUM SECURITY CONSIDERATIONS
879 CBOR::XS provides a "TO_CBOR" method for both Math::BigInt and
880 Math::BigFloat that tries to encode the number in the simplest possible
881 way, that is, either a CBOR integer, a CBOR bigint/decimal fraction (tag
882 4) or an arbitrary-exponent decimal fraction (tag 264). Rational numbers
883 (Math::BigRat, tag 30) can also contain bignums as members.
884
885 CBOR::XS will also understand base-2 bigfloat or arbitrary-exponent
886 bigfloats (tags 5 and 265), but it will never generate these on its own.
887
888 Using the built-in Math::BigInt::Calc support, encoding and decoding
889 decimal fractions is generally fast. Decoding bigints can be slow for
890 very big numbers (tens of thousands of digits, something that could
891 potentially be caught by limiting the size of CBOR texts), and decoding
892 bigfloats or arbitrary-exponent bigfloats can be *extremely* slow
893 (minutes, decades) for large exponents (roughly 40 bit and longer).
894
895 Additionally, Math::BigInt can take advantage of other bignum libraries,
896 such as Math::GMP, which cannot handle big floats with large exponents,
897 and might simply abort or crash your program, due to their code quality.
898
899 This can be a concern if you want to parse untrusted CBOR. If it is, you
900 might want to disable decoding of tag 2 (bigint) and 3 (negative bigint)
901 types. You should also disable types 5 and 265, as these can be slow
902 even without bigints.
903
904 Disabling bigints will also partially or fully disable types that rely
905 on them, e.g. rational numbers that use bignums.
906
493CBOR IMPLEMENTATION NOTES 907CBOR IMPLEMENTATION NOTES
494 This section contains some random implementation notes. They do not 908 This section contains some random implementation notes. They do not
495 describe guaranteed behaviour, but merely behaviour as-is implemented 909 describe guaranteed behaviour, but merely behaviour as-is implemented
496 right now. 910 right now.
497 911
… …
504 Only the double data type is supported for NV data types - when Perl 918 Only the double data type is supported for NV data types - when Perl
505 uses long double to represent floating point values, they might not be 919 uses long double to represent floating point values, they might not be
506 encoded properly. Half precision types are accepted, but not encoded. 920 encoded properly. Half precision types are accepted, but not encoded.
507 921
508 Strict mode and canonical mode are not implemented. 922 Strict mode and canonical mode are not implemented.
923
924LIMITATIONS ON PERLS WITHOUT 64-BIT INTEGER SUPPORT
925 On perls that were built without 64 bit integer support (these are rare
926 nowadays, even on 32 bit architectures, as all major Perl distributions
927 are built with 64 bit integer support), support for any kind of 64 bit
928 integer in CBOR is very limited - most likely, these 64 bit values will
929 be truncated, corrupted, or otherwise not decoded correctly. This also
930 includes string, array and map sizes that are stored as 64 bit integers.
509 931
510THREADS 932THREADS
511 This module is *not* guaranteed to be thread safe and there are no plans 933 This module is *not* guaranteed to be thread safe and there are no plans
512 to change this until Perl gets thread support (as opposed to the 934 to change this until Perl gets thread support (as opposed to the
513 horribly slow so-called "threads" which are simply slow and bloated 935 horribly slow so-called "threads" which are simply slow and bloated

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