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Revision 1.7 by root, Sun Mar 25 00:47:42 2007 UTC vs.
Revision 1.37 by root, Thu May 23 09:32:02 2013 UTC

…		…
1	NAME	1	NAME
2	JSON::XS - JSON serialising/deserialising, done correctly and fast	2	JSON::XS - JSON serialising/deserialising, done correctly and fast
3		3
		4	JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ
		5	(http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
		6
4	SYNOPSIS	7	SYNOPSIS
5	use JSON::XS;	8	use JSON::XS;
6		9
7	# exported functions, croak on error	10	# exported functions, they croak on error
		11	# and expect/generate UTF-8
8		12
9	$utf8_encoded_json_text = to_json $perl_hash_or_arrayref;	13	$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
10	$perl_hash_or_arrayref = from_json $utf8_encoded_json_text;	14	$perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
11		15
12	# oo-interface	16	# OO-interface
13		17
14	$coder = JSON::XS->new->ascii->pretty->allow_nonref;	18	$coder = JSON::XS->new->ascii->pretty->allow_nonref;
15	$pretty_printed_unencoded = $coder->encode ($perl_scalar);	19	$pretty_printed_unencoded = $coder->encode ($perl_scalar);
16	$perl_scalar = $coder->decode ($unicode_json_text);	20	$perl_scalar = $coder->decode ($unicode_json_text);
		21
		22	# Note that JSON version 2.0 and above will automatically use JSON::XS
		23	# if available, at virtually no speed overhead either, so you should
		24	# be able to just:
		25
		26	use JSON;
		27
		28	# and do the same things, except that you have a pure-perl fallback now.
17		29
18	DESCRIPTION	30	DESCRIPTION
19	This module converts Perl data structures to JSON and vice versa. Its	31	This module converts Perl data structures to JSON and vice versa. Its
20	primary goal is to be correct and its secondary goal is to be fast.	32	primary goal is to be correct and its secondary goal is to be fast.
21	To reach the latter goal it was written in C.	33	To reach the latter goal it was written in C.
		34
		35	Beginning with version 2.0 of the JSON module, when both JSON and
		36	JSON::XS are installed, then JSON will fall back on JSON::XS (this can
		37	be overridden) with no overhead due to emulation (by inheriting
		38	constructor and methods). If JSON::XS is not available, it will fall
		39	back to the compatible JSON::PP module as backend, so using JSON instead
		40	of JSON::XS gives you a portable JSON API that can be fast when you need
		41	and doesn't require a C compiler when that is a problem.
22		42
23	As this is the n-th-something JSON module on CPAN, what was the reason	43	As this is the n-th-something JSON module on CPAN, what was the reason
24	to write yet another JSON module? While it seems there are many JSON	44	to write yet another JSON module? While it seems there are many JSON
25	modules, none of them correctly handle all corner cases, and in most	45	modules, none of them correctly handle all corner cases, and in most
26	cases their maintainers are unresponsive, gone missing, or not listening	46	cases their maintainers are unresponsive, gone missing, or not listening
27	to bug reports for other reasons.	47	to bug reports for other reasons.
28		48
29	See COMPARISON, below, for a comparison to some other JSON modules.
30
31	See MAPPING, below, on how JSON::XS maps perl values to JSON values and	49	See MAPPING, below, on how JSON::XS maps perl values to JSON values and
32	vice versa.	50	vice versa.
33		51
34	FEATURES	52	FEATURES
35	* correct handling of unicode issues	53	* correct Unicode handling
		54
36	This module knows how to handle Unicode, and even documents how and	55	This module knows how to handle Unicode, documents how and when it
37	when it does so.	56	does so, and even documents what "correct" means.
38		57
39	* round-trip integrity	58	* round-trip integrity
		59
40	When you serialise a perl data structure using only datatypes	60	When you serialise a perl data structure using only data types
41	supported by JSON, the deserialised data structure is identical on	61	supported by JSON and Perl, the deserialised data structure is
42	the Perl level. (e.g. the string "2.0" doesn't suddenly become "2").	62	identical on the Perl level. (e.g. the string "2.0" doesn't suddenly
		63	become "2" just because it looks like a number). There are minor
		64	exceptions to this, read the MAPPING section below to learn about
		65	those.
43		66
44	* strict checking of JSON correctness	67	* strict checking of JSON correctness
		68
45	There is no guessing, no generating of illegal JSON texts by	69	There is no guessing, no generating of illegal JSON texts by
46	default, and only JSON is accepted as input by default (the latter	70	default, and only JSON is accepted as input by default (the latter
47	is a security feature).	71	is a security feature).
48		72
49	* fast	73	* fast
50	Compared to other JSON modules, this module compares favourably in
51	terms of speed, too.
52		74
		75	Compared to other JSON modules and other serialisers such as
		76	Storable, this module usually compares favourably in terms of speed,
		77	too.
		78
53	* simple to use	79	* simple to use
		80
54	This module has both a simple functional interface as well as an OO	81	This module has both a simple functional interface as well as an
55	interface.	82	object oriented interface interface.
56		83
57	* reasonably versatile output formats	84	* reasonably versatile output formats
		85
58	You can choose between the most compact guarenteed single-line	86	You can choose between the most compact guaranteed-single-line
59	format possible (nice for simple line-based protocols), a pure-ascii	87	format possible (nice for simple line-based protocols), a pure-ASCII
60	format (for when your transport is not 8-bit clean), or a	88	format (for when your transport is not 8-bit clean, still supports
61	pretty-printed format (for when you want to read that stuff). Or you	89	the whole Unicode range), or a pretty-printed format (for when you
62	can combine those features in whatever way you like.	90	want to read that stuff). Or you can combine those features in
		91	whatever way you like.
63		92
64	FUNCTIONAL INTERFACE	93	FUNCTIONAL INTERFACE
65	The following convinience methods are provided by this module. They are	94	The following convenience methods are provided by this module. They are
66	exported by default:	95	exported by default:
67		96
68	$json_text = to_json $perl_scalar	97	$json_text = encode_json $perl_scalar
69	Converts the given Perl data structure (a simple scalar or a	98	Converts the given Perl data structure to a UTF-8 encoded, binary
70	reference to a hash or array) to a UTF-8 encoded, binary string
71	(that is, the string contains octets only). Croaks on error.	99	string (that is, the string contains octets only). Croaks on error.
72		100
73	This function call is functionally identical to:	101	This function call is functionally identical to:
74		102
75	$json_text = JSON::XS->new->utf8->encode ($perl_scalar)	103	$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
76		104
77	except being faster.	105	Except being faster.
78		106
79	$perl_scalar = from_json $json_text	107	$perl_scalar = decode_json $json_text
80	The opposite of "to_json": expects an UTF-8 (binary) string and	108	The opposite of "encode_json": expects an UTF-8 (binary) string and
81	tries to parse that as an UTF-8 encoded JSON text, returning the	109	tries to parse that as an UTF-8 encoded JSON text, returning the
82	resulting simple scalar or reference. Croaks on error.	110	resulting reference. Croaks on error.
83		111
84	This function call is functionally identical to:	112	This function call is functionally identical to:
85		113
86	$perl_scalar = JSON::XS->new->utf8->decode ($json_text)	114	$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
87		115
88	except being faster.	116	Except being faster.
		117
		118	$is_boolean = JSON::XS::is_bool $scalar
		119	Returns true if the passed scalar represents either JSON::XS::true
		120	or JSON::XS::false, two constants that act like 1 and 0,
		121	respectively and are used to represent JSON "true" and "false"
		122	values in Perl.
		123
		124	See MAPPING, below, for more information on how JSON values are
		125	mapped to Perl.
		126
		127	A FEW NOTES ON UNICODE AND PERL
		128	Since this often leads to confusion, here are a few very clear words on
		129	how Unicode works in Perl, modulo bugs.
		130
		131	1. Perl strings can store characters with ordinal values > 255.
		132	This enables you to store Unicode characters as single characters in
		133	a Perl string - very natural.
		134
		135	2. Perl does not associate an encoding with your strings.
		136	... until you force it to, e.g. when matching it against a regex, or
		137	printing the scalar to a file, in which case Perl either interprets
		138	your string as locale-encoded text, octets/binary, or as Unicode,
		139	depending on various settings. In no case is an encoding stored
		140	together with your data, it is use that decides encoding, not any
		141	magical meta data.
		142
		143	3. The internal utf-8 flag has no meaning with regards to the encoding
		144	of your string.
		145	Just ignore that flag unless you debug a Perl bug, a module written
		146	in XS or want to dive into the internals of perl. Otherwise it will
		147	only confuse you, as, despite the name, it says nothing about how
		148	your string is encoded. You can have Unicode strings with that flag
		149	set, with that flag clear, and you can have binary data with that
		150	flag set and that flag clear. Other possibilities exist, too.
		151
		152	If you didn't know about that flag, just the better, pretend it
		153	doesn't exist.
		154
		155	4. A "Unicode String" is simply a string where each character can be
		156	validly interpreted as a Unicode code point.
		157	If you have UTF-8 encoded data, it is no longer a Unicode string,
		158	but a Unicode string encoded in UTF-8, giving you a binary string.
		159
		160	5. A string containing "high" (> 255) character values is not a UTF-8
		161	string.
		162	It's a fact. Learn to live with it.
		163
		164	I hope this helps :)
89		165
90	OBJECT-ORIENTED INTERFACE	166	OBJECT-ORIENTED INTERFACE
91	The object oriented interface lets you configure your own encoding or	167	The object oriented interface lets you configure your own encoding or
92	decoding style, within the limits of supported formats.	168	decoding style, within the limits of supported formats.
93		169
101		177
102	my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})	178	my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
103	=> {"a": [1, 2]}	179	=> {"a": [1, 2]}
104		180
105	$json = $json->ascii ([$enable])	181	$json = $json->ascii ([$enable])
		182	$enabled = $json->get_ascii
106	If $enable is true (or missing), then the "encode" method will not	183	If $enable is true (or missing), then the "encode" method will not
107	generate characters outside the code range 0..127 (which is ASCII).	184	generate characters outside the code range 0..127 (which is ASCII).
108	Any unicode characters outside that range will be escaped using	185	Any Unicode characters outside that range will be escaped using
109	either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL	186	either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
110	escape sequence, as per RFC4627.	187	escape sequence, as per RFC4627. The resulting encoded JSON text can
		188	be treated as a native Unicode string, an ascii-encoded,
		189	latin1-encoded or UTF-8 encoded string, or any other superset of
		190	ASCII.
111		191
112	If $enable is false, then the "encode" method will not escape	192	If $enable is false, then the "encode" method will not escape
113	Unicode characters unless required by the JSON syntax. This results	193	Unicode characters unless required by the JSON syntax or other
114	in a faster and more compact format.	194	flags. This results in a faster and more compact format.
		195
		196	See also the section ENCODING/CODESET FLAG NOTES later in this
		197	document.
		198
		199	The main use for this flag is to produce JSON texts that can be
		200	transmitted over a 7-bit channel, as the encoded JSON texts will not
		201	contain any 8 bit characters.
115		202
116	JSON::XS->new->ascii (1)->encode ([chr 0x10401])	203	JSON::XS->new->ascii (1)->encode ([chr 0x10401])
117	=> ["\ud801\udc01"]	204	=> ["\ud801\udc01"]
118		205
		206	$json = $json->latin1 ([$enable])
		207	$enabled = $json->get_latin1
		208	If $enable is true (or missing), then the "encode" method will
		209	encode the resulting JSON text as latin1 (or iso-8859-1), escaping
		210	any characters outside the code range 0..255. The resulting string
		211	can be treated as a latin1-encoded JSON text or a native Unicode
		212	string. The "decode" method will not be affected in any way by this
		213	flag, as "decode" by default expects Unicode, which is a strict
		214	superset of latin1.
		215
		216	If $enable is false, then the "encode" method will not escape
		217	Unicode characters unless required by the JSON syntax or other
		218	flags.
		219
		220	See also the section ENCODING/CODESET FLAG NOTES later in this
		221	document.
		222
		223	The main use for this flag is efficiently encoding binary data as
		224	JSON text, as most octets will not be escaped, resulting in a
		225	smaller encoded size. The disadvantage is that the resulting JSON
		226	text is encoded in latin1 (and must correctly be treated as such
		227	when storing and transferring), a rare encoding for JSON. It is
		228	therefore most useful when you want to store data structures known
		229	to contain binary data efficiently in files or databases, not when
		230	talking to other JSON encoders/decoders.
		231
		232	JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
		233	=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
		234
119	$json = $json->utf8 ([$enable])	235	$json = $json->utf8 ([$enable])
		236	$enabled = $json->get_utf8
120	If $enable is true (or missing), then the "encode" method will	237	If $enable is true (or missing), then the "encode" method will
121	encode the JSON result into UTF-8, as required by many protocols,	238	encode the JSON result into UTF-8, as required by many protocols,
122	while the "decode" method expects to be handled an UTF-8-encoded	239	while the "decode" method expects to be handled an UTF-8-encoded
123	string. Please note that UTF-8-encoded strings do not contain any	240	string. Please note that UTF-8-encoded strings do not contain any
124	characters outside the range 0..255, they are thus useful for	241	characters outside the range 0..255, they are thus useful for
125	bytewise/binary I/O. In future versions, enabling this option might	242	bytewise/binary I/O. In future versions, enabling this option might
126	enable autodetection of the UTF-16 and UTF-32 encoding families, as	243	enable autodetection of the UTF-16 and UTF-32 encoding families, as
127	described in RFC4627.	244	described in RFC4627.
128		245
129	If $enable is false, then the "encode" method will return the JSON	246	If $enable is false, then the "encode" method will return the JSON
130	string as a (non-encoded) unicode string, while "decode" expects	247	string as a (non-encoded) Unicode string, while "decode" expects
131	thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or	248	thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or
132	UTF-16) needs to be done yourself, e.g. using the Encode module.	249	UTF-16) needs to be done yourself, e.g. using the Encode module.
		250
		251	See also the section ENCODING/CODESET FLAG NOTES later in this
		252	document.
133		253
134	Example, output UTF-16BE-encoded JSON:	254	Example, output UTF-16BE-encoded JSON:
135		255
136	use Encode;	256	use Encode;
137	$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);	257	$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
…		…
156	2	276	2
157	]	277	]
158	}	278	}
159		279
160	$json = $json->indent ([$enable])	280	$json = $json->indent ([$enable])
		281	$enabled = $json->get_indent
161	If $enable is true (or missing), then the "encode" method will use a	282	If $enable is true (or missing), then the "encode" method will use a
162	multiline format as output, putting every array member or	283	multiline format as output, putting every array member or
163	object/hash key-value pair into its own line, identing them	284	object/hash key-value pair into its own line, indenting them
164	properly.	285	properly.
165		286
166	If $enable is false, no newlines or indenting will be produced, and	287	If $enable is false, no newlines or indenting will be produced, and
167	the resulting JSON text is guarenteed not to contain any "newlines".	288	the resulting JSON text is guaranteed not to contain any "newlines".
168		289
169	This setting has no effect when decoding JSON texts.	290	This setting has no effect when decoding JSON texts.
170		291
171	$json = $json->space_before ([$enable])	292	$json = $json->space_before ([$enable])
		293	$enabled = $json->get_space_before
172	If $enable is true (or missing), then the "encode" method will add	294	If $enable is true (or missing), then the "encode" method will add
173	an extra optional space before the ":" separating keys from values	295	an extra optional space before the ":" separating keys from values
174	in JSON objects.	296	in JSON objects.
175		297
176	If $enable is false, then the "encode" method will not add any extra	298	If $enable is false, then the "encode" method will not add any extra
…		…
182	Example, space_before enabled, space_after and indent disabled:	304	Example, space_before enabled, space_after and indent disabled:
183		305
184	{"key" :"value"}	306	{"key" :"value"}
185		307
186	$json = $json->space_after ([$enable])	308	$json = $json->space_after ([$enable])
		309	$enabled = $json->get_space_after
187	If $enable is true (or missing), then the "encode" method will add	310	If $enable is true (or missing), then the "encode" method will add
188	an extra optional space after the ":" separating keys from values in	311	an extra optional space after the ":" separating keys from values in
189	JSON objects and extra whitespace after the "," separating key-value	312	JSON objects and extra whitespace after the "," separating key-value
190	pairs and array members.	313	pairs and array members.
191		314
…		…
196		319
197	Example, space_before and indent disabled, space_after enabled:	320	Example, space_before and indent disabled, space_after enabled:
198		321
199	{"key": "value"}	322	{"key": "value"}
200		323
		324	$json = $json->relaxed ([$enable])
		325	$enabled = $json->get_relaxed
		326	If $enable is true (or missing), then "decode" will accept some
		327	extensions to normal JSON syntax (see below). "encode" will not be
		328	affected in anyway. *Be aware that this option makes you accept
		329	invalid JSON texts as if they were valid!*. I suggest only to use
		330	this option to parse application-specific files written by humans
		331	(configuration files, resource files etc.)
		332
		333	If $enable is false (the default), then "decode" will only accept
		334	valid JSON texts.
		335
		336	Currently accepted extensions are:
		337
		338	* list items can have an end-comma
		339
		340	JSON separates array elements and key-value pairs with commas.
		341	This can be annoying if you write JSON texts manually and want
		342	to be able to quickly append elements, so this extension accepts
		343	comma at the end of such items not just between them:
		344
		345	[
		346	1,
		347	2, <- this comma not normally allowed
		348	]
		349	{
		350	"k1": "v1",
		351	"k2": "v2", <- this comma not normally allowed
		352	}
		353
		354	* shell-style '#'-comments
		355
		356	Whenever JSON allows whitespace, shell-style comments are
		357	additionally allowed. They are terminated by the first
		358	carriage-return or line-feed character, after which more
		359	white-space and comments are allowed.
		360
		361	[
		362	1, # this comment not allowed in JSON
		363	# neither this one...
		364	]
		365
201	$json = $json->canonical ([$enable])	366	$json = $json->canonical ([$enable])
		367	$enabled = $json->get_canonical
202	If $enable is true (or missing), then the "encode" method will	368	If $enable is true (or missing), then the "encode" method will
203	output JSON objects by sorting their keys. This is adding a	369	output JSON objects by sorting their keys. This is adding a
204	comparatively high overhead.	370	comparatively high overhead.
205		371
206	If $enable is false, then the "encode" method will output key-value	372	If $enable is false, then the "encode" method will output key-value
207	pairs in the order Perl stores them (which will likely change	373	pairs in the order Perl stores them (which will likely change
208	between runs of the same script).	374	between runs of the same script, and can change even within the same
		375	run from 5.18 onwards).
209		376
210	This option is useful if you want the same data structure to be	377	This option is useful if you want the same data structure to be
211	encoded as the same JSON text (given the same overall settings). If	378	encoded as the same JSON text (given the same overall settings). If
212	it is disabled, the same hash migh be encoded differently even if	379	it is disabled, the same hash might be encoded differently even if
213	contains the same data, as key-value pairs have no inherent ordering	380	contains the same data, as key-value pairs have no inherent ordering
214	in Perl.	381	in Perl.
215		382
216	This setting has no effect when decoding JSON texts.	383	This setting has no effect when decoding JSON texts.
217		384
		385	This setting has currently no effect on tied hashes.
		386
218	$json = $json->allow_nonref ([$enable])	387	$json = $json->allow_nonref ([$enable])
		388	$enabled = $json->get_allow_nonref
219	If $enable is true (or missing), then the "encode" method can	389	If $enable is true (or missing), then the "encode" method can
220	convert a non-reference into its corresponding string, number or	390	convert a non-reference into its corresponding string, number or
221	null JSON value, which is an extension to RFC4627. Likewise,	391	null JSON value, which is an extension to RFC4627. Likewise,
222	"decode" will accept those JSON values instead of croaking.	392	"decode" will accept those JSON values instead of croaking.
223		393
…		…
230	"allow_nonref", resulting in an invalid JSON text:	400	"allow_nonref", resulting in an invalid JSON text:
231		401
232	JSON::XS->new->allow_nonref->encode ("Hello, World!")	402	JSON::XS->new->allow_nonref->encode ("Hello, World!")
233	=> "Hello, World!"	403	=> "Hello, World!"
234		404
		405	$json = $json->allow_unknown ([$enable])
		406	$enabled = $json->get_allow_unknown
		407	If $enable is true (or missing), then "encode" will not throw an
		408	exception when it encounters values it cannot represent in JSON (for
		409	example, filehandles) but instead will encode a JSON "null" value.
		410	Note that blessed objects are not included here and are handled
		411	separately by c<allow_nonref>.
		412
		413	If $enable is false (the default), then "encode" will throw an
		414	exception when it encounters anything it cannot encode as JSON.
		415
		416	This option does not affect "decode" in any way, and it is
		417	recommended to leave it off unless you know your communications
		418	partner.
		419
		420	$json = $json->allow_blessed ([$enable])
		421	$enabled = $json->get_allow_blessed
		422	If $enable is true (or missing), then the "encode" method will not
		423	barf when it encounters a blessed reference. Instead, the value of
		424	the convert_blessed option will decide whether "null"
		425	("convert_blessed" disabled or no "TO_JSON" method found) or a
		426	representation of the object ("convert_blessed" enabled and
		427	"TO_JSON" method found) is being encoded. Has no effect on "decode".
		428
		429	If $enable is false (the default), then "encode" will throw an
		430	exception when it encounters a blessed object.
		431
		432	$json = $json->convert_blessed ([$enable])
		433	$enabled = $json->get_convert_blessed
		434	If $enable is true (or missing), then "encode", upon encountering a
		435	blessed object, will check for the availability of the "TO_JSON"
		436	method on the object's class. If found, it will be called in scalar
		437	context and the resulting scalar will be encoded instead of the
		438	object. If no "TO_JSON" method is found, the value of
		439	"allow_blessed" will decide what to do.
		440
		441	The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
		442	returns other blessed objects, those will be handled in the same
		443	way. "TO_JSON" must take care of not causing an endless recursion
		444	cycle (== crash) in this case. The name of "TO_JSON" was chosen
		445	because other methods called by the Perl core (== not by the user of
		446	the object) are usually in upper case letters and to avoid
		447	collisions with any "to_json" function or method.
		448
		449	This setting does not yet influence "decode" in any way, but in the
		450	future, global hooks might get installed that influence "decode" and
		451	are enabled by this setting.
		452
		453	If $enable is false, then the "allow_blessed" setting will decide
		454	what to do when a blessed object is found.
		455
		456	$json = $json->filter_json_object ([$coderef->($hashref)])
		457	When $coderef is specified, it will be called from "decode" each
		458	time it decodes a JSON object. The only argument is a reference to
		459	the newly-created hash. If the code references returns a single
		460	scalar (which need not be a reference), this value (i.e. a copy of
		461	that scalar to avoid aliasing) is inserted into the deserialised
		462	data structure. If it returns an empty list (NOTE: not "undef",
		463	which is a valid scalar), the original deserialised hash will be
		464	inserted. This setting can slow down decoding considerably.
		465
		466	When $coderef is omitted or undefined, any existing callback will be
		467	removed and "decode" will not change the deserialised hash in any
		468	way.
		469
		470	Example, convert all JSON objects into the integer 5:
		471
		472	my $js = JSON::XS->new->filter_json_object (sub { 5 });
		473	# returns [5]
		474	$js->decode ('[{}]')
		475	# throw an exception because allow_nonref is not enabled
		476	# so a lone 5 is not allowed.
		477	$js->decode ('{"a":1, "b":2}');
		478
		479	$json = $json->filter_json_single_key_object ($key [=>
		480	$coderef->($value)])
		481	Works remotely similar to "filter_json_object", but is only called
		482	for JSON objects having a single key named $key.
		483
		484	This $coderef is called before the one specified via
		485	"filter_json_object", if any. It gets passed the single value in the
		486	JSON object. If it returns a single value, it will be inserted into
		487	the data structure. If it returns nothing (not even "undef" but the
		488	empty list), the callback from "filter_json_object" will be called
		489	next, as if no single-key callback were specified.
		490
		491	If $coderef is omitted or undefined, the corresponding callback will
		492	be disabled. There can only ever be one callback for a given key.
		493
		494	As this callback gets called less often then the
		495	"filter_json_object" one, decoding speed will not usually suffer as
		496	much. Therefore, single-key objects make excellent targets to
		497	serialise Perl objects into, especially as single-key JSON objects
		498	are as close to the type-tagged value concept as JSON gets (it's
		499	basically an ID/VALUE tuple). Of course, JSON does not support this
		500	in any way, so you need to make sure your data never looks like a
		501	serialised Perl hash.
		502
		503	Typical names for the single object key are "__class_whatever__", or
		504	"$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
		505	things like "__class_md5sum(classname)__", to reduce the risk of
		506	clashing with real hashes.
		507
		508	Example, decode JSON objects of the form "{ "__widget__" => <id> }"
		509	into the corresponding $WIDGET{<id>} object:
		510
		511	# return whatever is in $WIDGET{5}:
		512	JSON::XS
		513	->new
		514	->filter_json_single_key_object (__widget__ => sub {
		515	$WIDGET{ $_[0] }
		516	})
		517	->decode ('{"__widget__": 5')
		518
		519	# this can be used with a TO_JSON method in some "widget" class
		520	# for serialisation to json:
		521	sub WidgetBase::TO_JSON {
		522	my ($self) = @_;
		523
		524	unless ($self->{id}) {
		525	$self->{id} = ..get..some..id..;
		526	$WIDGET{$self->{id}} = $self;
		527	}
		528
		529	{ __widget__ => $self->{id} }
		530	}
		531
235	$json = $json->shrink ([$enable])	532	$json = $json->shrink ([$enable])
		533	$enabled = $json->get_shrink
236	Perl usually over-allocates memory a bit when allocating space for	534	Perl usually over-allocates memory a bit when allocating space for
237	strings. This flag optionally resizes strings generated by either	535	strings. This flag optionally resizes strings generated by either
238	"encode" or "decode" to their minimum size possible. This can save	536	"encode" or "decode" to their minimum size possible. This can save
239	memory when your JSON texts are either very very long or you have	537	memory when your JSON texts are either very very long or you have
240	many short strings. It will also try to downgrade any strings to	538	many short strings. It will also try to downgrade any strings to
241	octet-form if possible: perl stores strings internally either in an	539	octet-form if possible: perl stores strings internally either in an
242	encoding called UTF-X or in octet-form. The latter cannot store	540	encoding called UTF-X or in octet-form. The latter cannot store
243	everything but uses less space in general.	541	everything but uses less space in general (and some buggy Perl or C
		542	code might even rely on that internal representation being used).
		543
		544	The actual definition of what shrink does might change in future
		545	versions, but it will always try to save space at the expense of
		546	time.
244		547
245	If $enable is true (or missing), the string returned by "encode"	548	If $enable is true (or missing), the string returned by "encode"
246	will be shrunk-to-fit, while all strings generated by "decode" will	549	will be shrunk-to-fit, while all strings generated by "decode" will
247	also be shrunk-to-fit.	550	also be shrunk-to-fit.
248		551
…		…
251		554
252	In the future, this setting might control other things, such as	555	In the future, this setting might control other things, such as
253	converting strings that look like integers or floats into integers	556	converting strings that look like integers or floats into integers
254	or floats internally (there is no difference on the Perl level),	557	or floats internally (there is no difference on the Perl level),
255	saving space.	558	saving space.
		559
		560	$json = $json->max_depth ([$maximum_nesting_depth])
		561	$max_depth = $json->get_max_depth
		562	Sets the maximum nesting level (default 512) accepted while encoding
		563	or decoding. If a higher nesting level is detected in JSON text or a
		564	Perl data structure, then the encoder and decoder will stop and
		565	croak at that point.
		566
		567	Nesting level is defined by number of hash- or arrayrefs that the
		568	encoder needs to traverse to reach a given point or the number of
		569	"{" or "[" characters without their matching closing parenthesis
		570	crossed to reach a given character in a string.
		571
		572	Setting the maximum depth to one disallows any nesting, so that
		573	ensures that the object is only a single hash/object or array.
		574
		575	If no argument is given, the highest possible setting will be used,
		576	which is rarely useful.
		577
		578	Note that nesting is implemented by recursion in C. The default
		579	value has been chosen to be as large as typical operating systems
		580	allow without crashing.
		581
		582	See SECURITY CONSIDERATIONS, below, for more info on why this is
		583	useful.
		584
		585	$json = $json->max_size ([$maximum_string_size])
		586	$max_size = $json->get_max_size
		587	Set the maximum length a JSON text may have (in bytes) where
		588	decoding is being attempted. The default is 0, meaning no limit.
		589	When "decode" is called on a string that is longer then this many
		590	bytes, it will not attempt to decode the string but throw an
		591	exception. This setting has no effect on "encode" (yet).
		592
		593	If no argument is given, the limit check will be deactivated (same
		594	as when 0 is specified).
		595
		596	See SECURITY CONSIDERATIONS, below, for more info on why this is
		597	useful.
256		598
257	$json_text = $json->encode ($perl_scalar)	599	$json_text = $json->encode ($perl_scalar)
258	Converts the given Perl data structure (a simple scalar or a	600	Converts the given Perl data structure (a simple scalar or a
259	reference to a hash or array) to its JSON representation. Simple	601	reference to a hash or array) to its JSON representation. Simple
260	scalars will be converted into JSON string or number sequences,	602	scalars will be converted into JSON string or number sequences,
…		…
269		611
270	JSON numbers and strings become simple Perl scalars. JSON arrays	612	JSON numbers and strings become simple Perl scalars. JSON arrays
271	become Perl arrayrefs and JSON objects become Perl hashrefs. "true"	613	become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
272	becomes 1, "false" becomes 0 and "null" becomes "undef".	614	becomes 1, "false" becomes 0 and "null" becomes "undef".
273		615
		616	($perl_scalar, $characters) = $json->decode_prefix ($json_text)
		617	This works like the "decode" method, but instead of raising an
		618	exception when there is trailing garbage after the first JSON
		619	object, it will silently stop parsing there and return the number of
		620	characters consumed so far.
		621
		622	This is useful if your JSON texts are not delimited by an outer
		623	protocol (which is not the brightest thing to do in the first place)
		624	and you need to know where the JSON text ends.
		625
		626	JSON::XS->new->decode_prefix ("[1] the tail")
		627	=> ([], 3)
		628
		629	INCREMENTAL PARSING
		630	In some cases, there is the need for incremental parsing of JSON texts.
		631	While this module always has to keep both JSON text and resulting Perl
		632	data structure in memory at one time, it does allow you to parse a JSON
		633	stream incrementally. It does so by accumulating text until it has a
		634	full JSON object, which it then can decode. This process is similar to
		635	using "decode_prefix" to see if a full JSON object is available, but is
		636	much more efficient (and can be implemented with a minimum of method
		637	calls).
		638
		639	JSON::XS will only attempt to parse the JSON text once it is sure it has
		640	enough text to get a decisive result, using a very simple but truly
		641	incremental parser. This means that it sometimes won't stop as early as
		642	the full parser, for example, it doesn't detect mismatched parentheses.
		643	The only thing it guarantees is that it starts decoding as soon as a
		644	syntactically valid JSON text has been seen. This means you need to set
		645	resource limits (e.g. "max_size") to ensure the parser will stop parsing
		646	in the presence if syntax errors.
		647
		648	The following methods implement this incremental parser.
		649
		650	[void, scalar or list context] = $json->incr_parse ([$string])
		651	This is the central parsing function. It can both append new text
		652	and extract objects from the stream accumulated so far (both of
		653	these functions are optional).
		654
		655	If $string is given, then this string is appended to the already
		656	existing JSON fragment stored in the $json object.
		657
		658	After that, if the function is called in void context, it will
		659	simply return without doing anything further. This can be used to
		660	add more text in as many chunks as you want.
		661
		662	If the method is called in scalar context, then it will try to
		663	extract exactly one JSON object. If that is successful, it will
		664	return this object, otherwise it will return "undef". If there is a
		665	parse error, this method will croak just as "decode" would do (one
		666	can then use "incr_skip" to skip the errornous part). This is the
		667	most common way of using the method.
		668
		669	And finally, in list context, it will try to extract as many objects
		670	from the stream as it can find and return them, or the empty list
		671	otherwise. For this to work, there must be no separators between the
		672	JSON objects or arrays, instead they must be concatenated
		673	back-to-back. If an error occurs, an exception will be raised as in
		674	the scalar context case. Note that in this case, any
		675	previously-parsed JSON texts will be lost.
		676
		677	Example: Parse some JSON arrays/objects in a given string and return
		678	them.
		679
		680	my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
		681
		682	$lvalue_string = $json->incr_text
		683	This method returns the currently stored JSON fragment as an lvalue,
		684	that is, you can manipulate it. This only works when a preceding
		685	call to "incr_parse" in scalar context successfully returned an
		686	object. Under all other circumstances you must not call this
		687	function (I mean it. although in simple tests it might actually
		688	work, it will fail under real world conditions). As a special
		689	exception, you can also call this method before having parsed
		690	anything.
		691
		692	This function is useful in two cases: a) finding the trailing text
		693	after a JSON object or b) parsing multiple JSON objects separated by
		694	non-JSON text (such as commas).
		695
		696	$json->incr_skip
		697	This will reset the state of the incremental parser and will remove
		698	the parsed text from the input buffer so far. This is useful after
		699	"incr_parse" died, in which case the input buffer and incremental
		700	parser state is left unchanged, to skip the text parsed so far and
		701	to reset the parse state.
		702
		703	The difference to "incr_reset" is that only text until the parse
		704	error occured is removed.
		705
		706	$json->incr_reset
		707	This completely resets the incremental parser, that is, after this
		708	call, it will be as if the parser had never parsed anything.
		709
		710	This is useful if you want to repeatedly parse JSON objects and want
		711	to ignore any trailing data, which means you have to reset the
		712	parser after each successful decode.
		713
		714	LIMITATIONS
		715	All options that affect decoding are supported, except "allow_nonref".
		716	The reason for this is that it cannot be made to work sensibly: JSON
		717	objects and arrays are self-delimited, i.e. you can concatenate them
		718	back to back and still decode them perfectly. This does not hold true
		719	for JSON numbers, however.
		720
		721	For example, is the string 1 a single JSON number, or is it simply the
		722	start of 12? Or is 12 a single JSON number, or the concatenation of 1
		723	and 2? In neither case you can tell, and this is why JSON::XS takes the
		724	conservative route and disallows this case.
		725
		726	EXAMPLES
		727	Some examples will make all this clearer. First, a simple example that
		728	works similarly to "decode_prefix": We want to decode the JSON object at
		729	the start of a string and identify the portion after the JSON object:
		730
		731	my $text = "[1,2,3] hello";
		732
		733	my $json = new JSON::XS;
		734
		735	my $obj = $json->incr_parse ($text)
		736	or die "expected JSON object or array at beginning of string";
		737
		738	my $tail = $json->incr_text;
		739	# $tail now contains " hello"
		740
		741	Easy, isn't it?
		742
		743	Now for a more complicated example: Imagine a hypothetical protocol
		744	where you read some requests from a TCP stream, and each request is a
		745	JSON array, without any separation between them (in fact, it is often
		746	useful to use newlines as "separators", as these get interpreted as
		747	whitespace at the start of the JSON text, which makes it possible to
		748	test said protocol with "telnet"...).
		749
		750	Here is how you'd do it (it is trivial to write this in an event-based
		751	manner):
		752
		753	my $json = new JSON::XS;
		754
		755	# read some data from the socket
		756	while (sysread $socket, my $buf, 4096) {
		757
		758	# split and decode as many requests as possible
		759	for my $request ($json->incr_parse ($buf)) {
		760	# act on the $request
		761	}
		762	}
		763
		764	Another complicated example: Assume you have a string with JSON objects
		765	or arrays, all separated by (optional) comma characters (e.g. "[1],[2],
		766	[3]"). To parse them, we have to skip the commas between the JSON texts,
		767	and here is where the lvalue-ness of "incr_text" comes in useful:
		768
		769	my $text = "[1],[2], [3]";
		770	my $json = new JSON::XS;
		771
		772	# void context, so no parsing done
		773	$json->incr_parse ($text);
		774
		775	# now extract as many objects as possible. note the
		776	# use of scalar context so incr_text can be called.
		777	while (my $obj = $json->incr_parse) {
		778	# do something with $obj
		779
		780	# now skip the optional comma
		781	$json->incr_text =~ s/^ \s* , //x;
		782	}
		783
		784	Now lets go for a very complex example: Assume that you have a gigantic
		785	JSON array-of-objects, many gigabytes in size, and you want to parse it,
		786	but you cannot load it into memory fully (this has actually happened in
		787	the real world :).
		788
		789	Well, you lost, you have to implement your own JSON parser. But JSON::XS
		790	can still help you: You implement a (very simple) array parser and let
		791	JSON decode the array elements, which are all full JSON objects on their
		792	own (this wouldn't work if the array elements could be JSON numbers, for
		793	example):
		794
		795	my $json = new JSON::XS;
		796
		797	# open the monster
		798	open my $fh, "<bigfile.json"
		799	or die "bigfile: $!";
		800
		801	# first parse the initial "["
		802	for (;;) {
		803	sysread $fh, my $buf, 65536
		804	or die "read error: $!";
		805	$json->incr_parse ($buf); # void context, so no parsing
		806
		807	# Exit the loop once we found and removed(!) the initial "[".
		808	# In essence, we are (ab-)using the $json object as a simple scalar
		809	# we append data to.
		810	last if $json->incr_text =~ s/^ \s* \[ //x;
		811	}
		812
		813	# now we have the skipped the initial "[", so continue
		814	# parsing all the elements.
		815	for (;;) {
		816	# in this loop we read data until we got a single JSON object
		817	for (;;) {
		818	if (my $obj = $json->incr_parse) {
		819	# do something with $obj
		820	last;
		821	}
		822
		823	# add more data
		824	sysread $fh, my $buf, 65536
		825	or die "read error: $!";
		826	$json->incr_parse ($buf); # void context, so no parsing
		827	}
		828
		829	# in this loop we read data until we either found and parsed the
		830	# separating "," between elements, or the final "]"
		831	for (;;) {
		832	# first skip whitespace
		833	$json->incr_text =~ s/^\s*//;
		834
		835	# if we find "]", we are done
		836	if ($json->incr_text =~ s/^\]//) {
		837	print "finished.\n";
		838	exit;
		839	}
		840
		841	# if we find ",", we can continue with the next element
		842	if ($json->incr_text =~ s/^,//) {
		843	last;
		844	}
		845
		846	# if we find anything else, we have a parse error!
		847	if (length $json->incr_text) {
		848	die "parse error near ", $json->incr_text;
		849	}
		850
		851	# else add more data
		852	sysread $fh, my $buf, 65536
		853	or die "read error: $!";
		854	$json->incr_parse ($buf); # void context, so no parsing
		855	}
		856
		857	This is a complex example, but most of the complexity comes from the
		858	fact that we are trying to be correct (bear with me if I am wrong, I
		859	never ran the above example :).
		860
274	MAPPING	861	MAPPING
275	This section describes how JSON::XS maps Perl values to JSON values and	862	This section describes how JSON::XS maps Perl values to JSON values and
276	vice versa. These mappings are designed to "do the right thing" in most	863	vice versa. These mappings are designed to "do the right thing" in most
277	circumstances automatically, preserving round-tripping characteristics	864	circumstances automatically, preserving round-tripping characteristics
278	(what you put in comes out as something equivalent).	865	(what you put in comes out as something equivalent).
279		866
280	For the more enlightened: note that in the following descriptions,	867	For the more enlightened: note that in the following descriptions,
281	lowercase perl refers to the Perl interpreter, while uppcercase Perl	868	lowercase perl refers to the Perl interpreter, while uppercase Perl
282	refers to the abstract Perl language itself.	869	refers to the abstract Perl language itself.
283		870
284	JSON -> PERL	871	JSON -> PERL
285	object	872	object
286	A JSON object becomes a reference to a hash in Perl. No ordering of	873	A JSON object becomes a reference to a hash in Perl. No ordering of
287	object keys is preserved (JSON does not preserver object key	874	object keys is preserved (JSON does not preserve object key ordering
288	ordering itself).	875	itself).
289		876
290	array	877	array
291	A JSON array becomes a reference to an array in Perl.	878	A JSON array becomes a reference to an array in Perl.
292		879
293	string	880	string
294	A JSON string becomes a string scalar in Perl - Unicode codepoints	881	A JSON string becomes a string scalar in Perl - Unicode codepoints
295	in JSON are represented by the same codepoints in the Perl string,	882	in JSON are represented by the same codepoints in the Perl string,
296	so no manual decoding is necessary.	883	so no manual decoding is necessary.
297		884
298	number	885	number
299	A JSON number becomes either an integer or numeric (floating point)	886	A JSON number becomes either an integer, numeric (floating point) or
300	scalar in perl, depending on its range and any fractional parts. On	887	string scalar in perl, depending on its range and any fractional
301	the Perl level, there is no difference between those as Perl handles	888	parts. On the Perl level, there is no difference between those as
302	all the conversion details, but an integer may take slightly less	889	Perl handles all the conversion details, but an integer may take
303	memory and might represent more values exactly than (floating point)	890	slightly less memory and might represent more values exactly than
		891	floating point numbers.
		892
		893	If the number consists of digits only, JSON::XS will try to
		894	represent it as an integer value. If that fails, it will try to
		895	represent it as a numeric (floating point) value if that is possible
		896	without loss of precision. Otherwise it will preserve the number as
		897	a string value (in which case you lose roundtripping ability, as the
		898	JSON number will be re-encoded toa JSON string).
		899
		900	Numbers containing a fractional or exponential part will always be
		901	represented as numeric (floating point) values, possibly at a loss
		902	of precision (in which case you might lose perfect roundtripping
		903	ability, but the JSON number will still be re-encoded as a JSON
304	numbers.	904	number).
		905
		906	Note that precision is not accuracy - binary floating point values
		907	cannot represent most decimal fractions exactly, and when converting
		908	from and to floating point, JSON::XS only guarantees precision up to
		909	but not including the leats significant bit.
305		910
306	true, false	911	true, false
307	These JSON atoms become 0, 1, respectively. Information is lost in	912	These JSON atoms become "JSON::XS::true" and "JSON::XS::false",
308	this process. Future versions might represent those values	913	respectively. They are overloaded to act almost exactly like the
309	differently, but they will be guarenteed to act like these integers	914	numbers 1 and 0. You can check whether a scalar is a JSON boolean by
310	would normally in Perl.	915	using the "JSON::XS::is_bool" function.
311		916
312	null	917	null
313	A JSON null atom becomes "undef" in Perl.	918	A JSON null atom becomes "undef" in Perl.
314		919
315	PERL -> JSON	920	PERL -> JSON
…		…
317	truly typeless language, so we can only guess which JSON type is meant	922	truly typeless language, so we can only guess which JSON type is meant
318	by a Perl value.	923	by a Perl value.
319		924
320	hash references	925	hash references
321	Perl hash references become JSON objects. As there is no inherent	926	Perl hash references become JSON objects. As there is no inherent
322	ordering in hash keys, they will usually be encoded in a	927	ordering in hash keys (or JSON objects), they will usually be
323	pseudo-random order that can change between runs of the same program	928	encoded in a pseudo-random order that can change between runs of the
324	but stays generally the same within a single run of a program.	929	same program but stays generally the same within a single run of a
325	JSON::XS can optionally sort the hash keys (determined by the	930	program. JSON::XS can optionally sort the hash keys (determined by
326	canonical flag), so the same datastructure will serialise to the	931	the canonical flag), so the same datastructure will serialise to
327	same JSON text (given same settings and version of JSON::XS), but	932	the same JSON text (given same settings and version of JSON::XS),
328	this incurs a runtime overhead.	933	but this incurs a runtime overhead and is only rarely useful, e.g.
		934	when you want to compare some JSON text against another for
		935	equality.
329		936
330	array references	937	array references
331	Perl array references become JSON arrays.	938	Perl array references become JSON arrays.
332		939
		940	other references
		941	Other unblessed references are generally not allowed and will cause
		942	an exception to be thrown, except for references to the integers 0
		943	and 1, which get turned into "false" and "true" atoms in JSON. You
		944	can also use "JSON::XS::false" and "JSON::XS::true" to improve
		945	readability.
		946
		947	encode_json [\0, JSON::XS::true] # yields [false,true]
		948
		949	JSON::XS::true, JSON::XS::false
		950	These special values become JSON true and JSON false values,
		951	respectively. You can also use "\1" and "\0" directly if you want.
		952
333	blessed objects	953	blessed objects
334	Blessed objects are not allowed. JSON::XS currently tries to encode	954	Blessed objects are not directly representable in JSON. See the
335	their underlying representation (hash- or arrayref), but this	955	"allow_blessed" and "convert_blessed" methods on various options on
336	behaviour might change in future versions.	956	how to deal with this: basically, you can choose between throwing an
		957	exception, encoding the reference as if it weren't blessed, or
		958	provide your own serialiser method.
337		959
338	simple scalars	960	simple scalars
339	Simple Perl scalars (any scalar that is not a reference) are the	961	Simple Perl scalars (any scalar that is not a reference) are the
340	most difficult objects to encode: JSON::XS will encode undefined	962	most difficult objects to encode: JSON::XS will encode undefined
341	scalars as JSON null value, scalars that have last been used in a	963	scalars as JSON "null" values, scalars that have last been used in a
342	string context before encoding as JSON strings and anything else as	964	string context before encoding as JSON strings, and anything else as
343	number value:	965	number value:
344		966
345	# dump as number	967	# dump as number
346	to_json [2] # yields [2]	968	encode_json [2] # yields [2]
347	to_json [-3.0e17] # yields [-3e+17]	969	encode_json [-3.0e17] # yields [-3e+17]
348	my $value = 5; to_json [$value] # yields [5]	970	my $value = 5; encode_json [$value] # yields [5]
349		971
350	# used as string, so dump as string	972	# used as string, so dump as string
351	print $value;	973	print $value;
352	to_json [$value] # yields ["5"]	974	encode_json [$value] # yields ["5"]
353		975
354	# undef becomes null	976	# undef becomes null
355	to_json [undef] # yields [null]	977	encode_json [undef] # yields [null]
356		978
357	You can force the type to be a string by stringifying it:	979	You can force the type to be a JSON string by stringifying it:
358		980
359	my $x = 3.1; # some variable containing a number	981	my $x = 3.1; # some variable containing a number
360	"$x"; # stringified	982	"$x"; # stringified
361	$x .= ""; # another, more awkward way to stringify	983	$x .= ""; # another, more awkward way to stringify
362	print $x; # perl does it for you, too, quite often	984	print $x; # perl does it for you, too, quite often
363		985
364	You can force the type to be a number by numifying it:	986	You can force the type to be a JSON number by numifying it:
365		987
366	my $x = "3"; # some variable containing a string	988	my $x = "3"; # some variable containing a string
367	$x += 0; # numify it, ensuring it will be dumped as a number	989	$x += 0; # numify it, ensuring it will be dumped as a number
368	$x *= 1; # same thing, the choise is yours.	990	$x *= 1; # same thing, the choice is yours.
369		991
370	You can not currently output JSON booleans or force the type in	992	You can not currently force the type in other, less obscure, ways.
371	other, less obscure, ways. Tell me if you need this capability.	993	Tell me if you need this capability (but don't forget to explain why
		994	it's needed :).
372		995
373	circular data structures	996	Note that numerical precision has the same meaning as under Perl (so
374	Those will be encoded until memory or stackspace runs out.	997	binary to decimal conversion follows the same rules as in Perl,
		998	which can differ to other languages). Also, your perl interpreter
		999	might expose extensions to the floating point numbers of your
		1000	platform, such as infinities or NaN's - these cannot be represented
		1001	in JSON, and it is an error to pass those in.
375		1002
376	COMPARISON	1003	ENCODING/CODESET FLAG NOTES
377	As already mentioned, this module was created because none of the	1004	The interested reader might have seen a number of flags that signify
378	existing JSON modules could be made to work correctly. First I will	1005	encodings or codesets - "utf8", "latin1" and "ascii". There seems to be
379	describe the problems (or pleasures) I encountered with various existing	1006	some confusion on what these do, so here is a short comparison:
380	JSON modules, followed by some benchmark values. JSON::XS was designed
381	not to suffer from any of these problems or limitations.
382		1007
383	JSON 1.07	1008	"utf8" controls whether the JSON text created by "encode" (and expected
384	Slow (but very portable, as it is written in pure Perl).	1009	by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only
		1010	control whether "encode" escapes character values outside their
		1011	respective codeset range. Neither of these flags conflict with each
		1012	other, although some combinations make less sense than others.
385		1013
386	Undocumented/buggy Unicode handling (how JSON handles unicode values	1014	Care has been taken to make all flags symmetrical with respect to
387	is undocumented. One can get far by feeding it unicode strings and	1015	"encode" and "decode", that is, texts encoded with any combination of
388	doing en-/decoding oneself, but unicode escapes are not working	1016	these flag values will be correctly decoded when the same flags are used
		1017	- in general, if you use different flag settings while encoding vs. when
		1018	decoding you likely have a bug somewhere.
		1019
		1020	Below comes a verbose discussion of these flags. Note that a "codeset"
		1021	is simply an abstract set of character-codepoint pairs, while an
		1022	encoding takes those codepoint numbers and encodes them, in our case
		1023	into octets. Unicode is (among other things) a codeset, UTF-8 is an
		1024	encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets and
		1025	encodings at the same time, which can be confusing.
		1026
		1027	"utf8" flag disabled
		1028	When "utf8" is disabled (the default), then "encode"/"decode"
		1029	generate and expect Unicode strings, that is, characters with high
		1030	ordinal Unicode values (> 255) will be encoded as such characters,
		1031	and likewise such characters are decoded as-is, no canges to them
		1032	will be done, except "(re-)interpreting" them as Unicode codepoints
		1033	or Unicode characters, respectively (to Perl, these are the same
		1034	thing in strings unless you do funny/weird/dumb stuff).
		1035
		1036	This is useful when you want to do the encoding yourself (e.g. when
		1037	you want to have UTF-16 encoded JSON texts) or when some other layer
		1038	does the encoding for you (for example, when printing to a terminal
		1039	using a filehandle that transparently encodes to UTF-8 you certainly
		1040	do NOT want to UTF-8 encode your data first and have Perl encode it
		1041	another time).
		1042
		1043	"utf8" flag enabled
		1044	If the "utf8"-flag is enabled, "encode"/"decode" will encode all
		1045	characters using the corresponding UTF-8 multi-byte sequence, and
		1046	will expect your input strings to be encoded as UTF-8, that is, no
		1047	"character" of the input string must have any value > 255, as UTF-8
		1048	does not allow that.
		1049
		1050	The "utf8" flag therefore switches between two modes: disabled means
		1051	you will get a Unicode string in Perl, enabled means you get an
		1052	UTF-8 encoded octet/binary string in Perl.
		1053
		1054	"latin1" or "ascii" flags enabled
		1055	With "latin1" (or "ascii") enabled, "encode" will escape characters
		1056	with ordinal values > 255 (> 127 with "ascii") and encode the
		1057	remaining characters as specified by the "utf8" flag.
		1058
		1059	If "utf8" is disabled, then the result is also correctly encoded in
		1060	those character sets (as both are proper subsets of Unicode, meaning
		1061	that a Unicode string with all character values < 256 is the same
		1062	thing as a ISO-8859-1 string, and a Unicode string with all
		1063	character values < 128 is the same thing as an ASCII string in
389	properly).	1064	Perl).
390		1065
391	No roundtripping (strings get clobbered if they look like numbers,	1066	If "utf8" is enabled, you still get a correct UTF-8-encoded string,
392	e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that	1067	regardless of these flags, just some more characters will be escaped
393	will decode into the number 2.	1068	using "\uXXXX" then before.
394		1069
395	JSON::PC 0.01	1070	Note that ISO-8859-1-encoded strings are not compatible with UTF-8
396	Very fast.	1071	encoding, while ASCII-encoded strings are. That is because the
		1072	ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1
		1073	codeset being a subset of Unicode), while ASCII is.
397		1074
398	Undocumented/buggy Unicode handling.	1075	Surprisingly, "decode" will ignore these flags and so treat all
		1076	input values as governed by the "utf8" flag. If it is disabled, this
		1077	allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
		1078	strict subsets of Unicode. If it is enabled, you can correctly
		1079	decode UTF-8 encoded strings.
399		1080
400	No roundtripping.	1081	So neither "latin1" nor "ascii" are incompatible with the "utf8"
		1082	flag - they only govern when the JSON output engine escapes a
		1083	character or not.
401		1084
402	Has problems handling many Perl values (e.g. regex results and other	1085	The main use for "latin1" is to relatively efficiently store binary
403	magic values will make it croak).	1086	data as JSON, at the expense of breaking compatibility with most
		1087	JSON decoders.
404		1088
405	Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}"	1089	The main use for "ascii" is to force the output to not contain
406	which is not a valid JSON text.	1090	characters with values > 127, which means you can interpret the
		1091	resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about
		1092	any character set and 8-bit-encoding, and still get the same data
		1093	structure back. This is useful when your channel for JSON transfer
		1094	is not 8-bit clean or the encoding might be mangled in between (e.g.
		1095	in mail), and works because ASCII is a proper subset of most 8-bit
		1096	and multibyte encodings in use in the world.
407		1097
408	Unmaintained (maintainer unresponsive for many months, bugs are not	1098	JSON and ECMAscript
409	getting fixed).	1099	JSON syntax is based on how literals are represented in javascript (the
		1100	not-standardised predecessor of ECMAscript) which is presumably why it
		1101	is called "JavaScript Object Notation".
410		1102
411	JSON::Syck 0.21	1103	However, JSON is not a subset (and also not a superset of course) of
412	Very buggy (often crashes).	1104	ECMAscript (the standard) or javascript (whatever browsers actually
		1105	implement).
413		1106
414	Very inflexible (no human-readable format supported, format pretty	1107	If you want to use javascript's "eval" function to "parse" JSON, you
415	much undocumented. I need at least a format for easy reading by	1108	might run into parse errors for valid JSON texts, or the resulting data
416	humans and a single-line compact format for use in a protocol, and	1109	structure might not be queryable:
417	preferably a way to generate ASCII-only JSON texts).
418		1110
419	Completely broken (and confusingly documented) Unicode handling	1111	One of the problems is that U+2028 and U+2029 are valid characters
420	(unicode escapes are not working properly, you need to set	1112	inside JSON strings, but are not allowed in ECMAscript string literals,
421	ImplicitUnicode to different values on en- and decoding to get	1113	so the following Perl fragment will not output something that can be
422	symmetric behaviour).	1114	guaranteed to be parsable by javascript's "eval":
423		1115
424	No roundtripping (simple cases work, but this depends on wether the	1116	use JSON::XS;
425	scalar value was used in a numeric context or not).
426		1117
427	Dumping hashes may skip hash values depending on iterator state.	1118	print encode_json [chr 0x2028];
428		1119
429	Unmaintained (maintainer unresponsive for many months, bugs are not	1120	The right fix for this is to use a proper JSON parser in your javascript
430	getting fixed).	1121	programs, and not rely on "eval" (see for example Douglas Crockford's
		1122	json2.js parser).
431		1123
432	Does not check input for validity (i.e. will accept non-JSON input	1124	If this is not an option, you can, as a stop-gap measure, simply encode
433	and return "something" instead of raising an exception. This is a	1125	to ASCII-only JSON:
434	security issue: imagine two banks transfering money between each
435	other using JSON. One bank might parse a given non-JSON request and
436	deduct money, while the other might reject the transaction with a
437	syntax error. While a good protocol will at least recover, that is
438	extra unnecessary work and the transaction will still not succeed).
439		1126
440	JSON::DWIW 0.04	1127	use JSON::XS;
441	Very fast. Very natural. Very nice.
442		1128
443	Undocumented unicode handling (but the best of the pack. Unicode	1129	print JSON::XS->new->ascii->encode ([chr 0x2028]);
444	escapes still don't get parsed properly).
445		1130
446	Very inflexible.	1131	Note that this will enlarge the resulting JSON text quite a bit if you
		1132	have many non-ASCII characters. You might be tempted to run some regexes
		1133	to only escape U+2028 and U+2029, e.g.:
447		1134
448	No roundtripping.	1135	# DO NOT USE THIS!
		1136	my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
		1137	$json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
		1138	$json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
		1139	print $json;
449		1140
450	Does not generate valid JSON texts (key strings are often unquoted,	1141	Note that this is a bad idea: the above only works for U+2028 and
451	empty keys result in nothing being output)	1142	U+2029 and thus only for fully ECMAscript-compliant parsers. Many
		1143	existing javascript implementations, however, have issues with other
		1144	characters as well - using "eval" naively simply will cause problems.
452		1145
453	Does not check input for validity.	1146	Another problem is that some javascript implementations reserve some
		1147	property names for their own purposes (which probably makes them
		1148	non-ECMAscript-compliant). For example, Iceweasel reserves the
		1149	"__proto__" property name for its own purposes.
		1150
		1151	If that is a problem, you could parse try to filter the resulting JSON
		1152	output for these property strings, e.g.:
		1153
		1154	$json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
		1155
		1156	This works because "__proto__" is not valid outside of strings, so every
		1157	occurence of ""__proto__"\s*:" must be a string used as property name.
		1158
		1159	If you know of other incompatibilities, please let me know.
		1160
		1161	JSON and YAML
		1162	You often hear that JSON is a subset of YAML. This is, however, a mass
		1163	hysteria(*) and very far from the truth (as of the time of this
		1164	writing), so let me state it clearly: *in general, there is no way to
		1165	configure JSON::XS to output a data structure as valid YAML* that works
		1166	in all cases.
		1167
		1168	If you really must use JSON::XS to generate YAML, you should use this
		1169	algorithm (subject to change in future versions):
		1170
		1171	my $to_yaml = JSON::XS->new->utf8->space_after (1);
		1172	my $yaml = $to_yaml->encode ($ref) . "\n";
		1173
		1174	This will usually generate JSON texts that also parse as valid YAML.
		1175	Please note that YAML has hardcoded limits on (simple) object key
		1176	lengths that JSON doesn't have and also has different and incompatible
		1177	unicode character escape syntax, so you should make sure that your hash
		1178	keys are noticeably shorter than the 1024 "stream characters" YAML
		1179	allows and that you do not have characters with codepoint values outside
		1180	the Unicode BMP (basic multilingual page). YAML also does not allow "\/"
		1181	sequences in strings (which JSON::XS does not currently generate, but
		1182	other JSON generators might).
		1183
		1184	There might be other incompatibilities that I am not aware of (or the
		1185	YAML specification has been changed yet again - it does so quite often).
		1186	In general you should not try to generate YAML with a JSON generator or
		1187	vice versa, or try to parse JSON with a YAML parser or vice versa:
		1188	chances are high that you will run into severe interoperability problems
		1189	when you least expect it.
		1190
		1191	(*) I have been pressured multiple times by Brian Ingerson (one of the
		1192	authors of the YAML specification) to remove this paragraph, despite
		1193	him acknowledging that the actual incompatibilities exist. As I was
		1194	personally bitten by this "JSON is YAML" lie, I refused and said I
		1195	will continue to educate people about these issues, so others do not
		1196	run into the same problem again and again. After this, Brian called
		1197	me a (quote)complete and worthless idiot(unquote).
		1198
		1199	In my opinion, instead of pressuring and insulting people who
		1200	actually clarify issues with YAML and the wrong statements of some
		1201	of its proponents, I would kindly suggest reading the JSON spec
		1202	(which is not that difficult or long) and finally make YAML
		1203	compatible to it, and educating users about the changes, instead of
		1204	spreading lies about the real compatibility for many years and
		1205	trying to silence people who point out that it isn't true.
		1206
		1207	Addendum/2009: the YAML 1.2 spec is still incompatible with JSON,
		1208	even though the incompatibilities have been documented (and are
		1209	known to Brian) for many years and the spec makes explicit claims
		1210	that YAML is a superset of JSON. It would be so easy to fix, but
		1211	apparently, bullying people and corrupting userdata is so much
		1212	easier.
454		1213
455	SPEED	1214	SPEED
456	It seems that JSON::XS is surprisingly fast, as shown in the following	1215	It seems that JSON::XS is surprisingly fast, as shown in the following
457	tables. They have been generated with the help of the "eg/bench" program	1216	tables. They have been generated with the help of the "eg/bench" program
458	in the JSON::XS distribution, to make it easy to compare on your own	1217	in the JSON::XS distribution, to make it easy to compare on your own
459	system.	1218	system.
460		1219
461	First comes a comparison between various modules using a very short JSON	1220	First comes a comparison between various modules using a very short
462	string:	1221	single-line JSON string (also available at
		1222	<http://dist.schmorp.de/misc/json/short.json>).
463		1223
464	{"method": "handleMessage", "params": ["user1", "we were just talking"], "id": null}	1224	{"method": "handleMessage", "params": ["user1",
		1225	"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
		1226	1, 0]}
465		1227
466	It shows the number of encodes/decodes per second (JSON::XS uses the	1228	It shows the number of encodes/decodes per second (JSON::XS uses the
467	functional interface, while JSON::XS/2 uses the OO interface with	1229	functional interface, while JSON::XS/2 uses the OO interface with
468	pretty-printing and hashkey sorting enabled). Higher is better:	1230	pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink.
		1231	JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ uses
		1232	the from_json method). Higher is better:
469		1233
470	module \| encode \| decode \|	1234	module \| encode \| decode \|
471	-----------\|------------\|------------\|	1235	--------------\|------------\|------------\|
472	JSON \| 11488.516 \| 7823.035 \|	1236	JSON::DWIW/DS \| 86302.551 \| 102300.098 \|
473	JSON::DWIW \| 94708.054 \| 129094.260 \|	1237	JSON::DWIW/FJ \| 86302.551 \| 75983.768 \|
474	JSON::PC \| 63884.157 \| 128528.212 \|	1238	JSON::PP \| 15827.562 \| 6638.658 \|
475	JSON::Syck \| 34898.677 \| 42096.911 \|	1239	JSON::Syck \| 63358.066 \| 47662.545 \|
476	JSON::XS \| 654027.064 \| 396423.669 \|	1240	JSON::XS \| 511500.488 \| 511500.488 \|
477	JSON::XS/2 \| 371564.190 \| 371725.613 \|	1241	JSON::XS/2 \| 291271.111 \| 388361.481 \|
		1242	JSON::XS/3 \| 361577.931 \| 361577.931 \|
		1243	Storable \| 66788.280 \| 265462.278 \|
478	-----------+------------+------------+	1244	--------------+------------+------------+
479		1245
480	That is, JSON::XS is more than six times faster than JSON::DWIW on	1246	That is, JSON::XS is almost six times faster than JSON::DWIW on
481	encoding, more than three times faster on decoding, and about thirty	1247	encoding, about five times faster on decoding, and over thirty to
482	times faster than JSON, even with pretty-printing and key sorting.	1248	seventy times faster than JSON's pure perl implementation. It also
		1249	compares favourably to Storable for small amounts of data.
483		1250
484	Using a longer test string (roughly 18KB, generated from Yahoo! Locals	1251	Using a longer test string (roughly 18KB, generated from Yahoo! Locals
485	search API (http://nanoref.com/yahooapis/mgPdGg):	1252	search API (<http://dist.schmorp.de/misc/json/long.json>).
486		1253
487	module \| encode \| decode \|	1254	module \| encode \| decode \|
488	-----------\|------------\|------------\|	1255	--------------\|------------\|------------\|
489	JSON \| 273.023 \| 44.674 \|	1256	JSON::DWIW/DS \| 1647.927 \| 2673.916 \|
490	JSON::DWIW \| 1089.383 \| 1145.704 \|	1257	JSON::DWIW/FJ \| 1630.249 \| 2596.128 \|
491	JSON::PC \| 3097.419 \| 2393.921 \|	1258	JSON::PP \| 400.640 \| 62.311 \|
492	JSON::Syck \| 514.060 \| 843.053 \|	1259	JSON::Syck \| 1481.040 \| 1524.869 \|
493	JSON::XS \| 6479.668 \| 3636.364 \|	1260	JSON::XS \| 20661.596 \| 9541.183 \|
494	JSON::XS/2 \| 3774.221 \| 3599.124 \|	1261	JSON::XS/2 \| 10683.403 \| 9416.938 \|
		1262	JSON::XS/3 \| 20661.596 \| 9400.054 \|
		1263	Storable \| 19765.806 \| 10000.725 \|
495	-----------+------------+------------+	1264	--------------+------------+------------+
496		1265
497	Again, JSON::XS leads by far.	1266	Again, JSON::XS leads by far (except for Storable which non-surprisingly
		1267	decodes a bit faster).
498		1268
499	On large strings containing lots of high unicode characters, some	1269	On large strings containing lots of high Unicode characters, some
500	modules (such as JSON::PC) seem to decode faster than JSON::XS, but the	1270	modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
501	result will be broken due to missing (or wrong) unicode handling. Others	1271	result will be broken due to missing (or wrong) Unicode handling. Others
502	refuse to decode or encode properly, so it was impossible to prepare a	1272	refuse to decode or encode properly, so it was impossible to prepare a
503	fair comparison table for that case.	1273	fair comparison table for that case.
504		1274
505	RESOURCE LIMITS	1275	SECURITY CONSIDERATIONS
506	JSON::XS does not impose any limits on the size of JSON texts or Perl	1276	When you are using JSON in a protocol, talking to untrusted potentially
507	values they represent - if your machine can handle it, JSON::XS will	1277	hostile creatures requires relatively few measures.
508	encode or decode it. Future versions might optionally impose structure	1278
509	depth and memory use resource limits.	1279	First of all, your JSON decoder should be secure, that is, should not
		1280	have any buffer overflows. Obviously, this module should ensure that and
		1281	I am trying hard on making that true, but you never know.
		1282
		1283	Second, you need to avoid resource-starving attacks. That means you
		1284	should limit the size of JSON texts you accept, or make sure then when
		1285	your resources run out, that's just fine (e.g. by using a separate
		1286	process that can crash safely). The size of a JSON text in octets or
		1287	characters is usually a good indication of the size of the resources
		1288	required to decode it into a Perl structure. While JSON::XS can check
		1289	the size of the JSON text, it might be too late when you already have it
		1290	in memory, so you might want to check the size before you accept the
		1291	string.
		1292
		1293	Third, JSON::XS recurses using the C stack when decoding objects and
		1294	arrays. The C stack is a limited resource: for instance, on my amd64
		1295	machine with 8MB of stack size I can decode around 180k nested arrays
		1296	but only 14k nested JSON objects (due to perl itself recursing deeply on
		1297	croak to free the temporary). If that is exceeded, the program crashes.
		1298	To be conservative, the default nesting limit is set to 512. If your
		1299	process has a smaller stack, you should adjust this setting accordingly
		1300	with the "max_depth" method.
		1301
		1302	Something else could bomb you, too, that I forgot to think of. In that
		1303	case, you get to keep the pieces. I am always open for hints, though...
		1304
		1305	Also keep in mind that JSON::XS might leak contents of your Perl data
		1306	structures in its error messages, so when you serialise sensitive
		1307	information you might want to make sure that exceptions thrown by
		1308	JSON::XS will not end up in front of untrusted eyes.
		1309
		1310	If you are using JSON::XS to return packets to consumption by JavaScript
		1311	scripts in a browser you should have a look at
		1312	<http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/>
		1313	to see whether you are vulnerable to some common attack vectors (which
		1314	really are browser design bugs, but it is still you who will have to
		1315	deal with it, as major browser developers care only for features, not
		1316	about getting security right).
		1317
		1318	THREADS
		1319	This module is not guaranteed to be thread safe and there are no plans
		1320	to change this until Perl gets thread support (as opposed to the
		1321	horribly slow so-called "threads" which are simply slow and bloated
		1322	process simulations - use fork, it's much faster, cheaper, better).
		1323
		1324	(It might actually work, but you have been warned).
		1325
		1326	THE PERILS OF SETLOCALE
		1327	Sometimes people avoid the Perl locale support and directly call the
		1328	system's setlocale function with "LC_ALL".
		1329
		1330	This breaks both perl and modules such as JSON::XS, as stringification
		1331	of numbers no longer works correcly (e.g. "$x = 0.1; print "$x"+1" might
		1332	print 1, and JSON::XS might output illegal JSON as JSON::XS relies on
		1333	perl to stringify numbers).
		1334
		1335	The solution is simple: don't call "setlocale", or use it for only those
		1336	categories you need, such as "LC_MESSAGES" or "LC_CTYPE".
		1337
		1338	If you need "LC_NUMERIC", you should enable it only around the code that
		1339	actually needs it (avoiding stringification of numbers), and restore it
		1340	afterwards.
510		1341
511	BUGS	1342	BUGS
512	While the goal of this module is to be correct, that unfortunately does	1343	While the goal of this module is to be correct, that unfortunately does
513	not mean its bug-free, only that I think its design is bug-free. It is	1344	not mean it's bug-free, only that I think its design is bug-free. If you
514	still very young and not well-tested. If you keep reporting bugs they	1345	keep reporting bugs they will be fixed swiftly, though.
515	will be fixed swiftly, though.	1346
		1347	Please refrain from using rt.cpan.org or any other bug reporting
		1348	service. I put the contact address into my modules for a reason.
		1349
		1350	SEE ALSO
		1351	The json_xs command line utility for quick experiments.
516		1352
517	AUTHOR	1353	AUTHOR
518	Marc Lehmann <schmorp@schmorp.de>	1354	Marc Lehmann <schmorp@schmorp.de>
519	http://home.schmorp.de/	1355	http://home.schmorp.de/
520		1356

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Comparing JSON-XS/README (file contents): Revision 1.7 by root, Sun Mar 25 00:47:42 2007 UTC vs. Revision 1.37 by root, Thu May 23 09:32:02 2013 UTC

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Comparing JSON-XS/README (file contents):
Revision 1.7 by root, Sun Mar 25 00:47:42 2007 UTC vs.
Revision 1.37 by root, Thu May 23 09:32:02 2013 UTC