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Cpanel::JSON::XS - cPanel fork of JSON::XS, fast and correct serializing


 use Cpanel::JSON::XS;

 # exported functions, they croak on error
 # and expect/generate UTF-8

 $utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
 $perl_hash_or_arrayref  = decode_json $utf8_encoded_json_text;

 # OO-interface

 $coder = Cpanel::JSON::XS->new->ascii->pretty->allow_nonref;
 $pretty_printed_unencoded = $coder->encode ($perl_scalar);
 $perl_scalar = $coder->decode ($unicode_json_text);

 # Note that 5.6 misses most smart utf8 and encoding functionalities
 # of newer releases.

 # Note that L<JSON::MaybeXS> will automatically use Cpanel::JSON::XS
 # if available, at virtually no speed overhead either, so you should
 # be able to just:
 use JSON::MaybeXS;

 # and do the same things, except that you have a pure-perl fallback now.

 Note that this module will be replaced by a new JSON::Safe module soon,
 with the same API just guaranteed safe defaults.


This module converts Perl data structures to JSON and vice versa. Its primary goal is to be correct and its secondary goal is to be fast. To reach the latter goal it was written in C.

As this is the n-th-something JSON module on CPAN, what was the reason to write yet another JSON module? While it seems there are many JSON modules, none of them correctly handle all corner cases, and in most cases their maintainers are unresponsive, gone missing, or not listening to bug reports for other reasons.

See below for the cPanel fork.

See MAPPING, below, on how Cpanel::JSON::XS maps perl values to JSON values and vice versa.


  • correct Unicode handling

    This module knows how to handle Unicode with Perl version higher than 5.8.5, documents how and when it does so, and even documents what "correct" means.

  • round-trip integrity

    When you serialize a perl data structure using only data types supported by JSON and Perl, the deserialized data structure is identical on the Perl level. (e.g. the string "2.0" doesn't suddenly become "2" just because it looks like a number). There are minor exceptions to this, read the MAPPING section below to learn about those.

  • strict checking of JSON correctness

    There is no guessing, no generating of illegal JSON texts by default, and only JSON is accepted as input by default. the latter is a security feature.

  • fast

    Compared to other JSON modules and other serializers such as Storable, this module usually compares favourably in terms of speed, too.

  • simple to use

    This module has both a simple functional interface as well as an object oriented interface.

  • reasonably versatile output formats

    You can choose between the most compact guaranteed-single-line format possible (nice for simple line-based protocols), a pure-ASCII format (for when your transport is not 8-bit clean, still supports the whole Unicode range), or a pretty-printed format (for when you want to read that stuff). Or you can combine those features in whatever way you like.

cPanel fork

Since the original author MLEHMANN has no public bugtracker, this cPanel fork sits now on github.

src repo: original:

RT: or

Changes to JSON::XS

- bare hashkeys are now checked for utf8. (GH #209)

- stricter decode_json() as documented. non-refs are disallowed. safe by default. added a 2nd optional argument. decode() honors now allow_nonref.

- fixed encode of numbers for dual-vars. Different string representations are preserved, but numbers with temporary strings which represent the same number are here treated as numbers, not strings. Cpanel::JSON::XS is a bit slower, but preserves numeric types better.

- numbers ending with .0 stray numbers, are not converted to integers. [#63] dual-vars which are represented as number not integer (42+"bar" != 5.8.9) are now encoded as number (=> 42.0) because internally it's now a NOK type. However !!1 which is wrongly encoded in 5.8 as "1"/1.0 is still represented as integer.

- different handling of inf/nan. Default now to null, optionally with stringify_infnan() to "inf"/"nan". [#28, #32]

- added binary extension, non-JSON and non JSON parsable, allows \xNN and \NNN sequences.

- 5.6.2 support; sacrificing some utf8 features (assuming bytes all-over), no multi-byte unicode characters with 5.6.

- interop for true/false overloading. JSON::XS, JSON::PP and Mojo::JSON representations for booleans are accepted and JSON::XS accepts Cpanel::JSON::XS booleans [#13, #37] Fixed overloading of booleans. Cpanel::JSON::XS::true stringifies again to "1", not "true", analog to all other JSON modules.

- native boolean mapping of yes and no to true and false, as in YAML::XS. In perl !0 is yes, !1 is no. The JSON value true maps to 1, false maps to 0. [#39]

- support arbitrary stringification with encode, with convert_blessed and allow_blessed.

- ithread support. Cpanel::JSON::XS is thread-safe, JSON::XS not

- is_bool can be called as method, JSON::XS::is_bool not.

- performance optimizations for threaded Perls

- relaxed mode, allowing many popular extensions

- protect our magic object from corruption by wrong or missing external methods, like FREEZE/THAW or serialization with other methods.

- additional fixes for:

  - #208 - no security-relevant out-of-bounds reading of module memory
    when decoding hash keys without ending ':'

  - [cpan #88061] AIX atof without USE_LONG_DOUBLE

  - #10 unshare_hek crash

  - #7, #29 avoid re-blessing where possible. It fails in JSON::XS for
   READONLY values, i.e. restricted hashes.

  - #41 overloading of booleans, use the object not the reference.

  - #62 -Dusequadmath conversion and no SEGV.

  - #72 parsing of values followed \0, like 1\0 does fail.

  - #72 parsing of illegal unicode or non-unicode characters.

  - #96 locale-insensitive numeric conversion.

  - #154 numeric conversion fixed since 5.22, using the same strtold as perl5.

  - #167 sort tied hashes with canonical.

  - #212 fix utf8 object stringification

- public maintenance and bugtracker

- use ppport.h, sanify XS.xs comment styles, harness C coding style

- common::sense is optional. When available it is not used in the published production module, just during development and testing.

- extended testsuite, passes all tests. In fact it is the only know JSON decoder which does so, while also being the fastest.

- support many more options and methods from JSON::PP: stringify_infnan, allow_unknown, allow_stringify, allow_barekey, encode_stringify, allow_bignum, allow_singlequote, dupkeys_as_arrayref, sort_by (partially), escape_slash, convert_blessed, ... optional decode_json(, allow_nonref) arg. relaxed implements allow_dupkeys.

- support all 5 unicode BOM's: UTF-8, UTF-16LE, UTF-16BE, UTF-32LE, UTF-32BE, encoding internally to UTF-8.


The following convenience methods are provided by this module. They are exported by default:

$json_text = encode_json $perl_scalar, [json_type]

Converts the given Perl data structure to a UTF-8 encoded, binary string (that is, the string contains octets only). Croaks on error.

This function call is functionally identical to:

   $json_text = Cpanel::JSON::XS->new->utf8->encode ($perl_scalar, $json_type)

Except being faster.

For the type argument see Cpanel::JSON::XS::Type.

$perl_scalar = decode_json $json_text [, $allow_nonref [, my $json_type ] ]

The opposite of encode_json: expects an UTF-8 (binary) string of an json reference and tries to parse that as an UTF-8 encoded JSON text, returning the resulting reference. Croaks on error.

This function call is functionally identical to:

   $perl_scalar = Cpanel::JSON::XS->new->utf8->decode ($json_text, $json_type)

except being faster.

Note that older decode_json versions in Cpanel::JSON::XS older than 3.0116 and JSON::XS did not set allow_nonref but allowed them due to a bug in the decoder.

If the new 2nd optional $allow_nonref argument is set and not false, the allow_nonref option will be set and the function will act is described as in the relaxed RFC 7159 allowing all values such as objects, arrays, strings, numbers, "null", "true", and "false". See ""OLD" VS. "NEW" JSON (RFC 4627 VS. RFC 7159)" below, why you don't want to do that.

For the 3rd optional type argument see Cpanel::JSON::XS::Type.

$is_boolean = Cpanel::JSON::XS::is_bool $scalar

Returns true if the passed scalar represents either JSON::PP::true or JSON::PP::false, two constants that act like 1 and 0, respectively and are used to represent JSON true and false values in Perl. (Also recognizes the booleans produced by JSON::XS.)

See MAPPING, below, for more information on how JSON values are mapped to Perl.



from_json has been renamed to decode_json


to_json has been renamed to encode_json


Since this often leads to confusion, here are a few very clear words on how Unicode works in Perl, modulo bugs.

1. Perl strings can store characters with ordinal values > 255.

This enables you to store Unicode characters as single characters in a Perl string - very natural.

2. Perl does not associate an encoding with your strings.

... until you force it to, e.g. when matching it against a regex, or printing the scalar to a file, in which case Perl either interprets your string as locale-encoded text, octets/binary, or as Unicode, depending on various settings. In no case is an encoding stored together with your data, it is use that decides encoding, not any magical meta data.

3. The internal utf-8 flag has no meaning with regards to the encoding of your string.
4. A "Unicode String" is simply a string where each character can be validly interpreted as a Unicode code point.

If you have UTF-8 encoded data, it is no longer a Unicode string, but a Unicode string encoded in UTF-8, giving you a binary string.

5. A string containing "high" (> 255) character values is not a UTF-8 string.
6. Unicode noncharacters only warn, as in core.

The 66 Unicode noncharacters U+FDD0..U+FDEF, and U+*FFFE, U+*FFFF just warn, see But illegal surrogate pairs fail to parse.

7. Raw non-Unicode characters above U+10FFFF are disallowed.

Raw non-Unicode characters outside the valid unicode range fail to parse, because "A string is a sequence of zero or more Unicode characters" RFC 7159 section 1 and "JSON text SHALL be encoded in Unicode RFC 7159 section 8.1. We use now the UTF8_DISALLOW_SUPER flag when parsing unicode.

I hope this helps :)


The object oriented interface lets you configure your own encoding or decoding style, within the limits of supported formats.

$json = new Cpanel::JSON::XS

Creates a new JSON object that can be used to de/encode JSON strings. All boolean flags described below are by default disabled.

The mutators for flags all return the JSON object again and thus calls can be chained:

   my $json = Cpanel::JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
   => {"a": [1, 2]}
$json = $json->ascii ([$enable])
$enabled = $json->get_ascii

If $enable is true (or missing), then the encode method will not generate characters outside the code range 0..127 (which is ASCII). Any Unicode characters outside that range will be escaped using either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence, as per RFC4627. The resulting encoded JSON text can be treated as a native Unicode string, an ascii-encoded, latin1-encoded or UTF-8 encoded string, or any other superset of ASCII.

If $enable is false, then the encode method will not escape Unicode characters unless required by the JSON syntax or other flags. This results in a faster and more compact format.

See also the section ENCODING/CODESET FLAG NOTES later in this document.

The main use for this flag is to produce JSON texts that can be transmitted over a 7-bit channel, as the encoded JSON texts will not contain any 8 bit characters.

  Cpanel::JSON::XS->new->ascii (1)->encode ([chr 0x10401])
  => ["\ud801\udc01"]
$json = $json->latin1 ([$enable])
$enabled = $json->get_latin1

If $enable is true (or missing), then the encode method will encode the resulting JSON text as latin1 (or ISO-8859-1), escaping any characters outside the code range 0..255. The resulting string can be treated as a latin1-encoded JSON text or a native Unicode string. The decode method will not be affected in any way by this flag, as decode by default expects Unicode, which is a strict superset of latin1.

If $enable is false, then the encode method will not escape Unicode characters unless required by the JSON syntax or other flags.

See also the section ENCODING/CODESET FLAG NOTES later in this document.

The main use for this flag is efficiently encoding binary data as JSON text, as most octets will not be escaped, resulting in a smaller encoded size. The disadvantage is that the resulting JSON text is encoded in latin1 (and must correctly be treated as such when storing and transferring), a rare encoding for JSON. It is therefore most useful when you want to store data structures known to contain binary data efficiently in files or databases, not when talking to other JSON encoders/decoders.

  Cpanel::JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
  => ["\x{89}\\u0abc"]    # (perl syntax, U+abc escaped, U+89 not)
$json = $json->binary ([$enable])
$enabled = $json = $json->get_binary

If the $enable argument is true (or missing), then the encode method will not try to detect an UTF-8 encoding in any JSON string, it will strictly interpret it as byte sequence. The result might contain new \xNN sequences, which is unparsable JSON. The decode method forbids \uNNNN sequences and accepts \xNN and octal \NNN sequences.

There is also a special logic for perl 5.6 and utf8. 5.6 encodes any string to utf-8 automatically when seeing a codepoint >= 0x80 and < 0x100. With the binary flag enabled decode the perl utf8 encoded string to the original byte encoding and encode this with \xNN escapes. This will result to the same encodings as with newer perls. But note that binary multi-byte codepoints with 5.6 will result in illegal unicode character in binary string errors, unlike with newer perls.

If $enable is false, then the encode method will smartly try to detect Unicode characters unless required by the JSON syntax or other flags and hex and octal sequences are forbidden.

See also the section ENCODING/CODESET FLAG NOTES later in this document.

The main use for this flag is to avoid the smart unicode detection and possible double encoding. The disadvantage is that the resulting JSON text is encoded in new \xNN and in latin1 characters and must correctly be treated as such when storing and transferring, a rare encoding for JSON. It will produce non-readable JSON strings in the browser. It is therefore most useful when you want to store data structures known to contain binary data efficiently in files or databases, not when talking to other JSON encoders/decoders. The binary decoding method can also be used when an encoder produced a non-JSON conformant hex or octal encoding \xNN or \NNN.

  Cpanel::JSON::XS->new->binary->encode (["\x{89}\x{abc}"])
  5.6:   Error: malformed or illegal unicode character in binary string
  >=5.8: ['\x89\xe0\xaa\xbc']

  Cpanel::JSON::XS->new->binary->encode (["\x{89}\x{bc}"])
  => ["\x89\xbc"]

  Cpanel::JSON::XS->new->binary->decode (["\x89\ua001"])
  Error: malformed or illegal unicode character in binary string

  Cpanel::JSON::XS->new->decode (["\x89"])
  Error: illegal hex character in non-binary string
$json = $json->utf8 ([$enable])
$enabled = $json->get_utf8

If $enable is true (or missing), then the encode method will encode the JSON result into UTF-8, as required by many protocols, while the decode method expects to be handled an UTF-8-encoded string. Please note that UTF-8-encoded strings do not contain any characters outside the range 0..255, they are thus useful for bytewise/binary I/O. In future versions, enabling this option might enable autodetection of the UTF-16 and UTF-32 encoding families, as described in RFC4627.

If $enable is false, then the encode method will return the JSON string as a (non-encoded) Unicode string, while decode expects thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs to be done yourself, e.g. using the Encode module.

See also the section ENCODING/CODESET FLAG NOTES later in this document.

Example, output UTF-16BE-encoded JSON:

  use Encode;
  $jsontext = encode "UTF-16BE", Cpanel::JSON::XS->new->encode ($object);

Example, decode UTF-32LE-encoded JSON:

  use Encode;
  $object = Cpanel::JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
$json = $json->pretty ([$enable])

This enables (or disables) all of the indent, space_before and space_after (and in the future possibly more) flags in one call to generate the most readable (or most compact) form possible.

Example, pretty-print some simple structure:

   my $json = Cpanel::JSON::XS->new->pretty(1)->encode ({a => [1,2]})
      "a" : [
$json = $json->indent ([$enable])
$enabled = $json->get_indent

If $enable is true (or missing), then the encode method will use a multiline format as output, putting every array member or object/hash key-value pair into its own line, indenting them properly.

If $enable is false, no newlines or indenting will be produced, and the resulting JSON text is guaranteed not to contain any newlines.

This setting has no effect when decoding JSON texts.

$json = $json->indent_length([$number_of_spaces])
$length = $json->get_indent_length()

Set the indent length (default 3). This option is only useful when you also enable indent or pretty. The acceptable range is from 0 (no indentation) to 15

$json = $json->space_before ([$enable])
$enabled = $json->get_space_before

If $enable is true (or missing), then the encode method will add an extra optional space before the : separating keys from values in JSON objects.

If $enable is false, then the encode method will not add any extra space at those places.

This setting has no effect when decoding JSON texts. You will also most likely combine this setting with space_after.

Example, space_before enabled, space_after and indent disabled:

   {"key" :"value"}
$json = $json->space_after ([$enable])
$enabled = $json->get_space_after

If $enable is true (or missing), then the encode method will add an extra optional space after the : separating keys from values in JSON objects and extra whitespace after the , separating key-value pairs and array members.

If $enable is false, then the encode method will not add any extra space at those places.

This setting has no effect when decoding JSON texts.

Example, space_before and indent disabled, space_after enabled:

   {"key": "value"}
$json = $json->relaxed ([$enable])
$enabled = $json->get_relaxed

If $enable is true (or missing), then decode will accept some extensions to normal JSON syntax (see below). encode will not be affected in anyway. Be aware that this option makes you accept invalid JSON texts as if they were valid!. I suggest only to use this option to parse application-specific files written by humans (configuration files, resource files etc.)

If $enable is false (the default), then decode will only accept valid JSON texts.

Currently accepted extensions are:

  • list items can have an end-comma

    JSON separates array elements and key-value pairs with commas. This can be annoying if you write JSON texts manually and want to be able to quickly append elements, so this extension accepts comma at the end of such items not just between them:

          2, <- this comma not normally allowed
          "k1": "v1",
          "k2": "v2", <- this comma not normally allowed
  • shell-style '#'-comments

    Whenever JSON allows whitespace, shell-style comments are additionally allowed. They are terminated by the first carriage-return or line-feed character, after which more white-space and comments are allowed.

         1, # this comment not allowed in JSON
            # neither this one...
  • literal ASCII TAB characters in strings

    Literal ASCII TAB characters are now allowed in strings (and treated as \t) in relaxed mode. Despite JSON mandates, that TAB character is substituted for "\t" sequence.

         "Hello<TAB>World", # literal <TAB> would not normally be allowed
  • allow_singlequote

    Single quotes are accepted instead of double quotes. See the "allow_singlequote" option.

        { "foo":'bar' }
        { 'foo':"bar" }
        { 'foo':'bar' }
  • allow_barekey

    Accept unquoted object keys instead of with mandatory double quotes. See the "allow_barekey" option.

        { foo:"bar" }
  • allow_dupkeys

    Allow decoding of duplicate keys in hashes. By default duplicate keys are forbidden. See RFC 7159 section 4: "The names within an object should be unique." See the allow_dupkeys option.

$json = $json->canonical ([$enable])
$enabled = $json->get_canonical

If $enable is true (or missing), then the encode method will output JSON objects by sorting their keys. This is adding a comparatively high overhead.

If $enable is false, then the encode method will output key-value pairs in the order Perl stores them (which will likely change between runs of the same script, and can change even within the same run from 5.18 onwards).

This option is useful if you want the same data structure to be encoded as the same JSON text (given the same overall settings). If it is disabled, the same hash might be encoded differently even if contains the same data, as key-value pairs have no inherent ordering in Perl.

This setting has no effect when decoding JSON texts.

This is now also done with tied hashes, contrary to JSON::XS. But note that with most large tied hashes stored as tree it is advised to sort the iterator already and don't sort the hash output here. Most such iterators are already sorted, as such e.g. DB_File with DB_BTREE.

$json = $json->sort_by (undef, 0, 1 or a block)

This currently only (un)sets the canonical option, and ignores custom sort blocks.

This setting has no effect when decoding JSON texts.

This setting has currently no effect on tied hashes.

$json = $json->escape_slash ([$enable])
$enabled = $json->get_escape_slash

According to the JSON Grammar, the forward slash character (U+002F) "/" need to be escaped. But by default strings are encoded without escaping slashes in all perl JSON encoders.

If $enable is true (or missing), then encode will escape slashes, "\/".

This setting has no effect when decoding JSON texts.

$json = $json->unblessed_bool ([$enable])
$enabled = $json->get_unblessed_bool
    $json = $json->unblessed_bool([$enable])

If $enable is true (or missing), then decode will return Perl non-object boolean variables (1 and 0 as numbers or "1" and "" as strings) for JSON booleans (true and false). If $enable is false, then decode will return JSON::PP::Boolean objects for JSON booleans.

$json = $json->allow_singlequote ([$enable])
$enabled = $json->get_allow_singlequote
    $json = $json->allow_singlequote([$enable])

If $enable is true (or missing), then decode will accept JSON strings quoted by single quotations that are invalid JSON format.


This is also enabled with relaxed. As same as the relaxed option, this option may be used to parse application-specific files written by humans.

$json = $json->allow_barekey ([$enable])
$enabled = $json->get_allow_barekey
    $json = $json->allow_barekey([$enable])

If $enable is true (or missing), then decode will accept bare keys of JSON object that are invalid JSON format.

Same as with the relaxed option, this option may be used to parse application-specific files written by humans.

$json = $json->allow_bignum ([$enable])
$enabled = $json->get_allow_bignum
    $json = $json->allow_bignum([$enable])

If $enable is true (or missing), then decode will convert the big integer Perl cannot handle as integer into a Math::BigInt object and convert a floating number (any) into a Math::BigFloat.

On the contrary, encode converts Math::BigInt objects and Math::BigFloat objects into JSON numbers with allow_blessed enable.

   $bigfloat = $json->decode('2.000000000000000000000000001');
   print $json->encode($bigfloat);
   # => 2.000000000000000000000000001

See "MAPPING" about the normal conversion of JSON number.

$json = $json->allow_bigint ([$enable])

This option is obsolete and replaced by allow_bignum.

$json = $json->allow_nonref ([$enable])
$enabled = $json->get_allow_nonref

If $enable is true (or missing), then the encode method can convert a non-reference into its corresponding string, number or null JSON value, which is an extension to RFC4627. Likewise, decode will accept those JSON values instead of croaking.

If $enable is false, then the encode method will croak if it isn't passed an arrayref or hashref, as JSON texts must either be an object or array. Likewise, decode will croak if given something that is not a JSON object or array.

Example, encode a Perl scalar as JSON value with enabled allow_nonref, resulting in an invalid JSON text:

   Cpanel::JSON::XS->new->allow_nonref->encode ("Hello, World!")
   => "Hello, World!"
$json = $json->allow_unknown ([$enable])
$enabled = $json->get_allow_unknown

If $enable is true (or missing), then encode will not throw an exception when it encounters values it cannot represent in JSON (for example, filehandles) but instead will encode a JSON null value. Note that blessed objects are not included here and are handled separately by c<allow_nonref>.

If $enable is false (the default), then encode will throw an exception when it encounters anything it cannot encode as JSON.

This option does not affect decode in any way, and it is recommended to leave it off unless you know your communications partner.

$json = $json->allow_stringify ([$enable])
$enabled = $json->get_allow_stringify

If $enable is true (or missing), then encode will stringify the non-object perl value or reference. Note that blessed objects are not included here and are handled separately by allow_blessed and convert_blessed. String references are stringified to the string value, other references as in perl.

This option does not affect decode in any way.

This option is special to this module, it is not supported by other encoders. So it is not recommended to use it.

$json = $json->require_types ([$enable])
$enable = $json->get_require_types
     $json = $json->require_types([$enable])

If $enable is true (or missing), then encode will require either enabled type_all_string or second argument with supplied JSON types. See Cpanel::JSON::XS::Type. When type_all_string is not enabled or second argument is not provided (or is undef), then encode croaks. It also croaks when the type for provided structure in encode is incomplete.

$json = $json->type_all_string ([$enable])
$enable = $json->get_type_all_string
     $json = $json->type_all_string([$enable])

If $enable is true (or missing), then encode will always produce stable deterministic JSON string types in resulted output.

When $enable is false, then result of encoded JSON output may be different for different Perl versions and may depends on loaded modules.

This is useful it you need deterministic JSON types, independently of used Perl version and other modules, but do not want to write complicated type definitions for Cpanel::JSON::XS::Type.

$json = $json->allow_dupkeys ([$enable])
$enabled = $json->get_allow_dupkeys

If $enable is true (or missing), then the decode method will not die when it encounters duplicate keys in a hash. allow_dupkeys is also enabled in the relaxed mode.

The JSON spec allows duplicate name in objects but recommends to disable it, however with Perl hashes they are impossible, parsing JSON in Perl silently ignores duplicate names, using the last value found.

See RFC 7159 section 4: "The names within an object should be unique."

$json = $json->dupkeys_as_arrayref ([$enable])
$enabled = $json->get_dupkeys_as_arrayref

If enabled, allow decoding of duplicate keys in hashes and store the values as arrayref in the hash instead. By default duplicate keys are forbidden. Enabling this also enables the "allow_dupkeys" option, but disabling this does not disable the "allow_dupkeys" option.


    print encode_json ($json->decode ('{"a":"b","a":"c"}'));

      => {"a":["b","c"]}

This changes the result structure, thus cannot be enabled by default. The client must be aware of it. The resulting arrayref is not yet marked somehow (blessed or such).

$json = $json->allow_blessed ([$enable])
$enabled = $json->get_allow_blessed

If $enable is true (or missing), then the encode method will not barf when it encounters a blessed reference. Instead, the value of the convert_blessed option will decide whether null (convert_blessed disabled or no TO_JSON method found) or a representation of the object (convert_blessed enabled and TO_JSON method found) is being encoded. Has no effect on decode.

If $enable is false (the default), then encode will throw an exception when it encounters a blessed object without convert_blessed and a TO_JSON method.

This setting has no effect on decode.

$json = $json->convert_blessed ([$enable])
$enabled = $json->get_convert_blessed

If $enable is true (or missing), then encode, upon encountering a blessed object, will check for the availability of the TO_JSON method on the object's class. If found, it will be called in scalar context and the resulting scalar will be encoded instead of the object. If no TO_JSON method is found, a stringification overload method is tried next. If both are not found, the value of allow_blessed will decide what to do.

The TO_JSON method may safely call die if it wants. If TO_JSON returns other blessed objects, those will be handled in the same way. TO_JSON must take care of not causing an endless recursion cycle (== crash) in this case. The same care must be taken with calling encode in stringify overloads (even if this works by luck in older perls) or other callbacks. The name of TO_JSON was chosen because other methods called by the Perl core (== not by the user of the object) are usually in upper case letters and to avoid collisions with any to_json function or method.

If $enable is false (the default), then encode will not consider this type of conversion.

This setting has no effect on decode.

$json = $json->allow_tags ([$enable])
$enabled = $json->get_allow_tags


If $enable is true (or missing), then encode, upon encountering a blessed object, will check for the availability of the FREEZE method on the object's class. If found, it will be used to serialize the object into a nonstandard tagged JSON value (that JSON decoders cannot decode).

It also causes decode to parse such tagged JSON values and deserialize them via a call to the THAW method.

If $enable is false (the default), then encode will not consider this type of conversion, and tagged JSON values will cause a parse error in decode, as if tags were not part of the grammar.

$json = $json->filter_json_object ([$coderef->($hashref)])

When $coderef is specified, it will be called from decode each time it decodes a JSON object. The only argument is a reference to the newly-created hash. If the code references returns a single scalar (which need not be a reference), this value (i.e. a copy of that scalar to avoid aliasing) is inserted into the deserialized data structure. If it returns an empty list (NOTE: not undef, which is a valid scalar), the original deserialized hash will be inserted. This setting can slow down decoding considerably.

When $coderef is omitted or undefined, any existing callback will be removed and decode will not change the deserialized hash in any way.

Example, convert all JSON objects into the integer 5:

   my $js = Cpanel::JSON::XS->new->filter_json_object (sub { 5 });
   # returns [5]
   $js->decode ('[{}]')
   # throw an exception because allow_nonref is not enabled
   # so a lone 5 is not allowed.
   $js->decode ('{"a":1, "b":2}');
$json = $json->filter_json_single_key_object ($key [=> $coderef->($value)])

Works remotely similar to filter_json_object, but is only called for JSON objects having a single key named $key.

This $coderef is called before the one specified via filter_json_object, if any. It gets passed the single value in the JSON object. If it returns a single value, it will be inserted into the data structure. If it returns nothing (not even undef but the empty list), the callback from filter_json_object will be called next, as if no single-key callback were specified.

If $coderef is omitted or undefined, the corresponding callback will be disabled. There can only ever be one callback for a given key.

As this callback gets called less often then the filter_json_object one, decoding speed will not usually suffer as much. Therefore, single-key objects make excellent targets to serialize Perl objects into, especially as single-key JSON objects are as close to the type-tagged value concept as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not support this in any way, so you need to make sure your data never looks like a serialized Perl hash.

Typical names for the single object key are __class_whatever__, or $__dollars_are_rarely_used__$ or }ugly_brace_placement, or even things like __class_md5sum(classname)__, to reduce the risk of clashing with real hashes.

Example, decode JSON objects of the form { "__widget__" => <id> } into the corresponding $WIDGET{<id>} object:

   # return whatever is in $WIDGET{5}:
      ->filter_json_single_key_object (__widget__ => sub {
            $WIDGET{ $_[0] }
      ->decode ('{"__widget__": 5')

   # this can be used with a TO_JSON method in some "widget" class
   # for serialization to json:
   sub WidgetBase::TO_JSON {
      my ($self) = @_;

      unless ($self->{id}) {
         $self->{id} =;
         $WIDGET{$self->{id}} = $self;

      { __widget__ => $self->{id} }
$json = $json->shrink ([$enable])
$enabled = $json->get_shrink

Perl usually over-allocates memory a bit when allocating space for strings. This flag optionally resizes strings generated by either encode or decode to their minimum size possible. This can save memory when your JSON texts are either very very long or you have many short strings. It will also try to downgrade any strings to octet-form if possible: perl stores strings internally either in an encoding called UTF-X or in octet-form. The latter cannot store everything but uses less space in general (and some buggy Perl or C code might even rely on that internal representation being used).

The actual definition of what shrink does might change in future versions, but it will always try to save space at the expense of time.

If $enable is true (or missing), the string returned by encode will be shrunk-to-fit, while all strings generated by decode will also be shrunk-to-fit.

If $enable is false, then the normal perl allocation algorithms are used. If you work with your data, then this is likely to be faster.

In the future, this setting might control other things, such as converting strings that look like integers or floats into integers or floats internally (there is no difference on the Perl level), saving space.

$json = $json->max_depth ([$maximum_nesting_depth])
$max_depth = $json->get_max_depth

Sets the maximum nesting level (default 512) accepted while encoding or decoding. If a higher nesting level is detected in JSON text or a Perl data structure, then the encoder and decoder will stop and croak at that point.

Nesting level is defined by number of hash- or arrayrefs that the encoder needs to traverse to reach a given point or the number of { or [ characters without their matching closing parenthesis crossed to reach a given character in a string.

Setting the maximum depth to one disallows any nesting, so that ensures that the object is only a single hash/object or array.

If no argument is given, the highest possible setting will be used, which is rarely useful.

Note that nesting is implemented by recursion in C. The default value has been chosen to be as large as typical operating systems allow without crashing.

See "SECURITY CONSIDERATIONS", below, for more info on why this is useful.

$json = $json->max_size ([$maximum_string_size])
$max_size = $json->get_max_size

Set the maximum length a JSON text may have (in bytes) where decoding is being attempted. The default is 0, meaning no limit. When decode is called on a string that is longer then this many bytes, it will not attempt to decode the string but throw an exception. This setting has no effect on encode (yet).

If no argument is given, the limit check will be deactivated (same as when 0 is specified).

See "SECURITY CONSIDERATIONS", below, for more info on why this is useful.

$json->stringify_infnan ([$infnan_mode = 1])
$infnan_mode = $json->get_stringify_infnan

Get or set how Cpanel::JSON::XS encodes inf, -inf or nan for numeric values. Also qnan, snan or negative nan on some platforms.

null: infnan_mode = 0. Similar to most JSON modules in other languages. Always null.

stringified: infnan_mode = 1. As in Mojo::JSON. Platform specific strings. Stringified via sprintf(%g), with double quotes.

inf/nan: infnan_mode = 2. As in JSON::XS, and older releases. Passes through platform dependent values, invalid JSON. Stringified via sprintf(%g), but without double quotes.

"inf/-inf/nan": infnan_mode = 3. Platform independent inf/nan/-inf strings. No QNAN/SNAN/negative NAN support, unified to "nan". Much easier to detect, but may conflict with valid strings.

$json_text = $json->encode ($perl_scalar, $json_type)

Converts the given Perl data structure (a simple scalar or a reference to a hash or array) to its JSON representation. Simple scalars will be converted into JSON string or number sequences, while references to arrays become JSON arrays and references to hashes become JSON objects. Undefined Perl values (e.g. undef) become JSON null values. Neither true nor false values will be generated.

For the type argument see Cpanel::JSON::XS::Type.

$perl_scalar = $json->decode ($json_text, my $json_type)

The opposite of encode: expects a JSON text and tries to parse it, returning the resulting simple scalar or reference. Croaks on error.

JSON numbers and strings become simple Perl scalars. JSON arrays become Perl arrayrefs and JSON objects become Perl hashrefs. true becomes 1, false becomes 0 and null becomes undef.

For the type argument see Cpanel::JSON::XS::Type.

($perl_scalar, $characters) = $json->decode_prefix ($json_text)

This works like the decode method, but instead of raising an exception when there is trailing garbage after the first JSON object, it will silently stop parsing there and return the number of characters consumed so far.

This is useful if your JSON texts are not delimited by an outer protocol and you need to know where the JSON text ends.

   Cpanel::JSON::XS->new->decode_prefix ("[1] the tail")
   => ([1], 3)
$json->to_json ($perl_hash_or_arrayref)

Deprecated method for perl 5.8 and newer. Use encode_json instead.

$json->from_json ($utf8_encoded_json_text)

Deprecated method for perl 5.8 and newer. Use decode_json instead.


In some cases, there is the need for incremental parsing of JSON texts. While this module always has to keep both JSON text and resulting Perl data structure in memory at one time, it does allow you to parse a JSON stream incrementally. It does so by accumulating text until it has a full JSON object, which it then can decode. This process is similar to using decode_prefix to see if a full JSON object is available, but is much more efficient (and can be implemented with a minimum of method calls).

Cpanel::JSON::XS will only attempt to parse the JSON text once it is sure it has enough text to get a decisive result, using a very simple but truly incremental parser. This means that it sometimes won't stop as early as the full parser, for example, it doesn't detect mismatched parentheses. The only thing it guarantees is that it starts decoding as soon as a syntactically valid JSON text has been seen. This means you need to set resource limits (e.g. max_size) to ensure the parser will stop parsing in the presence of syntax errors.

The following methods implement this incremental parser.

[void, scalar or list context] = $json->incr_parse ([$string])

This is the central parsing function. It can both append new text and extract objects from the stream accumulated so far (both of these functions are optional).

If $string is given, then this string is appended to the already existing JSON fragment stored in the $json object.

After that, if the function is called in void context, it will simply return without doing anything further. This can be used to add more text in as many chunks as you want.

If the method is called in scalar context, then it will try to extract exactly one JSON object. If that is successful, it will return this object, otherwise it will return undef. If there is a parse error, this method will croak just as decode would do (one can then use incr_skip to skip the erroneous part). This is the most common way of using the method.

And finally, in list context, it will try to extract as many objects from the stream as it can find and return them, or the empty list otherwise. For this to work, there must be no separators between the JSON objects or arrays, instead they must be concatenated back-to-back. If an error occurs, an exception will be raised as in the scalar context case. Note that in this case, any previously-parsed JSON texts will be lost.

Example: Parse some JSON arrays/objects in a given string and return them.

   my @objs = Cpanel::JSON::XS->new->incr_parse ("[5][7][1,2]");
$lvalue_string = $json->incr_text (>5.8 only)

This method returns the currently stored JSON fragment as an lvalue, that is, you can manipulate it. This only works when a preceding call to incr_parse in scalar context successfully returned an object, and 2. only with Perl >= 5.8

Under all other circumstances you must not call this function (I mean it. although in simple tests it might actually work, it will fail under real world conditions). As a special exception, you can also call this method before having parsed anything.

This function is useful in two cases: a) finding the trailing text after a JSON object or b) parsing multiple JSON objects separated by non-JSON text (such as commas).


This will reset the state of the incremental parser and will remove the parsed text from the input buffer so far. This is useful after incr_parse died, in which case the input buffer and incremental parser state is left unchanged, to skip the text parsed so far and to reset the parse state.

The difference to incr_reset is that only text until the parse error occurred is removed.


This completely resets the incremental parser, that is, after this call, it will be as if the parser had never parsed anything.

This is useful if you want to repeatedly parse JSON objects and want to ignore any trailing data, which means you have to reset the parser after each successful decode.


All options that affect decoding are supported, except allow_nonref. The reason for this is that it cannot be made to work sensibly: JSON objects and arrays are self-delimited, i.e. you can concatenate them back to back and still decode them perfectly. This does not hold true for JSON numbers, however.

For example, is the string 1 a single JSON number, or is it simply the start of 12? Or is 12 a single JSON number, or the concatenation of 1 and 2? In neither case you can tell, and this is why Cpanel::JSON::XS takes the conservative route and disallows this case.


Some examples will make all this clearer. First, a simple example that works similarly to decode_prefix: We want to decode the JSON object at the start of a string and identify the portion after the JSON object:

   my $text = "[1,2,3] hello";

   my $json = new Cpanel::JSON::XS;

   my $obj = $json->incr_parse ($text)
      or die "expected JSON object or array at beginning of string";

   my $tail = $json->incr_text;
   # $tail now contains " hello"

Easy, isn't it?

Now for a more complicated example: Imagine a hypothetical protocol where you read some requests from a TCP stream, and each request is a JSON array, without any separation between them (in fact, it is often useful to use newlines as "separators", as these get interpreted as whitespace at the start of the JSON text, which makes it possible to test said protocol with telnet...).

Here is how you'd do it (it is trivial to write this in an event-based manner):

   my $json = new Cpanel::JSON::XS;

   # read some data from the socket
   while (sysread $socket, my $buf, 4096) {

      # split and decode as many requests as possible
      for my $request ($json->incr_parse ($buf)) {
         # act on the $request

Another complicated example: Assume you have a string with JSON objects or arrays, all separated by (optional) comma characters (e.g. [1],[2], [3]). To parse them, we have to skip the commas between the JSON texts, and here is where the lvalue-ness of incr_text comes in useful:

   my $text = "[1],[2], [3]";
   my $json = new Cpanel::JSON::XS;

   # void context, so no parsing done
   $json->incr_parse ($text);

   # now extract as many objects as possible. note the
   # use of scalar context so incr_text can be called.
   while (my $obj = $json->incr_parse) {
      # do something with $obj

      # now skip the optional comma
      $json->incr_text =~ s/^ \s* , //x;

Now lets go for a very complex example: Assume that you have a gigantic JSON array-of-objects, many gigabytes in size, and you want to parse it, but you cannot load it into memory fully (this has actually happened in the real world :).

Well, you lost, you have to implement your own JSON parser. But Cpanel::JSON::XS can still help you: You implement a (very simple) array parser and let JSON decode the array elements, which are all full JSON objects on their own (this wouldn't work if the array elements could be JSON numbers, for example):

   my $json = new Cpanel::JSON::XS;

   # open the monster
   open my $fh, "<bigfile.json"
      or die "bigfile: $!";

   # first parse the initial "["
   for (;;) {
      sysread $fh, my $buf, 65536
         or die "read error: $!";
      $json->incr_parse ($buf); # void context, so no parsing

      # Exit the loop once we found and removed(!) the initial "[".
      # In essence, we are (ab-)using the $json object as a simple scalar
      # we append data to.
      last if $json->incr_text =~ s/^ \s* \[ //x;

   # now we have the skipped the initial "[", so continue
   # parsing all the elements.
   for (;;) {
      # in this loop we read data until we got a single JSON object
      for (;;) {
         if (my $obj = $json->incr_parse) {
            # do something with $obj

         # add more data
         sysread $fh, my $buf, 65536
            or die "read error: $!";
         $json->incr_parse ($buf); # void context, so no parsing

      # in this loop we read data until we either found and parsed the
      # separating "," between elements, or the final "]"
      for (;;) {
         # first skip whitespace
         $json->incr_text =~ s/^\s*//;

         # if we find "]", we are done
         if ($json->incr_text =~ s/^\]//) {
            print "finished.\n";

         # if we find ",", we can continue with the next element
         if ($json->incr_text =~ s/^,//) {

         # if we find anything else, we have a parse error!
         if (length $json->incr_text) {
            die "parse error near ", $json->incr_text;

         # else add more data
         sysread $fh, my $buf, 65536
            or die "read error: $!";
         $json->incr_parse ($buf); # void context, so no parsing

This is a complex example, but most of the complexity comes from the fact that we are trying to be correct (bear with me if I am wrong, I never ran the above example :).


Detect all unicode Byte Order Marks on decode. Which are UTF-8, UTF-16LE, UTF-16BE, UTF-32LE and UTF-32BE.

The BOM encoding is set only for one specific decode call, it does not change the state of the JSON object.

Warning: With perls older than 5.20 you need load the Encode module before loading a multibyte BOM, i.e. >= UTF-16. Otherwise an error is thrown. This is an implementation limitation and might get fixed later.

See "JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32."

"Implementations MUST NOT add a byte order mark to the beginning of a JSON text", "implementations (...) MAY ignore the presence of a byte order mark rather than treating it as an error".

See also

Beware that Cpanel::JSON::XS is currently the only JSON module which does accept and decode a BOM.

The latest JSON spec forbid the usage of UTF-16 or UTF-32, the character encoding is UTF-8. Thus in subsequent updates BOM's of UTF-16 or UTF-32 will throw an error.


This section describes how Cpanel::JSON::XS maps Perl values to JSON values and vice versa. These mappings are designed to "do the right thing" in most circumstances automatically, preserving round-tripping characteristics (what you put in comes out as something equivalent).

For the more enlightened: note that in the following descriptions, lowercase perl refers to the Perl interpreter, while uppercase Perl refers to the abstract Perl language itself.



A JSON object becomes a reference to a hash in Perl. No ordering of object keys is preserved (JSON does not preserve object key ordering itself).


A JSON array becomes a reference to an array in Perl.


A JSON string becomes a string scalar in Perl - Unicode codepoints in JSON are represented by the same codepoints in the Perl string, so no manual decoding is necessary.


A JSON number becomes either an integer, numeric (floating point) or string scalar in perl, depending on its range and any fractional parts. On the Perl level, there is no difference between those as Perl handles all the conversion details, but an integer may take slightly less memory and might represent more values exactly than floating point numbers.

If the number consists of digits only, Cpanel::JSON::XS will try to represent it as an integer value. If that fails, it will try to represent it as a numeric (floating point) value if that is possible without loss of precision. Otherwise it will preserve the number as a string value (in which case you lose roundtripping ability, as the JSON number will be re-encoded to a JSON string).

Numbers containing a fractional or exponential part will always be represented as numeric (floating point) values, possibly at a loss of precision (in which case you might lose perfect roundtripping ability, but the JSON number will still be re-encoded as a JSON number).

Note that precision is not accuracy - binary floating point values cannot represent most decimal fractions exactly, and when converting from and to floating point, Cpanel::JSON::XS only guarantees precision up to but not including the least significant bit.

true, false

When unblessed_bool is set to true, then JSON true becomes 1 and JSON false becomes 0.

Otherwise these JSON atoms become JSON::PP::true and JSON::PP::false, respectively. They are JSON::PP::Boolean objects and are overloaded to act almost exactly like the numbers 1 and 0. You can check whether a scalar is a JSON boolean by using the Cpanel::JSON::XS::is_bool function.

The other round, from perl to JSON, !0 which is represented as yes becomes true, and !1 which is represented as no becomes false.

Via Cpanel::JSON::XS::Type you can now even force negation in encode, without overloading of !:

    my $false = Cpanel::JSON::XS::false;
    print($json->encode([!$false], [JSON_TYPE_BOOL]));
    => [true]

A JSON null atom becomes undef in Perl.

shell-style comments (# text)

As a nonstandard extension to the JSON syntax that is enabled by the relaxed setting, shell-style comments are allowed. They can start anywhere outside strings and go till the end of the line.

tagged values ((tag)value).

Another nonstandard extension to the JSON syntax, enabled with the allow_tags setting, are tagged values. In this implementation, the tag must be a perl package/class name encoded as a JSON string, and the value must be a JSON array encoding optional constructor arguments.

See "OBJECT SERIALIZATION", below, for details.


The mapping from Perl to JSON is slightly more difficult, as Perl is a truly typeless language, so we can only guess which JSON type is meant by a Perl value.

hash references

Perl hash references become JSON objects. As there is no inherent ordering in hash keys (or JSON objects), they will usually be encoded in a pseudo-random order that can change between runs of the same program but stays generally the same within a single run of a program. Cpanel::JSON::XS can optionally sort the hash keys (determined by the canonical flag), so the same datastructure will serialize to the same JSON text (given same settings and version of Cpanel::JSON::XS), but this incurs a runtime overhead and is only rarely useful, e.g. when you want to compare some JSON text against another for equality.

array references

Perl array references become JSON arrays.

other references

Other unblessed references are generally not allowed and will cause an exception to be thrown, except for references to the integers 0 and 1, which get turned into false and true atoms in JSON.

With the option allow_stringify, you can ignore the exception and return the stringification of the perl value.

With the option allow_unknown, you can ignore the exception and return null instead.

   encode_json [\"x"]        # => cannot encode reference to scalar 'SCALAR(0x..)'
                             # unless the scalar is 0 or 1
   encode_json [\0, \1]      # yields [false,true]

   allow_stringify->encode_json [\"x"] # yields "x" unlike JSON::PP
   allow_unknown->encode_json [\"x"]   # yields null as in JSON::PP
Cpanel::JSON::XS::true, Cpanel::JSON::XS::false

These special values become JSON true and JSON false values, respectively. You can also use \1 and \0 or !0 and !1 directly if you want.

   encode_json [Cpanel::JSON::XS::false, Cpanel::JSON::XS::true] # yields [false,true]
   encode_json [!1, !0], [JSON_TYPE_BOOL, JSON_TYPE_BOOL] # yields [false,true]

eq/ne comparisons with true, false:

false is eq to the empty string or the string 'false' or the special empty string !!0 or !1, i.e. SV_NO, or the numbers 0 or 0.0.

true is eq to the string 'true' or to the special string !0 (i.e. SV_YES) or to the numbers 1 or 1.0.

blessed objects

Blessed objects are not directly representable in JSON, but Cpanel::JSON::XS allows various optional ways of handling objects. See "OBJECT SERIALIZATION", below, for details.

See the allow_blessed and convert_blessed methods on various options on how to deal with this: basically, you can choose between throwing an exception, encoding the reference as if it weren't blessed, use the objects overloaded stringification method or provide your own serializer method.

simple scalars

Simple Perl scalars (any scalar that is not a reference) are the most difficult objects to encode: Cpanel::JSON::XS will encode undefined scalars or inf/nan as JSON null values and other scalars to either number or string in non-deterministic way which may be affected or changed by Perl version or any other loaded Perl module.

If you want to have stable and deterministic types in JSON encoder then use Cpanel::JSON::XS::Type.

Alternative way for deterministic types is to use type_all_string method when all perl scalars are encoded to JSON strings.

Non-deterministic behavior is following: scalars that have last been used in a string context before encoding as JSON strings, and anything else as number value:

   # dump as number
   encode_json [2]                      # yields [2]
   encode_json [-3.0e17]                # yields [-3e+17]
   my $value = 5; encode_json [$value]  # yields [5]

   # used as string, but the two representations are for the same number
   print $value;
   encode_json [$value]                 # yields [5]

   # used as different string (non-matching dual-var)
   my $str = '0 but true';
   my $num = 1 + $str;
   encode_json [$num, $str]           # yields [1,"0 but true"]

   # undef becomes null
   encode_json [undef]                  # yields [null]

   # inf or nan becomes null, unless you answered
   # "Do you want to handle inf/nan as strings" with yes
   encode_json [9**9**9]                # yields [null]

You can force the type to be a JSON string by stringifying it:

   my $x = 3.1; # some variable containing a number
   "$x";        # stringified
   $x .= "";    # another, more awkward way to stringify
   print $x;    # perl does it for you, too, quite often

You can force the type to be a JSON number by numifying it:

   my $x = "3"; # some variable containing a string
   $x += 0;     # numify it, ensuring it will be dumped as a number
   $x *= 1;     # same thing, the choice is yours.

Note that numerical precision has the same meaning as under Perl (so binary to decimal conversion follows the same rules as in Perl, which can differ to other languages). Also, your perl interpreter might expose extensions to the floating point numbers of your platform, such as infinities or NaN's - these cannot be represented in JSON, and thus null is returned instead. Optionally you can configure it to stringify inf and nan values.


As JSON cannot directly represent Perl objects, you have to choose between a pure JSON representation (without the ability to deserialize the object automatically again), and a nonstandard extension to the JSON syntax, tagged values.


What happens when Cpanel::JSON::XS encounters a Perl object depends on the allow_blessed, convert_blessed and allow_tags settings, which are used in this order:

1. allow_tags is enabled and the object has a FREEZE method.

In this case, Cpanel::JSON::XS uses the Types::Serialiser object serialization protocol to create a tagged JSON value, using a nonstandard extension to the JSON syntax.

This works by invoking the FREEZE method on the object, with the first argument being the object to serialize, and the second argument being the constant string JSON to distinguish it from other serializers.

The FREEZE method can return any number of values (i.e. zero or more). These values and the paclkage/classname of the object will then be encoded as a tagged JSON value in the following format:

   ("classname")[FREEZE return values...]



For example, the hypothetical My::Object FREEZE method might use the objects type and id members to encode the object:

   sub My::Object::FREEZE {
      my ($self, $serializer) = @_;

      ($self->{type}, $self->{id})
2. convert_blessed is enabled and the object has a TO_JSON method.

In this case, the TO_JSON method of the object is invoked in scalar context. It must return a single scalar that can be directly encoded into JSON. This scalar replaces the object in the JSON text.

For example, the following TO_JSON method will convert all URI objects to JSON strings when serialized. The fact that these values originally were URI objects is lost.

   sub URI::TO_JSON {
      my ($uri) = @_;
3. convert_blessed is enabled and the object has a stringification overload.

In this case, the overloaded "" method of the object is invoked in scalar context. It must return a single scalar that can be directly encoded into JSON. This scalar replaces the object in the JSON text.

For example, the following "" method will convert all URI objects to JSON strings when serialized. The fact that these values originally were URI objects is lost.

    package URI;
    use overload '""' => sub { shift->as_string };
4. allow_blessed is enabled.

The object will be serialized as a JSON null value.

5. none of the above

If none of the settings are enabled or the respective methods are missing, Cpanel::JSON::XS throws an exception.


For deserialization there are only two cases to consider: either nonstandard tagging was used, in which case allow_tags decides, or objects cannot be automatically be deserialized, in which case you can use postprocessing or the filter_json_object or filter_json_single_key_object callbacks to get some real objects our of your JSON.

This section only considers the tagged value case: I a tagged JSON object is encountered during decoding and allow_tags is disabled, a parse error will result (as if tagged values were not part of the grammar).

If allow_tags is enabled, Cpanel::JSON::XS will look up the THAW method of the package/classname used during serialization (it will not attempt to load the package as a Perl module). If there is no such method, the decoding will fail with an error.

Otherwise, the THAW method is invoked with the classname as first argument, the constant string JSON as second argument, and all the values from the JSON array (the values originally returned by the FREEZE method) as remaining arguments.

The method must then return the object. While technically you can return any Perl scalar, you might have to enable the enable_nonref setting to make that work in all cases, so better return an actual blessed reference.

As an example, let's implement a THAW function that regenerates the My::Object from the FREEZE example earlier:

   sub My::Object::THAW {
      my ($class, $serializer, $type, $id) = @_;

      $class->new (type => $type, id => $id)

See the "SECURITY CONSIDERATIONS" section below. Allowing external json objects being deserialized to perl objects is usually a very bad idea.


The interested reader might have seen a number of flags that signify encodings or codesets - utf8, latin1, binary and ascii. There seems to be some confusion on what these do, so here is a short comparison:

utf8 controls whether the JSON text created by encode (and expected by decode) is UTF-8 encoded or not, while latin1 and ascii only control whether encode escapes character values outside their respective codeset range. Neither of these flags conflict with each other, although some combinations make less sense than others.

Care has been taken to make all flags symmetrical with respect to encode and decode, that is, texts encoded with any combination of these flag values will be correctly decoded when the same flags are used - in general, if you use different flag settings while encoding vs. when decoding you likely have a bug somewhere.

Below comes a verbose discussion of these flags. Note that a "codeset" is simply an abstract set of character-codepoint pairs, while an encoding takes those codepoint numbers and encodes them, in our case into octets. Unicode is (among other things) a codeset, UTF-8 is an encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets and encodings at the same time, which can be confusing.

utf8 flag disabled

When utf8 is disabled (the default), then encode/decode generate and expect Unicode strings, that is, characters with high ordinal Unicode values (> 255) will be encoded as such characters, and likewise such characters are decoded as-is, no changes to them will be done, except "(re-)interpreting" them as Unicode codepoints or Unicode characters, respectively (to Perl, these are the same thing in strings unless you do funny/weird/dumb stuff).

This is useful when you want to do the encoding yourself (e.g. when you want to have UTF-16 encoded JSON texts) or when some other layer does the encoding for you (for example, when printing to a terminal using a filehandle that transparently encodes to UTF-8 you certainly do NOT want to UTF-8 encode your data first and have Perl encode it another time).

utf8 flag enabled

If the utf8-flag is enabled, encode/decode will encode all characters using the corresponding UTF-8 multi-byte sequence, and will expect your input strings to be encoded as UTF-8, that is, no "character" of the input string must have any value > 255, as UTF-8 does not allow that.

The utf8 flag therefore switches between two modes: disabled means you will get a Unicode string in Perl, enabled means you get an UTF-8 encoded octet/binary string in Perl.

latin1, binary or ascii flags enabled

With latin1 (or ascii) enabled, encode will escape characters with ordinal values > 255 (> 127 with ascii) and encode the remaining characters as specified by the utf8 flag. With binary enabled, ordinal values > 255 are illegal.

If utf8 is disabled, then the result is also correctly encoded in those character sets (as both are proper subsets of Unicode, meaning that a Unicode string with all character values < 256 is the same thing as a ISO-8859-1 string, and a Unicode string with all character values < 128 is the same thing as an ASCII string in Perl).

If utf8 is enabled, you still get a correct UTF-8-encoded string, regardless of these flags, just some more characters will be escaped using \uXXXX then before.

Note that ISO-8859-1-encoded strings are not compatible with UTF-8 encoding, while ASCII-encoded strings are. That is because the ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 codeset being a subset of Unicode), while ASCII is.

Surprisingly, decode will ignore these flags and so treat all input values as governed by the utf8 flag. If it is disabled, this allows you to decode ISO-8859-1- and ASCII-encoded strings, as both strict subsets of Unicode. If it is enabled, you can correctly decode UTF-8 encoded strings.

So neither latin1, binary nor ascii are incompatible with the utf8 flag - they only govern when the JSON output engine escapes a character or not.

The main use for latin1 or binary is to relatively efficiently store binary data as JSON, at the expense of breaking compatibility with most JSON decoders.

The main use for ascii is to force the output to not contain characters with values > 127, which means you can interpret the resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any character set and 8-bit-encoding, and still get the same data structure back. This is useful when your channel for JSON transfer is not 8-bit clean or the encoding might be mangled in between (e.g. in mail), and works because ASCII is a proper subset of most 8-bit and multibyte encodings in use in the world.

JSON and ECMAscript

JSON syntax is based on how literals are represented in javascript (the not-standardized predecessor of ECMAscript) which is presumably why it is called "JavaScript Object Notation".

However, JSON is not a subset (and also not a superset of course) of ECMAscript (the standard) or javascript (whatever browsers actually implement).

If you want to use javascript's eval function to "parse" JSON, you might run into parse errors for valid JSON texts, or the resulting data structure might not be queryable:

One of the problems is that U+2028 and U+2029 are valid characters inside JSON strings, but are not allowed in ECMAscript string literals, so the following Perl fragment will not output something that can be guaranteed to be parsable by javascript's eval:

   use Cpanel::JSON::XS;

   print encode_json [chr 0x2028];

The right fix for this is to use a proper JSON parser in your javascript programs, and not rely on eval (see for example Douglas Crockford's json2.js parser).

If this is not an option, you can, as a stop-gap measure, simply encode to ASCII-only JSON:

   use Cpanel::JSON::XS;

   print Cpanel::JSON::XS->new->ascii->encode ([chr 0x2028]);

Note that this will enlarge the resulting JSON text quite a bit if you have many non-ASCII characters. You might be tempted to run some regexes to only escape U+2028 and U+2029, e.g.:

   my $json = Cpanel::JSON::XS->new->utf8->encode ([chr 0x2028]);
   $json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
   $json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
   print $json;

Note that this is a bad idea: the above only works for U+2028 and U+2029 and thus only for fully ECMAscript-compliant parsers. Many existing javascript implementations, however, have issues with other characters as well - using eval naively simply will cause problems.

Another problem is that some javascript implementations reserve some property names for their own purposes (which probably makes them non-ECMAscript-compliant). For example, Iceweasel reserves the __proto__ property name for its own purposes.

If that is a problem, you could parse try to filter the resulting JSON output for these property strings, e.g.:

   $json =~ s/"__proto__"\s*:/"__proto__renamed":/g;

This works because __proto__ is not valid outside of strings, so every occurrence of "__proto__"\s*: must be a string used as property name.

Unicode non-characters between U+FFFD and U+10FFFF are decoded either to the recommended U+FFFD REPLACEMENT CHARACTER (see Unicode PR #121: Recommended Practice for Replacement Characters), or in the binary or relaxed mode left as is, keeping the illegal non-characters as before.

Raw non-Unicode characters outside the valid unicode range fail now to parse, because "A string is a sequence of zero or more Unicode characters" RFC 7159 section 1 and "JSON text SHALL be encoded in Unicode RFC 7159 section 8.1. We use now the UTF8_DISALLOW_SUPER flag when parsing unicode.

If you know of other incompatibilities, please let me know.


You often hear that JSON is a subset of YAML. in general, there is no way to configure JSON::XS to output a data structure as valid YAML that works in all cases. If you really must use Cpanel::JSON::XS to generate YAML, you should use this algorithm (subject to change in future versions):

   my $to_yaml = Cpanel::JSON::XS->new->utf8->space_after (1);
   my $yaml = $to_yaml->encode ($ref) . "\n";

This will usually generate JSON texts that also parse as valid YAML.


It seems that JSON::XS is surprisingly fast, as shown in the following tables. They have been generated with the help of the eg/bench program in the JSON::XS distribution, to make it easy to compare on your own system.

JSON::XS is with Data::MessagePack and Sereal one of the fastest serializers, because JSON and JSON::XS do not support backrefs (no graph structures), only trees. Storable supports backrefs, i.e. graphs. Data::MessagePack encodes its data binary (as Storable) and supports only very simple subset of JSON.

First comes a comparison between various modules using a very short single-line JSON string (also available at

   {"method": "handleMessage", "params": ["user1",
   "we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
   1,  0]}

It shows the number of encodes/decodes per second (JSON::XS uses the functional interface, while Cpanel::JSON::XS/2 uses the OO interface with pretty-printing and hash key sorting enabled, Cpanel::JSON::XS/3 enables shrink. JSON::DWIW/DS uses the deserialize function, while JSON::DWIW::FJ uses the from_json method). Higher is better:

   module        |     encode |     decode |
   JSON::DWIW/DS |  86302.551 | 102300.098 |
   JSON::DWIW/FJ |  86302.551 |  75983.768 |
   JSON::PP      |  15827.562 |   6638.658 |
   JSON::Syck    |  63358.066 |  47662.545 |
   JSON::XS      | 511500.488 | 511500.488 |
   JSON::XS/2    | 291271.111 | 388361.481 |
   JSON::XS/3    | 361577.931 | 361577.931 |
   Storable      |  66788.280 | 265462.278 |

That is, JSON::XS is almost six times faster than JSON::DWIW on encoding, about five times faster on decoding, and over thirty to seventy times faster than JSON's pure perl implementation. It also compares favourably to Storable for small amounts of data.

Using a longer test string (roughly 18KB, generated from Yahoo! Locals search API (

   module        |     encode |     decode |
   JSON::DWIW/DS |   1647.927 |   2673.916 |
   JSON::DWIW/FJ |   1630.249 |   2596.128 |
   JSON::PP      |    400.640 |     62.311 |
   JSON::Syck    |   1481.040 |   1524.869 |
   JSON::XS      |  20661.596 |   9541.183 |
   JSON::XS/2    |  10683.403 |   9416.938 |
   JSON::XS/3    |  20661.596 |   9400.054 |
   Storable      |  19765.806 |  10000.725 |

Again, JSON::XS leads by far (except for Storable which non-surprisingly decodes a bit faster).

On large strings containing lots of high Unicode characters, some modules (such as JSON::PC) seem to decode faster than JSON::XS, but the result will be broken due to missing (or wrong) Unicode handling. Others refuse to decode or encode properly, so it was impossible to prepare a fair comparison table for that case.

For updated graphs see

INTEROP with JSON and JSON::XS and other JSON modules

As long as you only serialize data that can be directly expressed in JSON, Cpanel::JSON::XS is incapable of generating invalid JSON output (modulo bugs, but JSON::XS has found more bugs in the official JSON testsuite (1) than the official JSON testsuite has found in JSON::XS (0)). Cpanel::JSON::XS is currently the only known JSON decoder which passes all tests, while being the fastest also.

When you have trouble decoding JSON generated by this module using other decoders, then it is very likely that you have an encoding mismatch or the other decoder is broken.

When decoding, JSON::XS is strict by default and will likely catch all errors. There are currently two settings that change this: relaxed makes JSON::XS accept (but not generate) some non-standard extensions, and allow_tags or allow_blessed will allow you to encode and decode Perl objects, at the cost of being totally insecure and not outputting valid JSON anymore.

JSON-XS-3.01 broke interoperability with JSON-2.90 with booleans. See JSON.

Cpanel::JSON::XS needs to know the JSON and JSON::XS versions to be able work with those objects, especially when encoding a booleans like {"is_true":true}. So you need to load these modules before.

true/false overloading and boolean representations are supported.

JSON::XS and JSON::PP representations are accepted and older JSON::XS accepts Cpanel::JSON::XS booleans. All JSON modules JSON, JSON, PP, JSON::XS, Cpanel::JSON::XS produce JSON::PP::Boolean objects, just Mojo and JSON::YAJL not. Mojo produces Mojo::JSON::_Bool and JSON::YAJL::Parser just an unblessed IV.

Cpanel::JSON::XS accepts JSON::PP::Boolean and Mojo::JSON::_Bool objects as booleans.

I cannot think of any reason to still use JSON::XS anymore.


When you use allow_tags to use the extended (and also nonstandard and invalid) JSON syntax for serialized objects, and you still want to decode the generated serialize objects, you can run a regex to replace the tagged syntax by standard JSON arrays (it only works for "normal" package names without comma, newlines or single colons). First, the readable Perl version:

   # if your FREEZE methods return no values, you need this replace first:
   $json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[\s*\]/[$1]/gx;

   # this works for non-empty constructor arg lists:
   $json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[/[$1,/gx;

And here is a less readable version that is easy to adapt to other languages:

   $json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/[$1,/g;

Here is an ECMAScript version (same regex):

   json = json.replace (/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/g, "[$1,");

Since this syntax converts to standard JSON arrays, it might be hard to distinguish serialized objects from normal arrays. You can prepend a "magic number" as first array element to reduce chances of a collision:

   $json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/["XU1peReLzT4ggEllLanBYq4G9VzliwKF",$1,/g;

And after decoding the JSON text, you could walk the data structure looking for arrays with a first element of XU1peReLzT4ggEllLanBYq4G9VzliwKF.

The same approach can be used to create the tagged format with another encoder. First, you create an array with the magic string as first member, the classname as second, and constructor arguments last, encode it as part of your JSON structure, and then:

   $json =~ s/\[\s*"XU1peReLzT4ggEllLanBYq4G9VzliwKF"\s*,\s*("([^\\":,]+|\\.|::)*")\s*,/($1)[/g;

Again, this has some limitations - the magic string must not be encoded with character escapes, and the constructor arguments must be non-empty.


Since this module was written, Google has written a new JSON RFC, RFC 7159 (and RFC7158). Unfortunately, this RFC breaks compatibility with both the original JSON specification on and RFC4627.

As far as I can see, you can get partial compatibility when parsing by using ->allow_nonref. However, consider the security implications of doing so.

I haven't decided yet when to break compatibility with RFC4627 by default (and potentially leave applications insecure) and change the default to follow RFC7159, but application authors are well advised to call ->allow_nonref(0) even if this is the current default, if they cannot handle non-reference values, in preparation for the day when the default will change.


JSON::XS and Cpanel::JSON::XS are not only fast. JSON is generally the most secure serializing format, because it is the only one besides Data::MessagePack, which does not deserialize objects per default. For all languages, not just perl. The binary variant BSON (MongoDB) does more but is unsafe.

It is trivial for any attacker to create such serialized objects in JSON and trick perl into expanding them, thereby triggering certain methods. Watch for an exploit demo for "CVE-2015-1592 SixApart MovableType Storable Perl Code Execution" for a deserializer which expands objects. Deserializing even coderefs (methods, functions) or external data would be considered the most dangerous.

Security relevant overview of serializers regarding deserializing objects by default:

                      Objects   Coderefs  External Data

    Data::Dumper      YES       YES       YES
    Storable          YES       NO (def)  NO
    Sereal            YES       NO        NO
    YAML              YES       NO        NO
    B::C              YES       YES       YES
    B::Bytecode       YES       YES       YES
    BSON              YES       YES       NO
    JSON::SL          YES       NO        YES
    JSON              NO (def)  NO        NO
    Data::MessagePack NO        NO        NO
    XML               NO        NO        YES

    Pickle            YES       YES       YES
    PHP Deserialize   YES       NO        NO

When you are using JSON in a protocol, talking to untrusted potentially hostile creatures requires relatively few measures.

First of all, your JSON decoder should be secure, that is, should not have any buffer overflows. Obviously, this module should ensure that.

Second, you need to avoid resource-starving attacks. That means you should limit the size of JSON texts you accept, or make sure then when your resources run out, that's just fine (e.g. by using a separate process that can crash safely). The size of a JSON text in octets or characters is usually a good indication of the size of the resources required to decode it into a Perl structure. While JSON::XS can check the size of the JSON text, it might be too late when you already have it in memory, so you might want to check the size before you accept the string.

Third, Cpanel::JSON::XS recurses using the C stack when decoding objects and arrays. The C stack is a limited resource: for instance, on my amd64 machine with 8MB of stack size I can decode around 180k nested arrays but only 14k nested JSON objects (due to perl itself recursing deeply on croak to free the temporary). If that is exceeded, the program crashes. To be conservative, the default nesting limit is set to 512. If your process has a smaller stack, you should adjust this setting accordingly with the max_depth method.

Also keep in mind that Cpanel::JSON::XS might leak contents of your Perl data structures in its error messages, so when you serialize sensitive information you might want to make sure that exceptions thrown by JSON::XS will not end up in front of untrusted eyes.

If you are using Cpanel::JSON::XS to return packets to consumption by JavaScript scripts in a browser you should have a look at to see whether you are vulnerable to some common attack vectors (which really are browser design bugs, but it is still you who will have to deal with it, as major browser developers care only for features, not about getting security right). You might also want to also look at Mojo::JSON special escape rules to prevent from XSS attacks.

"OLD" VS. "NEW" JSON (RFC 4627 VS. RFC 7159)

TL;DR: Due to security concerns, Cpanel::JSON::XS will not allow scalar data in JSON texts by default - you need to create your own Cpanel::JSON::XS object and enable allow_nonref:

   my $json = JSON::XS->new->allow_nonref;

   $text = $json->encode ($data);
   $data = $json->decode ($text);

The long version: JSON being an important and supposedly stable format, the IETF standardized it as RFC 4627 in 2006. Unfortunately the inventor of JSON Douglas Crockford unilaterally changed the definition of JSON in javascript. Rather than create a fork, the IETF decided to standardize the new syntax (apparently, so I as told, without finding it very amusing).

The biggest difference between the original JSON and the new JSON is that the new JSON supports scalars (anything other than arrays and objects) at the top-level of a JSON text. While this is strictly backwards compatible to older versions, it breaks a number of protocols that relied on sending JSON back-to-back, and is a minor security concern.

For example, imagine you have two banks communicating, and on one side, the JSON coder gets upgraded. Two messages, such as 10 and 1000 might then be confused to mean 101000, something that couldn't happen in the original JSON, because neither of these messages would be valid JSON.

If one side accepts these messages, then an upgrade in the coder on either side could result in this becoming exploitable.

This module has always allowed these messages as an optional extension, by default disabled. The security concerns are the reason why the default is still disabled, but future versions might/will likely upgrade to the newer RFC as default format, so you are advised to check your implementation and/or override the default with ->allow_nonref (0) to ensure that future versions are safe.


Cpanel::JSON::XS has proper ithreads support, unlike JSON::XS. If you encounter any bugs with thread support please report them.

From Version 4.00 - 4.19 you couldn't encode true with threads::shared magic.


While the goal of the Cpanel::JSON::XS module is to be correct, that unfortunately does not mean it's bug-free, only that the author thinks its design is bug-free. If you keep reporting bugs and tests they will be fixed swiftly, though.

Since the JSON::XS author refuses to use a public bugtracker and prefers private emails, we use the tracker at github, so you might want to report any issues twice. Once in private to MLEHMANN to be fixed in JSON::XS and one to our the public tracker. Issues fixed by JSON::XS with a new release will also be backported to Cpanel::JSON::XS and 5.6.2, as long as cPanel relies on 5.6.2 and Cpanel::JSON::XS as our serializer of choice.


This module is available under the same licences as perl, the Artistic license and the GPL.


The cpanel_json_xs command line utility for quick experiments.

JSON::PP, JSON, JSON::XS, JSON::MaybeXS, Mojo::JSON, Mojo::JSON::MaybeXS, JSON::SL, JSON::DWIW, JSON::YAJL, JSON::Any, Test::JSON, Locale::Wolowitz,


Reini Urban <>

Marc Lehmann <>,


Reini Urban <>