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= Lua CJSON 2.1devel Manual =
Mark Pulford <>
:revdate: 1st March 2012
The Lua CJSON module provides JSON support for Lua.
- Fast, standards compliant encoding/parsing routines
- Full support for JSON with UTF-8, including decoding surrogate pairs
- Optional run-time support for common exceptions to the JSON
specification (infinity, NaN,..)
- No dependencies on other libraries
- UTF-16 and UTF-32 are not supported
Lua CJSON is covered by the MIT license. Review the file +LICENSE+ for
The latest version of this software is available from the[Lua CJSON website].
Feel free to email me if you have any patches, suggestions, or comments.
Lua CJSON requires either[Lua] 5.1, Lua 5.2, or[LuaJIT] to build.
The build method can be selected from 4 options:
Make:: Unix (including Linux, BSD, Mac OSX & Solaris), Windows
CMake:: Unix, Windows
RPM:: Linux
LuaRocks:: Unix, Windows
The included +Makefile+ has generic settings.
First, review and update the included makefile to suit your platform (if
Next, build and install the module:
make install
Or install manually into your Lua module directory:
~~~~~[CMake] can generate build configuration for many
different platforms (including Unix and Windows).
First, generate the makefile for your platform using CMake. If CMake is
unable to find Lua, manually set the +LUA_DIR+ environment variable to
the base prefix of your Lua 5.1 installation.
While +cmake+ is used in the example below, +ccmake+ or +cmake-gui+ may
be used to present an interface for changing the default build options.
mkdir build
cd build
# Optional: export LUA_DIR=$LUA51_PREFIX
cmake ..
Next, build and install the module:
make install
# Or:
Review the[CMake documentation]
for further details.
Linux distributions using[RPM] can create a package via
the included RPM spec file. Ensure the +rpm-build+ package (or similar)
has been installed.
Build and install the module via RPM:
rpmbuild -tb lua-cjson-2.1devel.tar.gz
~~~~~~~~[LuaRocks] can be used to install and manage Lua
modules on a wide range of platforms (including Windows).
First, extract the Lua CJSON source package.
Next, install the module:
cd lua-cjson-2.1devel
luarocks make
LuaRocks does not support platform specific configuration for Solaris.
On Solaris, you may need to manually uncomment +USE_INTERNAL_ISINF+ in
the rockspec before building this module.
Review the[LuaRocks documentation]
for further details.
Build Options (#define)
Lua CJSON offers several +#define+ build options to address portability
issues, and enable non-default features. Some build methods may
automatically set platform specific options if required. Other features
should be enabled manually.
USE_INTERNAL_ISINF:: Workaround for Solaris platforms missing +isinf+.
DISABLE_INVALID_NUMBERS:: Recommended on platforms where +strtod+ /
+sprintf+ are not POSIX compliant (eg, Windows MinGW). Prevents
+cjson.encode_invalid_numbers+ and +cjson.decode_invalid_numbers+ from
being enabled. However, +cjson.encode_invalid_numbers+ may still be
set to +"null"+. When using the Lua CJSON built-in floating point
conversion this option is unnecessary and is ignored.
Built-in floating point conversion
Lua CJSON may be built with David Gay's[floating point conversion routines]. This can
increase overall performance by up to 50% on some platforms when
converting a large amount of numeric data. However, this option reduces
portability and is disabled by default.
USE_INTERNAL_FPCONV:: Enable internal number conversion routines.
IEEE_BIG_ENDIAN:: Must be set on big endian architectures.
MULTIPLE_THREADS:: Must be set if Lua CJSON may be used in a
multi-threaded application. Requires the _pthreads_ library.
API (Functions)
-- Module instantiation
local cjson = require "cjson"
local cjson2 =
local cjson_safe = require ""
-- Translate Lua value to/from JSON
text = cjson.encode(value)
value = cjson.decode(text)
-- Get and/or set Lua CJSON configuration
setting = cjson.decode_invalid_numbers([setting])
setting = cjson.encode_invalid_numbers([setting])
keep = cjson.encode_keep_buffer([keep])
depth = cjson.encode_max_depth([depth])
depth = cjson.decode_max_depth([depth])
convert, ratio, safe = cjson.encode_sparse_array([convert[, ratio[, safe]]])
Module Instantiation
local cjson = require "cjson"
local cjson2 =
local cjson_safe = require ""
Import Lua CJSON via the Lua +require+ function. Lua CJSON does not
register a global module table.
The +cjson+ module will throw an error during JSON conversion if any
invalid data is encountered. Refer to <<cjson_encode,+cjson.encode+>>
and <<cjson_decode,+cjson.decode+>> for details.
The module behaves identically to the +cjson+ module,
except when errors are encountered during JSON conversion. On error, the
+cjson_safe.encode+ and +cjson_safe.decode+ functions will return
+nil+ followed by the error message. can be used to instantiate an independent copy of the Lua
CJSON module. The new module has a separate persistent encoding buffer,
and default settings.
Lua CJSON can support Lua implementations using multiple preemptive
threads within a single Lua state provided the persistent encoding
buffer is not shared. This can be achieved by one of the following
- Disabling the persistent encoding buffer with
- Ensuring each thread calls <<encode,+cjson.encode+>> separately (ie,
treat +cjson.encode+ as non-reentrant).
- Using a separate +cjson+ module table per preemptive thread
Lua CJSON uses +strtod+ and +snprintf+ to perform numeric conversion as
they are usually well supported, fast and bug free. However, these
functions require a workaround for JSON encoding/parsing under locales
using a comma decimal separator. Lua CJSON detects the current locale
during instantiation to determine and automatically implement the
workaround if required. Lua CJSON should be reinitialised via if the locale of the current process changes. Using a
different locale per thread is not supported.
value = cjson.decode(json_text)
+cjson.decode+ will deserialise any UTF-8 JSON string into a Lua value
or table.
UTF-16 and UTF-32 JSON strings are not supported.
+cjson.decode+ requires that any NULL (ASCII 0) and double quote (ASCII
34) characters are escaped within strings. All escape codes will be
decoded and other bytes will be passed transparently. UTF-8 characters
are not validated during decoding and should be checked elsewhere if
JSON +null+ will be converted to a NULL +lightuserdata+ value. This can
be compared with +cjson.null+ for convenience.
By default, numbers incompatible with the JSON specification (infinity,
NaN, hexadecimal) can be decoded. This default can be changed with
.Example: Decoding
json_text = '[ true, { "foo": "bar" } ]'
value = cjson.decode(json_text)
-- Returns: { true, { foo = "bar" } }
Care must be taken after decoding JSON objects with numeric keys. Each
numeric key will be stored as a Lua +string+. Any subsequent code
assuming type +number+ may break.
setting = cjson.decode_invalid_numbers([setting])
-- "setting" must be a boolean. Default: true.
Lua CJSON may generate an error when trying to decode numbers not
supported by the JSON specification. _Invalid numbers_ are defined as:
- infinity
- NaN
- hexadecimal
Available settings:
+true+:: Accept and decode _invalid numbers_. This is the default
+false+:: Throw an error when _invalid numbers_ are encountered.
The current setting is always returned, and is only updated when an
argument is provided.
depth = cjson.decode_max_depth([depth])
-- "depth" must be a positive integer. Default: 1000.
Lua CJSON will generate an error when parsing deeply nested JSON once
the maximum array/object depth has been exceeded. This check prevents
unnecessarily complicated JSON from slowing down the application, or
crashing the application due to lack of process stack space.
An error may be generated before the depth limit is hit if Lua is unable
to allocate more objects on the Lua stack.
By default, Lua CJSON will reject JSON with arrays and/or objects nested
more than 1000 levels deep.
The current setting is always returned, and is only updated when an
argument is provided.
json_text = cjson.encode(value)
+cjson.encode+ will serialise a Lua value into a string containing the
JSON representation.
+cjson.encode+ supports the following types:
- +boolean+
- +lightuserdata+ (NULL value only)
- +nil+
- +number+
- +string+
- +table+
The remaining Lua types will generate an error:
- +function+
- +lightuserdata+ (non-NULL values)
- +thread+
- +userdata+
By default, numbers are encoded with 14 significant digits. Refer to
<<encode_number_precision,+cjson.encode_number_precision+>> for details.
Lua CJSON will escape the following characters within each UTF-8 string:
- Control characters (ASCII 0 - 31)
- Double quote (ASCII 34)
- Forward slash (ASCII 47)
- Blackslash (ASCII 92)
- Delete (ASCII 127)
All other bytes are passed transparently.
Lua CJSON will successfully encode/decode binary strings, but this is
technically not supported by JSON and may not be compatible with other
JSON libraries. To ensure the output is valid JSON, applications should
ensure all Lua strings passed to +cjson.encode+ are UTF-8.
Base64 is commonly used to encode binary data as the most efficient
encoding under UTF-8 can only reduce the encoded size by a further
&#126;8%. Lua Base64 routines can be found in the[LuaSocket] and[lbase64] packages.
Lua CJSON uses a heuristic to determine whether to encode a Lua table as
a JSON array or an object. A Lua table with only positive integer keys
of type +number+ will be encoded as a JSON array. All other tables will
be encoded as a JSON object.
Lua CJSON does not use metamethods when serialising tables.
- +rawget+ is used to iterate over Lua arrays
- +next+ is used to iterate over Lua objects
Lua arrays with missing entries (_sparse arrays_) may optionally be
encoded in several different ways. Refer to
<<encode_sparse_array,+cjson.encode_sparse_array+>> for details.
JSON object keys are always strings. Hence +cjson.encode+ only supports
table keys which are type +number+ or +string+. All other types will
generate an error.
Standards compliant JSON must be encapsulated in either an object (+{}+)
or an array (+[]+). If strictly standards compliant JSON is desired, a
table must be passed to +cjson.encode+.
By default, encoding the following Lua values will generate errors:
- Numbers incompatible with the JSON specification (infinity, NaN)
- Tables nested more than 1000 levels deep
- Excessively sparse Lua arrays
These defaults can be changed with:
- <<encode_invalid_numbers,+cjson.encode_invalid_numbers+>>
- <<encode_max_depth,+cjson.encode_max_depth+>>
- <<encode_sparse_array,+cjson.encode_sparse_array+>>
.Example: Encoding
value = { true, { foo = "bar" } }
json_text = cjson.encode(value)
-- Returns: '[true,{"foo":"bar"}]'
setting = cjson.encode_invalid_numbers([setting])
-- "setting" must a boolean or "null". Default: false.
Lua CJSON may generate an error when encoding floating point numbers not
supported by the JSON specification (_invalid numbers_):
- infinity
- NaN
Available settings:
+true+:: Allow _invalid numbers_ to be encoded using the Javascript
compatible values +NaN+ and +Infinity+. This will generate
non-standard JSON, but these values are supported by some libraries.
+"null"+:: Encode _invalid numbers_ as a JSON +null+ value. This allows
infinity and NaN to be encoded into valid JSON.
+false+:: Throw an error when attempting to encode _invalid numbers_.
This is the default setting.
The current setting is always returned, and is only updated when an
argument is provided.
keep = cjson.encode_keep_buffer([keep])
-- "keep" must be a boolean. Default: true.
Lua CJSON can reuse the JSON encoding buffer to improve performance.
Available settings:
+true+:: The buffer will grow to the largest size required and is not
freed until the Lua CJSON module is garbage collected. This is the
default setting.
+false+:: Free the encode buffer after each call to +cjson.encode+.
The current setting is always returned, and is only updated when an
argument is provided.
depth = cjson.encode_max_depth([depth])
-- "depth" must be a positive integer. Default: 1000.
Once the maximum table depth has been exceeded Lua CJSON will generate
an error. This prevents a deeply nested or recursive data structure from
crashing the application.
By default, Lua CJSON will generate an error when trying to encode data
structures with more than 1000 nested tables.
The current setting is always returned, and is only updated when an
argument is provided.
.Example: Recursive Lua table
a = {}; a[1] = a
precision = cjson.encode_number_precision([precision])
-- "precision" must be an integer between 1 and 14. Default: 14.
The amount of significant digits returned by Lua CJSON when encoding
numbers can be changed to balance accuracy versus performance. For data
structures containing many numbers, setting
+cjson.encode_number_precision+ to a smaller integer, for example +3+,
can improve encoding performance by up to 50%.
By default, Lua CJSON will output 14 significant digits when converting
a number to text.
The current setting is always returned, and is only updated when an
argument is provided.
convert, ratio, safe = cjson.encode_sparse_array([convert[, ratio[, safe]]])
-- "convert" must be a boolean. Default: false.
-- "ratio" must be a positive integer. Default: 2.
-- "safe" must be a positive integer. Default: 10.
Lua CJSON classifies a Lua table into one of three kinds when encoding a
JSON array. This is determined by the number of values missing from the
Lua array as follows:
Normal:: All values are available.
Sparse:: At least 1 value is missing.
Excessively sparse:: The number of values missing exceeds the configured
Lua CJSON encodes sparse Lua arrays as JSON arrays using JSON +null+ for
the missing entries.
An array is excessively sparse when all the following conditions are
- +ratio+ > +0+
- _maximum_index_ > +safe+
- _maximum_index_ > _item_count_ * +ratio+
Lua CJSON will never consider an array to be _excessively sparse_ when
+ratio+ = +0+. The +safe+ limit ensures that small Lua arrays are always
encoded as sparse arrays.
By default, attempting to encode an _excessively sparse_ array will
generate an error. If +convert+ is set to +true+, _excessively sparse_
arrays will be converted to a JSON object.
The current settings are always returned. A particular setting is only
changed when the argument is provided (non-++nil++).
.Example: Encoding a sparse array
cjson.encode({ [3] = "data" })
-- Returns: '[null,null,"data"]'
.Example: Enabling conversion to a JSON object
cjson.encode({ [1000] = "excessively sparse" })
-- Returns: '{"1000":"excessively sparse"}'
API (Variables)
The name of the Lua CJSON module (+"cjson"+).
The version number of the Lua CJSON module (+"2.1devel"+).
Lua CJSON decodes JSON +null+ as a Lua +lightuserdata+ NULL pointer.
+cjson.null+ is provided for comparison.
-[RFC 4627]
-[JSON website]
// vi:ft=asciidoc tw=72:
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