1
2JSMN
3====
4
5jsmn (pronounced like 'jasmine') is a minimalistic JSON parser in C. It can be
6easily integrated into resource-limited or embedded projects.
7
8You can find more information about JSON format at [json.org][1]
9
10Library sources are available at [bitbucket.org/zserge/jsmn][2]
11
12The web page with some information about jsmn can be found at
13[http://zserge.com/jsmn.html][3]
14
15Philosophy
16----------
17
18Most JSON parsers offer you a bunch of functions to load JSON data, parse it
19and extract any value by its name. jsmn proves that checking the correctness of
20every JSON packet or allocating temporary objects to store parsed JSON fields
21often is an overkill.
22
23JSON format itself is extremely simple, so why should we complicate it?
24
25jsmn is designed to be **robust** (it should work fine even with erroneous
26data), **fast** (it should parse data on the fly), **portable** (no superfluous
27dependencies or non-standard C extensions). An of course, **simplicity** is a
28key feature - simple code style, simple algorithm, simple integration into
29other projects.
30
31Features
32--------
33
34* compatible with C89
35* no dependencies (even libc!)
36* highly portable (tested on x86/amd64, ARM, AVR)
37* about 200 lines of code
38* extremely small code footprint
39* API contains only 2 functions
40* no dynamic memory allocation
41* incremental single-pass parsing
42* library code is covered with unit-tests
43
44Design
45------
46
47The rudimentary jsmn object is a **token**. Let's consider a JSON string:
48
49 '{ "name" : "Jack", "age" : 27 }'
50
51It holds the following tokens:
52
53* Object: `{ "name" : "Jack", "age" : 27}` (the whole object)
54* Strings: `"name"`, `"Jack"`, `"age"` (keys and some values)
55* Number: `27`
56
57In jsmn, tokens do not hold any data, but point to token boundaries in JSON
58string instead. In the example above jsmn will create tokens like: Object
59[0..31], String [3..7], String [12..16], String [20..23], Number [27..29].
60
61Every jsmn token has a type, which indicates the type of corresponding JSON
62token. jsmn supports the following token types:
63
64* Object - a container of key-value pairs, e.g.:
65 `{ "foo":"bar", "x":0.3 }`
66* Array - a sequence of values, e.g.:
67 `[ 1, 2, 3 ]`
68* String - a quoted sequence of chars, e.g.: `"foo"`
69* Primitive - a number, a boolean (`true`, `false`) or `null`
70
71Besides start/end positions, jsmn tokens for complex types (like arrays
72or objects) also contain a number of child items, so you can easily follow
73object hierarchy.
74
75This approach provides enough information for parsing any JSON data and makes
76it possible to use zero-copy techniques.
77
78Install
79-------
80
81To clone the repository you should have mercurial installed. Just run:
82
83 $ hg clone http://bitbucket.org/zserge/jsmn jsmn
84
85Repository layout is simple: jsmn.c and jsmn.h are library files, tests are in
86the jsmn\_test.c, you will also find README, LICENSE and Makefile files inside.
87
88To build the library, run `make`. It is also recommended to run `make test`.
89Let me know, if some tests fail.
90
91If build was successful, you should get a `libjsmn.a` library.
92The header file you should include is called `"jsmn.h"`.
93
94API
95---
96
97Token types are described by `jsmntype_t`:
98
99 typedef enum {
100 JSMN_PRIMITIVE = 0,
101 JSMN_OBJECT = 1,
102 JSMN_ARRAY = 2,
103 JSMN_STRING = 3
104 } jsmntype_t;
105
106**Note:** Unlike JSON data types, primitive tokens are not divided into
107numbers, booleans and null, because one can easily tell the type using the
108first character:
109
110* <code>'t', 'f'</code> - boolean
111* <code>'n'</code> - null
112* <code>'-', '0'..'9'</code> - number
113
114Token is an object of `jsmntok_t` type:
115
116 typedef struct {
117 jsmntype_t type; // Token type
118 int start; // Token start position
119 int end; // Token end position
120 int size; // Number of child (nested) tokens
121 } jsmntok_t;
122
123**Note:** string tokens point to the first character after
124the opening quote and the previous symbol before final quote. This was made
125to simplify string extraction from JSON data.
126
127All job is done by `jsmn_parser` object. You can initialize a new parser using:
128
129 jsmn_parser parser;
130 jsmntok_t tokens[10];
131
132 jsmn_init(&parser);
133
134 // js - pointer to JSON string
135 // tokens - an array of tokens available
136 // 10 - number of tokens available
137 jsmn_parse(&parser, js, tokens, 10);
138
139This will create a parser, and then it tries to parse up to 10 JSON tokens from
140the `js` string.
141
142A non-negative reutrn value of `jsmn_parse` is the number of tokens actually
143used by the parser.
144Passing NULL instead of the tokens array would not store parsing results, but
145instead the function will return the value of tokens needed to parse the given
146string. This can be useful if you don't know yet how many tokens to allocate.
147
148If something goes wrong, you will get an error. Error will be one of these:
149
150* `JSMN_ERROR_INVAL` - bad token, JSON string is corrupted
151* `JSMN_ERROR_NOMEM` - not enough tokens, JSON string is too large
152* `JSMN_ERROR_PART` - JSON string is too short, expecting more JSON data
153
154If you get `JSON_ERROR_NOMEM`, you can re-allocate more tokens and call
155`jsmn_parse` once more. If you read json data from the stream, you can
156periodically call `jsmn_parse` and check if return value is `JSON_ERROR_PART`.
157You will get this error until you reach the end of JSON data.
158
159Other info
160----------
161
162This software is distributed under [MIT license](http://www.opensource.org/licenses/mit-license.php),
163 so feel free to integrate it in your commercial products.
164
165[1]: http://www.json.org/
166[2]: https://bitbucket.org/zserge/jsmn/wiki/Home
167[3]: http://zserge.com/jsmn.html
168