2 Copyright (c) 2009 Dave Gamble
4 Permission is hereby granted, free of charge, to any person obtaining a copy
5 of this software and associated documentation files (the "Software"), to deal
6 in the Software without restriction, including without limitation the rights
7 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
8 copies of the Software, and to permit persons to whom the Software is
9 furnished to do so, subject to the following conditions:
11 The above copyright notice and this permission notice shall be included in
12 all copies or substantial portions of the Software.
14 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
15 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
16 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
17 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
18 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
19 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
25 cJSON aims to be the dumbest possible parser that you can get your job done with.
26 It's a single file of C, and a single header file.
28 JSON is described best here: http://www.json.org/
29 It's like XML, but fat-free. You use it to move data around, store things, or just
30 generally represent your program's state.
33 First up, how do I build?
34 Add cJSON.c to your project, and put cJSON.h somewhere in the header search path.
35 For example, to build the test app:
37 gcc cJSON.c test.c -o test -lm
41 As a library, cJSON exists to take away as much legwork as it can, but not get in your way.
42 As a point of pragmatism (i.e. ignoring the truth), I'm going to say that you can use it
43 in one of two modes: Auto and Manual. Let's have a quick run-through.
46 I lifted some JSON from this page: http://www.json.org/fatfree.html
47 That page inspired me to write cJSON, which is a parser that tries to share the same
48 philosophy as JSON itself. Simple, dumb, out of the way.
52 "name": "Jack (\"Bee\") Nimble",
62 Assume that you got this from a file, a webserver, or magic JSON elves, whatever,
63 you have a char * to it. Everything is a cJSON struct.
65 cJSON *root = cJSON_Parse(my_json_string);
67 This is an object. We're in C. We don't have objects. But we do have structs.
70 cJSON *format = cJSON_GetObjectItem(root,"format");
71 int framerate = cJSON_GetObjectItem(format,"frame rate")->valueint;
74 Want to change the framerate?
75 cJSON_GetObjectItem(format,"frame rate")->valueint=25;
78 char *rendered=cJSON_Print(root);
80 Finished? Delete the root (this takes care of everything else).
83 That's AUTO mode. If you're going to use Auto mode, you really ought to check pointers
84 before you dereference them. If you want to see how you'd build this struct in code?
86 root=cJSON_CreateObject();
87 cJSON_AddItemToObject(root, "name", cJSON_CreateString("Jack (\"Bee\") Nimble"));
88 cJSON_AddItemToObject(root, "format", fmt=cJSON_CreateObject());
89 cJSON_AddStringToObject(fmt,"type", "rect");
90 cJSON_AddNumberToObject(fmt,"width", 1920);
91 cJSON_AddNumberToObject(fmt,"height", 1080);
92 cJSON_AddFalseToObject (fmt,"interlace");
93 cJSON_AddNumberToObject(fmt,"frame rate", 24);
95 Hopefully we can agree that's not a lot of code? There's no overhead, no unnecessary setup.
96 Look at test.c for a bunch of nice examples, mostly all ripped off the json.org site, and
99 What about manual mode? First up you need some detail.
100 Let's cover how the cJSON objects represent the JSON data.
101 cJSON doesn't distinguish arrays from objects in handling; just type.
102 Each cJSON has, potentially, a child, siblings, value, a name.
104 The root object has: Object Type and a Child
105 The Child has name "name", with value "Jack ("Bee") Nimble", and a sibling:
106 Sibling has type Object, name "format", and a child.
107 That child has type String, name "type", value "rect", and a sibling:
108 Sibling has type Number, name "width", value 1920, and a sibling:
109 Sibling has type Number, name "height", value 1080, and a sibling:
110 Sibling hs type False, name "interlace", and a sibling:
111 Sibling has type Number, name "frame rate", value 24
113 Here's the structure:
114 typedef struct cJSON {
115 struct cJSON *next,*prev;
127 By default all values are 0 unless set by virtue of being meaningful.
129 next/prev is a doubly linked list of siblings. next takes you to your sibling,
130 prev takes you back from your sibling to you.
131 Only objects and arrays have a "child", and it's the head of the doubly linked list.
132 A "child" entry will have prev==0, but next potentially points on. The last sibling has next=0.
133 The type expresses Null/True/False/Number/String/Array/Object, all of which are #defined in
136 A Number has valueint and valuedouble. If you're expecting an int, read valueint, if not read
139 Any entry which is in the linked list which is the child of an object will have a "string"
140 which is the "name" of the entry. When I said "name" in the above example, that's "string".
141 "string" is the JSON name for the 'variable name' if you will.
143 Now you can trivially walk the lists, recursively, and parse as you please.
144 You can invoke cJSON_Parse to get cJSON to parse for you, and then you can take
145 the root object, and traverse the structure (which is, formally, an N-tree),
146 and tokenise as you please. If you wanted to build a callback style parser, this is how
147 you'd do it (just an example, since these things are very specific):
149 void parse_and_callback(cJSON *item,const char *prefix)
153 char *newprefix=malloc(strlen(prefix)+strlen(item->name)+2);
154 sprintf(newprefix,"%s/%s",prefix,item->name);
155 int dorecurse=callback(newprefix, item->type, item);
156 if (item->child && dorecurse) parse_and_callback(item->child,newprefix);
162 The prefix process will build you a separated list, to simplify your callback handling.
163 The 'dorecurse' flag would let the callback decide to handle sub-arrays on it's own, or
164 let you invoke it per-item. For the item above, your callback might look like this:
166 int callback(const char *name,int type,cJSON *item)
168 if (!strcmp(name,"name")) { /* populate name */ }
169 else if (!strcmp(name,"format/type") { /* handle "rect" */ }
170 else if (!strcmp(name,"format/width") { /* 800 */ }
171 else if (!strcmp(name,"format/height") { /* 600 */ }
172 else if (!strcmp(name,"format/interlace") { /* false */ }
173 else if (!strcmp(name,"format/frame rate") { /* 24 */ }
177 Alternatively, you might like to parse iteratively.
180 void parse_object(cJSON *item)
182 int i; for (i=0;i<cJSON_GetArraySize(item);i++)
184 cJSON *subitem=cJSON_GetArrayItem(item,i);
189 Or, for PROPER manual mode:
191 void parse_object(cJSON *item)
193 cJSON *subitem=item->child;
197 if (subitem->child) parse_object(subitem->child);
199 subitem=subitem->next;
203 Of course, this should look familiar, since this is just a stripped-down version
204 of the callback-parser.
206 This should cover most uses you'll find for parsing. The rest should be possible
207 to infer.. and if in doubt, read the source! There's not a lot of it! ;)
210 In terms of constructing JSON data, the example code above is the right way to do it.
211 You can, of course, hand your sub-objects to other functions to populate.
212 Also, if you find a use for it, you can manually build the objects.
213 For instance, suppose you wanted to build an array of objects?
217 cJSON *Create_array_of_anything(cJSON **items,int num)
219 int i;cJSON *prev, *root=cJSON_CreateArray();
222 if (!i) root->child=objects[i];
223 else prev->next=objects[i], objects[i]->prev=prev;
229 and simply: Create_array_of_anything(objects,24);
231 cJSON doesn't make any assumptions about what order you create things in.
232 You can attach the objects, as above, and later add children to each
235 As soon as you call cJSON_Print, it renders the structure to text.
239 The test.c code shows how to handle a bunch of typical cases. If you uncomment
240 the code, it'll load, parse and print a bunch of test files, also from json.org,
241 which are more complex than I'd care to try and stash into a const char array[].
247 - Dave Gamble, Aug 2009