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Copyright (c) 2009 Dave Gamble
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in
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all copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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cJSON aims to be the dumbest possible parser that you can get your job done with.
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It's a single file of C, and a single header file.
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JSON is described best here: http://www.json.org/
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It's like XML, but fat-free. You use it to move data around, store things, or just
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generally represent your program's state.
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First up, how do I build?
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Add cJSON.c to your project, and put cJSON.h somewhere in the header search path.
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For example, to build the test app:
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gcc cJSON.c test.c -o test -lm
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As a library, cJSON exists to take away as much legwork as it can, but not get in your way.
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As a point of pragmatism (i.e. ignoring the truth), I'm going to say that you can use it
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in one of two modes: Auto and Manual. Let's have a quick run-through.
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I lifted some JSON from this page: http://www.json.org/fatfree.html
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That page inspired me to write cJSON, which is a parser that tries to share the same
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philosophy as JSON itself. Simple, dumb, out of the way.
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"name": "Jack (\"Bee\") Nimble",
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Assume that you got this from a file, a webserver, or magic JSON elves, whatever,
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you have a char * to it. Everything is a cJSON struct.
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cJSON *root = cJSON_Parse(my_json_string);
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This is an object. We're in C. We don't have objects. But we do have structs.
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cJSON *format = cJSON_GetObjectItem(root,"format");
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int framerate = cJSON_GetObjectItem(format,"frame rate")->valueint;
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Want to change the framerate?
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cJSON_GetObjectItem(format,"frame rate")->valueint=25;
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char *rendered=cJSON_Print(root);
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Finished? Delete the root (this takes care of everything else).
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That's AUTO mode. If you're going to use Auto mode, you really ought to check pointers
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before you dereference them. If you want to see how you'd build this struct in code?
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root=cJSON_CreateObject();
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cJSON_AddItemToObject(root, "name", cJSON_CreateString("Jack (\"Bee\") Nimble"));
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cJSON_AddItemToObject(root, "format", fmt=cJSON_CreateObject());
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cJSON_AddStringToObject(fmt,"type", "rect");
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cJSON_AddNumberToObject(fmt,"width", 1920);
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cJSON_AddNumberToObject(fmt,"height", 1080);
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cJSON_AddFalseToObject (fmt,"interlace");
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cJSON_AddNumberToObject(fmt,"frame rate", 24);
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Hopefully we can agree that's not a lot of code? There's no overhead, no unnecessary setup.
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Look at test.c for a bunch of nice examples, mostly all ripped off the json.org site, and
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What about manual mode? First up you need some detail.
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Let's cover how the cJSON objects represent the JSON data.
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cJSON doesn't distinguish arrays from objects in handling; just type.
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Each cJSON has, potentially, a child, siblings, value, a name.
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The root object has: Object Type and a Child
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The Child has name "name", with value "Jack ("Bee") Nimble", and a sibling:
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Sibling has type Object, name "format", and a child.
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That child has type String, name "type", value "rect", and a sibling:
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Sibling has type Number, name "width", value 1920, and a sibling:
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Sibling has type Number, name "height", value 1080, and a sibling:
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Sibling hs type False, name "interlace", and a sibling:
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Sibling has type Number, name "frame rate", value 24
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Here's the structure:
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typedef struct cJSON {
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struct cJSON *next,*prev;
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By default all values are 0 unless set by virtue of being meaningful.
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next/prev is a doubly linked list of siblings. next takes you to your sibling,
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prev takes you back from your sibling to you.
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Only objects and arrays have a "child", and it's the head of the doubly linked list.
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A "child" entry will have prev==0, but next potentially points on. The last sibling has next=0.
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The type expresses Null/True/False/Number/String/Array/Object, all of which are #defined in
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A Number has valueint and valuedouble. If you're expecting an int, read valueint, if not read
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Any entry which is in the linked list which is the child of an object will have a "string"
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which is the "name" of the entry. When I said "name" in the above example, that's "string".
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"string" is the JSON name for the 'variable name' if you will.
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Now you can trivially walk the lists, recursively, and parse as you please.
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You can invoke cJSON_Parse to get cJSON to parse for you, and then you can take
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the root object, and traverse the structure (which is, formally, an N-tree),
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and tokenise as you please. If you wanted to build a callback style parser, this is how
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you'd do it (just an example, since these things are very specific):
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void parse_and_callback(cJSON *item,const char *prefix)
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char *newprefix=malloc(strlen(prefix)+strlen(item->name)+2);
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sprintf(newprefix,"%s/%s",prefix,item->name);
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int dorecurse=callback(newprefix, item->type, item);
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if (item->child && dorecurse) parse_and_callback(item->child,newprefix);
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The prefix process will build you a separated list, to simplify your callback handling.
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The 'dorecurse' flag would let the callback decide to handle sub-arrays on it's own, or
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let you invoke it per-item. For the item above, your callback might look like this:
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int callback(const char *name,int type,cJSON *item)
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if (!strcmp(name,"name")) { /* populate name */ }
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else if (!strcmp(name,"format/type") { /* handle "rect" */ }
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else if (!strcmp(name,"format/width") { /* 800 */ }
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else if (!strcmp(name,"format/height") { /* 600 */ }
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else if (!strcmp(name,"format/interlace") { /* false */ }
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else if (!strcmp(name,"format/frame rate") { /* 24 */ }
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Alternatively, you might like to parse iteratively.
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void parse_object(cJSON *item)
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int i; for (i=0;i<cJSON_GetArraySize(item);i++)
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cJSON *subitem=cJSON_GetArrayItem(item,i);
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Or, for PROPER manual mode:
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void parse_object(cJSON *item)
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cJSON *subitem=item->child;
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if (subitem->child) parse_object(subitem->child);
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subitem=subitem->next;
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Of course, this should look familiar, since this is just a stripped-down version
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of the callback-parser.
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This should cover most uses you'll find for parsing. The rest should be possible
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to infer.. and if in doubt, read the source! There's not a lot of it! ;)
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In terms of constructing JSON data, the example code above is the right way to do it.
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You can, of course, hand your sub-objects to other functions to populate.
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Also, if you find a use for it, you can manually build the objects.
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For instance, suppose you wanted to build an array of objects?
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cJSON *Create_array_of_anything(cJSON **items,int num)
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int i;cJSON *prev, *root=cJSON_CreateArray();
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if (!i) root->child=objects[i];
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else prev->next=objects[i], objects[i]->prev=prev;
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and simply: Create_array_of_anything(objects,24);
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cJSON doesn't make any assumptions about what order you create things in.
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You can attach the objects, as above, and later add children to each
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As soon as you call cJSON_Print, it renders the structure to text.
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The test.c code shows how to handle a bunch of typical cases. If you uncomment
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the code, it'll load, parse and print a bunch of test files, also from json.org,
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which are more complex than I'd care to try and stash into a const char array[].
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- Dave Gamble, Aug 2009
added
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src/hed/dmc/dq2/cJSON/README
