gpgme/src/cJSON.readme

/*
  Copyright (c) 2009 Dave Gamble

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*/

Welcome to cJSON.

cJSON aims to be the dumbest possible parser that you can get your job
done with.  It's a single file of C, and a single header file.

JSON is described best here: http://www.json.org/ It's like XML, but
fat-free. You use it to move data around, store things, or just
generally represent your program's state.


First up, how do I build?  Add cJSON.c to your project, and put
cJSON.h somewhere in the header search path.  For example, to build
the test app:

gcc cJSON.c test.c -o test -lm
./test


As a library, cJSON exists to take away as much legwork as it can, but
not get in your way.  As a point of pragmatism (i.e. ignoring the
truth), I'm going to say that you can use it in one of two modes: Auto
and Manual. Let's have a quick run-through.


I lifted some JSON from this page: http://www.json.org/fatfree.html
That page inspired me to write cJSON, which is a parser that tries to
share the same philosophy as JSON itself. Simple, dumb, out of the
way.

Some JSON:
{
    "name": "Jack (\"Bee\") Nimble",
    "format": {
        "type":       "rect",
        "width":      1920,
        "height":     1080,
        "interlace":  false,
        "frame rate": 24
    }
}

Assume that you got this from a file, a webserver, or magic JSON
elves, whatever, you have a char * to it. Everything is a cJSON
struct.  Get it parsed:

         cJSON *root = cJSON_Parse(my_json_string);

This is an object. We're in C. We don't have objects. But we do have
structs.  What's the framerate?

	cJSON *format = cJSON_GetObjectItem(root,"format");
	int framerate = cJSON_GetObjectItem(format,"frame rate")->valueint;

Want to change the framerate?

	cJSON_GetObjectItem(format,"frame rate")->valueint=25;

Back to disk?

	char *rendered=cJSON_Print(root);

Finished? Delete the root (this takes care of everything else).

	cJSON_Delete(root);

That's AUTO mode. If you're going to use Auto mode, you really ought
to check pointers before you dereference them. If you want to see how
you'd build this struct in code?

	cJSON *root,*fmt;
	root=cJSON_CreateObject();
	cJSON_AddItemToObject(root, "name",
                              cJSON_CreateString("Jack (\"Bee\") Nimble"));
	cJSON_AddItemToObject(root, "format", fmt=cJSON_CreateObject());
	cJSON_AddStringToObject(fmt,"type",		"rect");
	cJSON_AddNumberToObject(fmt,"width",		1920);
	cJSON_AddNumberToObject(fmt,"height",		1080);
	cJSON_AddFalseToObject (fmt,"interlace");
	cJSON_AddNumberToObject(fmt,"frame rate",	24);

Hopefully we can agree that's not a lot of code? There's no overhead,
no unnecessary setup.  Look at test.c for a bunch of nice examples,
mostly all ripped off the json.org site, and a few from elsewhere.

What about manual mode? First up you need some detail.  Let's cover
how the cJSON objects represent the JSON data.  cJSON doesn't
distinguish arrays from objects in handling; just type.  Each cJSON
has, potentially, a child, siblings, value, a name.

- The root object has: Object Type and a Child
- The Child has name "name", with value "Jack ("Bee") Nimble", and a sibling:
- Sibling has type Object, name "format", and a child.
- That child has type String, name "type", value "rect", and a sibling:
- Sibling has type Number, name "width", value 1920, and a sibling:
- Sibling has type Number, name "height", value 1080, and a sibling:
- Sibling hs type False, name "interlace", and a sibling:
- Sibling has type Number, name "frame rate", value 24

Here's the structure:

typedef struct cJSON {
	struct cJSON *next,*prev;
	struct cJSON *child;

	int type;

	char *valuestring;
	int valueint;
	double valuedouble;

	char *string;
} cJSON;

By default all values are 0 unless set by virtue of being meaningful.

next/prev is a doubly linked list of siblings. next takes you to your sibling,
prev takes you back from your sibling to you.

Only objects and arrays have a "child", and it's the head of the
doubly linked list.

A "child" entry will have prev==0, but next potentially points on. The
last sibling has next=0.

The type expresses Null/True/False/Number/String/Array/Object, all of
which are #defined in cJSON.h

A Number has valueint and valuedouble. If you're expecting an int,
read valueint, if not read valuedouble.

Any entry which is in the linked list which is the child of an object
will have a "string" which is the "name" of the entry. When I said
"name" in the above example, that's "string".  "string" is the JSON
name for the 'variable name' if you will.

Now you can trivially walk the lists, recursively, and parse as you
please.  You can invoke cJSON_Parse to get cJSON to parse for you, and
then you can take the root object, and traverse the structure (which
is, formally, an N-tree), and tokenise as you please. If you wanted to
build a callback style parser, this is how you'd do it (just an
example, since these things are very specific):

void parse_and_callback(cJSON *item,const char *prefix)
{
        while (item)
	{
		char *newprefix=malloc(strlen(prefix)+strlen(item->name)+2);
		sprintf(newprefix,"%s/%s",prefix,item->name);
		int dorecurse=callback(newprefix, item->type, item);
		if (item->child && dorecurse)
                    parse_and_callback(item->child,newprefix);
		item=item->next;
		free(newprefix);
	}
}

The prefix process will build you a separated list, to simplify your
callback handling.

The 'dorecurse' flag would let the callback decide to handle
sub-arrays on it's own, or let you invoke it per-item. For the item
above, your callback might look like this:

int callback(const char *name,int type,cJSON *item)
{
	if (!strcmp(name,"name"))	{ /* populate name */ }
	else if (!strcmp(name,"format/type")	{ /* handle "rect" */ }
	else if (!strcmp(name,"format/width")	{ /* 800 */ }
	else if (!strcmp(name,"format/height")	{ /* 600 */ }
	else if (!strcmp(name,"format/interlace")	{ /* false */ }
	else if (!strcmp(name,"format/frame rate")	{ /* 24 */ }
	return 1;
}

Alternatively, you might like to parse iteratively.
You'd use:

void parse_object(cJSON *item)
{
	int i; for (i=0;i<cJSON_GetArraySize(item);i++)
	{
		cJSON *subitem=cJSON_GetArrayItem(item,i);
		// handle subitem.
	}
}

Or, for PROPER manual mode:

void parse_object(cJSON *item)
{
	cJSON *subitem=item->child;
	while (subitem)
	{
		// handle subitem
		if (subitem->child) parse_object(subitem->child);

		subitem=subitem->next;
	}
}

Of course, this should look familiar, since this is just a
stripped-down version of the callback-parser.

This should cover most uses you'll find for parsing. The rest should
be possible to infer.. and if in doubt, read the source! There's not a
lot of it! ;)


In terms of constructing JSON data, the example code above is the
right way to do it.  You can, of course, hand your sub-objects to
other functions to populate.  Also, if you find a use for it, you can
manually build the objects.  For instance, suppose you wanted to build
an array of objects?

cJSON *objects[24];

cJSON *Create_array_of_anything(cJSON **items,int num)
{
	int i;cJSON *prev, *root=cJSON_CreateArray();
	for (i=0;i<24;i++)
	{
		if (!i)	root->child=objects[i];
		else	prev->next=objects[i], objects[i]->prev=prev;
		prev=objects[i];
	}
	return root;
}

and simply: Create_array_of_anything(objects,24);

cJSON doesn't make any assumptions about what order you create things
in.  You can attach the objects, as above, and later add children to
each of those objects.

As soon as you call cJSON_Print, it renders the structure to text.



The test.c code shows how to handle a bunch of typical cases. If you
uncomment the code, it'll load, parse and print a bunch of test files,
also from json.org, which are more complex than I'd care to try and
stash into a const char array[].


Enjoy cJSON!


- Dave Gamble, Aug 2009