Parsing JSON in C using JSMN

I recently came across this small and really useful library, JSMN (pronounced jasmine) which is really easy and useful for parsing JSON data in C with small but useful API.

From its website :

[JSMN is] is designed to be robust (it should work fine even with erroneous data), fast (it parses data on the fly and is re-entrant), portable (no superfluous dependencies or non-standard C extensions). And of course, simplicity is a key feature.

So let’s get started!

Installation

As I’ve already said, JSMN is really small. And it is opensource with MIT license. So you can basically just drop in the library inside your project and use it wherever you want.

Clone the git repository using

git clone https://github.com/zserge/jsmn.git

or mercurial using

hg clone http://bitbucket.org/zserge/jsmn jsmn

Run make to build the library.

That’s about it. Wherever you want to use the library include jsmn.h and compile with the libjsmn.a library.

Usage

Before doing any parsing, a jsmn_parser object needs to be created and then initialized. Like this :

#include <jsmn.h>

int main(){
	jsmn_parser parser; // creating the object
	// initializing
	// same function can also be used to reset an object
	jsmn_init(&parser);
	return 0;
}

Now we can get to actual parsing using the jsmn_parse function. Instead of assigning separate memory space for each token in the JSON what JSMN does is create pointers which point to start and end of each tokens. These tokens are stored in jsmntok_t structure which is defined as below :

typedef struct {
    jsmntype_t type; /* Token type */
    int start;       /* Token start position */
    int end;         /* Token end position */
    int size;        /* Number of child (nested) tokens */
} jsmntok_t;

It’s pretty straight forward. Now lets use the jsmn_parse.

jsmntok_t tokens[256];
int n;
char *json;

json = ... // store your JSON data in a character pointer

n = jsmn_parse(&parser, json, strlen(json), tokens, 256);
// n will now contain number of parsed tokens or a negative
// error value

If you don’t know the number of tokens beforehand you can pass NULL instead of tokens and it will return number of tokens without actually storing them so you can allocate memory appropriately.

If you’re reading input from a stream then you can continuously check if return value of jsmn_parse is JSMN_ERROR_NOMEM and if it is then reallocate more memory.

And that’s it. You can traverse through the scanned tokens array and do whatever you like with it.

Mind that JSMN does not perform unescaping on the tokens to UTF-8 escaped token will remain as it is.

You can find more information and example code here :

Installation

Usage

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