A tutorial on json manipulation in C on GNU/Linux (various BSD flavors should work too).

First things first

This tutorial will assume you have a basic knowledge of the json format and of the C programming langage. If you don’t know what json is, you should read json.org.

Examples have been tested on Archlinux with json-c (0.12-2) and clang (3.5.0).

Why json-c ?

Other libraries handle the json format in C, but json-c has the advantage of being light and doesn’t drag a whole set of gigantic dependencies (e.g. Glib). This is probably important if your program is oriented for embedded.

json-c is open source so check it out: github.com/json-c/json-c

Installation

I won’t cover this part in depth, you should be able to install a package on your system. Bear in mind that the naming of these packages aren’t consistent from one distribution to another. Here is a little list of all variants I’ve encountered:

json-c
libjson
libjson0

For the rest of the tutorial you will obviously need the development version of those packages.

Compilation

Add the json-c header to your program:

#include <json.h>

Here is an example on how to compile using the json-c library:

$ clang -I/usr/include/json-c/ -o test test.c -ljson-c

I prefer clang but gcc works just as well. Be sure to check the include path, depending on the packet name it may be different!

Documentation

The documentation is available online: json-c.github.io/json-c/. Files json_object.c and/or json_tokener.c is most likely what you are looking for.

Outline

I did not reinvent the wheel (shame on me), a good tutorial on json-c already exists: linuxprograms.wordpress.com/2010/05/20/json-c-libjson-tutorial/. However, the tutorial linked above has some issues mainly due to outdated versions, partial example files, memory leaks and symbol filtering preventing most copy/paste. This is too painful, I had to (try to) make a better tutorial!

Thus, the outline:

Any comments, improvements, typos / languages corrections are welcome!

Outline / Next