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json-c

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   &#160;<span id="projectnumber">0.16</span>
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<div class="title">json-c Documentation</div>  </div>
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<div class="textblock"><h1><code>json-c</code></h1>
<ol type="1">
<li><a href="#overview">Overview and Build Status</a></li>
<li><a href="#buildunix">Building on Unix</a><ul>
<li><a href="#installprereq">Prerequisites</a></li>
<li><a href="#buildcmds">Build commands</a></li>
</ul>
</li>
<li><a href="#CMake">CMake options</a></li>
<li><a href="#testing">Testing</a></li>
<li><a href="#buildvcpkg">Building with `vcpkg`</a></li>
<li><a href="#linking">Linking to libjson-c</a></li>
<li><a href="#using">Using json-c</a></li>
</ol>
<h2>JSON-C - A JSON implementation in C <a class="anchor" id="overview"></a></h2>
<p>JSON-C implements a reference counting object model that allows you to easily construct JSON objects in C, output them as JSON formatted strings and parse JSON formatted strings back into the C representation of JSON objects. It aims to conform to <a href="https://tools.ietf.org/html/rfc7159">RFC 7159</a>.</p>
<p>Skip down to <a href="#using">Using json-c</a> or check out the <a href="https://json-c.github.io/json-c/">API docs</a>, if you already have json-c installed and ready to use.</p>
<p>Home page for json-c: <a href="https://github.com/json-c/json-c/wiki">https://github.com/json-c/json-c/wiki</a></p>
<p>Build Status</p>
<ul>
<li><a href="https://ci.appveyor.com/project/hawicz/json-c">AppVeyor Build</a> <div class="image">
<img src="https://ci.appveyor.com/api/projects/status/github/json-c/json-c?branch=master&svg=true"  alt="AppVeyor Build Status"/>
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</li>
<li><a href="https://travis-ci.org/json-c/json-c">Travis Build</a> <div class="image">
<img src="https://travis-ci.org/json-c/json-c.svg?branch=master"  alt="Travis Build Status"/>
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</li>
</ul>
<p>Test Status</p>
<ul>
<li><a href="https://coveralls.io/github/json-c/json-c?branch=master">Coveralls</a> <a href="https://coveralls.io/github/json-c/json-c?branch=master">![Coverage Status](https://coveralls.io/repos/github/json-c/json-c/badge.svg?branch=master)</a></li>
</ul>
<h2>Building on Unix with <code>git</code>, <code>gcc</code> and <code>cmake</code> <a class="anchor" id="buildunix"></a></h2>
<p>If you already have json-c installed, see <a href="#linking">Linking to `libjson-c`</a> for how to build and link your program against it.</p>
<h3>Prerequisites: <a class="anchor" id="installprereq"></a></h3>
<ul>
<li><code>gcc</code>, <code>clang</code>, or another C compiler</li>
</ul>
<ul>
<li><code>cmake&gt;=2.8</code>, <code>&gt;=3.16</code> recommended, <code>cmake=&gt;3.1</code> for tests</li>
</ul>
<p>To generate docs you'll also need:</p>
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<li><code>doxygen&gt;=1.8.13</code></li>
</ul>
<p>If you are on a relatively modern system, you'll likely be able to install the prerequisites using your OS's packaging system.</p>
<h3>Install using apt (e.g. Ubuntu 16.04.2 LTS)</h3>
<pre class="fragment">sudo apt install git
sudo apt install cmake
sudo apt install doxygen  # optional
sudo apt install valgrind # optional
</pre><h3>Build instructions: <a class="anchor" id="buildcmds"></a></h3>
<p><code>json-c</code> GitHub repo: <a href="https://github.com/json-c/json-c">https://github.com/json-c/json-c</a> </p>
<pre class="fragment">$ git clone https://github.com/json-c/json-c.git
$ mkdir json-c-build
$ cd json-c-build
$ cmake ../json-c   # See CMake section below for custom arguments
</pre><p>Note: it's also possible to put your build directory inside the json-c source directory, or even not use a separate build directory at all, but certain things might not work quite right (notably, <code>make distcheck</code>)</p>
<p>Then: </p>
<pre class="fragment">$ make
$ make test
$ make USE_VALGRIND=0 test   # optionally skip using valgrind
$ make install
</pre><h3>Generating documentation with Doxygen:</h3>
<p>The library documentation can be generated directly from the source code using Doxygen tool: </p>
<pre class="fragment"># in build directory
make doc
google-chrome doc/html/index.html
</pre><h2>CMake Options <a class="anchor" id="CMake"></a></h2>
<p>The json-c library is built with <a href="https://cmake.org/cmake-tutorial/">CMake</a>, which can take a few options.</p>
<table class="doxtable">
<tr>
<th>Variable </th><th>Type </th><th>Description</th></tr>
<tr>
<td>CMAKE_INSTALL_PREFIX </td><td>String </td><td>The install location. </td></tr>
<tr>
<td>CMAKE_BUILD_TYPE </td><td>String </td><td>Defaults to "debug". </td></tr>
<tr>
<td>BUILD_SHARED_LIBS </td><td>Bool </td><td>The default build generates a dynamic (dll/so) library. Set this to OFF to create a static library only. </td></tr>
<tr>
<td>BUILD_STATIC_LIBS </td><td>Bool </td><td>The default build generates a static (lib/a) library. Set this to OFF to create a shared library only. </td></tr>
<tr>
<td>DISABLE_STATIC_FPIC </td><td>Bool </td><td>The default builds position independent code. Set this to OFF to create a shared library only. </td></tr>
<tr>
<td>DISABLE_BSYMBOLIC </td><td>Bool </td><td>Disable use of -Bsymbolic-functions. </td></tr>
<tr>
<td>DISABLE_THREAD_LOCAL_STORAGE </td><td>Bool </td><td>Disable use of Thread-Local Storage (HAVE___THREAD). </td></tr>
<tr>
<td>DISABLE_WERROR </td><td>Bool </td><td>Disable use of -Werror. </td></tr>
<tr>
<td>ENABLE_RDRAND </td><td>Bool </td><td>Enable RDRAND Hardware RNG Hash Seed. </td></tr>
<tr>
<td>ENABLE_THREADING </td><td>Bool </td><td>Enable partial threading support. </td></tr>
<tr>
<td>OVERRIDE_GET_RANDOM_SEED </td><td>String </td><td>A block of code to use instead of the default implementation of json_c_get_random_seed(), e.g. on embedded platforms where not even the fallback to time() works. Must be a single line. </td></tr>
</table>
<p>Pass these options as <code>-D</code> on CMake's command-line. </p>
<pre class="fragment"># build a static library only
cmake -DBUILD_SHARED_LIBS=OFF ..
</pre><h3>Building with partial threading support</h3>
<p>Although json-c does not support fully multi-threaded access to object trees, it has some code to help make its use in threaded programs a bit safer. Currently, this is limited to using atomic operations for <a class="el" href="json__object_8h.html#a675aa3a9cced685dbfd1c1a770a0c3e4">json_object_get()</a> and <a class="el" href="json__object_8h.html#afabf61f932cd64a4122ca8092452eed5">json_object_put()</a>.</p>
<p>Since this may have a performance impact, of at least 3x slower according to <a href="https://stackoverflow.com/a/11609063,">https://stackoverflow.com/a/11609063,</a> it is disabled by default. You may turn it on by adjusting your cmake command with: -DENABLE_THREADING=ON</p>
<p>Separately, the default hash function used for object field keys, lh_char_hash, uses a compare-and-swap operation to ensure the random seed is only generated once. Because this is a one-time operation, it is always compiled in when the compare-and-swap operation is available.</p>
<h3>cmake-configure wrapper script</h3>
<p>For those familiar with the old autoconf/autogen.sh/configure method, there is a <code>cmake-configure</code> wrapper script to ease the transition to cmake. </p>
<pre class="fragment">mkdir build
cd build
../cmake-configure --prefix=/some/install/path
make
</pre><p>cmake-configure can take a few options.</p>
<table class="doxtable">
<tr>
<th>options </th><th>Description</th></tr>
<tr>
<td>prefix=PREFIX </td><td>install architecture-independent files in PREFIX </td></tr>
<tr>
<td>enable-threading </td><td>Enable code to support partly multi-threaded use </td></tr>
<tr>
<td>enable-rdrand </td><td>Enable RDRAND Hardware RNG Hash Seed generation on supported x86/x64 platforms. </td></tr>
<tr>
<td>enable-shared </td><td>build shared libraries [default=yes] </td></tr>
<tr>
<td>enable-static </td><td>build static libraries [default=yes] </td></tr>
<tr>
<td>disable-Bsymbolic </td><td>Avoid linking with -Bsymbolic-function </td></tr>
<tr>
<td>disable-werror </td><td>Avoid treating compiler warnings as fatal errors </td></tr>
</table>
<h2>Testing: <a class="anchor" id="testing"></a></h2>
<p>By default, if valgrind is available running tests uses it. That can slow the tests down considerably, so to disable it use: </p>
<pre class="fragment">export USE_VALGRIND=0
</pre><p>To run tests a separate build directory is recommended: </p>
<pre class="fragment">mkdir build-test
cd build-test
# VALGRIND=1 causes -DVALGRIND=1 to be passed when compiling code
# which uses slightly slower, but valgrind-safe code.
VALGRIND=1 cmake ..
make

make test
# By default, if valgrind is available running tests uses it.
make USE_VALGRIND=0 test   # optionally skip using valgrind
</pre><p>If a test fails, check <code>Testing/Temporary/LastTest.log</code>, <code>tests/testSubDir/${testname}/${testname}.vg.out</code>, and other similar files. If there is insufficient output try: </p>
<pre class="fragment">VERBOSE=1 CTEST_OUTPUT_ON_FAILURE=1 make test
</pre><p>or </p>
<pre class="fragment">JSONC_TEST_TRACE=1 make test
</pre><p>and check the log files again.</p>
<h2>Building on Unix and Windows with <code>vcpkg</code> <a class="anchor" id="buildvcpkg"></a></h2>
<p>You can download and install JSON-C using the <a href="https://github.com/Microsoft/vcpkg/">vcpkg</a> dependency manager: </p>
<pre class="fragment">git clone https://github.com/Microsoft/vcpkg.git
cd vcpkg
./bootstrap-vcpkg.sh
./vcpkg integrate install
vcpkg install json-c
</pre><p>The JSON-C port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please <a href="https://github.com/Microsoft/vcpkg">create an issue or pull request</a> on the vcpkg repository.</p>
<h2>Linking to <code>libjson-c</code> <a class="anchor" id="linking"></a></h2>
<p>If your system has <code>pkgconfig</code>, then you can just add this to your <code>makefile</code>: </p>
<pre class="fragment">CFLAGS += $(shell pkg-config --cflags json-c)
LDFLAGS += $(shell pkg-config --libs json-c)
</pre><p>Without <code>pkgconfig</code>, you might do something like this: </p>
<pre class="fragment">JSON_C_DIR=/path/to/json_c/install
CFLAGS += -I$(JSON_C_DIR)/include/json-c
# Or to use lines like: #include &lt;json-c/json_object.h&gt;
#CFLAGS += -I$(JSON_C_DIR)/include
LDFLAGS+= -L$(JSON_C_DIR)/lib -ljson-c
</pre><p>If your project uses cmake:</p>
<ul>
<li><p class="startli">Add to your CMakeLists.txt file:</p>
<p class="startli">find_package(json-c CONFIG) target_link_libraries(${PROJECT_NAME} PRIVATE json-c::json-c)</p>
</li>
</ul>
<ul>
<li><p class="startli">Then you might run in your project:</p>
<p class="startli">cd build cmake -DCMAKE_PREFIX_PATH=/path/to/json_c/install/lib64/cmake ..</p>
</li>
</ul>
<h2>Using json-c <a class="anchor" id="using"></a></h2>
<p>To use json-c you can either include <a class="el" href="json_8h.html" title="A convenience header that may be included instead of other individual ones.">json.h</a>, or preferably, one of the following more specific header files:</p>
<ul>
<li><a class="el" href="json__object_8h.html" title="Core json-c API. Start here, or with json_tokener.h.">json_object.h</a> - Core types and methods.</li>
<li><a class="el" href="json__tokener_8h.html" title="Methods to parse an input string into a tree of json_object objects.">json_tokener.h</a> - Methods for parsing and serializing json-c object trees.</li>
<li><a class="el" href="json__pointer_8h.html" title="JSON Pointer (RFC 6901) implementation for retrieving objects from a json-c object tree...">json_pointer.h</a> - JSON Pointer (RFC 6901) implementation for retrieving objects from a json-c object tree.</li>
<li><a class="el" href="json__object__iterator_8h.html" title="An API for iterating over json_type_object objects, styled to be familiar to C++ programmers. Unlike json_object_object_foreach() and json_object_object_foreachC(), this avoids the need to expose json-c internals like lh_entry.">json_object_iterator.h</a> - Methods for iterating over single json_object instances. (See also <code><a class="el" href="json__object_8h.html#acf5f514a9e0061c10fc08055762639ee">json_object_object_foreach()</a></code> in <a class="el" href="json__object_8h.html" title="Core json-c API. Start here, or with json_tokener.h.">json_object.h</a>)</li>
<li><a class="el" href="json__visit_8h.html" title="Methods for walking a tree of objects.">json_visit.h</a> - Methods for walking a tree of json-c objects.</li>
<li><a class="el" href="json__util_8h.html" title="Miscllaneous utility functions and macros.">json_util.h</a> - Miscellaneous utility functions.</li>
</ul>
<p>For a full list of headers see <a href="https://json-c.github.io/json-c/json-c-current-release/doc/html/files.html">files.html</a></p>
<p>The primary type in json-c is json_object. It describes a reference counted tree of json objects which are created by either parsing text with a <a class="el" href="structjson__tokener.html">json_tokener</a> (i.e. <code><a class="el" href="json__tokener_8h.html#a61679f178111963a9ffa3c8179553f7a">json_tokener_parse_ex()</a></code>), or by creating (with <code><a class="el" href="json__object_8h.html#a68c383f54544fca19b5f2425be397600">json_object_new_object()</a></code>, <code><a class="el" href="json__object_8h.html#ae92f0770fb4b3c884ce35de52d3d7de8">json_object_new_int()</a></code>, etc...) and adding (with <code><a class="el" href="json__object_8h.html#a27bd808a022251059a43f1f6370441cd">json_object_object_add()</a></code>, <code><a class="el" href="json__object_8h.html#a18cdd9a7455e09f36cdf6e5756b7f586">json_object_array_add()</a></code>, etc...) them individually. Typically, every object in the tree will have one reference, from its parent. When you are done with the tree of objects, you call <a class="el" href="json__object_8h.html#afabf61f932cd64a4122ca8092452eed5">json_object_put()</a> on just the root object to free it, which recurses down through any child objects calling <a class="el" href="json__object_8h.html#afabf61f932cd64a4122ca8092452eed5">json_object_put()</a> on each one of those in turn.</p>
<p>You can get a reference to a single child (<code><a class="el" href="json__object_8h.html#a1a097805abb53b4c8a60d573730a8939">json_object_object_get()</a></code> or <code><a class="el" href="json__object_8h.html#a676711a76545d4ec65cc75f100f5fd19">json_object_array_get_idx()</a></code>) and use that object as long as its parent is valid. If you need a child object to live longer than its parent, you can increment the child's refcount (<code><a class="el" href="json__object_8h.html#a675aa3a9cced685dbfd1c1a770a0c3e4">json_object_get()</a></code>) to allow it to survive the parent being freed or it being removed from its parent (<code><a class="el" href="json__object_8h.html#ac6605fdafca20bd5d33c84f4f80a3bda">json_object_object_del()</a></code> or <code><a class="el" href="json__object_8h.html#a722eca9f578704d3af38b97549242c1f">json_object_array_del_idx()</a></code>)</p>
<p>When parsing text, the <a class="el" href="structjson__tokener.html">json_tokener</a> object is independent from the json_object that it returns. It can be allocated (<code><a class="el" href="json__tokener_8h.html#a5ac7e2c350bc592cf2fa7b9935b00ef5">json_tokener_new()</a></code>) used one or multiple times (<code><a class="el" href="json__tokener_8h.html#a61679f178111963a9ffa3c8179553f7a">json_tokener_parse_ex()</a></code>, and freed (<code><a class="el" href="json__tokener_8h.html#a887c4661906fc6b36cc366304e522534">json_tokener_free()</a></code>) while the json_object objects live on.</p>
<p>A json_object tree can be serialized back into a string with <code><a class="el" href="json__object_8h.html#a9db613127bd4ef7db42307e43a85fc1b">json_object_to_json_string_ext()</a></code>. The string that is returned is only valid until the next "to_json_string" call on that same object. Also, it is freed when the json_object is freed. </p>
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