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json-c

1. Overview and Build Status
2. Getting Help
3. Building on Unix
   • Prerequisites
   • Build commands
4. CMake options
5. Testing
6. Building with vcpkg
7. Building for Android
8. Building for Commodore Amiga or MorphOS
9. Linking to libjson-c
10. Using json-c

JSON-C - A JSON implementation in C

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 RFC 8259.

Skip down to Using json-c
or check out the API docs,
if you already have json-c installed and ready to use.

Home page for json-c: https://github.com/json-c/json-c/wiki

Getting Help

If you have questions about using json-c, please start a thread on
our forums at: https://groups.google.com/forum/#!forum/json-c

If you believe you’ve discovered a bug, report it at
(https://github.com/json-c/json-c/issues). Please be sure to include
the version of json-c you’re using, the OS you’re running on, and any
other relevant details. Fully reproducible test cases and/or patches
to fix problems are greatly appreciated.

Fixes for bugs, or small new features can be directly submitted as a
pull request. For major new
features or large changes of any kind, please first start a discussion
on the forums.

Building on Unix with git, gcc and cmake

If you already have json-c installed, see Linking to libjson-c
for how to build and link your program against it.

Build Status

• AppVeyor Build
• Travis Build

Test Status

• Coveralls

Prerequisites:

• gcc, clang, or another C compiler

• cmake>=2.8, >=3.16 recommended, cmake=>3.1 for tests

To generate docs you’ll also need:

• doxygen>=1.8.13

If you are on a relatively modern system, you’ll likely be able to install
the prerequisites using your OS’s packaging system.

Install using apt (e.g. Ubuntu 16.04.2 LTS)

sudo apt install git
sudo apt install cmake
sudo apt install doxygen  # optional
sudo apt install valgrind # optional

Build instructions:

json-c GitHub repo: https://github.com/json-c/json-c

$ 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

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, make distcheck)

Then:

$ make
$ make test
$ make USE_VALGRIND=0 test   # optionally skip using valgrind
$ sudo make install          # it could be necessary to execute make install

Generating documentation with Doxygen:

The library documentation can be generated directly from the source code using Doxygen tool:

# in build directory
make doc
google-chrome doc/html/index.html

CMake Options

The json-c library is built with CMake,
which can take a few options.

Pass these options as -D on CMake’s command-line.

# build a static library only
cmake -DBUILD_SHARED_LIBS=OFF ..

Building with partial threading support

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
json_object_get() and json_object_put().

Since this may have a performance impact, of at least 3x slower
according to https://stackoverflow.com/a/11609063, it is disabled by
default. You may turn it on by adjusting your cmake command with:
-DENABLE_THREADING=ON

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.

cmake-configure wrapper script

For those familiar with the old autoconf/autogen.sh/configure method,
there is a cmake-configure wrapper script to ease the transition to cmake.

mkdir build
cd build
../cmake-configure --prefix=/some/install/path
make

cmake-configure can take a few options.

Testing:

By default, if valgrind is available running tests uses it.
That can slow the tests down considerably, so to disable it use:

export USE_VALGRIND=0

To run tests a separate build directory is recommended:

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

If a test fails, check Testing/Temporary/LastTest.log,
tests/testSubDir/${testname}/${testname}.vg.out, and other similar files.
If there is insufficient output try:

VERBOSE=1 CTEST_OUTPUT_ON_FAILURE=1 make test

or

JSONC_TEST_TRACE=1 make test

and check the log files again.

Building on Unix and Windows with vcpkg

You can download and install JSON-C using the vcpkg dependency manager:

git clone https://github.com/Microsoft/vcpkg.git
cd vcpkg
./bootstrap-vcpkg.sh
./vcpkg integrate install
vcpkg install json-c

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 create an issue or pull request on the vcpkg repository.

Building for Android

Building on Android is now particularly well supported, but there
have been some reports of success using
https://developer.android.com/ndk/guides/cmake

mkdir json-c-build
cd json-c-build/
export NDK_HOME=~/Library/Android/sdk/ndk/22.1.7171670/
cmake \
    --toolchain=$NDK_HOME/build/cmake/android.toolchain.cmake \
    -DANDROID_STL=none \
    -DANDROID_ABI=arm64-v8a \
    -DANDROID_PLATFORM=android-29 \
    -DANDROID_LD=lld \
    -DCMAKE_BUILD_TYPE=MinSizeRel \
    -DCMAKE_INSTALL_PREFIX=<install prefix> \
    -DENABLE_THREADING=true \
    ..
make install

Building for Commodore Amiga or MorphOS

Building for Commodore Amiga is supported for both Motorola 68k (AmigaOS 3) and PowerPC (AmigaOS 4) architectures. MorphOS on compatible PowerPC hardware is also supported. You can set up a cross compiler locally, however it is much easier to use the already preconfigured Amiga development environment wtthin a Docker container.

Install Docker on your machine if you don’t already have it. You can download Docker Desktop for Windows/macOS/Linux here.

To build for Motorola 68k Amiga:

mkdir build
docker run --rm \
    -v ${PWD}:/work \
    -e USER=$( id -u ) -e GROUP=$( id -g ) \
    -it  sacredbanana/amiga-compiler:m68k-amigaos bash
cd build
cmake -DM68K_CRT=newlib ..
make

libjson-c.a will get created in the build directory.

You can change newlib to nix20, nix13, ixemul or clib2 if you would like to build the library suited for libnix or clib2 instead. Newlib is default.

To build for PowerPC Amiga:

mkdir build
docker run --rm \
    -v ${PWD}:/work \
    -e USER=$( id -u ) -e GROUP=$( id -g ) \
    -it  sacredbanana/amiga-compiler:ppc-amigaos bash
cd build
cmake ..
make

libjson-c.a will get created in the build directory.

To build for PowerPC MorphOS:

mkdir build
docker run --rm \
    -v ${PWD}:/work \
    -e USER=$( id -u ) -e GROUP=$( id -g ) \
    -it  sacredbanana/amiga-compiler:ppc-morphos bash
cd build
cmake -DNOIXEMUL=1 ..
make

If you are making an application that absolutely requires ixemul, then remove the -DNOIXEMUL=1.

libjson-c.a will get created in the build directory.

Linking to libjson-c

If your system has pkgconfig,
then you can just add this to your makefile:

CFLAGS += $(shell pkg-config --cflags json-c)
LDFLAGS += $(shell pkg-config --libs json-c)

Without pkgconfig, you might do something like this:

JSON_C_DIR=/path/to/json_c/install
CFLAGS += -I$(JSON_C_DIR)/include/json-c
# Or to use lines like: #include <json-c/json_object.h>
#CFLAGS += -I$(JSON_C_DIR)/include
LDFLAGS+= -L$(JSON_C_DIR)/lib -ljson-c

If your project uses cmake:

• Add to your CMakeLists.txt file:

find_package(json-c CONFIG)
target_link_libraries(${PROJECT_NAME} PRIVATE json-c::json-c)

• Then you might run in your project:

cd build
cmake -DCMAKE_PREFIX_PATH=/path/to/json_c/install/lib64/cmake ..

Using json-c

To use json-c you can either include json.h, or preferably, one of the
following more specific header files:

• json_object.h - Core types and methods.
• json_tokener.h - Methods for parsing and serializing json-c object trees.
• json_pointer.h - JSON Pointer (RFC 6901) implementation for retrieving

               objects from a json-c object tree.

• json_object_iterator.h - Methods for iterating over single json_object instances. (See also json_object_object_foreach() in json_object.h)
• json_visit.h - Methods for walking a tree of json-c objects.
• json_util.h - Miscellaneous utility functions.

For a full list of headers see files.html

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
json_tokener (i.e. json_tokener_parse_ex()), or by creating
(with json_object_new_object(), json_object_new_int(), etc…) and adding
(with json_object_object_add(), json_object_array_add(), 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 json_object_put() on just
the root object to free it, which recurses down through any child objects
calling json_object_put() on each one of those in turn.

You can get a reference to a single child
(json_object_object_get() or json_object_array_get_idx())
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 (json_object_get()) to allow it to survive
the parent being freed or it being removed from its parent
(json_object_object_del() or json_object_array_del_idx())

When parsing text, the json_tokener object is independent from the json_object
that it returns. It can be allocated (json_tokener_new())
used one or multiple times (json_tokener_parse_ex(), and
freed (json_tokener_free()) while the json_object objects live on.

A json_object tree can be serialized back into a string with
json_object_to_json_string_ext(). 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.