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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

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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

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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

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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

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

Variable	Type	Description
CMAKE_INSTALL_PREFIX	String	The install location.
CMAKE_BUILD_TYPE	String	Defaults to “debug”.
BUILD_SHARED_LIBS	Bool	The default build generates a dynamic (dll/so) library. Set this to OFF to create a static library only.
BUILD_STATIC_LIBS	Bool	The default build generates a static (lib/a) library. Set this to OFF to create a shared library only.
DISABLE_STATIC_FPIC	Bool	The default builds position independent code. Set this to OFF to create a shared library only.
DISABLE_BSYMBOLIC	Bool	Disable use of -Bsymbolic-functions.
DISABLE_THREAD_LOCAL_STORAGE	Bool	Disable use of Thread-Local Storage (HAVE___THREAD).
DISABLE_WERROR	Bool	Disable use of -Werror.
DISABLE_EXTRA_LIBS	Bool	Disable use of extra libraries, libbsd
DISABLE_JSON_POINTER	Bool	Omit json_pointer support from the build.
ENABLE_RDRAND	Bool	Enable RDRAND Hardware RNG Hash Seed.
ENABLE_THREADING	Bool	Enable partial threading support.
OVERRIDE_GET_RANDOM_SEED	String	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.

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.

options	Description
prefix=PREFIX	install architecture-independent files in PREFIX
enable-threading	Enable code to support partly multi-threaded use
enable-rdrand	Enable RDRAND Hardware RNG Hash Seed generation on supported x86/x64 platforms.
enable-shared	build shared libraries [default=yes]
enable-static	build static libraries [default=yes]
disable-Bsymbolic	Avoid linking with -Bsymbolic-function
disable-werror	Avoid treating compiler warnings as fatal errors

Testing:

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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

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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

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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

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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 within a Docker container.

Included below are instructions for building for specific individual systems, as well as how to build for all Amiga variants.

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.

By default the m68k build uses -fbaserel. You can select a different base-relative mode with -DM68K_BASEREL=:

• baserel (default) — -fbaserel
• baserel32 — -fbaserel32
• off — no base-relative flag

cmake -DM68K_CRT=newlib -DM68K_BASEREL=baserel32 ..
cmake -DM68K_CRT=newlib -DM68K_BASEREL=off ..

You can also set the target CPU with -DM68K_CPU=. This is passed to the compiler as -m<cpu> (for example 68020 becomes -m68020). If omitted, the compiler default is used.

cmake -DM68K_CRT=newlib -DM68K_CPU=68020 ..
cmake -DM68K_CRT=newlib -DM68K_CPU=68040 -DM68K_BASEREL=baserel32 ..

A note on base-relative addressing (-fbaserel, i.e. M68K_BASEREL=baserel) on m68k: a base-relative library keeps its globals at an offset from a4, and a4 is established once by the program’s startup code. Such a library is only safe inside a program that is itself base-relative and that declares every function the OS can call back into (BOOPSI/MUI dispatchers, struct Hook entries, interrupt servers) with __saveds, so a4 is re-established on entry. Without that, a hook entered from the OS runs with a foreign a4 and every a4-relative access made by this library – or by libnix’s malloc – reads from a wrong address.

Conversely, a library built without -fbaserel and compiled at -O1 or above may allocate a4 as a scratch register. That is harmless in a program that is also non-base-relative, but it corrupts the base pointer of a base-relative one.

In short: match M68K_BASEREL to the consuming program, and if that program is base-relative make sure its OS callbacks are __saveds.

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.

Select the C runtime with -DPPC_CRT=: newlib (default), clib2 or clib4. Anything other than newlib is passed to the compiler as -mcrt=<value>.

cmake -DPPC_CRT=clib4 ..

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.

Two further options apply to MorphOS:

• -DMORPHOS_CLIB= selects default (ixemul) or libnix (passed as -mclib=libnix). That is the same multilib -noixemul selects, and is the self-contained ABI – an ixemul build needs ixemul.library on the target.
• -DMORPHOS_BASEREL32=ON adds -mbaserel32.

cmake -DMORPHOS_CLIB=libnix ..
cmake -DMORPHOS_BASEREL32=ON ..

Building and packaging every variant

Two scripts in the amiga/ directory build the whole matrix across all three systems, so you do not have to drive the container by hand.

• amiga/build-amiga-sdk.sh [OS ...] — builds every variant and installs libjson-c.a plus the json-c/ headers into an SDK tree. The location comes from $AMIGA_SDK (default /opt/amiga); either a <target>/ or an AmigaSDK-gcc-style amigaos3/sdk/<target>/ layout is accepted, and an OS whose subdirectory is absent is skipped rather than failing.
• amiga/package-amiga.sh [OS ...] — builds the same matrix into a staging tree and produces dist/json-c.lha for distribution.

Both default to AmigaOS3 AmigaOS4 MorphOS, and both take the variant list from the same table in amiga/build-amiga-sdk.sh, so they cannot disagree. 22 libraries are built in total: 15 for AmigaOS 3 (newlib, libnix and clib2, each across the five multilib slots), 3 for AmigaOS 4 (newlib, clib2, clib4) and 4 for MorphOS (ixemul and native, each with and without -mbaserel32).

Which of them actually link is a separate question from whether they build, and was checked by compiling a program against each one:

target	links
AmigaOS 3, libnix	all 5 slots
AmigaOS 3, clib2	lib/libm020/ and lib/libb32/libm020/ only
AmigaOS 3, newlib	none – the m68k newlib tree has no gettimeofday()
AmigaOS 4	all 3 (newlib, clib2, clib4)
MorphOS	all 4 slots

The clib2 failures are inside clib2 itself: its 68000 multilib provides no strtoll/strtoull, and 16-bit -fbaserel overflows with truncated to fit: DREL16 once json-c’s data is added – -fbaserel32 has no 64 KB limit. Link with -ljson-c -lm after your own objects, and add -lunix for clib2.

The archive is a single json-c/ directory at the root, with the readme and one directory per system, each following the compiler’s own layout so it can be copied straight over an SDK:

json-c/json-c.readme
json-c/AmigaOS3/{include,lib,...}/     plus libnix/ and clib2/ subtrees
json-c/AmigaOS4/{include,lib,lib/clib2,lib/clib4}/
json-c/MorphOS/{include,lib,lib/libb32,lib/libnix}/

Note that m68k and PPC lay their C runtimes out differently, and the variant table follows each. On m68k a runtime is a separate tree (libnix/, clib2/), each with its own include/ and lib/. On PPC a runtime is a multilib slot beneath a single shared lib/: -mcrt=clib4 searches <target>/lib/clib4/ and takes headers from <target>/include/. This was read off -print-search-dirs rather than assumed, because installing to the wrong one produces a library the compiler silently never finds.

Linking to libjson-c

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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

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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.