cJSON

by DaveGamble

DaveGamble / cJSON

Ultralightweight JSON parser in ANSI C

5.1K Stars 2.0K Forks Last release: about 2 months ago (v1.7.14) MIT License 1.1K Commits 44 Releases

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cJSON

Ultralightweight JSON parser in ANSI C.

Table of contents

License

MIT License

Copyright (c) 2009-2017 Dave Gamble and cJSON contributors

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Usage

Welcome to cJSON.

cJSON aims to be the dumbest possible parser that you can get your job done with. It's a single file of C, and a single header file.

JSON is described best here: http://www.json.org/ It's like XML, but fat-free. You use it to move data around, store things, or just generally represent your program's state.

As a library, cJSON exists to take away as much legwork as it can, but not get in your way. As a point of pragmatism (i.e. ignoring the truth), I'm going to say that you can use it in one of two modes: Auto and Manual. Let's have a quick run-through.

I lifted some JSON from this page: http://www.json.org/fatfree.html That page inspired me to write cJSON, which is a parser that tries to share the same philosophy as JSON itself. Simple, dumb, out of the way.

Building

There are several ways to incorporate cJSON into your project.

copying the source

Because the entire library is only one C file and one header file, you can just copy

cJSON.h
and
cJSON.c
to your projects source and start using it.

cJSON is written in ANSI C (C89) in order to support as many platforms and compilers as possible.

CMake

With CMake, cJSON supports a full blown build system. This way you get the most features. CMake with an equal or higher version than 2.8.5 is supported. With CMake it is recommended to do an out of tree build, meaning the compiled files are put in a directory separate from the source files. So in order to build cJSON with CMake on a Unix platform, make a

build
directory and run CMake inside it.
mkdir build
cd build
cmake ..

This will create a Makefile and a bunch of other files. You can then compile it:

make

And install it with

make install
if you want. By default it installs the headers
/usr/local/include/cjson
and the libraries to
/usr/local/lib
. It also installs files for pkg-config to make it easier to detect and use an existing installation of CMake. And it installs CMake config files, that can be used by other CMake based projects to discover the library.

You can change the build process with a list of different options that you can pass to CMake. Turn them on with

On
and off with
Off
:
  • -DENABLE_CJSON_TEST=On
    : Enable building the tests. (on by default)
  • -DENABLE_CJSON_UTILS=On
    : Enable building cJSON_Utils. (off by default)
  • -DENABLE_TARGET_EXPORT=On
    : Enable the export of CMake targets. Turn off if it makes problems. (on by default)
  • -DENABLE_CUSTOM_COMPILER_FLAGS=On
    : Enable custom compiler flags (currently for Clang, GCC and MSVC). Turn off if it makes problems. (on by default)
  • -DENABLE_VALGRIND=On
    : Run tests with valgrind. (off by default)
  • -DENABLE_SANITIZERS=On
    : Compile cJSON with AddressSanitizer and UndefinedBehaviorSanitizer enabled (if possible). (off by default)
  • -DENABLE_SAFE_STACK
    : Enable the SafeStack instrumentation pass. Currently only works with the Clang compiler. (off by default)
  • -DBUILD_SHARED_LIBS=On
    : Build the shared libraries. (on by default)
  • -DBUILD_SHARED_AND_STATIC_LIBS=On
    : Build both shared and static libraries. (off by default)
  • -DCMAKE_INSTALL_PREFIX=/usr
    : Set a prefix for the installation.
  • -DENABLE_LOCALES=On
    : Enable the usage of localeconv method. ( on by default )
  • -DCJSON_OVERRIDE_BUILD_SHARED_LIBS=On
    : Enable overriding the value of
    BUILD_SHARED_LIBS
    with
    -DCJSON_BUILD_SHARED_LIBS
    .

If you are packaging cJSON for a distribution of Linux, you would probably take these steps for example:

mkdir build
cd build
cmake .. -DENABLE_CJSON_UTILS=On -DENABLE_CJSON_TEST=Off -DCMAKE_INSTALL_PREFIX=/usr
make
make DESTDIR=$pkgdir install

On Windows CMake is usually used to create a Visual Studio solution file by running it inside the Developer Command Prompt for Visual Studio, for exact steps follow the official documentation from CMake and Microsoft and use the online search engine of your choice. The descriptions of the the options above still generally apply, although not all of them work on Windows.

Makefile

NOTE: This Method is deprecated. Use CMake if at all possible. Makefile support is limited to fixing bugs.

If you don't have CMake available, but still have GNU make. You can use the makefile to build cJSON:

Run this command in the directory with the source code and it will automatically compile static and shared libraries and a little test program (not the full test suite).

make all

If you want, you can install the compiled library to your system using

make install
. By default it will install the headers in
/usr/local/include/cjson
and the libraries in
/usr/local/lib
. But you can change this behavior by setting the
PREFIX
and
DESTDIR
variables:
make PREFIX=/usr DESTDIR=temp install
. And uninstall them with:
make PREFIX=/usr DESTDIR=temp uninstall
.

Vcpkg

You can download and install cJSON using the vcpkg dependency manager:

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

The cJSON 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.

Including cJSON

If you installed it via CMake or the Makefile, you can include cJSON like this:

#include 

Data Structure

cJSON represents JSON data using the

cJSON
struct data type:
/* The cJSON structure: */
typedef struct cJSON
{
    struct cJSON *next;
    struct cJSON *prev;
    struct cJSON *child;
    int type;
    char *valuestring;
    /* writing to valueint is DEPRECATED, use cJSON_SetNumberValue instead */
    int valueint;
    double valuedouble;
    char *string;
} cJSON;

An item of this type represents a JSON value. The type is stored in

type
as a bit-flag (this means that you cannot find out the type by just comparing the value of
type
).

To check the type of an item, use the corresponding

cJSON_Is...
function. It does a
NULL
check followed by a type check and returns a boolean value if the item is of this type.

The type can be one of the following:

  • cJSON_Invalid
    (check with
    cJSON_IsInvalid
    ): Represents an invalid item that doesn't contain any value. You automatically have this type if you set the item to all zero bytes.
  • cJSON_False
    (check with
    cJSON_IsFalse
    ): Represents a
    false
    boolean value. You can also check for boolean values in general with
    cJSON_IsBool
    .
  • cJSON_True
    (check with
    cJSON_IsTrue
    ): Represents a
    true
    boolean value. You can also check for boolean values in general with
    cJSON_IsBool
    .
  • cJSON_NULL
    (check with
    cJSON_IsNull
    ): Represents a
    null
    value.
  • cJSON_Number
    (check with
    cJSON_IsNumber
    ): Represents a number value. The value is stored as a double in
    valuedouble
    and also in
    valueint
    . If the number is outside of the range of an integer,
    INT_MAX
    or
    INT_MIN
    are used for
    valueint
    .
  • cJSON_String
    (check with
    cJSON_IsString
    ): Represents a string value. It is stored in the form of a zero terminated string in
    valuestring
    .
  • cJSON_Array
    (check with
    cJSON_IsArray
    ): Represent an array value. This is implemented by pointing
    child
    to a linked list of
    cJSON
    items that represent the values in the array. The elements are linked together using
    next
    and
    prev
    , where the first element has
    prev.next == NULL
    and the last element
    next == NULL
    .
  • cJSON_Object
    (check with
    cJSON_IsObject
    ): Represents an object value. Objects are stored same way as an array, the only difference is that the items in the object store their keys in
    string
    .
  • cJSON_Raw
    (check with
    cJSON_IsRaw
    ): Represents any kind of JSON that is stored as a zero terminated array of characters in
    valuestring
    . This can be used, for example, to avoid printing the same static JSON over and over again to save performance. cJSON will never create this type when parsing. Also note that cJSON doesn't check if it is valid JSON.

Additionally there are the following two flags:

  • cJSON_IsReference
    : Specifies that the item that
    child
    points to and/or
    valuestring
    is not owned by this item, it is only a reference. So
    cJSON_Delete
    and other functions will only deallocate this item, not its
    child
    /
    valuestring
    .
  • cJSON_StringIsConst
    : This means that
    string
    points to a constant string. This means that
    cJSON_Delete
    and other functions will not try to deallocate
    string
    .

Working with the data structure

For every value type there is a

cJSON_Create...
function that can be used to create an item of that type. All of these will allocate a
cJSON
struct that can later be deleted with
cJSON_Delete
. Note that you have to delete them at some point, otherwise you will get a memory leak.
Important: If you have added an item to an array or an object already, you mustn't delete it with
cJSON_Delete
. Adding it to an array or object transfers its ownership so that when that array or object is deleted, it gets deleted as well. You also could use
cJSON_SetValuestring
to change a
cJSON_String
's
valuestring
, and you needn't to free the previous
valuestring
manually.

Basic types

  • null is created with
    cJSON_CreateNull
  • booleans are created with
    cJSON_CreateTrue
    ,
    cJSON_CreateFalse
    or
    cJSON_CreateBool
  • numbers are created with
    cJSON_CreateNumber
    . This will set both
    valuedouble
    and
    valueint
    . If the number is outside of the range of an integer,
    INT_MAX
    or
    INT_MIN
    are used for
    valueint
  • strings are created with
    cJSON_CreateString
    (copies the string) or with
    cJSON_CreateStringReference
    (directly points to the string. This means that
    valuestring
    won't be deleted by
    cJSON_Delete
    and you are responsible for its lifetime, useful for constants)

Arrays

You can create an empty array with

cJSON_CreateArray
.
cJSON_CreateArrayReference
can be used to create an array that doesn't "own" its content, so its content doesn't get deleted by
cJSON_Delete
.

To add items to an array, use

cJSON_AddItemToArray
to append items to the end. Using
cJSON_AddItemReferenceToArray
an element can be added as a reference to another item, array or string. This means that
cJSON_Delete
will not delete that items
child
or
valuestring
properties, so no double frees are occurring if they are already used elsewhere. To insert items in the middle, use
cJSON_InsertItemInArray
. It will insert an item at the given 0 based index and shift all the existing items to the right.

If you want to take an item out of an array at a given index and continue using it, use

cJSON_DetachItemFromArray
, it will return the detached item, so be sure to assign it to a pointer, otherwise you will have a memory leak.

Deleting items is done with

cJSON_DeleteItemFromArray
. It works like
cJSON_DetachItemFromArray
, but deletes the detached item via
cJSON_Delete
.

You can also replace an item in an array in place. Either with

cJSON_ReplaceItemInArray
using an index or with
cJSON_ReplaceItemViaPointer
given a pointer to an element.
cJSON_ReplaceItemViaPointer
will return
0
if it fails. What this does internally is to detach the old item, delete it and insert the new item in its place.

To get the size of an array, use

cJSON_GetArraySize
. Use
cJSON_GetArrayItem
to get an element at a given index.

Because an array is stored as a linked list, iterating it via index is inefficient (

O(n²)
), so you can iterate over an array using the
cJSON_ArrayForEach
macro in
O(n)
time complexity.

Objects

You can create an empty object with

cJSON_CreateObject
.
cJSON_CreateObjectReference
can be used to create an object that doesn't "own" its content, so its content doesn't get deleted by
cJSON_Delete
.

To add items to an object, use

cJSON_AddItemToObject
. Use
cJSON_AddItemToObjectCS
to add an item to an object with a name that is a constant or reference (key of the item,
string
in the
cJSON
struct), so that it doesn't get freed by
cJSON_Delete
. Using
cJSON_AddItemReferenceToArray
an element can be added as a reference to another object, array or string. This means that
cJSON_Delete
will not delete that items
child
or
valuestring
properties, so no double frees are occurring if they are already used elsewhere.

If you want to take an item out of an object, use

cJSON_DetachItemFromObjectCaseSensitive
, it will return the detached item, so be sure to assign it to a pointer, otherwise you will have a memory leak.

Deleting items is done with

cJSON_DeleteItemFromObjectCaseSensitive
. It works like
cJSON_DetachItemFromObjectCaseSensitive
followed by
cJSON_Delete
.

You can also replace an item in an object in place. Either with

cJSON_ReplaceItemInObjectCaseSensitive
using a key or with
cJSON_ReplaceItemViaPointer
given a pointer to an element.
cJSON_ReplaceItemViaPointer
will return
0
if it fails. What this does internally is to detach the old item, delete it and insert the new item in its place.

To get the size of an object, you can use

cJSON_GetArraySize
, this works because internally objects are stored as arrays.

If you want to access an item in an object, use

cJSON_GetObjectItemCaseSensitive
.

To iterate over an object, you can use the

cJSON_ArrayForEach
macro the same way as for arrays.

cJSON also provides convenient helper functions for quickly creating a new item and adding it to an object, like

cJSON_AddNullToObject
. They return a pointer to the new item or
NULL
if they failed.

Parsing JSON

Given some JSON in a zero terminated string, you can parse it with

cJSON_Parse
.
cJSON *json = cJSON_Parse(string);

Given some JSON in a string (whether zero terminated or not), you can parse it with

cJSON_ParseWithLength
.
cJSON *json = cJSON_ParseWithLength(string, buffer_length);

It will parse the JSON and allocate a tree of

cJSON
items that represents it. Once it returns, you are fully responsible for deallocating it after use with
cJSON_Delete
.

The allocator used by

cJSON_Parse
is
malloc
and
free
by default but can be changed (globally) with
cJSON_InitHooks
.

If an error occurs a pointer to the position of the error in the input string can be accessed using

cJSON_GetErrorPtr
. Note though that this can produce race conditions in multithreading scenarios, in that case it is better to use
cJSON_ParseWithOpts
with
return_parse_end
. By default, characters in the input string that follow the parsed JSON will not be considered as an error.

If you want more options, use

cJSON_ParseWithOpts(const char *value, const char **return_parse_end, cJSON_bool require_null_terminated)
.
return_parse_end
returns a pointer to the end of the JSON in the input string or the position that an error occurs at (thereby replacing
cJSON_GetErrorPtr
in a thread safe way).
require_null_terminated
, if set to
1
will make it an error if the input string contains data after the JSON.

If you want more options giving buffer length, use

cJSON_ParseWithLengthOpts(const char *value, size_t buffer_length, const char **return_parse_end, cJSON_bool require_null_terminated)
.

Printing JSON

Given a tree of

cJSON
items, you can print them as a string using
cJSON_Print
.
char *string = cJSON_Print(json);

It will allocate a string and print a JSON representation of the tree into it. Once it returns, you are fully responsible for deallocating it after use with your allocator. (usually

free
, depends on what has been set with
cJSON_InitHooks
).

cJSON_Print
will print with whitespace for formatting. If you want to print without formatting, use
cJSON_PrintUnformatted
.

If you have a rough idea of how big your resulting string will be, you can use

cJSON_PrintBuffered(const cJSON *item, int prebuffer, cJSON_bool fmt)
.
fmt
is a boolean to turn formatting with whitespace on and off.
prebuffer
specifies the first buffer size to use for printing.
cJSON_Print
currently uses 256 bytes for its first buffer size. Once printing runs out of space, a new buffer is allocated and the old gets copied over before printing is continued.

These dynamic buffer allocations can be completely avoided by using

cJSON_PrintPreallocated(cJSON *item, char *buffer, const int length, const cJSON_bool format)
. It takes a buffer to a pointer to print to and its length. If the length is reached, printing will fail and it returns
0
. In case of success,
1
is returned. Note that you should provide 5 bytes more than is actually needed, because cJSON is not 100% accurate in estimating if the provided memory is enough.

Example

In this example we want to build and parse the following JSON:

{
    "name": "Awesome 4K",
    "resolutions": [
        {
            "width": 1280,
            "height": 720
        },
        {
            "width": 1920,
            "height": 1080
        },
        {
            "width": 3840,
            "height": 2160
        }
    ]
}

Printing

Let's build the above JSON and print it to a string:

//create a monitor with a list of supported resolutions
//NOTE: Returns a heap allocated string, you are required to free it after use.
char *create_monitor(void)
{
    const unsigned int resolution_numbers[3][2] = {
        {1280, 720},
        {1920, 1080},
        {3840, 2160}
    };
    char *string = NULL;
    cJSON *name = NULL;
    cJSON *resolutions = NULL;
    cJSON *resolution = NULL;
    cJSON *width = NULL;
    cJSON *height = NULL;
    size_t index = 0;

cJSON *monitor = cJSON_CreateObject();
if (monitor == NULL)
{
    goto end;
}

name = cJSON_CreateString("Awesome 4K");
if (name == NULL)
{
    goto end;
}
/* after creation was successful, immediately add it to the monitor,
 * thereby transferring ownership of the pointer to it */
cJSON_AddItemToObject(monitor, "name", name);

resolutions = cJSON_CreateArray();
if (resolutions == NULL)
{
    goto end;
}
cJSON_AddItemToObject(monitor, "resolutions", resolutions);

for (index = 0; index < (sizeof(resolution_numbers) / (2 * sizeof(int))); ++index)
{
    resolution = cJSON_CreateObject();
    if (resolution == NULL)
    {
        goto end;
    }
    cJSON_AddItemToArray(resolutions, resolution);

    width = cJSON_CreateNumber(resolution_numbers[index][0]);
    if (width == NULL)
    {
        goto end;
    }
    cJSON_AddItemToObject(resolution, "width", width);

    height = cJSON_CreateNumber(resolution_numbers[index][1]);
    if (height == NULL)
    {
        goto end;
    }
    cJSON_AddItemToObject(resolution, "height", height);
}

string = cJSON_Print(monitor);
if (string == NULL)
{
    fprintf(stderr, "Failed to print monitor.\n");
}

end: cJSON_Delete(monitor); return string; }

Alternatively we can use the

cJSON_Add...ToObject
helper functions to make our lifes a little easier:
//NOTE: Returns a heap allocated string, you are required to free it after use.
char *create_monitor_with_helpers(void)
{
    const unsigned int resolution_numbers[3][2] = {
        {1280, 720},
        {1920, 1080},
        {3840, 2160}
    };
    char *string = NULL;
    cJSON *resolutions = NULL;
    size_t index = 0;

cJSON *monitor = cJSON_CreateObject();

if (cJSON_AddStringToObject(monitor, "name", "Awesome 4K") == NULL)
{
    goto end;
}

resolutions = cJSON_AddArrayToObject(monitor, "resolutions");
if (resolutions == NULL)
{
    goto end;
}

for (index = 0; index < (sizeof(resolution_numbers) / (2 * sizeof(int))); ++index)
{
    cJSON *resolution = cJSON_CreateObject();

    if (cJSON_AddNumberToObject(resolution, "width", resolution_numbers[index][0]) == NULL)
    {
        goto end;
    }

    if (cJSON_AddNumberToObject(resolution, "height", resolution_numbers[index][1]) == NULL)
    {
        goto end;
    }

    cJSON_AddItemToArray(resolutions, resolution);
}

string = cJSON_Print(monitor);
if (string == NULL)
{
    fprintf(stderr, "Failed to print monitor.\n");
}

end: cJSON_Delete(monitor); return string; }

Parsing

In this example we will parse a JSON in the above format and check if the monitor supports a Full HD resolution while printing some diagnostic output:

/* return 1 if the monitor supports full hd, 0 otherwise */
int supports_full_hd(const char * const monitor)
{
    const cJSON *resolution = NULL;
    const cJSON *resolutions = NULL;
    const cJSON *name = NULL;
    int status = 0;
    cJSON *monitor_json = cJSON_Parse(monitor);
    if (monitor_json == NULL)
    {
        const char *error_ptr = cJSON_GetErrorPtr();
        if (error_ptr != NULL)
        {
            fprintf(stderr, "Error before: %s\n", error_ptr);
        }
        status = 0;
        goto end;
    }

name = cJSON_GetObjectItemCaseSensitive(monitor_json, "name");
if (cJSON_IsString(name) && (name->valuestring != NULL))
{
    printf("Checking monitor \"%s\"\n", name->valuestring);
}

resolutions = cJSON_GetObjectItemCaseSensitive(monitor_json, "resolutions");
cJSON_ArrayForEach(resolution, resolutions)
{
    cJSON *width = cJSON_GetObjectItemCaseSensitive(resolution, "width");
    cJSON *height = cJSON_GetObjectItemCaseSensitive(resolution, "height");

    if (!cJSON_IsNumber(width) || !cJSON_IsNumber(height))
    {
        status = 0;
        goto end;
    }

    if ((width->valuedouble == 1920) && (height->valuedouble == 1080))
    {
        status = 1;
        goto end;
    }
}

end: cJSON_Delete(monitor_json); return status; }

Note that there are no NULL checks except for the result of

cJSON_Parse
because
cJSON_GetObjectItemCaseSensitive
checks for
NULL
inputs already, so a
NULL
value is just propagated and
cJSON_IsNumber
and
cJSON_IsString
return
0
if the input is
NULL
.

Caveats

Zero Character

cJSON doesn't support strings that contain the zero character

'\0'
or
\u0000
. This is impossible with the current API because strings are zero terminated.

Character Encoding

cJSON only supports UTF-8 encoded input. In most cases it doesn't reject invalid UTF-8 as input though, it just propagates it through as is. As long as the input doesn't contain invalid UTF-8, the output will always be valid UTF-8.

C Standard

cJSON is written in ANSI C (or C89, C90). If your compiler or C library doesn't follow this standard, correct behavior is not guaranteed.

NOTE: ANSI C is not C++ therefore it shouldn't be compiled with a C++ compiler. You can compile it with a C compiler and link it with your C++ code however. Although compiling with a C++ compiler might work, correct behavior is not guaranteed.

Floating Point Numbers

cJSON does not officially support any

double
implementations other than IEEE754 double precision floating point numbers. It might still work with other implementations but bugs with these will be considered invalid.

The maximum length of a floating point literal that cJSON supports is currently 63 characters.

Deep Nesting Of Arrays And Objects

cJSON doesn't support arrays and objects that are nested too deeply because this would result in a stack overflow. To prevent this cJSON limits the depth to

CJSON_NESTING_LIMIT
which is 1000 by default but can be changed at compile time.

Thread Safety

In general cJSON is not thread safe.

However it is thread safe under the following conditions:

  • cJSON_GetErrorPtr
    is never used (the
    return_parse_end
    parameter of
    cJSON_ParseWithOpts
    can be used instead)
  • cJSON_InitHooks
    is only ever called before using cJSON in any threads.
  • setlocale
    is never called before all calls to cJSON functions have returned.

Case Sensitivity

When cJSON was originally created, it didn't follow the JSON standard and didn't make a distinction between uppercase and lowercase letters. If you want the correct, standard compliant, behavior, you need to use the

CaseSensitive
functions where available.

Duplicate Object Members

cJSON supports parsing and printing JSON that contains objects that have multiple members with the same name.

cJSON_GetObjectItemCaseSensitive
however will always only return the first one.

Enjoy cJSON!

  • Dave Gamble (original author)
  • Max Bruckner and Alan Wang (current maintainer)
  • and the other cJSON contributors

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