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Ultralightweight JSON parser in ANSI C.

• License
• Usage
  • Welcome to cJSON
  • Building
    • Copying the source
    • CMake
    • Makefile
    • Meson
    • Vcpkg
  • Including cJSON
  • Data Structure
  • Working with the data structure
    • Basic types
    • Arrays
    • Objects
  • Parsing JSON
  • Printing JSON
  • Example
    • Printing
    • Parsing
  • Caveats
    • Zero Character
    • Character Encoding
    • C Standard
    • Floating Point Numbers
    • Deep Nesting Of Arrays And Objects
    • Thread Safety
    • Case Sensitivity
    • Duplicate Object Members
  • Enjoy cJSON!

MIT License

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

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

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

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS ORIMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THEAUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHERLIABILITY, 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 INTHE SOFTWARE.

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 justgenerally 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 itin 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.htmlThat page inspired me to write cJSON, which is a parser that tries to share the samephilosophy as JSON itself. Simple, dumb, out of the way.

There are several ways to incorporate cJSON into your project.

Because the entire library is only one C file and one header file, you can just copycJSON.h andcJSON.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.

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 abuild directory and run CMake inside it.

mkdir buildcd buildcmake ..

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

make

And install it withmake 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 withOn and off withOff:

• -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 withvalgrind. (off by default)
• -DENABLE_SANITIZERS=On: Compile cJSON withAddressSanitizer andUndefinedBehaviorSanitizer enabled (if possible). (off by default)
• -DENABLE_SAFE_STACK: Enable theSafeStack 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 ofBUILD_SHARED_LIBS with-DCJSON_BUILD_SHARED_LIBS.
• -DENABLE_CJSON_VERSION_SO: Enable cJSON so version. ( on by default )

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

mkdir buildcd buildcmake .. -DENABLE_CJSON_UTILS=On -DENABLE_CJSON_TEST=Off -DCMAKE_INSTALL_PREFIX=/usrmakemake 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.

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 usingmake 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 thePREFIX andDESTDIR variables:make PREFIX=/usr DESTDIR=temp install. And uninstall them with:make PREFIX=/usr DESTDIR=temp uninstall.

To make cjson work in a project using meson, the libcjson dependency has to be included:

project('c-json-example','c')cjson=dependency('libcjson')example=executable('example','example.c',dependencies: [cjson],)

You can download and install cJSON using thevcpkg dependency manager:

git clone https://github.com/Microsoft/vcpkg.gitcd vcpkg./bootstrap-vcpkg.sh./vcpkg integrate installvcpkg 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, pleasecreate an issue or pull request on the vcpkg repository.

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

#include<cjson/cJSON.h>

cJSON represents JSON data using thecJSON struct data type:

/* The cJSON structure: */typedefstructcJSON{structcJSON*next;structcJSON*prev;structcJSON*child;inttype;char*valuestring;/* writing to valueint is DEPRECATED, use cJSON_SetNumberValue instead */intvalueint;doublevaluedouble;char*string;}cJSON;

An item of this type represents a JSON value. The type is stored intype as a bit-flag (this means that you cannot find out the type by just comparing the value oftype).

To check the type of an item, use the correspondingcJSON_Is... function. It does aNULL 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 withcJSON_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 withcJSON_IsFalse): Represents afalse boolean value. You can also check for boolean values in general withcJSON_IsBool.
• cJSON_True (check withcJSON_IsTrue): Represents atrue boolean value. You can also check for boolean values in general withcJSON_IsBool.
• cJSON_NULL (check withcJSON_IsNull): Represents anull value.
• cJSON_Number (check withcJSON_IsNumber): Represents a number value. The value is stored as a double invaluedouble and also invalueint. If the number is outside of the range of an integer,INT_MAX orINT_MIN are used forvalueint.
• cJSON_String (check withcJSON_IsString): Represents a string value. It is stored in the form of a zero terminated string invaluestring.
• cJSON_Array (check withcJSON_IsArray): Represent an array value. This is implemented by pointingchild to a linked list ofcJSON items that represent the values in the array. The elements are linked together usingnext andprev, where the first element hasprev.next == NULL and the last elementnext == NULL.
• cJSON_Object (check withcJSON_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 instring.
• cJSON_Raw (check withcJSON_IsRaw): Represents any kind of JSON that is stored as a zero terminated array of characters invaluestring. 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 thatchild points to and/orvaluestring is not owned by this item, it is only a reference. SocJSON_Delete and other functions will only deallocate this item, not itschild/valuestring.
• cJSON_StringIsConst: This means thatstring points to a constant string. This means thatcJSON_Delete and other functions will not try to deallocatestring.

For every value type there is acJSON_Create... function that can be used to create an item of that type.All of these will allocate acJSON struct that can later be deleted withcJSON_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, youmustn't delete it withcJSON_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 usecJSON_SetValuestring to change acJSON_String'svaluestring, and you needn't to free the previousvaluestring manually.

• null is created withcJSON_CreateNull
• booleans are created withcJSON_CreateTrue,cJSON_CreateFalse orcJSON_CreateBool
• numbers are created withcJSON_CreateNumber. This will set bothvaluedouble andvalueint. If the number is outside of the range of an integer,INT_MAX orINT_MIN are used forvalueint
• strings are created withcJSON_CreateString (copies the string) or withcJSON_CreateStringReference (directly points to the string. This means thatvaluestring won't be deleted bycJSON_Delete and you are responsible for its lifetime, useful for constants)

You can create an empty array withcJSON_CreateArray.cJSON_CreateArrayReference can be used to create an array that doesn't "own" its content, so its content doesn't get deleted bycJSON_Delete.

To add items to an array, usecJSON_AddItemToArray to append items to the end.UsingcJSON_AddItemReferenceToArray an element can be added as a reference to another item, array or string. This means thatcJSON_Delete will not delete that itemschild orvaluestring properties, so no double frees are occurring if they are already used elsewhere.To insert items in the middle, usecJSON_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, usecJSON_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 withcJSON_DeleteItemFromArray. It works likecJSON_DetachItemFromArray, but deletes the detached item viacJSON_Delete.

You can also replace an item in an array in place. Either withcJSON_ReplaceItemInArray using an index or withcJSON_ReplaceItemViaPointer given a pointer to an element.cJSON_ReplaceItemViaPointer will return0 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, usecJSON_GetArraySize. UsecJSON_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 thecJSON_ArrayForEach macro inO(n) time complexity.

You can create an empty object withcJSON_CreateObject.cJSON_CreateObjectReference can be used to create an object that doesn't "own" its content, so its content doesn't get deleted bycJSON_Delete.

To add items to an object, usecJSON_AddItemToObject. UsecJSON_AddItemToObjectCS to add an item to an object with a name that is a constant or reference (key of the item,string in thecJSON struct), so that it doesn't get freed bycJSON_Delete.UsingcJSON_AddItemReferenceToArray an element can be added as a reference to another object, array or string. This means thatcJSON_Delete will not delete that itemschild orvaluestring properties, so no double frees are occurring if they are already used elsewhere.

If you want to take an item out of an object, usecJSON_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 withcJSON_DeleteItemFromObjectCaseSensitive. It works likecJSON_DetachItemFromObjectCaseSensitive followed bycJSON_Delete.

You can also replace an item in an object in place. Either withcJSON_ReplaceItemInObjectCaseSensitive using a key or withcJSON_ReplaceItemViaPointer given a pointer to an element.cJSON_ReplaceItemViaPointer will return0 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 usecJSON_GetArraySize, this works because internally objects are stored as arrays.

If you want to access an item in an object, usecJSON_GetObjectItemCaseSensitive.

To iterate over an object, you can use thecJSON_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, likecJSON_AddNullToObject. They return a pointer to the new item orNULL if they failed.

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

cJSON*json=cJSON_Parse(string);

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

cJSON*json=cJSON_ParseWithLength(string,buffer_length);

It will parse the JSON and allocate a tree ofcJSON items that represents it. Once it returns, you are fully responsible for deallocating it after use withcJSON_Delete.

The allocator used bycJSON_Parse ismalloc andfree by default but can be changed (globally) withcJSON_InitHooks.

If an error occurs a pointer to the position of the error in the input string can be accessed usingcJSON_GetErrorPtr. Note though that this can produce race conditions in multithreading scenarios, in that case it is better to usecJSON_ParseWithOpts withreturn_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, usecJSON_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 replacingcJSON_GetErrorPtr in a thread safe way).require_null_terminated, if set to1 will make it an error if the input string contains data after the JSON.

If you want more options giving buffer length, usecJSON_ParseWithLengthOpts(const char *value, size_t buffer_length, const char **return_parse_end, cJSON_bool require_null_terminated).

Given a tree ofcJSON items, you can print them as a string usingcJSON_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. (usuallyfree, depends on what has been set withcJSON_InitHooks).

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

If you have a rough idea of how big your resulting string will be, you can usecJSON_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 usingcJSON_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 returns0. 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.

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        }    ]}

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){constunsignedintresolution_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_tindex=0;cJSON*monitor=cJSON_CreateObject();if (monitor==NULL)    {        gotoend;    }name=cJSON_CreateString("Awesome 4K");if (name==NULL)    {        gotoend;    }/* 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)    {        gotoend;    }cJSON_AddItemToObject(monitor,"resolutions",resolutions);for (index=0;index< (sizeof(resolution_numbers) / (2*sizeof(int)));++index)    {resolution=cJSON_CreateObject();if (resolution==NULL)        {            gotoend;        }cJSON_AddItemToArray(resolutions,resolution);width=cJSON_CreateNumber(resolution_numbers[index][0]);if (width==NULL)        {            gotoend;        }cJSON_AddItemToObject(resolution,"width",width);height=cJSON_CreateNumber(resolution_numbers[index][1]);if (height==NULL)        {            gotoend;        }cJSON_AddItemToObject(resolution,"height",height);    }string=cJSON_Print(monitor);if (string==NULL)    {fprintf(stderr,"Failed to print monitor.\n");    }end:cJSON_Delete(monitor);returnstring;}

Alternatively we can use thecJSON_Add...ToObject helper functions to make our lives a little easier:

//NOTE: Returns a heap allocated string, you are required to free it after use.char*create_monitor_with_helpers(void){constunsignedintresolution_numbers[3][2]= {        {1280,720},        {1920,1080},        {3840,2160}    };char*string=NULL;cJSON*resolutions=NULL;size_tindex=0;cJSON*monitor=cJSON_CreateObject();if (cJSON_AddStringToObject(monitor,"name","Awesome 4K")==NULL)    {        gotoend;    }resolutions=cJSON_AddArrayToObject(monitor,"resolutions");if (resolutions==NULL)    {        gotoend;    }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)        {            gotoend;        }if (cJSON_AddNumberToObject(resolution,"height",resolution_numbers[index][1])==NULL)        {            gotoend;        }cJSON_AddItemToArray(resolutions,resolution);    }string=cJSON_Print(monitor);if (string==NULL)    {fprintf(stderr,"Failed to print monitor.\n");    }end:cJSON_Delete(monitor);returnstring;}

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 */intsupports_full_hd(constchar*constmonitor){constcJSON*resolution=NULL;constcJSON*resolutions=NULL;constcJSON*name=NULL;intstatus=0;cJSON*monitor_json=cJSON_Parse(monitor);if (monitor_json==NULL)    {constchar*error_ptr=cJSON_GetErrorPtr();if (error_ptr!=NULL)        {fprintf(stderr,"Error before: %s\n",error_ptr);        }status=0;        gotoend;    }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;            gotoend;        }if ((width->valuedouble==1920)&& (height->valuedouble==1080))        {status=1;            gotoend;        }    }end:cJSON_Delete(monitor_json);returnstatus;}

Note that there are no NULL checks except for the result ofcJSON_Parse becausecJSON_GetObjectItemCaseSensitive checks forNULL inputs already, so aNULL value is just propagated andcJSON_IsNumber andcJSON_IsString return0 if the input isNULL.

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.

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.

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.

cJSON does not officially support anydouble 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.

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

In general cJSON isnot thread safe.

However it is thread safe under the following conditions:

• cJSON_GetErrorPtr is never used (thereturn_parse_end parameter ofcJSON_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.

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 theCaseSensitive functions where available.

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.

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