Installation and use of cJSON Library

I Introduction to CJSON

1.1 JSON(JavaScript Object Notation) is a lightweight text data exchange format, which is easy to read. At the same time, it is also easy for machine analysis and generation. Although JSON is a subset of Javascript, it is a language independent text format and adopts some habits similar to the C language family.
1.2 JSON structure

 typedef struct cJSON
    struct cJSON *next;
    struct cJSON *prev;
    struct cJSON *child;
    int type;
    char *valuestring;
    int valueint;
    double valuedouble;
    char *string;
} cJSON;
next and prev : Cjson As a ring with two-way tables, the structure can be connected through next and prev Pointer for table traversal
child: Can be cJSON_Array,cJSON_Object Type data
type: Type of current item
valuestring: Content storage, when the type is cJSON_String and cJSON_Raw
valueint: Content storage, integer, can be cJSON_False,cJSON_True data
valuedouble: Content store, floating point, when the type is cJSON_Number
string: Key name

II Common library functions of CJSON Library


JSON *cJSON_Parse(const char *value);

Function: serialize a JSON packet according to the structure of the cJSON structure, and open up a memory in the heap to store the cJSON structure
NULL: a pointer to the success of the memory block, and a value returned from the failure

2.2. cJSON_Print

char  *cJSON_Print(cJSON *item);

Function: parse cJSON data into JSON string, and open up a memory space of char * in the heap to store JSON string

Return value: a char * pointer is returned successfully, which points to the JSON string in the heap, and NULL is returned in case of failure

2.3 cJSON_PrintUnformatted
And cjson_ The print function is similar. But it is unformatted output.

2.4 cJSON_CreateObject

CJSON_PUBLIC(cJSON *) cJSON_CreateObject(void);

After creating a JSON object, you can add data items such as string or int to this object. Using this function will open up a space in memory through malloc() function, which needs to be released manually after use.
Other creation

CJSON_PUBLIC(cJSON *) cJSON_CreateNull(void);
CJSON_PUBLIC(cJSON *) cJSON_CreateTrue(void);
CJSON_PUBLIC(cJSON *) cJSON_CreateFalse(void);
CJSON_PUBLIC(cJSON *) cJSON_CreateBool(cJSON_bool boolean);
CJSON_PUBLIC(cJSON *) cJSON_CreateNumber(double num);
CJSON_PUBLIC(cJSON *) cJSON_CreateString(const char *string);
/* raw json */
CJSON_PUBLIC(cJSON *) cJSON_CreateRaw(const char *raw);
CJSON_PUBLIC(cJSON *) cJSON_CreateArray(void);
CJSON_PUBLIC(cJSON *) cJSON_CreateObject(void);

/* Create a string where valuestring references a string so
 * it will not be freed by cJSON_Delete */
CJSON_PUBLIC(cJSON *) cJSON_CreateStringReference(const char *string);
/* Create an object/array that only references it's elements so
 * they will not be freed by cJSON_Delete */
CJSON_PUBLIC(cJSON *) cJSON_CreateObjectReference(const cJSON *child);
CJSON_PUBLIC(cJSON *) cJSON_CreateArrayReference(const cJSON *child);

/* These utilities create an Array of count items.
 * The parameter count cannot be greater than the number of elements in the number array, otherwise array access will be out of bounds.*/
CJSON_PUBLIC(cJSON *) cJSON_CreateIntArray(const int *numbers, int count);
CJSON_PUBLIC(cJSON *) cJSON_CreateFloatArray(const float *numbers, int count);
CJSON_PUBLIC(cJSON *) cJSON_CreateDoubleArray(const double *numbers, int count);
CJSON_PUBLIC(cJSON *) cJSON_CreateStringArray(const char *const *strings, int count);

add to
2.5 cJSON_AddItemToObject

CJSON_PUBLIC(cJSON_bool) cJSON_AddItemToObject(cJSON *object, const char *string, cJSON *item);

Function: add an object object item named string to an object object object

2.6 cJSON_AddItemToArray

CJSON_PUBLIC(cJSON_bool) cJSON_AddItemToArray(cJSON *array, cJSON *item);

Adding elements to an array
cJSON *array: indicates the array to be added
cJSON *item: array object to add
Other additions

/* Append item to the specified array/object. */
CJSON_PUBLIC(cJSON_bool) cJSON_AddItemToArray(cJSON *array, cJSON *item);
CJSON_PUBLIC(cJSON_bool) cJSON_AddItemToObject(cJSON *object, const char *string, cJSON *item);
/* Use this when string is definitely const (i.e. a literal, or as good as), and will definitely survive the cJSON object.
 * WARNING: When this function was used, make sure to always check that (item->type & cJSON_StringIsConst) is zero before
 * writing to `item->string` */
CJSON_PUBLIC(cJSON_bool) cJSON_AddItemToObjectCS(cJSON *object, const char *string, cJSON *item);
/* Append reference to item to the specified array/object. Use this when you want to add an existing cJSON to a new cJSON, but don't want to corrupt your existing cJSON. */
CJSON_PUBLIC(cJSON_bool) cJSON_AddItemReferenceToArray(cJSON *array, cJSON *item);
CJSON_PUBLIC(cJSON_bool) cJSON_AddItemReferenceToObject(cJSON *object, const char *string, cJSON *item);

2.7 cJSON_Delete() and cJSON_free
After the cjson object is created, the memory needs to be released after processing:
If it is an object in cjson, please use cJSON_Delete()
If not object: cJSON_free() or free()

CJSON_PUBLIC(void) cJSON_Delete(cJSON *item);
CJSON_PUBLIC(void) cJSON_free(void *object);

7.1: cJSON_GetObjectItem

cJSON *cJSON_GetObjectItem(cJSON *object,const char *string);

Function: get JSON string field value
Return value: a pointer to the structure of cJSON type is returned successfully, and NULL is returned in failure
Other acquisition

7.2: cJSON_GetArraySize

CJSON_PUBLIC(int) cJSON_GetArraySize(const cJSON *array);

Gets the format of the elements in the array and returns
7.3: cJSON_GetArrayItem

CJSON_PUBLIC(cJSON *) cJSON_GetArrayItem(const cJSON *array, int index);

Get the index data element in the array and return

/* Returns the number of items in an array (or object). */
CJSON_PUBLIC(int) cJSON_GetArraySize(const cJSON *array);
/* Retrieve item number "index" from array "array". Returns NULL if unsuccessful. */
CJSON_PUBLIC(cJSON *) cJSON_GetArrayItem(const cJSON *array, int index);
/* Get item "string" from object. Case insensitive. */
CJSON_PUBLIC(cJSON *) cJSON_GetObjectItem(const cJSON * const object, const char * const string);
CJSON_PUBLIC(cJSON *) cJSON_GetObjectItemCaseSensitive(const cJSON * const object, const char * const string);
CJSON_PUBLIC(cJSON_bool) cJSON_HasObjectItem(const cJSON *object, const char *string);
/* For analysing failed parses. This returns a pointer to the parse error. You'll probably need to look a few chars back to make sense of it. Defined when cJSON_Parse() returns 0. 0 when cJSON_Parse() succeeds. */
CJSON_PUBLIC(const char *) cJSON_GetErrorPtr(void);

/* Check item type and return its value */
CJSON_PUBLIC(char *) cJSON_GetStringValue(const cJSON * const item);
CJSON_PUBLIC(double) cJSON_GetNumberValue(const cJSON * const ite

Reference link: Introduction to common cJSON library functions

III. installation and use examples of CJSON Library

3.1 cJSON Library Download

git clone

Then we go to the cJSON folder and see cJSON c and cJSON h. When a c file needs to be called, move the two files to the corresponding c folder

Packaging instances using the cJSON Library

 *      Copyright:  (C) 2022 hubeiwuhan
 *                  All rights reserved.
 *       Filename:  cjsontest.c
 *    Description:  This file 
 *        Version:  1.0.0(03/03/22)
 *         Author:  yanp <>
 *      ChangeLog:  1, Release initial version on "03/03/22 12:49:44"
#include <stdio.h>
#include <stdlib.h>
#include "cJSON.h"

int main()
    cJSON *root, *js_body, *js_list;
    root = cJSON_CreateObject();
    cJSON_AddItemToObject(root, "body", js_body = cJSON_CreateArray());
    cJSON_AddItemToArray(js_body, js_list = cJSON_CreateObject());
    cJSON_AddStringToObject(js_list, "name", "kayshi");
    cJSON_AddNumberToObject(js_list, "status", 100);

    char *s  = cJSON_Print(root);
        printf("%s \n", s); 
    return 0;


Added by schoolmommy on Thu, 03 Mar 2022 15:19:21 +0200