JSON(JavaScript Object Notation)是一种轻量级的数据交换格式，易于人阅读和编写，同时也易于机器解析和生成。有关JSON的知识，请参考《介绍 JSON》。

cJSON是一个用C语言实现的Json处理程序，代码量不大，只有一个头文件cJSON.h和一个源文件cJSON.c,用起来比较方便。从sourceforge上可以下载。

在cJSON.h中定义了JSON的数据结构，如下：

typedef struct cJSON {
    struct cJSON *next,*prev;    /* next/prev allow you to walk array/object chains. Alternatively, use GetArraySize/GetArrayItem/GetObjectItem */
    struct cJSON *child;         /* An array or object item will have a child pointer pointing to a chain of the items in the array/object. */

    int type;                    /* The type of the item, as above. */

    char *valuestring;           /* The item's string, if type==cJSON_String */
    int valueint;                /* The item's number, if type==cJSON_Number */
    double valuedouble;          /* The item's number, if type==cJSON_Number */

    char *string;                /* The item's name string, if this item is the child of, or is in the list of subitems of an object. */
} cJSON;

定义还是蛮清楚的，用链表存储方式存储JSON，type代表这个结构体的类型，包括数、字符串、数组、json对象等。如果代表的是实数，则valuedouble就存储了这个实数取值，如果是int型的，则取valueint，其他类似。 type的值在cJSON.h中定义了相应的宏。 / cJSON Types: /

#define cJSON_False 0
#define cJSON_True 1
#define cJSON_NULL 2
#define cJSON_Number 3
#define cJSON_String 4
#define cJSON_Array 5
#define cJSON_Object 6

cJSON.h中的函数定义也比较明白，看函数名和注释应该能明白函数的功能，下面列举了几个常用的函数：

extern cJSON *cJSON_Parse(const char *value);      /* Supply a block of JSON, and this returns a cJSON object you can interrogate. Call cJSON_Delete when finished. */
extern char  *cJSON_Print(cJSON *item);            /* Render a cJSON entity to text for transfer/storage. Free the char* when finished. */
extern char  *cJSON_PrintUnformatted(cJSON *item); /* Render a cJSON entity to text for transfer/storage without any formatting. Free the char* when finished. */
extern void   cJSON_Delete(cJSON *c);              /* Delete a cJSON entity and all subentities. */

/*下面的这些函数主要用来创建json*/
extern cJSON *cJSON_CreateObject(void);

/* Macros for creating things quickly. */
#define cJSON_AddNullToObject(object,name)        cJSON_AddItemToObject(object, name, cJSON_CreateNull())
#define cJSON_AddTrueToObject(object,name)        cJSON_AddItemToObject(object, name, cJSON_CreateTrue())
#define cJSON_AddFalseToObject(object,name)        cJSON_AddItemToObject(object, name, cJSON_CreateFalse())
#define cJSON_AddBoolToObject(object,name,b)    cJSON_AddItemToObject(object, name, cJSON_CreateBool(b))
#define cJSON_AddNumberToObject(object,name,n)    cJSON_AddItemToObject(object, name, cJSON_CreateNumber(n))
#define cJSON_AddStringToObject(object,name,s)    cJSON_AddItemToObject(object, name, cJSON_CreateString(s))

下面是写了个测试程序，输出是

{
    "student":    {
        "name":    "Jack",
        "sex":    "male",
        "age":    20,
        "score":    {
            "Chinese":    90,
            "English":    85.500000
        }
    }
}

程序代码很简单，按照自己需要的json格式来添加相应的内容即可。

#include <stdio.h>
#include <stdlib.h>
#include "cJSON.h"
cJSON * create_json()
{
    cJSON * json = 0;
    cJSON * json_student = 0;
    cJSON * json_score = 0;
    //创建json的根节点
    json = cJSON_CreateObject();                        
    //创建json子节点json_student
    json_student = cJSON_CreateObject();
    cJSON_AddStringToObject(json_student,"name","Jack");     //为json_student添加name属性，值为Jack
    cJSON_AddStringToObject(json_student,"sex","male");      //为json_student添加sex属性，值为male
    cJSON_AddNumberToObject(json_student,"age",20);          //为json_student添加age属性，值为20
    //创建json_student的子节点json_score
    json_score = cJSON_CreateObject();
    cJSON_AddNumberToObject(json_score,"Chinese",90.0);      //为json_score添加Chinese属性，值为90
    cJSON_AddNumberToObject(json_score,"English",85.5);      //为json_score添加English属性，值为85.5

    cJSON_AddItemToObject(json_student,"score",json_score);  //json_student添加子节点json_score,键为score
    cJSON_AddItemToObject(json,"student",json_student);      //json添加子节点json_student,键为student
    return json;
}
int main(int argc,char ** argv)
{
    //创建json
    cJSON * json = create_json();
    //打印json
    char * str_json = cJSON_Print(json);
    printf("%s\n",str_json);

    //释放json
    cJSON_Delete(json);
    free(str_json);
    return 0;
}