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。 它基于JavaScript Programming Language, Standard ECMA-262 3rd Edition - December 1999的一个子集,最新的定义可以参考ECMA-404_2nd_edition_The JSON Data Interchange Syntax,这份标准只有16页,可以一次性读完。
JSON具有以下这些形式:
- {
- "FirstName": "John",
- "LastName": "Doe",
- "Age": 43,
- "Address": {
- "Street": "Downing Street 10",
- "City": "London",
- "Country": "Great Britain"
- },
- "Phone numbers": [
- "+44 1234567",
- "+44 2345678"
- ]
- }
-
具体可以参考jason官网:http://www.json.org/json-zh.html
CJSON是一个基于C语言实现的json库,为了兼容更多的平台,使用ANSI C (C89)标准来编写,其项目地址在http://github.com/DaveGamble/cJSON。
有两种方式:
CJSON提供的接口都在cJSON.h中声明出来了(以CJSON_PUBLIC的宏开头),还有一些直接是宏
- /* returns the version of cJSON as a string */
- CJSON_PUBLIC(const char*) cJSON_Version(void);
-
- /* Supply malloc, realloc and free functions to cJSON */
- CJSON_PUBLIC(void) cJSON_InitHooks(cJSON_Hooks* hooks);
-
- /* Memory Management: the caller is always responsible to free the results from all variants of cJSON_Parse (with cJSON_Delete) and cJSON_Print (with stdlib free, cJSON_Hooks.free_fn, or cJSON_free as appropriate). The exception is cJSON_PrintPreallocated, where the caller has full responsibility of the buffer. */
- /* Supply a block of JSON, and this returns a cJSON object you can interrogate. */
- CJSON_PUBLIC(cJSON *) cJSON_Parse(const char *value);
- CJSON_PUBLIC(cJSON *) cJSON_ParseWithLength(const char *value, size_t buffer_length);
- /* ParseWithOpts allows you to require (and check) that the JSON is null terminated, and to retrieve the pointer to the final byte parsed. */
- /* If you supply a ptr in return_parse_end and parsing fails, then return_parse_end will contain a pointer to the error so will match cJSON_GetErrorPtr(). */
- CJSON_PUBLIC(cJSON *) cJSON_ParseWithOpts(const char *value, const char **return_parse_end, cJSON_bool require_null_terminated);
- CJSON_PUBLIC(cJSON *) cJSON_ParseWithLengthOpts(const char *value, size_t buffer_length, const char **return_parse_end, cJSON_bool require_null_terminated);
-
- /* Render a cJSON entity to text for transfer/storage. */
- CJSON_PUBLIC(char *) cJSON_Print(const cJSON *item);
- /* Render a cJSON entity to text for transfer/storage without any formatting. */
- CJSON_PUBLIC(char *) cJSON_PrintUnformatted(const cJSON *item);
- /* Render a cJSON entity to text using a buffered strategy. prebuffer is a guess at the final size. guessing well reduces reallocation. fmt=0 gives unformatted, =1 gives formatted */
- CJSON_PUBLIC(char *) cJSON_PrintBuffered(const cJSON *item, int prebuffer, cJSON_bool fmt);
- /* Render a cJSON entity to text using a buffer already allocated in memory with given length. Returns 1 on success and 0 on failure. */
- /* NOTE: cJSON is not always 100% accurate in estimating how much memory it will use, so to be safe allocate 5 bytes more than you actually need */
- CJSON_PUBLIC(cJSON_bool) cJSON_PrintPreallocated(cJSON *item, char *buffer, const int length, const cJSON_bool format);
- /* Delete a cJSON entity and all subentities. */
- CJSON_PUBLIC(void) cJSON_Delete(cJSON *item);
-
- /* 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 item);
-
- /* These functions check the type of an item */
- CJSON_PUBLIC(cJSON_bool) cJSON_IsInvalid(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsFalse(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsTrue(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsBool(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsNull(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsNumber(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsString(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsArray(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsObject(const cJSON * const item);
- CJSON_PUBLIC(cJSON_bool) cJSON_IsRaw(const cJSON * const item);
-
- /* These calls create a cJSON item of the appropriate type. */
- 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);
-
- /* 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);
-
- /* Remove/Detach items from Arrays/Objects. */
- CJSON_PUBLIC(cJSON *) cJSON_DetachItemViaPointer(cJSON *parent, cJSON * const item);
- CJSON_PUBLIC(cJSON *) cJSON_DetachItemFromArray(cJSON *array, int which);
- CJSON_PUBLIC(void) cJSON_DeleteItemFromArray(cJSON *array, int which);
- CJSON_PUBLIC(cJSON *) cJSON_DetachItemFromObject(cJSON *object, const char *string);
- CJSON_PUBLIC(cJSON *) cJSON_DetachItemFromObjectCaseSensitive(cJSON *object, const char *string);
- CJSON_PUBLIC(void) cJSON_DeleteItemFromObject(cJSON *object, const char *string);
- CJSON_PUBLIC(void) cJSON_DeleteItemFromObjectCaseSensitive(cJSON *object, const char *string);
-
- /* Update array items. */
- CJSON_PUBLIC(cJSON_bool) cJSON_InsertItemInArray(cJSON *array, int which, cJSON *newitem); /* Shifts pre-existing items to the right. */
- CJSON_PUBLIC(cJSON_bool) cJSON_ReplaceItemViaPointer(cJSON * const parent, cJSON * const item, cJSON * replacement);
- CJSON_PUBLIC(cJSON_bool) cJSON_ReplaceItemInArray(cJSON *array, int which, cJSON *newitem);
- CJSON_PUBLIC(cJSON_bool) cJSON_ReplaceItemInObject(cJSON *object,const char *string,cJSON *newitem);
- CJSON_PUBLIC(cJSON_bool) cJSON_ReplaceItemInObjectCaseSensitive(cJSON *object,const char *string,cJSON *newitem);
-
- /* Duplicate a cJSON item */
- CJSON_PUBLIC(cJSON *) cJSON_Duplicate(const cJSON *item, cJSON_bool recurse);
- /* Duplicate will create a new, identical cJSON item to the one you pass, in new memory that will
- * need to be released. With recurse!=0, it will duplicate any children connected to the item.
- * The item->next and ->prev pointers are always zero on return from Duplicate. */
- /* Recursively compare two cJSON items for equality. If either a or b is NULL or invalid, they will be considered unequal.
- * case_sensitive determines if object keys are treated case sensitive (1) or case insensitive (0) */
- CJSON_PUBLIC(cJSON_bool) cJSON_Compare(const cJSON * const a, const cJSON * const b, const cJSON_bool case_sensitive);
-
- /* Minify a strings, remove blank characters(such as ' ', '\t', '\r', '\n') from strings.
- * The input pointer json cannot point to a read-only address area, such as a string constant,
- * but should point to a readable and writable address area. */
- CJSON_PUBLIC(void) cJSON_Minify(char *json);
-
- /* Helper functions for creating and adding items to an object at the same time.
- * They return the added item or NULL on failure. */
- CJSON_PUBLIC(cJSON*) cJSON_AddNullToObject(cJSON * const object, const char * const name);
- CJSON_PUBLIC(cJSON*) cJSON_AddTrueToObject(cJSON * const object, const char * const name);
- CJSON_PUBLIC(cJSON*) cJSON_AddFalseToObject(cJSON * const object, const char * const name);
- CJSON_PUBLIC(cJSON*) cJSON_AddBoolToObject(cJSON * const object, const char * const name, const cJSON_bool boolean);
- CJSON_PUBLIC(cJSON*) cJSON_AddNumberToObject(cJSON * const object, const char * const name, const double number);
- CJSON_PUBLIC(cJSON*) cJSON_AddStringToObject(cJSON * const object, const char * const name, const char * const string);
- CJSON_PUBLIC(cJSON*) cJSON_AddRawToObject(cJSON * const object, const char * const name, const char * const raw);
- CJSON_PUBLIC(cJSON*) cJSON_AddObjectToObject(cJSON * const object, const char * const name);
- CJSON_PUBLIC(cJSON*) cJSON_AddArrayToObject(cJSON * const object, const char * const name);
-
- /* When assigning an integer value, it needs to be propagated to valuedouble too. */
- #define cJSON_SetIntValue(object, number) ((object) ? (object)->valueint = (object)->valuedouble = (number) : (number))
- /* helper for the cJSON_SetNumberValue macro */
- CJSON_PUBLIC(double) cJSON_SetNumberHelper(cJSON *object, double number);
- #define cJSON_SetNumberValue(object, number) ((object != NULL) ? cJSON_SetNumberHelper(object, (double)number) : (number))
- /* Change the valuestring of a cJSON_String object, only takes effect when type of object is cJSON_String */
- CJSON_PUBLIC(char*) cJSON_SetValuestring(cJSON *object, const char *valuestring);
-
- /* If the object is not a boolean type this does nothing and returns cJSON_Invalid else it returns the new type*/
- #define cJSON_SetBoolValue(object, boolValue) ( \
- (object != NULL && ((object)->type & (cJSON_False|cJSON_True))) ? \
- (object)->type=((object)->type &(~(cJSON_False|cJSON_True)))|((boolValue)?cJSON_True:cJSON_False) : \
- cJSON_Invalid\
- )
-
- /* Macro for iterating over an array or object */
- #define cJSON_ArrayForEach(element, array) for(element = (array != NULL) ? (array)->child : NULL; element != NULL; element = element->next)
-
- /* malloc/free objects using the malloc/free functions that have been set with cJSON_InitHooks */
- CJSON_PUBLIC(void *) cJSON_malloc(size_t size);
- CJSON_PUBLIC(void) cJSON_free(void *object);
-
CJSON核心数据结构就只有一个:
- typedef struct cJSON
- {
- /* next/prev allow you to walk array/object chains. Alternatively, use GetArraySize/GetArrayItem/GetObjectItem */
- struct cJSON *next;
- struct cJSON *prev;
- /* An array or object item will have a child pointer pointing to a chain of the items in the array/object. */
- struct cJSON *child;
-
- /* The type of the item, as above. */
- int type;
-
- /* The item's string, if type==cJSON_String and type == cJSON_Raw */
- char *valuestring;
- /* writing to valueint is DEPRECATED, use cJSON_SetNumberValue instead */
- int valueint;
- /* The item's number, if type==cJSON_Number */
- double valuedouble;
-
- /* The item's name string, if this item is the child of, or is in the list of subitems of an object. */
- char *string;
- } cJSON;
-
可以看出,json是一个嵌套数据结构,这与json的定义所表达的思想一致。
- /**
- * @file simple-cjson.c
- * @author your name (you@domain.com)
- * @brief
- * @version 0.1
- * @date 2023-02-19
- *
- * @copyright Copyright (c) 2023
- *
- */
- #include "../cJSON.h"
- #include <stdio.h>
-
- char simple_json[] = "{\
- \"FirstName\": \"John\",\
- \"LastName\": \"Doe\",\
- \"Age\": 43,\
- \"Address\": {\
- \"Street\": \"Downing Street 10\",\
- \"City\": \"London\",\
- \"Country\": \"Great Britain\"\
- },\
- \"Phone numbers\": [\
- \"+44 1234567\",\
- \"+44 2345678\"\
- ]\
- }";
-
- int main()
- {
- cJSON* json = cJSON_Parse(simple_json);
- cJSON_AddStringToObject(json, "ID", "666");
- char* print_json_str = cJSON_Print(json);
- printf("simple json: %s", print_json_str);
-
- return 0;
- }
-
-
编译运行:
- $./simple-cjson
- simple json: {
- "FirstName": "John",
- "LastName": "Doe",
- "Age": 43,
- "Address": {
- "Street": "Downing Street 10",
- "City": "London",
- "Country": "Great Britain"
- },
- "Phone numbers": ["+44 1234567", "+44 2345678"],
- "ID": "666"
- }
-
这部分完全可以参考CJSON项目的README。
CJSON使用ANSI C编写的,具有很多的指针操作,如果不注意非常容易产生指针操作相关的问题,这点最好阅读下项目的README和源代码。