[FFH] openharmony gnawing paper growth plan -- Application of cjson in traditional c/s model

【 Brief review 】

①【FFH】OpenHarmony Gnawing paper growth plan — Why? JSON Will gradually replace XML？
②【FFH】OpenHarmony Gnawing paper growth plan — Several common JSON Parser comparison
③【FFH】OpenHarmony Gnawing paper growth plan —JSON-RPC
④【FFH】OpenHarmony Gnawing paper growth plan — On serialization specification
⑤【FFH】OpenHarmony Gnawing paper growth plan —Flatbuffers be applied to MQTT agreement
⑥【FFH】OpenHarmony Gnawing paper growth plan — Overview of the development and application of serialization technology
⑦【FFH】OpenHarmony Gnawing paper growth plan —Apache Avro And Twiste

【 This issue focuses on 】

​ Hello everyone , Review the previous articles , Those we see are the whole JSON The content of the field , This time, let's watch something with OpenHarmony dependent . Let's explore the content of this time OpenHarmony What is the system source code used JSON Parsing library .

• How to find OpenHarmony Referenced third-party open source libraries
• Literature scene summary
• cJSON Structural analysis
• cJSON In tradition C/S Application under the model

How to find OpenHarmony Referenced third-party open source libraries

​ Before the start , Let's see how to find OpenHarmony Referenced third-party open source libraries .

​ It's very simple , Only need Gitee Global search on OpenHarmony third party+ Key terms in related fields , For example, I want to see OpenHarmony With the heel JSON Related libraries , That is global search OpenHarmony third party json Field .

​ Then we can see our search results , as follows , What we are discussing this time is cJSON This C Open source language JSON Parsing library .

Literature scene summary

​ cJSON It is used in the underlying data communication C Linguistic JSON Data analysis library . For example, it is used at the bottom of a system socket Way to communicate , Both ends of the communication can be used cJSON To deliver JSON Data object processing , Now let's take a look at an article about cJSON In tradition C/S（Client/Server） Application of the model .

cJSON github The open source address is as follows ：

https://github.com/DaveGamble/cJSON

cJSON Structural analysis

Let's see cJSON The structure part of the source code ：

/* The cJSON 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/*prev: Point to the next / Previous key value pair （key-value） node .
• type: Indicates the data type of the value in the key value pair node .
• *valuestring/valueint/valuedouble： If type For the string / integer / Floating point type , Then it points to the key value .
• *string: Used to store all keys （key） The name of , Include child Medium key.
• *child: When the value type is complex data structure , Such as arrays , object , nesting JSON wait , Then the pointer points to the key value .

​ You can see cJSON It's through Linked list To store JSON Data , This design is quite ingenious , Compare with JSONcpp,cJSON It can ensure that the written data and the output data are in the same order , and JSONcpp The order consistency of writing and output cannot be guaranteed , Because when storing JSONcpp According to key-value Sort internally . So for some requirements JSON Data sequential business ,cJSON be relative to JSONcpp It's a better choice .

​ Through this data structure, we can also know each JSON All key value pairs in the object pass next and prev Pointers link adjacent key value pairs of nodes , To ensure the data order we just mentioned .

​ From this data structure, we can see another ingenious place , Namely *child The pointer , This pointer makes this cJSON Support complex data structures , For example, array , object , nesting JSON Data objects, etc .

cJSON In tradition C/S Application under the model

Pictured ,cJSON There are many function methods in the Library , A series of creation and processing can be carried out JSON Operations on objects .

Now let's take a look at the traditional client End to server End pass cJSON The specific process of data exchange processing .

Data packaging process

​ The data package needs to be encapsulated into JSON Format , And then over the network . therefore , You need to create a cJSON Object of type , And then call cJSON_AddStringToObject() function , Add the attribute value of information to the object , At this point, by calling cJSON_Print() The function of parses the attributes of the object into the cache , It can be thought that cJSON Objects are serialized as strings , So actually , A string is stored in the cache . here , According to Ascii The code table will be cached Ascii Values are sequentially converted to hexadecimal numbers . Last , After sending the hexadecimal number to the server , So far, the packaging work is completed .

The process of parsing packets

· Again , After the server sends the information to the client , Data packets need to be properly analyzed according to the format of the Protocol . therefore , Call at this time cJSON_Parse（） Function to parse JSON Data packets , Besides , You also need to create cJSON Object to receive the parsed data , Last , Use JSON_GetObjectItem（） Function to get cJSON Different attribute values of objects , This completes the unpacking of data packets . It should be noted that , After processing the data , Must call cJSON_Delete（） Function to release the created object . Otherwise, memory leaks can result , And lead to system instability .

At the end of the article

​ We are simply right OpenHarmony Apply this third-party open source library cJSON The scenario application analysis and simple source code analysis are carried out .

​ In the next article, let's make a horizontal comparison with other open source C/C++ JSON Parsing library .

reference

A method of cross-platform network data exchange