Use of cjson Library

MQTT+JSON It is the most commonly used communication protocol for data interaction of Internet of things devices , among JSON The protocol is very suitable for plaintext transmission 、 Scenario of short packet transmission .

Know, understand and apply design a protocol , Be sure to understand its principle first , Then go to know its advantages and disadvantages and scope of application .

Catalog

1、JSON Concept expression

2、cJSON Possible modifications to the Library

2.1 Memory modification

2.2 Print number The emergence of scientific counting （ Reference resources ）

3、JSON General methods of parsing and processing

3.1 Definition

3.2 analysis JSON

3.2 feedback / Generate JSON

3.4 Often used JSON Processing function

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1、JSON Concept expression

JSON A data structure is a serialized object or array of key value pairs .

key , Is a string ;

value , It can be the object , Array , Numbers , One of the strings ;

The object is enclosed by curly braces and separated by commas ;

The array is enclosed by square brackets , The string is surrounded by double quotation marks , Both are separated by commas ;

Numbers don't need to be enclosed , Separate them directly with commas .

Its essence is a string , Can cross language 、 Cross platform storage , And string key-value The form is easy to read .

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cJSON Library is one of the most typical C Language JSON Library code .

It is very suitable for transplantation and use on embedded devices .

2、cJSON Possible modifications to the Library

2.1 Memory modification

By default, this library uses C Memory function of standard library .

If you need to debug memory problems , Or you need to unify memory usage , You can change the code in the following location to replace the memory function .

#if defined(_MSC_VER)
/* work around MSVC error C2322: '...' address of dllimport '...' is not static */
static void * CJSON_CDECL internal_malloc(size_t size)
{
    return malloc(size);
}
static void CJSON_CDECL internal_free(void *pointer)
{
    free(pointer);
}
static void * CJSON_CDECL internal_realloc(void *pointer, size_t size)
{
    return realloc(pointer, size);
}
#else
#define internal_malloc malloc
#define internal_free free
#define internal_realloc realloc
#endif

2.2 Print number The emergence of scientific counting （ Reference resources ）

static cJSON_bool print_number(const cJSON * const item, printbuffer * const output_buffer);

Document No 511 That's ok , This function performs sscanf after , Will cause printing number Type value , The result is the problem of scientific counting （stm32F205 During chip debugging , At that time, memory resources were extremely tight ）.

The modification scheme is as follows （ Solve after modification ）：

I don't know the specific reason , Just list the solutions first .

#if 0
        /* Try 15 decimal places of precision to avoid nonsignificant nonzero digits */
        length = sprintf((char*)number_buffer, "%1.15g", d);

        /* Check whether the original double can be recovered */
        if ((sscanf((char*)number_buffer, "%lg", &test) != 1) || !compare_double((double)test, d))
        {
            /* If not, print with 17 decimal places of precision */
            length = sprintf((char*)number_buffer, "%1.17g", d);
        }
#else
		length = sprintf((char*)number_buffer, "%d", (unsigned int)d);
#endif

3、JSON General methods of parsing and processing

3.1 Definition

// The necessary contents of the header file include relationships and declarations 

#include "cJSON.h"

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

typedef enum
{
	ERROR_CODE_INTER = -1,	// internal error 
	ERROR_CODE_NONE = 0,	// empty /OK
	ERROR_CODE_PARAM,		// Parameter error 
} EnErrorCode;

typedef struct
{
	char *CmdStr;
	int (*CmdFun)(cJSON*);
} TyJsonCmdFun;

void JsonCmdAnalysis(uint8_t *pData, uint16_t unDataLen);
void JsonCmdResponse(cJSON *Json, int err, void *res);
void JsonCmdSend(uint8_t *pData, uint16_t unDataLen);

Above , Defined ① The error code returned by the function ,②json Function structure ,③ The entrance of function analysis and feedback .

3.2 analysis JSON

Based on this , Sure Define the following function table ,

When receiving data to parse , Judge json Fixed key value pair of - Such as "cmd" Field , To execute the corresponding processing function .

Here is a point to pay attention to ：cJSON_Parse and cJSON_Delete.

cJSON_Parse The function is used to flow bytes into JSON object , There is a memory request action , And can't get through free To release , Only through cJSON_Delete To release .

int JsonCmdDeal_version(cJSON *Json);
int JsonCmdDeal_ota_start(cJSON *Json);
int JsonCmdDeal_ota_data(cJSON *Json);
int JsonCmdDeal_ota_end(cJSON *Json);
int JsonCmdDeal_driver_test(cJSON *Json);

const static TyJsonCmdFun JsonCmdFunTable[] = 
{
	{"version",			JsonCmdDeal_version},
	{"ota_start",		JsonCmdDeal_ota_start},
	{"ota_data",		JsonCmdDeal_ota_data},
	{"ota_end",			JsonCmdDeal_ota_end},
	{"driver_test",		JsonCmdDeal_driver_test},
}; 

void JsonCmdAnalysis(uint8_t *pData, uint16_t unDataLen)
{
	uint8_t i;
	cJSON *JsonCmd = NULL;
	
	JsonCmd = cJSON_Parse((char*)pData);
	if(JsonCmd != NULL)
	{
		for(i=0;i<sizeof(JsonCmdFunTable)/sizeof(TyJsonCmdFun);i++)
		{
			if(strcmp(JsonCmdFunTable[i].CmdStr,cJSON_GetObjectItem(JsonCmd,"cmd")->valuestring) == 0)
			{
				JsonCmdFunTable[i].CmdFun(JsonCmd);
				break;
			}
		}
		cJSON_Delete(JsonCmd);
	}
}

3.2 feedback / Generate JSON

void JsonCmdResponse(cJSON *Json, int err, void *res)
{
	char *json_buf = NULL;
	int buf_len = 0;
	cJSON* respond_json = NULL;
	
	respond_json = cJSON_CreateObject();
	if(respond_json != NULL)
	{
		cJSON_AddStringToObject(respond_json, "cmd", cJSON_GetObjectItem(Json,"cmd")->valuestring);
	
		if(err == ERROR_CODE_NONE)
			cJSON_AddStringToObject(respond_json, "rsq", "ok");
		else
			cJSON_AddNumberToObject(respond_json, "rsq", err);
		
		json_buf = cJSON_PrintUnformatted(respond_json);
		if(json_buf != NULL)
		{
			buf_len = strlen((char*)json_buf);
			
			JsonCmdSend((uint8_t *)json_buf, buf_len);
			
			free(json_buf);
		}
		
		cJSON_Delete(respond_json);
	}
}

void JsonCmdSend(uint8_t *pData, uint16_t unDataLen)
{
	//do nothing
}

int JsonCmdDeal_version(cJSON *Json)
{
	int err = ERROR_CODE_NONE;
	
	JsonCmdResponse(Json, err, NULL);
	
	return err;
}

alike ,cJSON_CreateObject() Function to create JSON object , And with the subsequent user call cJSON_AddXXXXToObject Class generates a memory request （realloc）, Finally, only by cJSON_Delete Free memory .

A little extra , Namely cJSON Of print function , Yes, it can be directly determined by free Function to release .

among ,cJSON_PrintUnformatted Yes, it will JSON Object is printed as a string without formatting characters .

cJSON_Print Is with formatting characters （ There will be many spaces and line breaks ）.

If you need an intuitive view JSON data , Suggest using cJSON_Print;

But if it is application business execution , Suggest or use cJSON_PrintUnformatted, Minimize the amount of bytes transmitted .

3.4 Often used JSON Processing function

// Memory manipulation function 
CJSON_PUBLIC(cJSON *) cJSON_Parse(const char *value);// String rotation json
CJSON_PUBLIC(char *) cJSON_Print(const cJSON *item);// Print json character string 1
CJSON_PUBLIC(char *) cJSON_PrintUnformatted(const cJSON *item);// Print json character string 2
CJSON_PUBLIC(void) cJSON_Delete(cJSON *item);// Release json

// establish json object 
CJSON_PUBLIC(cJSON *) cJSON_CreateObject(void);// establish json object 
CJSON_PUBLIC(cJSON *) cJSON_CreateArray(void);// Create array 

// establish json object - Used to update key value pairs 
CJSON_PUBLIC(cJSON *) cJSON_CreateNumber(double num);
CJSON_PUBLIC(cJSON *) cJSON_CreateString(const char *string);
CJSON_PUBLIC(cJSON *) cJSON_CreateTrue(void);
CJSON_PUBLIC(cJSON *) cJSON_CreateFalse(void);

// obtain json object 
CJSON_PUBLIC(int) cJSON_GetArraySize(const cJSON *array);// Get array size 
CJSON_PUBLIC(cJSON *) cJSON_GetArrayItem(const cJSON *array, int index);// Get an element of the array 
CJSON_PUBLIC(cJSON *) cJSON_GetObjectItem(const cJSON * const object, const char * const string);// Get key value pairs 

// add to json Key value pair 
CJSON_PUBLIC(cJSON*) cJSON_AddObjectToObject(cJSON * const object, const char * const name);
CJSON_PUBLIC(cJSON*) cJSON_AddArrayToObject(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_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);

// Determine the value type of key value pair 
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_IsFalse(const cJSON * const item);
CJSON_PUBLIC(cJSON_bool) cJSON_IsTrue(const cJSON * const item);

// Replace ( to update ) Key value pair 
CJSON_PUBLIC(void) cJSON_ReplaceItemInArray(cJSON *array, int which, cJSON *newitem);
CJSON_PUBLIC(void) cJSON_ReplaceItemInObject(cJSON *object,const char *string,cJSON *newitem);