Solution: STM32 failed to parse data using cjson

One 、 problem

stay STM32 transplant cJSON Behind the library , Use cJSON_Parse(), Parse failure .

char cmd[512] = "{\"msg\":\"this is successful start up\",\"result\":1,\"action\":\"req_startUp\",\"responseData\":\"trustedDevice\",\"serial_no\":\"0341\",\"timestamp\":1656489886238}";

cJSON *pRoot = cJSON_Parse(cmd);
const char *error_ptr = cJSON_GetErrorPtr();
if (error_ptr != NULL)
{
    
    printf("Error before: %s\n", error_ptr);
}

Use cJSON_GetErrorPtr() The following errors were analyzed ：

Error before: ,"timestamp":1656489886238}

And when you remove serial_no Field after , Can be parsed successfully .

char cmd[512] = "{\"msg\":\"this is successful start up\",\"result\":1,\"action\":\"req_startUp\",\"responseData\":\"trustedDevice\",\"timestamp\":1656489886238}";

Two 、 reason

When the parsed data is long , Will parse failure , But short data is no problem , The later investigation is because cJSON Parsing requires a large amount of memory , Overflow causes parsing to fail .

see STM32 Startup file . Such as startup_stm32f407xx.s, Find out ：

• The stack area （stack）： Automatically allocated and released by the compiler , Stores the parameter values of the function 、 The value of a local variable, etc , It operates like a stack in a data structure .
• Heap area （heap）： Usually assigned and released by programmers , If programmers don't release , It may be recycled by the operating system at the end of the program . The allocation is similar to the linked list in the data structure .

** Be careful ：** Remember to cJSON_Delete Delete object , also cJSON_Print Will keep applying for memory , After all calls, be sure to free Release , Otherwise, memory overflow will occur after multiple calls . Lead to the following cJSON_Print Can not apply for memory .

3、 ... and 、 resolvent

After increasing space , Problem solving

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• from Leung Written in 2022 year 6 month 30 Japan

• Reference resources ：STM32 Engineering use cjson The reason why the library failed to parse data