Unexplained behavior of char array after using `deserializeJson`

Question

I'm using RF24 library to send messages between 2 MCU's : 1) ESP8266 (no Wifi, same code can be compiled to any Arduino board) 2) pro-Micro.

Message is sent in JSON format, while using ArduinoJSON library to serialize and deserialize messages.

Altough entire code works as expected, I get an error in "RFread" function, which _readmsg array loses its content after using deserializeJson.

Attached below relevant part of "RFread" function. Explanations in code below. Please notice "Mark [X]" in code and Serial output:

char _readmsg[32]; <---- Storing input in this array
 char t2[32]; <---- variable to clone value
 radio.read(&_readmsg, sizeof(_readmsg)); <---- getting RF input and store it in `_readmsg`
 strcpy(t2, _readmsg); <---- cloning, part of error seeking
 Serial.print("first: "); <---- printing initial value stored. OK. marked [A] in Serial output
 Serial.println(_readmsg);
 Serial.print("duplicate: "); <---- printing clone. OK. marked [B] in Serial output
 Serial.println(t2);
 
 if (key != nullptr) <--- check if "msg" key was sent in message
 {
 StaticJsonDocument<80> DOC;
 deserializeJson(DOC, _readmsg); <---- **error beyond this point**
 Serial.print("second: ");
 Serial.println(_readmsg); <---- printing wrong value. marked [C] in Serial output
 if (DOC.containsKey(key))
 {
 const char *outmsg = DOC[key];
 if (out != nullptr)
 {
 strcpy(out, outmsg);
 return true;
 }
 }
 else
 {
 Serial.print("third: "); 
 Serial.println(_readmsg); <---- still error. marked [D] in Serial output
 Serial.print("duplicate2: ");
 Serial.println(t2); <---- clone variable is OK. marked [E] in Serial output
 strcpy(out, t2); // if key not present, return entire message
 return 0;
 }

and in Serial port:

message sent OK <---- irrelevant for debug 
15:18:14.934 -> first: {"id":"Port","msg":"fff"} <----[A] 
15:18:14.934 -> duplicate: {"id":"Port","msg":"fff"} <----[B]
15:18:14.934 -> second: id <----[C]
15:18:14.934 -> third: id <----[D]
15:18:14.934 -> duplicate2: {"id":"Port","msg":"fff"} <----[E]
15:18:14.934 -> Error. no such key. got reply: {"id":"Port","msg":"fff"}

Is there a reason for that ? is it a bug ?

• Benoit Blanchon: Please see: Why is the input modified?
• Benoit Blanchon: Also, please check the error code returned by deserializeJson()
• guyd: @BenoitBlanchon Error gets "OK"

Answer 1

When you pass a char* input to deserializeJson(), it alters the input to avoid making useless copies.
This is the "zero-copy" mode of ArduinoJson.

In this mode, the JSON parser modifies the input in-place. The following modifications are performed:

1. '0円' are inserted at the end of each string
2. Escaped special characters (like \n) are unescaped

Example:

char[] json = "{\"hello\":\"world\"}";
deserializeJson(doc, json);

After executing the line above, the input variable probably contains something like: "hello0円world0円".

You can find more details in the documentation

• guyd: Thank you for your reply - I don't understand why _readmsg changes ( as a concept ). Whould you recommend to clone that variable as I did using t2 ?

Answer 2

To sum up Benoit Blanchon answer:

in original code:

deserializeJson(DOC, _readmsg);

changes to:

deserializeJson(DOC, (const char *)_readmsg);