trying to deserialize JSON unsuccesfully - json

Question

I am trying to deserialize an array of chars using the ArduinoJson Lib. The chars are coming from a I2C connection using Wire library. They come successfully so I don't think there is an issue there.
When trying to de-serialize at run time I get the message "deserializeJson() failed: EmptyInput".
From ArduinoJson Troubleshooter's report
The issue happens at run time
The issue concerns deserialization
deserializeJson() returns EmptyInput
Input comes neither from an HTTP response, nor a file, nor a stream
The part of code that deserialization fails is here:
void callback(char payload[], unsigned int msg_size) {
Serial.print("Message arrived ");
for (int i=0;i<msg_size;i++) {
Serial.print((payload[i]));
}
DeserializationError error = deserializeJson(doc, payload, msg_size);
if (error) {
Serial.print(F("deserializeJson() failed: "));
Serial.println(error.f_str());
return;
}
const char* command = doc["cmd"];
Serial.print("command ");
Serial.println(command);
//rest of code
}
void setup() {
//communications
Wire.begin(0x8); // join i2c bus with address #8
Wire.onReceive(receiveEvent); // register event
Serial.begin(9600); // start serial for output
Serial.print("Start_");
Serial.println(ver);
}
void loop() {
// function that executes whenever data is received from master
// this function is registered as an event, see setup()
void receiveEvent(int howMany) {
char buff[60];
Serial.print("how many=");
Serial.println(howMany);
//initialize buffers
buff[0] = '\0';
int i = 0;
while (0 < Wire.available()) { // loop through all but the last
//char c = Wire.read(); // receive byte as a character
char c = Wire.read();
Serial.print(c); // print the character
buff[i] = c;
buff[i+1] = '\0'; // Add a NULL after each character
i++;
}
int x = Wire.read(); // receive byte as an integer
Serial.println();
Serial.print("x:");
Serial.println(x); // print the integer
Serial.print("BUFFER:");
for (int j=0; j < howMany; j++) {
Serial.print(buff[j]);
}
Serial.println();
Serial.print("size:");
Serial.println(sizeof(buff));
callback(buff, howMany);
}
The print out from serial ide is here:
Start_v.0.1
how many=25
{"cmd":"demo","seq":"1"}
x:-1
BUFFER:{"cmd":"demo","seq":"1"}
size:60
Message arrived {"cmd":"demo","seq":"1"} deserializeJson() failed: EmptyInput
But the payload variable, as you see here is not empty.
Can you provide any debugging hints? Thank you
Edit: I have added more code that shows the i2c communication
from smbus import SMBus
addr = 0x8 # bus address
bus = SMBus(1) # indicates /dev/ic2-1
numb = 1
def StringToBytes(val):
retVal = []
for c in val:
retVal.append(ord(c))
return retVal
def writeData(arrayValue, i2c_address):
byteValue = StringToBytes(arrayValue)
# examples --->
bus.write_i2c_block_data(i2c_address,0x00,byteValue)
return -1
# main
print ("Enter command:")
while 1:
cmd = input(">>>> ")
writeData(str(cmd), addr)

Answer 1

I filtered the incoming i2c buffer for nulls and the de serialization of JSON worked. I am not sure why the nulls are sent by rpi/python.
void receiveEvent(int howMany) {
char buff[60];
Serial.print("how many=");
Serial.println(howMany);
//initialize buffers
buff[0] = '\0';
int i = 0;
while (0 < Wire.available()) { // loop through all but the last
char c = Wire.read(); // receive byte as a character
if (c == '\0') {
//ignore
} else {
//char c = Wire.read();
Serial.print(c); // print the character
buff[i] = c;
buff[i+1] = '\0'; // Add a NULL after each character
i++;
}
}
int x = Wire.read(); // receive byte as an integer
Serial.println();
Serial.print("x:");
Serial.println(x); // print the integer
Serial.print("BUFFER:");
for (int j=0; j < howMany; j++) {
Serial.print(buff[j]);
}
Serial.println();
Serial.print("size:");
Serial.println(sizeof(buff));
callback(buff, howMany);
}