ArduinoJson parsing fail when reading string from EEPROM - c++

Question

Before someone flags this as duplicate I have found these two links and neither totally apply although I have implemented at least a little of both.
Buffer gets overwritten
Arduino reading json from EEPROM / converting uint8_t to char
My problem is this. I am trying to read and write a JSON string to an Arduinos EEPROM using ArduinoJson library (https://github.com/bblanchon/ArduinoJson). In the below code I generate a JsonObject from a hard coded JSON string (this works). I then write it to EEPROM (this works). It then gets read back from EEPROM (this works) but when I try and parse the second string using ArduinoJSON i get a parse failure.
For the purpose of testing I also clear the EEPROM each time just in case although eventually this will be removed.
The code compiles with no errors. I am hoping that someone more knowledgable in C++ than myself will spot something really obvious. I am compiling this onto a NodeMCU (esp8266).
#include <ArduinoJson.h>
#include <EEPROM.h>
StaticJsonBuffer<400> jsonBuffer;
JsonObject *jsonObject;
JsonObject *config;
String dummyJson = "{\"name\":\"RGB LED 1\",\"io\":[\"pwm1\",\"pwm2\",\"pwm3\"],\"io_type\":\"output\",\"device\":\"pwm_output\",\"uuid\":\"5a81f424aaf8d1e951ae78d270668337\",\"ip\":\"255.255.255.255\"}";
void setup()
{
Serial.begin(9600);
while (!Serial)
{
continue;
}
EEPROM.begin(512);
Serial.println("\n\n");
clearEEPROM();
createDummyJsonObject();
writeJsonToEEPROM();
readJsonFromEEPROM();
}
void createDummyJsonObject()
{
jsonObject = &jsonBuffer.parseObject(dummyJson);
if (!jsonObject->success())
{
Serial.println("jsonBuffer.parseObject() failed");
return;
}
else
{
Serial.println("JSON object generated from dummy string");
jsonObject->prettyPrintTo(Serial);
Serial.println("\n\n");
}
}
void loop()
{
// not used
}
void clearEEPROM()
{
for (int i = 0; i < 512 + 1; i++)
{
EEPROM.write(i, 0);
}
EEPROM.commit();
}
void writeJsonToEEPROM()
{
String jsonStr;
jsonObject->printTo(jsonStr);
for (int i = 0; i < jsonStr.length(); ++i)
{
EEPROM.write(i, jsonStr[i]);
}
EEPROM.write(jsonStr.length(), byte(0));
EEPROM.commit();
}
void readJsonFromEEPROM()
{
String jsonStr;
for (int i = 0; i < 512; ++i)
{
char c = char(EEPROM.read(i));
if (c != 0)
{
jsonStr += c;
delay(1);
}
else
{
break;
}
}
Serial.println(jsonStr);
char charBuf[jsonStr.length()];
jsonStr.toCharArray(charBuf, jsonStr.length());
config = &jsonBuffer.parseObject(charBuf);
if (!config->success())
{
Serial.println("jsonObject.parseObject() failed ");
return;
}
else
{
// Never reaches this point!
Serial.println("\nJSON object generated from EEPROM data");
config->prettyPrintTo(Serial);
Serial.println("\n\n");
}
}

Answer 1

The size allocated for charBuf should be jsonStr.length() + 1 because you need space for a string terminator. Therefore you should also add something like charBuf[jsonStr.length()] = '\0'; to provide that string terminator:
int const jsonStringLengh = jsonStr.length();
char charBuf[jsonStringLengh + 1];
jsonStr.toCharArray(charBuf, jsonStringLengh);
charBuf[jsonStringLengh] = '\0';

Answer 2

Ok so this solved it. All I had to do was create a new StaticJsonBuffer for the second string parse. For anyone who is having a similar issue here's the working code.
#include <ArduinoJson.h>
#include <EEPROM.h>
StaticJsonBuffer<512> jsonBuffer;
JsonObject *jsonObject;
JsonObject *config;
String dummyJson = "{\"name\":\"RGB LED 1\",\"io\":[\"pwm1\",\"pwm2\",\"pwm3\"],\"io_type\":\"output\",\"device\":\"pwm_output\",\"uuid\":\"5a81f424aaf8d1e951ae78d270668337\",\"ip\":\"255.255.255.255\"}";
void setup()
{
Serial.begin(9600);
while (!Serial)
{
continue;
}
EEPROM.begin(512);
clearEEPROM();
createDummyJsonObject();
writeJsonToEEPROM();
readJsonFromEEPROM();
}
void createDummyJsonObject()
{
jsonObject = &jsonBuffer.parseObject(dummyJson);
if (!jsonObject->success())
{
Serial.println("jsonBuffer.parseObject() failed");
return;
}
else
{
Serial.println("JSON object generated from dummy string");
}
}
void loop()
{
// not used
}
void clearEEPROM()
{
for (int i = 0; i < 512 + 1; i++)
{
EEPROM.write(i, 0);
}
EEPROM.commit();
}
void writeJsonToEEPROM()
{
String jsonStr;
jsonObject->printTo(jsonStr);
for (int i = 0; i < jsonStr.length(); ++i)
{
EEPROM.write(i, jsonStr[i]);
}
EEPROM.write(jsonStr.length(), byte(0));
EEPROM.commit();
}
void readJsonFromEEPROM()
{
String jsonStr;
for (int i = 0; i < 512; ++i)
{
char c = char(EEPROM.read(i));
if (c != 0)
{
jsonStr += c;
delay(1);
}
else
{
break;
}
}
StaticJsonBuffer<512> jsonBuffer2;
config = &jsonBuffer2.parseObject(jsonStr);
if (!config->success())
{
Serial.println("jsonObject.parseObject() failed ");
return;
}
else
{
Serial.println("\nJSON object generated from EEPROM data");
config->prettyPrintTo(Serial);
Serial.println("\n\n");
}
}