ArduinoJSON serialization returns an empty string when serializing to char*

Question

I made a function that serializes settings and returns a char* containing serialized data.

First i'm packing all the values into a StaticJsonDocument, then determining size of the output string using measureJson, then allocating space for the output char out[strsize] and then serializing data into space allocated serializeJson(doc,out,strsize)

The problem is that output string remains empty for unknown reason.

Things i checked:

• Json document is constructed properly and actually contains configurations settings
• measureJson() function properly returns the size of output and space is being allocated, strsize is not 0

Code:

char* configSerialize(bool msgpack){

  StaticJsonDocument<settsize> doc;

  JsonArray ipk = doc.createNestedArray("ip");
  JsonArray gateipk = doc.createNestedArray("gateip");
  JsonArray dnsk = doc.createNestedArray("dns");
  JsonArray mack = doc.createNestedArray("mac");

  unsigned char i;
  for(i=0;i<4;i++){
    ipk.add(ip[i]);
    gateipk.add(gateip[i]);
    dnsk.add(dns[i]);
  }
  for(i=0;i<6;i++){
    mack.add(mac[i]);
  }

  doc["subnet"] = subnet;
  doc["dhcp"] = DHCP;
  doc["alertbuzz"] = alertbuzz;

  const size_t strsize = msgpack ? measureMsgPack(doc) : measureJson(doc);
  char out[strsize];

  if(msgpack) serializeMsgPack(doc,out,strsize);
  else serializeJson(doc,out,strsize);

  return out;

}

Answer 1

char out[strsize];

This is a local variable/array inside your configSerialize() function and is invalid once you return from that function.

One way would be to use new and delete to allocate/deallocate space on the heap, but I would not recommend that on Arduino.

Another way would be to use char out[FIXED_SIZE]; outside of your function - i.e. as a global variable.

Also, if you're planning to use out as a string pointer, you'll need to add a zero byte at the end (and allocate space for that extra byte).