ArduinoJson library: how much space to allocate for serialized buffer?

Question

I am using the fantastic ArduinoJson library for working with JSON data on my ESP8266. I have been facing issues with random software wdt resets and have been investigating memory leaks wherever I can. I want to trim down my use of Strings and DynamicJsonDocuments as much as possible. So I thought of using automatic variables: char buffers: char[N] and StatitcJsonDocuments instead.

I am using the ArduinoJson assitant service to help me figure out how many bytes I should reserve. The following is one response payload, I am expecting to receive:

// char input[MAX_INPUT_LENGTH];

StaticJsonDocument<128> doc;

DeserializationError error = deserializeJson(doc, input, MAX_INPUT_LENGTH);

if (error) {
  Serial.print(F("deserializeJson() failed: "));
  Serial.println(error.f_str());
  return;
}

int statusCode = doc["statusCode"]; // 200
bool needUpdate = doc["needUpdate"]; // true
const char* message = doc["message"]; // "update required"

JsonObject data = doc["data"];
const char* data_deviceId = data["deviceId"]; // "DLD00005"
const char* data_user_fw_version = data["user_fw_version"]; // "0.0.0"
const char* data_new_fw_version = data["new_fw_version"]; // "0.0.5"

As you can see I am using a char [N] type as the input, which is in fact being copied using strncpy() from the https.getString().c_str() from the response of a previous https.POST() request. My issue is that I do not know how much memory I should reserve for the char[N] array, ie what should be the value of N? The reasons I ask is that I am getting a deserialization() failed: IncompleteInput error, which I'm pretty sure is because the char[N] buffer is not large enough. So my question is if the StaticJsonDocument object is 128 bytes, then how big should I make the char[N] buffer (is there any relation between the sizes)? Or should I just go with an arbitrarily large value like char[512] which I want to avoid to avoid stack overflows?

StaticJsonDocument<128> payload;
char payloadSerial[???];  
...
int status = https.POST(payloadSerial);
strncpy(payloadSerial, https.getString().c_str(), sizeof(payloadSerial));
DeserializationError error = deserializeJson(payload, payloadSerial, sizeof(payloadSerial));
if (error) {
    Monitor.print(F("DeserializeJson() failed: "));
    Monitor.println(error.f_str());
}

romkey · Accepted Answer · 2023-08-10 14:46:35Z

You're asking about code that looks like this:

StaticJsonDocument<128> payload;
char payloadSerial[???];  
...
int status = https.POST(payloadSerial);
strncpy(payloadSerial, https.getString().c_str(), sizeof(payloadSerial));
DeserializationError error = deserializeJson(payload, payloadSerial, sizeof(payloadSerial));

The documentation for deserializeJson() shows that it can accept a String directly:

DeserializationError deserializeJson(JsonDocument& doc, const String& input);

So there's no need to copy https.getString().c_str() into payloadSerial. You can just pass the String that https.getString() returns:

StaticJsonDocument<128> payload;

int status = https.POST(request_body);

DeserializationError error = deserializeJson(payload, https.getString());

You were using the same character array here for both the body of the request that you passed to POST() and for deserializing the JSON response. While this is okay, for clarity I changed the POST call to use something else as the character array is completely unnecessary for processing the response.

Even if deserializeJson() didn't accept a String type as an argument, you still wouldn't need to copy the response into a buffer. You could just pass https.getString().c_str():

StaticJsonDocument<128> payload;

int status = https.POST(request_body);

DeserializationError error = deserializeJson(payload, https.getString().c_str());

You really should check the value of status. It indicates whether the POST succeeded or not. If it failed, you should report that and the reason for the failure, and not attempt to process the returned payload as there is no returned payload.

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