JsonConfigFile.ino

#include <ArduinoJson.h>
#include <SD.h>
#include <SPI.h>

// Our configuration structure.
//
// Never use a JsonDocument to store the configuration!
// A JsonDocument is *not* a permanent storage; it's only a temporary storage
// used during the serialization phase. See:
// https://arduinojson.org/v6/faq/why-must-i-create-a-separate-config-object/
struct Config {
  char hostname[64];
  int port;
};

const char *filename = "/config.txt";  // <- SD library uses 8.3 filenames
Config config;                         // <- global configuration object

// Loads the configuration from a file
void loadConfiguration(const char *filename, Config &config) {
  // Open file for reading
  File file = SD.open(filename);

  // Allocate a temporary JsonDocument
  // Don't forget to change the capacity to match your requirements.
  // Use arduinojson.org/v6/assistant to compute the capacity.
  StaticJsonDocument<512> doc;

  // Deserialize the JSON document
  DeserializationError error = deserializeJson(doc, file);
  if (error)
    Serial.println(F("Failed to read file, using default configuration"));

  // Copy values from the JsonDocument to the Config
  config.port = doc["port"] | 2731;
  strlcpy(config.hostname,                  // <- destination
          doc["hostname"] | "example.com",  // <- source
          sizeof(config.hostname));         // <- destination's capacity

  // Close the file (Curiously, File's destructor doesn't close the file)
  file.close();
}

// Saves the configuration to a file
void saveConfiguration(const char *filename, const Config &config) {
  // Delete existing file, otherwise the configuration is appended to the file
  SD.remove(filename);

  // Open file for writing
  File file = SD.open(filename, FILE_WRITE);
  if (!file) {
    Serial.println(F("Failed to create file"));
    return;
  }

  // Allocate a temporary JsonDocument
  // Don't forget to change the capacity to match your requirements.
  // Use arduinojson.org/assistant to compute the capacity.
  StaticJsonDocument<256> doc;

  // Set the values in the document
  doc["hostname"] = config.hostname;
  doc["port"] = config.port;

  // Serialize JSON to file
  if (serializeJson(doc, file) == 0) {
    Serial.println(F("Failed to write to file"));
  }

  // Close the file
  file.close();
}

// Prints the content of a file to the Serial
void printFile(const char *filename) {
  // Open file for reading
  File file = SD.open(filename);
  if (!file) {
    Serial.println(F("Failed to read file"));
    return;
  }

  // Extract each characters by one by one
  while (file.available()) {
    Serial.print((char)file.read());
  }
  Serial.println();

  // Close the file
  file.close();
}

void setup() {
  // Initialize serial port
  Serial.begin(9600);
  while (!Serial) continue;

  // Initialize SD library
  while (!SD.begin()) {
    Serial.println(F("Failed to initialize SD library"));
    delay(1000);
  }

  // Should load default config if run for the first time
  Serial.println(F("Loading configuration..."));
  loadConfiguration(filename, config);

  // Create configuration file
  Serial.println(F("Saving configuration..."));
  saveConfiguration(filename, config);

  // Dump config file
  Serial.println(F("Print config file..."));
  printFile(filename);
}

void loop() {
  // not used in this example
}