// http://forum.m5stack.com/topic/184/problems-writing-to-sd-card
#include <M5Stack.h>

//Micro SD / TF Card Test

void listDir(fs::FS &fs, const char * dirname, uint8_t levels) {

// Print blank line on screen
M5.Lcd.printf(" \n ");
M5.Lcd.printf(" \n ");

Serial.printf("Listing directory: %s\n", dirname);
M5.Lcd.printf("Listing directory: %s\n", dirname);

File root = fs.open(dirname);
if (!root) {
Serial.println("Failed to open directory");
M5.Lcd.println("Failed to open directory");
return;
}
if (!root.isDirectory()) {
Serial.println("Not a directory");
M5.Lcd.println("Not a directory");
return;
}

File file = root.openNextFile();
while (file) {
if (file.isDirectory()) {
Serial.print(" DIR : ");
M5.Lcd.print(" DIR : ");
Serial.println(file.name());
M5.Lcd.println(file.name());
if (levels) {
listDir(fs, file.name(), levels - 1);
}
} else {
Serial.print(" FILE: ");
M5.Lcd.print(" FILE: ");
Serial.print(file.name());
M5.Lcd.print(file.name());
Serial.print(" SIZE: ");
M5.Lcd.print(" SIZE: ");
Serial.println(file.size());
M5.Lcd.println(file.size());
}
file = root.openNextFile();
}
}

void readFile(fs::FS &fs, const char * path) {
Serial.printf("Reading file: %s\n", path);
M5.Lcd.printf("Reading file: %s\n", path);

File file = fs.open(path);
if (!file) {
Serial.println("Failed to open file for reading");
M5.Lcd.println("Failed to open file for reading");
return;
}

Serial.print("Read from file: ");
M5.Lcd.print("Read from file: ");
while (file.available()) {
int ch = file.read();
Serial.write(ch);
M5.Lcd.write(ch);
}
}

void writeFile(fs::FS &fs, const char * path, const char * message) {
Serial.printf("Writing file: %s\n", path);
M5.Lcd.printf("Writing file: %s\n", path);

File file = fs.open(path, FILE_WRITE);
if (!file) {
Serial.println("Failed to open file for writing");
M5.Lcd.println("Failed to open file for writing");
return;
}
if (file.print(message)) {
Serial.println("File written");
M5.Lcd.println("File written");
} else {
Serial.println("Write failed");
M5.Lcd.println("Write failed");
}
}

//////////////////////////////////////////////////////////////////////////////////////
bool suca = 1;
bool assai = 0;
int x = 1680;
//////////////////////////////////////////////////////////////////////////////////////

// the setup routine runs once when M5Stack starts up

void setup() {

// initialize the M5Stack object
M5.begin();

// M5.startupLogo();
Wire.begin();

// Lcd display
M5.Lcd.setBrightness(100);
M5.Lcd.fillScreen(BLACK);
M5.Lcd.setCursor(0, 10);
M5.Lcd.setTextColor(WHITE);
M5.Lcd.setTextSize(2);

// Page Header
M5.Lcd.fillScreen(BLACK);
M5.Lcd.setCursor(0, 05);
M5.Lcd.printf("Testing SD Card Functions:\r\n");

//////////////////////////////////////////////////////////////////////////////////////

store = String(brgt) + "," + String(unit) + "," + String(x) + "\r\n";

//////////////////////////////////////////////////////////////////////////////////////
// writeFile(SD, "/hello.txt", "Hello world from M5Stack !!");
writeFile(SD, "/hello.txt", store.c_str()); // questa funziona!!!!

M5.Lcd.printf("");

// Print blank line on screen
M5.Lcd.printf(" \n ");

// Print blank line on screen
M5.Lcd.printf(" \n ");

readFile(SD, "/hello.txt");
// store2[3] = (readFile(SD, "/hello.txt"));
}

void loop() {

// put your main code here, to run repeatedly:

M5.update();
}

is it correct to use String?
is it the best way to save/read my variables?

in the past, with esp8266, I saved my three variables on EEPROM but since M5Stack have sd card I would like to have infinite cycle of read/write