Preferences

Provides ESP32-compatible Preferences API for a wider variety of platforms:

• ESP8266 using LittleFS
• RP2040, RP2350 boards with Pico core
• Realtek boards with Ameba Arduino SDK
• Wio Terminal via SFUD
• Particle: Muon, Photon 2, Argon, Boron, Xenon, Tracker, BSOM, MSOM, P2

Available from: Arduino Library Manager, PlatformIO, Particle Build

#include <Preferences.h>
Preferences prefs;

void setup() {
  Serial.begin(115200);
  prefs.begin("my-app");

  int counter = prefs.getInt("counter", 1); // default to 1
  Serial.print("Reboot count: ");
  Serial.println(counter);
  counter++;
  prefs.putInt("counter", counter);
}

void loop() {}

Preferences are stored in the internal flash filesystem in a bunch of /nvs/{namespace}/{property} files.
Filesystem should handle flash wearing, bad sectors and atomic rename file operation.

• LittleFS handles all that, so this is the default FS driver for ESP8266. SPIFFS use is possible, but it is discouraged.
• Particle Gen3 devices also operate on a built-in LittleFS filesystem.
• Wio Terminal uses the first 8KB of external SPI flash, accessed via sfud. This is not a real filesystem: it's a simple append-only log that gets compacted once it runs out of space.

Check out ESP32 Preferences library API. Differences:

• partition_label argument is not supported in begin()
• getType() and freeEntries() methods are not supported (returning dummy values)
• putBytes() and putString() allow writing empty values (length = 0)
• get*() operations don't fail if the existing value has a different type, and a size mismatch is treated like a missing key (the provided default value is returned)