Arduino core for ESP8266, with multi-threading

WARNING! This is a clone of the official Arduino core for the ESP8266, with some highly experimental cooperative multi-threading support. It will likely make fun of you while you try to figure out why it doesn't work.

Motivation

Multi-threading can make it easier to write and maintain code, by avoiding complex state machines. There are some libraries out there for simulating this, but they typically don't have separate stacks, or they don't address the issue with blocking calls in the ESP8266 WiFi libraries. Since I had some spare time, and the support basically was already there in the core, I made a crude attempt to expose it.

How it works

This is a case of cooperative multi-threading, meaning each thread either need to exit its loop regularly, or call one of the yield() or delay(ms) functions, to let the other threads run. All threads have the same priority and are scheduled in a round-robin fashion. If a thread function exit it will be invoked again later, just like the regular loop() function. There is no way to terminate threads.

The stack for each new thread is allocated on the heap and is by default 1024 bytes. See ESP8266Scheduler.h for the function signature.

In addition to the thread support, a number of library call sites have been modified to avoid esp_yield() and just call yield() in a loop instead, to avoid the blocking behavior. One should note this enables (accidental) concurrent access to the libraries, which may be TOTALLY UNSAFE. It's probably best if you keep your threads responsibilities completely separate.

Example

#include <ESP8266Scheduler.h>

void serialLoop() {
  int num = 0;
  while (true) {
    Serial.printf("The number is %d\n", num++);
    delay(1000);
  }
}

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(115200);
  Scheduler.startLoop(serialLoop);
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
}

Installation

Since this is a modified version of the original core, you need to remove that one if you have it installed and then install this one instead. Don't expect any support on it though, especially not from the original authors.

Performance

Haven't checked; probably horrible.

Known issues

Using ArduinoOTA is currently not super-safe since it doesn't suspend other threads, but it should not be that hard to fix.
Since delay() has been replaced with yield() loops, the CPU will never enter any low power states. Unsure if it did that before though.

Original README follows below.

Arduino core for ESP8266 WiFi chip

This project brings support for ESP8266 chip to the Arduino environment. It lets you write sketches using familiar Arduino functions and libraries, and run them directly on ESP8266, no external microcontroller required.

ESP8266 Arduino core comes with libraries to communicate over WiFi using TCP and UDP, set up HTTP, mDNS, SSDP, and DNS servers, do OTA updates, use a file system in flash memory, work with SD cards, servos, SPI and I2C peripherals.

Contents

Installing options:
Documentation
Issues and support
Contributing
License and credits

Installing with Boards Manager

Starting with 1.6.4, Arduino allows installation of third-party platform packages using Boards Manager. We have packages available for Windows, Mac OS, and Linux (32 and 64 bit).

Install the current upstream Arduino IDE at the 1.8 level or later. The current version is at the Arduino website.
Start Arduino and open Preferences window.
Enter http://arduino.esp8266.com/stable/package_esp8266com_index.json into Additional Board Manager URLs field. You can add multiple URLs, separating them with commas.
Open Boards Manager from Tools > Board menu and install esp8266 platform (and don't forget to select your ESP8266 board from Tools > Board menu after installation).

Latest release

Boards manager link: http://arduino.esp8266.com/stable/package_esp8266com_index.json

Documentation: http://esp8266.github.io/Arduino/versions/2.3.0/

Using git version

Install Arduino 1.8.2 from the Arduino website.
Go to Arduino directory
Clone this repository into hardware/esp8266com/esp8266 directory (or clone it elsewhere and create a symlink)

cd hardware
mkdir esp8266com
cd esp8266com
git clone https://github.com/esp8266/Arduino.git esp8266

Download binary tools (you need Python 2.7)

cd esp8266/tools
python get.py

Restart Arduino

Using PlatformIO

PlatformIO is an open source ecosystem for IoT development with cross platform build system, library manager and full support for Espressif (ESP8266) development. It works on the popular host OS: macOS, Windows, Linux 32/64, Linux ARM (like Raspberry Pi, BeagleBone, CubieBoard).

What is PlatformIO?
PlatformIO IDE
Quick Start with PlatformIO IDE or PlatformIO Core
Advanced usage - custom settings, uploading to SPIFFS, Over-the-Air (OTA) or using stage version
Integration with Cloud and Standalone IDEs - Cloud9, Codeanywhere, Eclipse Che (Codenvy), Atom, CLion, Eclipse, Emacs, NetBeans, Qt Creator, Sublime Text, VIM and Visual Studio
Project Examples

Building with make

makeEspArduino is a generic makefile for any ESP8266 Arduino project. Using make instead of the Arduino IDE makes it easier to do automated and production builds.

Documentation

Documentation for latest development version: https://arduino-esp8266.readthedocs.io/en/latest/

Issues and support

ESP8266 Community Forum is a well established community for questions and answers about Arduino for ESP8266.

If you find this forum useful, please consider supporting it with a donation.

If you encounter an issue which you think is a bug in the ESP8266 Arduino Core or the associated libraries, you are welcome to submit it here on Github: https://github.com/esp8266/Arduino/issues.

Please provide as much context as possible:

ESP8266 Arduino core version which you are using (you can check it in Boards Manager)
your sketch code; please wrap it into a code block, see Github markdown manual
when encountering an issue which happens at run time, attach serial output. Wrap it into a code block, just like the code.
for issues which happen at compile time, enable verbose compiler output in the IDE preferences, and attach that output (also inside a code block)
ESP8266 development board model
IDE settings (board choich, flash size)

Contributing

For minor fixes of code and documentation, please go ahead and submit a pull request.

Check out the list of issues which are easy to fix — easy issues for 2.4.0. Working on them is a great way to move the project forward.

Larger changes (rewriting parts of existing code from scratch, adding new functions to the core, adding new libraries) should generally be discussed by opening an issue first.

Feature branches with lots of small commits (especially titled "oops", "fix typo", "forgot to add file", etc.) should be squashed before opening a pull request. At the same time, please refrain from putting multiple unrelated changes into a single pull request.

License and credits

Arduino IDE is developed and maintained by the Arduino team. The IDE is licensed under GPL.

ESP8266 core includes an xtensa gcc toolchain, which is also under GPL.

Esptool written by Christian Klippel is licensed under GPLv2, currently maintained by Ivan Grokhotkov: https://github.com/igrr/esptool-ck.

Espressif SDK included in this build is under Espressif MIT License.

ESP8266 core files are licensed under LGPL.

SPI Flash File System (SPIFFS) written by Peter Andersson is used in this project. It is distributed under MIT license.

umm_malloc memory management library written by Ralph Hempel is used in this project. It is distributed under MIT license.

axTLS library written by Cameron Rich, built from https://github.com/igrr/axtls-8266, is used in this project. It is distributed under BSD license.