The Arduino docs explain yield() at https://www.arduino.cc/en/Reference/Scheduler with regards to the Due. Apparently it is part of the Scheduler library:
#include <Scheduler.h>However, I can call yield() on my Nano or ESP8266 without including the Scheduler lib -- but only in my main program, not inside include files. Also, the include does not work on my non-Dues.
What's the secret that I'm missing about yield() or- what does yield() do on Arduino platforms other than Due?
Best Answer
However, I can call yield() on my Nano or ESP8266 without includingthe Scheduler lib
The yield() function is also implemented inside the ESP8266 libraries:
Yielding
This is one of the most critical differences between the ESP8266 and amore classical Arduino microcontroller. The ESP8266 runs a lot ofutility functions in the background – keeping WiFi connected, managingthe TCP/IP stack, and performing other duties. Blocking thesefunctions from running can cause the ESP8266 to crash and resetitself. To avoid these mysterious resets, avoid long, blocking loopsin your sketch.
The amazing creators of the ESP8266 Arduino libraries also implementeda yield() function, which calls on the background functions to allowthem to do their things.
That's why you can call yield() from within your main program where the ESP8266 header is included.
See ESP8266 Thing Hookup Guide.
Update:
yield() is defined in Arduino.h as:
void yield(void);yield() is also declared in hooks.h as follows:
/*** Empty yield() hook.** This function is intended to be used by library writers to build* libraries or sketches that supports cooperative threads.** Its defined as a weak symbol and it can be redefined to implement a* real cooperative scheduler.*/static void __empty() {// Empty}void yield(void) __attribute__ ((weak, alias("__empty")));So, on the Nano, it probably does nothing (unless you have other libraries #included).
yield is a "weak" function from Arduino core for AVR. I see one call for it inside wiring.c.
void delay(unsigned long ms){uint32_t start = micros();while (ms > 0) {yield();while ( ms > 0 && (micros() - start) >= 1000) {ms--;start += 1000;}}}This means that the yield() function will be executed during the loop of delay function. Thus, yield would be used for some background processing while the delay ends or for doing a function with timeout feature.
Note: yield must be defined in application/sketch
UPDATE: The question made me excited to make a little post about yield and other hidden features from arduino core.