Atomic Operators

Atomic Operators on ESP8266

(this is kinda old - I have not retried with the newer compiler) I tried to use an std::atomic_exchange(&lock, 1) function. The sketch compiles; however, it cannot link because of an "undefined reference to __atomic_exchange_4". Apparently, there are no test-and-set instructions on the ESP8266. So far it looks like, for memory exchanges that need to be atomic, you will need to disable and restore interrupts around the sensitive area.

• For multi-architecture Arduino platforms, this repo looks good. He creates an ATOMIC() macro similar to the ATOMIC_BLOCK() that is available for Arduino AVRs.
• In the Arduino ESP8266 core the interrupts.h file has two class-based options for doing this:
  • InterruptsLock - Disables all interrupts through level 15.

    {
         InterruptLock lock;
         // Do sensitive stuff with interrupts locked out.
    }
    // Back to normal interrupt operations in background.
    

  • A macro AutoInterruptLock(level) that defines a class inline, then uses it to start the process.

    {
         AutoInterruptLock(3);  // Allows interrupt levels above 3
         // Do sensitive stuff with interrupts up to 3 locked out.
    }
    // Back to normal interrupt operations in background.
    

  • Something like this also seems to compile down nicely:

    int interlocked_exchange_int(volatile int *target, int value) {
        uint32_t ps = xt_rsil(15);
        int oldValue = *target;
        *target = value;
        xt_wsr_ps(ps);
        return oldValue;
    }