Arduino Shared Variable Corruption in Interrupts

In this tutorial, you'll learn about Arduino Shared Variable Corruption in Interrupts. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

Variables shared between ISR and main loop are corrupted or give inconsistent values.

Quick Fix

Wrong

int counter = 0;  // Missing volatile!

Compiler optimizes away the read — counter never updates in loop().

Right

volatile int counter = 0;

void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), myISR, FALLING);
}

void loop() {
  int localCounter;
  noInterrupts();  // Disable interrupts for atomic read
  localCounter = counter;
  interrupts();
  Serial.println(localCounter);
  delay(100);
}

void myISR() {
  counter++;
}

Counter value reads consistently (no corruption).

Prevention

Always declare shared variables as volatile. Use noInterrupts()/interrupts() for atomic access to multi-byte variables. For simple reads of 1-byte variables on AVR, interrupts()/noInterrupts() may be optional but is still good practice. On 32-bit boards, use atomic operations or mutexes.

DodaTech engineers apply these same patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro for production IoT reliability.

FAQ