Integer overflow and underflow in Blockchain / Solidity

Introduction

Integer overflow and underflow happen when numbers go beyond their allowed limits. This can cause wrong results or bugs in programs.

When working with cryptocurrency balances that must not exceed or go below certain values.

When counting tokens or assets in a smart contract to avoid errors.

When performing math operations that could produce very large or very small numbers.

When validating user inputs that affect numbers stored on the blockchain.

When designing secure contracts to prevent attackers from exploiting number limits.

Syntax

Blockchain / Solidity

// Example in Solidity
uint8 smallNumber = 255;
smallNumber += 1; // causes overflow

int8 negativeNumber = -128;
negativeNumber -= 1; // causes underflow

Unsigned integers (like uint8) can only hold positive numbers and zero.

Signed integers (like int8) can hold positive and negative numbers.

Examples

Adding 1 to the max value 255 causes overflow and wraps around to 0.

Blockchain / Solidity

uint8 x = 255;
x += 1; // x becomes 0 due to overflow

Subtracting 1 from the min value -128 causes underflow and wraps around to 127.

Blockchain / Solidity

int8 y = -128;
y -= 1; // y becomes 127 due to underflow

Adding numbers safely when total stays within allowed range.

Blockchain / Solidity

uint256 balance = 1000;
balance += 500; // safe addition if within limits

Sample Program

This Solidity contract shows integer overflow on a uint8 variable. The addOne function adds 1 to 255, causing overflow to 0 inside an unchecked block.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract OverflowExample {
    uint8 public smallNumber = 255;

    function addOne() public {
        unchecked {
            smallNumber += 1; // overflow happens here
        }
    }

    function getNumber() public view returns (uint8) {
        return smallNumber;
    }
}

Important Notes

Modern Solidity versions (0.8.0+) check for overflow by default and throw errors.

Using unchecked disables these checks and allows overflow/underflow.

Always be careful with unchecked math to avoid security risks.

Summary

Integer overflow happens when a number goes above its max limit and wraps around.

Integer underflow happens when a number goes below its min limit and wraps around.

In blockchain, these can cause bugs or security problems if not handled properly.