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.

Practice

1. What happens in blockchain when an unsigned integer variable exceeds its maximum value?

easy

A. It wraps around to zero (integer overflow).

B. It throws an error automatically.

C. It becomes negative.

D. It stays at the maximum value.

Solution

1. Step 1: Understand unsigned integer limits

   Unsigned integers start at 0 and go up to a max value (like 2^256-1 in Solidity).

2. Step 2: What happens when max is exceeded?

   When the value goes above max, it wraps around to 0, causing integer overflow.

3. Final Answer:

   It wraps around to zero (integer overflow). -> Option A

4. Quick Check:

   Unsigned integer overflow = wrap to zero [OK]

Hint: Unsigned integers wrap to zero on overflow [OK]

Common Mistakes:

• Thinking overflow throws an error automatically
• Assuming value becomes negative
• Believing value stays at max

2. Which of the following Solidity code snippets correctly prevents integer underflow when subtracting 1 from a uint variable balance?

easy

A. balance -= 1; require(balance >= 0);

B. balance = balance - 1;

C. if (balance > 0) { balance -= 1; }

D. balance = balance + 1;

Solution

1. Step 1: Identify safe subtraction

   To avoid underflow, subtract only if balance is greater than zero.

2. Step 2: Check each option

   if (balance > 0) { balance -= 1; } checks balance > 0 before subtracting, preventing underflow. The other options either subtract without checking first (allowing underflow) or add instead of subtracting.

3. Final Answer:

   if (balance > 0) { balance -= 1; } -> Option C

4. Quick Check:

   Check before subtracting prevents underflow [OK]

Hint: Check value > 0 before subtracting to avoid underflow [OK]

Common Mistakes:

• Not checking before subtracting
• Using require after subtraction (too late)
• Adding instead of subtracting

3. Consider this Solidity code snippet:

uint8 x = 255;
x += 1;

What is the value of x after execution?

medium

A. 0

B. 1

C. 255

D. 256

Solution

1. Step 1: Understand uint8 max value

   uint8 can hold values from 0 to 255. 255 is the max.

2. Step 2: Add 1 to max value

   Adding 1 causes overflow, wrapping value back to 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   uint8 overflow wraps to 0 [OK]

Hint: uint8 max is 255; adding 1 wraps to 0 [OK]

Common Mistakes:

• Expecting 256 (out of range)
• Expecting value to stay at 255
• Confusing with signed integers

4. This Solidity function tries to subtract 1 from count. What is the bug?

function decrement(uint count) public pure returns (uint) {
    count -= 1;
    return count;
}

medium

A. No bug; works fine.

B. Underflow if count is 0, wrapping to max uint.

C. Syntax error due to missing semicolon.

D. Returns negative number which is invalid.

Solution

1. Step 1: Analyze subtraction without check

   The function subtracts 1 from count without checking if count is zero.

2. Step 2: Understand underflow risk

   If count is 0, subtracting 1 causes underflow, wrapping to max uint value.

3. Final Answer:

   Underflow if count is 0, wrapping to max uint. -> Option B

4. Quick Check:

   Subtracting 1 from 0 causes underflow [OK]

Hint: Check before subtracting to avoid underflow [OK]

Common Mistakes:

• Assuming no bug without checks
• Thinking Solidity throws error automatically
• Confusing underflow with negative return

5. You have a smart contract token balance stored as uint256. You want to safely add tokens without overflow. Which approach is best?

hard

A. Convert to int256 and add.

B. Use unchecked addition: balance += amount;

C. Subtract amount first, then add balance.

D. Use SafeMath library's add function to check overflow.

Solution

1. Step 1: Understand overflow risk in addition

   Adding tokens can overflow uint256 max value, causing wraparound bugs.

2. Step 2: Use SafeMath library

   SafeMath's add function checks for overflow and reverts if it happens, preventing bugs.

3. Step 3: Evaluate other options

   Unchecked addition risks overflow. Subtracting first or converting to int256 does not prevent overflow safely.

4. Final Answer:

   Use SafeMath library's add function to check overflow. -> Option D

5. Quick Check:

   SafeMath add prevents overflow [OK]

Hint: Use SafeMath add to prevent overflow safely [OK]

Common Mistakes:

• Using unchecked addition risking overflow
• Trying to subtract before adding
• Converting to signed int without checks