Integer underflow/overflow

Question

What is integer overflow and underflow and how it can be possible? I saw its example that coders avoid this bug by require(balanceOf(_to) +_value >= balanceOf(_to)) where _value is an unsigned integer so it will always be positive and will add to the balance, so this is an understood thing so why is this condition needed?

Answer 1

Solidity does not have any built-in protection from over/underflow. This means that if you hit the min or the max of a value (0 or 2^256-1 = 115792089237316195423570985008687907853269984665640564039457584007913129639935) then the value will "wrap" around and keep counting.

Put the following code in Remix to test it out.

pragma solidity ^0.5.16;

contract FlowTest {

    uint256 public a = 0;
    uint256 public b = 2**256-1; 

    function flow () external {
        a -= 1;
        b += 1;
    }
}

If you deploy this contract, you will see that a and b are 0 and 2^256-1, respectively. If you run flow(), you will see that they switch and a = 2^256 - 1 and b = 0. This is because a underflowed and b _overflowed`.

If you run flow() once more, you will see that a = 2^256-2 and b = 1.

OpenZeppelin's SafeMath.sol is a widely used library that helps with this.