> ## Documentation Index
> Fetch the complete documentation index at: https://docs.theblockchainlibrary.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Reentrancy Guard Pattern

> Implement a reentrancy guard to prevent one of the most common smart contract attacks.

# Reentrancy Guard Pattern

Implement a reentrancy guard to prevent one of the most common smart contract attacks.

Implement a reentrancy guard to prevent one of the most common smart contract attacks.

## Explanation

**Reentrancy** is the most famous smart contract vulnerability (the DAO hack, 2016). A malicious contract re-enters a function before state is updated, draining funds.

**How this guard works:**

* A `_locked` boolean is set `true` at function entry.
* If the external call triggers a re-entry, `nonReentrant` reverts.
* The boolean resets after the function completes.

OpenZeppelin provides `ReentrancyGuard` — prefer it in production. Always combine with checks-effects-interactions.

## Code

```solidity theme={null}
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

abstract contract ReentrancyGuard {
    bool private _locked;

    modifier nonReentrant() {
        require(!_locked, "Reentrancy: locked");
        _locked = true;
        _;
        _locked = false;
    }
}

contract SafeVault is ReentrancyGuard {
    mapping(address => uint256) public balances;

    function deposit() external payable {
        balances[msg.sender] += msg.value;
    }

    function withdraw(uint256 amount) external nonReentrant {
        require(balances[msg.sender] >= amount, "Insufficient");
        balances[msg.sender] -= amount;
        (bool ok, ) = msg.sender.call{value: amount}("");
        require(ok, "Failed");
    }
}
```

***

**Canonical knowledge ID:** `code-example:reentrancy-guard-pattern`