Reentrancy guard pattern in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Reentrancy Guard Pattern in blockchain smart contracts?

It is a technique used to prevent a function from being called again before the first call finishes, stopping attacks where a contract repeatedly calls itself to drain funds.

beginner

Why is reentrancy a problem in smart contracts?

Because it allows attackers to repeatedly enter a function before the previous execution finishes, potentially causing unexpected behavior like multiple withdrawals.

intermediate

How does a bool locked variable help in implementing a reentrancy guard?

It acts like a lock: when a function starts, it sets locked = true. If the function is called again before finishing, it sees the lock and stops execution.

intermediate

What Solidity keyword is commonly used to create a reentrancy guard modifier?

The modifier keyword is used to create reusable code that can check and set the lock before running the function body.

advanced

Give a simple example of a reentrancy guard modifier in Solidity.

```solidity bool private locked = false; modifier noReentrant() { require(!locked, "No reentrancy"); locked = true; _; locked = false; } ```

What does the reentrancy guard pattern primarily prevent?

A Multiple simultaneous calls to a function before it finishes

B Unauthorized access to private variables

C Overflow errors in arithmetic operations

D Slow transaction processing

The reentrancy guard stops a function from being called again before the first call finishes, preventing repeated entries.

In Solidity, which of these is commonly used to implement a reentrancy guard?

A A public state variable

B A boolean lock variable and a modifier

C A fallback function

D An event logger

A boolean lock variable combined with a modifier is the usual way to implement a reentrancy guard.

What happens if a reentrant call is detected in a function protected by a reentrancy guard?

A The contract self-destructs

B The call proceeds normally

C The call is rejected with an error

D The call is queued for later

The guard uses a require statement to reject any reentrant calls, stopping execution with an error.

Why is it important to set the lock variable back to false after the function finishes?

A To allow future calls to the function

B To save gas fees

C To increase contract balance

D To trigger an event

Resetting the lock allows the function to be called again later safely.

Which of these attacks does the reentrancy guard help prevent?

A Spam transactions

B Phishing attacks

C Sybil attacks

D Repeated withdrawal attacks

Reentrancy guards stop attackers from repeatedly withdrawing funds by reentering a function.

Explain how the reentrancy guard pattern works to protect a smart contract function.

Think about how a door lock stops someone from entering twice at the same time.

Describe a simple Solidity code example that uses a reentrancy guard modifier.

Focus on the lock variable and how it changes before and after the function runs.