Reentrancy guard pattern in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Reentrancy guard pattern

O(n)

Understanding Time Complexity

When using the reentrancy guard pattern, we want to know how the cost of checking and updating the guard changes as the number of calls grows.

We ask: How does the time spent grow when many function calls happen?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

bool locked = false;

function withdraw() public {
  require(!locked, "No reentrancy");
  locked = true;
  // send funds
  locked = false;
}
    

This code prevents a function from being called again before the first call finishes, protecting against repeated entry.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Checking and setting the locked flag on each call.
• How many times: Once per function call, no loops or recursion inside.

How Execution Grows With Input

Each call does a simple check and update, so the work grows directly with the number of calls.

Input Size (n)	Approx. Operations
10	10 checks and updates
100	100 checks and updates
1000	1000 checks and updates

Pattern observation: The time grows steadily as calls increase, with no extra loops or nested work.

Final Time Complexity

Time Complexity: O(n)

This means the time grows in a straight line with the number of calls, as each call does a fixed amount of work.

Common Mistake

[X] Wrong: "The reentrancy guard adds extra loops or slows down calls exponentially."

[OK] Correct: The guard only checks and sets a simple flag once per call, so it does not add loops or nested work.

Interview Connect

Understanding how simple checks like the reentrancy guard scale helps you explain smart contract safety clearly and confidently.

Self-Check

"What if the withdraw function included a loop over a list of recipients? How would the time complexity change?"

Practice

1.

What is the main purpose of the reentrancy guard pattern in blockchain smart contracts?

easy

A. To prevent a function from being called again before it finishes execution

B. To speed up the execution of smart contracts

C. To allow multiple users to access a function simultaneously

D. To reduce the gas cost of transactions

Solution

1. Step 1: Understand the problem reentrancy causes

   Reentrancy happens when a function is called again before the first call finishes, causing unexpected behavior or exploits.

2. Step 2: Identify the role of the reentrancy guard

   The reentrancy guard uses a lock to block repeated calls until the first call completes, preventing reentrancy attacks.

3. Final Answer:

   To prevent a function from being called again before it finishes execution -> Option A

4. Quick Check:

   Reentrancy guard = prevent repeated calls [OK]

Hint: Reentrancy guard stops repeated calls during execution [OK]

Common Mistakes:

• Thinking it speeds up contract execution
• Believing it allows simultaneous access
• Confusing it with gas optimization

2.

Which of the following is the correct way to declare a reentrancy guard variable in Solidity?

contract Example {
    // Fill in the blank
    _____ private locked = false;
}

easy

A. string

B. bool

C. int

D. address

Solution

1. Step 1: Identify the variable type for a lock

   The lock variable is used as a flag to indicate if the function is running, so it should be a boolean (true/false).

2. Step 2: Match the correct Solidity type

   In Solidity, bool is the type for true/false values, suitable for a lock variable.

3. Final Answer:

   bool -> Option B

4. Quick Check:

   Lock variable type = bool [OK]

Hint: Lock variables are boolean flags in Solidity [OK]

Common Mistakes:

• Using int instead of bool for lock
• Using string or address types incorrectly
• Forgetting to initialize the variable

3.

Consider this Solidity function using a reentrancy guard:

bool private locked = false;

function withdraw(uint amount) public {
    require(!locked, "Reentrant call detected");
    locked = true;
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Transfer failed");
    locked = false;
}

What happens if msg.sender.call triggers a fallback that calls withdraw again?

medium

A. The second call is blocked by the lock and reverts

B. The second call succeeds and causes reentrancy

C. The contract runs out of gas and fails

D. The lock variable resets before the second call

Solution

1. Step 1: Analyze the lock status during the first call

   When withdraw is called, locked is set to true before sending funds.

2. Step 2: Understand the effect on reentrant call

   If msg.sender.call triggers a fallback that calls withdraw again, the require(!locked) check fails because locked is true, blocking the call.

3. Final Answer:

   The second call is blocked by the lock and reverts -> Option A

4. Quick Check:

   Lock true blocks reentrant call [OK]

Hint: Lock true means no reentrant calls allowed [OK]

Common Mistakes:

• Assuming reentrancy still happens
• Thinking lock resets before second call
• Confusing gas failure with reentrancy

4.

Identify the bug in this reentrancy guard implementation:

bool private locked = false;

function withdraw(uint amount) public {
    require(!locked, "Reentrant call detected");
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Transfer failed");
    locked = true;
    locked = false;
}

medium

A. The require statement is missing

B. The lock variable is never set to true

C. The lock is set after the external call, allowing reentrancy

D. The call to msg.sender is incorrect syntax

Solution

1. Step 1: Check when the lock is set

   The lock is set to true only after the external call, which means the function is not protected during the call.

2. Step 2: Understand reentrancy risk

   Because the lock is set too late, a reentrant call can happen during the external call, causing vulnerability.

3. Final Answer:

   The lock is set after the external call, allowing reentrancy -> Option C

4. Quick Check:

   Lock must be set before external calls [OK]

Hint: Set lock before external calls to prevent reentrancy [OK]

Common Mistakes:

• Setting lock after external call
• Forgetting to set lock at all
• Misplacing require statements

5.

You want to protect a function that updates user balances and sends Ether. Which is the best way to apply the reentrancy guard pattern?

function safeWithdraw(uint amount) public {
    // Your code here
    balances[msg.sender] -= amount;
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Transfer failed");
}

hard

A. Update balances after sending Ether without any lock

B. Do not use any lock, just rely on require statements

C. Use a lock variable but set it after sending Ether

D. Set a lock variable before updating balances and sending Ether, then release it after

Solution

1. Step 1: Understand the order of operations for safety

   To prevent reentrancy, set a lock before any state changes and external calls, then release it after.

2. Step 2: Apply the reentrancy guard pattern correctly

   Set the lock variable to true before updating balances and sending Ether, then set it back to false after the transfer completes.

3. Final Answer:

   Set a lock variable before updating balances and sending Ether, then release it after -> Option D

4. Quick Check:

   Lock before state change and external call [OK]

Hint: Lock before balance update and transfer, unlock after [OK]

Common Mistakes:

• Setting lock after sending Ether
• Updating balances after external call
• Not using any lock at all