Following is code for a re-entrancy guard, but not sure if I understand it properly:

contract Test {
    using SafeMath for uint256;
    uint256 private counter = 1;
    modifier entrancyGuard() {
        counter = counter.add(1); // counter adds 1 to the existing 1 so becomes 2
        uint256 guard = counter; // assigns 2 to the "guard" variable
        require(guard == counter, "That is not allowed"); // 2 == 2? 

    function safeWithdraw(uint256 amount) external entrancyGuard() {
        // Withdraw code

I'm not sure how the modifier prevents re-entrancy.

  • Is this from Udacity course? I had a hard time trying to wrap my head around this when i first saw this code. I tried a few dry runs and it seemed to fail.Also it would be simple to use a boolean/mutex lock sort of solution like OpenZeppelin does. @abcoathup has already put the link in answer. I would recommend to follow the solution as given in OpenZeppelin contract instead.
    – Imdad
    May 15, 2020 at 4:27

2 Answers 2


The code:

modifier modi() {
function func() modi() {

Is equivalent to:

function func() {

If func is re-entered (i.e., called from within the execution of stuff), then prolog is executed a second time before epilog was executed for the first time.

So by identifying and reverting upon a scenario in which prolog is executed a second time before epilog was executed for the first time, we can guard func from being re-entered.


My understanding is that:

  1. The counter is incremented
  2. A copy of the value of counter is kept
  3. Control is returned (https://solidity.readthedocs.io/en/v0.6.8/contracts.html#function-modifiers)
  4. There could be reentrancy where the counter would be incremented
  5. A check is made that the copy of the counter value still equals the counter otherwise it reverts

You may want to look at the OpenZeppelin Contracts implementation: Documentation: https://docs.openzeppelin.com/contracts/3.x/api/utils#ReentrancyGuard Code: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.0.1/contracts/utils/ReentrancyGuard.sol

I also suggest looking at the following post on Reentrancy: https://forum.openzeppelin.com/t/reentrancy-after-istanbul/1742

Disclosure: I am the Community Manager at OpenZeppelin

If you have questions on using OpenZeppelin you can also ask in the Community Forum: https://forum.openzeppelin.com/

  • I'm still a bit confused. When there is reentrancy after the first run, does the counter outside of entrancyGuard() get reset to 1 through uint256 private counter = 1; and the variable uint256 guard, which is inside of entrancyGuard(), remain at 2? The Open Zeppelin document you referred to makes more sense to me because _notEntered = true; is set AFTER the function in question is run, whereas my example sets counter to be equal to the guard uint256 guard = counter;before the function is run.
    – Kevvv
    May 15, 2020 at 14:45
  • 1
    counter is updated to 3 when there is reentrancy whilst the original guard remains at 2 causing a revert.
    – abcoathup
    May 21, 2020 at 10:49

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