Checks-Effects-Interactions

Checks-Effects-Interactions Explained in Detail

Checks-Effects-Interactions is a control-flow pattern for functions that touch state and then call external code. The safe order is: validate the request, update internal state, then perform the external interaction.

The pattern reduces the chance that a callback can observe stale balances, shares, debt, or permissions.

Smart contract example

The unsafe version sends ETH before clearing the balance:

function withdraw() external {
    uint256 amount = balances[msg.sender];
    require(amount > 0, "empty");

    (bool ok,) = msg.sender.call{value: amount}("");
    require(ok, "send failed");

    balances[msg.sender] = 0;
}

The safer CEI order clears the balance before the external call:

function withdraw() external {
    uint256 amount = balances[msg.sender];
    require(amount > 0, "empty");

    balances[msg.sender] = 0;

    (bool ok,) = msg.sender.call{value: amount}("");
    require(ok, "send failed");
}

Checks-Effects-Interactions in Auditing

CEI is a baseline review technique. It gives auditors a fast way to spot callback windows before deeper invariant testing.

CEI is not a complete defense. Cross-function reentrancy, read-only reentrancy, token hooks, and multi-contract state machines can still break a protocol that appears to follow CEI in one function. Use the reentrancy attack guide to trace those callback variants beyond a single withdrawal function.

Red flags in code

• ETH transfer before balance, debt, share, or collateral updates.

• Token transfer before accounting is finalized.

• call, delegatecall, hooks, or callbacks inside sensitive functions.

• Partial state updates before an external interaction.

• CEI applied to one function while sibling functions touch the same state.

• Complex protocols relying on CEI without invariant tests.

How to test or review it

• Mark every external interaction in the function.

• List the state that is updated before and after each interaction.

• Try a malicious receiver that calls back during the interaction.

• Test same-function and cross-function callback paths.

• Check whether protocol invariants hold during the callback window.

• Pair CEI with a reentrancy guard when shared state remains exposed.