> For the complete documentation index, see [llms.txt](https://calnix.gitbook.io/eth-dev/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://calnix.gitbook.io/eth-dev/solidity-patterns/checks-effects-interactions-pattern.md).
# checks-effects-interactions pattern
## Objective: Negate Reentrancy attacks
### Negative Example
```solidity
function withdrawToken(uint256 amount) public {
require(userBalance[msg.sender] >= amount, "requested amount exceeds user balance");
require(iToken.transferFrom(address(this), msg.sender, amount), "failed on withdrawal");
userBalance[msg.sender] -= amount;
```
* `iToken.transferFrom` could be maliciously coded to call `withdrawToken` again,
* and the contract would transfer iToken out in a loop without modifying the user balance.
* The attacker would only have to be careful of stopping the loop before running out of gas, and it would drain your contract of iToken.
### Positive Example
```solidity
function withdraw(uint amount) external {
balances[msg.sender] -= amount;
bool success = wmdToken.transfer(msg.sender, amount);
require(success, "Withdrawal failed!");
emit Withdrawal(msg.sender, amount);
}
```
### Further Readings
*
---
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