> For the complete documentation index, see [llms.txt](https://docs.optimumsec.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.optimumsec.xyz/coding/reentrancy-guards.md).

# Use Reentrancy Guards

Reentrancy is generally not expected behavior in most smart contracts, with the exception of callback systems like those used in Uniswap. As a best practice, this behavior should be restricted unless explicitly necessary.

In practice, all state-changing functions that include external calls should be protected by a reentrancy guard by default, and any removal of this protection should only occur after a thorough assessment of its impact.

If gas optimization is a concern, consider using a reentrancy guard library that leverages [transient storage](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/ReentrancyGuardTransient.sol). Be aware that some EVM chains do not support transient storage. For details on opcode support across different chains, refer to [evmdiff.com](https://www.evmdiff.com/features?feature=opcodes).


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