Fallback Function

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Pronunciation

[fôl-bak fungk-shun]

Analogy

Think of the fallback function as a shopkeeper’s default clerk—when a customer arrives with an unrecognized request, the clerk handles it in a generic way.

Definition

A special function without name or arguments that is executed when a contract receives without data or when no other function matches the call signature.

Key Points Intro

Fallback functions provide a catch‑all mechanism in smart contracts via:

Key Points

Unnamed function: Declared as `fallback()` with external visibility.

Payable or not: Must be marked `payable` to receive Ether.

Gas stipend: Limited gas when called via `.send` or `.transfer`.

No arguments/return: Cannot accept data or return values.

Example

A contract implements `fallback() external payable { emit Received(msg.sender, ); }` to log unexpected transfers.

Technical Deep Dive

The fallback function is invoked if calldata is empty and no `receive()` exists, or if calldata does not match any function selector. It shares a 2300‑gas stipend when called via `transfer`/`send`, sufficient only for logging or simple changes. Defining logic here can catch erroneous calls or implement proxy forwarding.

Security Warning

Complex logic in fallback can enable reentrancy attacks due to inadvertent external calls; keep it minimal and use `ReentrancyGuard` where needed.

Caveat

Fallback functions have strict limits and can fail silently if over‑consuming .

Related Terms

Blockchain & Cryptocurrency Glossary

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