Solidity Function Selector Workbench

Function selectors, without the wall of text

When you call a function on an Ethereum smart contract, the EVM doesn't use the function name directly. Instead, each function is identified by its selector, a compact 4-byte identifier derived from the function's signature. This selector is the first 4 bytes of the keccak256 hash of the function's canonical signature.

For example, calling transfer(address,uint256) on an ERC-20 token doesn't send the string "transfer" on-chain. It sends 0xa9059cbb followed by ABI-encoded arguments. The contract's dispatcher matches this 4-byte prefix to the correct function's code.

How Function Selectors Are Computed

The computation follows three steps:

1. Canonicalize the signature. Write the function name followed by its parameter types in parentheses, with no spaces and no parameter names: transfer(address,uint256)
2. Hash with keccak256. Compute the keccak256 hash of the UTF-8 encoded signature string, producing 32 bytes.
3. Take the first 4 bytes. The function selector is bytes4(keccak256("transfer(address,uint256)")), which equals 0xa9059cbb.

Functions vs Events vs Errors

Functions use a 4-byte selector placed at the beginning of transaction calldata. The EVM's dispatcher uses this to route the call.

Events use the full 32-byte keccak256 hash as topic[0] in the transaction log. The full hash is needed because logs are indexed for efficient filtering, and 4 bytes would cause too many collisions across all contracts.

Custom Errors (introduced in Solidity 0.8.4) use a 4-byte selector just like functions. When a contract reverts with a custom error, the revert data starts with the error's 4-byte selector instead of the generic Error(string) selector.

4byte Lookup and Error Selectors

If you see a value like 0xef28f901 in revert data, calldata, a trace, or an explorer, treat it as a selector first. Paste it into the decode tab to check local known matches, then compare against 4byte.directory or Openchain when the selector is not in the local database.

For a custom error, compute the selector from the canonical error signature, such as NotOwner(address). For collision-sensitive systems, review the selector against proxy, router, diamond, and fallback dispatch code so a selector collision cannot route a call to the wrong logic.