bytes4keccak256onERC721Receivedaddressaddressuint256bytes

Bytes4 keccak256, in the context of Ethereum, refers to the 4-byte function selector used to identify which contract function to execute. The selector is a value of type bytes4 derived from the first four bytes of the keccak-256 hash of the function signature. The function signature is the function name followed by a parenthesized, comma-separated list of parameter types, for example "transfer(address,uint256)".

In an Ethereum call, the calldata begins with this 4-byte selector, followed by encoded arguments. The selector allows the Ethereum Virtual Machine to route a call to the correct function, even when multiple functions share the same name but differ in parameter types (overloads). Because the selector is computed from the full signature, overloads produce distinct selectors.

To compute a selector, one takes keccak256("functionSignature") and uses the first 4 bytes of the resulting 32-byte hash. For instance, keccak256("transfer(address,uint256)") starts with 0xa9059cbb, so the selector is 0xa9059cbb. Another common example is "approve(address,uint256)" which yields 0x095ea7b3. In practice, Solidity compilers and various libraries provide helpers to generate these selectors automatically, and developers often rely on ABI encoding utilities to assemble the complete call data.

Applications of the bytes4 selector include low-level calls, contract dispatch, and ABI-based function invocation. It is important to note that changing a function’s signature changes its selector, which can affect contract interfaces and interoperability.