RareSkills Solidity Interview Question #19 Answered: What is the difference between fallback and receive?

Faybian Byrd

Published in

Coinmonks

3 min readFeb 2, 2024

This series will provide answers to the list of Solidity interview questions that were published by RareSkills..

Question #19 (Easy): What is the difference between fallback and receive?

Answer: The difference between fallback and receive is that receive is executed on a call to the contract with empty calldata and fallback is executed on a call to the contract when no function selector matches the function selector given in the calldata, or if the calldata was empty and there is no receive function. Another difference is that there is only one version of receive and there are multiple versions of fallback that can be declared (one per contract). Also, receive must be declared as payable while fallback is optionally payable (true as of the latest version 0.8.24).

Demonstration:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.24;

contract TestPayable {
    uint256 public x;
    uint256 public y;

    /**
     * This function is called for all messages sent to this contract, except
     * plain Ether transfers (there is no other function except the receive
     * function). Any call with non-empty calldata to this contract will
     * execute the fallback function (even if Ether is sent along with the 
     * call).
     */
    fallback() external payable {
        x = 1;
        y = msg.value;
    }

    /**
     * This function is called for plain Ether transfers, i.e.
     * for every call with empty calldata.
     */
    receive() external payable {
        x = 2;
        y = msg.value;
    }

    /**
     * Commented out since only one fallback function can de declared in the
     * contract. However, this is also a valid declaration of a fallback
     * function that accepts `bytes calldata` as input and returns `bytes
     * memory`. 
     */
    /* fallback (bytes calldata input) external payable returns (bytes memory) {
        x = 3;
        y = msg.value;
        
        return input;
    }*/
}

Further Discussion:

Although a payable fallback function can be used to receive Ether, this method is not recommended since it would not fail due to interface confusions on the part of the sender. For example, if a user mistakenly sends Ether with a call to a contract function that does not exist, the payable fallback function would receive the Ether and not revert the transaction. This could result in a bad user experience due to a loss of funds.

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Disclosure: Some content may have been generated with the help of artificial intelligence.

RareSkills Solidity Interview Question #19 Answered: What is the difference between fallback and receive?

Written by Faybian Byrd