• receive()
• fallback()

Solidity Smart contracts have two built in fallback functions that are triggered when no other function in the contract matches the specified function in the call. This can happen for multiple reasons like a type, the use of Smart Contract Proxies or for Re-Entrancy attacks.

There are two fallback functions:

• received()
• fallback()

flowchart TD;

    CALLDATA --> A
    A{is msg.data empty?} --> |yes|B{receive exists?};
    A --> |no|C{function selector match?}
    B --> |yes|G((received))
    B --> |no|D{fallback exists?}
    D --> |yes|H((fallback))
    D --> |no|F((transaction is reverted))
    C --> |no|D
    C --> |yes|E((execute function))

receive()

• This is also an unnamed external payable function without any input or output parameters.
• This function is executed when the smart contract is called with ether but no calldata is provided.
• If there is no receive() function, then the fallback() is executed (and if there is no receive nor fallback, then the transaction reverts).

In the example below receive() is executed when we invoke the CALLDATA function without any argument.

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.7;

contract FallbackReceive {

    uint256 private value;

    receive() external payable {
        value += 1;
    }

    function getValue() public view returns(uint256) {
        return value;
    }
}

However, if we pass a hexadecimal value (e.g. 000A) to CALLDATA it will fail because it requires fallback().

fallback()

• This is an unnamed external function without any input or output parameters.
• This function is executed when no other functions match the intended function calls and calldata is sent.
• If ETH are sent to smart contracts with calldata’s and the function selector doesn’t match any existing function in the contract, the fallback() function is executed (again, if no fallback is implemented, the transaction reverts).

In the example below fallback() is executed when we invoke the CALLDATA function with (e.g. 000A) and without arguments.

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.7;

contract FallbackReceive {

    uint256 private value;

    fallback() external payable {
        value += 2;
    }

    function getValue() public view returns(uint256) {
        return value;
    }
}