Table of Contents for How Solidity function selectors work

How selectors work

When you write Solidity code, you will write contracts with functions

contract Example {
 function funcA() public {
 }
 function funcB(uint256 someNum) public {
 }
}

When this gets compiled, the original names (funcA, funcB in the above example) do not get stored.

Instead each function is changed to a selector. Instead the first four bytes of a hash of the function (and arguments) are stored, and these are used when the functions are called.

In psuedocode, it looks like this:

var $funcANameAndArgs = 'funcA()';
var $funcBNameAndArgs = 'funcB(uint256)';

var $funcAHash = keccack256(funcANameAndArgs);
var $funcBHash = keccack256(funcBNameAndArgs);

var $selectorFuncA = firstFourBytes($funcAHash);
var $selectorFuncB = firstFourBytes($funcBHash);

It hashes the function name, and the types of any arguements (but not the names), and takes the first 4 bytes (8 characters in hex).

Note: If you define a function as function something(uint aNumber), then you have to hash something(uint256) (note the full uint256 is used, not just uint).

How to call another contract’s function, using a selector

You can use abi.encodeWithSignature

addr.call(abi.encodeWithSignature("transfer(address,uint256)", yourAddressVariable, yourAmountVariable))

Selectors for library functions

If you have a library, with a function that has a struct then the selector is generated in a different way.

Normally if you have a function that has a param that is a struct, the selector will be the first 4 bytes of the keccak256 hash of each variable type within a struct.

e.g.

struct CoolStruct {
    uint256 coolValue;
    uint256 otherCoolValue;
}

function getHash(CoolStruct memory self) public pure returns (bytes32 hash) {
    hash = keccak256(abi.encodePacked(
        self.coolValue
    ));
}

You’d expect the selector to be keccak256("getHash((uint256,uint256))"), which is 0x31183ed7

but in a library it is the first four bytes of keccak256("getHash(LibStructTest.CoolStruct)") (0x54d08d7a)

For more info about this (and also the source that I found / copied examples from) is https://github.com/ethereum/solidity/issues/6420

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