What is a function signature and function selector in solidity (and EVM languages)?

Question

I've heard the term "function signature" as well as "function selector", what are they? What are they for?

Answer 1

Function Signature

A function signature is a combination of the function's name and the types of parameters it takes, combined together as a string with no spaces.

For example, let's say you have a function in solidity where the method looks like this:

function transfer(address sender, uint256 amount) public {
  // Some code here
}

This function would have a function signature of:

transfer(address,uint256)

These are important because we use function signatures to get the next part, function selectors. Additionally, you'll want to use uint256 instead of uint for computing a function signature (and function selector).

Function Selector

A function selector is the first 4 bytes of the call data for a function call that specifies the function to be called. A function selector is the hash of the same function's signature.

Which might be a little confusing, but let's break it down. When someone makes a call to an EVM smart contract, the smart contract needs to know which function it should execute. The piece of code that governs this is known as the function selector, and it might look like this:

0xa9059cbb //this is the function selector for the transfer function signature above. 

You can get the function selector by hashing the string of the function signature in solidity.

bytes4(keccak256(bytes(function_signature)))

If you are a Foundry user, you can calculate the selector using the cast sig CLI:

$ cast sig "transfer(address,uint256)"

Bonus

There are EVM Signature Databases that make it easier to find out the function signature of a selector. Understanding function signatures and selectors allow developers to call functions of any contract without having an ABI.

Answer 2

@patrick-collins answer is useful, but I thought I'd add code to calculate the selector in TypeScript/ethers/hardhat:

/**
 * Returns an Ethereum function selector given the function signature
 * @param functionSignature The name of the function (and its arguments in parens)
 */
function ethereumFunctionSelector(functionSignature: string): BytesLike {
  const functionSignatureAsUtf8Bytes = hardhat.ethers.utils.toUtf8Bytes(functionSignature)
  const signatureCksum = ethers.utils.keccak256(functionSignatureAsUtf8Bytes)
  // end is 10 because we want the first four bytes (8 characters), and
  // signatureCksum starts with 0x (two characters).  arrayify expects
  // what ethers calls a DataHexStringOrArrayish https://docs.ethers.org/v5/api/utils/bytes/
  // which is something like 0xdd365b8b
  const firstFourBytesOfChecksum = signatureCksum.slice(0, 10)
  const result = hardhat.ethers.utils.arrayify(firstFourBytesOfChecksum)
  return result
}