Is it possible to send an extra argument or data when interacting with a contract?

Question

Take the transfer function of the DAI contract as example. It allows only two arguments: dst and wad.

This is how the decoded data input looks like on a sample transaction:

Function: transfer(address dst, uint256 wad)

MethodID: 0xa9059cbb
[0]:  0000000000000000000000006262998ced04146fa42253a5c0af90ca02dfd2a3
[1]:  000000000000000000000000000000000000000000000002c3c465ca58ec0000

Is it possible to attach some extra data when calling the transfer function, so that the decoded data input looks as follows:

Function: transfer(address dst, uint256 wad)

MethodID: 0xa9059cbb
[0]:  0000000000000000000000006262998ced04146fa42253a5c0af90ca02dfd2a3
[1]:  000000000000000000000000000000000000000000000002c3c465ca58ec0000
[2]:  000000000000000000000000000000000000000000000000000000000000fede

(obviously without the transaction being reverted or something)

If not possible, is there any other way to attach a message to a transaction that interacts with a contract?

Answer 1

It is up to each contract what they will do with extra data in a transaction's input field.

• It can ignore any extra data
• It can revert if they receive more thant than they were expecting.

Unfortunatelly there is no standard way to add extra metadata to a transaction to a contract.

If target is an EOA you can add any information you want to input field.

---

Example to append data to a ERC20 transfer event

const message = '686f6c61'
const data = token.methods.transfer(recipient, amount).encodeABI()

const tx = {
  nonce: '0x00',
  gasPrice: '0x09184e72a000',
  gasLimit: '0xea60',
  to: token.options.address,
  value: '0x00',
  data: data + message
};

const signedTx = new Tx(tx, {'chain':'ropsten'});
signedTx.sign(privateKey);

const serializedTx = signedTx.serialize();
web3.eth.sendSignedTransaction('0x' + serializedTx.toString('hex'))
.on('receipt', console.log);