How do I make a smart contract send a message to a user along with a transaction?

Question

Smart contracts can obviously send transactions through the Ethereum network but can they also send a simple message along with the transaction?

Usaully you would put the message into hex code in the "data" field but how do you make a smart contract do that?

Thanks.

Answer 1

I'm assuming you would like to send a message along with a transaction that goes to a "normal" account, not a "smart contract" account. Maybe even from a "normal" account as well.

What you can do is convert your text message to hex, using any online converter, for example this one.

Most wallets have a data field for a transaction, where you then paste the hex. From within a contract, you would have to use the call method, as described here.

For example, a message like "3 o'clock at the tower" converts to "0x33206f27636c6f636b2061742074686520746f776572", which you send along in the "data" field.

If the receiver does not get these data in their wallet, they will have to look it up the transaction on the blockchain explorer like etherscan, scroll down and find the field Input Data, where your hex string will be displayed.

Be sure that the receiver know how to convert this back. They could use an online hex to text converter like this one.

Answer 2

The data field is used especially when calling functions in contracts - all the data (called function name and arguments) go in that field.

So your contract can simply call a function in another contract with parameters - assuming the function accepts such parameters. So contracts take in extra data but calls to EOAs (Externally Owned Account) don't take extra data as they don't understand what to do with the data. I'm a bit unsure what happens to a transaction to an EOA with extra data - I guess it will just go through even if the receiver doesn't understand the data.

Answer 3

Send a message to a user.

It depends on what you mean by "send a message" and "to a user".

If you mean SMS, Email, push notification, etc., then no. This is because a fundamental limitation is that contracts can have only very limited interaction with the outside world.

• Users sign transactions (input)
• Functions can return values to other contracts (but not the outside world)
• Functions can emit events.

You can create something similar to contracts sending messages to users based on listening to event logs, which are visible to everyone. Either the client app listens or an intermediary server listens, perchance to send an SMS, email or push notification.

Hope it helps.