Im trying to do what The DAO did and do something when a sender sent money to my contract address. But im not sure how to. The something I want to do is to just create a new user in a mapping with the string sent in the msg.data object.

I have heard about the fallback function, and thought I could extract the msg.data. But that seems to be bytes. Do I have to convert it? If so how do you do that?

Here is my code:


struct User {
    address user;
    bool paid;

mapping(string => User) PremiumUsers;
address owner;

function Premium() {
    owner = msg.sender;

function () {
  var mail = byteconverterToString(msg.data);
  var newUser = PremiumUsers[mail];
  newUser.user = msg.sender;
  newUser.paid = true;

1 Answer 1


IF the fallback function only gets 2300 gas, it can't write to contract storage and here are 2 ideas.

Option 1

You could add an explicit function like receiveEther(string senderName) in the contract. web3.js can be used like contractInstance.receiveEther("name of the sender", {value: web3.toWei(1, "ether"), ...}) and the contract could access the sender name and msg.value easily without parsing the ABI.

Option 2

If you want to avoid having a function the user has to call, then in your contract fallback function you emit an event with msg.data. The app listens for the event and can then parse and store the event data in a database, or if you want to store the data in the contract, the app makes a transaction to the contract to store the data.

A difference with option1 is that storing the data in the contract is a second transaction. It is easier for the user that they don't need to call a function, but the app needs to be built robustly so that the data gets stored (whatever is listening for the event might go down).

  • Ok, il try, but I managed to change a string with the fallback function. Had a string saying no. And changing it in the fallbackfunction, to yes. But is that not writing to the contract storage?
    – vonGohren
    Jun 15, 2016 at 10:47
  • So do you mean to have a filter inside the fallback function, then to have a app listening for that filter and running that function you mention here? I'm trying to avoid having a function the user has to call. I have used a function that uses a modifier which sets a price on the function. But then the user have to get the contract and call the function and everything. I would love to be able for people to just send money and instantly becomming a premium user with the given mailaddress.
    – vonGohren
    Jun 15, 2016 at 11:38
  • Yes, your idea is good for "to avoid having a function the user has to call". I added an option2 to the answer. For your first comment, I added missing details to ethereum.stackexchange.com/questions/5992/….
    – eth
    Jun 15, 2016 at 19:37
  • Thanks I will se what I have time for, holding workshop tomorrow! I will grant it as a final answere when I get it to work!
    – vonGohren
    Jun 15, 2016 at 19:58
  • @vonGohren No problem, good luck with the workshop!
    – eth
    Jun 15, 2016 at 20:03

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