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If someone creates a smart contract (and is owner of that contract), is it possible for that person to collect ETH and other tokens, that are sent to the address of smart contract?

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This is only possible, if the creator of a smart contract makes it possible. If a contract does not implement any withdrawal() function, then the funds may be irretrievable by the owner.

This is a feature, not a bug. Would you want to ether delta smart contract creator to withdraw all the ether and tokens from the contract at a whim? No.

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A smart contract does not have any owner or withdrawal mechanisms unless they have been coded in the contract. This gives smart contracts a power of being trustless: it is possible to use contracts without trusting the owner, by carefully understanding the code of the contract.

Withdrawing ERC-20 tokens

If you're a smart contract author, and want to be able to withdraw ERC-20 tokens, you need to do 2 steps.

Step 1: Add this interface (subset of ERC-20) above your contract

contract Token {
    function transfer(address _to, uint _value) returns (bool success);
    function balanceOf(address _owner) constant returns (uint balance);
}

Step 2: Here is a function you can add inside your contract

function withdrawTokens(address tokenContract) external onlyOwner {
    Token tc = Token(tokenContract);

    tc.transfer(owner, tc.balanceOf(this));
}

This assumes you've correctly written and configured yourself as the owner. Since your contract may receive many different types of tokens, you need to withdrawTokens from each tokenContract. If owner is another contract, it must also have a withdrawTokens function to prevent tokens from getting stuck in the owner contract.

Withdrawing ether

The code is simpler:

function withdrawEther() external onlyOwner {
    owner.transfer(this.balance);
}

Again, if owner is another contract, it must also have a withdrawEther function to prevent ether from getting stuck in the owner contract.

Another reference: How to withdraw Ether from a contract

  • Thanks, so if the contract is already up, and does not have such function, are funds stuck? Can it be upgraded, so that this can be added? – nikola ivkov Oct 7 '17 at 15:48
  • @nikolaivkov Yes, funds will be stuck. Similarly, a contract also cannot be upgraded unless it already has code allowing it to be upgraded in some way. – eth Oct 10 '17 at 8:47

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