## Preamble EIP: 20 Title: ERC-20 Token Standard Author: Fabian Vogelsteller , Vitalik Buterin Type: Standard Category: ERC Status: Accepted Created: 2015-11-19 ## Simple Summary A standard interface for tokens. ## Abstract The following standard allows for the implementation of a standard API for tokens within smart contracts. This standard provides basic functionality to transfer tokens, as well as allow tokens to be approved so they can be spent by another on-chain third party. ## Motivation A standard interface allows any tokens on Ethereum to be re-used by other applications: from wallets to decentralized exchanges. ## Specification ## Token ### Methods **NOTE**: Callers MUST handle `false` from `returns (bool success)`. Callers MUST NOT assume that `false` is never returned! #### name Returns the name of the token - e.g. `"MyToken"`. OPTIONAL - This method can be used to improve usability, but interfaces and other contracts MUST NOT expect these values to be present. ``` js function name() constant returns (string name) ``` #### symbol Returns the symbol of the token. E.g. "HIX". OPTIONAL - This method can be used to improve usability, but interfaces and other contracts MUST NOT expect these values to be present. ``` js function symbol() constant returns (string symbol) ``` #### decimals Returns the number of decimals the token uses - e.g. `8`, means to divide the token amount by `100000000` to get its user representation. OPTIONAL - This method can be used to improve usability, but interfaces and other contracts MUST NOT expect these values to be present. ``` js function decimals() constant returns (uint8 decimals) ``` #### totalSupply Returns the total token supply. ``` js function totalSupply() constant returns (uint256 totalSupply) ``` #### balanceOf Returns the account balance of another account with address `_owner`. ``` js function balanceOf(address _owner) constant returns (uint256 balance) ``` #### transfer Transfers `_value` amount of tokens to address `_to`, and MUST fire the `Transfer` event. The function SHOULD `throw` if the `_from` account balance does not have enough tokens to spend. A token contract which creates new tokens SHOULD trigger a Transfer event with the `_from` address set to `0x0` when tokens are created. *Note* Transfers of 0 values MUST be treated as normal transfers and fire the `Transfer` event. ``` js function transfer(address _to, uint256 _value) returns (bool success) ``` #### transferFrom Transfers `_value` amount of tokens from address `_from` to address `_to`, and MUST fire the `Transfer` event. The `transferFrom` method is used for a withdraw workflow, allowing contracts to transfer tokens on your behalf. This can be used for example to allow a contract to transfer tokens on your behalf and/or to charge fees in sub-currencies. The function SHOULD `throw` unless the `_from` account has deliberately authorized the sender of the message via some mechanism. *Note* Transfers of 0 values MUST be treated as normal transfers and fire the `Transfer` event. ``` js function transferFrom(address _from, address _to, uint256 _value) returns (bool success) ``` #### approve Allows `_spender` to withdraw from your account multiple times, up to the `_value` amount. If this function is called again it overwrites the current allowance with `_value`. **NOTE**: To prevent attack vectors like the one [described here](https://docs.google.com/document/d/1YLPtQxZu1UAvO9cZ1O2RPXBbT0mooh4DYKjA_jp-RLM/) and discussed [here](https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729), clients SHOULD make sure to create user interfaces in such a way that they set the allowance first to `0` before setting it to another value for the same spender. THOUGH The contract itself shouldn't enforce it, to allow backwards compatilibilty with contracts deployed before ``` js function approve(address _spender, uint256 _value) returns (bool success) ``` #### allowance Returns the amount which `_spender` is still allowed to withdraw from `_owner`. ``` js function allowance(address _owner, address _spender) constant returns (uint256 remaining) ``` ### Events #### Transfer MUST trigger when tokens are transferred, including zero value transfers. ``` js event Transfer(address indexed _from, address indexed _to, uint256 _value) ``` #### Approval MUST trigger on any successful call to `approve(address _spender, uint256 _value)`. ``` js event Approval(address indexed _owner, address indexed _spender, uint256 _value) ``` ## Implementation There are already plenty of ERC20-compliant tokens deployed on the Ethereum network. Different implementations have been written by various teams that have different trade-offs: from gas saving to improved security. #### Example implementations are available at - https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/token/StandardToken.sol - https://github.com/ConsenSys/Tokens/blob/master/contracts/StandardToken.sol #### Implementation of adding the force to 0 before calling "approve" again: - https://github.com/Giveth/minime/blob/master/contracts/MiniMeToken.sol ## Copyright Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).