ERC20接口下USDT代币的深入解析

ERC20代币合约规则简介

ERC20 是各个代币的标准接口。ERC20 代币仅仅是以太坊代币的子集，为了充分兼容 ERC20，开发者需要将一组特定的函数（接口）集成到他们的智能合约中，以便在高层面能够执行这些操作：获得代币总供应量、获得账户余额、转让代币、批准花费代币。

ERC20 让以太坊区块链上的其他智能合约和去中心化应用之间无缝交互。一些具有部分但非所有ERC20标准功能的代币被认为是部分ERC20兼容，这还要视其具体缺失的功能而定，但总体是它们仍然很容易与外部交互。

简单的来说：ERC20接口就是一套在智能合约上开发Token(代币)的规范协议。

需要说明的是：代币是运行在智能合约上的币，其没有自己的私链，交易代币需要调用函数而不是在以太坊上真正的发起一个交易，可以看做以太坊账户的附属品。每个账户可能在多个合约代币中有贮藏代币。

ERC20协议的详细讲解可见尚硅谷视频：/video/BV1NJ411D7rf?p=34

USDT代币

现在市值较大的代币都基本支持ERC20接口规范，ERC20接口规范构造简单。

本文主要通过研究目前在以太坊上运行的市值最高的USDT代币，以此来了解ERC20这一代币规范的使用。

USDT代币的合约架构

USDT代币功能

ERC20要求下的转账、余额查询、授权消费功能代币紧急暂停与重起代币用户黑名单合约便捷升级，可适应非ERC20协议代币代币管理者权限移交

USDT代币合约源码

对着源码撸了一遍，做了略微改动（适应编译器版本）以及详细的内容说明。

适应版本： 0.4.22版本编译器无warning无error

pragma solidity >0.4.17;/*** @title SafeMath 数学安全函数* @dev Math operations with safety checks that throw on error.*/library SafeMath {function mul(uint256 a, uint256 b) internal pure returns (uint256) {if (a == 0) {return 0;}uint256 c = a * b;assert(c / a == b);return c;}function div(uint256 a, uint256 b) internal pure returns (uint256) {// 分母大于0在solidity合约中已经会自动判定了// assert(b > 0); // Solidity automatically throws when dividing by 0uint256 c = a / b;// assert(a == b * c + a % b); // There is no case in which this doesn't holdreturn c;}function sub(uint256 a, uint256 b) internal pure returns (uint256) {assert(b <= a);return a - b;}function add(uint256 a, uint256 b) internal pure returns (uint256) {uint256 c = a + b;assert(c >= a);return c;}}/*** @title Ownable 代币的拥有者* @dev The Ownable contract has an owner address, and provides basic authorization control.* @dev functions, this simplifies the implementation of "user permissions".* @dev 这个合约主要是指明合约创建人为代币的创建者，还包括授权控制功能，简化“用户权限”.*/contract Ownable{//"拥有者"address public owner;/*** @dev The Ownable constructor sets the original `owner` of the contract to the sender* account.* @dev 把创建合约的人作为初始的“拥有者”.*/constructor() public{owner = msg.sender;}/*** @dev Throws if called by any account other than the owner.* @dev 暂时未知, 应该是只能拥有者进行的操作.*/modifier onlyOwner(){require(msg.sender == owner, "仅owner调用！");//这一行表示继承此合约中使用_;}/*** @dev Allows the current owner to transfer control of the contract to a newOwner.* @dev 权力转移给新的拥有者* @param newOwner The address to transfer ownership to.*/function transferOwnership(address newOwner) public onlyOwner{//先确保新用户不是0x0地址require(newOwner != address(0), "不能给地址0转移owner");owner = newOwner;}}/*** @title ERC20Basic 基于REC20，不是直接继承，而是类似的代码* @dev Simpler version of ERC20 interface 对于ERC20标准接口的简化版本* @dev see /ethereum/EIPs/issues/20* @dev 新版本的编译器0.6.1要求抽象合约前要加abstract，并且抽象函数要加上virtual*/contract ERC20Basic{//定义接口的一系列函数uint public _totalSupply;//总发行货币量function totalSupply() public view returns(uint);//查看总货币量函数function balanceOf(address who) public view returns(uint);//查某人余额function transfer(address to, uint value) public;//转账交易函数event Transfer(address indexed from, address indexed to, uint value);//定义转账记录事件}/*** @title ERC20 interface* @dev see /ethereum/EIPs/issues/20* @dev 继承与上面的接口*/contract ERC20 is ERC20Basic{//拓展了第三方授权功能//授权给别人用自己的钱，返回钱数？function allowance(address owner, address spender) public view returns(uint);//借助谁（from）向谁（to）转币function transferFrom(address from, address to, uint value) public;//授权使用额度函数function approve(address spender, uint value) public;//记录授权event Approval(address indexed owner, address indexed spender, uint value);}/*** @title Basic token 基础代币* @dev Basic version of StandardToken, with no allowances.* @dev 仅实现代币基本功能（没有第三方授权）* */contract BasicToken is Ownable, ERC20Basic{//使用安全数学函数using SafeMath for uint;mapping(address => uint) public balances;// additional variables for use if transaction fees ever became necessary// 如果有必要收取交易费用，可使用其他变量uint public basisPointsRate = 0; //基本利率uint public maximunFee = 0; //最大利息金额/*** @dev Fix for the ERC20 short address attack. 防止短地址攻击，具体可看博客ERC20文章* @dev 凡是涉及转账交易（合约调用）都需要加上这一限制*/modifier onlyPayloadSize(uint size){//msg.data就是data域（calldata）中的内容，一般来说都是4（函数名）+32（转账地址）+32（转账金额）=68字节//短地址攻击简单来说就是转账地址后面为0但故意缺省，导致金额32字节前面的0被当做地址而后面自动补0导致转账金额激增。//参数size就是除函数名外的剩下字节数//解决方法：对后面的的字节数的长度限制要求require(!(msg.data.length < size+4), "Invalid short address");_;}/*** @dev transfer token for a specified address 转给一个符合规定（非短地址）的地址* @param _to The address to transfer to. 转账地址* @param _value The amount to be transferred. 转账金额*/function transfer(address _to, uint _value) public onlyPayloadSize(2 * 32){//先算利息: （转账金额*基本利率)/10000 (ps:因为浮点会精度缺失，所以这样计算)uint fee = (_value.mul(basisPointsRate)).div(10000);//判断是否超最大额if (fee > maximunFee) fee = maximunFee;//计算剩下的钱uint sendAmount = _value.sub(fee);//转账的钱要够源码没加这个判断不知为何？//不需要检查，因为后面balances[msg.sender].sub(sendAmount)其中会检查，不够会报异常。//require(balances[msg.sender] >= _value);//有安全数学函数就不用判断溢出了//扣钱balances[msg.sender] = balances[msg.sender].sub(sendAmount);//加钱balances[_to] = balances[_to].add(sendAmount);//利息去向->ownerif (fee > 0){//因为继承于Ownable，所以可以拿到ownerbalances[owner] = balances[owner].add(fee);//继承于ERCBasic接口，其中申明了Transfer记录//记录利息去向emit Transfer(msg.sender, owner, fee);}//记录转账去向,注意记录的不是总金额而是去除交易费的金额emit Transfer(msg.sender, _to, sendAmount);}/*** @dev Gets the balance of the specified address. 查余额函数* @param _owner The address to query the the balance of.* @return An uint representing the amount owned by the passed address.*/function balanceOf(address _owner) public view returns(uint balance){return balances[_owner];}}/*** @title Standard ERC20 token ERC20标准代币** @dev Implementation of the basic standard token. 依据基本代币准则* @dev /ethereum/EIPs/issues/20* @dev Based oncode by FirstBlood: /Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol* @dev 借鉴了firstblood代币* @dev 对代币基础功能的拓展-> 添加了第三方授权功能*/contract StandardToken is BasicToken, ERC20{//授权金额映射：某人对其他所有人授权的金额的映射mapping(address => mapping(address => uint)) public allowed;//uint最大值uint public constant MAX_UINT = 2**256-1;/*** @dev Transfer tokens from one address to another 授权转账：从一个账户转到另一个账户* @param _from address The address which you want to send tokens from 已得到授权的账户* @param _to address The address which you want to transfer to 转向的账户* @param _value uint the amount of tokens to be transferred 转账金额*/function transferFrom(address _from, address _to, uint _value) public onlyPayloadSize(2 * 32){//授权金额：授权者对于当前调用者授权其可使用的金额量uint _allowance = allowed[_from][msg.sender];//在这里同样不需要检查授权金额是否足够,后面的sub函数这种情况会检测// require(_allowance >= _value);//1.先算利息uint fee = (_value.mul(basisPointsRate)).div(10000);if (fee > maximunFee) fee = maximunFee;//2.扣钱// 这里为什么要判断？if (_allowance < MAX_UINT){//注意这里扣去的是总金额，包括了利息都要从授权方的授权金额去除allowed[_from][msg.sender] = _allowance.sub(_value);}balances[_from] = balances[_from].sub(_value);//3.加钱uint sendAmount = _value.sub(fee);balances[_to] = balances[_to].add(sendAmount);//4.利息去向if (fee > 0){balances[owner] = balances[owner].add(fee);emit Transfer(_from, owner, fee);}//5.记录emit Transfer(_from, _to, sendAmount);}/*** @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.* @dev 调用者授权给他人可使用金额* @param _spender The address which will spend the funds. 被授权者* @param _value The amount of tokens to be spent. 金额*/function approve(address _spender, uint _value) public onlyPayloadSize(2 * 32){// To change the approve amount you first have to reduce the addresses`// allowance to zero by calling `approve(_spender, 0)` if it is not// already 0 to mitigate the race condition described here:// /ethereum/EIPs/issues/20#issuecomment-263524729//这里的限定条件是：不能将已设置过的授权金额改动，除非改为0。//也就是说对他人的授权金额只能是从0改为value,这一次机会，再改就只能改回到0require(!(_value != 0 && allowed[msg.sender][_spender] != 0), "You have only one chance to approve , you can only change it to 0 later");//1.改allowedallowed[msg.sender][_spender] = _value;//2. 记录emit Approval(msg.sender, _spender, _value);}/*** @dev Function to check the amount of tokens than an owner allowed to a spender.* @param _owner address The address which owns the funds. 地址拥有资金的地址。* @param _spender address The address which will spend the funds. 查看授权了多少钱* @return A uint specifying the amount of tokens still available for the spender.*/function allowance(address _owner, address _spender) public view returns(uint remaining){return allowed[_owner][_spender];}}/*** @title Pausable 中断* @dev Base contract which allows children to implement an emergency stop mechanism.* @dev 实现紧急停止机制*/contract Pausable is Ownable{event Pause();event Unpause();bool public paused = false;/*** @dev Modifier to make a function callable only when the contract is not paused.* @dev 限制条件：函数只能是在合约未停止情况下执行.*/modifier whenNotPaused(){require(!paused, "Must be used without pausing");_;}/*** @dev Modifier to make a function callable only when the contract is paused.* @dev 函数只能在停止条件下执行*/modifier whenPaused(){require(paused, "Must be used under pause");_;}/*** @dev called by the owner to pause, triggers stopped state* @dev 只能由代币管理者进行停止**/function pause() public onlyOwner whenNotPaused {paused = true;emit Pause();}/*** @dev called by the owner to unpause, returns to normal state* @dev 只能是代币管理者进行重开*/function unpause() public onlyOwner whenPaused{paused = false;emit Unpause();}}/*** @dev 列黑名单*/contract BlackList is Ownable, BasicToken{//黑名单映射mapping(address => bool) isBlackListed;//事件event DestroyedBlackFunds(address _blackListedUser, uint _balance);event AddedBlackList(address _user);event RemovedBlackList(address _user);//Getters to allow the same blacklist to be used also by other contracts (including upgraded Tether)//允许其他合约调用此黑名单(external)，查看此人是否被列入黑名单function getBlackListStatus(address _maker) external view returns(bool){return isBlackListed[_maker];}//获取当前代币的Ownerfunction getOwner() external view returns(address){return owner;}//增加黑名单function addBlackList(address _evilUser) public onlyOwner{isBlackListed[_evilUser] = true;emit AddedBlackList(_evilUser);}//去除某人黑名单function removeBlackList(address _clearUser) public onlyOwner{isBlackListed[_clearUser] = false;emit RemovedBlackList(_clearUser);}//去除掉黑名单账户的钱function destroyBlackFunds(address _blackListUser) public onlyOwner{//1. 检查是否在黑名单require(isBlackListed[_blackListUser], "You can only clear the money of users in the blacklist");//2. 查看要清除的钱uint dirtyFunds = balanceOf(_blackListUser);//3. 扣除清零balances[_blackListUser] = 0;//4. 总代币发行量减少_totalSupply = _totalSupply.sub(dirtyFunds);//5. 记录emit DestroyedBlackFunds(_blackListUser, dirtyFunds);}}//标准代币拓展(为了适应不支持ERC20的情况或者是拓展)contract UpgradedStandardToken is StandardToken{// those methods are called by the legacy contract// and they must ensure msg.sender to be the contract address// 这些拓展方法都是来自遗留合同// 并且合约调用者必须是合约地址function transferByLegacy(address from, address to, uint value) public;function transferFromByLegacy(address sender, address from, address spender, uint value) public;function approveByLegacy(address from, address spender, uint value) public;}//主体代币contract TetherToken is Pausable, StandardToken, BlackList{string public name; //代币名string public symbol; //标志uint public decimals; //精度/小数点后几位address public upgradedAddress; //升级合约的地址（必须是合约地址）bool public deprecated; //弃用（支持ERC20与否）// The contract can be initialized with a number of tokens 可初始化多个代币// All the tokens are deposited to the owner address//// @param _balance Initial supply of the contract// @param _name Token Name// @param _symbol Token symbol// @param _decimals Token decimalsconstructor(uint _initialSupply,string _name,string _symbol,uint _decimals) public {//总发行币都给owner_totalSupply = _initialSupply;balances[owner] = _initialSupply;name = _name;symbol = _symbol;decimals = _decimals;deprecated = false;}// Called when new token are issuedevent Issue(uint amount);// Called when tokens are redeemedevent Redeem(uint amount);// Called when contract is deprecatedevent Deprecate(address newAddress);// Called if contract ever adds feesevent Params(uint feeBasisPoints, uint maxFee);// Forward ERC20 methods to upgraded contract if this one is deprecated//如果不推荐使用ERC20方法，则将其转为升级的合同function transfer(address _to, uint _value) public whenNotPaused{//排除黑名单require(!isBlackListed[msg.sender], "The account you applied for is on the blacklist and cannot be transferred");//判断是否支持ERC20if(deprecated){//不支持的话就调用用upgradedAddress实例化的对象的transferByLegacy函数//不知道这里为什么要传msg.sender？//我猜测是重新升级适配函数的话调用此函数的人(msg.sender)也要转过去return UpgradedStandardToken(upgradedAddress).transferByLegacy(msg.sender, _to, _value);}else{//支持的话就直接调用ERC20的转账//这里没有返回值，不知道为什么还要加returnreturn super.transfer(_to, _value);}}//同理：// Forward ERC20 methods to upgraded contract if this one is deprecatedfunction transferFrom(address _from, address _to, uint _value) public whenNotPaused{require(!isBlackListed[_from], "The account you applied for is on the blacklist and cannot be transferred");if(deprecated){return UpgradedStandardToken(upgradedAddress).transferFromByLegacy(msg.sender, _from, _to, _value);}else{return super.transferFrom(_from, _to, _value);}}// Forward ERC20 methods to upgraded contract if this one is deprecated//注意这里查询余额在代币暂停的情况下也是可以使用的function balanceOf(address who) public view returns(uint){if(deprecated){return UpgradedStandardToken(upgradedAddress).balanceOf(who);}else{return super.balanceOf(who);}}// Forward ERC20 methods to upgraded contract if this one is deprecatedfunction approve(address _spender, uint _value) public whenNotPaused{//这里不用检查了，如果是在黑名单中，那么授权再多也没用，transferFrom的时候就检测出来了if(deprecated){return UpgradedStandardToken(upgradedAddress).approveByLegacy(msg.sender, _spender, _value);}else{return super.approve(_spender, _value);}}// Forward ERC20 methods to upgraded contract if this one is deprecatedfunction allowance(address _owner, address _spender) public view returns(uint){if(deprecated){return UpgradedStandardToken(upgradedAddress).allowance(_owner, _spender);}else{return super.allowance(_owner, _spender);}}// deprecate current contract in favour of a new one//反对现行合同，改用新合同. upgradedAddress新合同地址function deprecate(address _upgradedAddress) public onlyOwner{deprecated = true;upgradedAddress = _upgradedAddress;//记录emit Deprecate(_upgradedAddress);}// deprecate current contract if favour of a new one//反对现行合同，如果想换一个新合约,需要提前知道当前发行量function totalSupply() public view returns(uint){if(deprecated){return UpgradedStandardToken(upgradedAddress).totalSupply();}else{return _totalSupply;}}// Issue a new amount of tokens 发行新数量的代币// these tokens are deposited into the owner address//// @param _amount Number of tokens to be issuedfunction issue(uint _amount) public onlyOwner{//增加拥有者的量balances[owner] = balances[owner].add(_amount);//增加发行的总代币量_totalSupply = _totalSupply.add(_amount);//记录emit Issue(_amount);}// 调整利息率和最大利息限制function setParams(uint newBasisPoints, uint newMaxFee) public onlyOwner{// Ensure transparency by hardcoding limit beyond which fees can never be added//通过硬编码限制来确保透明度，超过这个限度就不能再增加费用了require(newBasisPoints < 20, "The new BasisPoints cannot exceed 20"); //0.002require(newMaxFee < 50, "The new MaxFee cannot exceed 50"); //5*10**(decimals+1)basisPointsRate = newBasisPoints;maximunFee = newMaxFee.mul(10**decimals);//记录emit Params(newBasisPoints, newMaxFee);}}