区块链神吐槽在编写智能合约的时候需要注意的一个主要的安全特性：防止溢出和下溢。为了防止这些情况，OpenZeppelin建立了一个叫做SafeMath的库(library)，默认情况下可以防止这些问题。
什么是溢出（overflow）
假设我们有一个uint8, 只能存储8 bit数据。这意味着我们能存储的最大数字就是二进制11111111(或者说十进制的2^8-1 =255).
来看看下面的代码。最后 number 将会是什么值？
在这个例子中，我们导致了溢出---虽然我们加了1，但是number出乎意料地等于0了。
下溢(underflow)也类似，如果你从一个等于0的uint8减去1, 它将变成255 (因为uint是无符号的，其不能等于负数)。
分析 SafeMath源码
1.加法，仅限内部调用，返回 uint256
require(c>=a && c>=b); //验证结果： 两个正数相加，和一定大于每个加数
2.减法，仅限内部调用，返回 uint256
require(b <= a) 因为返回值需要是 正数，所以此处判断 b必须小于等于a
3.乘法，仅限内部调用，返回 uint256
uint256 c = a * b; 容易溢出，比如 a=2,b=2^255 乘积 2^256 刚好溢出。结果取后面的256位(全为0)，导致 c=0。
所以使用 （a == 0 || c / a == b），验证结果的一致性。
4.除法，仅限内部调用，返回 uint256
require(b > 0), 确保被除数不能为0
require(a == b * c + a % b);防止溢出，验证结果的一致性
总结：
不要直接使用简单的 "+-*/" ，尽量使用 library SafeMath 中的函数，避免整数溢出的隐患。
附：完整源码
pragma solidity ^0.4.24;
library SafeMath {
/**
* @dev Returns the addition of two unsigned integers, reverting on
* overflow.
*
* Counterpart to Solidity's `+` operator.
*
* Requirements:
* - Addition cannot overflow.
*/
function safeAdd(uint256 a, uint256 b) internal pure returns (uint256) {
uint256 c = a + b;
require(c >= a, "SafeMath: addition overflow");
return c;
}
/**
* @dev Returns the subtraction of two unsigned integers, reverting on
* overflow (when the result is negative).
*
* Counterpart to Solidity's `-` operator.
*
* Requirements:
* - Subtraction cannot overflow.
*/
function safeSub(uint256 a, uint256 b) internal pure returns (uint256) {
return safeSub(a, b, "SafeMath: subtraction overflow");
}
/**
* @dev Returns the subtraction of two unsigned integers, reverting with custom message on
* overflow (when the result is negative).
*
* Counterpart to Solidity's `-` operator.
*
* Requirements:
* - Subtraction cannot overflow.
*
* NOTE: This is a feature of the next version of OpenZeppelin Contracts.
* @dev Get it via `npm install @openzeppelin/contracts@next`.
*/
function safeSub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
require(b <= a, errorMessage);
uint256 c = a - b;
return c;
}
/**
* @dev Returns the multiplication of two unsigned integers, reverting on
* overflow.
*
* Counterpart to Solidity's `*` operator.
*
* Requirements:
* - Multiplication cannot overflow.
*/
function safeMul(uint256 a, uint256 b) internal pure returns (uint256) {
// Gas optimization: this is cheaper than requiring 'a' not being zero, but the
// benefit is lost if 'b' is also tested.
if (a == 0) {
return 0;
}
uint256 c = a * b;
require(c / a == b, "SafeMath: multiplication overflow");
return c;
}
/**
* @dev Returns the integer division of two unsigned integers. Reverts on
* division by zero. The result is rounded towards zero.
*
* Counterpart to Solidity's `/` operator. Note: this function uses a
* `revert` opcode (which leaves remaining gas untouched) while Solidity
* uses an invalid opcode to revert (consuming all remaining gas).
*
* Requirements:
* - The divisor cannot be zero.
*/
function safeDiv(uint256 a, uint256 b) internal pure returns (uint256) {
return safeDiv(a, b, "SafeMath: division by zero");
}
/**
* @dev Returns the integer division of two unsigned integers. Reverts with custom message on
* division by zero. The result is rounded towards zero.
*
* Counterpart to Solidity's `/` operator. Note: this function uses a
* `revert` opcode (which leaves remaining gas untouched) while Solidity
* uses an invalid opcode to revert (consuming all remaining gas).
*
* Requirements:
* - The divisor cannot be zero.
* NOTE: This is a feature of the next version of OpenZeppelin Contracts.
* @dev Get it via `npm install @openzeppelin/contracts@next`.
*/
function safeDiv(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
// Solidity only automatically asserts when dividing by 0
require(b > 0, errorMessage);
uint256 c = a / b;
return c;
}
/**
* @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
* Reverts when dividing by zero.
*
* Counterpart to Solidity's `%` operator. This function uses a `revert`
* opcode (which leaves remaining gas untouched) while Solidity uses an
* invalid opcode to revert (consuming all remaining gas).
*
* Requirements:
* - The divisor cannot be zero.
*/
function safeMod(uint256 a, uint256 b) internal pure returns (uint256) {
return safeMod(a, b, "SafeMath: modulo by zero");
}
/**
* @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
* Reverts with custom message when dividing by zero.
*
* Counterpart to Solidity's `%` operator. This function uses a `revert`
* opcode (which leaves remaining gas untouched) while Solidity uses an
* invalid opcode to revert (consuming all remaining gas).
*
* Requirements:
* - The divisor cannot be zero.
*
* NOTE: This is a feature of the next version of OpenZeppelin Contracts.
* @dev Get it via `npm install @openzeppelin/contracts@next`.
*/
function safeMod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
require(b != 0, errorMessage);
return a % b;
}
}