什麼是整型溢出呢？在solidity編寫合約時，定義整型一般是用uint8, uint256。一個變數如果定義為uint8表示的無符號的8位整型，即取值範圍為0-255。當給這個變數賦值256時，即整型溢出變成了0，以此類推257變成了1。

			
pragma solidity ^0.4.24;

//author: netkiller <netkiller@msn.com>
//homepage: http://www.netkiller.cn

contract NetkillerOverflowTest{
    

    function add(uint8 a, uint8 b) pure public returns (uint8){
        
        uint8 result = a + b;
        
        return result;
        
    }
    
    function sub(uint8 a, uint8 b) pure public returns (uint8){
        
        uint8 result = a - b;
        
        return result;
        
    }
    
    function mul(uint8 a, uint8 b) pure public returns (uint8){
        
        uint8 result = a * b;
        
        return result;
        
    }
    
    function div(uint8 a, uint8 b) pure public returns (uint8){
        
        uint8 result = a / b;
        
        return result;
        
    }
    
}		
			
			

調用上面合約，運行結果

			
254 + 1 = 255
254 + 2 = 0
254 + 3 = 1
			
			

減法運行結果

			
10 - 20 = 246
			
			

乘法運行結果

			
51 * 5 = 255
51 * 6 = 50
			
			

再來測試乘法

			
255 / 10 = 25			
			
			

這有點想千年蟲問題，即99年變成00年後，你無法區分1900年還是2000年。

現在測試一下uint256，uint256支持的取值範圍是0到2^256-1

			
pragma solidity ^0.4.24;

//author: netkiller <netkiller@msn.com>
//homepage: http://www.netkiller.cn

contract TestUint256Overflow {
    // (2**256 – 1) + 1 = 0 向上溢出測試
    function overflow() pure public returns (uint256 _overflow) {
        uint256 max = 2 ** 256 - 1;
        return max + 1;
    }

    // 0 – 1 = 2**256 – 1 向下溢出測試
    function underflow() pure public returns (uint256 _underflow) {
        uint256 min = 0;
        return min - 1;
    }
}
			
			

運行結果

			
_overflow : 0
_underflow : 115792089237316195423570985008687907853269984665640564039457584007913129639935
			
			

第一個函數溢出為 0，第二個函數 0 - 1 = 115792089237316195423570985008687907853269984665640564039457584007913129639935

解決溢出問題使用SafeMath庫

			
pragma solidity ^0.4.24;

//author: netkiller <netkiller@msn.com>
//homepage: http://www.netkiller.cn

library SafeMath {

  function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
    if (a == 0) {
      return 0;
    }

    c = a * b;
    assert(c / a == b);
    return c;
  }

  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    return a / b;
  }

  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 c) {
    c = a + b;
    assert(c >= a);
    return c;
  }
}

contract NetkillerSafeMath {
 
    using SafeMath for uint256;    
    
    function add(uint256 a, uint256 b) pure public returns (uint256){
        uint256 result = a.add(b);
        return result;
    }
    function sub(uint256 a, uint256 b) pure public returns (uint256){
        uint256 result = a.sub(b);
        return result;
    }
    function mul(uint256 a, uint256 b) pure public returns (uint256){
        uint256 result = a.mul(b);
        return result;
    }
    function div(uint256 a, uint256 b) pure public returns (uint256){
        uint256 result = a.div(b);
        return result;
    }
}			
			
			

測試 SafeMath

			
add(115792089237316195423570985008687907853269984665640564039457584007913129639934,1)  => 115792089237316195423570985008687907853269984665640564039457584007913129639935
add(115792089237316195423570985008687907853269984665640564039457584007913129639935,1)  => 拋出異常