C语言的隐式类型转换

看了一篇博文,该文章中有如下描述

 1 #include <stdio.h>  
 2   
 3 int main()  
 4 {  
 5     unsigned short a = 1;  
 6     unsigned short b = 0;  
 7    
 8     if (a < (b-1)) //a和b-1的结果-1都被转换为int类型进行比较,1<-1,显然false  
 9     {  
10         printf("in if\n");  
11     }  
12   
13     return 0;  
14       
15 }  

预期结果时打印“in if”,但实际运行时却发现啥都没有输出。改为如下代码就可以正常输出“in if”,此时与预期相符

 1 #include <stdio.h>  
 2   
 3 int main()  
 4 {  
 5     unsigned short a = 1;  
 6     unsigned int b = 0;  
 7    
 8     if (a < (b-1)) //b-1的结果-1,被转换为0xffffffff,1<0xffffffff,显然true啊  
 9     {  
10         printf("in if\n");  
11     }  
12   
13     return 0;  
14       
15 }  

使用汇编,直接对比两者结果(公司安全不让发送外网图片..),发现主要差异在如下5行,前四行中对short进行了扩展,由2字节扩展为4字节,高位填充0。导致运行结果与与预期不符合的原因就在第5行:jge 8048461 

 1 08048425 <main>:
 2  8048425:      8d 4c 24 04              lea    0x4(%esp),%ecx
 3  8048429:      83 e4 f0                 and    $0xfffffff0,%esp
 4  804842c:      ff 71 fc                 pushl  -0x4(%ecx)
 5  804842f:      55                       push   %ebp
 6  8048430:      89 e5                    mov    %esp,%ebp
 7  8048432:      51                       push   %ecx
 8  8048433:      83 ec 14                 sub    $0x14,%esp
 9  8048436:      66 c7 45 f6 01 00        movw   $0x1,-0xa(%ebp)
10  804843c:      66 c7 45 f4 00 00        movw   $0x0,-0xc(%ebp)
11  8048442:      0f b7 45 f6              movzwl -0xa(%ebp),%eax
12  8048446:      0f b7 55 f4              movzwl -0xc(%ebp),%edx
13  804844a:      83 ea 01                 sub    $0x1,%edx
14  804844d:      39 d0                    cmp    %edx,%eax
15  804844f:      7d 10                    jge    8048461 <main+0x3c>
16  8048451:      83 ec 0c                 sub    $0xc,%esp
17  8048454:      68 00 85 04 08           push   $0x8048500
18  8048459:      e8 a2 fe ff ff           call   8048300 <puts@plt>
19  804845e:      83 c4 10                 add    $0x10,%esp
20  8048461:      b8 00 00 00 00           mov    $0x0,%eax
21  8048466:      8b 4d fc                 mov    -0x4(%ebp),%ecx
22  8048469:      c9                       leave  
23  804846a:      8d 61 fc                 lea    -0x4(%ecx),%esp
24  804846d:      c3                       ret    
25  804846e:      66 90                    xchg   %ax,%ax

 

汇编中的数值是没有符号之分的,但指令是有符号分别的,上述使用的jge即是有符号比较的方式,导致比较结果为:if (1<-1),为false,不会打印字符串

指令 含义 运算符号
jbe unsigned below or equal (lower or same) <=
jae unsigned above or equal (higher or same) >=
jb unsigned below (lower) <
ja unsigned above (higher) >
jle signed less or equal <=
jge signed greater or equal >=
jl signed less than <
jg signed greater than >
posted @ 2018-03-20 17:27  charlieroro  阅读(2157)  评论(0编辑  收藏  举报