问一个const的问题

jiyongguang

void main(void)
{
   const int a = 4;
   int *p = &a;
   *p = 5;
  printf("%d\n", a);
}
结果居然是5,为什么?a不是只读的吗?gcc4.1编译的

zhllg

const int *p

lucida

貌似gcc允许如此只是历史原因，程序不能这么写，结果不可预计
注意
a1.c:4: warning: initialization discards qualifiers from pointer target type

1. 
   
2. lucida@fancyworld ~/test $ cat a1.c
   
3. void main(void)
   
4. {
   
5. const int a = 4;
   
6. int *p = &a;
   
7. *p = 5;
   
8. printf("a=%d, *p=%d\n", a, *p);
   
9. }
   
10. lucida@fancyworld ~/test $ gcc a1.c
    
11. a1.c: In function ‘main’:
    
12. a1.c:4: warning: initialization discards qualifiers from pointer target type
    
13. a1.c:6: warning: incompatible implicit declaration of built-in function ‘printf’
    
14. a1.c:2: warning: return type of ‘main’ is not ‘int’
    
15. lucida@fancyworld ~/test $ ./a.out
    
16. a=5, *p=5
    
17. lucida@fancyworld ~/test $ gcc a1.c -O2
    
18. a1.c: In function ‘main’:
    
19. a1.c:4: warning: initialization discards qualifiers from pointer target type
    
20. a1.c:6: warning: incompatible implicit declaration of built-in function ‘printf’
    
21. a1.c:2: warning: return type of ‘main’ is not ‘int’
    
22. lucida@fancyworld ~/test $ ./a.out
    
23. a=4, *p=5
    

复制代码

g++就很好

1. 
   
2. lucida@fancyworld ~/test $ cat a1.cc
   
3. #include <iostream>
   
4. 
   
5. using namespace std;
   
6. 
   
7. int main(void)
   
8. {
   
9. const int a = 4;
   
10. const int* const p = &a;
    
11. *p = 5;
    
12. cout<< "%d\n"<< a<<endl;
    
13. return 0;
    
14. }
    
15. lucida@fancyworld ~/test $ gcc a1.cc
    
16. a1.cc: In function ‘int main()’:
    
17. a1.cc:9: error: assignment of read-only location
    

复制代码

jiyongguang

Post by zhllg
const int *p

用const int *p的话,把p*强制类型转换成int *p一样可以修改a的值

galilette

according to K&R[1988]:

Appendix A6.6:
A pointer may be converted to another pointer whose type is the same except for the addition or removal of qualifiers of the object type to which the pointer refers. ... If qualifiers are removed, operations on the underlying object remain subject to the qualifiers in its actual declaration

Chapter2.4:
The result is implementation-defined if an attempt is made to change a const

zhllg

Post by jiyongguang
用const int *p的话,把p*强制类型转换成int *p一样可以修改a的值

要是硬改就没辙了
毕竟这个变量是在stack里的，而且C is not a strong typed language，不像c++

所以为了保险，有人建议用gcc -Werror来编译

ahlongxp

我看一般的程序都是 -Wall

wall_john

[PHP]
// file: a1.cc
#include <iostream>

using namespace std;

int main(void)
{
const int a = 4;
const int *const b = &a;
__asm__ (
        " movl $5, %%ecx \n"
        " movl %%ecx, 0x0(%0) \n"
        : : "r" (b));
cout<<&a<<endl;
cout<<a<<endl;
cout<<b<<endl;
cout<<*b<<endl;
return 0;
}
[/PHP]
gcc a1.cc -lstdc++
./a.out

猜猜会输出什么：）

wall_john

看到输出了没？相同的地址内容确不一样。objdump代码自己看汇编吧，嘻嘻。

zhllg

Post by ahlongxp
我看一般的程序都是 -Wall

-Wall不能启用-Werror
两者无直接联系