发现C语言里面{}很奇妙

下面的代码在 ubuntu 14.04.3上可以编译，GCC 版本是 4.8.4

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1. #include <stdio.h>
   
2. #include <stdlib.h>
   
3. int main()
   
4. {
   
5.     int my_int;
   
6. 
   
7.     printf("Arithmetic Operator");
   
8. 
   
9.     my_int=6;
   
10.     printf("my_int=%d,-my_int=%d\n",my_int,-my_int);
    
11. 
    
12.     printf("int 1+2=%d\n",1+2);
    
13.     printf("int 5-1=%d\n",5-1);
    
14.     printf("int 5*2=%d\n",5*2);
    
15.     
    
16. // printf("\n9 div 4=2 remaind 1:\n");
    
17.     printf("int 9/4=%d\n",9/4);
    
18.     printf("int 9%%4=%d\n",9%4);
    
19.     printf("double 9/4=%f\n",9.0/4.0);
    
20.     
    
21. 
    
22.     my_int=10; //will overwrite previous my_int;
    
23.     int my_int;
    
24. 
    
25.     printf("Arithmetic Operator");
    
26. 
    
27.     my_int=6;
    
28.     printf("my_int=%d,-my_int=%d\n",my_int,-my_int);
    
29. 
    
30.     printf("int 1+2=%d\n",1+2);
    
31.     printf("int 5-1=%d\n",5-1);
    
32.     printf("int 5*2=%d\n",5*2);
    
33.     
    
34. // printf("\n9 div 4=2 remaind 1:\n");
    
35.     printf("int 9/4=%d\n",9/4);
    
36.     printf("int 9%%4=%d\n",9%4);
    
37.     printf("double 9/4=%f\n",9.0/4.0);
    
38.     
    
39. 
    
40.     my_int=10; //will overwrite previous my_int;
    
41.     printf("my_int=10: %d\n",my_int);
    
42. 
    
43.     my_int++;
    
44.     printf("my_int++: %d\n",my_int);
    
45. 
    
46.     my_int+=5;
    
47.     printf("my_int+=5: %d\n",my_int);
    
48. 
    
49.     my_int--;
    
50.     printf("my_int--: %d\n",my_int);
    
51. 
    
52.     my_int-=2;
    
53.     printf("my_int-=2: %d\n",my_int);
    
54. 
    
55.     my_int*=5;
    
56.     printf("my_int*=5: %d\n",my_int);
    
57. 
    
58.     my_int/=2;
    
59.     printf("my_int/=2: %d\n",my_int);
    
60. 
    
61.     my_int%=3;
    
62.     printf("my_int%%=3: %d\n",my_int);
    
63.     
    
64.     return 0;
    
65.     }

但是在 kali linux 上不能编译，gcc 版本是 5.4.0
编译报错如下。

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1. gcc arithmetic.c -o arithmetic
   
2. arithmetic.c: In function ‘main’:
   
3. arithmetic.c:23:9: error: redeclaration of ‘my_int’ with no linkage
   
4.      int my_int;
   
5.          ^
   
6. arithmetic.c:5:9: note: previous declaration of ‘my_int’ was here
   
7.      int my_int

但是用{}就能解决掉这个问题：
我在第二次声明变量my_int 之前加了{} ，当然按照道理来说，最后声明的同名变量应该会覆盖前面的变量，所以在GCC 4.8.4 上没有报错，即使没有{}, 而在GCC 5.4.0 我就得
加上{}才能编译通过。

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1. #include <stdio.h>
   
2. #include <stdlib.h>
   
3. int main()
   
4. {
   
5.     int my_int;
   
6. 
   
7.     printf("Arithmetic Operator");
   
8. 
   
9.     my_int=6;
   
10.     printf("my_int=%d,-my_int=%d\n",my_int,-my_int);
    
11. 
    
12.     printf("int 1+2=%d\n",1+2);
    
13.     printf("int 5-1=%d\n",5-1);
    
14.     printf("int 5*2=%d\n",5*2);
    
15. 
    
16.     // printf("\n9 div 4=2 remaind 1:\n");
    
17.     printf("int 9/4=%d\n",9/4);
    
18.     printf("int 9%%4=%d\n",9%4);
    
19.     printf("double 9/4=%f\n",9.0/4.0);
    
20. 
    
21. 
    
22.     my_int=10; //will overwrite previous my_int;
    
23.     {
    
24.         int my_int;
    
25. 
    
26.         printf("Arithmetic Operator");
    
27. 
    
28.         my_int=6;
    
29.         printf("my_int=%d,-my_int=%d\n",my_int,-my_int);
    
30. 
    
31.         printf("int 1+2=%d\n",1+2);
    
32.         printf("int 5-1=%d\n",5-1);
    
33.         printf("int 5*2=%d\n",5*2);
    
34. 
    
35.         // printf("\n9 div 4=2 remaind 1:\n");
    
36.         printf("int 9/4=%d\n",9/4);
    
37.         printf("int 9%%4=%d\n",9%4);
    
38.         printf("double 9/4=%f\n",9.0/4.0);
    
39. 
    
40. 
    
41.         my_int=10; //will overwrite previous my_int;
    
42.         printf("my_int=10: %d\n",my_int);
    
43. 
    
44.         my_int++;
    
45.         printf("my_int++: %d\n",my_int);
    
46. 
    
47.         my_int+=5;
    
48.         printf("my_int+=5: %d\n",my_int);
    
49. 
    
50.         my_int--;
    
51.         printf("my_int--: %d\n",my_int);
    
52. 
    
53.         my_int-=2;
    
54.         printf("my_int-=2: %d\n",my_int);
    
55. 
    
56.         my_int*=5;
    
57.         printf("my_int*=5: %d\n",my_int);
    
58. 
    
59.         my_int/=2;
    
60.         printf("my_int/=2: %d\n",my_int);
    
61. 
    
62.         my_int%=3;
    
63.         printf("my_int%%=3: %d\n",my_int);
    
64.     }
    
65.     return 0;
    
66. }