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

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int my_int;

    printf("Arithmetic Operator");

    my_int=6;
    printf("my_int=%d,-my_int=%d\n",my_int,-my_int);

    printf("int 1+2=%d\n",1+2);
    printf("int 5-1=%d\n",5-1);
    printf("int 5*2=%d\n",5*2);

//    printf("\n9 div 4=2 remaind 1:\n");
    printf("int 9/4=%d\n",9/4);
    printf("int 9%%4=%d\n",9%4);
    printf("double 9/4=%f\n",9.0/4.0);


    my_int=10; //will overwrite previous my_int;
    int my_int;

    printf("Arithmetic Operator");

    my_int=6;
    printf("my_int=%d,-my_int=%d\n",my_int,-my_int);

    printf("int 1+2=%d\n",1+2);
    printf("int 5-1=%d\n",5-1);
    printf("int 5*2=%d\n",5*2);

//    printf("\n9 div 4=2 remaind 1:\n");
    printf("int 9/4=%d\n",9/4);
    printf("int 9%%4=%d\n",9%4);
    printf("double 9/4=%f\n",9.0/4.0);


    my_int=10; //will overwrite previous my_int;
    printf("my_int=10: %d\n",my_int);

    my_int++;
    printf("my_int++: %d\n",my_int);

    my_int+=5;
    printf("my_int+=5: %d\n",my_int);

    my_int--;
    printf("my_int--: %d\n",my_int);

    my_int-=2;
    printf("my_int-=2: %d\n",my_int);

    my_int*=5;
    printf("my_int*=5: %d\n",my_int);

    my_int/=2;
    printf("my_int/=2: %d\n",my_int);

    my_int%=3;
    printf("my_int%%=3: %d\n",my_int);

    return 0;
    }

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

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

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int my_int;

    printf("Arithmetic Operator");

    my_int=6;
    printf("my_int=%d,-my_int=%d\n",my_int,-my_int);

    printf("int 1+2=%d\n",1+2);
    printf("int 5-1=%d\n",5-1);
    printf("int 5*2=%d\n",5*2);

    //    printf("\n9 div 4=2 remaind 1:\n");
    printf("int 9/4=%d\n",9/4);
    printf("int 9%%4=%d\n",9%4);
    printf("double 9/4=%f\n",9.0/4.0);


    my_int=10; //will overwrite previous my_int;
    {
        int my_int;

        printf("Arithmetic Operator");

        my_int=6;
        printf("my_int=%d,-my_int=%d\n",my_int,-my_int);

        printf("int 1+2=%d\n",1+2);
        printf("int 5-1=%d\n",5-1);
        printf("int 5*2=%d\n",5*2);

        //    printf("\n9 div 4=2 remaind 1:\n");
        printf("int 9/4=%d\n",9/4);
        printf("int 9%%4=%d\n",9%4);
        printf("double 9/4=%f\n",9.0/4.0);


        my_int=10; //will overwrite previous my_int;
        printf("my_int=10: %d\n",my_int);

        my_int++;
        printf("my_int++: %d\n",my_int);

        my_int+=5;
        printf("my_int+=5: %d\n",my_int);

        my_int--;
        printf("my_int--: %d\n",my_int);

        my_int-=2;
        printf("my_int-=2: %d\n",my_int);

        my_int*=5;
        printf("my_int*=5: %d\n",my_int);

        my_int/=2;
        printf("my_int/=2: %d\n",my_int);

        my_int%=3;
        printf("my_int%%=3: %d\n",my_int);
    }
    return 0;
}