c - 结构成员 - 地址重叠

Question

在这个程序中，我声明了一个结构体和几个成员，并且使用了一个包含两个元素的数组作为这个特定结构体的实例。我在此结构中使用了 5 元素数组。一个奇怪的事情是，在主函数中，当我尝试获取 5 元素数组中不存在的第 6 个元素的地址时，我得到了下一个结构的第一个成员整数的地址。为什么当我尝试访问不存在的数组元素时没有错误显示以及为什么两个成员具有相同的地址？

#include<stdio.h>
 struct {int num1,num2; char s1; int *ptr; int abc[5]; }a[2];

void main(){

      int start, last; 

      start = &a[1].num1;

      last = &a[0].num1;

      printf("\nSize of the Structure : %d Bytes", start-last);

      printf("\naddress of num1 in structure a[0] is %d", &a[0].num1);

      printf("\naddress of num2 in structure  a[0] is %d", &a[0].num2);

      printf("\naddress of char in structure a[0] is %d", &a[0].s1);

      printf("\naddress of ptr in structure a[0] is %d", &a[0].ptr);

      printf("\naddress of I element in array abc[5] in structure a[0] is %d", &a[0].abc[0]);

      printf("\naddress of II element in array abc[5] in structure a[0] is %d", &a[0].abc[1]);

      printf("\naddress of III element in array abc[5] in structure a[0] is %d", &a[0].abc[2]);

      printf("\naddress of IV element in array abc[5] in structure a[0] is %d", &a[0].abc[3]);

      printf("\naddress of V element in array abc[5] in structure a[0] is %d", &a[0].abc[4]);

      **printf("\naddress of VI element in the 5 element array abc[5] in structure a[0] is %d", &a[0].abc[5]);

      printf("\naddress of num1 in structure a[1] is %d", &a[1].num1);**

      printf("\n");


  }

kevin@kevin-desktop:~/Documents/programs$ gcc --version
gcc (Ubuntu 4.4.3-4ubuntu5.1) 4.4.3
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

kevin@kevin-desktop:~/Documents/programs$ gcc structure_size.c -o structure_size
structure_size.c: In function ‘main’:
structure_size.c:15: warning: assignment makes integer from pointer without a cast
structure_size.c:17: warning: assignment makes integer from pointer without a cast
structure_size.c:21: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
structure_size.c:23: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
structure_size.c:25: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘char *’
structure_size.c:27: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int **’
structure_size.c:29: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
structure_size.c:31: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
structure_size.c:33: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
structure_size.c:35: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
structure_size.c:37: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
structure_size.c:39: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
structure_size.c:41: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘int *’
kevin@kevin-desktop:~/Documents/programs$ ./structure_size

Size of the Structure : 36 Bytes
address of num1 in structure a[0] is 134520896
address of num2 in structure  a[0] is 134520900
address of char in structure a[0] is 134520904
address of ptr in structure a[0] is 134520908
address of I element in array abc[5] in structure a[0] is 134520912
address of II element in array abc[5] in structure a[0] is 134520916
address of III element in array abc[5] in structure a[0] is 134520920
address of IV element in array abc[5] in structure a[0] is 134520924
address of V element in array abc[5] in structure a[0] is 134520928
**address of VI element in the 5 element array abc[5] in structure a[0] is 134520932
address of num1 in structure a[1] is 134520932**
kevin@kevin-desktop:~/Documents/programs$ 

Answer 1

这就是 C 的工作原理。我认为这个现象在这个简化的代码中更容易表现出来:

char string[2][5];              // allocates a block of 10 bytes

string == string[0];            // true
string == &string[0][0];        // true
&string[0][5] == &string[1][0]; // true