一个奇怪的sizeof的问题

[yiyefangzhou24]

struct ArpHeader
{
    unsigned short hdType;   //硬件类型
    unsigned short proType;   //协议类型
    unsigned char hdSize;   //硬件地址长度
    unsigned char proSize;   //协议地址长度
    unsigned short op;   //操作类型，ARP请求（1），ARP应答（2），RARP请求（3），RARP应答（4）。
    u_char smac[6];   //源MAC地址
unsigned long sip;
    u_char dmac[6];   //目的MAC地址
unsigned long dip;
};

[ID870177103]

struct ArpHeader {
	unsigned short hdType;   //硬件类型
	unsigned short proType;   //协议类型
	unsigned char hdSize;   //硬件地址长度
	unsigned char proSize;   //协议地址长度
	unsigned short op;   //操作类型，ARP请求（1），ARP应答（2），RARP请求（3），RARP应答（4）。
	u_char smac[6];   //源MAC地址
	unsigned long sip;
	u_char dmac[6];   //目的MAC地址
	unsigned long dip;
};

#pragma pack(1)
struct ArpHeader_X {
	unsigned short hdType;   //硬件类型
	unsigned short proType;   //协议类型
	unsigned char hdSize;   //硬件地址长度
	unsigned char proSize;   //协议地址长度
	unsigned short op;   //操作类型，ARP请求（1），ARP应答（2），RARP请求（3），RARP应答（4）。
	u_char smac[6];   //源MAC地址
	unsigned long sip;
	u_char dmac[6];   //目的MAC地址
	unsigned long dip;
};
#pragma pack()

下面的就是28了

[战在春秋]

原因是编译器增加了字节以对齐结构体元素。 如果想深入学习，可以参考： http://blog.csdn.net/qq_21792169/article/details/50506449 如果想禁止编译器的这种行为： 对visual studio来说，可以在文件头部加上：

# pragma pack (1)

如果有帮助，请采纳答案好吗，谢谢！

[paschen]

因为你的结构体应该按8字节进行对齐的

[战在春秋]

是结果和你预想的不一样吗？ 那你最初期望的结果是什么呢？

[yiyefangzhou24]

应该是28，但是运行出来显示32

[赵4老师]

仅供参考：

#include <stdio.h>
#define field_offset(s,f) (int)(&(((struct s *)(0))->f))
struct AD  { int a; char b[13]; double c;};
#pragma pack(push)
#pragma pack(1)
struct A1  { int a; char b[13]; double c;};
#pragma pack(2)
struct A2  { int a; char b[13]; double c;};
#pragma pack(4)
struct A4  { int a; char b[13]; double c;};
#pragma pack(8)
struct A8  { int a; char b[13]; double c;};
#pragma pack(16)
struct A16 { int a; char b[13]; double c;};
#pragma pack(pop)
int main() {
    printf("AD.a %d\n",field_offset(AD,a));
    printf("AD.b %d\n",field_offset(AD,b));
    printf("AD.c %d\n",field_offset(AD,c));
    printf("AD sizeof %d\n", sizeof(AD));
    printf("\n");
    printf("A1.a %d\n",field_offset(A1,a));
    printf("A1.b %d\n",field_offset(A1,b));
    printf("A1.c %d\n",field_offset(A1,c));
    printf("A1 sizeof %d\n", sizeof(A1));
    printf("\n");
    printf("A2.a %d\n",field_offset(A2,a));
    printf("A2.b %d\n",field_offset(A2,b));
    printf("A2.c %d\n",field_offset(A2,c));
    printf("A2 sizeof %d\n", sizeof(A2));
    printf("\n");
    printf("A4.a %d\n",field_offset(A4,a));
    printf("A4.b %d\n",field_offset(A4,b));
    printf("A4.c %d\n",field_offset(A4,c));
    printf("A4 sizeof %d\n", sizeof(A4));
    printf("\n");
    printf("A8.a %d\n",field_offset(A8,a));
    printf("A8.b %d\n",field_offset(A8,b));
    printf("A8.c %d\n",field_offset(A8,c));
    printf("A8 sizeof %d\n", sizeof(A8));
    printf("\n");
    printf("A16.a %d\n",field_offset(A16,a));
    printf("A16.b %d\n",field_offset(A16,b));
    printf("A16.c %d\n",field_offset(A16,c));
    printf("A16 sizeof %d\n", sizeof(A16));
    printf("\n");
    return 0;
}
//AD.a 0
//AD.b 4
//AD.c 24
//AD sizeof 32
//
//A1.a 0
//A1.b 4
//A1.c 17
//A1 sizeof 25
//
//A2.a 0
//A2.b 4
//A2.c 18
//A2 sizeof 26
//
//A4.a 0
//A4.b 4
//A4.c 20
//A4 sizeof 28
//
//A8.a 0
//A8.b 4
//A8.c 24
//A8 sizeof 32
//
//A16.a 0
//A16.b 4
//A16.c 24
//A16 sizeof 32
//
//

[yelinCoder]

    cout << "ArpHeader1.hdType = 0x " << &ArpHeader1.hdType << endl;
    cout << "ArpHeader1.proType = 0x" << &ArpHeader1.proType << endl;
    cout << "ArpHeader1.hdSize = 0x " << &ArpHeader1.hdSize << endl;
    cout << "ArpHeader1.proSize = 0x" << &ArpHeader1.proSize << endl;
    cout << "ArpHeader1.op = 0x" << &ArpHeader1.op << endl;
    cout << "ArpHeader1.smac = 0x" << &ArpHeader1.smac << endl;
    cout << "ArpHeader1.sip = 0x" << &ArpHeader1.sip << endl;
    cout << "ArpHeader1.dmac = 0x" << &ArpHeader1.dmac << endl;
    cout << "ArpHeader1.dip = 0x" << &ArpHeader1.dip << endl;

前面关于原理的事情，几位大大都描述了很清晰，我这里就补充一下，关于 结构体实际地址分布的说明。