C语言宏offsetof

/* REQ_MSG_SIZE computes size of 'mtext' part of 'requestMsg' structure.
   We use offsetof() to handle the possibility that there are padding
   bytes between the 'clientId' and 'pathname' fields. */

#include <stddef.h>
#include <stdio.h>

struct address {
   char name;
   int phone;
   char street;
};
   
int main()
{
   printf("address 结构中的 name 偏移 = %ld 字节\n",
   offsetof(struct address, name));
   
   printf("address 结构中的 phone 偏移 = %ld 字节\n",
   offsetof(struct address, phone));
   
   printf("address 结构中的 street 偏移 = %ld 字节\n",
   offsetof(struct address, street));
   
   return(0);
}

运行结果如下：

root@52coder:~/workspace# gcc -g -o offset offset.c
root@52coder:~/workspace# ./offset
address 结构中的 name 偏移 = 0 字节
address 结构中的 phone 偏移 = 4 字节
address 结构中的 street 偏移 = 8 字节

示例代码二：

#include <stddef.h>
#include <stdio.h>

struct address {
   char name;
   char street;
   int phone;
};
   
int main()
{
   printf("address 结构中的 name 偏移 = %ld 字节\n",
   offsetof(struct address, name));
   
   printf("address 结构中的 street 偏移 = %ld 字节\n",
   offsetof(struct address, street));
   
   printf("address 结构中的 phone 偏移 = %ld 字节\n",
   offsetof(struct address, phone));

   return(0);
}

执行结果：

root@52coder:~/workspace# ./offset 
address 结构中的 name 偏移 = 0 字节
address 结构中的 street 偏移 = 1 字节
address 结构中的 phone 偏移 = 4 字节