Q：关于结构体的对齐，到底遵循什么原则
A：首先先不讨论结构体按多少字节对齐，先看看只以1字节对齐的情况：
#include
#include
#define PRINT_D(intValue) printf(#intValue" is %d\n", (intValue));
#define OFFSET(struct,member) ((char *)&((struct *)0)->member - (char *)0)
#pragma pack(1)
typedef struct
{
char sex;
short score;
int age;
}student;
int main()
{
PRINT_D(sizeof(student))
PRINT_D(OFFSET(student,sex))
PRINT_D(OFFSET(student,score))
PRINT_D(OFFSET(student,age))
return 0;
}
输出：
sizeof(student) is 7
OFFSET(student,sex) is 0
OFFSET(student,score) is 1
OFFSET(student,age) is 3
可以看到，如果按1字节对齐，那么结构体内部的成员紧密排列，sizeof(char) == 1, sizeof(short) == 2, sizeof(int) == 4.
修改上面的代码， 去掉#pragma pack语句，代码如下：
#include
#include
#define PRINT_D(intValue) printf(#intValue" is %d\n", (intValue));
#define OFFSET(struct,member) ((char *)&((struct *)0)->member - (char *)0)
typedef struct
{
char sex;
short score;
int age;
}student;
int main()
{
PRINT_D(sizeof(student))
PRINT_D(OFFSET(student,sex))
PRINT_D(OFFSET(student,score))
PRINT_D(OFFSET(student,age))
return 0;
}
运行结果：
sizeof(student) is 8
OFFSET(student,sex) is 0
OFFSET(student,score) is 2
OFFSET(student,age) is 4