packed的作用是取消位元組對齊

#include<stdio.h>

typedef struct {

    int i;  //4
    char a; //1
    char b; //1
    char c; //1
            //1

}Test1;

typedef struct {

    int i;  //4
    char a; //1
    char b; //1
    char c; //1

}__attribute__((packed))Test2;

typedef struct {

    int i;  //4
    char a; //1
    char b; //1
    char c; //1

}__attribute__((__packed__))Test3;

int
main(int argc, char *argv[])
{

    printf("Test1 = %d \r\n",sizeof(Test1));
    printf("Test2 = %d \r\n",sizeof(Test2));
    printf("Test3 = %d \r\n",sizeof(Test3));

    return 0;
}
/**輸出結果：
Test1 = 8
Test2 = 7
Test3 = 7
*/