C语言结构体成员指定存储位数

#include <stdio.h>

struct Test
{
	int a:4;
	char ch:1;
};

struct A
{
	int a;
	char ch;
};

int main()
{
	printf("sizeof(struct Test)=%d\n",sizeof(struct Test));
	printf("sizeof(struct A)=%d\n",sizeof(struct A));
	struct Test t1={0};
	//printf("sizeof(t1.a)=%d\n",sizeof(t1.a));//编译出错
	t1.a=15;//二进制的1111，按照有符号数就是-1
	t1.ch=3;
	printf("t1.a=%d\n",t1.a);
	printf("t1.ch=%d\n",t1.ch);
	printf("0x%X\n",*((unsigned int*)&t1));
	return 0;
}

以上代码在64位的linux上gcc编译后的运行结果为：
sizeof(struct Test)=4
sizeof(struct A)=8
t1.a=-1
t1.ch=-1
0x1F