1. C语言实现的HTTP协议的解析源码中有下面的结构体定义：
struct http_parser { unsigned int type : 2; unsigned int flags : 8; unsigned int state : 7; unsigned int header_state : 7; unsigned int index : 7; unsigned int lenient_http_headers : 1; unsigned int nread; unsigned int content_length; unsigned short http_major; unsigned short http_minor; unsigned int status_code : 16; unsigned int method : 8; unsigned int http_errno : 7; unsigned int upgrade : 1; void *data; };当前运行环境是Windows7 64位运行环境
unsigned int字节长度是4，unsigned short字节长度是2，void *字节长度是4
如果没有使用unsigned int type : 2;这种声明方法，sizeof(struct http_parser) = 12 * 4 + 2 * 2 + 4 = 56
现在这种实现：sizeof(struct http_parser) = 4 + 8 + 4 + 4 + 4 = 24
2. 理解
unsigned int type : 2; 表示type变量使用unsinged int 32位中的低两位。
该结构体中的前5个字段总共占用4个字节
