共用体

共用体又叫做联合体，是C语言中几个不同的变量共占一段内存的结构。

下面是在OSPF协议代码中定义的一个共用体，并且使用到了段域的概念。

#if (_BYTE_ORDER == _LITTLE_ENDIAN )
typedef _struct

{
        BIT_FIELD (enum, BOOLEAN)       transit:1;                      /* This is a transit area */
        BIT_FIELD (enum, BOOLEAN)       virtual_up:1;           /* One or more virtual links in this area are up */
        BIT_FIELD (enum, BOOLEAN)       stub:1;                         /* This is a  area (no external routes) */
        BIT_FIELD (enum, BOOLEAN)       nssa:1;                         /* This is a not so stubby area */
        BIT_FIELD (enum, BOOLEAN)       stub_default:1;         /* Inject default into this stub area */
        BIT_FIELD (enum,BOOLEAN)        not_used:3;
} _pack OSPF_AREA_FLAGS;
#else /* _BYTE_ORDER == _BIG_ENDIAN */
typedef _struct OSPF_AREA_FLAGS
{
        BIT_FIELD (enum,BOOLEAN)        not_used:3;
        BIT_FIELD (enum, BOOLEAN)       stub_default:1;         /* Inject default into this stub area */
        BIT_FIELD (enum, BOOLEAN)       nssa:1;                         /* This is a not so stubby area */
        BIT_FIELD (enum, BOOLEAN)       stub:1;                         /* This is a  area (no external routes) */
        BIT_FIELD (enum, BOOLEAN)       virtual_up:1;           /* One or more virtual links in this area are up */
        BIT_FIELD (enum, BOOLEAN)       transit:1;                      /* This is a transit area */
} _pack OSPF_AREA_FLAGS;
#endif /* _BYTE_ORDER == _LITTLE_ENDIAN */

typedef _union  UNION_OSPF_AREA_FLAGS
{
        OSPF_AREA_FLAGS _bit;
        BYTE                    _byte;
} _pack UNION_OSPF_AREA_FLAGS;

下面是rip协议中使用套接字接收数据，存放数据包的结构使用了共用体。

#define MAXPACKETSIZE           512     /* From rip/ripLib.h */

typedef struct rip_pkt {
	u_char	rip_cmd;		/* request/response */
	u_char	rip_vers;		/* protocol version # */
	u_char	rip_domain[2];		/* Version 2 routing domain. */
	union {
		struct	netinfo ru_nets[1];	/* variable length... */
	} ripun;
#define	rip_nets	ripun.ru_nets
} RIP_PKT;

LOCAL void process (int fd)
{
	struct sockaddr from;
	int fromlen, cc;
	union {
		char	buf[MAXPACKETSIZE+1];
		RIP_PKT rip;
	} inbuf;	//定义了一个存放数据包的共用体，将char数组和rip数据包格式的结构体共用

        bzero ((char *)&from, sizeof (from));
	for (;;) 
	{
		fromlen = sizeof (from);
		cc = recvfrom (fd, (char *)&inbuf, sizeof (inbuf), 0, 
                               &from, &fromlen);	//将从socket中接收到的数据放入共用体buf中
		if (cc <= 0) 
		{
			...
			//出错处理
		}
		if (fromlen != sizeof (struct sockaddr_in)) break;
		
		routedInput (&from, &inbuf.rip, cc);	//使用共用体中的rip数据包格式变量作为参数
	}
}
	

枚举类型

枚举类型是ANSI C新标准中增加的。

下面是OSPF协议中定义的枚举的类型。

/* the OSPF interface configure parameters enum */
typedef enum{
	swOspfIfParam_cost = 1,
	swOspfIfParam_retranInter = 2,
	swOspfIfParam_tranDelay = 3,
	swOspfIfParam_prio = 4,
	swOspfIfParam_helloInter = 5,
	swOspfIfParam_deadInter = 6,
	swOspfIfParam_authKey = 7,
	swOspfIfParam_digestKey =8
}swOspfIfIfParam_t;