使用某类型是否用声明的思考

Posted 2022-11-13 Ijuan_0712

原因：

由于看linux协议栈内核源码使遇到了如下情况：

#ifndef _NET_NEIGHBOUR_H
#define _NET_NEIGHBOUR_H

#include <linux/neighbour.h>

/*
 *	Generic neighbour manipulation
 *
 *	Authors:
 *	Pedro Roque		<[e-mail]>
 *	Alexey Kuznetsov	<[e-mail]>
 *
 * 	Changes:
 *
 *	Harald Welte:		<[e-mail]>
 *		- Add neighbour cache statistics like rtstat
 */

#include <asm/atomic.h>
#include <linux/netdevice.h>
#include <linux/skbuff.h>
#include <linux/rcupdate.h>
#include <linux/seq_file.h>

#include <linux/err.h>
#include <linux/sysctl.h>
#include <linux/workqueue.h>
#include <net/rtnetlink.h>

/*
 * NUD stands for "neighbor unreachability detection"
 */

#define NUD_IN_TIMER	(NUD_INCOMPLETE|NUD_REACHABLE|NUD_DELAY|NUD_PROBE)
#define NUD_VALID	(NUD_PERMANENT|NUD_NOARP|NUD_REACHABLE|NUD_PROBE|NUD_STALE|NUD_DELAY)
#define NUD_CONNECTED	(NUD_PERMANENT|NUD_NOARP|NUD_REACHABLE)

struct neighbour;

struct neigh_parms

#ifdef CONFIG_NET_NS
	struct net *net;
#endif
	struct net_device *dev;
	struct neigh_parms *next;
	int	(*neigh_setup)(struct neighbour *);
	void	(*neigh_cleanup)(struct neighbour *);
	struct neigh_table *tbl;

	void	*sysctl_table;

	int dead;
	atomic_t refcnt;
	struct rcu_head rcu_head;

	int	base_reachable_time;
	int	retrans_time;
	int	gc_staletime;
	int	reachable_time;
	int	delay_probe_time;

	int	queue_len;
	int	ucast_probes;
	int	app_probes;
	int	mcast_probes;
	int	anycast_delay;
	int	proxy_delay;
	int	proxy_qlen;
	int	locktime;
;

neigh_parms中用的struct neighbour类型在使用前声明了下，而struct neigh_table类型却没有进行任何声明，这样难道不会出错吗？

验证：

验证一：

#include <stdio.h>

struct str2;

struct str1

    struct str2 st;
	int i1;
;

struct str2

	int i2;
;

int main()

	struct str1 stest1;
	stest1.st.i2 = 2;
	stest1.i1 = 1;
	
	printf("%d----%d\\n", stest1.st.i2, stest1.i1);

	return 0;

运行结果为：

define1.c:7:17: 错误：字段‘st’的类型不完全
     struct str2 st;
                 ^
验证二：

#include <stdio.h>

//typedef struct str2* ST;
//struct str2;

struct str1

//	ST  pst;
    struct str2 * pst;
	int i1;
;

struct str2

	int i2;
;


int main()

	struct str1 stest1;
	struct str2 stest2;
	stest1.pst = &stest2;
	stest1.pst->i2 = 2;
	stest1.i1 = 1;

	printf("%d-----%d\\n", stest1.pst->i2, stest1.i1);

	return 0;

运行结果：

$ gcc define.c
$ ./a.out
2-----1
结论：

用的是指针类型无妨，若是实体类型将会报错