#define的一个精妙用途，用来标记起始和结束

在NTP项目中，NTP.h中，

#ifdef AUTOKEY
#define clear_to_zero opcode
	u_int32	opcode;		/* last request opcode */
	associd_t assoc;	/* peer association ID */
	u_int32	crypto;		/* peer status word */
	EVP_PKEY *pkey;		/* public key */
	const EVP_MD *digest;	/* message digest algorithm */
	char	*subject;	/* certificate subject name */
	char	*issuer;	/* certificate issuer name */
	struct cert_info *xinfo; /* issuer certificate */
	keyid_t	pkeyid;		/* previous key ID */
	keyid_t	hcookie;	/* host cookie */
	keyid_t	pcookie;	/* peer cookie */
	const struct pkey_info *ident_pkey; /* identity key */
	BIGNUM	*iffval;	/* identity challenge (IFF, GQ, MV) */
	const BIGNUM *grpkey;	/* identity challenge key (GQ) */
	struct value cookval;	/* receive cookie values */
	struct value recval;	/* receive autokey values */
	struct exten *cmmd;	/* extension pointer */
	u_long	refresh;	/* next refresh epoch */

	/*
	 * Variables used by authenticated server
	 */
	keyid_t	*keylist;	/* session key ID list */
	int	keynumber;	/* current key number */
	struct value encrypt;	/* send encrypt values */
	struct value sndval;	/* send autokey values */
#else	/* !AUTOKEY follows */
#define clear_to_zero status
#endif	/* !AUTOKEY */

	/*
	 * Ephemeral state variables
	 */
	u_char	status;		/* peer status */
	u_char	new_status;	/* under-construction status */
	u_char	reach;		/* reachability register */
	int	flash;		/* protocol error test tally bits */
	u_long	epoch;		/* reference epoch */
	int	burst;		/* packets remaining in burst */
	int	retry;		/* retry counter */
	int	flip;		/* interleave mode control */
	int	filter_nextpt;	/* index into filter shift register */
	double	filter_delay[NTP_SHIFT]; /* delay shift register */
	double	filter_offset[NTP_SHIFT]; /* offset shift register */
	double	filter_disp[NTP_SHIFT]; /* dispersion shift register */
	u_long	filter_epoch[NTP_SHIFT]; /* epoch shift register */
	u_char	filter_order[NTP_SHIFT]; /* filter sort index */
	l_fp	rec;		/* receive time stamp */
	l_fp	xmt;		/* transmit time stamp */
	l_fp	dst;		/* destination timestamp */
	l_fp	aorg;		/* origin timestamp */
	l_fp	borg;		/* alternate origin timestamp */
	double	offset;		/* peer clock offset */
	double	delay;		/* peer roundtrip delay */
	double	jitter;		/* peer jitter (squares) */
	double	disp;		/* peer dispersion */
	double	xleave;		/* interleave delay */
	double	bias;		/* programmed offset bias */

	/*
	 * Variables used to correct for packet length and asymmetry.
	 */
	double	t21;		/* outbound packet delay */
	int	t21_bytes;	/* outbound packet length */
	int	t21_last;	/* last outbound packet length */
	double	r21;		/* outbound data rate */
	double	t34;		/* inbound packet delay */
	int	t34_bytes;	/* inbound packet length */
	double	r34;		/* inbound data rate */

	/*
	 * End of clear-to-zero area
	 */
	u_long	update;		/* receive epoch */
#define end_clear_to_zero update

用两个个#define来标记 结尾 和 开始， 以后用的时候直接用这两个宏，相减便可以得到大小。

#define	CLEAR_TO_ZERO(p)	((char *)&((p)->clear_to_zero))
#define	END_CLEAR_TO_ZERO(p)	((char *)&((p)->end_clear_to_zero))
#define	LEN_CLEAR_TO_ZERO	(END_CLEAR_TO_ZERO((struct peer *)0) \
				    - CLEAR_TO_ZERO((struct peer *)0))
#define CRYPTO_TO_ZERO(p)	((char *)&((p)->clear_to_zero))
#define END_CRYPTO_TO_ZERO(p)	((char *)&((p)->end_clear_to_zero))
#define LEN_CRYPTO_TO_ZERO	(END_CRYPTO_TO_ZERO((struct peer *)0) \
				    - CRYPTO_TO_ZERO((struct peer *)0))