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From: gwyn@smoke.BRL.MIL (Doug Gwyn)
Newsgroups: comp.lang.c
Subject: Re: offsetof() macro
Message-ID: <10827@smoke.BRL.MIL>
Date: 28 Aug 89 03:33:13 GMT
References: <70@motto.UUCP> <175@crdos1.crd.ge.COM>
Reply-To: gwyn@brl.arpa (Doug Gwyn)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 22

In article <175@crdos1.crd.ge.COM> davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr) writes:
>  What good is offsetof?

Extracted from <sys/dirent.h>:

struct dirent				/* data from getdents()/readdir() */
	{
	long		d_ino;		/* inode number of entry */
	off_t		d_off;		/* offset of disk directory entry */
	unsigned short	d_reclen;	/* length of this record */
	char		d_name[1];	/* name of file */	/* non-POSIX */
	};

/* The following nonportable ugliness could have been avoided by defining
   DIRENTSIZ and DIRENTBASESIZ to also have (struct dirent *) arguments. */
#define	DIRENTBASESIZ		(((struct dirent *)0)->d_name \
				- (char *)&((struct dirent *)0)->d_ino)
#define	DIRENTSIZ( namlen )	((DIRENTBASESIZ + sizeof(long) + (namlen)) \
				/ sizeof(long) * sizeof(long))

This, as the comment indicates, is nonportable and ugly.  I leave it as a
simple exercise to the reader to figure out how offsetof() can improve this.