From owner-freebsd-fs  Fri Apr 27  5:49: 2 2001
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To: Igor Shmukler <shmukler@mail.ru>
Cc: freebsd-fs@FreeBSD.ORG, iedowse@maths.tcd.ie
Subject: Re: more on FFS 
In-Reply-To: Your message of "Thu, 26 Apr 2001 21:05:23 EDT."
             <03b901c0ceb6$27ceae00$7b02a8c0@tp600e> 
Date: Fri, 27 Apr 2001 13:48:58 +0100
From: Ian Dowse <iedowse@maths.tcd.ie>
Message-ID:  <200104271348.aa54585@salmon.maths.tcd.ie>
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In message <03b901c0ceb6$27ceae00$7b02a8c0@tp600e>, Igor Shmukler writes:
>
>How do I find where on disk ROOT inode is located.
>I tried to thru sources, but FS structure quite large and I get lost in  =
>offsets.

The `ino_to_fsba' and `ino_to_fsbo' macros defined in fs.h are probably
what you want. `ino_to_fsba' returns the offset from the start of the
partition of the block containing the specified inode. This offset is
in units of the filesystem fragment size, and you are expected to read
a full fs_bsize block starting at that offset. Then `ino_to_fsbo' tells
you the inode number within that block. e.g. (untested, and has no
error checking):

	struct fs *fs;
	char *sbbuf;
	char *blockbuf;
	struct dinode *dip;
	off_t offset;

	fd = open("/dev/da0s1a", O_RDONLY);

	/* read superblock and point `fs' at it. */
	sbbuf = malloc(SBSIZE);
	pread(fd, sbbuf, SBSIZE, SBOFF);
	fs = (struct fs *)sbbuf;

	/* Get the root inode */
	offset = (off_t)ino_to_fsba(fs, ROOTINO) * (off_t)fs->fs_fsize;
	blockbuf = malloc(fs->fs_bsize);
	pread(fd, blockbuf, fs->fs_bsize, offset);
	dip = &((struct dinode *)blockbuf)[ino_to_fsbo(fs, ROOTINO)];

	/* `dip' now points to the root inode */

Ian

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