2.9BSD/usr/man/man5/filsys.5
.TH FILSYS 5
.UC
.SH NAME
filsys, fblk, ino \- format of file system volume
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/fblk.h>
.br
.B #include <sys/filsys.h>
.br
.B #include <sys/ino.h>
.SH DESCRIPTION
Every
file system storage volume
(e.g. HP disk, RL disk, RK disk)
has a common format for certain vital information.
Every such volume is divided into a certain number
of BSIZE blocks (in bytes; specified in <sys/param.h>).
Block 0 is unused and is available to contain
a bootstrap program, pack label, or other information.
.PP
Block 1 (SUPERB) is the
.I "super block."
The layout of the super block as defined by the include file
.I <sys/filsys.h>
is:
.PP
.nf
.ta \w'struct 'u +\w'daddr_t 'u +\w's_inode[NICINOD]; 'u
/*
* Structure of the super-block
*/
struct filsys
{
u_short s_isize; /* size in blocks of i-list */
daddr_t s_fsize; /* size in blocks of entire volume */
short s_nfree; /* number of addresses in s_free */
daddr_t s_free[NICFREE]; /* free block list */
short s_ninode; /* number of i-nodes in s_inode */
ino_t s_inode[NICINOD]; /* free i-node list */
char s_flock; /* lock during free list manipulation */
char s_ilock; /* lock during i-list manipulation */
char s_fmod; /* super block modified flag */
char s_ronly; /* mounted read-only flag */
time_t s_time; /* last super block update */
daddr_t s_tfree; /* total free blocks*/
ino_t s_tinode; /* total free inodes */
short s_dinfo[2]; /* interleave stuff */
#define s_m s_dinfo[0]
#define s_n s_dinfo[1]
char s_fsmnt[12]; /* ordinary file mounted on */
ino_t s_lasti; /* start place for circular search */
ino_t s_nbehind; /* est # free inodes before s_lasti */
};
#ifdef KERNEL
struct filsys *getfs();
#endif
.fi
.PP
.I S\_isize
is the address of the first block after the i-list,
which starts just after the super-block, in block 2.
Thus is i-list is
.IR s\_isize \-2
blocks long.
.I S\_fsize
is the address of the first block not potentially
available for allocation
to a file.
These numbers are used by the system to
check for bad block addresses;
if an `impossible' block address is allocated from the free list
or is freed,
a diagnostic is written on the on-line console.
Moreover, the free array is cleared, so as to prevent further
allocation from a presumably corrupted free list.
.PP
The free list for each volume is maintained as
follows.
The
.I s\_free
array contains, in
.I "s\_free[1], ... , s\_free[s\_nfree\-1],"
up to NICFREE free block numbers.
NICFREE is a configuration constant.
.I S\_free[0]
is the block address of the head
of a chain of blocks constituting the free list.
The layout of each block of the free chain as defined
in the include file
.I <sys/fblk.h>
is:
.PP
.nf
struct fblk
{
short df_nfree;
daddr_t df_free[NICFREE];
};
.fi
.PP
The fields
.I df\_nfree
and
.I df\_free
in a free block are used exactly like
.I s\_nfree
and
.I s\_free
in the super block.
To allocate a block:
decrement
.I s\_nfree,
and the new block number is
.I s\_free[s\_nfree].
If the new block address is 0,
there are no blocks left, so give an error.
If
.I s\_nfree
became 0,
read the new block into
.I s\_nfree
and
.I s\_free.
To free a block, check if
.I s\_nfree
is NICFREE; if so,
copy
.I s\_nfree
and the
.I s\_free
array into it,
write it out, and set
.I s\_nfree
to 0.
In any event set
.I s\_free[s\_nfree]
to the freed block's address and
increment
.I s\_nfree.
.PP
.I S\_ninode
is the number of free i-numbers in the
.I s\_inode
array.
To allocate an i-node:
if
.I s\_ninode
is greater than 0,
decrement it and return
.I s\_inode[s\_ninode].
If it was 0, read the i-list
and place the numbers of all free inodes
(up to NICINOD) into the
.I s\_inode
array,
then try again.
To free an i-node,
provided
.I s\_ninode
is less than NICINODE,
place its number into
.I s\_inode[s\_ninode]
and increment
.I s\_ninode.
If
.I s\_ninode
is already NICINODE, don't bother to enter the freed i-node into any table.
This list of i-nodes is only to speed
up the allocation process; the information
as to whether the inode is really free
or not is maintained in the inode itself.
.PP
.I S\_flock
and
.I s\_ilock
are flags maintained in the core
copy of the file system
while it is mounted
and their values on disk are immaterial.
The value of
.I s\_fmod
on disk is likewise immaterial;
it is used as a flag to indicate that the super-block has
changed and should be copied to
the disk during the next periodic update of file
system information.
.I S\_ronly
is a write-protection indicator; its disk value is also immaterial.
.PP
.I S\_time
is the last time the super-block of the file system was changed.
During a reboot,
.I s\_time
of the super-block for the root file system
is used to set the system's idea of the time.
.PP
I-numbers begin at 1, and the storage for i-nodes
begins in block 2.
.tr |
I-nodes are 64 bytes long, so 8 of them fit into a block.
I-node 2 is reserved for the root directory of the file
system, but no other i-number has a built-in
meaning.
Each i-node represents one file.
The format of an i-node as given in the include file
.I <sys/ino.h>
is:
.PP
.nf
.ta \w'#define 'u +\w'time_t 'u +\w'di_addr[40]; 'u
/*
* Inode structure as it appears on
* a disk block.
*/
struct dinode
{
u_short di_mode; /* mode and type of file */
short di_nlink; /* number of links to file */
short di_uid; /* owner's user id */
short di_gid; /* owner's group id */
off_t di_size; /* number of bytes in file */
char di_addr[40]; /* disk block addresses */
time_t di_atime; /* time last accessed */
time_t di_mtime; /* time last modified */
time_t di_ctime; /* time created */
};
#ifndef UCB_NKB
#define INOPB 8 /* 8 inodes per block */
/*
* 39 of the address bytes are used;
* 13 addresses of 3 bytes each.
*/
#endif
#if UCB_NKB==1
#define INOPB 16 /* 16 inodes per BSIZE block */
/*
* 21 of the address bytes are used;
* 7 addresses of 3 bytes each.
*/
#endif
.fi
.PP
.I Di\_mode
tells the kind of file; it
is encoded identically to the
.I st\_mode field of
.IR stat (2).
.I Di\_nlink
is the number of directory entries
(links) that refer to this i-node.
.I Di\_uid
and
.I di\_gid
are the owner's user and group IDs.
.I Size
is the number of bytes in the file.
.I Di\_atime
and
.I di\_mtime
are the times of last access and modification
of the file contents (read, write or create)
(see
.IR times (2));
.I Di\_ctime
records the time of last modification
to the inode or to the file, and is used
to determine whether it should be dumped.
.PP
Special files are recognized by their modes
and not by i-number.
A block-type special file is one which
can potentially be mounted as a file system;
a character-type special file cannot, though it is
not necessarily character-oriented.
For special files, the
.I di\_addr
field is occupied by the device code
(see
.IR types (5)).
The device codes
of block and character special files overlap.
.PP
Disk addresses of plain files and directories
are kept in the array
.I di\_addr
packed into 3 bytes each.
The first 10 addresses specify device blocks directly.
The last 3 addresses are singly, doubly, and triply
indirect and point to blocks of 128 block pointers.
Pointers in indirect blocks have the type
.I daddr\_t
(see
.IR types (5)).
.PP
For block
.I b
in a file to exist,
it
is not necessary that all blocks less than
.I b
exist.
A zero block number either in the address words of
the i-node or in an indirect block indicates that the
corresponding block has never been allocated.
Such a missing block reads as if it contained all zero words.
.SH "SEE ALSO"
stat(2), dir(5), types(5), dcheck(8), fsck(8), icheck(8), mount(8)