V7/usr/man/man5/filsys.5

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.TH FILSYS  5
.SH NAME
filsys, flblk, ino \- format of file system volume
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/flbk.h>
.br
.B #include <sys/filsys.h>
.br
.B #include <sys/ino.h>
.SH DESCRIPTION
Every
file system storage volume
(e.g. RF disk, RK disk, RP disk, DECtape reel)
has a common format for certain vital information.
Every such volume is divided into a certain number
of 512-byte blocks.
Block 0 is unused and is available to contain
a bootstrap program, pack label, or other information.
.PP
Block 1 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
.so /usr/include/sys/filsys.h
.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
.so /usr/include/sys/fblk.h
.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
The fields
.I s_tfree, s_tinode, s_fname
and
.I s_fpack
are not currently maintained.
.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
.so /usr/include/sys/ino.h
.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"
icheck(1), dcheck(1), dir(5), mount(1), stat(2), types(5)