AT&T UNIX™PC Unix System V Users Manual - tenox

FS (4) FS (4)
S_type indicates the file system type. Currently, two types of file
systems are supported: the original 512-byte oriented and the new
improved 1024-byte oriented. S_magic is used to distinguish the
original 512-byte oriented file systems from the newer file systems.
If this field is not equal to the magic number, FsMAGIC, the type
is assumed to be Fsl b, otherwise the s_type field is used. In the
following description, a block is then determined by the type. For
the original 512-byte oriented file system, a block is 512 bytes.
For the 1024-byte oriented file system, a block is 1024 bytes or
two sectors. The operating system takes care of all conversions
from logical block numbers to physical sector numbers.
S_isize is the address of the first data block after the i-list; the ilist
starts just after the super-block, namely in block 2; thus the
i-list is s_isize- 2 blocks long. SJsize is the first block not potentially
available for allocation to a file. These numbers are used by
the system to check for bad block numbers; if an "impossible"
block number 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.
The free list for each volume is maintained as follows. The sJree
array contains, in sJree[1], ... , sJree [s_nfree-1], up to 49
numbers of free blocks. SJree [OJ is the block number of the head
of a chain of blocks constituting the free list. The first long in
each free-chain block is the number (up to 50) of free-block
numbers listed in the next 50 longs of this chain member. The
first of these 50 blocks is the link to the next member of the chain.
To allocate a block: decrement s_nfree, and the new block is
sJree [s_nfree ]. If the new block number is 0, there are no blocks
left, so give an error. If s_nfree became 0, read in the block
named by the new block number, replace s_nfree by its first
word, and copy the block numbers in the next 50 longs into the
sJree array. To free a block, check if s_nfree is 50; if so, copy
s_nfree and the sJree array into it, write it out, and set s_nfree
to 0. In any event set sJree [s_nfree] to the freed block's number
and increment s_nfree .
s_tfree is the total free blocks available in the file system.
S_ninode is the number of free i-numbers in the s_inode array.
To allocate an i-node: if s_ninode is greater than 0, decrement it
and return s_inode [s_ninode]. If it was 0, read the i-list and
place the numbers of all free inodes (up to 100) into the s_inode
array, then try again. To free an i-node, provided s_ninode is less
than 100, place its number into s_inode [ s_ninode] and increment
s_ninode . If s_ninode is already 100, do not 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.
S_tinode is the total free inodes available in the file system.
S_fiock and s_ilock are flags maintained in the core copy of the
file system while it is mounted and their values on disk are
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