freebsd-nq/share/doc/psd/02.implement/implement

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UNIX Implementation
.AU "MH 2C-523" 2394
K. Thompson
.AI
AT&T Bell Laboratories
Murray Hill, NJ
.AB
This paper describes in high-level terms the
implementation of the resident
.UX
kernel.
This discussion is broken into three parts.
The first part describes
how the
.UX
system views processes, users, and programs.
The second part describes the I/O system.
The last part describes the
.UX
file system.
.AE
.NH
INTRODUCTION
.PP
The
.UX
kernel consists of about 10,000
lines of C code and about 1,000 lines of assembly code.
The assembly code can be further broken down into
200 lines included for
the sake of efficiency
(they could have been written in C)
and 800 lines to perform hardware
functions not possible in C.
.PP
This code represents 5 to 10 percent of what has
been lumped into the broad expression
``the
.UX
operating system.''
The kernel is the only
.UX
code that
cannot be substituted by a user to his
own liking.
For this reason,
the kernel should make as few real
decisions as possible.
This does not mean to allow the user
a million options to do the same thing.
Rather, it means to allow only one way to
do one thing,
but have that way be the least-common divisor
of all the options that might have been provided.
.PP
What is or is not implemented in the kernel
represents both a great responsibility and a great power.
It is a soap-box platform on
``the way things should be done.''
Even so, if
``the way'' is too radical,
no one will follow it.
Every important decision was weighed
carefully.
Throughout,
simplicity has been substituted for efficiency.
Complex algorithms are used only if
their complexity can be localized.
.NH
PROCESS CONTROL
.PP
In the
.UX
system,
a user executes programs in an
environment called a user process.
When a system function is required,
the user process calls the system
as a subroutine.
At some point in this call,
there is a distinct switch of environments.
After this,
the process is said to be a system process.
In the normal definition of processes,
the user and system processes are different
phases of the same process
(they never execute simultaneously).
For protection,
each system process has its own stack.
.PP
The user process may execute
from a read-only text segment,
which is shared by all processes
executing the same code.
There is no
.IT functional
benefit
from shared-text segments.
An
.IT efficiency
benefit comes from the fact
that there is no need to swap read-only
segments out because the original
copy on secondary memory is still current.
This is a great benefit to interactive
programs that tend to be swapped while
waiting for terminal input.
Furthermore,
if two processes are
executing
simultaneously
from the same copy of a read-only segment,
only one copy needs to reside in
primary memory.
This is a secondary effect,
because
simultaneous execution of a program
is not common.
It is ironic that this effect,
which reduces the use of primary memory,
only comes into play when there is
an overabundance of primary memory,
that is,
when there is enough memory
to keep waiting processes loaded.
.PP
All current read-only text segments in the
system are maintained from the
.IT "text table" .
A text table entry holds the location of the
text segment on secondary memory.
If the segment is loaded,
that table also holds the primary memory location
and the count of the number of processes
sharing this entry.
When this count is reduced to zero,
the entry is freed along with any
primary and secondary memory holding the segment.
When a process first executes a shared-text segment,
a text table entry is allocated and the
segment is loaded onto secondary memory.
If a second process executes a text segment
that is already allocated,
the entry reference count is simply incremented.
.PP
A user process has some strictly private
read-write data
contained in its
data segment.
As far as possible,
the system does not use the user's
data segment to hold system data.
In particular,
there are no I/O buffers in the
user address space.
.PP
The user data segment has two growing boundaries.
One, increased automatically by the system
as a result of memory faults,
is used for a stack.
The second boundary is only grown (or shrunk) by
explicit requests.
The contents of newly allocated primary memory
is initialized to zero.
.PP
Also associated and swapped with
a process is a small fixed-size
system data segment.
This segment contains all
the data about the process
that the system needs only when the
process is active.
Examples of the kind of data contained
in the system data segment are:
saved central processor registers,
open file descriptors,
accounting information,
scratch data area,
and the stack for the system phase
of the process.
The system data segment is not
addressable from the user process
and is therefore protected.
.PP
Last,
there is a process table with
one entry per process.
This entry contains all the data
needed by the system when the process
is
.IT not
active.
Examples are
the process's name,
the location of the other segments,
and scheduling information.
The process table entry is allocated
when the process is created, and freed
when the process terminates.
This process entry is always directly
addressable by the kernel.
.PP
Figure 1 shows the relationships
between the various process control
data.
In a sense,
the process table is the
definition of all processes,
because
all the data associated with a process
may be accessed
starting from the process table entry.
.KF
.if t .in .375i
.so fig1.pic
.if t .in -.375i
.sp 2v
.ce
Fig. 1\(emProcess control data structure.
.KE
.NH 2
Process creation and program execution
.PP
Processes are created by the system primitive
.UL fork .
The newly created process (child) is a copy of the original process (parent).
There is no detectable sharing of primary memory between the two processes.
(Of course,
if the parent process was executing from a read-only
text segment,
the child will share the text segment.)
Copies of all writable data segments
are made for the child process.
Files that were open before the
.UL fork
are
truly shared after the
.UL fork .
The processes are informed as to their part in the
relationship to
allow them to select their own
(usually non-identical)
destiny.
The parent may
.UL wait
for the termination of
any of its children.
.PP
A process may
.UL exec
a file.
This consists of exchanging the current text and data
segments of the process for new text and data
segments specified in the file.
The old segments are lost.
Doing an
.UL exec
does
.IT not
change processes;
the process that did the
.UL exec
persists,
but
after the
.UL exec
it is executing a different program.
Files that were open
before the
.UL exec
remain open after the
.UL exec .
.PP
If a program,
say the first pass of a compiler,
wishes to overlay itself with another program,
say the second pass,
then it simply
.UL exec s
the second program.
This is analogous
to a ``goto.''
If a program wishes to regain control
after
.UL exec ing
a second program,
it should
.UL fork
a child process,
have the child
.UL exec
the second program, and
have the parent
.UL wait
for the child.
This is analogous to a ``call.''
Breaking up the call into a binding followed by
a transfer is similar to the subroutine linkage in
SL-5.
.[
griswold hanson sl5 overview
.]
.NH 2
Swapping
.PP
The major data associated with a process
(the user data segment,
the system data segment, and
the text segment)
are swapped to and from secondary
memory, as needed.
The user data segment and the system data segment
are kept in contiguous primary memory to reduce
swapping latency.
(When low-latency devices, such as bubbles,
.UC CCD s,
or scatter/gather devices,
are used,
this decision will have to be reconsidered.)
Allocation of both primary
and secondary memory is performed
by the same simple first-fit algorithm.
When a process grows,
a new piece of primary memory is allocated.
The contents of the old memory is copied to the new memory.
The old memory is freed
and the tables are updated.
If there is not enough primary memory,
secondary memory is allocated instead.
The process is swapped out onto the
secondary memory,
ready to be swapped in with
its new size.
.PP
One separate process in the kernel,
the swapping process,
simply swaps the other
processes in and out of primary memory.
It examines the
process table looking for a process
that is swapped out and is
ready to run.
It allocates primary memory for that
process and
reads its segments into
primary memory, where that process competes for the
central processor with other loaded processes.
If no primary memory is available,
the swapping process makes memory available
by examining the process table for processes
that can be swapped out.
It selects a process to swap out,
writes it to secondary memory,
frees the primary memory,
and then goes back to look for a process
to swap in.
.PP
Thus there are two specific algorithms
to the swapping process.
Which of the possibly many processes that
are swapped out is to be swapped in?
This is decided by secondary storage residence
time.
The one with the longest time out is swapped in first.
There is a slight penalty for larger processes.
Which of the possibly many processes that
are loaded is to be swapped out?
Processes that are waiting for slow events
(i.e., not currently running or waiting for
disk I/O)
are picked first,
by age in primary memory,
again with size penalties.
The other processes are examined
by the same age algorithm,
but are not taken out unless they are
at least of some age.
This adds
hysteresis to the swapping and
prevents total thrashing.
.PP
These swapping algorithms are the
most suspect in the system.
With limited primary memory,
these algorithms cause total swapping.
This is not bad in itself, because
the swapping does not impact the
execution of the resident processes.
However, if the swapping device must
also be used for file storage,
the swapping traffic severely
impacts the file system traffic.
It is exactly these small systems
that tend to double usage of limited disk
resources.
.NH 2
Synchronization and scheduling
.PP
Process synchronization is accomplished by having processes
wait for events.
Events are represented by arbitrary integers.
By convention,
events are chosen to be addresses of
tables associated with those events.
For example, a process that is waiting for
any of its children to terminate will wait
for an event that is the address of
its own process table entry.
When a process terminates,
it signals the event represented by
its parent's process table entry.
Signaling an event on which no process
is waiting has no effect.
Similarly,
signaling an event on which many processes
are waiting will wake all of them up.
This differs considerably from
Dijkstra's P and V
synchronization operations,
.[
dijkstra sequential processes 1968
.]
in that
no memory is associated with events.
Thus there need be no allocation of events
prior to their use.
Events exist simply by being used.
.PP
On the negative side,
because there is no memory associated with events,
no notion of ``how much''
can be signaled via the event mechanism.
For example,
processes that want memory might
wait on an event associated with
memory allocation.
When any amount of memory becomes available,
the event would be signaled.
All the competing processes would then wake
up to fight over the new memory.
(In reality,
the swapping process is the only process
that waits for primary memory to become available.)
.PP
If an event occurs
between the time a process decides
to wait for that event and the
time that process enters the wait state,
then
the process will wait on an event that has
already happened (and may never happen again).
This race condition happens because there is no memory associated with
the event to indicate that the event has occurred;
the only action of an event is to change a set of processes
from wait state to run state.
This problem is relieved largely
by the fact that process switching can
only occur in the kernel by explicit calls
to the event-wait mechanism.
If the event in question is signaled by another
process,
then there is no problem.
But if the event is signaled by a hardware
interrupt,
then special care must be taken.
These synchronization races pose the biggest
problem when
.UX
is adapted to multiple-processor configurations.
.[
hawley meyer multiprocessing unix
.]
.PP
The event-wait code in the kernel
is like a co-routine linkage.
At any time,
all but one of the processes has called event-wait.
The remaining process is the one currently executing.
When it calls event-wait,
a process whose event has been signaled
is selected and that process
returns from its call to event-wait.
.PP
Which of the runable processes is to run next?
Associated with each process is a priority.
The priority of a system process is assigned by the code
issuing the wait on an event.
This is roughly equivalent to the response
that one would expect on such an event.
Disk events have high priority,
teletype events are low,
and time-of-day events are very low.
(From observation,
the difference in system process priorities
has little or no performance impact.)
All user-process priorities are lower than the
lowest system priority.
User-process priorities are assigned
by an algorithm based on the
recent ratio of the amount of compute time to real time consumed
by the process.
A process that has used a lot of
compute time in the last real-time
unit is assigned a low user priority.
Because interactive processes are characterized
by low ratios of compute to real time,
interactive response is maintained without any
special arrangements.
.PP
The scheduling algorithm simply picks
the process with the highest priority,
thus
picking all system processes first and
user processes second.
The compute-to-real-time ratio is updated
every second.
Thus,
all other things being equal,
looping user processes will be
scheduled round-robin with a
1-second quantum.
A high-priority process waking up will
preempt a running, low-priority process.
The scheduling algorithm has a very desirable
negative feedback character.
If a process uses its high priority
to hog the computer,
its priority will drop.
At the same time, if a low-priority
process is ignored for a long time,
its priority will rise.
.NH
I/O SYSTEM
.PP
The I/O system
is broken into two completely separate systems:
the block I/O system and the character I/O system.
In retrospect,
the names should have been ``structured I/O''
and ``unstructured I/O,'' respectively;
while the term ``block I/O'' has some meaning,
``character I/O'' is a complete misnomer.
.PP
Devices are characterized by a major device number,
a minor device number, and
a class (block or character).
For each class,
there is an array of entry points into the device drivers.
The major device number is used to index the array
when calling the code for a particular device driver.
The minor device number is passed to the
device driver as an argument.
The minor number has no significance other
than that attributed to it by the driver.
Usually,
the driver uses the minor number to access
one of several identical physical devices.
.PP
The use of the array of entry points
(configuration table)
as the only connection between the
system code and the device drivers is
very important.
Early versions of the system had a much
less formal connection with the drivers,
so that it was extremely hard to handcraft
differently configured systems.
Now it is possible to create new
device drivers in an average of a few hours.
The configuration table in most cases
is created automatically by a program
that reads the system's parts list.
.NH 2
Block I/O system
.PP
The model block I/O device consists
of randomly addressed, secondary
memory blocks of 512 bytes each.
The blocks are uniformly addressed
0, 1, .\|.\|. up to the size of the device.
The block device driver has the job of
emulating this model on a
physical device.
.PP
The block I/O devices are accessed
through a layer of buffering software.
The system maintains a list of buffers
(typically between 10 and 70)
each assigned a device name and
a device address.
This buffer pool constitutes a data cache
for the block devices.
On a read request,
the cache is searched for the desired block.
If the block is found,
the data are made available to the
requester without any physical I/O.
If the block is not in the cache,
the least recently used block in the cache is renamed,
the correct device driver is called to
fill up the renamed buffer, and then the
data are made available.
Write requests are handled in an analogous manner.
The correct buffer is found
and relabeled if necessary.
The write is performed simply by marking
the buffer as ``dirty.''
The physical I/O is then deferred until
the buffer is renamed.
.PP
The benefits in reduction of physical I/O
of this scheme are substantial,
especially considering the file system implementation.
There are,
however,
some drawbacks.
The asynchronous nature of the
algorithm makes error reporting
and meaningful user error handling
almost impossible.
The cavalier approach to I/O error
handling in the
.UX
system is partly due to the asynchronous
nature of the block I/O system.
A second problem is in the delayed writes.
If the system stops unexpectedly,
it is almost certain that there is a
lot of logically complete,
but physically incomplete,
I/O in the buffers.
There is a system primitive to
flush all outstanding I/O activity
from the buffers.
Periodic use of this primitive helps,
but does not solve, the problem.
Finally,
the associativity in the buffers
can alter the physical I/O sequence
from that of the logical I/O sequence.
This means that there are times
when data structures on disk are inconsistent,
even though the software is careful
to perform I/O in the correct order.
On non-random devices,
notably magnetic tape,
the inversions of writes can be disastrous.
The problem with magnetic tapes is ``cured'' by
allowing only one outstanding write request
per drive.
.NH 2
Character I/O system
.PP
The character I/O system consists of all
devices that do not fall into the block I/O model.
This includes the ``classical'' character devices
such as communications lines, paper tape, and
line printers.
It also includes magnetic tape and disks when
they are not used in a stereotyped way,
for example, 80-byte physical records on tape
and track-at-a-time disk copies.
In short,
the character I/O interface
means ``everything other than block.''
I/O requests from the user are sent to the
device driver essentially unaltered.
The implementation of these requests is, of course,
up to the device driver.
There are guidelines and conventions
to help the implementation of
certain types of device drivers.
.NH 3
Disk drivers
.PP
Disk drivers are implemented
with a queue of transaction records.
Each record holds a read/write flag,
a primary memory address,
a secondary memory address, and
a transfer byte count.
Swapping is accomplished by passing
such a record to the swapping device driver.
The block I/O interface is implemented by
passing such records with requests to
fill and empty system buffers.
The character I/O interface to the disk
drivers create a transaction record that
points directly into the user area.
The routine that creates this record also insures
that the user is not swapped during this
I/O transaction.
Thus by implementing the general disk driver,
it is possible to use the disk
as a block device,
a character device, and a swap device.
The only really disk-specific code in normal
disk drivers is the pre-sort of transactions to
minimize latency for a particular device, and
the actual issuing of the I/O request.
.NH 3
Character lists
.PP
Real character-oriented devices may
be implemented using the common
code to handle character lists.
A character list is a queue of characters.
One routine puts a character on a queue.
Another gets a character from a queue.
It is also possible to ask how many
characters are currently on a queue.
Storage for all queues in the system comes
from a single common pool.
Putting a character on a queue will allocate
space from the common pool and link the
character onto the data structure defining the queue.
Getting a character from a queue returns
the corresponding space to the pool.
.PP
A typical character-output device
(paper tape punch, for example)
is implemented by passing characters
from the user onto a character queue until
some maximum number of characters is on the queue.
The I/O is prodded to start as
soon as there is anything on the queue
and, once started,
it is sustained by hardware completion interrupts.
Each time there is a completion interrupt,
the driver gets the next character from the queue
and sends it to the hardware.
The number of characters on the queue is checked and,
as the count falls through some intermediate level,
an event (the queue address) is signaled.
The process that is passing characters from
the user to the queue can be waiting on the event, and
refill the queue to its maximum
when the event occurs.
.PP
A typical character input device
(for example, a paper tape reader)
is handled in a very similar manner.
.PP
Another class of character devices is the terminals.
A terminal is represented by three
character queues.
There are two input queues (raw and canonical)
and an output queue.
Characters going to the output of a terminal
are handled by common code exactly as described
above.
The main difference is that there is also code
to interpret the output stream as
.UC  ASCII
characters and to perform some translations,
e.g., escapes for deficient terminals.
Another common aspect of terminals is code
to insert real-time delay after certain control characters.
.PP
Input on terminals is a little different.
Characters are collected from the terminal and
placed on a raw input queue.
Some device-dependent code conversion and
escape interpretation is handled here.
When a line is complete in the raw queue,
an event is signaled.
The code catching this signal then copies a
line from the raw queue to a canonical queue
performing the character erase and line kill editing.
User read requests on terminals can be
directed at either the raw or canonical queues.
.NH 3
Other character devices
.PP
Finally,
there are devices that fit no general category.
These devices are set up as character I/O drivers.
An example is a driver that reads and writes
unmapped primary memory as an I/O device.
Some devices are too
fast to be treated a character at time,
but do not fit the disk I/O mold.
Examples are fast communications lines and
fast line printers.
These devices either have their own buffers
or ``borrow'' block I/O buffers for a while and
then give them back.
.NH
THE FILE SYSTEM
.PP
In the
.UX
system,
a file is a (one-dimensional) array of bytes.
No other structure of files is implied by the
system.
Files are attached anywhere
(and possibly multiply)
onto a hierarchy of directories.
Directories are simply files that
users cannot write.
For a further discussion
of the external view of files and directories,
see Ref.\0
.[
ritchie thompson unix bstj 1978
%Q This issue
.].
.PP
The
.UX
file system is a disk data structure
accessed completely through
the block I/O system.
As stated before,
the canonical view of a ``disk'' is
a randomly addressable array of
512-byte blocks.
A file system breaks the disk into
four self-identifying regions.
The first block (address 0)
is unused by the file system.
It is left aside for booting procedures.
The second block (address 1)
contains the so-called ``super-block.''
This block,
among other things,
contains the size of the disk and
the boundaries of the other regions.
Next comes the i-list,
a list of file definitions.
Each file definition is
a 64-byte structure, called an i-node.
The offset of a particular i-node
within the i-list is called its i-number.
The combination of device name
(major and minor numbers) and i-number
serves to uniquely name a particular file.
After the i-list,
and to the end of the disk,
come free storage blocks that
are available for the contents of files.
.PP
The free space on a disk is maintained
by a linked list of available disk blocks.
Every block in this chain contains a disk address
of the next block in the chain.
The remaining space contains the address of up to
50 disk blocks that are also free.
Thus with one I/O operation,
the system obtains 50 free blocks and a
pointer where to find more.
The disk allocation algorithms are
very straightforward.
Since all allocation is in fixed-size
blocks and there is strict accounting of
space,
there is no need to compact or garbage collect.
However,
as disk space becomes dispersed,
latency gradually increases.
Some installations choose to occasionally compact
disk space to reduce latency.
.PP
An i-node contains 13 disk addresses.
The first 10 of these addresses point directly at
the first 10 blocks of a file.
If a file is larger than 10 blocks (5,120 bytes),
then the eleventh address points at a block
that contains the addresses of the next 128 blocks of the file.
If the file is still larger than this
(70,656 bytes),
then the twelfth block points at up to 128 blocks,
each pointing to 128 blocks of the file.
Files yet larger
(8,459,264 bytes)
use the thirteenth address for a ``triple indirect'' address.
The algorithm ends here with the maximum file size
of 1,082,201,087 bytes.
.PP
A logical directory hierarchy is added
to this flat physical structure simply
by adding a new type of file, the directory.
A directory is accessed exactly as an ordinary file.
It contains 16-byte entries consisting of
a 14-byte name and an i-number.
The root of the hierarchy is at a known i-number
(\fIviz.,\fR 2).
The file system structure allows an arbitrary, directed graph
of directories with regular files linked in
at arbitrary places in this graph.
In fact,
very early
.UX
systems used such a structure.
Administration of such a structure became so
chaotic that later systems were restricted
to a directory tree.
Even now,
with regular files linked multiply
into arbitrary places in the tree,
accounting for space has become a problem.
It may become necessary to restrict the entire
structure to a tree,
and allow a new form of linking that
is subservient to the tree structure.
.PP
The file system allows
easy creation,
easy removal,
easy random accessing,
and very easy space allocation.
With most physical addresses confined
to a small contiguous section of disk,
it is also easy to dump, restore, and
check the consistency of the file system.
Large files suffer from indirect addressing,
but the cache prevents most of the implied physical I/O
without adding much execution.
The space overhead properties of this scheme are quite good.
For example,
on one particular file system,
there are 25,000 files containing 130M bytes of data-file content.
The overhead (i-node, indirect blocks, and last block breakage)
is about 11.5M bytes.
The directory structure to support these files
has about 1,500 directories containing 0.6M bytes of directory content
and about 0.5M bytes of overhead in accessing the directories.
Added up any way,
this comes out to less than a 10 percent overhead for actual
stored data.
Most systems have this much overhead in
padded trailing blanks alone.
.NH 2
File system implementation
.PP
Because the i-node defines a file,
the implementation of the file system centers
around access to the i-node.
The system maintains a table of all active
i-nodes.
As a new file is accessed,
the system locates the corresponding i-node,
allocates an i-node table entry, and reads
the i-node into primary memory.
As in the buffer cache,
the table entry is considered to be the current
version of the i-node.
Modifications to the i-node are made to
the table entry.
When the last access to the i-node goes
away,
the table entry is copied back to the
secondary store i-list and the table entry is freed.
.KF
.if t .in .25i
.so fig2.pic
.if t .in -.25i
.sp 2v
.ce
Fig. 2\(emFile system data structure.
.sp
.KE
.PP
All I/O operations on files are carried out
with the aid of the corresponding i-node table entry.
The accessing of a file is a straightforward
implementation of the algorithms mentioned previously.
The user is not aware of i-nodes and i-numbers.
References to the file system are made in terms of
path names of the directory tree.
Converting a path name into an i-node table entry
is also straightforward.
Starting at some known i-node
(the root or the current directory of some process),
the next component of the path name is
searched by reading the directory.
This gives an i-number and an implied device
(that of the directory).
Thus the next i-node table entry can be accessed.
If that was the last component of the path name,
then this i-node is the result.
If not,
this i-node is the directory needed to look up
the next component of the path name, and the
algorithm is repeated.
.PP
The user process accesses the file system with
certain primitives.
The most common of these are
.UL open ,
.UL create ,
.UL read ,
.UL write ,
.UL seek ,
and
.UL close .
The data structures maintained are shown in Fig. 2.
In the system data segment associated with a user,
there is room for some (usually between 10 and 50) open files.
This open file table consists of pointers that can be used to access
corresponding i-node table entries.
Associated with each of these open files is
a current I/O pointer.
This is a byte offset of
the next read/write operation on the file.
The system treats each read/write request
as random with an implied seek to the
I/O pointer.
The user usually thinks of the file as
sequential with the I/O pointer
automatically counting the number of bytes
that have been read/written from the file.
The user may,
of course,
perform random I/O by setting the I/O pointer
before reads/writes.
.PP
With file sharing,
it is necessary to allow related
processes to share a common I/O pointer
and yet have separate I/O pointers
for independent processes
that access the same file.
With these two conditions,
the I/O pointer cannot reside
in the i-node table nor can
it reside in the list of
open files for the process.
A new table
(the open file table)
was invented for the sole purpose
of holding the I/O pointer.
Processes that share the same open
file
(the result of
.UL fork s)
share a common open file table entry.
A separate open of the same file will
only share the i-node table entry,
but will have distinct open file table entries.
.PP
The main file system primitives are implemented as follows.
.UL \&open
converts a file system path name into an i-node
table entry.
A pointer to the i-node table entry is placed in a
newly created open file table entry.
A pointer to the file table entry is placed in the
system data segment for the process.
.UL \&create
first creates a new i-node entry,
writes the i-number into a directory, and
then builds the same structure as for an
.UL open .
.UL \&read
and
.UL write
just access the i-node entry as described above.
.UL \&seek
simply manipulates the I/O pointer.
No physical seeking is done.
.UL \&close
just frees the structures built by
.UL open
and
.UL create .
Reference counts are kept on the open file table entries and
the i-node table entries to free these structures after
the last reference goes away.
.UL \&unlink
simply decrements the count of the
number of directories pointing at the given i-node.
When the last reference to an i-node table entry
goes away,
if the i-node has no directories pointing to it,
then the file is removed and the i-node is freed.
This delayed removal of files prevents
problems arising from removing active files.
A file may be removed while still open.
The resulting unnamed file vanishes
when the file is closed.
This is a method of obtaining temporary files.
.PP
There is a type of unnamed
.UC  FIFO
file called a
.UL pipe.
Implementation of
.UL pipe s
consists of implied
.UL seek s
before each
.UL read
or
.UL write
in order to implement
first-in-first-out.
There are also checks and synchronization
to prevent the
writer from grossly outproducing the
reader and to prevent the reader from
overtaking the writer.
.NH 2
Mounted file systems
.PP
The file system of a
.UX
system
starts with some designated block device
formatted as described above to contain
a hierarchy.
The root of this structure is the root of
the
.UX
file system.
A second formatted block device may be
mounted
at any leaf of
the current hierarchy.
This logically extends the current hierarchy.
The implementation of
mounting
is trivial.
A mount table is maintained containing
pairs of designated leaf i-nodes and
block devices.
When converting a path name into an i-node,
a check is made to see if the new i-node is a
designated leaf.
If it is,
the i-node of the root
of the block device replaces it.
.PP
Allocation of space for a file is taken
from the free pool on the device on which the
file lives.
Thus a file system consisting of many
mounted devices does not have a common pool of
free secondary storage space.
This separation of space on different
devices is necessary to allow easy
unmounting
of a device.
.NH 2
Other system functions
.PP
There are some other things that the system
does for the user\-a
little accounting,
a little tracing/debugging,
and a little access protection.
Most of these things are not very
well developed
because our use of the system in computing science research
does not need them.
There are some features that are missed in some
applications, for example, better inter-process communication.
.PP
The
.UX
kernel is an I/O multiplexer more than
a complete operating system.
This is as it should be.
Because of this outlook,
many features are
found in most
other operating systems that are missing from the
.UX
kernel.
For example,
the
.UX
kernel does not support
file access methods,
file disposition,
file formats,
file maximum size,
spooling,
command language,
logical records,
physical records,
assignment of logical file names,
logical file names,
more than one character set,
an operator's console,
an operator,
log-in,
or log-out.
Many of these things are symptoms rather than features.
Many of these things are implemented
in user software
using the kernel as a tool.
A good example of this is the command language.
.[
bourne shell 1978 bstj
%Q This issue
.]
Each user may have his own command language.
Maintenance of such code is as easy as
maintaining user code.
The idea of implementing ``system'' code with general
user primitives
comes directly from
.UC  MULTICS .
.[
organick multics 1972
.]
.LP
.[
$LIST$
.]