1.
How
are devices represented in UNIX?
Answer:
All devices
are represented by files called special files that are located in/dev
directory. Thus, device files and other files are named and accessed in the
same way. A 'regular file' is just an ordinary data file in the disk. A 'block
special file' represents a device with characteristics similar to a disk (data
transfer in terms of blocks). A 'character special file' represents a device
with characteristics similar to a keyboard (data transfer is by stream of bits
in sequential order).
2.
What
is 'inode'?
Answer:
All UNIX
files have its description stored in a structure called 'inode'.
The inode contains info about the file-size, its
location, time of last access, time of last modification, permission and so on.
Directories are also represented as files and have an associated inode. In addition to descriptions about the file, the inode contains pointers to the data blocks of the file. If
the file is large, inode has indirect pointer to a
block of pointers to additional data blocks (this further aggregates for larger
files). A block is typically 8k.
Inode consists of the following fields:
o
File
owner identifier
o
File
access permissions
o
File
access times
o
Number
of links
o
File
size
o
Location
of the file data
3.
Brief
about the directory representation in UNIX
Answer:
A Unix directory is a file containing a correspondence between
filenames and inodes. A directory is a special file
that the kernel maintains. Only kernel modifies directories, but processes can
read directories. The contents of a directory are a list of filename and inode number pairs. When new directories are created,
kernel makes two entries named '.' (refers to the
directory itself) and '..' (refers to parent
directory). System call for creating directory is mkdir
(pathname, mode).
4.
What
are the Unix system calls for I/O?
Answer:
o
open(pathname,flag,mode) - open file
o
creat(pathname,mode) - create file
o
close(filedes) - close an open file
o
read(filedes,buffer,bytes) - read data from an open file
o
write(filedes,buffer,bytes) - write data to an open file
o
lseek(filedes,offset,from) - position an open file
o
dup(filedes) - duplicate an existing file descriptor
o
dup2(oldfd,newfd) - duplicate to a desired file descriptor
o
fcntl(filedes,cmd,arg) - change properties of an
open file
o
ioctl(filedes,request,arg) - change the behaviour of an open file
The
difference between fcntl anf ioctl is that the
former is intended for any open file, while the latter is for device-specific
operations.
5.
How
do you change File Access Permissions?
Answer:
Every file
has following attributes:
o
owner's
user ID ( 16 bit integer )
o
owner's
group ID ( 16 bit integer )
o
File
access mode word
'r w x -r w x- r w x'
(user permission-group permission-others permission)
r-read, w-write, x-execute
To change the access mode, we use chmod(filename,mode).
Example
1:
To
change mode of myfile to 'rw-rw-r--'
(ie. read, write permission for user - read,write permission for group - only read permission for
others) we give the args as:
chmod(myfile,0664) .
Each operation is represented by discrete values
'r' is 4
'w' is 2
'x' is 1
Therefore, for 'rw' the value is 6(4+2).
Example
2:
To change mode of myfile to 'rwxr--r--'
we give the args as:
chmod(myfile,0744).
6.
What
are links and symbolic links in UNIX file system?
Answer:
A link is a
second name (not a file) for a file. Links can be used to assign more than one
name to a file, but cannot be used to assign a directory more than one name or
link filenames on different computers.
Symbolic
link 'is' a file that only contains the name of another file.Operation
on the symbolic link is directed to the file pointed by the it.Both
the limitations of links are eliminated in symbolic links.
Commands
for linking files are:
Link ln filename1 filename2
Symbolic link ln -s filename1 filename2
7.
What
is a FIFO?
Answer:
FIFO are otherwise called as 'named pipes'. FIFO
(first-in-first-out) is a special file which is said to be data transient. Once
data is read from named pipe, it cannot be read again. Also, data can be read
only in the order written. It is used in interprocess
communication where a process writes to one end of the pipe (producer) and the
other reads from the other end (consumer).
8.
How
do you create special files like named pipes and device files?
Answer:
The system
call mknod creates special files in the following
sequence.
1.
kernel
assigns new inode,
2.
sets
the file type to indicate that the file is a pipe,
directory or special file,
3.
If
it is a device file, it makes the other entries like major, minor device
numbers.
For
example:
If the device is a disk, major device number refers to the disk controller and
minor device number is the disk.
9.
Discuss
the mount and unmount system calls
Answer:
The
privileged mount system call is used to attach a file system to a directory of
another file system; the unmount system call detaches
a file system. When you mount another file system on to your directory, you are
essentially splicing one directory tree onto a branch in another directory
tree. The first argument to mount call is the mount point, that is , a directory in the current file naming system. The
second argument is the file system to mount to that point. When you insert a cdrom to your unix
system's drive, the file system in the cdrom
automatically mounts to /dev/cdrom in your system.
10.
How
does the inode map to data block of a file?
Answer:
Inode has
13 block addresses. The first 10 are direct block addresses of the first 10
data blocks in the file. The 11th address points to a one-level index block.
The 12th address points to a two-level (double in-direction) index block. The
13th address points to a three-level(triple
in-direction)index block. This provides a very large maximum file size with
efficient access to large files, but also small files are accessed directly in
one disk read.
11.
What
is a shell?
Answer:
A shell is an
interactive user interface to an operating system services
that allows an user to enter commands as character strings or through a
graphical user interface. The shell converts them to system calls to the OS or
forks off a process to execute the command. System call results and other
information from the OS are presented to the user through an interactive
interface. Commonly used shells are sh,csh,ks
etc.
12.
Brief
about the initial process sequence while the system boots up.
Answer:
While
booting, special process called the 'swapper' or 'scheduler' is created with
Process-ID 0. The swapper manages memory allocation for processes and
influences CPU allocation.
The
swapper inturn creates 3 children:
the process dispatcher,
vhand and
dbflush
with IDs 1,2 and 3 respectively.
This
is done by executing the file /etc/init. Process dispatcher gives birth to the
shell. Unix keeps track of all the processes in an
internal data structure called the Process Table (listing command is ps -el).
13.
What
are various IDs associated with a process?
Answer:
Unix
identifies each process with a unique integer called ProcessID.
The process that executes the request for creation of a process is called the
'parent process' whose PID is 'Parent Process ID'.
Every process is associated with a particular user called the 'owner' who has
privileges over the process. The identification for the user is 'UserID'. Owner is the user who executes the process.
Process also has 'Effective User ID' which determines the access privileges for
accessing resources like files.
getpid()
-process id
getppid() -parent process id
getuid() -user id
geteuid() -effective user id
14.
Explain
fork() system call.
Answer:
The 'fork()' used to create a new process from an existing
process. The new process is called the child process, and the existing process
is called the parent. We can tell which is which by checking the return value
from 'fork()'. The parent gets the child's pid returned to him, but the child
gets 0 returned to him.
15.
Predict
the output of the following program code
16. main()17. {18. fork();
19. printf("Hello World!");
}
Answer:
Hello World!Hello World!
Explanation:
The fork
creates a child that is a duplicate of the parent process. The child begins
from the fork().All the statements after the call to
fork() will be executed twice.(once by the parent process and other by child).
The statement before fork() is executed only by the
parent process.
20.
Predict
the output of the following program code
21. main()22. {23. fork(); fork(); fork();24. printf("Hello World!");
}
Answer:
"Hello
World" will be printed 8 times.
Explanation:
2^n times
where n is the number of calls to fork()
17.
What
are the process states in Unix?
Answer:
As a process
executes it changes state according to its circumstances. Unix processes have
the following states:
Running : The process is either running or it is ready
to run .
Waiting : The process is waiting for an event or for a
resource.
Stopped : The process has been stopped, usually by
receiving a signal.
Zombie : The process is dead but have not been removed
from the process table.
18.
What
Happens when you execute a program?
Answer:
When you
execute a program on your UNIX system, the system creates a special environment
for that program. This environment contains everything needed for the system to
run the program as if no other program were running on the system. Each process
has process context, which is everything that is unique about the state of the
program you are currently running. Every time you execute a program the UNIX
system does a fork, which performs a series of operations to create a process
context and then execute your program in that context. The steps include the
following: Allocate a slot in the process table, a list of currently running
programs kept by UNIX. Assign a unique process identifier (PID) to the process.
iCopy the context of the
parent, the process that requested the spawning of the new process. Return the
new PID to the parent process. This enables the parent process to examine or
control the process directly.
After the fork is complete, UNIX runs your program.
19.
What
Happens when you execute a command?
Answer:
When you
enter 'ls' command to look at the contents of your
current working directory, UNIX does a series of things to create an
environment for ls and the run it: The shell has UNIX
perform a fork. This creates a new process that the shell will use to run the ls program. The shell has UNIX perform an exec of the ls program. This replaces the shell program and data with
the program and data for ls and then starts running
that new program. The ls program is loaded into the
new process context, replacing the text and data of the shell. The ls program performs its task, listing the contents of the
current directory.
24.
What
is a Daemon?
Answer:
A daemon is a
process that detaches itself from the terminal and runs, disconnected, in the
background, waiting for requests and responding to them. It can also be defined
as the background process that does not belong to a terminal session. Many
system functions are commonly performed by daemons, including the sendmail daemon, which handles mail, and the NNTP daemon,
which handles USENET news. Many other daemons may exist. Some of the most
common daemons are: init: Takes over the basic running of the system when the
kernel has finished the boot process. inetd:
Responsible for starting network services that do not have their own
stand-alone daemons. For example, inetd usually takes
care of incoming rlogin, telnet, and ftp connections. cron: Responsible for running repetitive tasks on a
regular schedule.
25.
What
is 'ps' command for?
Answer:
The ps command prints the process status for some or all of the
running processes. The information given are the process identification number
(PID),the amount of time that the process has taken to
execute so far etc.
26.
How
would you kill a process?
Answer:
The kill
command takes the PID as one argument; this identifies which process to
terminate. The PID of a process can be got using 'ps'
command.
27.
What
is an advantage of executing a process in background?
Answer:
The most
common reason to put a process in the background is to allow you to do
something else interactively without waiting for the process to complete. At
the end of the command you add the special background symbol, &. This
symbol tells your shell to execute the given command in the background.
Example:
cp *.* ../backup& (cp is for copy)
28.
How
do you execute one program from within another?
Answer:
The system
calls used for low-level process creation are execlp() and execvp(). The execlp call
overlays the existing program with the new one , runs
that and exits. The original program gets back control only when an error
occurs.
execlp(path,file_name,arguments..); //last argument must be NULL
A variant of execlp called execvp
is used when the number of arguments is not known in advance. execvp(path,argument_array); //argument array should be terminated
by NULL
29.
What
is IPC? What are the various schemes available?
Answer:
The term IPC
(Inter-Process Communication) describes various ways by which different process
running on some operating system communicate between each other. Various
schemes available are as follows:
Pipes:
One-way communication scheme through which different process can communicate.
The problem is that the two processes should have a common ancestor
(parent-child relationship). However this problem was fixed with the
introduction of named-pipes (FIFO).
Message
Queues :
Message queues can be used between related and unrelated processes running on a
machine.
Shared
Memory:
This is the fastest of all IPC schemes. The memory to be shared is mapped into
the address space of the processes (that are sharing). The speed achieved is
attributed to the fact that there is no kernel involvement. But this scheme
needs synchronization.
Various
forms of synchronisation are mutexes,
condition-variables, read-write locks, record-locks, and semaphores.
30.
What
is the difference between Swapping and Paging?
Answer:
Swapping:
Whole process is moved from the swap device to the main memory for execution.
Process size must be less than or equal to the available main memory. It is
easier to implementation and overhead to the system. Swapping systems does not
handle the memory more flexibly as compared to the paging systems.
Paging:
Only the required memory pages are moved to main memory from the swap device
for execution. Process size does not matter. Gives the concept
of the virtual memory. It provides greater flexibility in mapping the
virtual address space into the physical memory of the machine. Allows more number of processes to fit in the main memory
simultaneously. Allows the greater process size than
the available physical memory. Demand paging systems handle the memory
more flexibly.
31.
What
is major difference between the Historic Unix and the new BSD release of Unix
System V in terms of Memory Management?
Answer:
Historic Unix
uses Swapping – entire
process is transferred to the main memory from the swap device, whereas the
Unix System V uses Demand Paging – only the part of
the process is moved to the main memory. Historic Unix
uses one Swap Device and Unix System V allow multiple Swap Devices.
32.
What
is the main goal of the Memory Management?
Answer:
It decides
which process should reside in the main memory, Manages the parts of the
virtual address space of a process which is non-core resident, Monitors the available
main memory and periodically write the processes into the swap device to
provide more processes fit in the main memory simultaneously.
33.
What
is a Map?
Answer:
A Map is an
Array, which contains the addresses of the free space in the swap device that
are allocatable resources, and the number of the
resource units available there. This allows First-Fit allocation of contiguous
blocks of a resource. Initially the Map contains one entry – address (block offset from the starting of the swap area)
and the total number of resources.
Kernel
treats each unit of Map as a group of disk blocks. On the allocation and
freeing of the resources Kernel updates the Map for accurate information.
34.
What
scheme does the Kernel in Unix System V follow while choosing a swap device
among the multiple swap devices?
Answer:
Kernel
follows Round Robin scheme choosing a swap device among the multiple swap
devices in Unix System V.
35.
What
is a Region?
Answer:
A Region is a
continuous area of a process's address space (such as text, data and stack).
The kernel in a 'Region Table' that is local to the process maintains region.
Regions are sharable among the process.
36.
What
are the events done by the Kernel after a process is being swapped out from the
main memory?
Answer:
When Kernel
swaps the process out of the primary memory, it performs the following:
o
Kernel
decrements the Reference Count of each region of the process. If the reference
count becomes zero, swaps the region out of the main memory,
o
Kernel
allocates the space for the swapping process in the swap device,
o
Kernel
locks the other swapping process while the current swapping operation is going
on,
o
The
Kernel saves the swap address of the region in the region table.
37.
Is
the Process before and after the swap are the same? Give reason.
Answer:
Process
before swapping is residing in the primary memory in its original form. The
regions (text, data and stack) may not be occupied fully by the process, there
may be few empty slots in any of the regions and while swapping Kernel do not
bother about the empty slots while swapping the process out.
After
swapping the process resides in the swap (secondary memory) device. The regions
swapped out will be present but only the occupied region slots but not the
empty slots that were present before assigning.
While
swapping the process once again into the main memory, the Kernel referring to the Process Memory
Map, it assigns the main memory accordingly taking care of the empty slots in
the regions.
38.
What
do you mean by u-area (user area) or u-block?
Answer:
This contains
the private data that is manipulated only by the Kernel. This is local to the
Process, i.e. each process is allocated a u-area.
39.
What
are the entities that are swapped out of the main memory while swapping the
process out of the main memory?
Answer:
All memory
space occupied by the process, process's u-area, and Kernel stack are swapped out,
theoretically.
Practically,
if the process's u-area contains the Address Translation Tables for the process
then Kernel implementations do not swap the u-area.
40.
What
is Fork swap?
Answer:
fork() is
a system call to create a child process. When the parent process calls fork()
system call, the child process is created and if there is short of memory then
the child process is sent to the read-to-run state in the swap device, and
return to the user state without swapping the parent process. When the memory
will be available the child process will be swapped into the main memory.
41.
What
is Expansion swap?
Answer:
At the time
when any process requires more memory than it is currently allocated, the
Kernel performs Expansion swap. To do this Kernel reserves enough space in the
swap device. Then the address translation mapping is adjusted for the new
virtual address space but the physical memory is not allocated. At last Kernel
swaps the process into the assigned space in the swap device. Later when the
Kernel swaps the process into the main memory this assigns memory according to
the new address translation mapping.
42.
How
the Swapper works?
Answer:
The swapper
is the only process that swaps the processes. The Swapper operates only in the
Kernel mode and it does not uses System calls instead it uses internal Kernel
functions for swapping. It is the archetype of all kernel process.
43.
What
are the processes that are not bothered by the swapper? Give Reason.
Answer:
Zombie
process: They do not take any up physical memory.
Processes locked in memories that are updating the region of the process.
Kernel swaps only the sleeping processes rather than the 'ready-to-run'
processes, as they have the higher probability of being scheduled than the
Sleeping processes.
44.
What
are the requirements for a swapper to work?
Answer:
The swapper
works on the highest scheduling priority. Firstly it will look for any sleeping
process, if not found then it will look for the ready-to-run process for
swapping. But the major requirement for the swapper to work the ready-to-run
process must be core-resident for at least 2 seconds before swapping out. And
for swapping in the process must have been resided in the swap device for at
least 2 seconds. If the requirement is not satisfied then the swapper will go
into the wait state on that event and it is awaken once in a second by the
Kernel.
45.
What
are the criteria for choosing a process for swapping into memory from the swap
device?
Answer:
The resident
time of the processes in the swap device, the priority of the processes and the
amount of time the processes had been swapped out.
46.
What
are the criteria for choosing a process for swapping out of the memory to the
swap device?
Answer:
The process's
memory resident time,
Priority of the process and
The nice value.
47.
What
do you mean by nice value?
Answer:
Nice value is
the value that controls {increments or decrements} the priority of the process.
This value that is returned by the nice () system call.
The equation for using nice value is:
Priority = ("recent CPU usage"/constant) + (base- priority) + (nice
value)
Only the administrator can supply the nice value. The
nice () system call works for the running process only. Nice value of one
process cannot affect the nice value of the other process.
48.
What
are conditions on which deadlock can occur while swapping the processes?
Answer:
All processes
in the main memory are asleep.
All 'ready-to-run' processes are swapped out.
There is no space in the swap device for the new incoming process that are swapped out of the main memory.
There is no space in the main memory for the new incoming process.
49.
What
are conditions for a machine to support Demand Paging?
Answer:
Memory
architecture must based on Pages,
The machine must support the 'restartable'
instructions.
50.
What
is 'the principle of locality'?
Answer:
It's the
nature of the processes that they refer only to the small subset of the total
data space of the process. i.e. the process frequently
calls the same subroutines or executes the loop instructions.
51.
What
is the working set of a process?
Answer:
The set of
pages that are referred by the process in the last 'n', references, where 'n'
is called the window of the working set of the process.
52.
What
is the window of the working set of a process?
Answer:
The window of
the working set of a process is the total number in which the process had
referred the set of pages in the working set of the process.
53.
What
is called a page fault?
Answer:
Page fault is
referred to the situation when the process addresses a page in the working set
of the process but the process fails to locate the page in the working set. And
on a page fault the kernel updates the working set by reading the page from the
secondary device.
54.
What
are data structures that are used for Demand Paging?
Answer:
Kernel
contains 4 data structures for Demand paging. They are,
o
Page
table entries,
o
Disk
block descriptors,
o
Page
frame data table (pfdata),
o
Swap-use
table.
55.
What
are the bits that support the demand paging?
Answer:
Valid,
Reference, Modify, Copy on write, Age. These bits are the part of the page
table entry, which includes physical address of the page and protection bits.
|
Page address |
Age |
Copy on write |
Modify |
Reference |
Valid |
Protection |
56.
How
the Kernel handles the fork() system call in
traditional Unix and in the System V Unix, while swapping?
Answer:
Kernel in
traditional Unix, makes the duplicate copy of the
parent's address space and attaches it to the child's process, while swapping.
Kernel in System V Unix, manipulates the region tables, page table, and pfdata table entries, by incrementing the reference count
of the region table of shared regions.
57.
Difference
between the fork() and vfork()
system call?
Answer:
During the fork() system call the Kernel makes a copy of the parent
process's address space and attaches it to the child process.
But the vfork() system call do not makes any copy of the parent's address
space, so it is faster than the fork() system call. The child process as a
result of the vfork() system call executes exec() system call. The child
process from vfork() system call executes in the parent's address space (this
can overwrite the parent's data and stack ) which suspends the parent process
until the child process exits.
58.
What
is BSS(Block Started by Symbol)?
Answer:
A data
representation at the machine level, that has initial values when a program
starts and tells about how much space the kernel allocates for the
un-initialized data. Kernel initializes it to zero at run-time.
59.
What
is Page-Stealer process?
Answer:
This is the
Kernel process that makes rooms for the incoming pages, by swapping the memory
pages that are not the part of the working set of a process. Page-Stealer is
created by the Kernel at the system initialization and invokes it throughout
the lifetime of the system. Kernel locks a region when a process faults on a
page in the region, so that page stealer cannot steal the page, which is being
faulted in.
60.
Name
two paging states for a page in memory?
Answer:
The two
paging states are:
The page is aging and is not yet eligible for swapping,
The page is eligible for swapping but not yet eligible for reassignment to
other virtual address space.
61.
What
are the phases of swapping a page from the memory?
Answer:
Page stealer
finds the page eligible for swapping and places the page number in the list of
pages to be swapped.
Kernel
copies the page to a swap device when necessary and clears the valid bit in the
page table entry, decrements the pfdata reference
count, and places the pfdata table entry at the end
of the free list if its reference count is 0.
62.
What
is page fault? Its types?
Answer:
Page fault
refers to the situation of not having a page in the main memory when any
process references it.
There
are two types of page fault :
Validity fault,
Protection fault.
63.
In
what way the Fault Handlers and the Interrupt handlers are different?
Answer:
Fault handlers
are also an interrupt handler with an exception that the interrupt handlers
cannot sleep. Fault handlers sleep in the context of the process that caused
the memory fault. The fault refers to the running process and no arbitrary
processes are put to sleep.
64.
What
is validity fault?
Answer:
If a process
referring a page in the main memory whose valid bit is not set, it results in
validity fault.
The
valid bit is not set for those pages: that are outside the virtual address
space of a process, that are the part of the virtual address space of the
process but no physical address is assigned to it.
65.
What
does the swapping system do if it identifies the illegal page for swapping?
Answer:
If the disk
block descriptor does not contain any record of the faulted page, then this
causes the attempted memory reference is invalid and the kernel sends a
"Segmentation violation" signal to the offending process. This
happens when the swapping system identifies any invalid memory reference.
66.
What
are states that the page can be in, after causing a page fault?
Answer:
On a swap
device and not in memory,
On the free page list in the main memory,
In an executable file,
Marked "demand zero",
Marked "demand fill".
67.
In
what way the validity fault handler concludes?
Answer:
It sets the
valid bit of the page by clearing the modify bit.
It recalculates the process priority.
68.
At
what mode the fault handler executes?
Answer:
At the Kernel Mode.
69.
What
do you mean by the protection fault?
Answer:
Protection
fault refers to the process accessing the pages, which do not have the access
permission. A process also incur the protection fault when it attempts to write
a page whose copy on write bit was set during the fork()
system call.
70.
How
the Kernel handles the copy on write bit of a page, when the bit is set?
Answer:
In situations
like, where the copy on write bit of a page is set and that page is shared by
more than one process, the Kernel allocates new page and copies the content to
the new page and the other processes retain their references to the old page.
After copying the Kernel updates the page table entry with the new page number.
Then Kernel decrements the reference count of the old pfdata
table entry.
In
cases like, where the copy on write bit is set and no processes are sharing the
page, the Kernel allows the physical page to be reused by the processes. By
doing so, it clears the copy on write bit and disassociates the page from its
disk copy (if one exists), because other process may share the disk copy. Then
it removes the pfdata table entry from the page-queue
as the new copy of the virtual page is not on the swap device. It decrements
the swap-use count for the page and if count drops to 0, frees the swap space.
71.
For
which kind of fault the page is checked first?
Answer:
The page is
first checked for the validity fault, as soon as it is found that the page is
invalid (valid bit is clear), the validity fault handler returns immediately,
and the process incur the validity page fault. Kernel handles the validity fault
and the process will incur the protection fault if any one is present.
72.
In
what way the protection fault handler concludes?
Answer:
After
finishing the execution of the fault handler, it sets the
modify and protection bits and clears the copy on write bit. It
recalculates the process-priority and checks for signals.
73.
How
the Kernel handles both the page stealer and the fault handler?
Answer:
The page
stealer and the fault handler thrash because of the shortage of the memory. If
the sum of the working sets of all processes is greater that the physical
memory then the fault handler will usually sleep because it cannot allocate
pages for a process. This results in the reduction of the system throughput
because Kernel spends too much time in overhead, rearranging the memory in the
frantic pace.