1. How are devices represented in UNIX?
Ans: 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’?
Ans: 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:
File owner identifier
File type
File access permissions
File access times
Number of links
File size
Location of the file data
3. Brief about the directory representation in UNIX?
Ans: 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?
Ans: open(pathname,flag,mode) – open file
creat(pathname,mode) – create file
close(filedes) – close an open file
read(filedes,buffer,bytes) – read data from an open file
write(filedes,buffer,bytes) – write data to an open file
lseek(filedes,offset,from) – position an open file
dup(filedes) – duplicate an existing file descriptor
dup2(oldfd,newfd) – duplicate to a desired file descriptor
fcntl(filedes,cmd,arg) – change properties of an open file
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?
Ans: Every file has following attributes:
owner’s user ID ( 16 bit integer )
owner’s group ID ( 16 bit integer )
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?
Ans: 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?
Ans: 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?
Ans: 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 ?
Ans : 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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
main()
{
fork();
printf(“Hello World!”);
}
Ans: 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.
16. Predict the output of the following program code
main()
{
fork(); fork(); fork();
printf(“Hello World!”);
}
Ans: “Hello World” will be printed 8 times.
Explanation:
2^n times where n is the number of calls to fork()
17. List the system calls used for process management:
Ans: System calls Description
fork() To create a new process
exec() To execute a new program in a process
wait() To wait until a created process completes its execution
exit() To exit from a process execution
getpid() To get a process identifier of the current process
getppid() To get parent process identifier
nice() To bias the existing priority of a process
brk() To increase/decrease the data segment size of a process
18. How can you get/set an environment variable from a program?
Ans: Getting the value of an environment variable is
done by using `getenv()’. Setting the value of an environment variable
is done by using `putenv()’.
19. How can a parent and child process communicate?
Ans: A parent and child can communicate through any of
the normal inter-process communication schemes (pipes, sockets, message
queues, shared memory), but also have some special ways to communicate
that take advantage of their relationship as a parent and child. One of
the most obvious is that the parent can get the exit status of the
child.
20. What is a zombie?
Ans: When a program forks and the child finishes before
the parent, the kernel still keeps some of its information about the
child in case the parent might need it – for example, the parent may
need to check the child’s exit status. To be able to get this
information, the parent calls `wait()’; In the interval between the
child terminating and the parent calling `wait()’, the child is said to
be a `zombie’ (If you do `ps’, the child will have a `Z’ in its status
field to indicate this.)
21. What are the process states in Unix?
Ans: 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.
22. What Happens when you execute a program?
Ans: 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.
23. What Happens when you execute a command?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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 synchronization are mutexes, condition-variables, read-write locks, record-locks, and semaphores.
30. What is the difference between Swapping and Paging?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: Kernel follows Round Robin scheme choosing a swap device among the multiple swap devices in Unix System V.
35. What is a Region?
Ans: 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?
Ans: When Kernel swaps the process out of the primary memory, it performs the following:
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,Kernel allocates the space for the swapping process in the swap
device,Kernel locks the other swapping process while the current
swapping operation is going on,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.
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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.
Ans: 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?
Ans: 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?
Ans: 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?
Ans: The process’s memory resident time,Priority of the process and the nice value.
47. What do you mean by nice value?
Ans: 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?
Ans: 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?
Ans: Memory architecture must based on Pages, The machine must support the ‘restartable’ instructions.
50. What is ‘the principle of locality’?
Ans: 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.