How
are devices represented in UNIX?
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).
What
is 'inode'?
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
Brief
about the directory representation in UNIX
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).
What
are the Unix system calls for I/O?
·
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.
How
do you change File Access Permissions?
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).
What
are links and symbolic links in UNIX file system?
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
What
is a FIFO?
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).
How
do you create special files like named pipes and device files?
The system call mknod
creates special files in the following sequence.
kernel
assigns new inode,
sets
the file type to indicate that the file is a pipe, directory or special file,
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.
Discuss
the mount and unmount system calls
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.
How
does the inode map to data block of a file?
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.
What
is a shell?
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.
SECTION
- II
PROCESS MODEL and IPC
1. Brief about the initial process sequence
while the system boots up.
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).
2. What are various IDs associated with a
process?
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
3. Explain fork() system call.
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.
4. Predict the output of the following program
code
main()
{
fork();
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.
5. Predict the output of the following program code
main()
{
fork(); fork(); fork();
printf("Hello World!");
}
Answer:
"Hello World" will be printed 8 times.
Explanation:
2^n
times where n is the number of calls to fork()
6. List the system calls used for process
management:
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
7. How can you get/set an environment variable
from a program?
Getting the value of an environment variable is done by using
`getenv()'.
Setting the value of an environment variable is done by using
`putenv()'.
8. How can a parent and child process
communicate?
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.
9. What is a zombie?
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.)
10. What are the process states in Unix?
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.
11. What Happens when you execute a program?
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.
12. What Happens when you execute a command?
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.
13. What is a Daemon?
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.
14. What is 'ps' command for?
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.
15. How would you kill a process?
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.
16. What is an advantage of executing a process in
background?
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)
17. How do you execute one program from within
another?
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
18. What is IPC? What are the various schemes
available?
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.
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