1)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
2)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.
3) Predict the output of the following program code.
main()
{
fork();
printf("Hello World!");
}
Hello World!Hello World!
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.
4)Predict the output of the following program code
main()
{
fork(); fork();
fork();
printf("Hello World!");
}
"Hello World" will be printed 8 times.
2^n times where n is the number of calls to fork();
5)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
6)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()"
7)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.
8)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.)
9)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.
10)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.
11)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.
12)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.
13)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
14)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.
15)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)
16)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
17)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).
18)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
19)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).
20)How do you change File Access Permissions?
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)
To change the access mode, we use chmod(filename,mode).
21)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 imbolic links.
Commands for linking files are:
Link "ln filename1 filename2"
Symbolic link "ln -s filename1 filename2"
22)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).
23)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.
24)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.
25)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.