UNIX File
Management
1. 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).
2. 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
3. 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).
4. 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.
5. 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)
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).
(user permission) - (group permission) - (others permission)
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?
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"
Link "ln filename1 filename2"
Symbolic link "ln -s filename1 filename2"
7. 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).
8. 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.
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.
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?
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?
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.
UNIX Memory
Management
1. What is the difference
between Swapping and Paging?
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.
2. What is major
difference between the Historic Unix and the new BSD release of Unix System V
in terms of Memory Management?
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.
3. What is the main
goal of the Memory Management?
- 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.
4. What is a Map?
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.
Address Units
1 10,000
1 10,000
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.
5. What scheme does
the Kernel in Unix System V follow while choosing a swap device among the
multiple swap devices?
Kernel follows Round Robin scheme choosing a swap device among the
multiple swap devices in Unix System V.
6. What is a
Region?
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.
7. What are the
events done by the Kernel after a process is being swapped out from the main
memory?
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.
8. Is the Process
before and after the swap are the same? Give reason.
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.
9. What do you mean
by u-area (user area) or u-block?
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.
10. What are the
entities that are swapped out of the main memory while swapping the process out
of the main memory?
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.
11. What is Fork
swap?
"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.
12. What is
Expansion swap?
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
13. How the Swapper
works?
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.
14. What are the
processes that are not bothered by the swapper? Give Reason.
- 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.
15. What are the
requirements for a swapper to work?
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.
16. What are the
criteria for choosing a process for swapping into memory from the swap device?
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.
17. What are the
criteria for choosing a process for swapping out of the memory to the swap
device?
- The
process's memory resident time,
- Priority
of the process and
- The
nice value.
18. What do you
mean by nice value?
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
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
19. What are
conditions on which deadlock can occur while swapping the processes?
- 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.
20. What are
conditions for a machine to support Demand Paging?
- Memory
architecture must based on Pages,
- The machine
must support the 'restartable' instructions.
21. What is
"the principle of locality"?
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.
22. What is the
working set of a process?
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.
23. What is the
window of the working set of a process?
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.
24. What is called
a page fault?
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.
25. What are data
structures that are used for Demand Paging?
Kernel contains 4 data structures for Demand paging. They are,
- Page
table entries,
- Disk
block descriptors,
- Page
frame data table (pfdata),
- Swap-use
table.
26. What are the
bits that support the demand paging?
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.
27. How the Kernel
handles the fork() system call in traditional Unix and in the System V Unix,
while swapping?
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.
28. Difference
between the fork() and vfork() system call?
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.
29. What is
BSS(Block Started by Symbol)?
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.
30. What is
Page-Stealer process?
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
31. Name two paging
states for a page in memory?
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.
32. What are the
phases of swapping a page from the memory?
- 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.
33. What is page
fault? Its types?
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.
34. In what way the
Fault Handlers and the Interrupt handlers are different?
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.
35. What is
validity fault?
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.
36. What does the
swapping system do if it identifies the illegal page for swapping?
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
37. What are states
that the page can be in, after causing a page fault?
- 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"
38. In what way the
validity fault handler concludes?
- It
sets the valid bit of the page by clearing the modify bit.
- It
recalculates the process priority.
39. At what mode
the fault handler executes?
At the Kernel Mode
40. What do you
mean by the protection fault?
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.
41. How the Kernel
handles the copy on write bit of a page, when the bit is set?
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.
42. For which kind
of fault the page is checked first?
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.
43.
In what way the protection fault handler concludes?
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.
44.
How the Kernel handles both the page stealer and the fault handler?
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.
UNIX
Process Management
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!");
}
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.
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!");
}
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();
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
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()"
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.
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:
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:
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)
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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