file organization sheet - wordpress.com · file organization sheet 1. what is a file? ... briefly...
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1 | P a g e w i t h o u r b e s t w i s h e s D r . A y m a n E l s h e n a w y
File Organization Sheet
1. What is a File?
A collection of data is placed under permanent or non-volatile storage
Examples: anything that you can store in a disk, hard drive, tape, optical
media, and any other medium which doesn’t lose the information when the
power is turned off.
2. Compare between Logical and physical files?
Physical file: physically exists on secondary storage; known by the
operating system; appears in its file directory
Logical file, what your program actually uses, a ‘pipe’ though which
information can be extracted, or sent.
Type Physical File Logical File
collection of
bytes stored on a disk or tape
A “Channel” (like a telephone line) that hides the details of the file’s
location and physical format to the program
3. What is a Basic File System?
A Basic File System (BFS) is the only software that interfaces directly with the
peripheral devices attached to the system. The function of a basic file system
is sometimes referred to as physical I/O. This is because it deals with the
physical reading and writing of blocks of data on the peripheral devices.
However, the BFS does not understand the content and meaning of the data
being transferred.
4. Why Users and Computers Have a need for Basic File System?
Users and computer systems need a BFS for the following reasons:
To provide a common interface to perform block I/O.
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To provide a centralized control program to ensure that the devices
connected to the system are being allocated and scheduled properly.
To provide a central resource to ensure data integrity by providing error
recovery and exception handling for all users.
5. What are the Sources of File System inputs?
The data the BFS needs to perform its work must come from one of the
following sources:
• User Programs
• Command Language Files
• Job Control Languages (JCL), utilized at the user’s site a stand-alone utility
program that interactively request the data from the user. Such a program
can prompt the user for each of the required parameters, check inputs for
validity, and then store the information within the file itself.
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6. What are the basic Functions of the Basic File System?
• The allocation of all space on the devices
• The centralization of all I/O through the BFS
• The control of the devices themselves, which usually requires a rather
detailed knowledge of device-specific details.
• The processing of all the I/O terminations to ensure that the correct
processing is performed. Also, it must then initiate the next I/O request
targeted for that device.
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• Finally, upon completion of the user’s I/O request, the BFS must notify the
user as to whether the I/O request completed correctly. This is done by
returning a status field, which indicates how the I/O request terminated.
7. Briefly describe the fundamental file processing operations (write code
examples)?
Opening Files
code in C and C++ -> fd = open(filename, flags [, pmode]);
That makes the file ready to use by the program.
Closing Files
Files are usually closed automatically by the operating system (unless the
program is abnormally interrupted).
it Makes the logical file name available for another physical file.
Reading
Code -> Read(Source_file, Destination_addr, Size);
it reads data from the file which we define in “source_file” parameter which
located in the “Destination_addr” and we also specify the “Size”.
Write
Code-> Write(Destination_file, Source_addr, Size);
this function writes into the file which we specify it at the parameters.
Seeking
Code-> Seek(Source_file, Offset);
it’s The action of moving directly to a certain position in a file.
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Create file operation
Required Data
The following data are needed to create a file:
Device/user name: tells where to place the file when it is
created.
Filename identifies the file among all the other files in the
computer system.
File size tell how much room to allocate when it is creating the
file
File organization needed so that the file system, or more
specifically the access methods, can know how to insert and
later retrieve the blocks within the file
Block size amount of data that will be read or written in every
I/O operation.
Tasks performed by the file system to create a file?
Validate the User’s Input Data
Check to see if the file already exists
Allocate space on the medium for the file:
The BFS can tell if enough space exists by comparing the
projected file size, from the vantage point of the user, against
the device allocation table on the target device. If there is not
enough space on the medium, the BFS has no choice but to
abort its processing.
Insert a pointer to the first block allocated to the file
Allocate and initialize I/O buffers and control blocks
First, it must allocate space in memory large enough of READ
in the largest block in the file. The blocks of memory, which
are dedicated to the transferring of the data from memory to
and from the peripheral devices, are called I/O buffers.
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Next, the BFS must allocate and build a table, also known as
a file control block, in which the critical information about
the file will be saved.
Return success or fail status back to the user.
Errors:
Another file with the same name already exists.
The device has no available space to create another new file.
The device is, or has become, nonoperational.
There was a hardware problem which aborted the processing
One or more of the input parameters given to the BFS were
wrong.
Open File Operation
Required Data
Device name
User name, or account identification
Filename
The tasks for processing an OPEN file command are:
Allocate space for Internal Control Blocks (ICB)
Get the correct volumes mounted on the devices
Locate the file on the devices
Allocate internal memory space for I/O buffers
Protect against unauthorized users
Maintain a list of the entire open file in the system.
Errors:
File not found on the specified device and in the specified user
account or directory. This could be because of an incorrectly
specified filename or device identifier.
File already in use by some other user application.
Hardware error has occurred from which the BFS could not
recover.
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Extend File Operation
The purpose of the EXTEND file function is to allow the user(s) to explicitly
increase the size of a file.
Required Data
Device and directory name where the file is located
Name of the file
Amount by which the file should be increased in size.
The Tasks must be done as follows:
Validate the parameters in this request
Check to see if there is enough room on this device to increase
the size of the file by the requested amount.
If space exists, take the space from the medium’s general space
pool, and mark it in such a way that no other user can take this
space.
Add the space to the file in question, and to the internal
information kept on the file, so that can be correctly processed
at a later date.
Return a success or failure status code to the user.
Errors:
Device has no space available in which to extend the file
File was not previously Opened, indicating that the user has
inadvertently gotten the sequence of I/O processing out of
order.
Close File Operation
The purpose of a CLOSE file function is to allow the user to decrease, stop using
the file,
Required Data
The name of the file to be closed.
An indication of which currently open file is to be closed.
The Tasks must be done as follows:
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Write out any I/O buffers that have been modified and are still
in main memory but which have not yet been written out to the
specified device
Write out any updated information about the file that would be
useful or required to know in the future in order to process the
file correctly.
Release any internal memory space dedicated to the processing
of that particular file.
Return status to the user, indicating success or failure.
Errors:
The file to be closed was not currently open
Any of the errors that can occur on a WRITE operation could
occur, since we are writing blocks back into the file
A hardware problem could occur that could abort the CLOSE
processing.
Delete File Operation
The function of the DELETE file request is to erase or get rid of a specific file from
the system. This occurs when the user no longer needs a particular file and wishes
to reuse the file space from some other purpose.
Required Data
Device and user account name
File name
The Tasks must be done as follows:
Validate the user’s input parameters
Return all the space on the media taken up by the file to the
space pool on the media
Remove the file name from the directory where it resides.
Return status to the user.
Errors:
File could not be found, either because of an invalid file or user
account name.
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Other users were currently active on the file
A hardware problem was detected.
Block Operation
A block of data is the unit of data that is transferred to, or from, a device.
The amount of data (i.e., size of the block) is definable by the user. The larger the
block, the more data that will be transferred in that I/O operation and the greater
the performance of the program.
This is true but must be taken from the perspective that the memory resources
required to hold these blocks of data is not unlimited and should be considered
to be a scarce commodity.
Write Operation
Provide users with the capability of transferring data out of the computer’s main
memory and onto one of the devices connected to the system.
Required Data
Number of the block to be written
Address of the I/O buffer in many memory that contains the
block to be written
Size or amount of the block to be written
Name of the status field into which the file system will insert
the termination status of the I/O request.
The Tasks must be done as follows:
The WRITE routine must perform the following tasks as part of
the job of processing the request:
Validate the input parameters
Convert the logical block number to a physical device address
Check if space exists in the file for this block
If space exists, execute the WRITE I/O operation to the device.
Reset the buffer-modified flag for that specific I/O buffer, since
that block has now been written out to the device.
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If space does not exist in the file, extend the file. This is called
an implicit extend to the
Errors:
No space available for the operation to complete
Hardware unrecoverable error has occurred
Mismatch from READ after WRITE validation sequence.
Read Operation
The function of the READ routine is to transfer a specific block of data from the
medium into main memory. The unit of data transfer is the block.
Required Data
Number of the block to be read into memory
Address of the I/O block buffer in main memory into which the
block will be read
Size or amount of data to be read into main memory
The address of a field into which the file system can store the
status of the I/O request
The Tasks must be done as follows:
Validate input parameters
Convert the logical block number to a physical block
Read the block into an I/O buffer in main memory
Return status to the user program
Errors:
Read error detected by the device
Invalid device block number requested
Hardware failure detected
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8. Write a code for opening a file and reading form all data from the file
and then print all file content on the screen, then close the file?
9. Write a code for calculating the size of a specific file?
10. How can the files are treated?
A file can be treated as
o a stream of bytes (as we have seen before)
o a collection of records with fields
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11. What is meant by field, record, key, give an example?
Field: a data value, smallest unit of data with logical meaning
Record: A group of fields that forms a logical unit
Key: a subset of the fields in a record used to uniquely identify the record
Primary Key: A key that uniquely identifies a record.
Secondary Key: Other keys that may be used for search
12. What are the common methods for organizing fields in files?