Often it is not enough to just display the data on the screen.This is because if the data is large, only a limited amount of it can be stored in memory and only a limited amount of it can be displayed on the screen.It would be in appropriate to store this data in the memory for one more reason.Memory is volatile(RAM) an its contents are lost once the program me is terminate. so if we need the same data again it would have t either entered through the key-board again or would have to regenerated pragmatically.obviously both these operations would be tedious.At such times it become necessary to store the data in the manner that can be later retrieved and displayed either in part or in whole.The medium is usually "file" or "disk".Basic Introduction:-
We
frequently use files for storing information which can be processed by our
programs. In order to store information permanently and retrieve it we need to use files.
Files are
not only used for data. Our programs are also stored in files.
The editor
which you use to enter your program and save it, simply manipulates files for
you.
The Unix
commands cat, cp, cmp are all programs which process your files.
In order to
use files we have to learn about File I/O i.e. how to write information
to a file and how to read information from a file.
We will see
that file I/O is almost identical to the terminal I/O that we have being using
so far.
The primary
difference between manipulating files and doing terminal I/O is that we must
specify in our programs which files we wish to use.
As you know,
you can have many files on your disk. If you wish to use a file in your
programs, then you must specify which file or files you wish to use.
Specifying
the file you wish to use is referred to as opening the file.
When you
open a file you must also specify what you wish to do with it i.e. Read from the file, Write to the file, or both.
Because you
may use a number of different files in your program, you must specify when
reading or writing which file you wish to use. This is accomplished by using a
variable called a file pointer.
Every file
you open has its own file pointer variable. When you wish to write to a file
you specify the file by using its file pointer variable.
You declare
these file pointer variables as follows:
FILE
*fopen(), *fp1, *fp2, *fp3;
The
variables fp1, fp2, fp3 are file pointers. You may use any name you wish.
The file
<stdio.h> contains declarations for the Standard I/O library and should
always be included at the very
beginning of C programs using files.
Constants
such as FILE, EOF and NULL are defined in <stdio.h>.
You should
note that a file pointer is simply a variable like an integer or character.
It does not point
to a file or the data in a file. It is simply used to indicate which file your
I/O operation refers to.
A file
number is used in the Basic language and a unit number is used in Fortran for
the same purpose.
fp = fopen( “prog.c”, “r”) ;
The above
statement opens a file called prog.c
for reading and associates the file
pointer fp with the file.
When we wish
to access this file for I/O, we use the file pointer variable fp to refer to
it.
You can have
up to about 20 files open in your program - you need one file pointer for each
file you intend to use.
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