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File Handling
The console oriented I/O operations pose
two major problems:1. It becomes cumbersome and time
consuming to handle large volumes ofdata through terminals.
2. The entire data is lost when either theprogram is terminated or the computer isturned o.
o it is necessar! to have a more"e#ible approach where data can bestored on dis$s and read whenevernecessar!%without destro!ing the
data.This method emplo!es the concept
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File Operations
There are dierent operations that
can be carried out on a &le. Theseare: (reation of a new &le
Opening an e#isting &le
)eading from a &le
*riting to a &le
+oving to a speci&c location in a &le
,see$ing- (losing a &le
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Defning and Opening a File If we want to store data in the &le in the secondar!
memor!%we must specif! certain things about the&le%to the operating s!stem.The! include:
1. ilename
2. ata tructure
0. urpose ata structure of a &le is de&ned as FILE in
the librar! of standard I/O functionde&nitions.Therefore %all &les should be
declared as t!pe FILE before the! areused.FILE is a de&ned data t!pe.
FILE *p;
p=open(“flename”,”mode”);
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+ode can be one of the following:
“r” Open an e#isting &le for reading onl!.
“!” Open a new &le for writing onl!. If a &le with the
speci&ed fle-name currently exists, it will be destro!edand a new &le created in its place.
“ a “ Open an e#isting &le for appending ,i.e.% for addingnew information at the end of the &le-. ' new &le
will be created if the &le with the speci&ed fle-name does
not exist.“r"“ Open an e#isting &le for both reading and writing.
“!" 3 Open a new &le for both reading and writing. If a &lewith the speci&ed fle-name currently exists, it will bedestro!ed and a new &le created in its place.
3 a" 3 Open an e#isting &le for both reading andappending. ' new &le will be created if the &le with thespeci&ed fle-name does not exist.
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4 include 5stdio.h5
main, -
6
I78 9fp char ch
fp ; fopen , 5)1.(5% 5r5 -
while , 1 -
6ch ; fgetc , fp -
if , ch ;; 8O -
brea$
printf , 5
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open( ) perorms t#ree important tas$s !#en %o&open t#e fle in “r” mode'
irstl! it searches on the dis$ the &le to be opened.
Then it loads the &le from the dis$ into a place inmemor! called buer.
It sets up a character pointer that points to the &rstcharacter of the buer.
To read the &le>s contents from memor! there e#ists afunction called get( ) This has been used in ourprogram as%
# = get ( p ) ;
get( ) reads the character from the current pointer
position% advances the pointer position so that it nowpoints to the ne#t character% and returns the characterthat is read% which we collected in the variable ch. ?otethat once the &le has been opened% we no longer referto the &le b! its name% but through the &le pointer p
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• In place of the function get( ) we could have aswell used the maro get( ) with the same eect
• *e have used the function get( ) !it#in an
indefnite !#ile loop There has to be a wa! tobrea$ out of this while. *hen shall we brea$ out...the moment we reach the end of &le. &t !#atis end o fle+ speial #arater, !#ose
-.II /al&e is 01, signifes end o fle Thischaracter is inserted be!ond the last character inthe &le% when it is created.
• *hile reading from the &le% when get( )encounters this special character% instead ofreturning the character that it has read, itret&rns t#e maro EOF 2#e EOF maro #as3een defned in t#e fle “stdio#
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If the &le opening fails due to an! of the reasons % theopen( ) &ntion ret&rns a /al&e 45LL (defned in“stdio#” as 6defne 45LL 7-. @ere is how this can behandled in a program...
4include 5stdio.h5
main, -
6
I78 9fp
fp ; fopen , 5)1.(5% 5r5 -
if , fp ;; ?A77 -
6
puts , 5cannot open &le5 - e#it, -
=
=
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.losing t#e File*hen we have &nished reading from the &le% we
need to close it. This is done using the function
lose( ) through the statement,
fclose , fp -
Once we close the &le we can no longer read from itusing get( ) &nless !e reopen t#e fle ?ote
that to close the &le we don>t use the &lenamebut the &le pointer fp. On closing the &le thebuer associated with the &le is removed frommemor!.
*hen we close this &le using lose( )t#ree operations !o&ld 3e perormed'
The characters in the buer would be written tothe &le on the dis$.
't the end of &le a character with '(II value 2B
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E4D OF FILE
*hile reading data from a &le% if we want to read thecomplete &le i.e. the &le should be read till the end
of data in it% then an end of &le mar$er of the &leshould be chec$ed.
Two wa!s:
8EOF' This character is present in &le at the end. 8g:
while,a;getc ,fp-C;8O-
putc ,a%fp-
2. eo' feof,- is a macro which returns D if end of &leis not reached.If end of &le is reached%it returns non
Eero value.eg:if,feof ,fp--
printf ,3The end of &le has reachedF-
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9* .o&nt #ars, spaes, ta3s and ne!lines in afle *9
4 include 5stdio.h5
main, -
6
I78 9fp
char ch int nol ; D% not ; D% nob ; D% noc ; D
fp ; fopen , 5)1.(5% 5r5 -
while , 1 -
6ch ; fgetc , fp -
if , ch ;; 8O -
brea$
nocGG
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if , ch ;; H H -
nobGG
if , ch ;; HnH -
nolGG
if , ch ;; HtH -
notGG
=
fclose , fp -
printf , 5n?umber of characters ;
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I9O Operations on Files2#e get() and p&t() F&ntions
These are analogous to get#ar and p&t#ar
functions and handle one character at a time.
'ssume that a &le is opened with mode w and the &lepointer p. Then % the statement:
p&t(,p);
writes the character contained in the charactervariable c to the &le associated with FILE pointer fp.
imilarl! % get is used to read a character from a &lethat has been opened in read mode.
=get(p); The &le pointer moves b! one character position for
ever! operation of get or p&t.
The reading should be terminated when EOF isencounterd.
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if , ft ;; ?A77 -
6
puts , 5(annot open target &le5 -
fclose , fs - e#it, -
=
while , 1 -
6
ch ; fgetc , fs -
if , ch ;; 8O -
brea$
else
fputc , ch% ft - =
fclose , fs -
fclose , ft -
=
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2#e get! and p&t! F&ntions
The get! and p&t! are integer
oriented functions. The! are similarto get and p&t functions and areused to read and write integer
values. These functions would be useful when
we deal with integer data onl!.
p&t!(integer,p);get!(p);
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-tring (line) I9O in Files)eading or writing strings of characters from and to &les is as
eas! as reading and writing individual characters. @ere is aprogram that writes strings to a &le using the function p&ts( )
9* :eei/es strings rom $e%3oard and !rites t#em to fle*9
4include 5stdio.h5
main, -
6
I78 9fp
char sJKDL
if , fp ;; ?A77 -
6puts , 5(annot open &le5 -
e#it, -
=
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printf , 5n8nter a few lines of te#t:n5 -
while , strlen , gets , s - - M D -
6
fputs , s% fp -
fputs , 5n5% fp -
=
fclose , fp - =
The p&ts( - function then writes the contents ofthe arra! to the dis$. ince p&ts( ) does not
automaticall! add a newline character to the endof the string% we must do this e#plicitl! to ma$eit easier to read the string bac$ from the &le.
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9* :eads strings rom t#e fle and displa%s t#em on sreen*9
4include 5stdio.h5
main, -
6
I78 9fp
char sJKDL
fp ; fopen , 5O8+.TNT5% 5r5 -
if , fp ;; ?A77 -
6
puts , 5(annot open &le5 -
e#it, -
=while , fgets , s% P% fp - C; ?A77 -
printf , 5
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'nd here is the output...
hining and bright% the! are forever%
so true about diamonds%
more so of memories%
especiall! !ours C
The function gets( ) ta$es t#ree arg&ments
The frst is t#e address where the string isstored% and the seond is t#e maim&mlengt# o t#e string. This argumentprevents fgets, - from reading in too long a
string and over"owing the arra!. The t#irdarg&ment is t#e pointer to t#e str&t&reFILE. *hen all the lines from the &le havebeen read% we attempt to read one more line%in which case fgets, - returns a ?A77.
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2#e print and san F&ntionsprint( ) and san( ) li3rar% &ntions to read9!rite data rom9to
fle
This function is similar to print( ), e#cept that a FILE pointer is included asthe &rst argument. 's in print( ), we can format the data in a variet! ofwa!s% b! using print( ) In fact all the format conventions of print( -function wor$ with print( ) as well.
print(p,”ontrol strings”,list);
9*
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struct emp e
fp ; fopen , 58+7OQ88.'T5% 5w5 -
if , fp ;; ?A77 -
6
puts , 5(annot open &le5 -
e#it, -
=
while , another ;; HQH -
6printf , 5n8nter name% age and basic salar!: 5 -
scanf , 5
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9* :ead reords rom a fle &sing str&t&re *9
4include 5stdio.h5
main, -
6
I78 9fp struct emp
6
char nameJRDL
int age
"oat bs
=
struct emp e
fp ; fopen , 58+7OQ88.'T5% 5r5 -
if , fp ;; ?A77 -
6
puts , 5(annot open &le5 - e#it, -
=
while , fscanf , fp% 5
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2et Files and inar% Files
' te#t &le contains onl! te#tual
information li$e alphabets% digits andspecial s!mbols. In actualit! the'(II codes of these characters are
stored in te#t &les. ' good e#ampleof a te#t &le is an! ( program % sa!)1.(.
's against this% a binar! &le ismerel! a collection of b!tes. Thiscollection might be a compiledversion of a ( program ,sa!
)1.8N8-% or music data stored in a
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4include 5stdio.h5
main, -
6
I78 9fs% 9ft int ch
fs ; fopen , 5pr1.e#e5% 5rb5 -
if , fs ;; ?A77 -
6
puts , 5(annot open source &le5 - e#it, -
=
ft ; fopen , 5newpr1.e#e5% 5wb5 -
if , ft ;; ?A77 -
6puts , 5(annot open target &le5 -
fclose , fs -
e#it, -
=
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while , 1 -
6
ch ; fgetc , fs -
if , ch ;; 8O -
brea$
else
fputc , ch% ft -
=
fclose , fs -
fclose , ft -
=
Asing this program we can comfortabl! cop! te#t as well as binar!&les. ?ote that here we have opened the source and target &lesin 3rbF and 3wbF modes respectivel!. *hile opening the &le in te#tmode we can use either 3rF or 3rtF% but since te#t mode is thedefault mode we usuall! drop the Ut>.
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rom the programming angle there are three main areaswhere te#t and binar! mode &les are dierent. Theseare:
@andling of newlines )epresentation of end of &le
torage of numbers
2et /ers&s inar% ode' 4e!lines• *e have alread! seen that% in te#t mode% a newline
character is converted into the carriage returnlinefeedcombination before being written to the dis$.
• 7i$ewise% the carriage returnlinefeed combination on
the dis$ is converted bac$ into a newline when the &leis read b! a ( program.
• @owever% if a &le is opened in binar! mode% asopposed to te#t mode% these conversions will not ta$eplace.
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2et /ers&s inar% ode' End o File
• The second dierence between te#t and binar!modes is in the wa! the endof&le is detected. In
te#t mode% a special character% whose '(IIvalue is 2B% is inserted after the last character inthe &le to mar$ the end of &le. If this character isdetected at an! point in the &le% the read
function would return the 8O signal to theprogram.
• 's against this% there is no such specialcharacter present in the binar! mode &les to
mar$ the end of &le. The binar! mode &les $eeptrac$ of the end of &le from the number ofcharacters present in the director! entr! of the&le.
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2et /ers&s inar% ode' -torage o4&m3ers
• The advantage of storing an integer number in binar! formis that it saves a lot of dis$ space.
8g: If !ou want to store an integer value 1BR then it willconsume R b!tes of storage as each digit will be convertedto a '(II character which reVuires one b!te of storage.
In binar! &les %1BR will consume onl! 2 b!tes for storage.
'(II ormat
R b!tes
inar% Format
2 b!tes
1 B R
D D D D D 1 D 1 11 11
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