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CS1061 C ProgrammingLecture 18: Sequential File Processing

A. O’Riordan, 2004, 2007 updated

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 The FILE structure C views each file as a sequence of btes

!iles end with the end"of"file #ar$er %&O!'

(trea# created when a file is opened

Openin) a file returns a pointer to a !*+& structure

he !*+& structure

!ile descriptor  inde- into operatin) sste# arra called the open file table

!ile Control loc$ %!C' found in ever arra ele#ent, sste# uses it to ad#inister the file

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Creating a FileFILE *myPtr;

creates a !*+& pointer called #/tr 

myPtr = fopen("myFile.dat", openmode);

!unction fopen%' returns a !*+& pointer and ta$es two ar)u#ents file to openand file open #ode.

*f open fails, 1++ returned.

fclose(FILE ointer!

!unction fclose%' closes specified file.

&ven thou)h it’s perfor#ed auto#aticall when pro)ra# ends it is )ood practiceto close files e-plicitl.

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"ea#ing a sequential access $leA sequential access file is written and read fro# the start to the end of the file

usin) a file pointer.

Create a !*+& pointer, tie it to the file to read

myPtr = fopen("myFile.dat", "r");

se fscanf%' to read fro# the file.

+i$e scanf%', e-cept first ar)u#ent is a !*+& pointer, e.).3

fscanf(myPtr, "%d%s%f", &myInt, mytrin!, &myFloat);

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foen(!&ver call to fopen%' will tpicall be followed with a test, li$e this3

ifp = fopen("inpt.dat", "r");

if(ifp == #$LL)printf("cant open file'n");

eit or retrn

or 

if((ifp = fopen("inpt.dat", "r")) == #$LL)

printf("cant open file'n");

eit or retrn

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File %o#es!iles are open in a certain #ode.

%&'E SE' F&" FILE C"E)TE'* E+ISTI,- FILE*

.a. )en#ing /es )en#e# to

.a. "ea#ingaen#ing /es )en#e# to

.r. "ea# onl2 ,o /es

.r. "ea#ing3riting ,o /es

.3. 4rite onl2 /es 'estro2e#

.3. "ea#ing3riting /es 'estro2e#

he nu#ber of strea#s that #a be open si#ultaneousl is specified b the #acro

!O/&15A6 %in stdio.h'. !O/&15A6 #ust be at least ei)ht in *(O C.

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fscanf(! an# frintf(!fscanf%' and fprintf%' are file processin) equivalents of scanf%' and printf%'.

/rototpes %as in stdio.h'3

int fprintf(FILE *stream, const c+ar *format, ...);

int fscanf(FILE *stream, const c+ar *format, ...);

fprintf%stdout, 8ello' is eqivalent to printf%98ello'

fscanf%stdin, 9:d;,

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frintf(! e5amleinclde -stdio.+

inclde -stdli/.+

int main()

int i, score0123;

FILE *4icores;

for(i=2; i-12; i55) score0i3 = 122 6 i;

if(7(4icores = fopen("scores.tt", "8"))== #$LL)

for(i=2; i-12; i55)

fprintf(4icores, "#m/er %d9't %d'n", i51, score0i3);

retrn 2;

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fscanf(! e5amleinclde -stdio.+

int main()

int i;FILE *fp;

fp = fopen("scores.tt", "r");

8+ile(fscanf(fp, "%d", &i) 7= E:F)

printf("%d'n", i);

retrn 2;

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fgetc(! an# futc(!f)etc%' reads one character fro# a file.

a$es a !*+& pointer as an ar)u#ent.

f)etc%' and )etc%' are identical, e-cept that )etc%' is usuall i#ple#ented as a#acro for efficienc.

fputc%' writes one character to a file

a$es a !*+& pointer and a character to write as an ar)u#ent.

+i$ewise fputc%' and putc%' are identical.

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fgetline(!Read one line fro# data.t-t, copin) it to line arra %not #ore than #a- chars'.

=oes not place ter#inatin) >n in line arra. Returns line len)th, or 0 for e#ptline, or &O! for end"of"file.

 1ow we could read one line fro# ifp b callin)3

define r?);

...

f!etline(ifp, line,

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fgetc(! e5amleint f!etline(FILE *fp, c+ar line03, int ma)

int nc+ = 2, c;

ma = ma 6 1; * lea@e room for '2 *

8+ile((c = f!etc(fp)) 7= E:F)

if(c == 'n)

/reaA;

if(nc+ - ma)

line0nc+3 = c;

nc+ = nc+ 5 1;

if((c == E:F) && (nc+ == 2))

retrn E:F;

line0nc+3 = '2;

retrn nc+;

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Caution(equential access file cannot be #odified without the ris$ of destroin) other

data.

!ields can var in sie "different representation in files and screen than internalrepresentation.

?, B4, "D0 are all ints, but have different sies on dis$.

ecause of these li#itations C also has rando# access file processin) features.

(ee ne-t lecture.