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File Handling

January 5, 2013 Programming and 1

File handling in C

• In C we use FILE * to represent a pointer to a file.
• fopen is used to open a file. It returns the special
value NULL to indicate that it couldn't open the file.

FILE *fptr;
char filename[]= "file2.dat";
fptr= fopen (filename,"w");
if (fptr == NULL) {
printf (“ERROR IN FILE CREATION”);
/* DO SOMETHING */
}


Modes for opening files

• The second argument of fopen is the mode in
which we open the file. There are three
• "r" opens a file for reading
• "w" creates a file for writing - and writes over
all previous contents (deletes the file so be
careful!)
• "a" opens a file for appending - writing on the
end of the file
• “rb” read binary file (raw bytes)
• “wb” write binary file


The exit() function
• Sometimes error checking means we want an
"emergency exit" from a program. We want it
to stop dead.
• In main we can use "return" to stop.
• In functions we can use exit to do this.
• Exit is part of the stdlib.h library

exit(-1);
in a function is exactly the same as
return -1;
in the main routine

Usage of exit( )

FILE *fptr;
char filename[]= "file2.dat";
if (fptr == NULL) {
printf (“ERROR IN FILE CREATION”);
/* Do something */
exit(-1);
}


Writing to a file using fprintf( )

• fprintf( ) works just like printf and sprintf
except that its first argument is a file pointer.

FILE *fptr;
fptr= fopen ("file.dat","w");
/* Check it's open */
fprintf (fptr,"Hello World!n");


Reading Data Using fscanf( )
•We also read data from a file using fscanf( ).

FILE *fptr;
input.dat
fptr= fopen (“input.dat”,“r”);
/* Check it's open */
20 30
if (fptr==NULL)
{
printf(“Error in opening file n”);
}
x=20
fscanf(fptr,“%d%d”,&x,&y); y=30


Reading lines from a file using fgets( )
We can read a string using fgets ( ).
FILE *fptr;
char line [1000];
/* Open file and check it is open */
while (fgets(line,1000,fptr) != NULL) {
printf ("Read line %sn",line);
}

fgets( ) takes 3 arguments, a string, a maximum
number of characters to read and a file pointer.
It returns NULL if there is an error (such as EOF).


Closing a file

• We can close a file simply using fclose( ) and
the file pointer.

FILE *fptr;
char filename[]= "myfile.dat";
if (fptr == NULL) { Opening
printf ("Cannot open file to write!n");
exit(-1);
}
fprintf (fptr,"Hello World of filing!n"); Access
fclose (fptr);
closing

Three special streams
• Three special file streams are defined in the
<stdio.h> header
• stdin reads input from the keyboard
• stdout send output to the screen
• stderr prints errors to an error device
(usually also the screen)
• What might this do?
fprintf (stdout,"Hello World!n");


An example program
Give value of i
#include <stdio.h>
15
main() Value of i=15
{ No error: But an example to show error message.
int i;

fprintf(stdout,"Give value of i n");
fscanf(stdin,"%d",&i);
Display on
fprintf(stdout,"Value of i=%d n",i);
The screen
fprintf(stderr,"No error: But an example to
show error message.n");
}


Input File & Output File redirection

• One may redirect the input and output files to
other files (other than stdin and stdout).
• Usage: Suppose the executable file is a.out
$ ./a.out <in.dat >out.dat

No error: But an example to show error message.

Give value of i Display
15 Value of i=15 screen

in.dat out.dat

Reading and Writing a character

• A character reading/writing is equivalent to
reading/writing a byte.
int getchar( );
int fgetc(FILE *fp);
int putchar(int c);
int fputc(int c, FILE *fp);
• Example:
char c;
c=getchar( );
putchar(c);

Example: use of getchar() and putchar()

#include <stdio.h>
main()
{
int c;

printf("Type text and press return to see it again n");
printf("For exiting press <CTRL D> n");
while((c=getchar( ))!=EOF) putchar(c);
}

End of file

Command Line Arguments

• Command line arguments may be passed by
specifying them under main( ).

int main(int argc, char *argv[ ]);

Argument
Count Array of Strings
as command line
arguments including
the command itself.


Example: Reading command line arguments

#include <stdio.h>
#include <string.h>

int main(int argc,char *argv[])
{
FILE *ifp,*ofp;
int i,c;
char src_file[100],dst_file[100];

if(argc!=3){
printf("Usage: ./a.out <src_file> <dst_file> n");
exit(0);
}
else{
strcpy(src_file,argv[1]);
strcpy(dst_file,argv[2]);
}

Example: Contd.
if((ifp=fopen(src_file,"r"))==NULL)
{
printf("File does not exist.n"); ./a.out s.dat d.dat
exit(0);
}
if((ofp=fopen(dst_file,"w"))==NULL) argc=3
{
printf("File not created.n");
exit(0); ./a.out
} argv s.dat
while((c=getc(ifp))!=EOF){
putc(c,ofp);
d.dat
}
fclose(ifp);
fclose(ofp);
}


Getting numbers from strings
• Once we've got a string with a number in it
(either from a file or from the user typing) we
can use atoi or atof to convert it to a
number
• The functions are part of stdlib.h
char numberstring[]= "3.14";
int i;
double pi;
pi= atof (numberstring);
i= atoi ("12");
Both of these functions return 0 if they have a problem

Example: Averaging from Command Line
#include <stdio.h>
#include <stdlib.h> $ ./a.out 45 239 123

int main(int argc,char *argv[])
Average=135.666667
{
float sum=0;
int i,num;

num=argc-1;
for(i=1;i<=num;i++)
sum+=atof(argv[i]);
printf("Average=%f n",sum/(float) num);
}


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