The ftell(), fseek(), rewind() Functions

THE ftell(), fseek(), rewind() FUNCTIONS

We have seen the files that are useful for reading and writing data. When we are interested in accessing only a particular part of a file and not in reading the other parts. This can be achieved with the help of the following functions.

i) ftell()

ii) fseek()

iii) rewind()

i) ftell(): This function is used to specify the current position of a file. It takes the file pointer and returns a number of type long, that corresponds to the current position.

Syntax: n = ftell (fp);

Description: n is the relative offset of the current position in bytes.

ii) fseek(): It is used to move the file position to a desired location within the file.

Syntax: fseek(file ptr, offset, position);

Description:

file ptr is a pointer in the file concerned.

offset is a number or variable of type long.

position is an integer number. It specifies the number of positions to be moved from the location specified by position.

These functions are very useful to handle the Random Access data files.

iii) rewind(): It is used to takes a file pointer to the starting position of the file and resets the position to the start of the file.

Example:

rewind (fp);

ftell(fp);

Where, 0 is assigned to n because the file position has been set to the start of the file by rewind.

This function is used to read a file more than once, without closing and opening the file.

NOTE: Remember that whenever a file is opened for reading or writing, a rewind is done implicitly.

Example: Program using ftell(), fseek() and rewind() functions.

/* Program using ftell(), fseek() and rewind() functions */

#include <stdio.h>

main()

{

/* Local definitions */

long int x = 0, m;

/* Statements */

FILE *fptr = fopen("vrb.dat", "r");

if (fptr != NULL)

{

        fseek(fptr, 0L, 0); /* fseek() is a function used to position desired point */

        while(x = getc(fptr) != EOF)  /* check while x=getc(fptr) is not equalled to the end of file */

        putchar(c);

        fseek(fptr, 0L, 2);

        x = ftell (fptr); /*ftell() used to specify the current position*/

        printf("\n The size of the file is %d", x);

        rewind(fptr); /*rewind to move control to the beginning */

        x = ftell (fptr);

        printf("\n The size of the file is %d", x);

} /* if */

else

        printf("\n The file does not exists");

fclose(fptr);  /* file close function closes a file associated with file pointer */

} /* main */

OUTPUT

vrb publishers pvt ltd.

The size of the file is 23

The size of the file is 0.

EXPLANATION: This program stores some text into the file vrb.dat, and using ftell, fseek(), and rewind() functions it displays the length of the file in concern with the file pointer.