FILE ACCESSING TECHNIQUES (or) FILE TYPES

FILE ACCESSING TECHNIQUES (or) FILE TYPES

            Every open file has an associated file position indicator, which describes where read and write operations take place in the file.  The position is always specified in bytes from the beginning of the file.

            When a file is opened in either read (or) write mode, the position indicator is always at beginning of the file i.e., at position ‘0’.  If the file is opened in append mode, the position indicator is at the end of the file.

           

File accessing techniques can be classified into two types as:

1.                  Sequential file processing

2.                  Random access file processing

1. Sequential File Processing:           In sequential file processing, the file pointer moves character by character without skipping data. i.e., read or write operations performed sequentially.  getc(), putc(), fgets(), fputs() etc., functions support sequential file processing.

2. Random Access File Processing:              In random access file processing, the file pointer can change from one location to another location according to user requirements.  Here, operations are performed in random access manner.  Most important functions used in random access file processing are:

            a)         fseek()

            b)         ftell()

            c)         rewind()

a)  fseek() function:    fseek() function is used to move the position indicator to a desired location within the file.  The general format of the fseek() function is:

            Syntax:            fseek(FilePointer, OffSet, Position);

Where,

Ø  OffSet is the number of bytes to be moved and it must be long integer.  The value may be positive of negative.  If the OffSet value be positive, file pointer moves to forward direction; otherwise, file pointer moves to backward direction.

Ø  Position specifies the starting position in the file to move.  Position can take any one of the following three values.

VALUE	MEANING
0	Beginning of File
1	Current Position
2	End of File

When the operation is successful, fseek() function returns zero; otherwise, it return -1.

Examples:

1.         fseek(fp,0L,0)             -           Go to beginning of the file

2.         fseek(fp,0L,1)             -           Stay at the current position

3.         fseek(fp,0L,2)             -           Go to end of the file

4.         fseek(fp,m,0)               -           Move to m bytes from beginning of the file

5.         fseek(fp,m,1)               -           Go to forward by m bytes from current position

6.         fseek(fp,-m,2)             -           Go to backward by m bytes from the end of file.

b)  ftell() function:      ftell() function is used to return the current position of the file pointer in the file.  The general format of the ftell() function is:

            Syntax:            N = ftell(FilePointer);

Where,

            N is a long integer variable.

Function returns current position (in bytes) as long integer value.  If any error encountered, then the function returns -1.

c)  rewind() function:             rewind() function is used to reset the file pointer to beginning of the file.  The general format of the rewind() function is:

            Syntax:            rewind(FilePointer);

Function reset the file pointer at the beginning of the file.

WRITE A PROGRAM TO PRINT THE CONTENTS OF A FILE IN REVERSE ORDER 

#include<stdio.h>

main()

{

            FILE *fp;

            long n,i;

            char ch;

            clrscr();

            fp=fopen("den.txt","r");

            if(fp==NULL)

            {

                        printf("\nFile Opening Error");

                        exit();

            }

            fseek(fp,-1L,2);

            n=ftell(fp);

            printf("\nFile Contents in Reverse Order:\n");

            for(i=1;i<=n+1;i++)

            {

                        fseek(fp,-i,2);

                        putchar(fgetc(fp));

            }

            fclose(fp);

}