Random Access File

Random Access File

Normally data in file read in sequential manner , but if need to access only a particular part of a file and not reading the other part. For that random access functions like  fseek() , ftell() and rewind() available in I/O library.

ftell()

This function returns relative offset (in bytes ) of the current position of a file. This function useful in saving the current position of a file , which can be used  later in program.

Syntax:

                   long  ftell(File-ptr);

As shown in above syntax File-ptr is the pointer to the file structure.

It returns the current file position of the given stream. If it is a binary stream, then the value is the number of bytes from the beginning of the file. If it is a text stream, then the value is a value useable by the fseek() function to return the file position to the current position.

On success the current file position is returned. On error a value of -1L is returned .

Ex,

          long n= ftell(fp);

Here, variable n stores the current position and n bytes have already been read and written.

fseek()

This function is used to move file position to a desire location within a file.

Syntax:

fseek(File-ptr,offset,position);

Here, File-ptr is the pointer to the file.

Offset specifies the number of positions (byte) to be moved from the location specified by position. Offset is a number or variable of long data type.

Position is an integer number. It has following three values.

Value	Meaning
0        Or       SEEK_SET	Beginning of file
1        Or       SEEK_CUR	Current position
2        Or       SEEK_END	End of file

If offset is positive , it moves file pointer in forward direction. If offet is negative , it moves file pointer in backward direction.

This function returns zero if operation is successful. If the file pointer is end of file means EOF and though file pointer tries to move data then error occurs and returns -1

Operations of fseek()

Statement	Meaning
feek(fp,0L,0)	Go to the beginning.
feek(fp,0L,1)	Stay at current position.
feek(fp,0L,2)	Go to the end of file.
feek(fp,m,0)	Move (m+1) th byte in the file.
feek(fp,m,1)	Go forward by m bytes
feek(fp,-m,1)	Go backward by m bytes from current position
feek(fp,-m,2)	Go backward by m bytes from the end.

rewind()

This function resets the file position to the beginning of the file.

Syntax:

                   void rewind(File-ptr);

Here, File pointer is the pointer to file structure.

Ex,

            rewind(fp);

            long n=ftell(fp);

In above example it will assigns 0 to n because the file position has been set to the start of the file.

With the use of this function , without closing or opening a file , file can be read more then one time. File is opened fpr both reading and writing.

                   

Posted By : Ruchita Pandya