Random Access Files

Random Access Files

Random access to files are required when we want

to access only particular part of a file instead of reading and writing data sequentially.This can be
achieved by using the functions like : fseek,ftell,rewind.

Ftell:

1.This function is used to find the current position of a file.

2.It takes file pointer as an argument.

3.It returns a number of type long.

            n=ftell(fp);

      where fp is a file pointer

4.This ftell is used to save the current position of a file which can be used for later use.

5.Here  n returns the relative position of a file in bytes i.e., relative offset. This means that n bytes
have already been read or written.

Rewind:

1.It takes the file pointer as an argument and resets  the position to the start of a file.

2.   rewind(fp)
        n=ftell(fp)
   The above two statements result the value of n to be zero, since the rewind function has set the
filepointer to start of the file,where first byte is set to 0.

3.rewind(fp) function is used to reading and writing a file  more than once without having
to open and close a file.

4.When a file is opened for reading and writing rewind is  done implicitly.

Fseek:

1.This function is used to move the file position to a desired location within the file.

      fseek(fileptr,offset,position)

fileptr  is pointer to the file concerned offset is a number or variable of type long .It specifies the no.of positions(bytes) to be moved from the location specified by position. 

Position is an integer number.It takes one of the three values.

    Vlaue            Meaning     
     0    beginning of the file.
     1    current postion.
     2    end of file.

3.The offset may be positive or negative .If positive move forwards else move backwards.

4.Examples on Fseek:

fseek(fp,0L,0) --->  Go to the beginning.
fseek(fp,0L,1) ---> Stay at the current position.
fseek(fp,0L,2) ---> Goto the end of the file,past
                            last character of file.
fseek(fp,m,0) --->  Move to the (m+1)th byte in the
                            file.
fseek(fp,m,1) ---> Go forward by m bytes.
fseek(fp,m,-1) ---> Go backwards by m bytes from
                            current position.
fseek(fp,-m,2) ---> Go backwards by m bytes from end
                             of file.

Opening a file under Random access:

    fp=fopen("Random","w")