CareerCup

I think it can be done better than this... This is my solution:
lets say you can read/store N MB of data in memory(N < 1 Giga).

while not EOF
  read and reverse in memory the N MB of characters;
  save it in a temporary file and add it to some sort of file stack;

open final file;

while file_stack not empty
  pop the last file;
  open it and write its content in the final file;
  close and delete temp file

close final file;

I think we can start reading from the last of the file. Suppose we read last m characters from the file and then reverse them and write it to another file. Then again read last but one m characters from the file reverse it and write them to second file and so on.

Since the file is very large, and say the memory is of N Megabytes; get the first N/2 Megabytes from the front and N/2 Megabytes from the last (assuming user space in the memory is of N megabytes)..and swap first character with the last, 2nd character with N-1 and so on. When finished get the next N/2 MB from the front and next N/2 from the back , repeat the process. (Termination condition is i>j where i is the pointer denoting the end of the first chunk and j is the pointer pointing to the beginning of the last chunk as viewed from the start).

mmap the file?

Not sure if it is a pure algorithm question.
What about some low level stuff ?
A file is stored in blocks in the harddisk . Any filesystem will have the sequential data blocks holding the data of the file .
So , we have reverse the content of each of the data blocks and also reverse the sequence number of the data blocks ( i.e block 0 becomes block n , block 1 becomes block n-1 .... )

Friends, you people are forgetting that you can not EDIT a file in C, just like you edit an array.
You can open files in C, either in Write Mode (which we can't use here, since it will truncate the contents of the file), or append mode (which always write new Data, at the end of the file, regardless of any calls to fseek)

So the only possible solution is to open another file in the secondary memory, have your read pointer at the beginning in the 1st file, and do a character to character copy.

If its a WORD TO WORD copy, the job becomes slightly complicated, as you need to read the chars in a char buffer, in backward direction, till u get a space or any other delimiter, reverse the string in the buffer, an then write it to the 2nd file.

Shabbas Saurabh...

Cannot we do it like reversing of link list?? Just start storing the data in list and store the address of that chunk and finally use reverse link list..