c++ - What is the most efficient way to remove first N bytes from a file on Windows?

Question

Say, I have a file of an arbitrary length S and I need to remove first of its N bytes (where N is much less than S.) What is the most efficient way to do it on Windows?

I'm looking for a WinAPI to do this, if one is available.

Otherwise, what are my options -- to load it into RAM and then re-write the existing file with the remainder of data? (In this case I cannot be sure that the PC has enough RAM?) Or write the remainder of file data into a new file, erase the old one, and rename the new file into the old one. (In this case what to do if any of these steps fail? Plus how about defragmentation that this method causes on disk?)

Answer 1

There is no general way to do this built into the OS. There are theoretical ways to edit the file system's data structures underneath the operating system on sector or cluster boundaries, but this is different for each file system, and would need to violate any security model.

To accomplish this you can read in the data starting at byte N in chunks of say 4k, and then write them back out starting at byte zero, and then use the file truncate command (setendoffile) to set the new smaller end of file when you are finished copying the data.