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I was using lsof to track down deleted files that were still taking up space and I realized that I wasn't quite sure what an offset is with respect to a file. lsof's man page was less than helpful in this regard and searching around I couldn't get a clear picture of what it is.
What is a file offset and why is it useful to have that piece of information?
The offset is the current position in the file, as maintained by the kernel for a given file description (see the lseek(2) and open(2) manpages for details).
As to why it's useful in lsof's output, I'm not really sure. It can give some idea of a process's progress through a file, although it won't cover all cases (memory-mapped files won't show offset changes).
Stephen Kitt's answer is close, but not quite correct; if we read lseek(2) more carefully, we will see that it actually says (bold mine):
The lseek() function repositions the file offset of the open file description associated with the file descriptor fd [...]
So, what's an "open file description" and how does it relate to a "file descriptor"?
Well, down in the NOTES section, we find:
See open(2) for a discussion of the relationship between file descriptors, open file descriptions, and files.
Perfect! However, rather than pasting a wall of text from the manpage, I'll just link to a good answer to "What is an open file description?" that explains the same thing.
