I thought all files were seekable, but I’m going through a codebase that handles cases where a file is not seekable.
Why is that necessary?
In what scenarios would a file be unseekable, and why?
1
- 1Pipes are the most common example: unix.stackexchange.com/q/30885/70524, unix.stackexchange.com/a/18135/70524, etc., etc. What would it even mean to seek on a socket?muru– muru2025-03-17 04:00:16 +00:00Commented Mar 17 at 4:00
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1 Answer
In Unix, almost everything that you do data exchange with is treated like a file, but not everything is a file.
If you are handed an open file descriptor, it might be:
- A regular file stored on a POSIX conforming file system (and most others) is always seekable
- A character device (such as a tty) is generally not seekable
- A block device, which is seekable
- A FIFO which is not seekable
- A socket of any type (unix, tcp, udp / stream, datagram), which is not seekable
- A pipe, which is not seekable
All but the last two can be opened and treated almost like a regular file, until you try to seek on it. (The last two use a function call other than open() that is specific to their type)
All of the non-seekable types above are basically character streams that are consumable rather than repeatable, so seeking on them makes no sense.
- Fifos are named pipes, not a distinct type, meaning that pipes still use open(). Only sockets don't.grawity– grawity2025-03-17 05:03:17 +00:00Commented Mar 17 at 5:03
- What's an actual file? If you mean a regular file for which
S_ISREG(mode)is true, then they are generally seekable, but you can find special filesystems (I've seen some fuse based ones) where they may not be.Stéphane Chazelas– Stéphane Chazelas2025-03-17 07:13:47 +00:00Commented Mar 17 at 7:13 - 1Block devices are generally fully seekable, not only by block incrementStéphane Chazelas– Stéphane Chazelas2025-03-17 07:14:19 +00:00Commented Mar 17 at 7:14
- A fifo may be called a named pipe, but in every other way it doesn't act like a pipe. It acts much more like a file than a pipe.user10489– user104892025-03-17 11:38:09 +00:00Commented Mar 17 at 11:38
- I would be interested to know the name of your hypothetical non-seekable filesystem. There's no point in listing ones that it does work for, because it's all the standard ones.user10489– user104892025-03-17 11:40:30 +00:00Commented Mar 17 at 11:40