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An amazing directory: /proc (europe.pub)

submitted 5 days ago by cm0002@europe.pub to c/linux@programming.dev

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[-] Unleaded8163@fedia.io 51 points 5 days ago

The creator of this image is Julia Evans. She's awesome. She's also really active on Mastodon @b0rk@jvns.ca and has a bot that posts a bunch of images similar to this one @b0rk_reruns@jvns.ca. She also sells bigger, more in-depth zines at wizardzines.com. I don't know her personally, just think she's great. Both of those mastodon accounts are worth following IMO.

[-] rimu@piefed.social 1 points 5 days ago

Thank you!

[-] state_electrician@discuss.tchncs.de 21 points 5 days ago

/proc is one of the things I keep learning about again and again. I always read about it when I don't need it and when I would need it, I have forgotten about it.

[-] sik0fewl@piefed.ca 8 points 5 days ago

How can the symlinks work if the file has been deleted?

[-] princess 13 points 5 days ago

They aren't symbolic links. They're closer to hard links, IIRC, in that they share the inode with other file paths.

ofc /proc isn't a "real" file system so the metaphors break down a little; it's not backed by disk, it's a view into the kernel's process information exposed as a file system

[-] trem 11 points 5 days ago

Just to kind of confirm this:

ls -l does not show them like hard links, but rather like soft/symbolic links.

But when I do tail -f /path/to/file.txt on a file (to keep it open in a process) and then delete the file, I get this output in ls -l:

3 -> '/path/to/file.txt (deleted)'

The 3 seems to just be an incrementing number for each file opened by the process. And then, well, obviously the file isn't now called "file.txt (deleted)". That is just a name the kernel makes up when ls -l asks it what's in that directory.

So, presumably the kernel keeps a separate copy of that file in memory until the process closes the file or is terminated. And then exposes it through this pseudo-softlink rather than making use of hardlinks.

[-] korthrun@piefed.social 7 points 5 days ago* (last edited 5 days ago)

The 3 seems to just be an incrementing number for each file opened by the process.

FWIW: https://en.wikipedia.org/wiki/File_descriptor#file_descriptor_table

If you throw an strace on a process that interacts with the filesystem you'll see that stuff like open() returns a file descriptor and stuff like read() takes a file descriptor as an argument.

These are also generally the numbers you're using when you do I/O redirection in your shell with stuff like 2>&1 or exec 5< ./some_file.txt