Things I wish Linux had (incomplete):
* SIGINFO (from BSD)
* Firmlinks (from HURD)
* Doors (from Solaris)

SIGINFO is a kill signal that is triggered with ^T (like how ^C triggers SIGINT) that makes the process spit out information.
A particular program that would greatly benefit from it would be dd, which must be *externally* sent SIGUSR1 to make it print I/O status.

Firmlinks are like symlinks in that they point to a file instead of an inode, but instead of just pointing to a path (valid or not), it points to the file itself, avoiding breakage.
Doors are a special file type (like named pipes or sockets) that allow for simple RPC, eschewing necessity for things like D-Bus for simple IPC operations. A Linux implementation of doors exists but it's abandoned since it was never merged.

Unlike a hard link, it's not a separate file pointing to the same inode, but a special file pointing at a file pointing at an inode instead.

@erkin So hard links not dependent on being on the same FS and more compatible?

Not sure how they handle cross-filesystem links but that's the gist. Unlike hardlinks, firmlinks die if the original file is deleted.

@erkin Also finding it hard to find documentation on that.

I don't think any OS other than HURD implements it.

@erkin I meant documentation on it in general, HURD docs would work.

This page explains how HURD's filesystem concept works:
It's got rather novel features.

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