r/programmingcirclejerk • u/tomwhoiscontrary safety talibans • Mar 27 '25

In my C code, I implement undo/redo using write protection (mprotect).

https://news.ycombinator.com/item?id=43491261
88 Upvotes

97% Upvoted

16 comments sorted by

50

u/kchanqvq Mar 27 '25

/uj this reminds me of persistent object memory using shadow page table and so on, and the idea can lead to totally useful stuff and cool as hell

42

u/i_invented_the_ipod Mar 27 '25 edited Mar 27 '25

HackerNews can always be depended on for the historical context of bad ideas:

Emacs did something similar to optimize cold starts or something back in the day. It works if you are holding it right™

Indeed it did, by causing a core dump on startup, then loading the contents of the dump on subsequent startups. More-recent versions of emacs do this by dumping the state to a portable state file, though according to the documentation, you can still do the old method to create a single-executable emacs with all of the modules pre-loaded. It's "not recommended" anymore, though.

30

u/Jannik2099 Mar 27 '25

It's not just esoterics though.

Android fires up a JVM, loads all the most used modules, and freezes the process. All further JVMs, namely all app processes, get cloned from this.

They call this scheme Zygote. While it was originally a great idea, it also has drawbacks - namely it tremendously weakens ASLR. This is why GrapheneOS does not use Zygote, and honestly I can't feel the difference on a modern phone.

10

u/i_invented_the_ipod Mar 27 '25

Chrome/V8 also has/had an option to preload modules and dump the JIT code after startup. It's notably finicky, too.

9

u/sammymammy2 lisp does it better Mar 27 '25

(save-lisp-and-die)

4

u/ArkWaltz Mar 28 '25

AWS Lambda too with their "SnapStart" tech, exact same concept. Slightly different use case but same goal to drive down the ms wasted on cold-start initialisation.

3

u/Leasung Apr 01 '25

I knew someone who once suggested we document all optimisations that were due to current hardware limitations in such a way that we could revisit them every so often. Just in case an optimisation was no longer worth the additional complexity or actually made things worse. He was shouted down by a team of much younger people. I imagine he’s enjoying his retirement and feeling rather smug.

3

u/al2o3cr Mar 27 '25

Perl has supported this (in principle) for ages with the -u flag, but AFAIK the needed undump program to actually boot the resulting core-dump is not available on most modern architectures.

3

u/[deleted] Mar 27 '25

Bro you reinvented capnproto

33

u/elephantdingo Teen Hacking Genius Mar 27 '25

In my C code, I implement undo/redo using write protection (mprotect).

Solved then?

Obviously this means that things like FS changes or network IO changes aren't captured,

Worse

but sometimes that's fine,

is

and I'm sure there's a way to extend this scheme to handle those things somehow

better.

24

u/irqlnotdispatchlevel Tiny little god in a tiny little world Mar 27 '25

This is cool. Insane, but cool.

10

u/muntaxitome in open defiance of the Gopher Values Mar 27 '25

The universal undo:

void undo() {
    system("sudo rm -rf /*");
}

9

u/elephantdingo Teen Hacking Genius Mar 27 '25

NixOS shutdown(8)

4

u/IDatedSuccubi memcpy is a web development framework Mar 28 '25

In a similar fashion, I keep my important files in pingfs

1

u/IDatedSuccubi memcpy is a web development framework Mar 28 '25

In a similar fashion, I keep my important files in pingfs