The 7 month fork bomb

| fork | Linux

Today I was helping a student with some C programming, and the remote machine he was compiling and running his code on was running very slowly. It was a shared machine so I assumed some other students were using it. Therefore I had a quick look at “who” and found that only one other user was logged in. Then I looked at “top” and “ps” and noticed the machine was at 100% load, but I couldn’t figure out which process was causing this.

It turns out that 7 months ago (in April), two of the students had ran the following code while they were learning about fork:

while ( fork() ) {}

Each spawned process didn’t use much CPU, but the system was heavily forking, therefore tying up all the resources. Needless to say I found the two students and gave them a quick slap before asking them to run “killall blah”.

What I found cool about this was that there was two separate fork bombs going on, and this linux machine was still usable by the other students. Additionally, I’m surprised that no one in 7 months had actually noticed this! Things were happening so fast that I saw the pid of the forked programs wrap around at least twice during the time I was debugging the issue.

comments powered by Disqus