Fork Bomb

CS 241

IMPORTANT: Fork Bomb Warning!

If your code fork()s inside a loop that runs forever and without any sort of a pause, you will end up creating a fork bomb. You can read more about them here:

The machines limit the number of processes a user can spawn to prevent a fork bomb for taking down the host machine when on the remlnx machines. However, if you fork bomb, you will reach that number of processes and will be unable to do anything with your user account until EWS cleans it up. We strongly suggest you use a bash shell when doing any testing of this MP and setting the bash shell to limit the maximum number of processes that can be sub-process of that bash shell. Thus even if you reach the limit for your bash shell, you have not reached your machine limit. Therefore, you can launch another shell (eg: another instance of `putty`) and go in and kill your processes.

To limit the number of processes that can be spawned from the bash shell, use the following commands:

%> bash                # to enter the bash shell
%> ulimit -u 20        # max processes to 20
%> make                # make/compile your program
%> ./shell             # run your program)
After accidental fork bomb, open a new bash shell, and identify the IDs of the processes that you have to kill:

%> ps -aef | grep NETID
root      13116     7645   0 Sep07 ?        00:00:00 sshd: NETID [priv]
NETID     13123     13116  0 Sep07 ?        00:00:00 sshd: NETID@pts/4
NETID     13124     13123  0 Sep07 pts/4    00:00:00 ./shell
NETID     14281     13124  0 00:07 pts/4    00:00:00 ./shell
NETID     14282     13124  0 00:07 pts/4    00:00:00 ./shell

The second column (the first column with numbers) shows the process ID of the running process. You should go about stopping all the processes that are running ./shell and then killing all the processes running ./shell.

To stop a running process with ID = PID, use the command:

%> kill -STOP PID

And, to kill the process, use the command:

%> kill -KILL PID

And at this point, you're now set to start working again. Not fun, and thus the importance of making doubly sure that your fork() code is correct.

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