Understanding Force Quit On The Mac

How many times have you had an application go haywire and then needed to "Force Quit".  Ever wondered what that means and why it is necessary?  You probably thought,  "why can't I just close the app and let it die?"   For those folks with a deep need to tinker and understand, this blog post is for you.

One of the greatest things Apple ever did was to decide to bring OSX to the macintosh.  OSX is based on the Unix operating system. Specifically, it's based on the NeXTSTEP OS which was a very special version of Unix tailored by Steve Job's NeXT corporation.

All versions of Unix allow "Pre-Emptive MultiTasking" which means many different processes can by many different users at the same time.  Apple OS X,  Linux, Sun Solaris, Cray Unicos, IBM AIX, HP uX, and any other version of Unix will support this feature.  The wonderful thing about this is that you can have one user running one process and another user running another process and they won't normally interfere with each other.

You actually can see this if you open a Terminal.  On your Mac, open Terminal and type "ps".  The "ps" command will show you all the processes that are running.

Every process has a "Process ID" that is given to it.  It also will have a parent process and the process ID of the parent is shown as well.

Though, sometimes, things go wrong. Sometimes a process will find a way to hog system resources or behave erratically. Sometimes, a process would go so wrong, that you had to force it to die.  For this reason, Unix needed a command to talk to send messages to the process, and, if necessary, shut the process down. So, the "kill" command was created.


Kill takes a "signal" which is sent to the process. If you just type "kill" and your process ID, that signal will be "SIGTERM" which tells the process to terminate. Though, if the process is really wedged tight, that won't be enough. So, there is a more lethal and powerful signal, "SIGKILL" which can not be ignored.  In unix, each signal has a number. For "SIGKILL" the number is 9 so a SIGKILL is usually referred to as a "kill -9".


When you are doing a "Force Quit" on the Mac, this is exactly what you are doing.  It's a "kill -9" wrapped in a nice user interface.  Though, if you wanted to play Unix Nerd, you could do the same thing from a shell.

The Kill Command

The Kill Command used to send a signal to a process. The signal sent might not necessarily be to terminate execution. The following table is a list of some of the signals supported.

SIGNAL	NUMBER	DEFAULT	EVENT	Explanation
HUP	1	Exit	Hangup	Tells the program to reread all of its configuration files.
INT	2	Exit	Interrupt
QUIT	3	Core	Quit	Terminates Program and leaves a core file.
ILL	4	Core	Illegal Instruction
TRAP	5	Core	Trace/Breakpoint Trap
ABRT	6	Core	Abort
EMT	7	Core	Emulation Trap
FPE	8	Core	Arithmetic Exception
KILL	9	Exit	Killed	Cannot be caught or ignored.
BUS	10	Core	Bus Error	When used with Celpaint, will cause it to save out the planes.
SEGV	11	Core	Segmentation Fault	Will cause Softimage, Maya and Photoshop to save out and then exit.
SYS	12	Core	Bad System Call
PIPE	13	Exit	Broken Pipe
ALRM	14	Exit	Alarm Clock
TERM	15	Exit	Terminated	Terminates, but is more gentle. It gives the process a chance to clean up. This is the default.
USR1	16	Exit	User Signal 1	Causes the font server to re read the configuration file.
USR2	17	Exit	User Signal 2	Causes the font server to fluch the font cache.
CHLD	18	Ignore	Child Status Changed
PWR	19	Ignore	Power Fail/Restart
WINCH	20	Ignore	Window Size Change
URG	21	Ignore	Ugrent Socket Condition
POLL	22	Exit	Pollable Event
IO	22	Exit	input/output possible signal
STOP	23	Stop	Stopped (Signal)
TSTP	24	Stop	Stopped (User)
CONT	25	Ignore	Continued
TTIN	26	Stop	Stopped (tty input)
TTOU	27	Stop	Stopped (tty output)
VTALRM	28	Exit