How Do Task Killers Work

How do task killers work?

In a nutshell, Automatic Task Killers work by polling the OS for a list of currently running processes and the memory they are consuming. Then either with an intelligent algorithm or with user input the Task Killers issue a call to the system telling the system to kill the process. There are two apis you can do this.

They are

• Process.killProcess(int pid)
• ActivityManager.killBackgroundProcesses(String packageName)

This first works by invoking Process.killProcess(int pid) where pid is the unique identifier for a specific process. Android kills processes in the same way that linux does; however, a user may only kill processes that they own. In Android each app is run with a unique UID (UserID). Apps using this API an App can only kill their own processes, hence the following explanation in the docs for Process.killProcess(int pid):

Kill the process with the given PID. Note that, though this API allows
us to request to kill any process based on its PID, the kernel will
still impose standard restrictions on which PIDs you are actually able
to kill. Typically this means only the process running the caller's
packages/application and any additional processes created by that app;
packages sharing a common UID will also be able to kill each other's
processes.

When this method is called the signal is generated by the OS and sent to the process. Whenever a process receives a signal from the OS it must either handle that signal or immediately die. Signals such as SIG_KILL cannot be handled and result in the immediate death of the recipient process. If you want to kill processes that you don't have privileges to kill, i.e. its not your process, then you must switch users or escalate your privileges (on android this requires root privileges on the device).

The second API works by telling the built in ActivityManager that you wan to kill processes associated with a specific Package. This API gets around the need for your UID to match the UID of the process because it requires the user to accept the KILL_BACKGROUND_PROCESSES permission. This permission signals to the OS that an app has been approved by the user as a task killer. When a task killer wants to kill an app, it tells the OS to kill the process allowing an app to get around the problem of only being able to kill processes that it owns.

In the Android Docs it says that this API actually uses the first Process.killProcess API

Have the system immediately kill all background processes associated
with the given package. This is the same as the kernel killing those
processes to reclaim memory; the system will take care of restarting
these processes in the future as needed.

If you want to know more I suggest you read about the Posix Signals and The Linux kill command

How can I account for users using Task Killers?

There's no way to account for this, so I've re-built my app around it.

How do Task Managers kill apps?

You can send the signal using:

Process.sendSignal(pid, Process.SIGNAL_KILL);

To completely kill the process, it's recommended to call:

ActivityManager.killBackgroundProcesses(PackageName)

before sending the signal.

Really killing a process in Windows

"End Process" on the Processes-Tab calls TerminateProcess which is the most ultimate way Windows knows to kill a process.

If it doesn't go away, it's currently locked waiting on some kernel resource (probably a buggy driver) and there is nothing (short of a reboot) you could do to make the process go away.

Have a look at this blog-entry from wayback when: http://blogs.technet.com/markrussinovich/archive/2005/08/17/unkillable-processes.aspx

Unix based systems like Linux also have that problem where processes could survive a kill -9 if they are in what's known as "Uninterruptible sleep" (shown by top and ps as state D) at which point the processes sleep so well that they can't process incoming signals (which is what kill does - sending signals).

Normally, Uninterruptible sleep should not last long, but as under Windows, broken drivers or broken userpace programs (vfork without exec) can end up sleeping in D forever.

How does process killing work in android?

Here is a wild guess from a novice:

Android sits on top of the linux operating system as shown here. If you connect to your android virtual device (avd) and open the terminal emulator or connect through the ADB, you will find that you have access to various linux commands.

C:\Users\james>adb -e shell
# cd /system/bin
# ls -a -l
...
lrwxr-xr-x root     shell             2010-06-30 15:32 insmod -> toolbox
-rwxr-xr-x root     shell       18172 2010-06-30 15:32 installd
lrwxr-xr-x root     shell             2010-06-30 15:32 ioctl -> toolbox
lrwxr-xr-x root     shell             2010-06-30 15:32 ionice -> toolbox
-rwxr-xr-x root     shell       10036 2010-06-30 15:33 keystore
-rwxr-xr-x root     shell        6520 2010-06-30 15:33 keystore_cli
lrwxr-xr-x root     shell             2010-06-30 15:32 kill -> toolbox

Notice that most of the commands are just links to one small program (toolbox).
I can list the running programs with the ps command but if I try to kill them with the kill command... it says i do not have permission. You might be able to create a task manager that uses the underlying ps and kill commands in order to do the dirty work for you. You may have to overcome some permission issues though.

I expect you could run the system commands with something like this:

Runtime.getRuntime().exec("ls");

Edit:

I skimmed through this open source task manager but I didn't see how it goes about killing tasks. Maybe someone more experienced can explain it.

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