Create button and functions for decreasing NI

[LauraAubin]

What does this do?

Part of #13

This creates a button that will decrease the selected process' NI value.

What I learned:

Through this, I discovered that I could not lower the priority without having root privileges.

From the man page for setpriority:

EACCES
The caller attempted to lower a process priority, but did not have the required privilege.

And from the man page for renice:

Bugs
Non super-users can not increase scheduling priorities of their own processes, even if they were the ones that decreased the priorities in the first place.

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What we learn here is that no non-privileged users can lower the niceness value, which in turn would increase it's priority. This is a security measure to protect vulnerability from applications taking up too much of the computer's CPU.

How to overcome this?

First I compiled the app in Xcode with created a .app file. When I tried to use sudo to run the app with root privileges:

sudo ./Xcode.app/Contents/MacOS/Xcode

It returned an error: Illegal instruction: 4. This error means that the OS did not understand the compiled application.

As a result, I had to take the app out of sandbox mode. Basically, sandboxing is a feature in Xcode that protects your system by limiting the app's privileges. In our case, we do not want this. These settings were found in the entitlements:

I removed both the security and the sandbox settings. The entitlements section is now empty.

After doing this, I recompiled a new .app file, and ran this command again:

sudo ./Xcode.app/Contents/MacOS/Xcode

Decreasing the NI value now works because sudo gives us the proper permissions and removing the sandbox allows the app to be understood by the OS.

[LauraAubin]

Removed sandbox entitlement

[LauraAubin]

Removed security entitlement