GATE IT 2005 | Question: 19, UGCNET-June2012-III: 57

Question

A user level process in Unix traps the signal sent on a Ctrl-C input, and has a signal handling routine that saves appropriate files before terminating the process. When a Ctrl-C input is given to this process, what is the mode in which the signal handling routine executes?

• A. User mode
• B. Kernel mode
• C. Superuser mode
• D. Privileged mode

Comments

signals may be handled by two possible handlers,default or if user defined overriding default one.Here which is handling the signal?default or user defined?

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I think this question could be done by option elimination method:

Since b and d are the same hence they both cannot be the answer as multiple options not allowed.

Now, superuser mode is a mode that requires an administrator to do certain tasks.

Now think of a situation when you are using a computer in guest mode. Now you have written an infinite loop program and running in cmd. Now to stop cmd you will press ctrl+c and immediately the program stops without asking any permission. Now, this means CPU does not ask you for admin permission to do ctrl+c. This means the task can be done without going superuser mode.

Now, you have only one option left. Hence, A will be the answer.

Please correct if I'm wrong.

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is this in syllabus?

Answer 1

Answer = Kernel mode.
Explanation.
Hardware interrupts (also called external/asynchronous interrupts), are ones in which the notification originates from a hardware device such as a keyboard, mouse , etc.
The most common way of sending signals to processes is using the keyboard:
• Ctrl-C: Causes the system to send an INT signal (SIGINT) to the running process.
• Ctrl-Z: causes the system to send a TSTP signal (SIGTSTP) to the running process.
The kernel handles signals in the context of the process that receives them, so
a process must run to handle signals.

Answer 2

The question says "signal handling routine that saves appropriate files BEFORE TERMINATING THE PROCESS" . This means we are still in user mode. So answer is A.

Answer 3

In UNIX and UNIX-based systems, CTRL+C sends a SIGINT (Signal Interrupt) signal to the process. Its up to the receiving process how he wants to handle the signal. Most processes handle this signal by terminating gracefully, after performing cleaning up tasks before terminating.

But a process may or may not terminate upon receiving the signal. It is up to that process how he wants to handle it. Nothing is being forced on the process. It may also ignore this signal if it wants to. Programmers can customize how their processes handle the SIGINT signal by setting up signal handlers. These handlers are specialized functions that are invoked when the process receives a specific signal, such as SIGINT. By defining a custom SIGINT handler, programmers can control how their processes react to user interrupts.

For this reason, UNIX systems don’t require the sender of this signal to have special privileges. So A) is the correct answer.

For most signals like SIGTERM, SIGQUIT, etc, no special privileges are required. However, for more forceful signals like SIGKILL, special privileges will be required.