Any process can either be terminated normally or abnormally.

A process can be terminated in one of the following ways:

• by exiting (i.e., the process terminates itself calling exit system call)- Normal Termination
• by being signaled –Abnormal Termination
• by having no running threads (i.e. the thread count goes to 0)

Normal Terminations:

• This reflects that the process has completed its task and has terminated gracefully.
• The exit() system call is used by most operating systems for process termination.
• The process leaves the processor and releases all its resources

Abnormal Terminations:

This reflects that the process has not completed its task rather it has terminated in between because of any other reasons.

Some of the common reasons for abnormal terminations are:

• The process is trying to access some memory locations which it should not, resulting in a SIGSEGV signal.
• Termination because of permissions that is read-write permissions.
• Another process explicitly sends a signal to the process that is with kill(pid, SIGTERM).
• Trying to bypass the System call API accessing the kernel.

The most common utility to terminate a process is “kill” command.

• The kill command is used to send a signal to processes.
• The most frequently used signal is SIGKILL or -9, which terminates the given processes.

Syntax:

$ kill -<signal_name_or_signal_number> PID_OF_PROCESS

Options can be fetched by command kill -l

Signals can be specified in three different ways:

• Using numbers (eg : -1 or -9)
• Using the “SIG” prefix(eg: SIGHUP or SIGKILL or SIGTERM)
• Without using the “SIG” prefix( eg: HUP or KILL or TERM)

Example:

• kill -9 <pid> or kill -SIGTERM <pid>
• kill -SIGTERM <pid>
• kill -TERM <pid>

All the above are equivalent.

Values of PID:

• If the value of PID is greater than 0, the signal is sent to a specific process with that ID
• If the value of PID is 0, the signal is sent to all the processes in the current group.
• If the value is -1 the signal is sent to all processes with the same UID as the user invoking the command.
• If PID is less than -1, the signal is sent to all processes in the process group equal to the absolute value of the PID that is if PID is -9 then the signal will be sent to process id 9 in the current process group eq to GID.

Related Posts:

• Process vs program
• Sections of a process
• States of a process
• Process Control Block(PCB)
• Attributes of a process
• Context switching

Categories: Operating system (OS)