20.2 Process States

   

---

A process is always in one of the many states. A state shows the current activity of a process. In a multitasking system, although to a user it appears that all processes are running simultaneously , strictly speaking only one process is active on one CPU at a time. The kernel schedules the processes such that every process gets a time slice for its execution. During the life cycle of a process, it changes its state many times.

Running

A process is in a running state when its text (code) is being executed. The kernel schedules processes such that each process has its time slice during which the kernel executes its code. A process may go to another state from the running state depending on several factors.

Sleeping

A process may have to wait for a resource or event that is currently not available. It makes a sleep system call that puts the process on a queue of sleeping processes. This system call also changes the state of the process. When the resource becomes available, the kernel wakes up the process.

Ready to Run Process

When the kernel wakes up a sleeping process, its state changes and becomes runnable. It means that the process becomes ready to run and waits for its scheduled time slice.

Stopped

The stop signal puts a process in the stopped or suspended state. A stop signal changes the state of the process immediately. The process remains stopped until its gets a continue signal. After getting this signal, the stopped process goes into a runnable (or ready to run) state. If a process receives a stop signal when it is sleeping, it goes back to a sleep state after receiving a continue signal.

Zombie

When a process terminates, it sends an exit system call. At this time, the kernel releases resources used by the process. The process becomes a zombie and remains in the zombie state until its parent process gives it permission to die. If the parent process is not responding, the process may be left in a zombie state indefinitely.

---

   

Top