Through the course of its lifetime, a process transits through
several states. Queues in main memory keep track of the process
by its process ID. A process resides on a queue according to its
state; process states are defined in the proc.h header file. Events
such as receipt of a signal cause the process to transit from one
state to another.
Table 1-3 Process states
| State | What Takes Place |
|---|
| idle (SIDL) | Process is created by a call to fork,
vfork, or exec;
can be scheduled to run. |
| run (SRUN) | Process is on a run queue, available to execute
in either kernel or user mode. |
| stopped (SSTOP) | Executing process is stopped by a signal or
parent process |
| sleep (SSLEEP) | Process is not executing; may be waiting for
resources |
| zombie (SZOMB) | Having exited, the process no longer exists,
but leaves behind for the parent process some record of its execution. |
The following figure demonstrates something of the transitions
between process states.
When a program starts up a process, the kernel allocates a
structure for it from the process table. The process is now in idle
state, waiting for system resources. Once it acquires the resource,
the process is linked onto a run queue and made runnable. When
the process acquires a time-slice, it runs, switching as necessary
between kernel mode and user mode. If a running process receives
a SIGSTOP signal (as with control-Z in vi) or is being traced, it
enters a stop state. On receiving a SIGCONT signal, the process
returns to a run queue (in-core, runnable). If a running process
must wait for a resource (such as a semaphore or completion of I/O),
the process goes on a sleep queue (sleep state) until getting the
resource, at which time the process wakes up and is put on a run
queue (in-core, runnable). A sleeping process might also be swapped
out, in which case, when it receives its resource (or wakeup signal)
the process might be made runnable, but remain swapped out. The
process is swapped in and is put on a run queue. Once a process
ends, it exits into a zombie state.
![[Process states and transitions]](img/gfx4.gif)