UNIX Process Management

1
UNIX Process Management

• Chapter 3 - Section 4

2
System V Execution

• Most of the OS executes within the context of the
  user process, as in Figure 3.15 (b)
• Dual mode execution protects the OS routines from
  the user process code.
• The user process executes in user mode when it
  executes its own code, in kernel mode when it
  executes OS code.

3
UNIX SVR4 Process Management

• UNIX has two kinds of processes
• System processes
• run in kernel mode and perform functions that
  benefit the system as a whole (memory allocation,
  process swapping...)
• User processes
• run in user mode when executing user program code
• run in kernel mode when responding to system
  calls, traps, and interrupts

4
UNIX Process State Diagram
5
UNIX 9-State Model

• UNIX has two run states user and kernel.
• The UNIX sleep states (asleep in memory, asleep
  swapped) correspond to our blocked and blocked
  suspended states.
• The Preempted and Ready states correspond to our
  Ready state
• Zombie state is similar to the Exit state.

6
UNIX Run States

• The Kernel Run state is entered whenever there is
  a mode switch from user to kernel mode.
• Traditional UNIX systems did not preempt
  processes running in kernel mode, in order to
  protect system data structures.
• Drawback bad for real time systems

7
UNIX Ready States

• Ready to Run and Preempted states are
  essentially the same.
• Processes enter the Ready state when they are
  first created or when they unblock.
• Processes enter the Preempted state when the
  kernel preempts them due to presence of higher
  priority process.

8
UNIX Process Creation

• fork() is the UNIX system call to create a new
  process. (see p. 148 for steps)
• The new process (the child) is an exact copy of
  the parent, so there are now two virtually
  identical processes.
• exec() system call can be used to overlay code
  segment in child with a new program, so parent
  and child are different.

9
UNIX Process Creation

• When fork() complete, the OS can
• return control to parent process (usual result)
• schedule the child process
• schedule an entirely different process
• fork() returns 0 to the child, childs pid to the
  parent

10
Creating a New UNIX Process

• int pid
• pid fork()
• if (pid 0) / parent and child return here /
• / child process executes here /
• exit(0)
• else if (pid gt 0)
• / parent process executes here /
• wait() / if parent wants to wait for child
  to finish /
• else / pid lt0 indicates an error /