20180913
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Processes
=================================

What is a process?
A running program, includes its machine state

What's part of a process's machine state?
Process's state
Open files
Registers
+ program counter
+ stack pointer
+ frame pointer
Address space - the memory for the process

With time sharing (multiprogramming), the OS switches between processes every 10-100 milliseconds
Pseudo-parallelism
True parallelism requires a multi-processor system

What is the API in the OS for a process:
-----------
Create
Destroy
Wait
Control
+ Suspend
+ Kill
+ Etc.
Status

Process Creation
~~~~~~~~~~~~~~~~~~~~~~
Processes are created when:
1) User requested a new process (opened an app)
2) A running process wants help (e.g., using fork())
3) System initialization
4) Starting a batch job (only on mainframes)

OS steps to run a programs
-----------
Load (from disk) the code and any static data (initialized variables) into memory
Allocate memory for the stack
Initialize the stack (with argc and argv)
Allocate some memory for the heap
Setup file descriptors (stdin, stdout, stderr)
Start executing main


Process Hierarchies
-----------
UNIX: init starts running after booting, a new process for each terminal, building a process tree
Windows: No process hierarchy, but the parent process is given a special token (handle) to control the child process

Process Termination
~~~~~~~~~~~~~~~~~~~~~~
Processes are terminated when:
1) Normal completion
   exit() or ExitProcess()
2) Error
3) Fatal error (involuntary)
4) Killed (involuntary)
   kill() or TerminateProcess()

Process States
~~~~~~~~~~~~~~~~~~~~~~

Main process states
Blocked
Running
Ready

Possible process state transitions
Exercise

Implementation of Processes
~~~~~~~~~~~~~~~~~~~~~~

Process Table
-----------
OS tracks processes in the process table (aka Process Control Block (PCB))
Stores a process's:
+ program counter
+ stack pointer
+ memory allocation
+ open files
+ scheduling information
+ etc.


20180918
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Process API
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fork()
~~~~~~~~~~~~~~~~~~~~~~
UNIX:
-----------
Makes a clone of the current process (with the same memory image, environment strings and open files)
Memory is mapped so that a copy only happens when the child writes to (changes) the memory
The child generally calls exec() and replaces the memory
2 steps: needed fro redirecting standard output and error

Windows
-----------
1 step: CreateProcess() (so we have a different address space to begin with)

man fork

Example: fork-example-02.c


Possible orderings:

1)
[pid: 33070] Original process
[pid: 33070] Parent created process with PID 33071
[pid: 33070] Final statement
[pid: 33071] Hello, world (from the child process)!
[pid: 33071] Final statement

2)
[pid: 33070] Original process
[pid: 33071] Hello, world (from the child process)!
[pid: 33071] Final statement
[pid: 33070] Parent created process with PID 33071
[pid: 33070] Final statement

3)
[pid: 33070] Original process
[pid: 33071] Hello, world (from the child process)!
[pid: 33070] Parent created process with PID 33071
[pid: 33071] Final statement
[pid: 33070] Final statement

4)
[pid: 33070] Original process
[pid: 33071] Hello, world (from the child process)!
[pid: 33070] Parent created process with PID 33071
[pid: 33070] Final statement
[pid: 33071] Final statement

5)
[pid: 33070] Original process
[pid: 33070] Parent created process with PID 33071
[pid: 33071] Hello, world (from the child process)!
[pid: 33070] Final statement
[pid: 33071] Final statement

6)
[pid: 33070] Original process
[pid: 33070] Parent created process with PID 33071
[pid: 33071] Hello, world (from the child process)!
[pid: 33071] Final statement
[pid: 33070] Final statement


wait()
~~~~~~~~~~~~~~~~~~~~~~
Suspends execution until a child process completes


exec()
~~~~~~~~~~~~~~~~~~~~~~
Overwrites the current process

Example: fork-exec-example.c


UNIX Commands
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