Process¶

Program is passive, a binary code
Process is active, a program that is executing in memory

Process

A process includes:

Code (machine code)
Data section
Stack
Heap
Program counter
Associated resources

Threads¶

Threads belong to the same process share

Every processes are called threads on linux!!

Process States¶

States

New: process is being created
Ready: the process is in the memory waiting to be assigned to a processor
Running: executing
Waiting: waiting for events to occur
Terminated: terminated

Process Control Block (PCV)¶

Created by OS

PCB

Process state
PC
CPU registers
Scheduling info
Memory management info
IO status
Accounting info

Context switch¶

Context switch

把現在在執行的process 0的state存在PCB0，然後把另一個需要被執行的process 1的PCB1讀出來

Problem

在Context switch的時候CPU是idle的，而且因為要分享資源所以是必須的

Solution

提高memory speed
加更多register
用一個instruction把所有的register存好
multiple sets of registers

Note

Context switch is slow

Process Creation¶

Maybe:

Parent and child share all resources
Child process shares subset of parent's resources
Parent and child share no resources

Execution¶

Parent and children execute concurrently
Parent waits until children terminate

Address space¶

Child duplicate of parent, communication via sharing variables
Child has a program loaded into it, communication via message passing

UNIX/Linux Process Creation¶

fork¶

Create a new child process
Child duplicates the address space of its parent
Child and parent execute concurrently after fork
Child: return value of fork is 0
Parent: return value of fork is PID of the child

execlp¶

Load a new binary file, destroying the old code

wait¶

Parent waits for one of its child process to complete

Example

#include <stdio.h>
#include <unistd.h>

void main() {
int pid = fork();
if (pid == 0)
    printf("Child pid = %d\n", pid);
else
    printf("Parent pid = %d\n", pid);
return;
}

Process termination¶

Terminates when the last statement is executed or exit() is called
All resources of process includeing physical & virtual memory, open files, IO buffers are deallocated by the OS
Parent can terminate execution of children processes by specifying its pid

Killing parent kills all its children