03

Processes Overview

• Chapters 3-6 in CS 4410 Operating Systems by R. Agarwal et al. discuss processes in operating systems.

Definitions

• Program: Contains code and data specified in a programming language, typically stored in a file on disk.

• Process: Occurs when a program is run; it is an instance of a program in execution, and multiple processes of the same program can run concurrently.

Executables

• An executable is a file with:

  • CPU instructions (executable code)

  • Data manipulated by these instructions

• Created through the compilation of a program and linked with necessary libraries.

Process Abstraction

• A process runs on:

  • Address Space: Memory and execution context (registers including Program Counter (PC) and Stack Pointer (SP))

  • Environment: Clocks, files, networks, manipulated through system calls.

  • Good abstractions should hide implementation details, provide easy interfaces, be instantiated multiple times, and be efficient.

Lifecycle of a Process vs Program

• A program is passive (just code and data).

• A process is active (it changes data, registers, files).

• Same program can yield multiple processes (e.g., ./program &).

Process Memory Layout

• Memory is organized in segments:

  • Text Segment: Holds executable code and constants.

  • Data Segment: Contains global variables.

  • Heap: Used for dynamic memory allocation.

  • Stack: Used for function calls and local variables.

Process Stack and Heap

• Stack: Manages function calls, maintains local variables, return addresses, and temporary data.

• Heap: Used with functions related to dynamic memory allocation to manage memory.

System Calls

• Processes interact with the OS kernel through system calls to perform tasks such as reading input, writing to screens, creating processes, and managing files.

• System calls have a skinny interface which enhances portability and security.

Process Creation

• In UNIX, process creation involves:

  • fork(): Creates a new child process that is a clone of the parent process.

  • exec(): Loads a new program into the address space of a process.

• Process control block (PCB) contains information such as process ID, user ID, and the current state of the process.

Threading

• Threads are lightweight processes that share the same memory space and can run concurrently.

• Two types of threads:

  • Kernel Threads: Managed by the operating system; can block calls.

  • User Threads: Managed entirely in user space, require less overhead during context switching.

Thread Management API

• Example functions for thread management include:

  • thread_create(func, arg): Creates a new thread.

  • thread_yield(): Yields execution of the CPU to allow other threads to run.

  • thread_exit(): Finishes the execution of the thread.

Shell Interface

• A shell is an interpreter that allows users to create and manage processes through executing commands.

• Environment variables hold information about the process environment and include paths to executables and other system settings.

Understanding Processes and Threads

• Processes represent an execution environment whereas threads represent separate execution paths within the same process's memory space.

• Proper management of threads can provide advantages in responsiveness and resource sharing, important for applications such as web servers.

Communication and Control in Processes

• Various system calls allow processes to communicate with the OS and manage their execution states, helping in creating more sophisticated and responsive applications.