Processes Creation and Threads Creation of Processes Creation

Processes Creation and Threads

Creation of Processes • Creation is very specific to an operating system. • OS needs to: – Reserve memory for process – Add entry in process table – Load the application code (optional? ) • All systems start from a single process.

Process Creation Calc. exe My. App. exe Create. Process(“calc”)

Linux Process • Based on a system call named fork(). • Syntax: – pid_t fork(); • Duplicates the process calling this function. • The new process is called the child, the original process is called the parent.

Linux Processes • The child process will see 0 returned from the fork() call, the parent will see the process id of the new process. • Often the new process will want to execute other command rather than just being a duplicate. Uses exec() function.

Example Unix call Main() { do_something(); if (fork() == 0) { // this is the child process execl(“xcalculator”); } // the child process will never make it here! do_morestuff(); }

What Happens Here? main() { while (1) fork(); }

My. App fork() exec() Timeline xcalculator

Terminating a Process • Memory needs to be released • Entry needs to be removed from process table. • Usually the process asks to terminate itself! • Program calls Exit. Process() or exit(). • If your program doesn’t call this, the library (Java, C run time) will call it.

Terminating Other Processes • Can one process ask another to stop? – If the first process has enough permission then yes. – OS will not deliver the message otherwise. • The process sends a special signal to the other process. – Terminate. Process (Windows) – kill (Linux)

Other Techniques • The OS will remove any process that has caused a fault. – “This application has performed an illegal operating and will be shut down. ”

Design of an E-mail Client • We design an e-mail client like Outlook. • Problem: – Our e-mail client will notify the user when new mail comes in (even if they are creating a new message).

Attempt #1 Forever() { wait for a key add key to the new message if (new mail) { play notification } }

Attempt #2 Forever() { wait for a key but no longer than 1 second if (key pressed) { add character to message } if (new mail) { play notification } }

Divide The Problem! // checking for new mail Forever() { sleep for an amount of time (or better yet block) if (new mail) play notification } // Handling keyboard Forever() { wait for key (yes, block waiting for the key) add character to mail message }

Two separate programs? • If we could run both of these loops at the same time, our problem would be easier! • Two processes are possible but sharing information between them is not convenient.

Thread • A thread is a sequence of instructions. • Each process contains at least 1 thread. • Many OS allow for multiple threads within a single process. • Often called a lightweight process. • Provides multi-tasking within a single process.

Threads • Threads have access to all variables that are within the same program. • If you have a global, you can read it or change it. • No security between threads of a single process – Easy to share information – Easy to make mistakes!

Reasons for Threads • Can simplify problems • On multi-core processors two threads can run simultaneously. – Programs need to take advantage of this