Week 11 - July 22-26

Introduction

This week we study threads and mutexes. Threads allow us to perform parallel programming within a process. Mutexes provide us with a synchronization mechanism to protect shared resources used by multiple threads. We will also look at server design.

Videos

Quiz

• Quiz 9 will be on threads. Study chapter 29 and 30 of The Linux Programming Interface.

Lecture Material

• Threads - Chapter 29 of The Linux Programming Interface.
• Thread synchronization - Chapter 30 of The Linux Programming Interface.
• Sockets and the Implementation of a World Wide Messaging System.
• What’s the Diff: Programs, Processes, and Threads.

Labs

• Lab 8 - client/server with messaging.
• Lab 9 - Server with Multiple Clients and a Receive Thread with Mutexing.

Assignment

Introduction to Assignment 2.

Sample Code

Threads and Mutexes

• A simple introduction to threads can be found in counter.cpp. This program simply starts off 5 threads with a 1 second delay between each. Each thread counts 10 seconds.
  Note that in the Makefile a link to the pthread library is made (LIBS=-lpthread). This is required wherever threads are used in the code.
• For a good discussion of processes vs threads, see the article What’s the Diff: Programs, Processes, and Threads.
• This week we study threads and mutexes. Mutexing is to ensure our threads are synchronized and operations are atomic. Atomic operations are the foundation on which other synchronization methods are built, they provide instructions that execute atomically, without interruption. Atomic operators are indivisible instructions.
• Code that demonstrates the need for mutexes can be found in counterNoMutex.cpp, counterMutex.cpp, and Makefile.
• In synchronizing threads with mutexes, one has to avoid thread deadlock. Deadlock is a situation where a set of processes are blocked because each process is holding a resource and waiting for another resource acquired by some other process.
• Code that demonstrates deadlock can be found in deadlock.cpp and Makefile.
• Code that demonstrates stream sockets with a separate receive thread for socket reads can be found in client1.cpp, server.cpp, and Makefile. Note that common resources are mutexed.
• Code that demonstrates INET sockets with a separate receive thread for socket reads can be found in msgClient.cpp, msgServer.cpp, and Makefile. Note that common resources are mutexed.
• A server can act as a message pump or dispatcher if it accepts messages from a number of clients and redirects these messages to a number of clients. A client creates a message with the source client information and destination client information embedded in the messsage. The client sends this message to a server which redirects it to the destination client. Code that demonstrates this message pump or dispatcher using INET sockets with a separate receive thread for socket reads can be found in msgClient.cpp, msgPump.cpp, startClient.sh, stopClient.sh, and Makefile. Note that common resources are mutexed.
  For the rationale behind such a central server, see the first six slides of Sockets and the Implementation of a World Wide Messaging System.