Session 06: Processes and Threads

This session is about getting the operating system to run things for you: a second program, started from your own, and a second flow of control inside your own program.

• Starting a new process with fork() and replacing its program with exec().
• Waiting for a process with waitpid(), and reading how it ended.
• Starting threads with pthread_create() and waiting for them with pthread_join().
• Splitting a piece of work between threads so that no two of them touch the same memory.

Learning objectives

By the end of this session you should be able to:

• Start a program from your own with fork() and execvp(), and explain why it takes two calls rather than one.
• Wait for a child process with waitpid(), take its exit status apart, and act on it.
• Write a shell that runs commands with arguments and chains them with ;, && and ||.
• Start threads with pthread_create(), pass each one its own work through a struct, and collect what it returns with pthread_join().
• Split a memory region, or an array of tasks, between threads so that each one works on its own part, with nothing shared for writing and therefore nothing to lock.
• Say what a process and a thread each have of their own, and what they share.

Prerequisites and required tools

• Session 03 concepts: what mmap() does with a file. All file handling in this session is written for you; you call it.
• Comfort with C pointers, structs and arrays of strings (argv).
• A Linux environment with gcc, make, strace and ps installed.

Check that your system has all it needs for the lab, by downloading and running the check-prerequisites.sh script:

wget http://raw.githubusercontent.com/cs-pub-ro/operating-systems/refs/heads/main/scripts/check-prerequisites.sh
chmod a+x check-prerequisites.sh
./check-prerequisites.sh

The script installs nothing. It reports what is missing and prints the command that installs it on your distribution.

If something is missing, be sure to install and configure it.

Getting the lab archive

Download 06-processes-threads.zip, then unzip it and change into the directory it creates:

wget https://github.com/cs-pub-ro/operating-systems/raw/lab-archives/06-processes-threads.zip
unzip 06-processes-threads.zip
cd 06-processes-threads/

Work inside that directory for the rest of the session.

Task order

The two demos come first, solved together with the teaching assistant: one for processes, one for threads. The core exercises are then solved individually or in teams, in the numeric order shown. Bonus exercises are optional: start them if you finish the core exercises, or take them home.

Order	Task	Type	Objective
1	demo-minimal-shell	Demo	Run a command as a new process: fork(), exec(), waitpid().
2	demo-substring-search	Demo	Search a mapped file with several threads, each on its own part of it.
3	01-shell-chaining	Core	Commands with arguments, and ;, &&, \|\| driven by the exit status.
4	02-aggregate-matches	Core	Threads that return a result, and a main thread that adds them up.
5	03-file-encryptor	Core	Threads that each take a batch of files.
6	bonus-incomplete-batches	Bonus	57 files in batches of 10: the last batch is smaller.
7	bonus-background-processes	Bonus	&: run a command without waiting, and collect it later.

The two demos each start a thread of their own through the session. The shell demo becomes 01-shell-chaining and then bonus-background-processes; the search demo becomes 02-aggregate-matches, and 03-file-encryptor applies the same split to an array of files instead of a region of bytes. bonus-incomplete-batches starts from your finished encryptor.c of 03-file-encryptor, and bonus-background-processes from your finished shell.c of 01-shell-chaining, so do those core exercises first.

One rule holds for every thread in this session: each thread works on memory that no other thread touches, and the results are read only after the thread has been joined. That is why no mutex appears anywhere; sharing memory for writing, and the tools for it, are the subject of session 08.

Each exercise directory has a README.md with the task itself. Where there is also a FURTHER.md, it holds optional extensions and questions to dig into once the task is done.