Session 02: The Operating System Interface

This session is about system calls: how a user-space program crosses into the kernel, and how the C library's convenience functions are built on top of it.

• What a system call is, and the x86-64 Linux calling convention.
• Writing raw syscall wrappers over a single generic my_syscall().
• Building libc-style helpers (my_puts, my_sleep, my_time) on your own wrappers.
• Confirming which calls a program really makes, with strace.

Learning objectives

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

• Explain what a system call is and how a user-space program reaches the kernel through it.
• Describe the x86-64 Linux system call convention: which registers carry the number, the arguments and the result.
• Implement raw syscall wrappers (my_write, my_getpid, my_nanosleep) on top of a generic my_syscall().
• Build higher-level helpers (my_puts, my_sleep, my_time) on top of your own wrappers.
• Read the kernel's return-value convention, including how a single register carries both a result and an error.
• Use strace to confirm which system calls a program actually performs.

Prerequisites and required tools

• Session 01 concepts: the software stack, and buffered versus unbuffered output.
• Basic familiarity with C pointers.
• A Linux environment on x86-64, with gcc, make and strace installed. The inline assembly in this session is specific to x86-64 Linux and will not work elsewhere.
• No prior assembly experience is required. The inline assembly is given to you and explained; you never have to write any.

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 02-os-interface.zip, then unzip it and change into the directory it creates:

wget https://github.com/cs-pub-ro/operating-systems/raw/lab-archives/02-os-interface.zip
unzip 02-os-interface.zip
cd 02-os-interface/

Work inside that directory for the rest of the session.

Task order

The demo comes first, solved together with the teaching assistant. 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-puts-write	Demo	Build the three-layer pattern: my_puts over my_write over my_syscall.
2	01-getpid	Core	Implement the simplest possible syscall wrapper.
3	02-nanosleep	Core	Implement a syscall that takes pointers, and a libc-style sleep() on top of it.
4	bonus-clock_gettime	Bonus	Implement clock_gettime and a libc-style time(), across a header and an implementation file.
5	bonus-printf	Bonus	Implement _putchar so a portable printf implementation writes through the write syscall.

The demo produces the my_syscall() function that every later task reuses unchanged, so do not skip it.

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.

The bonus-printf/utils/printf/ directory contains a third-party portable printf implementation (mpaland/printf, MIT licensed). It is support code for that exercise, not an exercise in itself.