Fundamentals of Application Programming: OS Concepts, System Calls, Processes, Threads, and Coroutines

This article, part of the "Application Programming Basics" series by the 360OpsDev team, provides a comprehensive overview of foundational concepts every programmer should understand.

1. Operating System Introduction

It begins with a diagram of the Unix system architecture, explaining the layers from the kernel (hardware control) to system calls, shell/library functions, and user applications.

The architecture is broken down into kernel, system calls, shell/library functions, and applications.

2. System Calls and Library Functions

System calls are the kernel's programming interfaces, while library functions provide higher‑level, user‑friendly wrappers (e.g., printf wraps the write system call).

3. User Mode and Kernel Mode

Code runs in user mode until a system call transfers execution to kernel mode, where the kernel performs privileged operations.

4. How a Program Executes

Using a classic hello.c example, the article walks through compilation, linking, loading into memory (via DMA), and execution, illustrating each step with diagrams.

5. Processes, Threads, and Coroutines

The memory layout of a program (text, initialized data, BSS, heap, stack) is shown, followed by explanations of processes (independent execution contexts), threads (OS‑scheduled lightweight execution flows), and coroutines (user‑mode cooperative scheduling).

Threads share most memory but have separate stacks; coroutines run within a single thread and cannot exploit multiple CPU cores directly.

6. Parent‑Child Process Sharing

Diagrams illustrate how file descriptors and kernel structures are shared between a parent and its forked child, a key technique for smooth service restarts.

Conclusion

The author emphasizes that mastering these fundamentals enables developers to reason about many problems independently and invites readers to provide feedback for continuous improvement.