Neutrino's Blog

A Detailed Guide to the Unix/Linux `top` Command

April 16, 2020
Liu, An-Chi 劉安齊

¶ TOP Overview

The top command is one of the most fundamental tools on Unix/Linux. It is similar to Windows Task Manager and lets you monitor the execution status of currently running programs. top is also one of the simplest ways to monitor a program: you can use it to observe how much memory and CPU a program consumes, along with many other details. There are many similar tools (such as htop and gtop) which are extended versions. If you’re interested, you can look them up, but for basic needs, top is more than sufficient.

Configure DNS on Linux/Unix from the Command Line

April 14, 2020
Liu, An-Chi 劉安齊

Here’s what happened: starting yesterday, a machine in my lab suddenly couldn’t access the Internet. I usually use that machine as a proxy, because sometimes I need an NCTU IP address for looking up papers. At first I thought Squid was broken, so I spent a long time tweaking the configuration—only to realize the issue didn’t seem to be Squid. Then I was shocked to find that I could SSH into the machine, but it couldn’t connect outbound.

2D Pattern Camera Calibration

April 6, 2020
Liu, An-Chi 劉安齊

A Concise Guide to Shell Internals and Implementation

January 18, 2020
Liu, An-Chi 劉安齊

¶ Shell Introduction

Whether you are using a local Unix machine, Windows, or connecting to a remote server, you will open a terminal (Terminal). After you log in, the initial screen is a shell, where developers can enter commands to run programs. In a sense, when people say “using the terminal,” they often mean “using the shell,” even though those terms are not originally identical. For more details about shells, see the English Wikipedia page “C shell”.

NCTU Observations and Reflections (1): First-Year Master's (Fall)

January 16, 2020
Liu, An-Chi 劉安齊

How I Became Interested in Computer Science

December 2, 2019
Liu, An-Chi 劉安齊

I am currently pursuing a graduate degree in computer science at NCTU. Deciding to go down the CS path was a long process for me. This story starts from the very beginning and describes how my interest in computer science gradually took shape. Everyone’s story is unique; I hope mine can offer some inspiration and lessons for others.

Red-Black Tree (RBT) Introduction

November 27, 2019
Liu, An-Chi 劉安齊

¶ Red-Black Tree Overview

A red-black tree is a special data structure that can automatically balance itself, similar to an AVL tree.

A red-black tree has the following properties:

Each node is either red or black
The root must be black
Every leaf (NIL) is black
If a node is red, its children must be black
Every path from the root to a leaf has the same number of black nodes

Following these rules ensures the tree stays balanced during insertions and deletions. A red-black tree guarantees that when the number of nodes is n, the height is at most 2log(n+1).

In practice, it is often used to implement sets and dictionaries. In C++, std::set and std::map are backed by red-black trees.

An Example of Signals with `sigaction` on Unix

November 24, 2019
Liu, An-Chi 劉安齊

There are many ways for processes to communicate on Unix. This post introduces a simple way to use signals. You can first read Beej’s introduction. As the name suggests, a signal is a notification sent and received by processes. For example, when you use a shell, Ctrl-C can interrupt a program because the shell catches the SIGINT signal triggered by Ctrl-C, recognizes it as an interrupt signal, and terminates the program.

To send signals, you can use sigaction() or signal(). I recommend sigaction because it’s newer. For a detailed comparison, see the Stack Overflow thread “What is the difference between sigaction and signal?”.

If you want to trigger a signal, you can use kill() or sigqueue(). The difference is that the latter is Linux-only, but it lets you attach additional information via siginfo. In addition, some system calls can trigger signals as well. For example, if you try to send() to a non-existent socket, you’ll get a SIGPIPE.

Using `mmap` to Create Shared Objects

November 20, 2019
Liu, An-Chi 劉安齊

When you have many processes and want to implement shared memory to handle shared data, you can use shared memory to build the solution. To create shared memory, you can use mmap or System V shmget. However, according to the Stack Overflow answer “(How to use shared memory with Linux in C)”, shmget is somewhat outdated, while mmap is newer and more flexible.

Shared memory allows us to create a region of memory that can be shared. mmap returns a pointer to that region, with type void *. If we want to put data into it, we can use memcpy to copy objects, strings, or anything else into the shared region. We can also cast the void * directly to an object pointer—this way we create a shared object, and different processes can access the object directly.

NTU Observations and Reflections (9): Senior Year (Spring) & Logging Out

June 26, 2019
Liu, An-Chi 劉安齊

This was my eighth semester at National Taiwan University (NTU), and also my last. For most people, senior spring is relaxed: they have already completed their credits and have no graduation pressure. But because I changed departments and kept taking courses in other departments, I still had not completed my credits by senior spring. Because of that, I still had 12 credits to take, and I still had to go to campus almost every day.

Overall, senior spring felt a bit absurd to me. It was as if I did not do anything particularly well, and there was not much I could be proud of. There were a few things worth being happy about, and a few setbacks that left a stronger impression.