The past week was so exciting that I skipped a couple of meals and had sleepless nights. Apparently, I am late to the party: there have been many significent moments last year or ealier that should have marked the beginning of the AI era for human being (much more appropriate than last week), while for me, it was not until last week that I got the feeling of advencing into AI era.

I learnt a plenty of things last week in order to get to the speed. This post notes down those AI techniques which excite me.

When coding, I often need to search for the references of a variable or a function across multiple files. Usually I enter command mode of vim and do a grep command, memorize the search results and open the files having hits. This common pattern could be dramatically accelerated by a user-defined vim command:
:command Gitgrep :lex system("git grep -n " . shellescape(getreg('/'))).

TL;DR The variable MAKEOVERRIDES passes the variables defined in the make command line to sub-makes (even those make invoked indirectly). Emptying MAKEOVERRIDES can prevent sub-makes from picking up variables unexpected.

It is usually easier to write and debug Python scripts than do it in C/C++, while for heavy computation tasks, C or C++ programs can run much faster. A common strategy to get the best of both worlds is writing the computation kernels in C/C++ and binding Python APIs to invoke the computation kernels in Python scripts. Python C API exists exactly for the purpose of binding Python with C/C++.

I learnt Python C API recently, and this post records my practice of creating a Python package, txt2qr, that uses C library libqrencode. Txt2QR generates the QR code (in SVG format) of a given text, and is easily integrated into a Django app.

About a year ago, I started a side project for fun as well as the need to keep track of my daily expenses in my own way. The outcome, Abacus, is an expenses bookkeeping web service (still under active development, but in good shape), which I have been using for months.

Abacus uses Django as the website backend engine, providing webpages for users to record, view and manage their daily expenses transactions in groups. Additionally, Abacus also has a Telegram bot interface to accomplish the same bookkeeping actions in Telegram chat. Abacus is meant to be deployed on a home PC as a self-hosted web service for personal use.

Hugo is another popular static site generator that runs faster than Jekyll. Recently, I migrated my blog from Github Page + Jekyll Now to Cloudflare Pages + Hugo.

I got to know Linux From Scratch soon after I bought my desktop machine. My first attampt failed at making gcc to setup the “cross-compile” environment. About four years later I picked it up and finally went through my first pass of building LFS. My destop machine has been running LFS happily since then. Two experiences are briefly note down: 1) break dependency circle with building twice; 2) reconfig kernel and build with additional modules.

It is convenient to index data stored in map, and hash table by name, and the theoretical time complexity is as low as O(logn) or O(1). Nevertheless, the overhead to convert a string name into the key to index the map or hash table structure could hurt the performance sometimes. This post demonstrates a technique do the hash computation on constant strings earlier at compile time.

Polymorphism in C++ means a function or object of the same identity/name behaves differently under different scenarios. A common example of polymorphism is a derived class could override the method definition in the base class if it is marked virtual and a base class pointer would invoke the proper definition according to which class instance it is actually pointing to.

Polymorphism reduces trivial programming efforts in some cases and is very useful. Template method for generic programming is another style of polymorphism done at compile time to avoid unnecessary code duplication and eases the burden of programmers (see my earlier post series). However, C++ forbids template methods to be marked as virtual. So could somehow we have it all?

Since C++11, the template parameter list could accept non-fixed number of arguments, (parameter pack), which is similar to variadic arguments for function arguments. Those are usually handled with recursion.

The following excellent articles in the past two days taught me a lot about how the loader does relocation for ELF executables and how thread local storage is handled:

• Load-time relocation of shared libraries
• Position Independent Code (PIC) in shared libraries
• Position Independent Code (PIC) in shared libraries on x64
• A Deep dive into (implicit) Thread Local Storage
• All about thread-local storage

The Ext4 file system on our server is corrupted, causing the folder having all my latest simulator code and experimental data disappeared.

This post tells how I get the data back and fix the file system.

A pitfall of using C++ class empty constructor to instantiate an object. It is easy to make the mistake and not easy to realize without knowing the reason behind.

This post has more container types from STL (Standard Template Library), including map, unordered_map, set, unordered_set. Comparison examples are made to show their differences.

GCC has its version written in the .comment section of the executable binaries as shown by commands:

objdump -s --section .comment /path/to/binary
# or pretty print
readelf -p .comment /path/to/binary

This post introduces how apply Linux commands to selective targets.

A server machine failed at upgrading from Ubuntu14.04 to Ubuntu16.04, and many packages are left in half-installed status, even apt is not working. I struggled several days to fix those broken packages, and this post documents some tricks I learnt.

I attended MPI Foundation, a nice training given by TACC staff, at the end of March. This post briefly documents the basic MPI programming knowledge I learnt (😓 should have been written long ago, but did not even start until the day before follow-up training).

Jekyll Gist is a plugin to embed a gist into the static pages (Jekyll-Gist Repository).

It is enabled at the backend of GitHub Page by default, and can be used as a liquid tag as follows:

{% gist 46b4401513cdbd8b2390172344157b7a UnionFind.cpp %}

The long hash code can be found in the URL of a specific gist repo. The Jekyll backend would render it to be a JavaScript tag. In addition, gist-it.appspot.com is a website providing JavaScript to embed files from a public github repository like a gist.

This post introduces how Linux command find is used to find file or directory with certain attributes.

This post introduces how to use escape code to get colorful output in a terminal.

4b825dc642cb6eb9a060e54bf8d69288fbee4904 is the hash code to index the “empty tree” (before your first commit) in git version control system.

Upon the strong suggestion from a senior student, I recently studied and practiced a very advanced syntax sugar, Lambda expressions, introduced in C++11. This post talks about the basic of lambda expressions.

Strongly connected components refer to connected components in a directed graph, and is useful to simplify a directed graph, for instance, merging all vertices in the same strongly connected components to be a big vertex, could produce a Directed Acyclic Graph (DAG) with less number of vertices. This post documents two well-known algorithms (i.e., Kosaraju’s Algorithm and Tarjan’s Algorithm) that solve the strongly connected component problem.

This post contains three shortcuts of bash.

Topological sorting is an operation performed on Directed Acyclic Graph (DAG) to generate a order of all vertices, so that vertex v would not appear before vertex u if there exists a path from vertex u to vertex v.

C++ introduced a built-in method to dynamically manage memory, as an alternative to library based malloc, realloc, free functions in C. This is achieved by two keywords, new and delete.

Weakly connected components refer to connected components in an undirected graph, also known as friend circle problem. This post documents three common solutions to figure out the number of connected components in graphs.

TACC Symposium at J.J Pickle Research Campus, Austin TX.

Tip 102:
The terminal text can be cleared by typing Ctrl+L. It is the same as the command clear.

This post contains a tip from TACC helping debug the shell script.

This post talks about how to get patch work with git.

After switched to use Linux, I use vim-latex to extand vim for LaTeX editing. However, sometimes I would encounter errors like package not found. I thought that is because vim-latex supports limited packages (under ~/.vim/ftplugin/latex-suite/packages/), but actually vim-latex is just the plugin to turn vim into a LaTeX editor, while the complier behind it is something else, for example TeX Live.
TeX Live has its own package manager called tlmgr. This post documents some issues I had to install absent package for TeX Live.

Message Passing Interface (MPI) is a standard for parallel computing, which means it could be the dependency of many programs developed for cluster. Determining the version of MPI is helpful to solve the dependency issues.

Under some circumstances, I would like to launch the process, then let other programs attach to it. No matter how fast can I type, it is impossible for me to bind the processes from the very beginning. The solution is let the script to launch the process but suspend it immediately.

#!/bin/bash
$@ &
PID=$!
kill -STOP $PID
echo $PID
wait $PID

The script earns enough time for you to do your operations in another terminal, then to resume the process:

kill -CONT $PID

Note: need to replace $PID with the number printed in the first terminal.

With huge files, tasks that as simple as removing first or last several lines could become hard. If you try to open a huge file with vim, the chance is that the system gets stuck struggling to load the huge file into memory. This post has several alternative approaches to delete lines from huge files.

I had a post, Container Types in C++, where I introduced a bit about one of the C++ containters std::map.
Yesterday I ran into an interesting use case of map, namely, I wanted to use map with customized key, bi-match unsigned integer pair. This post talks about the way to customize key for std::map, and issues and solution to get a bi-match unsigned integer pair work correctly as the key.

To invoke an editor for a long command, try ctrl-x e (that is holding control key press x then e).

This post contains four tips from TACC making the shell command cd more versatile.

In the post A Bite of Template in C++ (1), I mentioned a constraint for template member functions in class that explicit specialization is not allowed. However, I realized there are more constraints on function templates later when I wanted to make two template varients with slightly difference on only one member function. I tried specializing member function template, as well as inheriting a template member function from base class. Both of them failed due to some constraints. Briefly, the constraints are

1. partial specialization is not allowed for function template,
2. using declaration plus template disambiguator is not the right way to inherit a template member function from a template class.

Tips about some special variables used in makefile: automatic variables, $@, $<, $^; wildcard placeholder % in targets and dependencies and $* in recipe; and .DEFAULT_GOAL.

Two useful tips from TACC tips regarding compiler macro expanding.

Tip 120:
Want to know what MACROS are predefined by the GCC and Intel compilers? Do

gcc -dM -E - < /dev/null > gcc_defined_macros.txt
icc -dM -E - < /dev/null > icc_defined_macros.txt

The flag -E in the tip is telling the compiler to expand macros.

Tip 69:
Having trouble figuring out how a macro is being expanded? Try pasting the line into a terminal and adding a -E. For example try changing mpicc -DFOO -DBAR -c foo.c into mpicc -DFOO -DBAR -E foo.c > foo.i and look at the foo.i file.

I encountered two issues when using C++ template. One is about why sometimes typename has to be prefixed to declarations, and another is how to access the types in the parameter list from outside of the template. This post has some ideas for those two questions, as well as some discussions about nested templates.

The bash-completion package does not handle command make, while we can extend the completion support for make by adding bash configurations with keyword complete.

The keyword const is a qualifier used to claim the values of variables are constant. It is useful, because compiler recognizes it and help programmers to avoid some bugs made unconsciously. However, I am always confused about where to put the const, especially using it with special types, for example pointer. This post is about declarations with const qualifier (also applicable to volatile of course).

From recent C++ programming, I acquired two features of C++ template, which are new to me. Namely, not only types are allowed to be template arguments, and partial or explicit specialization of template.

The command line prompt in Linux could be customized to user’s need. It is achieved by modifying a series of environment variables, such as PS1, PROMPT_COMMAND and PS2.

Device node is a file-like abstraction in Linux for user to interact with devices. It could map the operations (read, write, fasync etc.) performed on the file to the associated device. Otherwise, user application needs to be executed with privilege to do memory map before using the device, and programer needs to know the physical address.

After reinstall OS, the SD card reader on my laptop stops work again. Because this time I use Linux Mint, which is based on higher version kernel, I encountered more troubles to drive the SD card reader.

Three steps, five errors and solutions are recorded.

Unlike inclusive cache, it is harder to implement an exclusive policy on cache. This post introduces two schemes for exclusive cache.

WayBack Machine is one of the services offered by Internet Archive, which documents all kinds of data on the Internet, such as web pages, books, videos, musics and so on.

WayBack Machine might help you find a website that is no longer up, or show you the evolution of it.

DFS (Deepth First Search) and BFS (Breadth First Search) are two approaches to traverse tree. It is relatively easy to implement BFS in an iterative manner (with the help of queue). While it is a bit challenging to do DFS without reursion.

This post compares the differences between built-in array, linked list and several STL (Standard Template Library) containers (array, vector, list, stack, queue, map). Some examples for their basic usage are included as well.

Reuse distance calculation is a problem related to cache design and evaluation. With a sequence of address references, reuse distance is defined as the number of unique addresses between two references to the same address. For example, in “A B C A C B”, the first B has an infinite reuse distance since it has never been rereferenced, while the second B has a reuse distance of 2.

Several algorithms were discovered long time ago. The following three are included in this post:

• Stack approach - Mattson et al. in 1970;
• Bit Vector approach - Bennett & Kruskal in 1975;
• AVL Tree approach - Olken in 1981;

Technical problems about C/C++ programming and microarchitecture design.

David Black-Schaffer from Uppsala Univerity made an impressing presentation about their TLC (Tag-Less Cache) work.

TLC is a novel cache hierarchy without tag array. Instead, it stores metadata separately to avoid unnecessary cache access.

Mainly about my project experiences.

After reinstall OS, the SD card reader on my laptop stops work again. Because this time I use Linux Mint, which is based on higher version kernel, I encountered more troubles to drive the SD card reader.

Three steps, five errors and solutions are recorded.

Mainly about how CPU interacts with GPU.

Two tips, one to find processor information, another for detailed system configurations.
Updated with two more tips on January 23, 2018. One to find memory information, another for interrupt information.
Updated with one more tip from TACC on July 23, 2018. It gives HTML view.

Jekyll is a static site generator that’s perfect for GitHub hosted blogs (Jekyll Repository).

Jekyll Now makes it easier to create your Jekyll blog, by eliminating a lot of the up front setup.