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The best articles by Ciro Santilli Updated 2025-07-16
These are the best articles ever authored by Ciro Santilli, most of them in the format of Stack Overflow answers.
Ciro posts update about new articles on his Twitter accounts.
Some random generally less technical in-tree essays will be present at: Section "Essays by Ciro Santilli".
- Trended on Hacker News:
- CIA 2010 covert communication websites on 2023-06-11. 190 points, a mild success.
- x86 Bare Metal Examples on 2019-03-19. 513 points. The third time something related to that repo trends. Hacker news people really like that repo!
- again 2020-06-27 (archive). 200 points, repository traffic jumped from 25 daily unique visitors to 4.6k unique visitors on the day
- How to run a program without an operating system? on 2018-11-26 (archive). 394 points. Covers x86 and ARM
- ELF Hello World Tutorial on 2017-05-17 (archive). 334 points.
- x86 Paging Tutorial on 2017-03-02. Number 1 Google search result for "x86 Paging" in 2017-08. 142 points.
- x86 assembly
- What does "multicore" assembly language look like?
- What is the function of the push / pop instructions used on registers in x86 assembly? Going down to memory spills, register allocation and graph coloring.
- Linux kernel
- What do the flags in /proc/cpuinfo mean?
- How does kernel get an executable binary file running under linux?
- How to debug the Linux kernel with GDB and QEMU?
- Can the sys_execve() system call in the Linux kernel receive both absolute or relative paths?
- What is the difference between the kernel space and the user space?
- Is there any API for determining the physical address from virtual address in Linux?
- Why do people write the
#!/usr/bin/envpython shebang on the first line of a Python script? - How to solve "Kernel Panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)"?
- Single program Linux distro
- QEMU
- gcc and Binutils:
- How do linkers and address relocation works?
- What is incremental linking or partial linking?
- GOLD (
-fuse-ld=gold) linker vs the traditional GNU ld and LLVM ldd - What is the -fPIE option for position-independent executables in GCC and ld? Concrete examples by running program through GDB twice, and an assembly hello world with absolute vs PC relative load.
- How many GCC optimization levels are there?
- Why does GCC create a shared object instead of an executable binary according to file?
- C/C++: almost all of those fall into "disassemble all the things" category. Ciro also does "standards dissection" and "a new version of the standard is out" answers, but those are boring:
- What does "static" mean in a C program?
- In C++ source, what is the effect of
extern "C"? - Char array vs Char Pointer in C
- How to compile glibc from source and use it?
- When should
static_cast,dynamic_cast,const_castandreinterpret_castbe used? - What exactly is
std::atomicin C++?. This answer was originally more appropriately entitled "Let's disassemble some stuff", and got three downvotes, so Ciro changed it to a more professional title, and it started getting upvotes. People judge books by their covers. notmain.o 0000000000000000 0000000000000017 W MyTemplate<int>::f(int) main.o 0000000000000000 0000000000000017 W MyTemplate<int>::f(int)Code 1.. From: What is explicit template instantiation in C++ and when to use it?nmoutputs showing that objects are redefined multiple times across files if you don't use template instantiation properly
- IEEE 754
- What is difference between quiet NaN and signaling NaN?
- In Java, what does NaN mean?
Without subnormals: +---+---+-------+---------------+-------------------------------+ exponent | ? | 0 | 1 | 2 | 3 | +---+---+-------+---------------+-------------------------------+ | | | | | | v v v v v v ----------------------------------------------------------------- floats * **** * * * * * * * * * * * * ----------------------------------------------------------------- ^ ^ ^ ^ ^ ^ | | | | | | 0 | 2^-126 2^-125 2^-124 2^-123 | 2^-127 With subnormals: +-------+-------+---------------+-------------------------------+ exponent | 0 | 1 | 2 | 3 | +-------+-------+---------------+-------------------------------+ | | | | | v v v v v ----------------------------------------------------------------- floats * * * * * * * * * * * * * * * * * ----------------------------------------------------------------- ^ ^ ^ ^ ^ ^ | | | | | | 0 | 2^-126 2^-125 2^-124 2^-123 | 2^-127
- Computer science
- Algorithms
- Is it necessary for NP problems to be decision problems?
- Polynomial time and exponential time. Answered focusing on the definition of "exponential time".
- What is the smallest Turing machine where it is unknown if it halts or not?. Answer focusing on "blank tape" initial condition only. Large parts of it are summarizing the Busy Beaver Challenge, but some additions were made.
- Algorithms
- Git
| 0 | 4 | 8 | C | |-------------|--------------|-------------|----------------| 0 | DIRC | Version | File count | ctime ...| 0 | ... | mtime | device | 2 | inode | mode | UID | GID | 2 | File size | Entry SHA-1 ...| 4 | ... | Flags | Index SHA-1 ...| 4 | ... |tree {tree_sha} {parents} author {author_name} <{author_email}> {author_date_seconds} {author_date_timezone} committer {committer_name} <{committer_email}> {committer_date_seconds} {committer_date_timezone} {commit message}- How do I clone a subdirectory only of a Git repository?
- Python
- Web technology
- OpenGL
- Node.js
- Ruby on Rails
- POSIX
- What is POSIX? Huge classified overview of the most important things that POSIX specifies.
- Systems programming
- What do the terms "CPU bound" and "I/O bound" mean?
Figure 12. Plot of "real", "user" and "sys" mean times of the output of time for CPU-bound workload with 8 threads. Source. From: What do 'real', 'user' and 'sys' mean in the output of time?+--------+ +------------+ +------+ | device |>---------------->| function 0 |>----->| BAR0 | | | | | +------+ | |>------------+ | | | | | | | +------+ ... ... | | |>----->| BAR1 | | | | | | +------+ | |>--------+ | | | +--------+ | | ... ... ... | | | | | | | | +------+ | | | |>----->| BAR5 | | | +------------+ +------+ | | | | | | +------------+ +------+ | +--->| function 1 |>----->| BAR0 | | | | +------+ | | | | | | +------+ | | |>----->| BAR1 | | | | +------+ | | | | ... ... ... | | | | | | +------+ | | |>----->| BAR5 | | +------------+ +------+ | | | ... | | | +------------+ +------+ +------->| function 7 |>----->| BAR0 | | | +------+ | | | | +------+ | |>----->| BAR1 | | | +------+ | | ... ... ... | | | | +------+ | |>----->| BAR5 | +------------+ +------+Code 5.Logical struture PCIe device, functions and BARs. From: What is the Base Address Register (BAR) in PCIe?
- Electronics
- Raspberry Pi
Figure 13. . Image from answer to: How to hook up a Raspberry Pi via Ethernet to a laptop without a router? 
Figure 14. . Image from answer to: How to hook up a Raspberry Pi via Ethernet to a laptop without a router? 
Figure 15. . Image from answer to: How to emulate the Raspberry Pi 2 on QEMU? 
Figure 16. . Image from answer to: How to run a C program with no OS on the Raspberry Pi?
- Raspberry Pi
- Computer security
- Media
Video 2. . Source. The original question was deleted, lol...: How to programmatically synthesize music?- How to resize a picture using ffmpeg's sws_scale()?
- Is there any decent speech recognition software for Linux? ran a few examples manually on
vosk-apiand compared to ground truth.
- Eclipse
- Computer hardware
- Scientific visualization software
Figure 17. VisIt zoom in 10 million straight line plot with some manually marked points. Source. From: Section "Survey of open source interactive plotting software with a 10 million point scatter plot benchmark by Ciro Santilli"
- Numerical analysis
Video 3. Real-time heat equation OpenGL visualization with interactive mouse cursor using relaxation method by Ciro Santilli (2016)Source.
- Computational physics
- Register transfer level languages like Verilog and VHDL
- Verilog:
Figure 19. . See also: Section "Verilator interactive example"
- Verilog:
- Android
- Debugging
- Program optimization
- What is tail call optimization?
Figure 21. . Source. The answer compares gprof, valgrind callgrind, perf and gperftools on a single simple executable.
- Data
Figure 22. Mathematics dump of Wikipedia CatTree. Source. In this project, Ciro Santilli explored extracting the category and article tree out of the Wikipedia dumps.
- Mathematics
Figure 23. Diagram of the fundamental theorem on homomorphisms by Ciro Santilli (2020)Shows the relationship between group homomorphisms and normal subgroups.- Section "Formalization of mathematics": some early thoughts that could be expanded. Ciro almost had a stroke when he understood this stuff in his teens.
Figure 24. Simple example of the Discrete Fourier transform. Source. That was missing from Wikipedia page: en.wikipedia.org/wiki/Discrete_Fourier_transform!
- Network programming
- Physics
- Biology
- Quantum computing
- Section "Quantum computing is just matrix multiplication"
Figure 28. Visualization of the continuous deformation of states as we walk around the Bloch sphere represented as photon polarization arrows. From: Understanding the Bloch sphere.
- Bitcoin
- GIMP
Figure 29. GIMP screenshot part of how to combine two images side-by-side in GIMP?.
- Home DIY
Figure 30. Total_Blackout_Cassette_Roller_Blind_With_Curtains.Source. From: Section "How to blackout your window without drilling"
- China
Updates Link to CIA website autism-news.org found on Wikipedia from back in 2011 Created 2026-06-14 Updated 2026-06-14 2026-06-14
Imagine that it is 2011 and you are editing the linked to it back in 2011 in a Talk of the "Causes of autism" Wikipedia page. You are researching about the far fetched theory that it was Neanderthal mixture with earlier humans which led to the appearance of autism. You Google it up and list some links. One of them happens to be
autism-news.org. This is how it would have looked like at the time according to archive.org. You mark it as "non-legitimate" and move on.And imagine that, unknown to you however, this page was actually created by CIA employees as of of many to allow their agents to communicate information back to the CIA! These pages would later be discovered by the target countries such as China and Iran, leading to the imprisonment and execution of many agents!
Well, this is exactly what happened to Wikipedia user Slartibartfastibast!
After the revelations of CIA 2010 covert communication websites, security researcher Ellie intentionally registered the domain autism-news.org, and she found out that Wikipedia Slartibartfastibast had linked to it back in 2011 in a Talk of the "Causes of autism" page discussing the legitimacy of the "Neanderthal Admixture Hypothesis".
Ellie created a Accidentally found a CIA covert communications website badge for them which his exceedingly cute.
Attempts to reach Slartibartfastibast to ask how he found the website failed, x.com/Slartibartfas12 is a possible account, and there is also a known similar Gmail address from a data leak which might match.
It is almost certain that they just happened to find the website while Googling for it, since we know that the websites were Googleable because it has been reported that the Iranians used Google dorking to find some of them.[ref] This is presumably because Google had access to zone files, which are big dumps of registered domains that some registrars make available to some organizations.
Ciro Santilli subsequently searched for any more hits of the Websites on the Wikipedia dumps by grepping gives a few dozen hits, all legit. The next step would be to try and download full Wikipedia history for the period, but that would require more patience and would likely not yield anything of much interest either.
enwiki-latest-externallinks.sql which contains all live external links outgoing from Wikipedia. The only two hits were autism-news.org itself and inkfreenews.com which became a legitimate local news website for Northern Indiana as early as 2013. Inspection of:grep -Po "inkfreenews.*?'.*?'.*?'" enwiki-latest-externallinks.sqlIt is understandable why
autism-news.org is would be one of the few websites most prone to being randomly Googled: unlike most websites which are quite generic news aggregator websites, this one contains a surprising amount of text. Much like the Star Wars website starwarsweb.net, we would like to think that the contractor who made this one up actually put their hearts into it, and might have had a real personal interest in the matter. The quotes about Neanderthal Admixture Hypothesis are in the Causes subpage:The Neanderthal Theory is that autism and other psychiatric conditions evolved from interbreeding between Homo Sapiens and Neanderthals. This interbreeding is believed to have caused genetic material from the Neanderthals to enter the Homo Sapiens genome. Whether such interbreeding ever actually occurred is controversial, and this theory is extremely speculative.
The username "Slartibartfastibast" seems to be a variation of Slartibartfast, a character from The Hitchhiker's Guide to the Galaxy who according to Gemini is a:So it's probably an autistic dude from the UK. Their Wiki homepage says simply however:and as of writing their latest contribution was from 2014 and they are typical STEM nerd based on their edits.
venerable "planetary designer" from the planet Magrathea, most famously known for his award-winning work designing coastlines and his deep, personal passion for creating fjords
2011 Wayback Machine archive of autism-news.org
. Source. Wikipedia CatTree Updated 2025-07-16
This mini-project walks the category hierarchy Wikipedia dumps and dumps them in various simple formats, HTML being the most interesting!
Mathematics dump of Wikipedia CatTree
. Source.