Older blog entries for rkrishnan (starting at number 264)

Neat hack to find offsets of C Structure members

Posted on November 13, 2013 by rkrishnan

Suppose we have a C structure with some elements like this:

  struct T {
    int foo;
    long bar;
    float baz;

C, being a portable assembler, arranges these data sequentially in the memory and aligns them appropriately. What if you want to find the offsets from the base of each of the element?

This macro does the trick. There are other more explicit ways to calculate it. But I found this macro very neat.

    #define OFFSET(x, y)  &((x *)0)->y

We use it this way:

    int offset;
  offset = OFFSET(struct T, bar);

How does this work? The idea is based on the fact that if the structure is put in memory starting from address 0, a pointer to the element inside the structure is also the same as the offset, since the base address is 0. So, we cast an integer (0 in this case) to a pointer pointing to the structure and just find the offset to the element whose offset we are interested in. The address of that element should be the same as offset, as the structure is assumed to be laid in the memory starting at address 0. That’s it.

The linux kernel defines a similar macro in the include/linux/stddef.h called offsetof:

  #ifdef __compiler_offsetof
#define offsetof(TYPE,MEMBER) __compiler_offsetof(TYPE,MEMBER)
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

Syndicated 2013-11-13 00:00:00 from Ramakrishnan Muthukrishnan

Turning off from internet

Posted on November 7, 2013 by rkrishnan

I had been doing a few MOOC courses in the past few weeks. Given the fact that I have to juggle between family, work, commute and the courses, I decided to put some breaks on the information intake.

I turned myself off from email, news etc (well, not completely but perhaps ~90%) in the past few weeks by unsubscribing from mailing lists and also by closing browser tabs that I am unlikely to read or benefit from in short term. I consistently kept number of open tabs in the browser to <= 5. I also installed browser plugins to warn myself and block twitter/hackernews etc after 10 minutes of usage per day from 6 AM to 10 PM. (I didn’t put restrictions after 10pm). Overall my information intake was much less.

The result is that I was much happy and got a lot of things done which made me even more happy. I didn’t miss anything (that would have helped me). I had a lot of withdrawal symptoms initially when I unsubscribed from some of the mailing lists which I had been reading for the past ~4 years. But I really didn’t miss anything at all that is of immediate use to me.

I felt a big void when the course ended. For a day or two, I didn’t know what to do with the new found free time. But after that I quickly filled it with trivia. But I learnt a bit from those intense periods of learning. I now have closed down all those pdfs and tabs that I had open and am getting back to work. I had temptations to re-join mailing lists. But I deliberately decided not to do so.

Working on something every day and getting into the “flow” helped me greatly in getting something done and contributing to some happiness. There are many sources of unhappiness in my life which I cannot do much about. But there are a handful that I can do something about, I felt working on some interesting and mind-bending problem certainly was worth it.

Syndicated 2013-11-07 00:00:00 from Ramakrishnan Muthukrishnan

Guy Steele on learning programming languages

Posted on October 26, 2013 by rkrishnan

About an year ago, while casually browsing, I wandered into Oracle Labs website and found Guy Steele’s page. If you don’t know who Guy Steele is, perhaps watching this video is a good start. In summary, Guy Steele was a student of Prof. Gerald Sussman and they together invented the legendary language SCHEME. I am a big fan of Scheme and its simplicity. I think I have watched almost every Guy Steele lecture videos freely available out there. In particular, I am a big fan of the Dan Friedman 60th birthday lecture, “Growing a language” lecture and so on. Youtube is your friend.

Coming back to the topic, I couldn’t resist emailing him and just say that I am a big fan of Scheme. With his permission, I am reproducing the email conversation we had and his great advice.

  > On Jul 6, 2012, at 6:53 AM, Ramakrishnan Muthukrishnan wrote:
> Thank you for the reply. Delighted to see a reply from you. I think I
> have watched all the public videos of talks you have given on various
> things Scheme related (Growing a language, Dan Friedman 60th birthday
> lecture etc) and am a big fan! Scheme has totally changed my
> perception about programming. I still have to learn a lot of things
> more deeply but I think I finally found something that I seem to
> really like -- Programming languages and thanks to you and your work
> for that. I also plan to read the "Lambda, the Ultimate" AI memos too.
> Enough to keep me busy for the next few years! Any advice from you in
> my endeavour in Programming language research will be highly
> appreciated.
> Thanks again
> Ramakrishnan

The main advice I have is what you are already doing: keep reading!
The "Lambda, the Ultimate" papers have held up pretty well over the years,
I think, but there is much, much more.  I recommend any paper that has
Phil Wadler, Simon Peyton Jones, or Charles Leiserson as a co-author.
Also, study more programming languages.  Any will do, but there seem
to be a lot of good ideas nowadays in Haskell, Clojure, Python, and Scala.
You are much better off, I think,  knowing three good programming languages
than one really great one.  And read code in each of these languages, maybe the
code for their standard libraries as well as an application.  Good luck!

Guy Steele

Syndicated 2013-10-26 00:00:00 from Ramakrishnan Muthukrishnan


Posted on October 25, 2013 by rkrishnan

Okay, I decided to throw away my previous blog posts and start afresh. I plan to write more frequently (let us see how that goes). Last time I put a lot of restrictions on myself on what to write about. I think I wasn’t very successful with that.

I decided to try Hakyll, a static webpage generator written in Haskell. I have always been very bad at creating “eye-candy” web pages. So this is a bare bones first version. I haven’t bothered to even change the default CSS file. Instead I want to spend my energy in writing some content.

The nice thing about Hakyll is that it is so easy to build and install using cabal. The website can be compiled into a binary, which is so convenient.

Syndicated 2013-10-25 00:00:00 from Ramakrishnan Muthukrishnan

Installing Plan9 on Qemu (Oct/2013 Edition)

Posted on October 25, 2013 by rkrishnan

I have had several attempts at learning plan9, none succeeded. About 2 months ago, I tried it on my work MacBook Air with VMWare Fusion but didn’t get it going. I decided to try again, this time on my GNU/Linux box. Installing Plan9 is just one small step. Real fun starts once one starts to use it. I have just started and am already learning a lot, fondly bringing back the memories of the times when I was installing Slackware on my 486/DX66 with 4MB of RAM in 1996.

Which Plan9?

There are a few versions of Plan9.

  • the original from Bell Labs
  • 9front, a fork from the Bell Labs version.
  • 9atom (supports a lot of hardware)
  • 9legacy (Bell labs + patch set)
  • NxM (designed for multicore)

I got into the #plan9 irc channel and mischief (of NoiseBridge, SF) was kind enough to offer some suggestions. He suggested me to install 9front. So, I went off and downloaded the ISO image.

Host machine setup

I installed plan9 as a guest on a host Debian GNU/Linux system. All I had to install was Qemu.

Booting the ISO

First you need to create a qcow image (virtual hard disk) for qemu to install plan9 into.

  $ qemu-img create -f qcow2 9front.qcow2.img 20G

Now, boot the iso image:

  $ qemu -hda 9front.qcow2.img -cdrom ~/iso/9front.iso -boot d -vga std -m 768

Just follow the instructions in the 9front install wiki page. You start the installation by doing inst/start. Once installation is done, stop the qemu reboot and start with the following switches.

  $ qemu -hda 9front.qcow2.img -boot c -vga std -m 768

Also I configured my screen with 1920x1080x16 which works just fine. I also used ps2intellimouse as my mouse, that gives me nice scrolling.

To take mouse control off Qemu, just press CTRL+ALT.


To make good use of the Plan9 graphical console (rio is the graphical shell of plan9), one need a 3-button mouse. acme need a real 3 botton mouse for some of the “chording” operations. One could simulate button-2 (the middle button) with SHIFT+right click. But it is not very pleasant. So, I got a Microsoft Notebook Optical Mouse (from ebay.in for Rs.1000) which has a side button that acts as the third button.

Unix to Plan9

Moving to Plan9 as a regular OS can be daunting (at least for me that was/is the case, but I am slowly making my way in). There is a nice Unix to Plan9 command transition page. Just keep it handy. Plan9 takes Unix philosophy to the extreme and that shows in the commands.

The terminal does not have a separate pager like more or less, infact there is an automatic pager and the command output blocks until one scroll the terminal, though this behaviour of the terminal can be changed using the menu. There is no commandline history. The shell is rc and is very nicely integrated with plan9 in all respects.

Unlike most Unix/GNU programs, plan9 programs need very less customizations. In many ways, GNU programs are antithesis of the Unix philosophy (of doing one thing well). I realized it only after I read and used plan9 a bit.


  • get an irc client working inside 9front.
  • get email client working with upas.
  • 9fs works like a charm. Play more with it.
  • how to compile C programs?
  • APE layer
  • plumber
  • read all the papers on the bell labs website.
  • ACME, sam, structured regexps, rc.
  • … many more.

Community and Support

There is a small but very passionate community around plan9. 9fans list and the #cat-v irc channel on freenode are great places to hang out with other plan9 enthusiasts.

Compiling programs

I tried to compile a few programs. I could rebuild the plan9 kernel and bootloader just fine. Mercurial seem to be the defacto version control system used by plan9 folks.

I tried to compile the Go compiler but that didn’t succeed. I needed a bunch of patches and even after applying those, it didn’t compile and finally I lost interest. A lot of plan9 folks use the paste service called sprunge for sharing error messages and patches. One could send error messages or recieve patches using the hget/hpost commands. For example, to post a file containing an error message into sprunge, this works:

  term% hpost -u http://sprunge.us -p http://sprunge.us sprunge@errors.txt # hit enter

The URL returned by hpost can be shared on the IRC.


Installing, using and reading the motivations behind Plan9 makes one understand how broken our day-today computing infrastructure is. Plan9 was a nice attempt at fixing Unix and bringing computing to modern ages. In plan9, everything is a file. Even environment is a file. Each process has its own private namespace. This means that one can get rid of the ugly hack called sudo from the system. Venti, the network file storage system, is a content addressible system, which works similar to git.

How ever the strenghts of Plan9 also turned out to be its weakness. The world had already moved well into Unix and making something too different from Unix, is not for someone without enough marketing muscle. Had ANSI Posix Environment (APE) been done and had there been a linux emulation layer, things would have looked quite different. Also Plan9 doesn’t have a C++ compiler. Almost every web browser out there is written in C++ and the lack of C++ compiler means that one cannot use modern web from Plan9.

I am extremely sorry to see plan9 die a silent death. Or dare I say, it is already dead? There are handful of people using it at the moment. But hopefully, Plan9 from Userspace will live on.

Syndicated 2013-10-25 00:00:00 from Ramakrishnan Muthukrishnan

Understanding the GNU/Linux graphics

Sometime ago, after reading some LWN articles (highly recomment an LWN subscription if you are interested in Linux kernel). But I found the going tough. There are lots of scattered information. Even though the software is all Free, it was impossible to comprehend why certain things were done the way it was done. It not only required reading articles (paying close attention to the date on which it was written), it also required digging into the past on how things looked before and how it changed. Most of the time, a long list of APIs are given which can only be comprehended by those working on it for a long time. It was very frustrating. I even wondered how anyone new can contribute to such projects after a few years when the current crop of experts have all lost interest in these projects or have passed away.

If you find yourself also in the same position as me, here are some pointers to some gems I found in my journey that does give a big picture of the GNU/Linux graphics/display sub-system. No, I am not competent enough to explain it myself yet, I would rather leave that to masters who have actually worked on it.

To get an overview of the various terminologies involved (DRM, DRI2, KMS, EGL, X, XRender, Wayland, pixman, cairo and other alphabet soup) start reading this overview article ”The Linux Graphics Stack”. Another great overview is this little PDF file which has short explanations of all the key pieces of the graphics stack from the hardware bits to the application. Once you read it, head straight to Wayland Architecture page which explains how X draws the screen and how Wayland is simplifying the picture. Pay particular attention to the journey of an event and its effect on the screen.

Now you are reading to watch this great LCA 2013 video on X and Wayland by X/Wayland hacker, Daniel Stone and look at the corresponding slides.

Another great video is the Episode 6 of “The luminosity of Free Software”, a Google Hangout series by KDE uberhacker Aaron Seigo. Another great article written in 2005 about the state of GNU/Linux graphics by Job Smirl.

And then we have the great LWN, which is an essential reference to every linux kernel programmer. There is a bunch of links to the relevant LWN articles and other discussions and slides on thie Linaro Memory Management page.


Again I am a journeyman into Graphics, trying to make sense of various terminologies. There are two pages, that I found helpful.

Hopefully, these links will give a good “big picture” view of the low level parts of rendering/video/graphics inside a modern GNU/Linux desktop. Also remember to watch the date on which this post was written (because The Internet does not forget anything and you, the reader, may be reading this page many months/years after the day this post was written). The display side of things being the most user visible and sensitive thing, is ever changing. The picture may look entirely different after a few years.

Syndicated 2013-03-08 07:08:00 from Ramakrishnan Muthukrishnan

My recent experiences with online courses

Last year, when the online AI Class was announced by Sebastian Thrun and Peter Norvig, I was thrilled and immediately signed up. Soon two other courses were offered. At work, I do low level software and have not formally studied AI or Databases or Machine Learning, not did I really see a need, in immediate future, to apply them in my job. Neverthless, I was thrilled at the possibility to hear and work with Stanford professors through these courses and enrolled for the AI course.

The AI class started and I think I could keep up my motivation level for the first 3 or 4 units. There were others distractions like family and work. I had to do a bit to extra work to learn some background mathematics and also read up the text to keep up with the lectures. Somehow, at that point of time, it all couldn’t fit together in my scheme of things, so I decided to discontinue. When I think back, I think I could have completed the course with a bit of extra effort, which I was not really putting at that point of time, instead I came up with some excuses! One of my cynical friends had predicted that I and some others at work who enrolled with me would all discontinue the course and I was sad that he was right.

Then the creation of Coursera and Udacity were announced. When I saw the announcement for the Design and analysis of Algorithms - 1 course from Stanford, I was extremely thrilled. I had always wanted to learn about analysis of algorithms but have never taken a formal course. I enrolled for the course and started working on the lectures. Tim Roughgarden, who was the lecturer for the course was going at a bit fast rate than what I could keep up. But somehow I caught up with lectures by working on them late nights and early mornings. I took notes as I went along. Taking notes meant, I had to watch the same lecture two or three times in some cases. It quickly blew up the time required to complete one week worth of lectures and sometime spilled over to the next week. But for me, it was liking playing a game. The problem sets and programming assignments were staggered by a week and so I could submit them on time. I was looking forward to the lectures and what new stuff Tim is going to throw at us, students. I did not find much time to participate in the forums. The programming assignments were mostly easy and was something I was really looking forward to. I used Racket for my programs and turned out that some others taking the course were also using Racket. It was a joy to program in Racket through out the course. During the last week of the course, I was with my parents and didn’t have a working Internet connection. After struggling with the phone company and wasting a lot of time on it, I decided to download the vides from elsewhere and work offline. In the end, I used my phone to connect to the Internet using GPRS and use my laptop via tethering to submit answer to the problem set and programming assignments. Overall, I think I did the tests very well.

Here are somethings I liked/disliked about the course:

  • Teacher is the most important element in a class. If teacher is uninteresting, everything else is. No amount of technology can save the situation. Tim is a great teacher. He talks a bit fast but after a while I started loving his style of talking and teaching. Some other classes (don’t want to point fingers at any specific course) didn’t have as good a teacher as Tim.
  • Free style writing on White/Black board instead of powerpoint was one of the highlights of the course. I think it was crucial for the success of the course. Many other courses which I signed up at Coursera were using powerpoints and the teachers (some of the greatest names in CS) were reading out from the powerpoint slides. I couldn’t sustain interest in such courses, how much ever great those teachers are. The way Tim taught the class is a role model and brought back memories of some of the best classes I had taken in real classroom years ago when I was a fulltime student.
  • Timing and difficulty level of the exercises within the lecture is another extremely important element.
  • A good teacher is far far better than self-learning from a book. I learnt tons of new things in these 5 weeks than I would have ever learnt in 5 weeks of reading.
  • The importance of taking notes cannot be overstated. This was the single best decision I ever took. I carried the notebooks around along with my laptop and used it whenever I got free time (sometime, even at work, when I am waiting for compilation to finish or in the evenings). The notes were handy while doing problem sets and programming assignments for a quick revisit to some particular lecture or to look up specific algorithm etc.
  • I didn’t use any text book though Tim recommended a few. I have CLR with me, but surprisingly I didn’t use it much while doing the course.
  • If I have seen one single use of Technology in recent times that positively affects the human beings, then that is this new experiment of online teaching.

Overall, it was a great experience with this course and I would like to thank Tim and Coursera for offering this great course online. I am looking forward to the part 2 of this course.

I also signed up for some of the new courses offered at Udacity. One of them that I am really excited about is the Web design course by Steve Huffman. I really like the style of presentation at Udacity. It is direct. It is short. The listener is tested at the end of (almost) every video. That makes it extremely interesting. Just like playing a video game!

Syndicated 2012-05-01 07:00:00 from Ramakrishnan Muthukrishnan

Helping the FSF

I like new gadgets and have been tempted many times to acquire some of them (like new Android phones and tablets). Usually these gadgets have a short life though (until it becomes obsolete, but still useful) and then newer gadgets come along. One can go on spending money and chasing these gadgets.

These days, I do a bit more analysis. I really think hard if I really need those gadgets and whether I can live without one. More things in one’s life definitely means less available time for doing other things (like spending time with family or reading books) and certainly more pain maintaining them.

Moreover, most of these gadgets do not respect the user’s freedom. When you buy a stock Android phone, chances are that the bootloader of that phone is locked. What this means is that only binaries of the bootloader signed by the manufacturer can be installed. This is true of most (all?) phones available in the market currently. Clever people have deviced ways to keep the bootloader intact and still load alternate OS images (like the excellent CyanogenMod firmware for Android phones).

When one runs these modern gadgets, the Applications (or apps, as they are called these days) are tied to the users account. The Application distributor (like Google or Amazon) can remotely kill any of those application or the phone itself. This kind of application distribution is very different from the way a desktop computer application is distributed.

These kinds of scenarios are coming to the good old personal computing as well. The UEFI comes with similar restrictions (I have to admit that I haven’t read in-depth about UEFI itself).

The good folks at the FSF have been doing a lot of work on Software Freedom and educating the users on these issues (in addition to supporting a number of Free Software projects and defending the rights of the copyright holders as well). They need to pay the staff, host the machines and support various campaigns (print documents, flyers etc). All these needs money. Projects like Android are successful because they are standing on the shoulders of the great work done for the past few years on various Free Software projects (eventhough Android strives hard to avoid GPL for the userspace projects).

Please consider donating some money to the FSF. I have been a proud associate member of the FSF for many years now. Contributing money is the easiest thing one can do to help the cause. A better way would be to work on Free Software projects itself.

If you are thinking of buying a gadget, think carefully if you really need one and if so, choose one which respects your freedom and don’t become a slave of the manufacturer. Also please think of donating 10% of the cost you plan to spend to organisations like the FSF.

Syndicated 2012-01-18 08:00:00 from Ramakrishnan Muthukrishnan

Using cKanren in Racket

cKanren is a wonderful system created by Clair Alvis and the group at IU for relational programming. The definitive work about cKanren is this paper. cKanren builds on another wonderful system called miniKanren created by William Byrd and Prof. Dan Friedman of IU.

Off late, I started reading “The Little Schemer” series and started reading the awesome ”The Reasoned Schemer”, also by the same team that wrote miniKanren. cKanren is written in R6RS scheme and is developed on Chez, evidently. Since I wanted to use Racket and DrRacket environment, I started looking at changes to be done to make it run on Racket. What follows below are the instructions to setup DrRacket for cKanren programming.

  • Download my fork of cKanren

    $ git clone git://github.com/vu3rdd/cKanren.git

  • Switch to the ‘racketification branch’

    $ cd cKanren $ git checkout -b racketification racketification

  • Now, make cKanren module visible in the Racket ‘collections’.

    $ raco link .

  • Now, fireup DrRacket. In the definitions window, use the following as the language.

    #lang cKanren

  • Hack away in cKanren!

Syndicated 2012-01-09 08:00:00 from Ramakrishnan Muthukrishnan

Sicp Challenge Progress

2011 had been an extremely interesting year. I feel very happy to have made good progress on my Programming Language Theory learning. I am also qute happy with my SICP challenge project, which was my only noteworthy side project. I am somewhere in the initial portions of chapter 4 right now and it had been worth every minute I spent on it.

I also started reading many books connected with SICP and Programming Language theory, like TaPL and EoPL. Discovering the work of Dan Friedman was an eye opener and I hope to spend many many hours in the next few years learning from his books. I also read “The Little Schemer” and am well into “The Seasoned Schemer” and on to “The Reasoned Schemer” as soon as I am done. I will happily recommend these great books for anyone starting with Scheme.

Syndicated 2011-12-23 08:00:00 from Ramakrishnan Muthukrishnan

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