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Karel as an adult


Just a collection of random rants that I can't really place anywhere else.

2016-07-20 Fswatch 1.09
2016-05-04 Another fswatch update
2016-03-24 fswatch updated
2016-02-22 OTG Price Claimed
2016-02-01 File System Watcher fswatch
2016-01-05 Commandline flag parsing in Go
2015-11-19 Phoning the Dutch Vehicle Registration
2015-11-09 FE 1.05 released
2015-09-28 Bad Horse
2015-09-24 A generic attribute holder class in Python
2015-08-05 Autorotating jpegs according to EXIF tags
2015-04-15 0MQ in Python
2015-04-13 FE update
2015-02-23 Perl Course notes moved
2015-01-13 For my dad
2014-12-11 Peugeot 307 te koop
2014-11-20 Combining tornado.web StaticFileHandler with dynamic handlers
2014-11-05 Pushing scripts to servers using ssh
2014-11-03 Python3 argparse Mini HOWTO
2014-10-30 Handling secure httponly cookies with Python3 and tornado.web
2014-10-29 Overloading libc Calls
2014-10-27 Python Bootcamp Notes
2014-10-06 Custom Squid Proxy Authentication
2014-08-30 CyclicLog
2014-05-02 Playing with Python
2014-04-08 Kalk 1.38
2014-03-18 Closing C++ lecture
2014-02-26 Spring Cleanup
2014-02-26 Gravity error
2014-02-03 More on the knight tour
2014-01-28 The circular knight tour
2014-01-24 A Perl IBAN Checker
2013-10-16 Go2SEPA is live
2013-07-24 AJAXy Fileuploads with JQuery
2013-05-23 The Ticket
2013-04-24 Who is leeching my Facebook
2013-04-15 Time for a compact Coolpix
2013-04-15 Selling my Peavey amp
2013-03-28 encfs Helper Revisited
2013-03-15 c conf 1.17
2013-02-25 Flitsers op de Nederlandse wegen
2013-01-14 OTG Revisited
2013-01-11 Kalk revisited
2012-12-12 God is coming
2012-11-29 Strangely disturbing
2012-11-27 How to sneeze on a motorbike
2012-11-23 Een aanslag uit de toekomst
2012-11-22 More voluntary layoffs
2012-10-17 Browser fingerprint
2012-10-15 Site layout revamped
2012-10-14 Crossroads performing well
2012-10-08 The saddle seat
2012-09-30 Sudo Exploit
2012-09-26 Technomona is live
2012-09-19 A Perl to HTML Prettyprinter
2012-09-17 Playing with Moose
2012-08-22 How do you export photos from Adobe Photoshop Elements
2012-08-21 Security hole at the Empire State Building
2012-07-15 Matrix Sunglasses
2012-07-12 Two Morgans
2012-07-12 Review of the BMW R1200RT Motorbike
2012-06-13 Passwords: maak het hackers moeilijk
2012-06-13 Mijndomein foutje
2012-06-07 YOLNT
2012-06-06 Human Origins
2012-05-23 Me on Facebook
2012-05-16 Voluntary Layoffs
2012-05-07 RBS Bike Tour
2012-04-27 Room with a view
2012-04-23 Browser cookies and Javascript revisited
2012-02-17 Schrodingers Cat
2012-02-14 KPN heeft problemen
2012-01-12 Memebase forever
2012-01-11 Strange squares
2011-12-22 TVV zuigt ezel
2011-12-08 Dilbert vs Skype
2011-11-29 The uncanny resilience of bulshytt
2011-11-23 Another silly Trojan attempt
2011-10-29 ACTA is coming our way
2011-10-28 Burgernet in the Netherlands
2011-10-27 Facepalm art
2011-10-26 Do not drag this image
2011-10-22 Off The Grid Challenge
2011-10-12 PI like a boss
2011-10-07 Once upon a time
2011-07-13 Dutch eticket system for trains
2011-07-12 Is Hell exothermic or endothermic
2011-04-27 Optical Illusions
2011-04-19 Odd lyrics
2011-04-16 Band Revival at MON
2011-03-13 Protests in the Middle East and you
2011-03-10 Mac OSX Hotkey for locking your system
2011-02-12 dnspb 0.06 is out
2011-02-08 Would I buy this fridge
2011-02-06 InstaYouth
2011-02-05 The Thinker is back
2011-01-17 Math challenge
2011-01-11 Zero tolerance and zero intelligence
2011-01-05 My interest income in 1991
2011-01-01 Your horoscope by Eddie
2010-12-22 New York City Tours might be half price for you
2010-12-20 Weather Forecast
2010-12-14 World Economy Collapse explained in 3 minutes
2010-12-13 The Salvation Army and its choice of toys
2010-12-08 Elizabeth thinks highly of me
2010-12-06 Should I trust my government with my data
2010-12-05 Announcing dnspb
2010-12-03 Realistic piechart
2010-11-26 Crossroads 2.71 is out
2010-11-24 8 bit Starwars
2010-11-17 Six to eight black men
2010-11-16 Canada wants backdoors and data and everything
2010-11-11 Autumn storm over the Netherlands
2010-10-08 USA wants backdoors to everything
2010-10-05 Sudoku solver in Perl
2010-10-02 Finally wrote up a Syscheck page
2010-09-28 Neon sign fail
2010-09-27 The Renault Eco Team
2010-09-23 Crossroads 2.68 is out
2010-09-20 How to suppress Flash cookies
2010-09-15 Meanwhile on Facebook
2010-09-09 The Yes Men Fix The World
2010-09-07 ed is not dead
2010-08-26 Installing Perl modules in a non root environment
2010-08-22 Magic self leviation
2010-08-20 Google Chrome does not support offline Gmail
2010-08-19 The number 48
2010-08-12 Welsh trout mini HOWTO
2010-08-04 Fooling a NetCache proxy into fetching forbidden files
2010-07-30 The world will end on May 21, 2011
2010-07-28 Hiding or showing a textbox with image animation using JQuery
2010-07-27 Manipulating browser cookies using Javascript
2010-07-25 Survival of the fittest book
2010-07-23 Pastafarians in Spain
2010-07-22 You have two sheep
2010-07-09 Highway bank fire
2010-07-08 Setting up a remote git repository
2010-07-06 Bye bye trusted old Macbook
2010-06-28 John Cleese on Football
2010-06-23 ABN Amro and the Pathetic Customer Service Dept.
2010-06-22 Wally does not like criticism
2010-06-14 Soccermatch Netherlands vs Denmark
2010-06-13 Lazy Cat
2010-06-08 Reading public Buzz using the Google API
2010-06-07 A Personal Letter from Steve Martin
2010-06-05 Sushi Saturday
2010-06-04 Suppressing the Enter key with Javascript
2010-05-31 Temporal spacial anomaly on the Dutch highway
2010-05-23 Greenhost will not log your traffic
2010-05-10 Jarlsberg Webapp Exploits
2010-05-04 A Thought Experiment
2010-05-03 SafeEdit information updated
2010-05-01 Microproxy now supports ftp
2010-04-30 What could get Data angry
2010-04-29 Lego Mindstorm solving the Rubik Cube
2010-04-28 Crossroads 2.65 is out
2010-04-17 Goggomobil in its natural habitat
2010-04-14 Bacon Time
2010-04-11 104 More friends to connect with
2010-04-10 Bacteria infested radio reporter
2010-04-07 The Kubat STAR
2010-03-30 Homework Essay
2010-03-29 C++ mutexes again
2010-03-20 Weird Eyechart
2010-03-15 Microproxy 1.01
2010-03-05 Microproxy
2010-03-03 Sven Kramer and the wrong lane
2010-02-26 Endearing Babe Magnet
2010-02-17 Speed of light measured using chocolate and a microwave
2010-02-17 Never again expires after 65 years
2010-02-16 encfs on the Mac
2010-02-15 and sexual predators
2010-02-10 Funny textbook
2010-02-09 DNS failing after sleep wake cycle
2010-02-06 Blast from the past
2010-01-28 Simple and straight Perl HTTP::Proxy
2010-01-15 Avatar the Movie
2010-01-08 Slightly NSFW Linux Ad
2010-01-07 WTF
2010-01-05 Stop Software Patents in the EU
2009-12-05 HammerServer 1.02
2009-11-28 Perls Automagical Autoloading
2009-10-07 Office Poster
2009-10-06 The nr 1 Nerdjoke
2009-10-04 WoW Startscript for my Mac
2009-09-27 HammerServer section is online
2009-09-26 The BING HQ
2009-09-26 Digging a WOW Tunnel
2009-06-29 Wee Todd
2009-06-23 The On Off Switch Revisited
2009-06-22 Meatspace
2009-05-30 My old houses
2009-05-11 LOLcats are funny
2009-05-11 Civic Duty WIN
2009-05-10 Vote for the baby, Sky Radio promo FAIL
2009-05-05 My secure data center
2009-02-15 My Valentine is sending me a dot exe
2009-02-05 MacPorts trash: .mp_123456 savefiles cleaning
2009-02-01 Truecrypt 6 on Linux and the ext3 filesystem
2009-01-28 www versus
2009-01-27 Songsmith and The Police
2009-01-25 My own Ministery of Silly Walks
2009-01-09 CoolIris Mini HOWTO
2008-11-04 UDP and DNS balancing
2008-11-02 Life in graphs
2008-11-01 Skeined yet?
2008-10-30 New Crossroads on the horizon
2008-10-28 Thread safe or not
2008-10-15 WOW patch 3 on a case sensitive MacOSX filesystem
2008-10-15 Surprising C++ optimizations
2008-10-14 Weird system message
2008-10-08 Data mining against terrorism does not work
2008-09-16 Crossroads at the top of
2008-09-09 Stupid spammers at Computable
2008-09-06 Spam prevention with Postfix and Postgrey
2008-09-03 The Gnomish Flying Machine
2008-08-27 Bank customer data on eBay
2008-08-26 Mutexes in C++ Threads
2008-08-22 4M dataloss in the UK last year
2008-08-21 Dropping spam with Postfix and Spamassassin
2008-08-18 Bayes and the War on Photography
2008-08-13 Good marital advice
2008-08-12 Squid proxy for personal usage

2008-08-11 Posix threads in C++

A short while ago I rewrote my load balancer Crossroads to a multi-threaded daemon in C++. I tried to find a neat and elegant way to handle POSIX threads in C++. Here's what I came up with; feel free to tinker around with it, and if you have suggestions or improvements, let me know!
I received a very good review from good friend and former collegue Frank B., and I've taken most of his comments at heart. So here's "Version 2". At this time we're still mailing about the approach, and the (non)elegant parts in it. I'll keep you posted.
I furthermore received comments from Lisa R., which identified a race condition in my original code. Fixed now - thanks Lisa!
Also from Frank, I got very neat code for mutex handling. The code below uses one global variable for locking; Frank's code is way smarter. Check out the new version at the blog entry Mutexes in C++ Threads.

If you want to download the code, get the source file It has all the necessary code in one file.

General approach

I designed a pure virtual class, aptly named Thread, which does all the work for you and hides the gory details. The class Thread has a pure virtual member act(), which of course must be implemented to do something useful.

The class implements the following member functions:

  • A virtual destructor, incase derived classes want to implement their own.
  • A member function lock() and unlock(), to "synchronize" thread execution.
  • And finally, there's member function run(), which is called to start thread execution. This function returns an ID (of type a pthread_t), so that the starter of the thread can wait for the thread to finish, if they wish so.

A sample MyThread class

Nothing's complete without a Hello World example. So there we go. Before diving into what Thread is supposed to look like, here's how it's used in a derived class. Each Thread-derived class must implement act(). and it may implement a constructor, destructor or any other method it wants. So here's MyThread:
class MyThread: public Thread {
    void act();

void MyThread::act() {
    for (int i = 0; i < 10; i++) {
	cout << "Hello World from thread " << hex << pthread_self()
	     << ", counter is " << i << "\n";
	sleep (1);
The sample class of course implements act() to do whatever's necessary. Note that inside act() locks are used to "protect" cout. In threads, all access to globals or statics must be protected by locking. If act() wouldn't do this, then the output from several MyThread runs would get garbled. The standard POSIX function pthread_self() is used to display the thread ID inside act().

According to Frank's remarks: I have to admit that lock() and unlock() don't really belong into the Thread class. A separate class Mutex is in real life more appropriate. Please consider my squeezing the locks into Thread as laziness... or "the demo effect"...

How to start MyThread

Here's how a Thread-derived class is "started". The caller must allocate new objects using new and then call the run() method to fire it up. That starts the thread, dissociates it from the main program, and calls act() to do whatever is supposed to be done.

The caller mustn't delete the thread objects! That is handled entirely within the Thread class. The reason is of course that the threads run separately - the caller doesn't know the right timing to call delete.

(Actually, POSIX threads have a way of "waiting" for a thread to terminate, using pthread_join(). The caller is of course at liberty to use this approach. However, I still chose to implement deallocation entirely within Thread, so that the caller doesn't need to worry. The caller can just "fire and forget". According to Frank, this breaks the rule of thumb "if you pack it in, pack it out yourself". I agree, normally I'd want the one to allocate something to also de-allocate it. This situation breaks that rule...)

Here's the first example. The following main() function fires up two threads, waits 15 seconds, and stops. The necessary waiting is there to let the two threads finish:

void test1() {
    cout << "Starting two threads,\n"
	 << "and sleeping for 15 seconds to allow threads to finish.\n";
    // Instantiate two thread objects and fire them up.
    (new MyThread())->run();
    (new MyThread())->run();

    // Wait for the threads to finish.
    sleep (15);

Here's the second example. This invocation actually waits for the threads to finish. It involves pthread_join() and is of course more precise than sleeping for 15 seconds.

void test2() {
    cout << "Starting two threads,\n"
	 << "and waiting for them to finish.\n";
    // Instantiate two thread objects.
	*a = new MyThread(),
	*b = new MyThread();

    // Start 'em up.
    pthread_t id_a = a->run();
    pthread_t id_b = b->run();

    // Wait for the threads to finish.
    pthread_join (id_a, 0);
    pthread_join (id_b, 0);

Class Thread

Finally, here's the class Thread itself. First the class declaration:
class Thread {
    virtual ~Thread();				// dtor

    virtual void act() = 0;			// Pure virtual act()

    pthread_t run(bool joinable = true);	// The starter

    void lock();				// Synchronization
    void unlock();
    static void *start(void *obj);

    static bool initialized;			// Mutex initialized yet?
    static pthread_mutex_t thread_mutex;	// Mutex itself

Trivial Thread member functions

The constructor and destructor are pretty trivial. The purpose of the destructor is only to provide a dummy; the constructor initializes the static data member thread_mutex unless this was already done before.
pthread_mutex_t Thread::thread_mutex;
bool Thread::initialized = false;

Thread::~Thread() {
The members lock() and unlock() use the normal pthread mutex functions, so that's easy too:
void Thread::lock() {
    int res;
    if (! initialized) {
	if ( (res = pthread_mutex_init (&thread_mutex, 0)) )
	    throw (static_cast<string>("Failed to initialize static mutex: ") +
	initialized = true;
    if ( (res = pthread_mutex_lock (&thread_mutex)) )
	throw (static_cast<string>("Failed to obtain mutex lock: ") +

void Thread::unlock() {
    int res;
    if ( (res = pthread_mutex_unlock (&thread_mutex)) )
	throw (static_cast<string>("Failed to release mutex lock: ") +

Thread::run() and Thread::start()

And now for the member functions that actually starts a thread, run() and start(). The "tour te force".

POSIX threads are started using pthread_create(). This C function requires a "thread entry point", which is a pointer to a function that accepts a void* argument, and returns a void*. That's actually the reason that Thread::start() is there, it converts to the right prototype and it's static, hence "object-less".

So first, there's Thread::run(). It has an optional argument bool joinable, default true, which sets the thread's "detach state". The caller can choose to start the threads as either totally detached, or not. When threads are started in detached mode, i.e., not-joinable, then the caller cannot use pthread_join() to wait for a thread to finish.

To be more precise, detached or joinable threads each have advantages and disadvantages:

  • When threads are started as joinable, then the caller can wait for them to finish. However, the caller will then have to "remember" what threads were started. These are the ID's returned by run() when starting the threads.
  • In my experiments, joinable threads ate more resources. Repetitive calls to pthread_create() would fail. Detached threads didn't; it seems that you can have more of these. But then, joining detached threads isn't possible.

Oh, just one more thing. When a thread is started, then error EAGAIN can be the result. In that case, there are no resources to start the thread at this time. When this happens, run() will wait a second and retry. If this fails three times in a row, then run() gives up.

pthread_t Thread::run(bool joinable) {
    pthread_attr_t attr;
    int res;
    pthread_t th;

    if (pthread_attr_init (&attr))
	throw ("Cannot initialize thread attributes");
    if (joinable) {
	if (pthread_attr_setdetachstate (&attr, PTHREAD_CREATE_JOINABLE))
	    throw ("Cannot set thread state as joinable");
    } else {
	if (pthread_attr_setdetachstate (&attr, PTHREAD_CREATE_DETACHED))
	    throw ("Cannot set thread state as detached");
    for (int i = 0; i < 3; i++) {
	res = pthread_create(&th, &attr,
			     reinterpret_cast<void *(*)(void *)>(start),
	if (!res) {
	    return (th);
	if (res == EAGAIN) {
	    cout << "Failed to start thread: " << strerror(res) <<
		", retrying\n";
	pthread_attr_destroy (&attr);
	throw (static_cast<string>("Failed to start thread: ") +
    throw ("Failed to start thread: "
	   "Resources unavailable after 3 tries, giving up");
And now Thread::start(). This function receives the this pointer as its argument t via pthread_start(). Therefore, even though it's a static (object-less) function, it knows what object is being threaded. So it just calls the right act() method and presto. Finally, once act() returns, the object is deleted by performing something like a virtual seppuku.
void *Thread::start(Thread *t) {
    try {
    } catch (string s) {
	cerr << s << "\n";
    } catch (...) {
	cerr << "Thread threw an exception\n";

    // Seppuku
    delete (t);
    return 0;

Caveats and Improvements

  • Exception handling in the above code is string-based only. After all, it's only a demo. If you want to incorporate this code into your own programs, make sure that the caught exceptions match whatever your program is throwing.
  • The code is pretty much tailored to my needs in Crossroads. E.g., the automatic delete in Thread::start() might not be what you desire... A more generic class Thread could make this optional, e.g., using some boolean flag.

2008-08-09 Crossroads mailing list
2008-08-08 Crossroads 2.00 is out
2008-08-01 Fail Pics
2008-07-14 The Fish Dance
2008-07-01 Big Bother and Massive Data Storage
2008-06-30 MMV One of omitted Unix tools
2008-06-08 Even anonymous breadcrumbs can give you away
2008-05-29 Crossroads in Argentina
2008-05-20 The Party at the Company Outing
2008-05-19 Crossroads 1.80 is out
2008-05-18 Where does technical innovation really come from
2008-05-16 Corporate bs generator
2008-05-15 Even the Vatican has to adapt
2008-05-12 Big Brother is watching your dog
2008-05-09 666 all over the place
2008-04-17 Security and privacy are incompatible
2008-04-16 The Hallmark E Card
2008-04-15 Crosroads Solaris port is out
2008-04-04 Identity theft can cost you dearly
2008-04-03 Crossroads can already do that
2008-03-31 A dagerous safari
2008-03-28 Why some Java J2EE projects are inefficient
2008-03-26 The Hummingbird
2008-03-25 The Easter delusion
2008-03-18 McAfee detects mass hack of 200.000 webpages
2008-03-17 More predictive statistics
2008-03-10 Backwards conclusions even on Slashdot
2008-02-18 A fractal photograph
2008-02-15 Kaprekar revisited
2008-02-14 Kaprekar numbers
2008-02-12 A tale of the criminal ineptitude
2008-02-10 Irritating Selfregistered users in PHPBB
2008-02-08 B2B Spam in the Netherlands
2008-02-06 Surprising iSight Capture
2008-02-05 Breadcrumbs at
2008-01-29 iSight Capture Utility
2008-01-28 The Male Brain
2008-01-26 Searching for the next Uri Geller
2008-01-24 Opt in for b2b spam
2008-01-14 Bokito Revisited
2008-01-13 Top Crossroads User
2008-01-12 World of Warcraft Dancing
2008-01-12 Justice dispensed better late than never
2008-01-11 Jeremy Clarkson and Identity Theft
2008-01-10 Terrorism in the Netherlands
2007-12-07 The mind and bodysnatchers are among us
2007-12-05 Bruce Schneier and Hildo
2007-12-04 Bye bye, good Christian soul
2007-12-03 Confusing mail message
2007-11-30 Medion MD 85276 reviewed
2007-11-29 Recent cases of data exposure
2007-11-20 Bayes bites
2007-11-19 Japan starts fingerprinting foreigners
2007-11-14 Privacy, Yahoo and the Strange World
2007-11-14 Privacy, Fall through algorithms, and Securing data
2007-11-07 European airlines to retain data
2007-11-03 BloggEd
2007-10-30 Wilders and
2007-10-28 The goldplated Mac
2007-10-26 More morons
2007-10-26 Dilbert nails it again
2007-10-23 Rough yet funny
2007-10-05 Another silly Trojan mail
2007-10-01 So ugly it is beautiful
2007-09-28 Here is a nickel kid
2007-09-23 Spy Shredder
2007-08-29 Web svn view 1.08
2007-08-24 Caught in THE Process
2007-08-21 Stupid Trojan attack
2007-08-21 Back in 1994
2007-08-20 A girly iPod
2007-08-17 Crossroads for RDP connections
2007-08-15 Firewall art
2007-08-14 jpeginfo
2007-08-13 Good People
2007-08-07 The Real Crossroads
2007-07-30 BBC Documentaries in the Netherlands
2007-07-12 No problems with Crossroads so far
2007-07-11 Politically correct ad nauseam
2007-07-02 Waka Waka Poem
2007-07-02 Voyage of the rubber ducks
2007-06-28 The On Off Switch
2007-06-27 No free lunch
2007-06-25 Crossroads web interface
2007-06-25 Blinkenlights
2007-06-21 There is no silver bullet
2007-06-18 Motto of the week
2007-06-18 Do not feed the troll
2007-06-17 Which programming language are you
2007-06-13 Crossroads support request
2007-06-12 Bokito glasses
2007-06-07 Apache mod_proxy balancer description
2007-06-05 A ticketnumber is not support
2007-06-05 403 Hammertime
2007-06-04 Playground Fun
2007-05-24 Ascii man
2007-05-07 Cannot find the damn server
2007-05-02 The BFG200
2007-04-27 Crossroads Top User
2007-03-30 Crossroads Usage
2007-03-25 The guy with the dark motorhelmet
2007-03-22 The Process and The Result
2007-03-21 Quotes attributed to Jos
2007-03-20 A really nice comment about Crossroads
2007-03-18 Kubat in the air