Who ever thought that having a silly old flag flying was boring does not know what we do!

Things we have done  and learned about flags

• Flags can be incorporated into games.
  • Links of Unity,
  • World Trefoil,
  • World Guide Flag
  • World Guide Song has our Guiding flag unfurling.
• Chosen the colour of different meeting times flags
  • Churchill Guide Unit Flag (now the flag for the 5 to 8’s [Purple])
  • Hazelwood Guide Unit Flag  (now the flag for the 8 to the 13 Guides)
  • Churchill Guide Unit Flag (now the flag for the 13 to 17 Guides [ mid blue with light purple trim])
• Raising the Flag,
• Protection of Flag.
• Why a flag.
• Carrying our Flag.
• Ceremonial flag preperations.
• Colour Party
• Giving instructions.
• Different games and challenges.
• Different Cermonies attended

1. Melbourne Guide Parade through Elizabeth Street….for B.P and Queens Guide reciepents.
2. Australian Day Celebrations at Morwell and Churchill
3. A.N.Z.A.C. Day celebrations at Morwell and Yinnar.
4. Victory in the Pacific (VP) 50th Birthday Celebrations.
5. The return of the Hazelwood Roll of Honour to the Hazelwood South Hall.
6. Camp Flags.
7. Parade
8. Presentations.

• Send coded messages.
• History of flags
• Flag designs
  • Why this goes there.
  • World Girl Guide and Girl Scout Flag.
  • Australia Flag
  • Victorian State Flag
  • Aboriginal Flag
  • Latrobe City Council Flag
• History of Flags
  • Old World Girl Guide and Girl Scout Flags
  • Old Australian Brownie ‘Flags’ of Chruchill Brownie Guide Unit
• How does having a Flag become a symbol of who we are and what we as Guides stand for.

This exercise poses the questions in a relatively simple manner so i will answer in the same way.

Finding Definitions for Eight Terms and Concepts used in Threaded Programming.

The term or concept will be followed by the definition.

Thread Synchronization

Thread synchronization refers to the simultaneous execution of multiple threads (defined further down) and/or processes, this allows a program to provide a wider range of services than using singular processes.

Locks

Basically a lock is a synchronization mechanism put in place to control access to resources between threads and processes. These locks are put in place to ensure the integrity of the data resources and that no 2 threads can access one data source at the same time.

Dead Locks

Wikipedia defines a deadlock as:

In computer science, deadlock refers to a specific condition when two or more processes are each waiting for each other to release a resource, or more than two processes are waiting for resources in a circular chain. Deadlock is a common problem in multiprocessing where many processes share a specific type of mutually exclusive resource known as a software lock or soft lock. Computers intended for the time-sharing and/or real-time markets are often equipped with a hardware lock (or hard lock) which guarantees exclusive access to processes, forcing serialized access. Deadlocks are particularly troubling because there is no general solution to avoid (soft) deadlocks.

Semaphores

Wikipedia defines a semaphore as:

In computer science, a semaphore is a protected variable or abstract data type which constitutes the classic method for restricting access to shared resources such as shared memory in a parallel programming environment. A counting semaphore is a counter for a set of available resources, rather than a locked/unlocked flag of a single resource. It was invented by Edsger Dijkstra^[1]. Semaphores are the classic solution to preventing race conditions in the dining philosophers problem, although they do not prevent resource deadlocks.

Mutual Exclusion (Mutex)

Mutual Exclusion is the concept of locking resources and restricting access.

Wikipedia defines the concept of mutual exclusion as:

Mutual exclusion (often abbreviated to mutex) algorithms are used in concurrent programming to avoid the simultaneous use of a common resource, such as a global variable, by pieces of computer code called critical sections. A critical section is a piece of code where a process or thread accesses a common resource. The critical section by itself is not a mechanism or algorithm for mutual exclusion. A program, process, or thread can have critical section in it without any mechanism or algorithm, which implements mutual exclusion. Examples of such resources are fine-grained flags, counters or queues, used to communicate between code that runs concurrently, such as an application and its interrupt handlers. The synchronization of access to those resources is an acute problem because a thread can be stopped or started at any time. To illustrate: suppose a section of code is altering a piece of data over several program steps, when another thread, perhaps triggered by some unpredictable event, starts executing. If this second thread reads from the same piece of data, the data, which is in the process of being overwritten, is in an inconsistent and unpredictable state. If the second thread tries overwriting that data, the ensuing state will probably be unrecoverable. These shared data being accessed by critical sections of code, must therefore be protected, so that other processes which read from or write to the chunk of data are excluded from running.

Thread

Basically threads are a lightweight processes. Darrel Ince defines a thread as The execution of a chunk of code which can be carried out in parallel with the execution of other chunks of code.

Event

Wikipedia defines an event as:

In computing an event is an action that is usually initiated outside the scope of a program and that is handled by a piece of code inside the program. Typically events are handled synchronous with the program flow, that is, the program has one or more dedicated places where events are handled. Typical sources of events include the user (who presses a key on the keyboard, in other words, through a keystroke). Another source is a hardware devices such as a timer. A computer program that changes its behavior in response to events is said to be event-driven, often with the goal of being interactive.

Waitable Timer

The Microsoft Developer Network defines a waitable timer object as:

A waitable timer object is a synchronization object whose state is set to signaled when the specified due time arrives. There are two types of waitable timers that can be created: manual-reset and synchronization. A timer of either type can also be a periodic timer.

Demonstration of Threading in Python

This program creates 10 tasks and sets a bounded semaphore so no more than 3 of the 10 processes can run at the same time. A mutex lock is also put in place. Each thread acquires the semaphore resource and lock and then releases it and runs until all taks have been executed successfully.

References

http://en.wikipedia.org/wiki/Deadlock

http://en.wikipedia.org/wiki/Semaphore_%28programming%29

http://en.wikipedia.org/wiki/Mutual_exclusion

http://en.wikipedia.org/wiki/Thread_%28computer_science%29

http://en.wikipedia.org/wiki/Event_%28computing%29

http://msdn.microsoft.com/en-us/library/ms687012%28VS.85%29.aspx

Dopo “Blank Fade”, lavoro che abbiamo trovato un pò apatico e scialbo, torna nei nostri impianti, nuovamente su M_nus, Gaiser, con 4 traccie, pronte ad infiammare la pista.
L’ EP si compone di Pullshup, Oolooloo, Mfnstmp, Semaphore, 4 bassline robuste, kick serrati, e dei synth che spezzano.
Questo è Pullshup, niente di nuovo all’orizzonte, il disco incarna perfettamente il filone M_nus, insomma un’altra release  da tirar fuori a metà dj-set per far impazzire l’intero dancefloor.

Tracklist:
a1 – Pullshup
a2 – Oolooloo
d.o. – Mfnstmp
d.o. – Semaphore

Artist: Gaiser

Title: Pullpush

Label: Minus

Cat. No.: MINUS81

Genre: Electronic

Playlist: Pullpush, Oolooloo, Mfnstmp, Semaphore

Va recomand un EP foarte bun, de la unul dintre artistii mei preferati… Gaiser

Les Voyeurs #79 – Enseignement du Cinéma – Les Options CAV

Émission diffusée le jeudi 17 septembre 2009 à 18h
rediffusée le samedi 19 septembre 2009 à 10h
en téléchargement pendant une semaine ici.
et en écoute permanente là.

Actualité

Patrick Swayze : Petit hommage à l’acteur hollywoodien ayant joué dans Dirty Dancing, Ghost et Point Break. Son dernier rôle aura été dans la série policière “The Beast” encore inédite en France. Il est décédé le 14 septembre suite à un cancer du pancréas.
Rien de Personnel : sortie du film de Mathias Gokalp, qui sera présent au Sémaphore à Nîmes pour présenter son film, le mardi 22 septembre à 20h30.
A l’affiche aux Arcades : L’Armée du Crime de Robert Guédiguian, Inglourious Basterds de Quentin Tarantino, Un Prohète de Jacques Audiard, A Deriva de Heitor Dhalia (en VOSTF), Tu n’aimeras point de Haim Tabakman.
Lucky Luke : avant-première aux Arcades le dimanche 27 septembre à 20h du film en présence du réalisateur James Huth, de Jean Dujardin et Alexandra Lamy. réservations des places au 04.66.52.63.03.
Avatar : Enfin la BA tant attendu du nouveau film qui l’est encore plus de James Cameron. Sortie France 16 décembre 2009.

Dossier de la semaine : L’enseignement du cinéma – Les Options CAV

Invité : Olivier Gineste, professeur de Terminale des options CAV.

Depuis que les Voyeurs parlent de films, d’acteurs, de réalisateurs, de compositeurs, de bandes-annonces… etc, ils n’avaient pas encore pensé à parler de l’enseignement de celui-ci.
Les Voyeurs ont donc décidé de vous présenter les classes d’options en cinéma audiovisuel (options CAV) du lycée Jean-Baptiste Dumas d’Alès. Et quoi de mieux pour en parler que d’avoir pour invité le coordonnateur de ses options, le professeur de Terminale M. Olivier Gineste.

Avec lui sont donc abordés les questions de cet enseignement que les élèves peuvent suivre dès la 2nde.
Quels sont les critères pour y entrer ? Quel est le contenu des cours ?

La théorie et la pratique, les intervenants, le programme, les tournages, les festivals, les épreuves du bac, etc…
Écoutez cette émission et vous saurez tout sur la manière d’apprendre le cinéma dès e le lycée.

La bande originale de la semaine :

Ecstasy of Gold composé par Ennio Morricone
Extrait de la bande originale du film Le Bon, la Brute et le Truand de Sergio Leone

La Chronique de la semaine : La Carte Postale Sétoise

Ghosts of the Civil Dead

Musique :
Made into a Movie – ZZ Top

Extraits :
Un cours de TP de seconde CAV
Père-Noël (court-métrage)

Présentation : Erik Antolin, Jérôme Bauzon, Cédric Cance
Réalisation : Jérémie Adrian
Invité : Olivier Gineste

Yesterday I briefly touched on the fact that there are limited places in NZ where a writer of speculative fiction can go to publish. For the most part, Kiwi writers look elsewhere – and it’s a wonderful thing that the internet exists, because it makes submitting work to anywhere in the world a much simpler (and cheaper) thing.

That said, there are a few places here that do publish spec-fic. The first one that I want to highlight is Semaphore, which has been around for a couple of years now, and is run by Marie. There have been some amazing stories published through here and the covers always feature fantastic artwork – I urge you to check out the site, whether you are a reader or a writer (or both), the e-zine is free so you have nothing to lose!

Second up we have a new market, SubSpaceTV.com. I first heard about this through this months SpecFicNZ newsletter (which, if you want to get on the mailing list drop me an email and I’ll get Ripley to add your email). They are open to submissions now and are looking for speculative fiction.

It seems a somewhat limited list, I know, but I’m sure that in the future there will be more NZ options. There are a few competitions throughout the year though, which cater to spec-fic, one in particular which I’d like to give a little more publicity to is the short story competition for Au Contraire – they are looking for science fiction and fantasy stories for an Anthology, to be released at the convention next year. Check out the details here, entries close at the end of January 2010, so there is still plenty of time to get something in.

Recently, over at Kiwi Writers, we decided to give some of our members an opportunity to get published. Okay, so the idea was born last year and it’s only recently come out, but we ran a challenge asking members to submit stories to us for a short story Collection. We didn’t get as many entries as we had hoped for, but really enjoyed a lot of them and had a lot of ‘fun’ putting the Collection together. You can find it here, the e-book is free to download, and there is a print version available through lulu. There are stories in it from a variety of writers, some of who have never been published before and it was so great to give them an opportunity to see their writing in print.

Anyways, I’m going to sign off here for today – tune in tomorrow for an interview with the lovely PJ Ballentine. She was kind enough to answer a few questions, which I really appreciate. She was also our guest judge on the Kiwi Writers Challenge Collection, and is an all round wonderful writer and person.

STEUERN DER .NET-THREADPOOL-AUSFÜHRUNGSREIHENFOLGE

private class BlockingQueue {
  private Queue _queue = new Queue();
  private Semaphore _gate = new Semaphore(0, Int32.MaxValue);

  public void Enqueue(T item) {
    lock (_queue)
      _queue.Enqueue(item);
    _gate.Release();
  }
  public T Dequeue() {
    _gate.WaitOne();
    lock (_queue)
      return _queue.Dequeue();
  }
}

Es gibt nichts, was es nicht gibt, insbesondere gibt es alles schon,
was ich mir selbst gebaut habe, weil ich dachte, dass es das so nicht
gibt.

Go MELVINS! This track is off of a 7″ I Can’t Shake It/Some Girls. It’s awesome, and you can hear it if you listen tonight from 8 to 10 pm on KRUI 89.7 fm or online here.

1. The Wake, Harmony + Singles, Give Up (1982)
2. Mission of Burma, The Horrible Truth About Burma, Trem Two (1985)
3. Sleater-Kinney, All Hands On the Bad One (2000), Leave you behind
4. The Flatmates, Potpourri: Hits Mixes, and Demos, I could be in Heaven (Single came out in 1986)
5. Love of Diagrams, EP, Pace or the Patience (2007)
6. The Wedding Present, Tommy (1985-1987 compilation), Felicity
7. Dinosaur Jr., Farm, Imagination Blind (2009)
8. Melvins, I can’t Shake it/Some Girls (Gearhead Records 1998), I Can’t Shake It
9. The Dirtbombs, Tanzen Gehn, Tanzen Gehn (2005)
10. Sebadoh, Bakesale, Dreams (1994)
11. Kinski, Sub Pop- Infecting the Galaxy One Planet at a Time, Semaphore

12. The Church, Priest = Aura, Ripple (1992)
13. Knapsack, Day Three of My New Live, Simple Favor (1997)
14. Elvis Costello & The Attractions, Get Happy!!, Riot Act (1980)

15. Stephen Malkmus, Matador at 15, Church on White
16. Bauhaus, Mask, Mask (1981)

17. The Tights, Cherry Red: The Punk Singles Collection, Cracked (1978)

18. Sonic Youth, the eternal, Antenna (2009)
19. Siouxsie & The Banshees, Kaleidoscope, Red Light (1980)
20. Cocteau Twins, Stars and Topsoil-A Collection (1982-1990), Blind Dumb Deaf
21. Catherine Wheel, Chrome, Fripp (1993)

22. My Bloody Valentine, Isn’t Anything, Feed me with your Kiss (1988)
23. Xymox (or Clan of Xymox???), Twist of Shadows, Blind Hearts (Polygram subsidiary label Wing Records 1989)
24. New Order, Movement, Hurt (12” Version) (originally came out in 1981)
25. Talking Heads, Remain in Light, Listening Wind
26. David Byrne and Brian Eno, Everything that Happens will Happen Today, Strange Overtones

We three intrepid graduates had five things on our to-do list today. Number four was start a blaug, even though blaugs are dead and everyone is microblauging now. Number five, get a job. We are still working on that one, but the other four are in the bag.

We haven’t made actual contact with Hans yet, our friend across the building who works really long hours and likes antique maps, but we do have a few strategies for getting jobs from his office. It involves post-it notes and semaphore. Also, learning semaphore.

Traffic light…you think it is as simple as red=stop green=go but no…that dreadful monster likes to mix things up a bit and add to your daily stress and chiropractor bill, for the first time we unveil its tactics…

Suicidal Yellow: A suicidal yellow is when you are approaching a traffic light that is green and the light changes to yellow at a precise distance that’s just suicidal because you are too far away to make it through the yellow yet too close for a smooth full stop. Very stressful situation my friends, very stressful. If you decide to go through you will be that idiot who goes through the intersection an almost gets hit by other cars. If you instead decide to stop, which pretty much involves slamming on your brakes (unless you want to come to a full stop in the middle of the intersection) you will be the reason why the insurance of the driver behind you will go up or you will be the recipient of many profanities due to his coffee being spilled because his intentions are to squeeze through the light behind you. I hate Suicidal Yellows.

Neck Breaker Green: Neck Breaker Green is a mean one…you are approaching a red light and you start slowing down your ride to eventually come to a full stop. As you are about to come to a full stop, at about two and half inches from accomplishing such maneuver, the light switches to green forcing you to quickly go from slowing down to acceleration in a split second. This causes a neck jerk that if repeated multiple time your nickname will soon be ‘Joe bobblehead’. The effects of this light are increased by 87.2% if you are the driver on front, all the pressure is on you to quickly get the traffic going again. I believe Chiropractors love this one.

3-2-1 Pedestrian: This one is a a developing trend, you might have noticed that some lights, not all of them, have a countdown for the pedestrian light. This feature is received with mixed feelings among fellow drivers, I’ve heard some drivers get stressed when they see the evil countdown decreasing its numbers. Others like me love it, because I know whether I have a chance at beating the evil light or not. What’s the other option? the orange hand flashing? How do you know if it’s going to flash ten more times or just two…believe me, once it stops flashing you better be ready to halt your horse.

How do you counterattack?

Below is an implementation of a File based mutex in Java

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.nio.channels.OverlappingFileLockException;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * A file based mutual exclusion lock that allows to acquire and release the underlying
 * resource. On acquiring the lock the file is created with the given contents. The
 * lock is free on construction.
 *
 * The lock is also associated with a maximum unused period. If the lock is not
 * updated during the given period, any other thread waiting to acquire the lock
 * will be given the lock.
 *
 * The lock is said to be acquired only when the file exists and file last used
 * timestamp is within unusedMsec. permitted
 *
 * The read and write operation to the mutex file are synchronized across
 * different process by encapsulating that with a create & lock operation
 * on a separate file. So before a process can acquire/read mutex (update mutex
 * content & timestamp) , it needs to create & lock another predefined
 * file and also delete the lock file after the operation on mutex are done. This is
 * to ensure that multiple processes/threads dont acquire the mutex at the same time
 */
public class FileMutex {
    /** The lock file name **/
    private String mutex = System.getProperty("java.io.tmpdir") + "app.lck";

    /** Msecs. file can be left unused before another thread acquires the lock File **/
    private long unusedMSec;

    /** file contents telling which component/application is holding the lock **/
    private String mutexContent;

    /** FileName of the lock for mutex **/
    private String lockOnMutex = mutex + ".lck";

    public static final long ONE_SECOND = 1000;
    public static final long ONE_MINUTE = 60 * ONE_SECOND;

    /**
     * @param lockFileContent
     * @param unusedMSec
     */
    public FileMutex(String lockFileContent, long unusedMSec) {
        this.mutexContent = lockFileContent;
        this.unusedMSec = unusedMSec;
    }

    public void acquire() throws InterruptedException {
        try {
            while (isInuseMutex())
                Thread.sleep(ONE_SECOND);
            createMutex();
        } catch (Exception e) {
            throw new InterruptedException("Error acquiring lock: " + e.getMessage());
        }
    }

    public synchronized void release() {
        releaseMutex();
    }

    public boolean tryAcquire() throws InterruptedException {
        try {
            if (!isInuseMutex()) {
                boolean created = createMutex();
                return created;
            }
        } catch (Exception e) {
            throw new InterruptedException("Error acquiring lock: " + e.getMessage());
        }
        return false;
    }

    public boolean update() {
        createMutex();
        return true;
    }

    /**
     * @return
     */
    private boolean isInuseMutex() {
        File lockOnMutexFile = new File(lockOnMutex);
        FileChannel channel = null;
        FileLock lock = null;
        try {
            channel = new RandomAccessFile(lockOnMutexFile, "rw").getChannel();
            lockOnMutexFile.createNewFile();
            try {
                lock = channel.tryLock();
            } catch (OverlappingFileLockException e) {
                // File is already locked in this thread or virtual machine
                return true;
            }

            File mutexFile = new File(mutex);
            if (!mutexFile.exists()) {
                return false;
            }

            Date lockLastUsedDate = new Date(mutexFile.lastModified());
            Date oldestAllowedLockUsedDate =
                         new Date(System.currentTimeMillis() - unusedMSec);
            if (lockLastUsedDate.before(oldestAllowedLockUsedDate)) {
                return false;
            }
        } catch (Exception e) {
            System.out.println("Error acquiring lock: " + e.getMessage());
        } finally {
            try {
                if (lock != null)
                    lock.release();
            } catch (IOException e) {
            }
            try {
                channel.close();
            } catch (IOException e) {
            }
            lockOnMutexFile.delete();
        }
        return true;
    }

    /**
     * Creates the lock file with the given contents.
     *
     */
    private boolean createMutex() {
        File lockOnMutexFile = new File(lockOnMutex);
        FileChannel channel = null;
        FileLock lock = null;
        try {
            channel = new RandomAccessFile(lockOnMutexFile, "rw").getChannel();
            lockOnMutexFile.createNewFile();
            try {
                lock = channel.tryLock();
            } catch (OverlappingFileLockException e) {
                // File is already locked in this thread or virtual machine
                return false;
            }

            File mutexFile = new File(mutex);
            mutexFile.createNewFile();
            FileWriter writer = new FileWriter(mutexFile);
            writer.append(mutexContent);
            writer.flush();
            writer.close();
        } catch (IOException e) {
            System.out.println("Error acquiring lock: " + e.getMessage());
        } finally {
            try {
                if (lock != null)
                    lock.release();
            } catch (IOException e) {
            }
            try {
                channel.close();
            } catch (IOException e) {
            }
            lockOnMutexFile.delete();
        }
        return true;
    }

    /**
     * Release the mutex .. that is Delete the underlying lock file, if it exists
     */
    private void releaseMutex() {
        File mutexFile = new File(mutex);
        mutexFile.delete();
    }
}

La Pointe Saint Gildas, on l’avait déjà fait tous les quatre il y a  un mois.  Ayant profité du véhicule de courtoisie pour se promener à six, l’endroit fut tout désigné pour une petite promenade…
En bonus, on a découvert des choses qu’on avait pas vu la première fois que nous y sommes allés
.

.
[ Infos sur le sémaphore : Wikipedia  ]

Une petite photo de groupe pour commencer
.
Au large ( en blanc ), un ferry se dirigeant vers le port de Montoir.
Après quelques ( loooooongues ) recherches sur le net, il s’agirait du Super-Fast Levante, qui peut transporter jusqu’à 117 voitures.
.
Voici la table d’orientation dont je vous avais parlé dans un précédent post sans vous la montrer.
.
Entre Leeloo avec son chapeau qui lui tombe sans cesse sur les yeux et David qui a une casquette trop petite, je crois qu’il est temps d’agir ^^
.
.
“Visite” d’un des blockhaus du site.
.
Juste devant le blockhaus, le polygone qui servait de pied à un radar.
.
Près du sémaphore des canons de la Seconde Guerre Mondiale.
.
D’ailleurs, en parlant du sémaphore, on s’y est attardés lors de cette balade, lui qu’on avait un peu laissé de coté lors de notre premier venue à la Pointe Saint Gildas.
Autour du bâtiment, des canons donc mais aussi des balises maritimes.
.

Le sémaphore fut construit en 1861.
En 1941, les troupes allemandes firent installer un feu que la Marine Française stoppa en 1949. Mais les navigateurs réclamant un balisage nocturne, un feu temporaire fut mis en place en 1954, sur un des blockhaus et non sur le sémaphore.

En 1958, le sémaphore étant mis à la disposition du Service des phares et balises, le feu est déplacé à l’intérieur de celui-ci, dans l’ancienne salle de veille. Un logement de gardiennage est aménagé : le feu sera gardé de 1959 à 1986, date de son automatisation. Une radiobalise y fonctionnera de 1959 à 1999.
En 1993, la structure en béton surmontant l’ancien sémaphore est démontée et remplacée par pylône portant le feu actuel.

Le phare actuel est un pylône métallique surmontant l’ancien sémaphore, en maçonnerie de pierre, peinte en blanc. Le pylône et la lanterne sont peints en vert. Il est automatisé et télécontrôlé à partir de Saint-Nazaire..
A l’intérieur, un petit musée océanographique ( payant ) et une salle avec une exposition gratuite. C’est cette derniere que nous avons été voir, avec pour thème le sauvetage en mer.
.
Les Hospitaliers Sauveteurs Bretons ( ou HSB ) ont fusionné avec la Société nationale de sauvetage en mer ( SNCM )  en 1967.
.
Encore un détail non vu lors de notre premiere visite en juin : un enclos avec une dizaine de moutons noirs
.
Quand je vous disais qu’on avait le chic pour se balader ou pique-niquer, toujours à marée basse ^^
Verrons nous un jour la Pointe St Gildas à marée haute ?
.

First of all…you can see an example here which may clarify your concept!

Their synchronization semantics are very different:

* mutexes allows serialization of access to a given resource i.e. multiple threads wait for a lock, one at a time, the thread owns the lock until it is done, most importantly only this particular thread can unlocks it.

* a binary semaphore is a counter with value 0 and 1, a task blocking on it until any task does a sem_post. The semaphore advertise that a resource is available, and it provides the mechanism to wait until it is signaled as being available.

As such one can see a mutex as a token passed from task to tasks and a semaphore as traffic red-light (it signals someone that it can proceed).
There is a lot of confusion around these two concepts. They permit the same kind of feature i.e. synchronization but as I explained with different semantics.

Semaphores are usually more expensive to use than mutex, but they provide you sometimes more reliability. For example, let’s say you have two threads, thread A and thread B trying to serialize access to resource R.

The mutex way would be;

step 00 – create a lock object
step 01 – A request the lock
step 02 – B request the lock
step 03 – A grab the lock
step 04 – A uses R
step 05 – A release the lock
step 06 – B grab the lock
step 07 – B uses R
step 08 – A request the lock
step 09 – B release the lock
step 10 – A grab the lock

Q: What happens if thread B returns between step 7 and step 9 ?
A: step 10 may never occurs …

In this scenario there is no way to recover, the mutex is an opaque object and you are stuck in a deadlock situation.

With semaphores things are a little bit different, you can make step 10 happen even if your thread leaves early.

Since people are confused by semaphores or misuse them more frequently, here are a few examples on how to use them.

Serializing access using a semaphore

Note that when using a binary semaphore to serialize access to a resource, you need to initialize it with the value 1, meaning resource available.

Requesting the lock is done by calling sem_wait() on it, and releasing the lock is done by calling sem_post():

// thirst get a semaphore and initialize it with the value 1
sem_init( … )
…

// lock the resource
sem_wait( …)

… do some stuff …

// release the resource
sem_post( … )

The consumer / producer model

if you initialize the semaphore withe the value 0 and call sem_wait on it, the semantic is different, you are waiting for some one to signal you that there is some job to do.
In this scenario, the semaphore is not used to serialize access to a particular resource, it is used as signaling mechanism.
A consumer is waiting for a consumer to signal him that some data is available or that there is work to do.

very simple and effective, that’s why semaphore are used so frequently for producer/consumer work flows.

// thirst get a semaphore and initialize it with the value 0
sem_init( … )
…

while( true )
{
// wait for some job to do
sem_wait( …)

… work hard on it …
}

// yep there is no sem_post here: the producer, that gives this task
// work to do is supposed to be the one calling sem_post()

check your platform semaphore implementation, sem_post(), sem_wait() and sem_init() are the API to use POSIX semaphores.

Your platform API may be different, but the concepts are the same.

Le 1er juin, on avait fait toute la cote Nord du département… Ce coup ci, on s’est dit qu’on allait faire la cote Sud ( bien moins sympa soit dit en passant )
Petite promenade en photos à la Pointe Saint Gildas.

[ Infos from Wikipédia ]

Zhom souffrant actuellement de vidéotite aigue, vous aurez droit au film de la préparation des sandwiches. Public sensible s’abstenir

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En ce moment, on a de la chance, les passants nous proposent souvent de nous prendre en photo tous les quatre
Petit pique nique avant notre marche digestive ( vous noterez David plus préoccupé par son casse croute que par la photo ^^ )
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La Pointe Saint-Gildas est une pointe rocheuse, dont la principale curiosité sont les quelques blockhaus datant de la Seconde Guerre mondiale qui parsèment la lande
Jusqu’en 1750, cette pointe portait aussi les noms de « pointe de Chevesché » ou « pointe de Chevêché ».
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Ils faisaient partie de la forteresse de la Pointe Saint-Gildas dont le quartier général était dans des blockhaus souterrains situés dans l’actuel camping.
Dés 1940 les troupes allemandes pour mener à bien la guerre sous-marine décident d’implanter une base sous-marine importante à Saint-Nazaire en même temps que les défenses destinées à assurer sa protection. Cette défense était confiée au 280° M.A.A (Marine Artillerie Abteilung) chargé d’interdire l’approche de St-Nazaire et l’embouchure de la Loire.
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Le 280° M.A.A avait 5 batteries dont une, la cinquième, sur la Pointe Saint-Gildas.
Cette batterie avait deux implantations, une sur la Pointe Saint-Gildas armée de 4 canons de 75 mm et une batterie lourde armée de 2 canons de 240 mm situés sur le chemin de la Raize sur le coté du l’actuel camping Eléovic.
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Le Sémaphore de la Pointe… Mais kézako un sémaphore ?
A l’époque, au 19eme siècle, c’était un poste de guet établi sur la côte, chargé de surveiller les approches maritimes et de signaler par signaux optiques toute activité ennemie.
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Un kilomètre à pieds, ça use ça use, un kilomètre à pieds, ca use les souliers !
Mais ca n’use pas, l’énergie des Lutins !
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La pointe Saint-Gildas a récemment fait l’objet d’un important programme de réhabilitation, afin de réduire la destruction progressive du biotope par la fréquentation touristique. De nombreuses allées balisées ont été tracées et la lande a été resemée et stabilisée.
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Un des toits des blockhaus a été transformé en terrasse panoramique, avec une table d’orientation.
A notre droite, Saint Nazaire avec le pont et les chantiers, à quelques 28 kms de là …
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Et à notre gauche, toujours à 28 kms, l’ile de Noirmoutier.
Si on avait pensé en être si proches à vol d’oiseau !
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Qui dit marée basse, dit pêcheurs à pieds.
Ca fait deux ans qu’on se dit qu’on va s’y mettre … Quand les Lutins seront plus grands surement ( car avec leur peur chronique de l’eau, c’est pas gagné pour le moment … )
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Après notre petite balade, on s’est dit que les Lutins avaient mérité de jouer dans le sable. Ils se sont alors attaqués à la destruction de châteaux déjà présents et David nous a fait une création en pierres, surement inspiré par sa visite de Carnac l’an passé ^^
[ Non la dame au panier jaune n'est pas topless, c'est son maillot qui est presque couleur peau ]

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Lucu juga kalau bisa, seperti apa ya?  Coba saksikan bagaimana milyaran transistor 2 komputer, yang kita ibaratkan lampu mini, terhampar luas membentuk lautan lampu di depan kita, satu di kiri, satu di kanan. Mula-mula senyap, gelap gulita. Tiba-tiba sebagian lautan lampu kiri menyala berkeredip seperti percikan kembang api di langit. Gelap, kembali senyap. Tidak terjadi apa-apa. Tak lama, sebagian lautan lampu kanan mulai merespon, berkeredip, sekali. Berkeredip sekali lagi, kali ini pola kedip lain. Disambut lautan lampu kiri dengan membalas kirim keredip, dibalas lagi oleh lautan kanan, balas lagi oleh kiri, oleh kanan, kiri, kanan, …. keredip bersahutan-sahutan. Begitu rupanya komputer ‘ngomong itu ya, ramai, indah, seperti pesta kembang api akbar! Andai saja kita bisa intip di dalamnya…

Keredipnya tidak sembarang, ada pesan di baliknya yang kalau diterjemahkan kira-kira begini, “Hello”, dijawab “Hello, how are you?”, dibalas “Fine, thank you, happy birthday dear, I love you”, langsung dibalas “Thank you, I love you too!”. Ooh gitu, bisa mesra juga, ya. (Tidak perlu diterjemahkan ‘kan.)

Begitulah cara komputer berkomunikasi, menyusun ‘0’ dan ‘1’ dari milyaran transistor dengan pola tertentu sedemikian rupa hingga terbentuk kata atau sandi yang dapat difahami komputer mitranya. Teknik sandi bukan hal baru, sudah dikenal sejak jaman dulu. Ketika kita ikut kegiatan pramuka (ada yang tidak masuk?), kita diajari menirukan cara suku Indian berkomunikasi dengan asap. Kita juga diajari cara berkomunikasi dengan memainkan cermin untuk memantulkan sinar matahari, memakai lampu sorot, ada semaphore memakai bendera, memakai peluit, bersuit, memukul lonceng. Setelah ditemukan listrik ada kode Morse. Masih banyak lagi.

Kita juga punya teknologi sandi dengan perangkat tak kalah canggih, teknologi kentongan! Tidak perlu api unggun, sinar matahari, lampu sorot, kain bendera. Sangat hemat energi (cukup energi nasi). Trok – trok.trok.trok – trok.trok… . “Aman!” Sangat efektif.

Untuk memulai komunikasi tidak bisa sembarang. Ada protokolnya, ada aturannya, ada etikanya. Seperti kita manusia saja, harus memperoleh ijin dulu, bertegur sapa saling menghormati, barulah pembicaraan bisa dimulai. Kehadiran kopi dan penganan melancarkan komunikasi. Setelah selesai, perlu ada pamit basa-basi.

Komunikasi komputer mengikuti peradaban manusia dengan protokol kurang lebih serupa, karena manusialah pembuatnya. Itulah teknologi komunikasi informasi, ICT (information communication technology). Komputer harus santun, komunikasi bergantian, ketika yang satu talk (bicara), yang lain listen (dengar). Dengan berebut omong, …ngomong apa ’sih! Berisik, ah.

Aneh juga kita ini, bisa-bisanya kita  mengatur komputer agar patuh dan tertib, tetapi kita tidak konsekuen senang melanggar sendiri, berebut omong. We are talking, not communicating. Masih saja senang pamer siapa master (‘juragan’), bukan kebalikannya jadi panutan.

Komputer, ternyata bisa juga ‘ngomong. (Dan dengar.)