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Topic: Semaphore


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In the News (Tue 23 Jul 19)

  
  Semaphore (communication) - Wikipedia, the free encyclopedia
Semaphores were adopted and widely used (with hand-held flags replacing the mechanical arms) in the maritime world in the early 1800s.
Semaphore signals were used, for example, at the Battle of Trafalgar.
The semaphores were successful enough that Samuel Morse failed to sell the electrical telegraph to the French government.
en.wikipedia.org /wiki/Semaphore_(communication)   (1563 words)

  
 Semaphore (programming) - Wikipedia, the free encyclopedia
Semaphores are the classic solution to the Dining philosophers problem, although they do not prevent all deadlocks.
In addition to their inadequacies in dealing with deadlocks, semaphores do not protect the programmer from the easy mistakes of taking a semaphore that is already held by the same process, and forgetting to release a semaphore that has been taken.
A blocking semaphore is a semaphore that is initialised with the value of 0.
en.wikipedia.org /wiki/Semaphore_(programming)   (2218 words)

  
 Semaphore (Java 2 Platform SE 5.0)
Semaphores are often used to restrict the number of threads than can access some (physical or logical) resource.
A semaphore initialized to one, and which is used such that it only has at most one permit available, can serve as a mutual exclusion lock.
methods for this semaphore, the current thread is next to be assigned permits and the number of available permits satisfies this request.
java.sun.com /j2se/1.5.0/docs/api/java/util/concurrent/Semaphore.html   (2587 words)

  
 RN Semaphore Procedure
It must be remembered by the Sender that the semaphore arms should be presented to the observer with a strongly contrasting background; it is best to have the sky as a background, but where this is not possible, the background should be that which throws the semaphore arms into the greatest relief.
The semaphore arms must be placed at the exact position indicating the letters or signs, the arms being moved from sign to sign by the shortest route; a distinct pause being made at each sign, according to the rate of sending.
When using the mechanical semaphore it is important that the positions of the arms should exactly agree with the positions of the handles, and that there is a minimum of backlash in the chains.
www.gwpda.org /naval/s0900000.htm   (1006 words)

  
 GNU CommonC++: Semaphore class Reference
A semaphore is generally used as a synchronization object between multiple threads or to protect a limited and finite resource such as a memory or thread pool.
One or more threads can wait on a semaphore until it is no longer 0, and hence the semaphore can be used as a simple thread synchronization object to enable one thread to pause others until the thread is ready or has provided data for them.
Semaphores are typically used as a counter for protecting or limiting concurrent access to a given resource, such as to permitting at most "x" number of threads to use resource "y", for example.
www.gnu.org /software/commoncpp/docs/refman/html/class_semaphore.html   (484 words)

  
 Semaphore
Semaphore is a means of line-of-sight communication dating from the early 1800s.
The basic technique of semaphore is to hold each arm at one of eight possible angles--straight up, straight down, left, right, or one of the diagonals.
Semaphore has three teaching features: (I.) You can type a letter on the keyboard and see the flag code displayed; (2.) Beginners can test on random five-letter groups; (3.) Experienced readers can test on messages taken from memory at random.
www.atarimagazines.com /v7n8/semaphore.html   (1256 words)

  
 Thread::Semaphore - thread-safe semaphores   (Site not responding. Last check: 2007-10-21)
Semaphores, unlike locks, aren't tied to particular scalars, and so may be used to control access to anything you care to use them for.
Semaphores don't limit their values to zero or one, so they can be used to control access to some resource that there may be more than one of.
This is the semaphore ``P operation'' (the name derives from the Dutch word ``pak'', which means ``capture'' -- the semaphore operations were named by the late Dijkstra, who was Dutch).
kobesearch.cpan.org /htdocs/perl/Thread/Semaphore.html   (266 words)

  
 How to Use Win32* Functions to Manage Semaphores - Intel® Software Network   (Site not responding. Last check: 2007-10-21)
A semaphore is a kernel object that has a numerical count associated with it to manage a finite number of system resources.
Semaphores are used mainly for solving producer/consumer problems where reads and writes are done at the same time.
When the count is greater than zero the semaphore is in a signaled state, and when the count is zero the semaphore is in a nonsignaled state.
www.intel.com /cd/ids/developer/asmo-na/eng/technologies/threading/knowledgebase/43836.htm   (418 words)

  
 The Origin of the Railway Semaphore
It is common knowledge that the first railway semaphore was erected by Charles Hutton Gregory on the London and Croydon Railway (later the Brighton) at New Cross, southeast London, in the winter of 1842-1843 on the newly enlarged layout also accommodating the South Eastern Railway.
The railway semaphore is an immediate descendant of the apparatus for optical telegraphs that were used from about 1793 until superseded by the electromagnetic telegraph, a process that was complete on land lines by 1850.
General Pasley suggested the semaphore to Mr Gregory as a reliable means to communicate with the driver, more certain than the flag signals previously used, and able to be seen at a greater distance.
www.du.edu /~jcalvert/railway/semaphor/semhist.htm   (2333 words)

  
 Setting our sights on semaphores
Semaphore undo structures are maintained in the event of a process terminating that has made some semaphore value adjustments.
It needs to be signed because semaphore operations can increment or decrement the value of a semaphore, and thus the system may need to apply a negative or positive adjustment value to the semaphore in order to do a successful adjust-on-exit operation.
For operations on semaphores in the same set, the kernel needs to ensure atomicity for the application, which is a guarantee that a semaphore operation initiated by a process will complete without interference from another process, whether the operation is on a single semaphore, or multiple semaphores in the same set.
www.carumba.com /talk/random/swol-10-insidesolaris-2.html   (2111 words)

  
 No. 757: Semaphore Telegraphy
Semaphore telegraphy was impractical outside a few densely populated areas.
Still, those old semaphore relays were once a huge enterprise -- even in parts of America.
In a few years the dying dinosaur of semaphore telegraphy would be utterly gone.
www.uh.edu /engines/epi757.htm   (421 words)

  
 : Class Semaphore
A semaphore has a name which is determined by the string argument of its constructor.
A semaphore can also be created for enforcing mutual exclusion by invoking the constructor with an argument of 1.
For this use, the semaphore is created by calling the constructor with an argument equal to the initial number of instances of the counted resource.
www.d.umn.edu /~cprince/courses/cs5631spring02/labs/doc2-3/Semaphore.html   (335 words)

  
 RTAI API: Semaphore functions   (Site not responding. Last check: 2007-10-21)
Negative value of a semaphore shows how many tasks are blocked on the semaphore queue, waiting to be awaken by calls to rt_sem_signal().
It should be also remarked that real time programming practitioners were using semaphores a long time before Dijkstra formalized P and V. "In Italian semaforo" means a traffic light, so that semaphores have an intuitive appeal and their use and meaning is easily understood.
The task that acquires a resource semaphore becomes its owner, also called resource owner, since it is the only one capable of manipulating the resource the semaphore is protecting.
www.rtai.org /documentation/kilauea/html/group__sem.html   (2410 words)

  
 GNN - Government News Network
The £15 million pilot scheme - known as Project Semaphore - is the first stage in the Government's e-Borders (electronic borders) programme, which will use 21st century technology to strengthen border control, effectively recording people as they travel into and out of the UK, and improve security.
Project Semaphore will initially target six million passengers a year travelling on a number of international air routes to and from the UK.
Semaphore will test and confirm the technical and business process design for the main e-Borders programme as well deliver immediate operational improvements across participating agencies.
www.gnn.gov.uk /content/detail.asp?NewsAreaID=2&ReleaseID=130801   (677 words)

  
 semaphore from FOLDOC   (Site not responding. Last check: 2007-10-21)
The value of a semaphore is the number of units of the resource which are free (if there is only one resource a "binary semaphore" with values 0 or 1 is used).
If a process does a P on a semaphore which is zero the process is added to the semaphore's queue.
When another process increments the semaphore by doing a V and there are tasks on the queue, one is taken off and resumed.
foldoc.org /?semaphore   (271 words)

  
 IPC:Semaphores
Semaphores can be operated on as individual units or as elements in a set.
A semaphore set consists of a control structure and an array of individual semaphores.
If a process with exclusive use of a semaphore terminates abnormally and fails to undo the operation or free the semaphore, the semaphore stays locked in memory in the state the process left it.
www.cs.cf.ac.uk /Dave/C/node26.html   (2948 words)

  
 Semaphore - Make   (Site not responding. Last check: 2007-10-21)
Semaphore “make” an album that is very slow in starting, and has tracks present during “Make” that really start over each time.
Still, while the arrangements might be sequenced in an interesting way, the sad fact is that the tracks on “Make” show a band in Semaphore that continually taps the same font of inspiration, meaning that tracks tend to morph together.
The further lack of vocals (Semaphore is an instrumental band) delete one further out for the band from this bleak reality; the music on “Make” is compelling when taken one track at a time, but taken as a collection, the disc suffers.
www.neufutur.com /Reviews/semaphore.html   (418 words)

  
 Semaphore   (Site not responding. Last check: 2007-10-21)
Semaphore is a signalling system which involves waving a pair of hand-held flags in various positions to indicate letters of the alphabet or numerals.
The flags used in modern semaphore are usually square, red and yellow and divided diagonaly with the red part on the top left.
Semaphore in various forms has been used to send messages over distances since at least Roman times.
www.omniglot.com /writing/semaphore.htm   (72 words)

  
 Australian Museum - Wild Kids - Mangroves - Semaphore Crab
Semaphore is a way of signalling using your hands or flags.
These crabs are called Semaphore Crabs because they can often be seen standing outside burrows signalling with their raised claws.
Semaphore Crabs will scoop up sand and mud with their front claws.
www.amonline.net.au /wild_kids/seashores/semaphore.htm   (135 words)

  
 Semaphore
However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly.
A semaphore initialized to 1 can serve as a mutual exclusion lock.
Here is a class that uses a semaphore to help manage access to a pool of items.
gee.cs.oswego.edu /dl/classes/EDU/oswego/cs/dl/util/concurrent/Semaphore.html   (347 words)

  
 msyed : System.Threading.Semaphore   (Site not responding. Last check: 2007-10-21)
A semaphore can allow more than one thread (and process) to simultaneously be in a critical section[3].
In the simplest case, when a semaphore is created, its constructor is passed two integeral arguments (lets call then IC and MC).
Critical sections should be guarded by concurrency constructs like locks, mutexes, or semaphores, based on their use case.
blogs.msdn.com /msyed/archive/2005/08/31/458795.aspx   (643 words)

  
 Digitrax.com: Installing Semaphore Type Signals with SE8C
These 8 semaphore signals are available in addition to the standard 32 LED signal heads on the 8 signal control cables and occupy a separate semaphore address control range.
Each semaphore should move to the green position when you set its first semaphore control address to c (closed) and to the red position it is set to t (thrown).
If the semaphore can be driven to red and green limits by Sw65 in the test but Sw66=t or c does not drive to the yellow mid-point position, check the wiring to the slow motion turnout machine from the +common line and also the position sense line.
www.digitrax.com /se8csemaphore.php   (905 words)

  
 Semaphore   (Site not responding. Last check: 2007-10-21)
The Semaphore, as produced on paper, is a costly enterprise and we are at the mercy of the printing company when it comes to final quality.
The key difference will be burning an Adobe Acrobat version of the Semaphore to a compact disc and mailing the CD to the members instead of the printed version.
Members can read the Semaphore on their computer screen after opening the file from the CD and if the choose, can print their own copy to read if they want it on paper.
gtwhs.tripod.com /semaphore.htm   (459 words)

  
 Semaphore Inc.
Semaphore, Inc. is a premier full-service advertising, marketing, branding, public relations and Internet development firm.
Semaphore produced a full range of marketing products for the Historic Columbia Foundation.
Semaphore takes a look at the reservations these small firms might have about using a logo.
www.semaphoreinc.com   (176 words)

  
 Port Windows IPC apps to Linux, Part 2: Semaphores and events
as -1, the semaphore count is decreased by 1 and if the value falls less than zero (since semaphore values cannot go less than zero), the semaphore count is not decreased but the calling thread/process is blocked until the semaphore is signaled.
POSIX semaphores are count semaphores, but when the count is set to 1 they provide similar functionality to the Windows event object.
POSIX semaphores are preferred when the timeout is not the factor in the porting.
www.ibm.com /developerworks/library/l-ipc2lin2.html?ca=dnt-621   (3133 words)

  
 Command Semaphore
A semaphore is a flag shared among workstations (each user's computer) or among processes on the same workstation.
A local semaphore is accessible by all processes on the same workstation and only on the workstation.
For example, a local semaphore can be used to monitor access to an interprocess array shared by all the processes in your single-user database or on the workstation.
www.4d.com /docs/CMU/CMU00143.HTM   (612 words)

  
 semaphore : Java Glossary
A semaphore is a flag used to check whether a resource is currently being used by another thread or process.
The process of grabbing a semaphore for semi-exclusive use is called downing the semaphore because they are implemented with a countdown integer that decrements for each lock and increments for each unlock.
For a semaphore to work, the check for full, and the decrement must be done all in one atomic uninterruptible instruction.
mindprod.com /jgloss/semaphore.html   (328 words)

  
 semop(2): semaphore operations - Linux man page
Each operation is performed on the sem_num-th semaphore of the semaphore set, where the first semaphore of the set is semaphore 0.
On successful completion, the sempid value for each semaphore specified in the array pointed to by sops is set to the process ID of the calling process.
When a semaphore's value is directly set using the SETVAL or SETALL request to semctl(2), the corresponding semadj values in all processes are cleared.
www.die.net /doc/linux/man/man2/semop.2.html   (1298 words)

  
 Semaphore
initial - the initial count of the semaphore (must be 0 to max).
name - name of the Semaphore, default is unnamed (empty string does the same).
Releases the semaphore object and increments its count by the specified amount.
www.newobjects.com /pages/ndl/axpack1/Semaphore.htm   (107 words)

  
 semaphore. The Columbia Encyclopedia, Sixth Edition. 2001-05
The marine semaphore, used by day between ships or between a ship and the shore, consists essentially of a post at the top of which are two pivoted arms.
In the railroad semaphore a single projecting arm pivoted at one end and attached to a vertical post is devised to take three positions.
Horizontal indicates stop, and vertical, all clear; the inclined position indicates that the locomotive may go ahead under control expecting to be stopped.
www.bartleby.com /65/se/semaphor.html   (188 words)

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