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Topic: Semaphore (communication)

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In the News (Thu 18 Jul 19)

  Semaphore (communication) - Wikipedia, the free encyclopedia
The semaphore or optical telegraph is an apparatus for conveying information by means of visual signals, with towers with pivoting blades or paddles, shutters, in a matrix, or hand-held flags etc. Information is encoded by the position of the mechanical elements; it is read when the blade or flag is in a fixed position.
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.
en.wikipedia.org /wiki/Semaphore_(communication)   (2217 words)

 Semaphore - Wikipedia, the free encyclopedia
Semaphore communication by means of flags or other positional indicators
Semaphores in computer science, in which only one part of a concurrent system may access a limited resource; this application of mutual exclusion in computing was invented by Edsger Dijkstra
Semaphore, South Australia, an historic seaside community in southern Australia
en.wikipedia.org /wiki/Semaphore   (141 words)

 [No title]
A semaphore is a particular structure containing a value grater or equal to zero and that manages a queue of processes waiting for a particular condition on the semaphore itself.
Semaphores can be used to control resource access: the value of the semaphore represents the number of processes which can access the resource; any time a process accesses the resource the value of the semaphore shall be decremented and incremented again when the resource is released.
If the absolute value of the semaphore is greater than or equal to that of sem_op the operation goes on and sem_op is added to the value of the semaphore (actually it is subtracted, negative number).
www.tldp.org /linuxfocus/English/January2003/article281.meta.shtml   (2296 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)

 Interprocess Communication APIs
A thread waiting on a semaphore in a semaphore set appears to be in a semaphore wait state (SEMW) on the Work with Threads display (requested using the WRKJOB command and taking option 20).
Once a semaphore is created and a pointer to the semaphore is obtained, the same operations are used to manipulate the values of both types of semaphores.
After the value of the semaphore is incremented, it may be decremented immediately by threads that have blocked trying to decrement the semaphore.
www.redbooks.ibm.com /pubs/html/as400/v4r5/ic2924/info/apis/unix3.htm   (3474 words)

 File 97289
Semaphores are generally not used one at a time, so Unix uses the concept of semaphore sets to make it easier to allocate and refer to semaphores.
Semaphores are also used to control two-task communication between the user process and shadow process if the fast (shared memory) driver is used.
Since it's hard to tell which semaphore set belongs to which process, you may have to wait until everyone logs off the database (and perhaps for the database to be shut down) before you can tell which semaphore set you need to free.
www.uaex.edu /srea/osee/97289.htm   (1186 words)

 Inter-Process Communication - Part II LG #105
The processes may communicate data among themselves, or it may be communicated by a shared data object.
Generally, sockets are associated with the concept of network communication in the form of client-server programming; a pair of processes of which one will be a client and one a server.
Exploring the use of sockets in an connectionless-oriented communication is left as an exercise to the readers.
linuxgazette.net /105/ramankutty.html   (3820 words)

 Learn Semaphore   (Site not responding. Last check: 2007-11-06)
Semaphore is a communication medium by which a person signals with two hand flags, moving his arms through various positions to represent letters, numerals, and special signs.
Semaphore and flashing light can be used interchangeably for many purposes, but semaphore is more rapid for short-distance transmission in clear daylight and may be used to send messages to several addressees at once if they are in suitable positions.
Wigwag, a semaphore system of signal communication, was invented in the United States during the 19th century and used in the Civil War.
www.lpickard.com /sem.htm   (379 words)

 CodeNotes® - Article Display
A semaphore is a variable that performs the operations of the locking mechanism.
Any other thread that queries the semaphore will now be blocked by the operating system and must wait until the semaphore returns to its open state.
The counting semaphore is initialized to a value representing the total number of threads allowed through the door at one time.
www.codenotes.com /articles/articleAction.aspx?articleID=491   (1448 words)

 [No title]
You could use a semaphore with an initial value of 1 and, around the file operating code, put two semaphore operations, the first to test and decrement the semaphore's value and the second to test and increment it.
The semaphore index is an index into the semaphore array and the operation value is a numerical value that will be added to the current value of the semaphore.
If all of the semaphore operations would have succeeded and the current process does not need to be suspended, Linux goes ahead and applies the operations to the appropriate members of the semaphore array.
www.tldp.org /LDP/tlk/ipc/ipc.html   (3796 words)

 [No title]
Interprocess communication in UNIX ================================================================================ Overview: When processes are coordinating their efforts to solve a problem or implement some application (like a distributed database, communica- tion system, etc.) they need to "talk" to each other.
P is the standard semaphore name for acquiring a semaphore, or waiting for it to become free.
C programmers might be tempted to use a file as a semaphore variable or a flag.
www-cs.canisius.edu /OLDSITE1997/PL_TUTORIALS/C/ADVANCED/ipc   (3045 words)

 Overview of Semaphore Operations
Semaphores are typically incremented by a process to block other processes while it is performing a critical operation or using a shared resource.
Semaphores can be configured as binary semaphores which have only two values: 0 and 1, or they can serve as general semaphores (or counters) where one process increments the semaphore and one or more cooperating processes decrement it.
Since the value of any semaphore when such conditions occur is unpredictable, the system enforces a limit on how much the value of a semaphore can change any undo operation.
docs.hp.com /en/TKP-90202/ch06s03.html   (392 words)

 semctl(3): XSI semaphore control operations - Linux man page
Return the value of semval for each semaphore in the semaphore set and place into the array pointed to by arg.array, where arg is the fourth argument to semctl().
Set the value of semval for each semaphore in the semaphore set according to the array pointed to by arg.array, where arg is the fourth argument to semctl().
The value of semid is not a valid semaphore identifier, or the value of semnum is less than 0 or greater than or equal to sem_nsems, or the value of cmd is not a valid command.
www.die.net /doc/linux/man/man3/semctl.3.html   (826 words)

 Semaphore Corporation: Acceptable Usage Policy
Semaphore reserves the right to vary this process on a case-by-case basis or to change this process at any time.
Semaphore receives (at abuse@semaphore.com) complaints ("Complaints") from third parties regarding Prohibited Activities allegedly being conducted by a Customer or its Users.
Semaphore reserves the right to suspend and/or terminate a Customer's Service at any time for any material failure of Customer, its Representatives or its Users to comply with these Rules and Regulations.
www.semaphore.com /abuse.html   (651 words)

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)

 [No title]
Counting semaphore is just like the binary semaphore with the exception that it keeps track of the number of times a semaphore has been given or taken.
For named semaphores, a symbolic name is assigned to a semaphore and all references to that semaphore are made using the symbolic name.
Semaphores must be used to provide mutual exclusion or synchronization for shared memory segments.
www.nvc.cs.vt.edu /~cyrus/f00/POSIX_IPC.doc   (1885 words)

 CO42018 Lab 4 - Java Semaphores   (Site not responding. Last check: 2007-11-06)
Semaphores are an elegant way of achieving inter process synchronisation and mutual exclusion.
Semaphores are a programming technique first devised by E. Dijkstra in the late 1960s.
Semaphores were originally described by Dijkstra in his article Co-operating Sequential Processes (published in Programming Languages (Genuys F., ed.) Academic Press, 1968).
www.dcs.napier.ac.uk /~shaun/rtse/labs/lab04.html   (854 words)

 Semaphore communication - Wikipedia, the free encyclopedia   (Site not responding. Last check: 2007-11-06)
Look for Semaphore communication in Wiktionary, our sister dictionary project.
Look for Semaphore communication in the Commons, our repository for free images, music, sound, and video.
Check for Semaphore communication in the deletion log, or visit its deletion vote page if it exists.
www.sciencedaily.com /encyclopedia/semaphore__communication_   (146 words)

Communication mechanisms allow processes to exchange information, which in the object-oriented case will be in the form of objects or object structures.
A semaphore is an object on which two operations are available: reserve and free (traditionally called P and V, but more suggestive names are preferable).
But if you devise a good communication mechanism you might very well have solved synchronization too: because two units cannot communicate unless the sender is ready to send and the receiver ready to receive, communication implies synchronization; pure synchronization may be viewed as the extreme case of communicating an empty message.
archive.eiffel.com /doc/manuals/technology/concurrency/concurrency-05.html   (1950 words)

 NQCIPC Documentation
This package is a very simple implementation of semaphores and intertask messages for Lego Mindstorms designed for use with the NQC package.
In accordance with the prevailing practice in the MindStorms developer community, NQCIPC is (c)1999 Brian Connors, made available under the terms of the Mozilla Public License.
A semaphore is like a "talking stick": it's a variable that several tasks have access to that lets them know not to all talk at once.
www.geocities.com /ResearchTriangle/Station/2266/nqcipc/nqcipcdoc.html   (1173 words)

 Semaphore   (Site not responding. Last check: 2007-11-06)
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)

 semaphore | esources | 2002 | june | brand semaphore   (Site not responding. Last check: 2007-11-06)
As for the Semaphore brand, our name is literally a symbol for communication as our business is the practice of communication.
(Semaphore is defined as a system of visual communication through the positioning of flags.) And although the only flags we use are in our own corporate branding design, we have created successful visual communication through branding and other marketing efforts for a wide variety of clients in more than 30 years in business.
However, the latest incarnation is the first attempt to make full use of the literal definition of Semaphore and to incorporate flag imagery as part of the overall design.
www.semaphoreinc.com /esources/newsletters/2002_06_c.html   (200 words)

 History of Telecommunications
Their semaphore system consisted of movable arms on a pole whose positions denoted letters of the alphabet.
Semaphores weren't very successful in England because of the fog and smog caused by the Industrial Revolution.
There were semaphore systems in the U.S., especially from Martha's Vineyard (an island near Cape Cod) and Boston, reporting to Boston's Custom House on the movement of sailing ships.
www.cclab.com /billhist.htm   (6066 words)

 Port Windows IPC apps to Linux, Part 2: Semaphores and events   (Site not responding. Last check: 2007-11-06)
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-128.ibm.com /developerworks/library/l-ipc2lin2.html?ca=drs-t2205   (3123 words)

 semaphore | esources | july
Semaphore Account Executive Marian Douglas Pearce, APR, recently earned recognition as a professional communicator in a national program offered by the Council of Rural Electric Communicators, which is affiliated with the National Rural Electric Cooperative Association (NRECA).
This is done through the establishment of professional development goals, identification of a body of knowledge and skills necessary to the practice of electric co-ops communication, and recognition of those individuals who have demonstrated a professional level of excellence.
Her portfolio was judged by communicators in the electric co-op industry who have already earned the CCC designation.
www.semaphoreinc.com /esources/newsletters/2003_07_esources.html   (552 words)

 Assignment 1
A constructor Semaphore (int limit), where limit, signifies the number times the critical region can be entered before blocking, i.e., a value of 1 for limit would imply a binary semaphore.
In order to do the communication, the server will have to open a datagram socket specifying a port number and keep listening to it in an infinite loop.
Remember that in all the above methods, the distributed semaphore object simply sends a request to the server and waits for the reply and returns the status as in the reply.
www.cs.cornell.edu /Courses/cs414/2000su/asgn2.html   (1190 words)

 Multitasking & communication   (Site not responding. Last check: 2007-11-06)
If the semaphore has a negative value, then this can represent the number of tasks waiting for the semaphore.
The commonest use of semaphores is for mutual exclusion.
Note, very similar to semaphore, but many systems use a separate mechanism to make it clear that this is synchronising tasks.
www.dcs.napier.ac.uk /~alistair/EE42005/EE42005_3.htm   (561 words)

 Semaphore Flag Signalling System
The Semaphore flag signaling system is an alphabet signalling system based on the waving of a pair of hand-held flags in a particular pattern.
The flags are usually square, red and yellow, divided diagonaly with the red portion in the upper hoist.
The pattern resembles a clock face divided into eight positions: up, down, out, high, low, for each of the left and right hands (LH and RH) six letters require the hand to be brought across the body so that both flags are on the same side.
www.anbg.gov.au /flags/semaphore.html   (304 words)

 More on 'Semaphore Communication'.
To progress your search for the term semaphore communication, a visit to the Connected Earth website is likely to help you.
It's a complete multi-media experience, where you can switch between clearly-written stories, more detailed study, images of exhibits in 3D, reminiscences in oral and written form from people who - in the past - worked in the telecommunications industry, film clips, and simple explanations or interactive animations of the way that things work.
Connected Earth is the place to continue your study of the term semaphore communication.
www.connected-earth.com /content/semaphore_communication.html   (287 words)

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