semaphore

noun

1. (countable, uncountable)Any equipment used for visual signalling by means of flags, lights, or mechanically moving arms, which are used to represent letters of the alphabet, or words.

   “We must here take the liberty of expostulating with Sir Home [Riggs] Popham and the first Lord of the Admiralty, for having given to the telegraphic machine, invented by that gallant officer, the barbarous name of Semaphore, instead of Sematophore or Semophore—either of them ugly enough.”

   “The large Silver Medal of the Society was this Session voted to Nic[h]olas Harris Nicolas, Esq. of the Inner Temple, for an Improvement on the Vertical Semaphore, and for his method of adapting a shifting Key to Telegraphic Communications, for the purpose of insuring their Secrecy. A Model of Mr. N's Semaphore has been placed in the Repository of the Society.”

   “That the systems of telegraph and semaphore now in use are in a great measure use-less by night, and totally so in a fog, cannot be doubted; and that a mode, both rapid and secret, would could be put into practice at small expense, in fact little more than the first cost, would be of essential utility to the Government of the country adopting it, is equally true.”

   “The arms of the semaphore at the settlement were, in fact, gesticulating with comical vehemence.”

   “When, on long lines of stations, towers or other structures are used, it may be necessary, for greater speed, to sometimes employ semaphores for aerial telegraphy. [...] Semaphores consist of a post with arms. The arms starting with about three feet in length, to be increased one foot for every mile. These arms are made movable by ropes passing over wheels or pulleys, and moved by a crank below.”

2. (countable, uncountable)A visual system for transmitting information using the above equipment; especially, by means of two flags held one in each hand, using an alphabetic and numeric code based on the position of the signaller's arms; flag semaphore.

   “Buried treasures are naturally among the problems which have come to Mr. [Sherlock] Holmes. One genuine case was accompanied by a diagram here reproduced. [...] Each Indiaman in those days had its own semaphore code, and it is conjectured that the three marks upon the left are signals from a three-armed semaphore.”

   “A system of communication called semaphore uses flags or flashing lights to send messages over distances. [...] Code flags are less common today but are sometimes used by ships that have lost radio contact.”

   “As always in Lisbon, my heart throbbed semaphores to call Tónio.”

   “Consider the wagging tail, the most basic semaphore in dog/human communication.”

3. (countable, uncountable)A bit, token, fragment of code, or some other mechanism which is used to restrict access to a shared function or device to a single process at a time, or to synchronize and coordinate events in different processes.

   “The thread increments the semaphore to prevent other threads from entering the critical section at the same time.”

   “Several synchronization techniques have been proposed and many of them have been adopted by parallel and distributed operating systems or parallel programming platforms. [...] Semaphores represent another synchronization technique that is mainly used by traditional stand-alone operating systems.”

   “A semaphore makes sure that the different processes don't step on each other's toes when they access the shared memory segment. Before a process can use the segment, it needs to get control of the semaphore. When it's done with the segment, it releases the semaphore for another process to grab.”

   “It is up to the programmer to ensure that if one thread updates multiple dependent memory locations (for example, writing a string, or updating a table) that another thread might read, some protection is put in place to ensure that the two threads don't update and read at the same time. [...] Several techniques are used by programmers to prevent this, including locks, semaphores, and mutexes.”

verb

1. (ambitransitive, figuratively)To signal using, or as if using, a semaphore, with the implication that it is done nonverbally.

   “These gunny-sack chaps are not animals or savages. Look, Henry! They are semaphoring! See that tree there, and that big one across the cañon. Watch the branches wave.”

   “Minutes later, unseen by the defenders, he semaphored back across the valley that he was going to make a fresh attempt.”

   “Near the concession stand on the east side of the park, she saw two familiar figures sitting on a bench in the shade of a large oak. Her father hailed her, wildly semaphoring with his hot dog.”

   “He can't hear what they're saying, but Jake is making angry movements with his hands and his taut posture semaphores, even to Daniel, his distinct displeasure.”