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semaphore    
n. 臂式号志,信号,手旗信号
vt. 以信号机联络
vi. 打信号

臂式号志,信号,手旗信号以信号机联络打信号

semaphore
信号; 信号标志; 信号灯

semaphore
旗号

semaphore
n 1: an apparatus for visual signaling with lights or
mechanically moving arms
v 1: send signals by or as if by semaphore
2: convey by semaphore, of information

Semaphore \Sem"a*phore\, n. [Gr. sh^ma a sign fe`rein to bear:
cf. F. s['e]maphore.]
A signal telegraph; an apparatus for giving signals by the
disposition of lanterns, flags, oscillating arms, etc.
[1913 Webster] Semaphoric

72 Moby Thesaurus words for "semaphore":
Roman candle, aid to navigation, alarm, amber light, balefire,
beacon, beacon fire, bell, bell buoy, blinker, blue peter, buoy,
caution light, flare, fog bell, fog signal, fog whistle, foghorn,
glance, go light, gong buoy, green light, heliograph, high sign,
international alphabet flag, international numeral pennant, kick,
leer, marker beacon, nod, nudge, parachute flare, pilot flag, poke,
police whistle, quarantine flag, radio beacon, red flag, red light,
rocket, sailing aid, semaphore flag, semaphore telegraph, sign,
signal, signal beacon, signal bell, signal fire, signal flag,
signal gong, signal gun, signal lamp, signal light, signal mast,
signal post, signal rocket, signal shot, signal siren,
signal tower, spar buoy, stop light, the nod, the wink, touch,
traffic light, traffic signal, watch fire, white flag, wigwag,
wigwag flag, wink, yellow flag

The classic method for
restricting access to shared resources (e.g. storage) in a
{multi-processing} environment. They were invented by
{Dijkstra} and first used in {T.H.E} {operating system}.

A semaphore is a {protected variable} (or {abstract data
type}) which can only be accessed using the following
operations:

P(s)
Semaphore s;
{
while (s == 0) ; /* wait until s>0 */
s = s-1;
}

V(s)
Semaphore s;
{
s = s1;
}

Init(s, v)
Semaphore s;
Int v;
{
s = v;
}

P and V stand for Dutch "Proberen", to test, and "Verhogen",
to increment. 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). The P
operation {busy-waits} (or maybe {sleeps}) until a resource is
available whereupon it immediately claims one. V is the
inverse, it simply makes a resource available again after the
process has finished using it. Init is only used to
initialise the semaphore before any requests are made. The P
and V operations must be {indivisible}, i.e. no other process
can access the semaphore during the their execution.

To avoid {busy-wait}ing, a semaphore may have an associated
{queue} of processes (usually a {FIFO}). 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.

(1995-02-01)


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  • multithreading - What is a semaphore? - Stack Overflow
    A semaphore has two parts : a counter, and a list of tasks waiting to access a particular resource A semaphore performs two operations : wait (P) [this is like acquiring a lock], and release (V)[ similar to releasing a lock] - these are the only two operations that one can perform on a semaphore
  • Semaphores - University of Wisconsin–Madison
    1 #include <semaphore h> 2 sem_t s; 3 sem_init( s, 0, 1); Figure 31 1: Initializing A Semaphore In the figure, we declare a semaphore s and initialize it to the value 1 bypassing1inasthethirdargument Thesecondargumenttoseminit() will be set to 0 in all of the examples we’ll see; this indicates that the
  • java - How does semaphore work? - Stack Overflow
    Some other implementations of a semaphore may have an idea of a "maximum" number of permits, and a call to release beyond that maximum would fail The Java Semaphore class allows a reverse situation, where a semaphore can start off with a negative number of permits, and all acquire() calls will fail until there have been enough release() calls
  • c++ - Why use a mutex and not a semaphore? - Stack Overflow
    The cppreference documentation doesn't state that binary_semaphore is more efficient than mutex - it says it may be more efficient than a generic counting_semaphore<1> Differences: Mutex: For mutual exclusion - same thread must lock and unlock Semaphore: For signaling between threads - acquire and release can happen on different threads
  • Difference between binary semaphore and mutex - Stack Overflow
    In counting semaphore handles access to n number of resources where n can be defined before the use Each semaphore has a count variable, which keeps the count of the number of resources in use, initially, it is set to n Each process that wishes to uses a resource performs a wait() operation on the semaphore (thereby decrementing the count)
  • CONCURRENCY: SEMAPHORES - University of Wisconsin–Madison
    SEMAPHORE OPERATIONS Wait or Test: sem_wait(sem_t*) Decrements semvalue by 1, Waits if value of semis negative (< 0) Signal or Post: sem_post(sem_t*) Increment semvalue by 1, then wake a single waiter if exists Value of the semaphore, when negative = the number of waiting threads
  • When should we use mutex and when should we use semaphore
    Binary semaphore and Mutex are different From OS perspective, a binary semaphore and counting semaphore are implemented in the same way and a binary semaphore can have a value 0 or 1 Mutex-> Can only be used for one and only purpose of mutual exclusion for a critical section of code Semaphore-> Can be used to solve variety of problems A
  • multithreading - When should I use semaphores? - Stack Overflow
    The semaphore cannot be bypassed, because it would break the blocking case Passing through the semaphore only if the pool is not empty may lead to a deadlock if done under the lock, but as soon as the lock is released, the result of the check for emptiness becomes useless
  • c - semaphore implementation - Stack Overflow
    the semaphore resources are pretty limited on every unixes you can check these with the 'ipcs' command there is an undo feature of the System V semaphores, so you can make sure that abnormal program termination doesn't leave your semaphores in an undesired state
  • Semaphore: understanding initial and maximum number of requests
    you can change semaphore count between 0 and maximum value, so when you are setting maximum value to 6, and initial value to 4 it means your potential for concurrency is 6 but currently you offer 4 concurrent resources, if in some cases you decide that you want to increase concurrency you can call Release which will open another slot for you however you cannot exceed maximum value set for





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