Semaphore

created by Webster 1913

(thing) by art...or something (7.2 y) (print) ? (I like it!) Thu Jul 20 2000 at 7:03:16

The signalling with flags, each position indicative of a different letter, to eventually spell out words or (if you hang around for long enough) sentences. Possibly the slowest and yet most visually entertaining form of communication in the world.

Also featuring in the highly amusing Monty Python skit entitled 'Wuthering Heights in Semaphore'.

(thing) by rp (2.9 hr) (print) ? (I like it!) Thu Jul 20 2000 at 10:29:57

Based on the railroad signpost, semaphores are also a software mechanism to regulate process synchronization, proposed by Edsger W. Dijkstra.

A semaphore is a software object. Its state is a nonnegative integer. Two methods are defined: decrement (known as P, for passeren, "pass"), and increment (known as V, for vrijgeven, "release").

A semaphore is shared by processes that wish to use a common resource. It is initialized with the number of processes that may use the resource concurrently. At any time, its state represents its availability.

Each participating process guards its critical sections of code, that is, the sections using the shared resource, by putting a P on the guarding semaphore in front of it, and a V at the end. A P blocks if the state is 0 (i.e. no more processes may use the resource at this time).

Semaphores are implemented as part of SysV IPC. man semop.

(idea) by everyone (3.2 mon) (print) ? (I like it!) Sun Nov 05 2000 at 3:58:37

There are also POSIX semaphores, completely independent of IPC stuff, though they are not very well implmented on Linux SPARC due to hardware issues.

man sem_init.

(idea) by Tristan (1.1 mon) (print) ? (I like it!) 8 C!s Mon Mar 05 2001 at 9:02:43

The flag positions

There are 5 positions for each hand - down, low, out, high and up. Some letters require both arms to be on the same side of the body.
LH = left hand
RH = right hand

(These diagrams are of a person facing you so the left and right sides are swapped.)

A and 1 - LH down RH low

           _ 
          / \
        __\ /__
       /       |
      / /|   | |
  /\ |_/ |   | |
 /  \/   |___|\/
 \  /    | | | |__
  \/     | | | |  |
         |_|_| |__|

B and 2 - LH down RH out

           _
          / \
      ____\ /__
 ____|___      |
|  |     |   | |
|__|     |   | |
         |___|\/
         | | | |__
         | | | |  |
         |_|_| |__|

C and 3 - LH down RH high


  /\
 /  \
 \  /\_
  \/ | \   _
      \ \ / \
       \ \\ /__
        \      |
         |   | |
         |   | |
         |___|\/
         | | | |__
         | | | |  |
         |_|_| |__|

D and 4 - LH down RH up (or LH up RH down)

        __
       |  |
       |__|
       |_
       | | _
       | |/ \
       | |\ /__
       |_      |
         |   | |
         |   | |
         |___|\/
         | | | |__
         | | | |  |
         |_|_| |__|

E and 5 - LH high RH down

                   /\
                  /  \
                _/\  /
           _   / | \/
          / \ / /
        __\ // /
       |      /
       | |   |
       | |   |
       \/|___|
     __| | | |
    |  | | | |
    |__| |_|_|

F and 6 - LH out RH down

           _
          / \
        __\ /____
       |      ___|____
       | |   |     |  |
       | |   |     |__|
       \/|___|
     __| | | |
    |  | | | |
    |__| |_|_|

G and 7 - LH low RH down

           _
          / \
        __\ /__
       |       \
       | |   |\ \
       | |   | \_| /\
       \/|___|   \/  \
      __|| | |    \  /
     |  || | |     \/
     |__||_|_|

H and 8 - LH across low RH out

           _
          / \
      ____\ /_
 ____|___     |
|  |     | / /
|__|     |/ /|
     /\  |_/_|
    /  \/| | |
    \  / | | |
     \/  |_|_|

I and 9 - LH across low RH up

        __
       |  |
       |__|
       |_
       | | _
       | |/ \
       | |\ /_
       |_     |
         | / /
         |/ /|
     /\  |_/_|
    /  \/| | |
    \  / | | |
     \/  |_|_|

J and 'alphabetic' - LH out RH up

        __
       |  |
       |__|
       |_
       | | _
       | |/ \
       | |\ /____
       |_     ___|____ 
         |   |     |  |
         |   |     |__|
         |___|
         | | |
         | | |
         |_|_|

K and 0 - LH up RH low

             __
            |  |
            |__|
              _|
           _ | |
          / \| |
        __\ /| |
       /      _|
      / /|   |
  /\ |_/ |   |
 /  \/   |___|
 \  /    | | |
  \/     | | |
         |_|_|

L - LH high RH low


                   /\
                  /  \
                _/\  /
           _   / | \/
          / \ / /
        __\ // /
       /      /
      / /|   |
  /\ |_/ |   |
 /  \/   |___|
 \  /    | | |
  \/     | | |
         |_|_|

M - LH out RH low

           _
          / \
        __\ /____
       /      ___|____
      / /|   |     |  |
  /\ |_/ |   |     |__|
 /  \/   |___|
 \  /    | | |
  \/     | | |
         |_|_|

N - LH low RH low

           _
          / \
        __\ /__
       /       \
      / /|   |\ \
  /\ |_/ |   | \_| /\
 /  \/   |___|   \/  \
 \  /    | | |    \  /
  \/     | | |     \/
         |_|_|

O - LH across high RH out

       
      /\
     /  \
     \  /\_
      \/ | \
          \ \
      ____\\ \
 ____|___    |
|  |     |   |
|__|     |   |
         |_ _|
         | | |
         | | |
         |_|_|

P - LH up RH out

             __
            |  |
            |__|
              _|
           _ | |
          / \| |
      ____\ /| |
 ____|___     _|
|  |     |   |
|__|     |   |
         |_ _|
         | | |
         | | |
         |_|_|

Q - LH high RH out


                   /\
                  /  \
                _/\  /
           _   / | \/
          / \ / /
      ____\ // /
 ____|___     /
|  |     |   |
|__|     |   |
         |___|
         | | |
         | | |
         |_|_|

R - LH out RH out

           _
          / \
      ____\ /____
 ____|___     ___|____
|  |     |   |     |  |
|__|     |   |     |__|
         |___|
         | | |
         | | |
         |_|_|

S - LH low RH out

           _
          / \
      ____\ /__
 ____|___      \
|  |     |   |\ \
|__|     |   | \_| /\
         |___|   \/  \
         | | |    \  /
         | | |     \/
         |_|_|

T - LH up RH high

             __
  /\        |  |
 /  \       |__|
 \  /\_       _|
  \/ | \   _ | |
      \ \ / \| |
       \ \\ /| |
        \     _|
         |   |
         |   |
         |___|
         | | |
         | | |
         |_|_|

U - LH high RH high


  /\               /\
 /  \             /  \
 \  /\_         _/\  /
  \/ | \   _   / | \/
      \ \ / \ / /
       \ \\ // /
        \     /
         |   |
         |   |
         |___|
         | | |
         | | |
         |_|_|

V - LH low RH up

        __
       |  |
       |__|
       |_
       | | _
       | |/ \
       | |\ /__
       |_      \
         |   |\ \
         |   | \_| /\
         |___|   \/  \
         | | |    \  /
         | | |     \/
         |_|_|

W - LH out RH across high


               /\
              /  \
            _/\  /
           / | \/
          / /
         / //____
         |    ___|____
         |   |     |  |
         |   |     |__|
         |_ _|
         | | |
         | | |
         |_|_|

X - LH low RH across high


               /\
              /  \
            _/\  /
           / | \/
          / /
         / //__
         |     \
         |   |\ \
         |   | \_| /\
         |_ _|   \/  \
         | | |    \  /
         | | |     \/
         |_|_|

Y - LH out RH high


  /\
 /  \
 \  /\_
  \/ | \   _
      \ \ / \
       \ \\ /____
        \     ___|____
         |   |     |  |
         |   |     |__|
         |___|
         | | |
         | | |
         |_|_|

Z - LH out RH across low

           _
          / \
         _\ /____
        |     ___|____
         \ \ |     |  |
         |\ \|     |__|
         |_\_|  /\
         | | |\/  \
         | | | \  /
         |_|_|  \/

Numerical sign - LH high RH up

        __
       |  |        /\
       |__|       /  \
       |_       _/\  /
       | | _   / | \/
       | |/ \ / /
       | |\ // /
       |_     /
         |   |
         |   |
         |___|
         | | |
         | | |
         |_|_|

Annul sign - LH low RH high


  /\
 /  \
 \  /\_
  \/ | \   _
      \ \ / \
       \ \\ /__  
        \      \       
         |   |\ \      
         |   | \_| /\  
         |___|   \/  \
         | | |    \  /
         | | |     \/
         |_|_|

Error - LH and RH raised and lowered together


  /\               /\
 /  \             /  \
 \  /\_         _/\  /
  \/ | \   _   / | \/
 /    \ \ / \ / /   \
|      \ \\ // /     |
|      /       \     | 
 \    / /|   |\ \   /  
  /\ |_/ |   | \_| /\  
 /  \/   |___|   \/  \
 \  /    | | |    \  /
  \/     | | |     \/
         |_|_|

(definition) by Webster 1913 (print) Wed Dec 22 1999 at 3:00:34

Sem"a*phore (?), n. [Gr. a seign + to bear: cf. F. s'emaphore.]

A signal telegraph; an apparatus for giving signals by the disposition of lanterns, flags, oscillating arms, etc.

printable version
chaos

Edsger W. Dijkstra	process synchronization	IPC	Sleeping barber problem
Well if it's gonna be THAT kinda party, then I'm gonna stick my dick in the mashed potatoes!	What ever happened to all the fun in the world	visible language	critical section
mutex	Monty Python	monitor	Pilots' Jargon
POSIX	Wuthering Heights	condition variable	Block
mutual exclusion	A Convoluted History of Early Telecommunications	ENOENT	futex
Aldis lamp	sock on the door knob	signaling system	Nexus

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