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'.

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.

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.

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

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

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.

 

© Webster 1913.