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semaphoremore about semaphore

semaphore

 

  3  definitions  found 
 
  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
 
  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 
 
  From  WordNet  r  1.6  [wn]: 
 
  semaphore 
  n  :  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 
 
  From  The  Free  On-line  Dictionary  of  Computing  (13  Mar  01)  [foldoc]: 
 
  semaphore 
 
  operating  system>  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  =  s+1; 
  } 
 
  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-wait}s  (or  maybe  {sleep}s)  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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