Re: "Real" Multitasking (Re: GNO questions...)

[jimwong@owlnet.rice.edu (Jim Wong)]

In article <jmk3.735453502@crux1.cit.cornell.edu> jmk3@crux3.cit.cornell.edu (Jay Krell) writes:
>
>What's a semaphore? (Maybe I should read comp.unix.*)
>
A semaphore is a pretty simple data structure that can be used to solve
synchronization probles, such as multiple processes accessing a global
variable.

A semaphore can be initialized as follows:
	
Semaphore s = InitialValue;

Each semaphore, then, supports two atomic (that is, they can't be interrupted
by the scheduler) operations, P and V.  They have the following effects:

P(s)
{
  while (s <= 0)
     ;
  s--;
}

V(s)
{
  s++
}


Note that in an actual implementation, a process doesn't busy wait in the 
while loop inside P; instead, the process would be blocked once it
became apparent that it was going to be stuck.
-- 
Jim Wong (jimwong@owlnet.rice.edu)