Implementing counting semaphores using binary semaphores

Implementation:

CSem(K) cs { // counting semaphore initialized to K 
int val ← K; // the value of csem 
BSem gate(min(1,val)); // 1 if val > 0; 0 if val = 0 
BSem mutex(1); // protects val 

Pc(cs) { 
P(gate) 
a1: P(mutex); 
val ← val − 1; 
if val > 0 V(gate); 
V(mutex); 
} 

Vc(cs) { 
P(mutex); 
val ← val + 1; 
if val = 1 
V(gate); 
V(mutex); 
}
}

Another Solution:

var mutex=1: binary-semaphore; 
delay={min(1,initvalue)}: binary-semaphore; 
C={initvalue}: integer; 

Procedure Wait() 
begin 
wait(delay); 
wait(mutex); 
C:=C-1; 
if C>0 then 
signal(delay); 
signal(mutex); 
end 

Procedure Signal() 
begin 
wait(mutex); 
C:=C+1; 
if C=1 then 
signal(delay) 
signal(mutex) 
end 

Conclusions 

• The implementation of general semaphores using binary semaphores must be implemented carefully so no concurrency errors are introduced 

• Various solutions exist, when choosing a solution examine the performance characteristics of each that best suits your needs 

• Implementation of general semaphores using binary semaphores is not recommended when efficiency is a concern