Binary Semaphore or Mutex

In counting semaphore, Mutual exclusion was not provided because we has the set of processes which required to execute in the critical section simultaneously.

However, Binary Semaphore strictly provides mutual exclusion. Here, instead of having more than 1 slots available in the critical section, we can only have at most 1 process in the critical section. The semaphore can have only two values, 0 or 1.

Let's see the programming implementation of Binary Semaphore.

StructBsemaphore
{
	enum Value(0,1); //value is enumerated data type which can only have two values 0 or 1.
	Queue type L;
}
/* L contains all PCBs corresponding to process 
Blocked while processing down operation unsuccessfully. 
*/ 
Down (Bsemaphore S) 
{
	if (s.value == 1) // if a slot is available in the 
	//critical section then let the process enter in the queue. 
	{
		S.value = 0; // initialize the value to 0 so that no other process can read it as 1. 
	}
	else
	{
		put the process (PCB) in S.L; //if no slot is available 
		//then let the process wait in the blocked queue. 
		sleep(); 
	}
}
Up (Bsemaphore S) 
{
	if (S.L is empty) //an empty blocked processes list implies that no process 
	//has ever tried to get enter in the critical section. 
	{
		S.Value =1; 
	}
	else
	{
		Select a process from S.L; 
		Wakeup(); // if it is not empty then wake the first process of the blocked queue. 
	} 
}

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