Semaphores in Os (Operating Systems) with example

What are Semaphores in Os?

• Semaphores are a synchronization mechanism introduced by Edsger Dijkstra in the late 1960s.
• Semaphores provide a way to achieve mutual exclusion and coordinate access to shared resources.
• Semaphores come in two main types: binary semaphores and counting semaphores.

What is Wait Operation in Semaphores?

• The "wait" operation, often denoted as P, is used by a process to request entry into a critical section or to acquire a resource.
• The steps involved in the wait operation are as follows:

1wait(M)
2{
3    while (M <= 0);
4    M--;
5}

• It checks if the semaphore value M is greater than 0.
• If M is not greater than 0, the process enters a loop, continuously checking the condition until it becomes true.
• Once the condition is true (i.e., M becomes greater than 0), the process decrements M and proceeds, indicating that it has acquired the semaphore.

• The process is allowed to proceed into the critical section or access the resource.
• The semaphore value is decremented by 1, indicating that one instance of the resource is now in use.
  

• The process is blocked, indicating that another process currently uses the resource.
• The process is placed in a waiting queue until the semaphore value becomes greater than 0.
  

What is Signal Operation in Semaphores?

1signal(M)
2{
3    M++;
4}

• The signal operation increments the semaphore value M.
• Signal operation is typically used to release the semaphore, indicating that the shared resource or critical section is now available for other processes to access.
• If there are processes waiting for the semaphore (blocked by the wait operation), the increment allows one of them to proceed.

• The steps involved in the signal operation are as follows:
  Release the Resource: The process leaves the critical section or releases the resource it was using.
• Increment Semaphore Value: The semaphore value is incremented by 1, indicating that the resource is now available for other processes.
• Unblock a Waiting Process (if any): If there are processes waiting in the queue (blocked by the wait operation), one of them is unblocked, allowing it to proceed.

Binary Semaphores

• Binary semaphores are a type of semaphore that can take on only two values: 0 and 1.
• They are primarily used for managing mutual exclusion, where the objective is to control access to a critical section of code.
• The binary semaphore acts as a simple lock that can be either locked (1) or unlocked (0).
  
  Key Characteristics:
  
• Values:
• Binary semaphores can only have values 0 or 1.
• 0 typically signifies that the resource is unavailable or the critical section is occupied,
• while 1 indicates that the resource is available or the critical section is free.
  
• Mutual Exclusion: Binary semaphores are well-suited for scenarios where only one process should access a critical section at any given time.
• They help prevent race conditions by allowing or denying access to the critical section based on the current value of the semaphore.
  
• Operations: The standard operations associated with binary semaphores are "wait" (P operation) and "signal" (V operation).

Counting Semaphores

• Counting semaphores can take on non-negative integer values.
• They are used for managing resources where multiple instances can be available simultaneously.
• Counting semaphores provides a way to control access to a pool of resources rather than a single resource.
  
  Key Characteristics:
  
• Values: Counting semaphores can have values greater than 1, allowing multiple processes to access a resource simultaneously.
• Resource Management: Counting semaphores are suitable for scenarios where a fixed number of identical resources need to be shared among multiple processes.
• Operations: Similar to binary semaphores, counting semaphores use "wait" (P operation) and "signal" (V operation) operations.

Advantages of Semaphores

1. Synchronization

2. Mutual Exclusion

3. Flexibility

4. Efficiency

5. Deadlock Prevention

Disadvantages of Semaphores

1. Complexity

2. Programming Errors

3. Priority Inversion

4. Overhead

5. Resource Contention