Number of semaphore keys. created keys are not matching with get and put

SystemVerilog
1

Kindly help me to understand how this is possible to get and put the number of keys which is not initialized in new as per given below:

module tb();
  semaphore sem = new(3);
  
  task process_A();
    sem.get(3);
    $display("process_A is started");
    #5ns;
    $display("process_A is completed");
    sem.put(5);
  endtask

  task process_B();
    sem.get(5);  // Accessing more keys than initialized
    $display("process_B is started");
    #5ns;
    $display("process_B is completed");
    sem.put(5);
  endtask
  
  task process_C();
    sem.get(6);  // Accessing more keys than available in the bucket
    $display("process_C is started");
    #5ns;
    $display("process_C is completed");
    sem.put(5);
  endtask
  
  initial begin
    fork
      process_A();
      process_B();
      process_C();
    join_none
  end
endmodule

It gives the below output:

process_A is started
process_A is completed
process_B is started
process_B is completed

2

When you write semaphore sem = new(3); it initializes the semaphore with 3 keys.

process_A() :

  • sem.get(3) Acquires all 3 available keys.
  • Executes its logic, then calls sem.put(5) returning 5 keys.
  • A semaphore does not restrict you from putting back more keys than were originally acquired.

process_B() :

  • sem.get(5) When this process starts, all keys have been taken by process_A, so it blocks and waits.
  • After process_A completes and puts back 5 keys, the semaphore has enough keys.
  • process_B then resumes execution, successfully acquires 5 keys, and completes its task.

process_C() :

  • sem.get(6)This process attempts to acquire 6 keys.
  • However, at no point does the semaphore hold 6 or more keys (maximum was 5).
  • As a result, process_C waits indefinitely and never proceeds, unless additional keys (at least 6) are put into the semaphore.
3

Hi Hardik,
Thanks, I understood your points. One more doubt is there:

We are creating 3 keys in new. Then what is the point in putting back 5 keys, we have not created these 2 extra keys. Considering the process_A() only, how we can putting(or put()) which we are not getting . Is it good practice to code in this way.

4

This is a weakness in the functionality of the semaphore class. Important is you should be aware of this and double-check how many keys you are putting back.

5

The semaphore class is a primitive construct serving various purposes, such as process synchronization and resource allocation. The user of the semaphore is responsible for managing the allocation of keys. Typically, a semaphore is encapsulated within another class that manages the keys. In most applications, the key count is always set to 1.