In reply to dave_59:
Thank you, Dave Rich.
I have read through your provided link, it has help me but not entirely. I’m a bit puzzled about how semaphores are used to access control ends of queues in mailbox. An answer in your link reads, when multiple threads want to write and read data, mailbox uses atomic test-and-set operation of semaphore which I’m not sure, how?. If you could give a brief idea about atomic test-and-set operation of semaphore. It should convince me to believe that mailbox is used for both synchronisation and as a communication channel between processes.