This is called synchronous because the messages are read in the order they were sent. There are four system calls associated with message queues . The key is a unique number and identifies the message queue. Each processes that wishes to communicate with the message queue must know its key. The id is a parameter to the other message queue commands.
Message queues are relatively simple to use.
The operating system handles the internal details of communication. When you send a message on the queue, any process waiting for a message is alerted. Locking the message queues are unnecessary as the OS verifies the integrity of the queue and will not allow two processes to access the queue in a destructive manner. Although the message queues do have these advantages, they have two distinct disadvantages. Message queues are slow at transferring large amounts of data, and there is a hard limit to the size of the data packet that can be transfered.