Prerequisite : Inter Process Communication
A message queue is a linked list of messages stored within the kernel and identified by a message queue identifier. A new queue is created or an existing queue opened by msgget().
New messages are added to the end of a queue by msgsnd(). Every message has a positive long integer type field, a non-negative length, and the actual data bytes (corresponding to the length), all of which are specified to msgsnd() when the message is added to a queue. Messages are fetched from a queue by msgrcv(). We don’t have to fetch the messages in a first-in, first-out order. Instead, we can fetch messages based on their type field.
All processes can exchange information through access to a common system message queue. The sending process places a message (via some (OS) message-passing module) onto a queue which can be read by another process. Each message is given an identification or type so that processes can select the appropriate message. Process must share a common key in order to gain access to the queue in the first place.
System calls used for message queues:
- ftok(): is use to generate a unique key.
- msgget(): either returns the message queue identifier for a newly created message queue or returns the identifiers for a queue which exists with the same key value.
- msgsnd(): Data is placed on to a message queue by calling msgsnd().
- msgrcv(): messages are retrieved from a queue.
- msgctl(): It performs various operations on a queue. Generally it is use to destroy message queue.
MESSAGE QUEUE FOR WRITER PROCESS
MESSAGE QUEUE FOR READER PROCESS