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
C
`
include <stdio.h> `
`
include <sys/ipc.h> `
`
include <sys/msg.h> `
`
define MAX 10 `
struct mesg_buffer {
long mesg_type;
`
include <sys/ipc.h> 0
include <sys/ipc.h> `1
`
include <sys/ipc.h> `2
`
include <sys/ipc.h> 3
include <sys/ipc.h> `4
`
include <sys/ipc.h> `5
`
include <sys/ipc.h> `7
`
include <sys/ipc.h> 3
include <sys/msg.h> `0
`
include <sys/msg.h> 2
include <sys/msg.h> 3
include <sys/msg.h> `4
`
include <sys/msg.h> `6
`
include <sys/msg.h> `8
`
define MAX 10 0
define MAX 10 1
define MAX 10 2
define MAX 10 `3
`
define MAX 10 5
define MAX 10 `6
`
define MAX 10 8
define MAX 10 `9`struct`0
`
define MAX 10 0
define MAX 10 `1`struct`4`struct`5
`struct`7 `struct`8
`struct`9
MESSAGE QUEUE FOR READER PROCESS
C
`
include <stdio.h> `
`
include <sys/ipc.h> `
`
include <sys/msg.h> `
struct mesg_buffer {
long mesg_type;
`
include <sys/ipc.h> 0
include <sys/ipc.h> `1
`
include <sys/ipc.h> `2
`
include <sys/ipc.h> 3
include <sys/ipc.h> `4
`
include <sys/ipc.h> `5
`
include <sys/ipc.h> `7
`
include <sys/ipc.h> 3
include <sys/msg.h> `0
`
include <sys/msg.h> 2
include <sys/msg.h> 3
include <sys/msg.h> `4
`
include <sys/msg.h> `6
long`7
define MAX 10 `9`long`9
`
define MAX 10 0
define MAX 10 `1`mesg_type; `3`mesg_type; `4
`mesg_type; `5`mesg_type; `6
`mesg_type; `8
`struct`7 `struct`8
`struct`9
Output:
Last Updated : 25 Nov, 2020
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