Message queues enable communication between processes, allowing multiple writers and readers to handle small messages and synchronize processes. System V and POSIX provide specific system calls for creating and managing message queues, which offer benefits such as reliable delivery, inter-application connectivity, and improved security. They are used extensively in various applications to facilitate efficient process communication and can be particularly advantageous in web service scenarios.

1. MESSAGE QUEUES
UNIX SYSTEM PROGRAMMING

2. INTRODUCTION
Message queues allow messages to be passed from one process to another. There can be multiple
writers to the queue as well as multiple readers.
Message queues are often used for passing small messages between processes. They can also be
used for process synchronization.
System V Message and POSIX message queue
Message Queue provides an asynchronous communications protocol, a system that puts a message
onto a message queue does not require an immediate response to continue processing”

3. WHY DO WE NEED ?
➢ As understood, once the message is received by a process it would be no longer available for any
other process. Whereas in shared memory, the data is available for multiple processes to access.
➢ If we want to communicate with small message formats.
➢ Shared memory data need to be protected with synchronization when multiple processes
communicating at the same time.
➢ The order of message queue is FIFO (First In First Out).

4. ARCHITECTURE

5. HOW WILL IT PROCESS ?
➢ Writing into the shared memory by
one process and reading from the
shared memory by another process.
➢ Writing into the shared memory by
one process with different data
packets and reading from it by
multiple processes.

6. SYSTEM CALL
Step 1 : msgget()
Step 2 : msgsnd()
Step 3 : msgrcv()
Step 4 : msgctl()

7. STEP 1
The first function normally called is msgget to either open an existing queue or create a new queue.
#include <sys/msg.h>
int msgget(key_t key, int flag);
Returns: message queue ID if OK, 1 on error
On success, msgget returns the non-negative queue ID. This value is then used with the other three
message queue functions.

8. STEP 2
Data is placed onto a message queue by calling msgsnd.
#include <sys/msg.h>
int msgsnd(int msqid, const void *ptr, size_t nbytes, int flag);
Returns: 0 if OK, 1 on error.
The ptr argument is then a pointer to a mymesg structure. The message type can be used by the
receiver to fetch messages in an order other than first in, first out.

9. STEP 3
Messages are retrieved from a queue by msgrcv.
#include<sys/msg.h>
ssize_t msgrcv(int msqid, void *ptr, size_t nbytes, long type, int flag);
Returns: size of data portion of message if OK, 1 on error.

10. STEP 4
The msgctl function performs various operations on a queue.
#include<sys/msg.h>
int msgctl(int msqid, int cmd, struct msqid_ds *buf );
Returns: 0 if OK, 1 on error.

11. BENEFITS
● Reliable message delivery: With this feature in place, additional de-duplication or
loss-prevention logic is unnecessary.
● Inter-application connectivity: This simplifies application development and enables
disparate architectures to work together.

12. BENEFITS
● Versatility: Message queue solutions can support multiple languages.They can also
support numerous application programing interfaces (APIs) and protocols.
● Resilience: Asynchronous messaging ensures application-specific faults won’t impact
the system.

13. BENEFITS
● Improved security: This can contribute to the overall security of the applications
and/or infrastructure.
● Integrated file transfers: This can be used as an alternative to FTP within enterprises
where such solutions are in use.

14. WEB SERVICE vs MESSAGE QUEUE

15. LIMITS
1. Total size of all messages in a queue.
2. Number of messages in a queue.
3. Size of a message
4. Number of queues.

16. USE CASE
Web service always has to be highly available and ready to receive new requests instead of being
locked by the processing of previous received requests.
In this case it is ideal to put a queue between the web service and the processing service.

17. UNIX APIs for Messages
POSIX message queue system calls:
mq_open
mq_close
mq_unlink
mq_send
mq_receive
mq_timedsend
mq_timedreceive
mq_notify
mq_getattr
mq_setattr
System V message queue system calls:
msgget
msgctl
msgsnd
msgrcv

18. CONCLUSION
Message queues are often used for passing small messages between processes. They can also be used
for process synchronization.
One process establishes a message queue that others may access. Often a server will establish a
message queue that multiple clients can access
Processes must share a common key in order to gain access to the queue in the first place.

19. THANK YOU