C++ || Snippet – How To Use Message Queues For Interprocess Communication

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The following is sample code which demonstrates the use of the msgsnd, msgrcv, and msgget function calls for use with message queues on Unix based systems.

Message queues are used for interprocess communication. They provide two (or more) separate programs (i.e: sender and receiver) with the ability to interact with eachother. This is ideal because the sender and receiver do not need to interact with the message queue at the same time. Messages can be sent at one point in time, be placed on the queue until the receiver is ready for it, and then be removed at another point in time when the program(s) that service the queue are finally ready to receive a message.

This concept is different from threaded and forked process IPC procedures, which often times process data in a more streamlined manner. Rather than following a strict pattern as to when data is to be sent and received, queuing is a mechanism in which messages are held until the receiving application is ready to process them.

The example on this page demonstrates the above use of a message queue to transferrer data between two separate programs. The sending program (client.cpp) sends an integer and string variable to the receiving program (server.cpp), which then displays that received data to the screen.

NOTE: The server program must be ran before the client program!

=== 1. Server.cpp (Receiver) ===

=== 2. Client.cpp (Sender) ===


QUICK NOTES:
The highlighted lines are sections of interest to look out for.

You can view all allocated message queues using the ipcs command. You can delete a message queue from command line using ipcrm -q [KEY SHOWN BY IPCS]. Message queues are a finite resource. If something goes wrong during the execution of your program, you must manually delete all your queues.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

The following is sample output.

SERVER OUTPUT:

The server has started!

Waiting for someone to connect to server id #753664 with the key 1258295474

someNumber = 0 buff = Message queues are awesome!
someNumber = 1 buff = Message queues are awesome!
someNumber = 2 buff = Message queues are awesome!
someNumber = 3 buff = Message queues are awesome!
someNumber = 4 buff = Message queues are awesome!
someNumber = 5 buff = Message queues are awesome!
someNumber = 6 buff = Message queues are awesome!
someNumber = 7 buff = Message queues are awesome!
someNumber = 8 buff = Message queues are awesome!
someNumber = 9 buff = Message queues are awesome!

Server is now shutting down!

CLIENT OUTPUT:

Successfully connected to server id #753664 with the key 1258295474

Now sending messages....Sending complete!

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