IPC Using Message Queues in Linux

Introduction to POSIX Message Queues

POSIX message queues are a form of interprocess communication (IPC) mechanism present in Linux systems. They make it easy and effective for processes to send and receive messages. The POSIX.1b standard describes POSIX message queues, which are part of the POSIX real-time extensions.

POSIX message queues store messages in queues that different processes can use. Each message in the queue has a priority, which makes it possible to get messages in a certain order. Processes can send messages to a queue and get messages from a queue. This lets different processes talk to each other and share info at the same time.

One of the best things about POSIX message queues is that they allow for asynchronous contact. This means that processes can send and receive messages without waiting for each other. This makes them good for situations in which processes need to talk to each other without having to wait for each other to answer.

POSIX message queues are given a unique name in the file system. This makes it possible for multiple processes to use the same queue by referring to its name. They can also be made as secret queues that only the process of making them can see.

Overall, POSIX message queues are a type of IPC mechanism present in Linux systems. Processes can send information to each other through named or unnamed queues. Each message in the queue has a priority, and messages can be read either in the order they were added to the queue (FIFO) or by priority.

POSIX Message Queues Library Functions

The POSIX message queues library provides several functions that allow processes to create, manage, send, and receive messages in message queues. Here are the key library functions used in POSIX message queues:

mq_open(): This function is used to open an existing message queue or create a new one in POSIX message queues. It is part of the POSIX message queues library and allows processes to access message queues for interprocess communication.

The function signature of mq_open() is as follows:

The parameters of mq_open() are:

• name: The name of the message queue. It should start with a forward slash (/) and be unique within the system. If the message queue doesn't exist, a new one will be created with the specified name.

• oflag: The open flags that control the behavior of mq_open(). Common flags include O_RDONLY (open for reading), O_WRONLY (open for writing), and O_CREAT (create the message queue if it doesn't exist).

• mode: The permissions to be set on the message queue if it is created. It is specified using the same format as file permissions (e.g., 0666).

• attr: A pointer to a struct mq_attr object that specifies the desired attributes of the message queue. If set to NULL, the default attributes will be used.

The return value of mq_open() is a message queue descriptor of type mqd_t. If the call is successful, the function returns a non-negative value representing the descriptor. On failure, it returns -1, and you can check the specific reason by examining the value of errno.

mq_close(): This function is used to close a message queue in POSIX message queues. It is responsible for releasing any resources associated with the message queue and making the message queue descriptor available for reuse.

The function signature of mq_close() is as follows:

The parameter mqdes is the message queue descriptor obtained when you create or open the message queue using functions like mq_open(). It represents the message queue that you want to close.

The return value of mq_close() is an integer indicating the success or failure of the operation. It returns 0 on success, indicating that the message queue was closed successfully. If an error occurs, it returns -1, and you can check the specific reason by examining the value of errno.

mq_unlink(): This function is used to remove a message queue from the system in POSIX message queues. It allows you to unlink and delete a named message queue, freeing up system resources associated with it.

The function signature of mq_unlink() is as follows:

The name parameter is a string representing the name of the message queue to be unlinked. The name should start with a forward slash (/) and be unique within the system.

The return value of mq_unlink() is an integer indicating the success or failure of the operation. It returns 0 on success, indicating that the message queue was successfully unlinked and removed from the system. If an error occurs, it returns -1, and you can check the specific reason by examining the value of errno.

mq_send(): This function is used to send a message to a message queue in POSIX message queues. It allows processes to send data and communicate with other processes through the message queue.

The function signature of mq_send() is as follows:

The parameters of mq_send() are:

• mqdes: The message queue descriptor obtained when you create or open the message queue using functions like mq_open(). It represents the message queue to which the message will be sent.

• msg_ptr: A pointer to the message data that you want to send. The message data should be a character array or a pointer to a structure.

• msg_len: The size of the message data in bytes.

• msg_prio: The priority of the message. Messages in the message queue can have different priorities, and the priority value is used to determine the order of message retrieval. Higher priority values indicate higher priority messages.

The return value of mq_send() is an integer indicating the success or failure of the operation. It returns 0 on success, indicating that the message was successfully sent to the message queue. If an error occurs, it returns -1, and you can check the specific reason by examining the value of errno.

mq_receive(): This function is used to receive a message from a message queue in POSIX message queues. It allows processes to retrieve and read messages that have been sent to the message queue by other processes.

The function signature of mq_receive() is as follows:

The parameters of mq_receive() are:

• mqdes: The message queue descriptor obtained when you create or open the message queue using functions like mq_open(). It represents the message queue from which the message will be received.

• msg_ptr: A pointer to a buffer where the received message will be stored. The received message will be copied into this buffer.

• msg_len: The size of the buffer in bytes, indicating the maximum length of the message that can be received.

• msg_prio: A pointer to an unsigned integer where the priority of the received message will be stored. Upon successful completion of mq_receive(), the priority of the received message will be stored in this variable.

The return value of mq_receive() is the number of bytes received and copied into the buffer. If an error occurs, it returns -1, and you can check the specific reason by examining the value of errno.

mq_getattr(): This function is used to retrieve the attributes of a message queue in POSIX message queues. It allows you to obtain information about the message queue, such as its current size, maximum number of messages, and maximum message size.

The function signature of mq_getattr() is as follows:

The parameters of mq_getattr() are:

• mqdes: The message queue descriptor obtained when you create or open the message queue using functions like mq_open(). It represents the message queue from which you want to retrieve the attributes.

• attr: A pointer to a struct mq_attr object where the attributes of the message queue will be stored. The struct mq_attr contains the following members:

• mq_flags: Flags associated with the message queue.

• mq_maxmsg: The maximum number of messages that the message queue can hold.

• mq_msgsize: The maximum size of each message in the message queue.

• mq_curmsgs: The current number of messages in the message queue.

The return value of mq_getattr() is an integer indicating the success or failure of the operation. It returns 0 on success, indicating that the attributes of the message queue were successfully retrieved and stored in the provided struct mq_attr object. If an error occurs, it returns -1, and you can check the specific reason by examining the value of errno.

All these above mentioned functions provide the necessary operations to create, open, close, send, and receive messages in POSIX message queues. They offer flexibility and control over the message queue behavior, such as setting attributes and managing queues. By using these functions, processes can effectively communicate and exchange data through message queues in a Linux environment.

Steps to Perform IPC Using Message Queues in Linux

To perform Interprocess Communication (IPC) using message queues in Linux, you can follow these steps:

Step 1: Include the necessary header files:

Step 2: Define a structure for your message:

Step 3: Generate a unique key for the message queue using the ftok() function:

Step 4: Create a message queue using the msgget() function:

Step 5: Send a message to the queue using the msgsnd() function:

Step 6: Receive a message from the queue using the msgrcv() function:

Step 7: Remove the message queue using the msgctl() function:

These steps outline the basic process of performing IPC using message queues in Linux. Remember to handle errors and perform necessary error checking throughout your code.

Examples

Here are some examples that demonstrate how to use message queues for IPC in Linux:

Example 1: Sending and receiving a message between two processes

Process 1 (Sender):

Process 2 (Receiver):

Compile and run the sender process in one terminal:

Compile and run the receiver process in another terminal:

You should see the output Received message: Hello, Process 2! in the receiver's terminal.

Example 2: Sending and receiving multiple messages in a loop

Process 1 (Sender):

Process 2 (Receiver):

Compile and run the processes as described in Example 1. The receiver process will receive and display messages Message 1 through Message 5.

These examples demonstrate the basic usage of message queues for IPC in Linux. You can expand upon these concepts to suit your specific needs.