Message Queue in Linux

In Linux, a message queue is a data structure that is used for interprocess communication (IPC). A message queue allows processes to exchange messages, with each message consisting of a fixed-sized block of data.

A message queue is created using the mq_open() function, which takes a name and some other parameters as arguments and returns a message queue descriptor. The message queue descriptor is a handle that is used to refer to the message queue in subsequent function calls.

Once a message queue has been created, processes can use the mq_send() and mq_receive() functions to add messages to the queue and retrieve messages from the queue, respectively. These functions take the message queue descriptor, a pointer to the message, and the length of the message as arguments.

The messages in a message queue are stored in a FIFO (first-in, first-out) manner, which means that the first message that is added to the queue is also the first message that is retrieved from the queue. This allows processes to communicate and coordinate their execution in a predictable manner.

The msgget() system call creates a new message queue or opens an existing one, and it returns a message queue identifier (MQID) that is used to access the queue. The msgsnd() system call is used to send a message to the queue, and the msgrcv() system call is used to receive a message from the queue.

Here is an example of how to create and use a message queue in Linux:

#include <sys/msg.h>

// Define a structure for the message
struct message {
    long type;
    char data[1024];
};

int main() {
    // Create a message queue
    int mqid = msgget(IPC_PRIVATE, 0600 | IPC_CREAT);
    if (mqid == -1) {
        // Error creating message queue
        // Handle the error
    }

    // Send a message to the queue
    struct message msg = {1, "Hello world!"};
    if (msgsnd(mqid, &msg, sizeof(msg.data), 0) == -1) {
        // Error sending message
        // Handle the error
    }

    // Receive a message from the queue
    if (msgrcv(mqid, &msg, sizeof(msg.data), 0, 0) == -1) {
        // Error receiving message
        // Handle the error
    } else {
        printf("Received message: %s\n", msg.data);
    }

    // Delete the message queue
    if (msgctl(mqid, IPC_RMID, NULL) == -1) {
        // Error deleting message queue
        // Handle the error
    }

    return 0;
}

In summary, a message queue in Linux is a data structure that allows processes to exchange messages using IPC. A message queue is created using the mq_open() function, and messages are added to and retrieved from the queue using the mq_send() and mq_receive() functions. Messages in a message queue are stored in a FIFO manner, which allows processes to communicate and coordinate their execution in a predictable manner.