Message Queues in Inter-Process Communication (IPC)

What Are Message Queues?

Message Queues are a form of inter-process communication that allows processes to send and receive discrete messages to one another via a queue maintained by the operating system or middleware. These queues operate like mailboxes: one process sends a message, and another retrieves it when ready.

Unlike shared memory, message queues are typically asynchronous and buffer-based, meaning the sender and receiver don’t need to run at the same time.

🔄 Real-Life Analogy

Imagine a mailbox system in an office. One person writes a note and drops it into a coworker’s mailbox. The coworker retrieves and reads it later, possibly when the sender is no longer around. Message queues work the same way—decoupling sender and receiver in both time and execution.

🧰 How Message Queues Work

1. A message queue is created (usually by the sender or a controller process).
2. One or more processes write messages into the queue.
3. One or more receiver processes read messages from the queue.
4. The OS ensures the messages are stored until read or deleted.
5. Each message may include priority, type, or metadata.

🛠️ Message Queue Implementations

🧪 Example: POSIX Message Queue in C

#include <mqueue.h>
#include <stdio.h>
#include <fcntl.h>

int main() {
    mqd_t mq;
    struct mq_attr attr;
    char buffer[1024];
    const char *message = "Hello from POSIX message queue!";

    attr.mq_flags = 0;
    attr.mq_maxmsg = 10;
    attr.mq_msgsize = 1024;
    attr.mq_curmsgs = 0;

    mq = mq_open("/myqueue", O_CREAT | O_WRONLY, 0644, &attr);
    mq_send(mq, message, strlen(message), 0);
    printf("Message sent.\n");
    mq_close(mq);
    return 0;
}

This sends a message into a named POSIX message queue. A separate program can open it in read-only mode and retrieve the message.

🔐 Message Queue Security & Permissions

• Access Control: Set permissions (e.g., 0644) on queues to restrict usage.
• Message Validation: Validate incoming message content to avoid injection or corruption.
• Expiration: Some implementations support timeouts or expiry on undelivered messages.

Features of Message Queues

✅ Advantages of Message Queues

❌ Disadvantages / Limitations

🔍 Use Cases for Message Queues

Message Queues vs Other IPC Methods

Best Practices

• ✅ Clean up queues after use (delete with mq_unlink() or msgctl()).
• ✅ Handle message overflows and timeouts gracefully.
• ✅ Use message priorities to ensure urgent processing.
• ✅ In long-running systems, monitor and rotate queues to avoid memory bloat.

🏁 Final Thoughts

Message queues are a robust, flexible, and language-agnostic form of IPC that fit perfectly into event-driven and distributed application architectures. Whether you’re building a desktop app, microservices, or an embedded system, message queues help maintain clear, decoupled communication between processes.

🧰 Choose message queues when you need:
🧾 structured messages,
🕰️ asynchronous delivery, and
🔌 inter-process or inter-service messaging.