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IPC: POSIX Message Queues — Priority-Ordered Framed Messages

POSIX message queues (mq_open) are kernel-managed named queues under /dev/mqueue. mq_send(mq, msg, len, priority) enqueues a discrete message with a priority. mq_receive returns the highest-priority message (ties: FIFO within priority). Unlike pipes: message framing is automatic (no length-prefix needed), priority is built-in. Blocks when full (mq_send) or empty (mq_receive); use O_NONBLOCK + EAGAIN to avoid blocking. mq_notify: receive an async signal (SIGEV_SIGNAL) or thread callback (SIGEV_THREAD) when the queue transitions from empty to non-empty — must re-register after each notification. mq_unlink removes the name; attributes (mq_maxmsg, mq_msgsize) set at creation. Link with -lrt on Linux. Use for: alerting systems, job queues, event prioritization.

The code

// POSIX Message Queue — typed messages + priority + async notify
#include <mqueue.h>

// Sender process
struct mq_attr attr{};
attr.mq_maxmsg = 10;   // max messages in queue
attr.mq_msgsize = 256; // max bytes per message

mqd_t mq = mq_open("/myqueue", O_CREAT | O_WRONLY, 0644, &attr);

// Priority: higher number = higher priority = dequeued first
mq_send(mq, "CRITICAL: CPU >90%", 18, 10); // priority 10
mq_send(mq, "WARN: mem 80%", 13, 5);       // priority 5
mq_send(mq, "INFO: heartbeat", 15, 1);     // priority 1
mq_close(mq);

// Receiver process — gets highest priority first
mqd_t mq = mq_open("/myqueue", O_RDONLY);
char buf[256];
unsigned int prio;
mq_receive(mq, buf, sizeof(buf), &prio);
// → buf = "CRITICAL: CPU >90%", prio = 10
mq_receive(mq, buf, sizeof(buf), &prio);
// → buf = "WARN: mem 80%", prio = 5
mq_close(mq);
mq_unlink("/myqueue"); // remove from /dev/mqueue

// Async notification — signal when message arrives
struct sigevent sev{};
sev.sigev_notify = SIGEV_SIGNAL;
sev.sigev_signo = SIGUSR1;
mq_notify(mq, &sev); // MUST re-register after each!
                     // Or: SIGEV_THREAD to call a function in a new thread

What this lesson walks through

1. 01mq_open — create a persistent named queue
2. 02mq_send with priority — highest priority dequeued first
3. 03Queue with 3 messages — sorted by priority
4. 04mq_receive — dequeues highest priority first
5. 05Queue full — mq_send blocks
6. 06mq_notify — async SIGEV_SIGNAL or SIGEV_THREAD

mq_open('/myqueue', O_CREAT|O_WRONLY) creates a named message queue under /dev/mqueue. Unlike pipes, it persists until mq_unlink() even if all processes close it. It has a name — any process can open it by that name. Set mq_maxmsg and mq_msgsize limits.