39. IPC — Message Queues

Processes exchange discrete messages through a kernel-managed queue — unlike a pipe's raw byte stream, message boundaries are preserved (send("Hello") then send("World") is read back as two distinct messages, not one merged blob).

Key features: message boundaries, priority (higher-priority messages can jump the queue), persistence (messages wait even if the consumer isn't currently running), and blocking/non-blocking/timeout receive modes.

POSIX System V
API	mq_open/send/receive	msgget/msgsnd/msgrcv
Identification	Name (/myqueue)	Integer key
Priority	Built-in	Manual
Preferred	Modern	Legacy

Message Queue Pipe Shared Memory Socket
Message boundaries	Yes	No	Manual	No (TCP)
Persistence	Yes	No	Yes	No
Cross machine	No	No	No	Yes
Speed	Medium	Fast	Fastest	Slowest

Real-world use: Windows GUI event queues (keyboard/mouse/paint events); microservice message brokers like RabbitMQ/Kafka, where one message can fan out to multiple consumers and the queue absorbs load spikes.

---