What are message queues and pipes ?

Answer 1

Message Queues are:

UNIDIRECTIONAL
Fixed number of entries
Each entry has a maximum size
All the queue memory (# entries * entry size) allocated at creation
Datagram-like behavior: reading an entry removes it from the queue. If you don't read the entire data, the rest is lost. For example: send a 20 byte message, but the receiver reads 10 bytes. The remaining 10 bytes are lost.
Task can only pend on a single queue using msqQReceive (there are ways to change that with alternative API)
When sending, you will pend if the queue is full (and you don't do NO_WAIT)
When receiving, you will pend if the queue is empty (and you don't do NO_WAIT)
Timeouts are supported on receive and send

Pipes

Are a layer over message Queues <--- Unidirectional!
Have a maximum number of elements and each element has maximum size
is NOT A STREAMING INTERFACE. Datagram semantics, just list message Queues
On read, WILL PEND until there is data to read
On write, WILL PEND until there is space in the underlying message queue
Can use select facility to wait on multiple pipes

Answer 2

1. Pipes aren't limited in size, message queues are.
2. Pipes can be integrated in systems using file descriptors, message queues have their own set of functions, though linux supports select(), poll(), epoll() and friends on the mqd_t.
3. Pipes, once closed, require some amount of cooperation on both sides to reestablish them, message queues can be closed and reopened on either side without the coorporation of the other side.
4. Pipes are flat, much like a stream, to impose a message structure you would have to implement a protocol on both sides, message queues are message oriented already, no care has to be taken to get, say, the fifth message in the queue.