Question

What are all the differences between pipes and message queues?

Please explain both from vxworks & unix perspectives.

I think pipes are unidirectional but message queues aren t.

But don t pipes internally use message queues, then how come pipes are unidirectional but message queues are not?

What are the other differences you can think of (from design or usage or other perspectives)?

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

That s what I can think of right now.

Answer 2

"VxWorks pipes differ significantly from UNIX pipes", says the vxWorks documentation, and they ain t kidding. Here s the manpages.

It looks like it would not be exaggerating much to say that the only similarity between Unix pipes and vxWorks pipes are that they re a form of IPC. The features are different, the APIs are different, and the implementations are surely very different.

Answer 3

I also found this difference in IPC in UNIX. It states that the difference between them is that Message queues and pipes is that the first stores/retrieves info in packets. While pipes do it character by character.

Msg queue:

Message queue: An anonymous data stream similar to the pipe, but stores and retrieves information in packets.

Pipe

Pipe: A two-way data stream interfaced through standard input and output and is read character by character

I also found this question here: Pipe vs msg queue

Answer 4

Comparison of message queues and pipes: - ONE message queue may be used to pass data in both directions - the message needn t to be read on first in-first out basis but can be processed selectively instead source: see http://www.cs.vsb.cz/grygarek/dosys/IPC.txt

Answer 5

MQs have kernel persistence, and can be opened by multiple processes.

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