Pipe vs msg queue

Question

What is the difference between message queues and a pipe in Linux?

Answer 1

Off the top of my head and assuming you talk about posix message queues (not the SysV ones):

• Pipes aren't limited in size, message queues are.
• 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.
• 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.
• 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.

Answer 2

They are very different things, really.

The biggest practical difference is that a pipe doesn't have the notion of "messages", it's just a pipe to write() bytes to and read() bytes from. The receiving end must have a way to know what piece of data constitute a "message" in your program, and you must implement that yourself. Furthermore the order of bytes is defined: bytes will come out in the order you put them in. And, generally speaking, it has one input and one output.

A message queue is used to transfer "messages", which have a type and size. So the receiving end can just wait for one "message" with a certain type, and you don't have to worry if this is complete or not. Several processes may send to and receive from the same queue.

see man mq_overview and/or man svipc for more information.