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What is the difference between message queues and a pipe in Linux?
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The pipe has no name; it is created for one use and both ends must be inherited from the single process which created the pipe. A FIFO special file is similar to a pipe, but instead of being an anonymous, temporary connection, a FIFO has a name or names like any other file.
Message queue has inherent synchronization overhead, guarantee of safety at cost of performance. Shared memory has no safeguards - if two threads access it simultaneously, they will possibly conflict (write inconsistent data) unless you assure thread safety yourself.
It's message passing. You specify a buffer to write into the socket buffer, and you find out how much space it has available beforehand using getTxAvailable() or whatever. It's not really shared memory as it does a buffer blit operation to help encapsulation of the socket.
A traditional pipe is “unnamed” and lasts only as long as the process. A named pipe, however, can last as long as the system is up, beyond the life of the process. It can be deleted if no longer used. Usually a named pipe appears as a file and generally processes attach to it for inter-process communication.
Off the top of my head and assuming you talk about posix message queues (not the SysV ones):
select(), poll(), epoll() and friends on the mqd_t.
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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.
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