difference between message queue and shared memory?

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. This may be insignificant in case minor errors are allowed (say, the data goes to analog output and some noise is acceptable), so you can skip error checking altogether, and go along with "good enough" approach at quite high performance gain. Also, shared memory allows for exchange of big pieces of data and common and persistent storage of data common to several apps saving memory storage. Message queues are for lower throughput - you can for example utilize them for safeguarding access to shared memory.

Both shared memory and message queues can be used to exchange information between processes. The difference is in how they are used.

Shared memory is exactly what you'd think: it's an area of storage that can be read and written by more than one process. It provides no inherent synchronization; in other words, it's up to the programmer to ensure that one process doesn't clobber another's data. But it's efficient in terms of throughput: reading and writing are relatively fast operations.

A message queue is a one-way pipe: one process writes to the queue, and another reads the data in the order it was written until an end-of-data condition occurs. When the queue is created, the message size (bytes per message, usually fairly small) and queue length (maximum number of pending messages) are set. Access is slower than shared memory because each read/write operation is typically a single message. But the queue guarantees that each operation will either processes an entire message successfully or fail without altering the queue. So the writer can never fail after writing only a partial message, and the reader will either retrieve a complete message or nothing at all.