Regarding the applicability or suitability of message queue vs. Shared Memory in this situation

Question

This is regarding the applicability or suitability of message queue vs. Shared Memory in this situation:

1. multiple DLLs or shared libraries

2. each library will try to communicate with My main application DLL or shared library, eg, the I/P to and O/P from all the DLLs or Shared libraries are to be communicated through my main application's Shared library. These communications are ASYNCHRONOUS.

3. some of the DLLs or shared libraries, other than my application's .so, will create multiple threads, and output from each such thread needs to be communicated back to my application library. The output from these threads are again ASYNCHRONOUS.

4. my main application DLL / .so will continue with its other work which is very likely that it communicates with some server over the network and it responds accordingly

5. The functioning of all other DLLs/ .so's are asynchronous

Q-1: In the above situation which is the best fit ? (I) a message queue , (II) a shared memory ?

Q-2: Any reference or link which enforces synchronization between several shared libraries using shared memory ?

Answer 1

I guess your understanding of the problem is wrong:

• Shared memory is a "channel" to connect different processes, like sockets, pipes, or normal memory.
• A message queue is a "protocol" for passing messages, like TCP or a ring buffer. You may create it over a socket (like 0MQ) or using a synchronized queue (like Intel TBB, see below) in shared or "normal" memory.

You do not need shared memory with the specifications you give. Shared memory is useful if one of the following is true:

• You have several processes (you don't, all your so/dlls will share the same memory)
• You need to persist the memory of your process if it crashes (you may need that but didn't mention).

Now, you need to choose a protocol for your code to talk over it. I'd recommend using Intel Thread Building Blocks ( TBB , that would answer Q2). They provide different layers of abstractions for what you want to achieve, I do not know enough to choose for you, though, take some time to read the (long) docs.

Answer 2

With the caveat that I only understand your application in the most general sense, I think message queues are a no-brainer provided the msgs you are passing are bounded and appropriate for a queue to being with.

The two primary things you seem to be concerned with are synchronization and asynchronousity. (1) POSIX message queues have the queue synchronization already built-in. That's one big headache removed. (2) Under Linux, the queue id mqd_t is a file descriptor which means it can be used in a select statement. That takes care of the asynchronous concerns. In your main DLL you can load up the mqd_t descriptors for all your queues in select statement and process your DLL msgs as they arrive with a consistent, debugged and well understood mechanism. (3) The tiny bit (and it is tiny for most apps) of efficiency you lose compared to shared memory is more than made up by the relative ease of msg queue use and the fact that your main application DLL is going to spend a relatively long time waiting on network I/O with the server anyway.