Stream Buffers vs Message Buffers vs Queues

Let me expand a bit on Richard’s reply, based on being a user (as opposed to the designer). One big difference between the Queue and the Buffers (Stream/Message) is, as was mentioned, that Queues support innately multiple senders or receivers while the buffers are somewhat lightweighted by having a restriction that at any given time only 1 task can be sending to it and 1 task can be receiving. You can get around this problem a bit by wrapping the access (on a given side) in a mutex, which ensures that you can’t get concurrent access, but also breaks somewhat the priority model as if first a low priority task tries to read/write a message but gets blocked, a higher priority task can’t get in front of that in line, due to the mutex. With a Queue, the fact that the queue itself handles the multiple access says that it can order the access based on priority.

Besides that, if we look at what sort of thing each can send:

  • A Queue deals with a fixed sized message, generally a struct but it could also be a base type including a pointer.
  • A Stream Buffer deals with messages as streams of bytes. A Message will be put in the queue as a unit, and doesn’t have any fixed size (just a maximum as it should fit the buffer), and when pulling out, it doesn’t need to be retrieved chunked the same way.
  • A Message buffer is internally built on a Stream Buffer, but is designed to handle a variable-sized message that is held together so it comes out in the same chunks as it went in.

Personally, if the message is always a fixed-sized, i.e. always the same structure, I will use a queue and not a message buffer, largely because I use a C++ type-safe wrapper which doesn’t work for the potentially variable-sized message of a message buffer.

The difference between a stream buffer and a message buffer is that with stream buffers, the message is mostly amorphous, the messages really are just a stream of characters or bytes, and there isn’t a strong need to keep messages distinct. A Messgage buffer deals with messages with definite structure, but perhaps after a common header to the message, there is a variable-length data block based on some field or fields in the header section.