Under the resource manager's covers

The resource manager is a server that uses the QNX Neutrino send/receive/reply messaging protocol to receive and reply to messages. The following is pseudo-code for a resource manager:

initialize the resource manager
register the name with the process manager
DO forever
    receive a message
    SWITCH on the type of message
            call io_open handler
        CASE _IO_READ:
            call io_read handler
        CASE _IO_WRITE:
            call io_write handler
        .   /* etc. handle all other messages */
        .   /* that may occur, performing     */
        .   /* processing as appropriate      */

Many of the details in the above pseudo-code are hidden from you by a resource manager library that you'll use. For example, you won't actually call a MsgReceive*() function — you'll call a library function, such as resmgr_block() or dispatch_block(), that does it for you. If you're writing a single-threaded resource manager, you might provide a message handling loop, but if you're writing a multithreaded resource manager, the loop is hidden from you.

You don't need to know the format of all the possible messages, and you don't have to handle them all. Instead, you register “handler functions,” and when a message of the appropriate type arrives, the library calls your handler. For example, suppose you want a client to get data from you using read() — you'll write a handler that's called whenever an _IO_READ message is received. Since your handler handles _IO_READ messages, we'll call it an io_read handler.

The resource manager library:

  1. Receives the message.
  2. Examines the message to verify that it's an _IO_READ message.
  3. Calls your io_read handler.

However, it's still your responsibility to reply to the _IO_READ message. You can do that from within your io_read handler, or later on when data arrives (possibly as the result of an interrupt from some data-generating hardware).

The library does default handling for any messages that you don't want to handle. After all, most resource managers don't care about presenting proper POSIX filesystems to the clients. When writing them, you want to concentrate on the code for talking to the device you're controlling. You don't want to spend a lot of time worrying about the code for presenting a proper POSIX filesystem to the client.