Custom messages

Although most of the time your resource manager will handle messages from clients, there still could be times when you need to control the behavior of the resource manager itself. For example, if the resource manager is for a serial port, you'll likely need a way to change the baud rate, and so on.

There are various ways you could send control information to a resource manager:

• Stop the resource manager, and then restart it with new command-line options. This is an awkward method that leaves the device unavailable while the resource manager is restarting. It isn't a viable solution if the resource manager controls various devices and you need to set different options (at different times) for different devices.
• Accept write() messages that include control instructions. This lets you control the resource manager from the command line or a script; you can simply echo a command to a path that the resource manager has registered.

  However, the write() messages might be broken into smaller messages, so your resource manager would have to be prepared to save the pieces, reassemble them, and then act on them. Your io_write handler also has to parse the commands, in addition to handling write() messages from the clients.

• Send an IPC message, via MsgSend(), directly to the resource manager. This message would contain a data structure that includes the control instructions. This method is fast, simple, and flexible, but it isn't portable because MsgSend() isn't a POSIX function.
• Send control instructions via a devctl() command. Such messages won't conflict with any other messages sent to the resource manager, but this approach might not be portable because devctl() isn't a POSIX function (it was in a draft standard, but wasn't adopted). See Handling devctl() messages, below.
• Set up a handler for other I/O messages when you call resmgr_attach(), as described in Setting resource manager attributes in the Fleshing Out the Skeleton chapter. We don't recommend this approach.
• Use message_attach() or pulse_attach() to attach a specified message range or pulse code for the dispatch handle. When a message with a type in that range is received, the dispatch_block() function calls the handler function that you specify. The message range must be outside the I/O range, and the messages or pulses aren't associated with an OCB. For more information, see Handling private messages and pulses, later in this chapter.
• Use an out-of-band I/O message, _IO_MSG. This type of message is associated with an OCB, and is handled through the resource manager's framework. For more information, see Handling out-of-band (_IO_MSG) messages, later in this chapter.