In general, you should avoid using this member. This member, if non-NULL, represents a routine that will get called with the current message received by the resource manager library when the library doesn't recognize the message. While you could use this to implement “private” or “custom” messages, this practice is discouraged (use either the _IO_DEVCTL or _IO_MSG handlers, see below). If you wish to handle pulses that come in, I recommend that you use the pulse_attach() function instead.
You should leave this member with the value NULL.