[boost] [MPI?] which API should i use for this?

I have a need to send an "event" to the "main thread's event loop". this may be done from a preemptive thread, a cooperative thread, a timer, or even the main thread itself. i want it to be posted into the event stream and handled in the order that events get processed on the main thread. would i use 'MPI'? or 'interprocess'? or is there a 'thread' way to do this? if it's MPI, what would be the right call? i'm coming from Mac land where i have traditionally used PostEventToQueue()