Here is a revised version.
Since there is no portable way to block signals, and no way at all on 
windows (apparently) the simplest way is to simply use NOWAIT_LOCK when 
adding a new pending call.  While this does not guarantee that we are 
always able to append a call (we may be unlucky in all our N tries and 
there is also no portable way to yield the thread timeslice between 
tries) such are also the semantics of the method.  Sometimes the buffer 
may be full, or we fail to get the lock in time, and a -1 is returned.  
It is up to the caller to respond appropriately, perhaps by trying again 
later.