Other than manipulating a process's signal mask (for example, using 
signal(3)), what can prevent delivery of a signal?

In particular, I've occassionally seen processes "hang" waiting for 
something (I/O completion?), and typing Control/C doesn't work.  Once 
the Control/C signal doesn't work, not even a `kill -KILL` can abort
that process!  Yet, if instead of typing Control/C first you simply
try `kill -KILL` then the target process is killed.

Can someone explain what's going on?

Thanks in advance!

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