if interrupted, fgetc returns EOF because the underlying read system call returns -1 (with error string "interrupted").

System calls are interrupted by a note (see notify(2)). Your loop will therefore stop and exit.

Also, see /sys/doc/comp.ms for some other details of the Plan 9 C environment. For instance, main is

      void main(int, char**);

and you need an explicit call to exits [sic] at the end of main, not a return 0 (or you'll get an error status "main"

returned to the shell).