Thanks for umbraticus and Richard.

The value 72 is experimentally OK.
However I am not convinced yet.
I can't find the value in the manual.

Kenji Arisawa

2023年5月9日(火) 17:00 Richard Miller <9fans@hamnavoe.com>:
> ; read -c 72 /dev/wctl

Or, if your chosen flavour of Plan 9 doesn't have the 'read -c'
option, one of these will do:

dd -if /dev/wctl -bs 72 -count 1 -quiet 1
syscall -o read 0 buf 72 </dev/wctl >[2]/dev/null


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