Non blocking reads in lua

So I wanted to try reading some midi data from my keyboard with lua and ubuntu. I installed lua 5.3 and lua posix with "sudo apt install lua5.3 lua-posix"


Then I plugged in my M-Audio Keyrig 49 to the usb and tested it with "aseqdump -l" to list the midi devices then "aseqdump -p 24:0" since it came up on port 24:0.

Ok, so now for the lua (something quick and hacky to test it):

The keyrig will generate 0x90 key on and always 3 bytes for each midi message (0x90 status, then pitch, the velocity) so that makes it really easy.

posix = require("posix")

j={}

function printj() if j==nil then j={} end for i=12*3,12*7 do if j[i]==nil or j[i]==0 then io.write(".") else io.write("*") end end print() end

processbyte=coroutine.create(function(b) if j==nil then j={} end while true do print(b) b=coroutine.yield() if string.byte(b)==0x90 then b=coroutine.yield() key=string.byte(b) b=coroutine.yield() velocity=string.byte(b) j[key]=velocity  print("KEY="..key,"VELOCITY="..velocity) printj() end end end)


f = posix.open("/dev/midi2",posix.O_RDONLY|posix.O_NONBLOCK)
print("file is "..f)
while true do 
a = posix.read(f,1) 
if a ~= nil then print(string.format("%x",string.byte(a))) coroutine.resume(processbyte,a) 
else  posix.rpoll(f,10)
end 
end

-- when done, just do a posix.close(f)

and some sample output:

................................................

90
3b
39
KEY=59 VELOCITY=57
.......................*.........................
9
90
3c
36
KEY=60 VELOCITY=54
.......................**........................
6
90
3e
23
KEY=62 VELOCITY=35
.......................**.*......................