While developing a client using the raw MIDI interface, bumped into an issue that went away after rebooting Ubuntu 19.10 - 64 bit. Using a MIDISPORT 4x4 one of the 4 ports could not be opened. The client (written in C) call hang and the client thus went unresponsive. No timeout, no error, no nothing. CTRL-BREAK the client and retrying many times gave the exact same result, no other apps were using the port. The other 3 ports on the interface and any other MIDI USB device connected could be opened and used without any problem.

When opening the port with amidi, an error was reported, it seems amidi did not gracefully exit as well.

amidi -receive -p hw:3,0,1 ALSA lib rawmidi_hw.c:233:(snd_rawmidi_hw_open) open /dev/snd/midiC3D0 failed: Device or resource busy cannot open port "hw:3,0,1": Device or resource busy

Looking at the rawmidi_hw.c code there is a GOTO and SYSERR statement. Not sure if those caused this .

if (fd < 0) { snd_ctl_close(ctl); SYSERR("open %s failed", filename); return -errno; . After reboot the problem went away. The problem could have been caused while fixing some by coding issue. Maybe someone with more understanding of the code can spot an issue in the code. While many applications use the raw MIDI interface. believe Bitwig and most others use the sequencer API to open ports.

Just wanted to log this just in case anyone else experiences this

Thanks.