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Crystal C bindings for the PortMIDI cross-platform MIDI I/O library
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examples ran code through `crystal tool format` Apr 19, 2019


port_midi is a wrapper around the cross-platform PortMidi MIDI I/O library.

The low-level wrapper around PortMidi is in the lib LibPortMIDI.

The module PortMIDI contains a few helper functions that are useful when converting between MIDI data and PortMidi messages. It also contains the function PortMIDI#list_all_devices which prints out all of the input and output devices available to PortMidi.

At a higher level of abstraction, InputStream and OutputStream represent the two kinds of streams that do reading and writing of MIDI data.

Finally, a SimpleMIDIDevice wraps one input stream and one output stream.


  1. Install the PortMidi library. If you are on MacOS and you use Homebrew, you can run brew install portmidi. Otherwise, download the PortMidi source from and compile it.
  2. Add the dependency to your shard.yml:
    github: jimm/crystal_port_midi
  1. Run shards install


require "port_midi"


See the examples directory for a few sample applications that use port_midi and the InputStream, OutputStream, and SimpleMIDIDevice classes.

To build the examples, run shards build.

Development and Testing

To run the tests, run crystal spec.

When running tests on Travis CI or CircleCI, we limit ourselves to tests that do not return PortMidi errors. That is because the installation of libportmidi available to Travis through apt-get is compiled with the PM_CHECK_ERRORS flag enabled. When an error such as an invalid device ID happens, the flag causes the PortMidi code to display a message, prompt the user to hit ENTER, and exit immediately. I can work around the prompt by running /usr/bin/yes | crystal spec, but I can't work around the call to exit.


To generate the docs, run crystal docs.


  1. Fork the repo
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request


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