MIDI on Python / PyGame, Ubuntu 12.04

Question

Trying to get a MIDI interface to work with pygame on Ubuntu 12.04. I know the keyboard works because it can control vkeybd and works with PyGame on OSX, so the issue with with MIDI in python.

$ python -m pygame.examples.midi --list

Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/lib/python2.7/dist-packages/pygame/examples/midi.py", line 820, in <module>
    print_device_info()
  File "/usr/lib/python2.7/dist-packages/pygame/examples/midi.py", line 25, in     print_device_info
    pygame.midi.init()
  File "/usr/lib/python2.7/dist-packages/pygame/midi.py", line 71, in init
    import pygame.pypm
ImportError: /usr/lib/libportmidi.so.0: undefined symbol: snd_seq_event_input_pending

python-pygame installed through the package manager, as is python-pm.

Any ideas? :)

Answer 1

Although this won't exactly answer your question, it may help you debug the problem yourself.

The error is this :

ImportError: /usr/lib/libportmidi.so.0: undefined symbol: snd_seq_event_input_pending

The undefined symbol is a failure of the dynamic linker to find the code required for the snd_seq_event_input_pending function.

On an example 32 bit Oneiric system we can do this to look at some symbols of libportmidi.so.0.

nm -DC /usr/lib/libportmidi.so.0 | grep snd_seq_event_input_pending

U snd_seq_event_input_pending

This tells us that the libportmidi library requires the code for snd_seq_event_input_pending but the symbol is undefined. So for libportmidi to function it must also load an additional library which contains this function.

On Oneiric I've found that this symbol is defined in libasound2.so.2.

nm -DC /usr/lib/i386-linux-gnu/libasound.so.2 | grep snd_seq_event_input_pending

000a0fa0 T snd_seq_event_input_pending

The T indicates that the function exists and is in the text (code) segment.

Usually, linking of associated libraries occurs automatically as libasound.so.2 should be referenced by libportmidi. On the same system.

ldd /usr/lib/libportmidi.so.0

....
libasound.so.2 => /usr/lib/i386-linux-gnu/libasound.so.2 (0x00e35000)

which shows that libmidi depends on libasound. In the ldd output list in your comments there is no reference to libasound, and so it won't try to automatically dynamically link libasound.so.2 when it is loaded, resulting in your error.

There's a few reasons why there may be an error:

• The way linking from libportmidi may have change from Oneiric to Precise. eg libportmidi may attempt to find its own dependencies for libasound. (Unlikely).
• There is a bug in packaging of libportmidi where it doesn't reference libasound.so.2 as it should. This may be platform specific (eg only an error on 64 bit systems).

I'd suggest that you try to find out the library on your system that contains the snd_seq_event_input_pending function and then work backwards to try and determine why it has not been linked with libportmidi.

The following bash command will help you find the libaries implementing snd_seq_event_input_pending. If you don't find anything, there's a problem with the libraries installed on your machine.

find /lib /usr/lib -name "lib*.so.*" | while read f; do
    if nm -DC "$f" | grep -q 'T snd_seq_event_input_pending'; then
        echo "$f"
    fi
done

Answer 2

I have exactly the same problem (on Ubuntu 12.04.1), using e.g. the MIDI playback tool in Frescobaldi (which is a Python application). This used to work fine, but doesn't anymore.

This is quite obviously a miscompiled portmidi package which was pushed out on on 2013-01-25, see https://launchpad.net/ubuntu/+source/portmidi/1:200-0ubuntu1.12.04.1. Downgrading to the previous 1:200-0ubuntu1 package solved the issue for me.

I guess that the proper course of action would be to file a bug report against the 1:200-0ubuntu1.12.04.1 version on Launchpad at https://bugs.launchpad.net/ubuntu/+source/portmidi/+bugs. If it doesn't get fixed, we might also ask falkTX if he would be willing to provide a working package in his KXStudio PPAs instead.

Just for the record, here's what ldd gives for the 1:200-0ubuntu1 libportmidi on my system:

linux-vdso.so.1 =>  (0x00007fffe9bff000)
libasound.so.2 => /usr/lib/x86_64-linux-gnu/libasound.so.2 (0x00007f26264cb000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f26262ae000)
libporttime.so.0 => /usr/lib/libporttime.so.0 (0x00007f26260ab000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f2625cec000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f26259f0000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f26257eb000)
librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f26255e3000)
/lib64/ld-linux-x86-64.so.2 (0x00007f26269f4000)

And the broken 1:200-0ubuntu1.12.04.1 version:

linux-vdso.so.1 =>  (0x00007fff9e3ff000)
libporttime.so.0 => /usr/lib/libporttime.so.0 (0x00007fb84ac71000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb84a8b2000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fb84a694000)
/lib64/ld-linux-x86-64.so.2 (0x00007fb84b0af000)

So any application which doesn't happen to link in libasound2 by itself will be hosed. Specifically, that seems to be the case for the Python portmidi module. (This kind of error is also aggravated by the fact that, at least from Ubuntu 12.04 onwards, gcc uses the --as-needed linker flag by default. I bet that there are still quite a few packages in the Ubuntu repos which are broken because of that.)