Fiddle + CoreMIDI + Segmentation fault

Ruby
1

Hello people,

I’m playing around with Fiddle and CoreMIDI on OSX.

ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin13]

require ‘fiddle’
require ‘fiddle/import’

module CoreMIDI
extend Fiddle::Importer
dlload
‘/System/Library/Frameworks/CoreMIDI.framework/Versions/Current/CoreMIDI’

#from MIDIServices.h
#extern MIDIEndpointRef MIDIGetSource(  ItemCount sourceIndex0 )
extern "int MIDIGetSource(int)"

# from MIDIServices.h
# extern OSStatus MIDIObjectGetStringProperty(MIDIObjectRef obj,

CFStringRef propertyID, CFStringRef *str);
extern “int MIDIObjectGetStringProperty(int, char *, char *)”

end

from MIDIServices.h

extern const CFStringRef kMIDIPropertyOffline

irb(main)> m = CoreMIDI.MIDIGetSource(0)
=> 273731602
irb(main)> str = “”
irb(main)> CoreMIDI.MIDIObjectGetStringProperty(m,"",str)
=> -10835 #kMIDIUnknownProperty = -10835

from MIDIServices.h

extern const CFStringRef kMIDIPropertyOffline

irb(main)>
CoreMIDI.MIDIObjectGetStringProperty(m,‘kMIDIPropertyOffline’,str)
[BUG] Segmentation fault at 0x00000000000000

#file ruby_2015-05-03-150759_nautilus.crash
Exception Type: EXC_BAD_ACCESS (SIGABRT)
Exception Codes: EXC_I386_GPFLT

The first try

CoreMIDI.MIDIObjectGetStringProperty(m,"",str)

returns -10835 which is kMIDIUnknownProperty. Looks ok for a response.
But then:

CoreMIDI.MIDIObjectGetStringProperty(m,‘kMIDIPropertyOffline’,str)

Exit with a Segmentation fault.

This function MIDIObjectGetStringProperty has an argument “CFStringRef
*str” which I understand is a reference. But how can I give that
reference from ruby. Is it possible?

Should I provide the BUG report. I’m not sure this is a BUG but me not
passing the right parameters type or references.

Many thanks in advance!
Matias Repetti.

2

Hello Juan M.,

I do not understand the subject matter, but from a formal point of view,
given that the last two arguments of the original function declaration
are
CFStringRef propertyID
and
CFStringRef *str
(one without an asterisk and one with an asterisk),
then it seems odd to me, that in your “extern” statement
they are both getting the same type, namely “char *”.

Would it not be more logical to add one astersk more
to the last argument, ie “char **”?

So long
Sven

Juan M. wrote in post #1172996:

Hello people,

I’m playing around with Fiddle and CoreMIDI on OSX.

ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin13]

require ‘fiddle’
require ‘fiddle/import’

module CoreMIDI
extend Fiddle::Importer
dlload
‘/System/Library/Frameworks/CoreMIDI.framework/Versions/Current/CoreMIDI’

#from MIDIServices.h
#extern MIDIEndpointRef MIDIGetSource(  ItemCount sourceIndex0 )
extern "int MIDIGetSource(int)"

# from MIDIServices.h
# extern OSStatus MIDIObjectGetStringProperty(MIDIObjectRef obj,

CFStringRef propertyID, CFStringRef *str);
extern “int MIDIObjectGetStringProperty(int, char *, char *)”

end

from MIDIServices.h

extern const CFStringRef kMIDIPropertyOffline

irb(main)> m = CoreMIDI.MIDIGetSource(0)
=> 273731602
irb(main)> str = “”
irb(main)> CoreMIDI.MIDIObjectGetStringProperty(m,"",str)
=> -10835 #kMIDIUnknownProperty = -10835

from MIDIServices.h

extern const CFStringRef kMIDIPropertyOffline

irb(main)>
CoreMIDI.MIDIObjectGetStringProperty(m,‘kMIDIPropertyOffline’,str)
[BUG] Segmentation fault at 0x00000000000000

#file ruby_2015-05-03-150759_nautilus.crash
Exception Type: EXC_BAD_ACCESS (SIGABRT)
Exception Codes: EXC_I386_GPFLT

The first try

CoreMIDI.MIDIObjectGetStringProperty(m,"",str)

returns -10835 which is kMIDIUnknownProperty. Looks ok for a response.
But then:

CoreMIDI.MIDIObjectGetStringProperty(m,‘kMIDIPropertyOffline’,str)

Exit with a Segmentation fault.

This function MIDIObjectGetStringProperty has an argument “CFStringRef
*str” which I understand is a reference. But how can I give that
reference from ruby. Is it possible?

Should I provide the BUG report. I’m not sure this is a BUG but me not
passing the right parameters type or references.

Many thanks in advance!
Matias Repetti.

3

Thanks Sven for your answer.

Apologies, I wrote this post in a hurry.

I’ll rewrite my question again when I’m back.

4

In short, I don’t know how to cast the types to be parsed by Fiddle and
also, how to pass variables as a reference.

This is the header on from MIDIServices.h file shows:

extern OSStatus MIDIObjectGetStringProperty(MIDIObjectRef obj,

CFStringRef propertyID, CFStringRef *str);

I tried to import the above function in ruby using Fiddle, to be honest,
I tried different types, I don’t know which types should I use on the
ruby side. For example OSStatus is a SInt32 then I though that using int
will be ok, MIDIObjectRef is a UInt32, then also int. But with
CFStringRef I have no idea.

I couldn’t find many examples on the topic, then I’m trying to guess.

Now I found some examples using DL:Import from the book: Practical Ruby
Projects: Ideas for the Eclectic Programmer(Topher Cyll) he is using
(void *) for the CFStringRef types. I’ll try that.

Another thing is, the function MIDIObjectGetStringProperty is setting
the result in: CFStringRef *str.
From ruby, I should provide a variable to receive the result, but I
don’t know how.

For example from Ruby:

str = “”
m = CoreMIDI.MIDIGetSource(0)
CoreMIDI.MIDIObjectGetStringProperty(m,<property_to_get>,str)

I expect “str” to contain the result of MIDIObjectGetStringProperty
call. But doesn’t happen.
If someone have some advice on this, I’ll appreciate.

Thanks.