Change OS X system volume programmatically

Question

How can I change the volume programmatically from Objective-C?

I found this question, Controlling OS X volume in Snow Leopard which suggests to do:

Float32 volume = 0.5;
UInt32 size = sizeof(Float32);

AudioObjectPropertyAddress address = {
    kAudioDevicePropertyVolumeScalar,
    kAudioDevicePropertyScopeOutput,
    1 // Use values 1 and 2 here, 0 (master) does not seem to work
};

OSStatus err;
err = AudioObjectSetPropertyData(kAudioObjectSystemObject, &address, 0, NULL, size, &volume);
NSLog(@"status is %i", err);

This does nothing for me, and prints out status is 2003332927.

I also tried using values 2 and 0 in the address structure, same result for both.

How can I fix this and make it actually decrease the volume to 50%?

Answer 1

You need to get the default audio device first:

#import <CoreAudio/CoreAudio.h>

AudioObjectPropertyAddress getDefaultOutputDevicePropertyAddress = {
  kAudioHardwarePropertyDefaultOutputDevice,
  kAudioObjectPropertyScopeGlobal,
  kAudioObjectPropertyElementMaster
};

AudioDeviceID defaultOutputDeviceID;
UInt32 volumedataSize = sizeof(defaultOutputDeviceID);
OSStatus result = AudioObjectGetPropertyData(kAudioObjectSystemObject,
                                             &getDefaultOutputDevicePropertyAddress,
                                             0, NULL,
                                             &volumedataSize, &defaultOutputDeviceID);

if(kAudioHardwareNoError != result)
{
  // ... handle error ...
}

You can then set your volume on channel 1 (left) and channel 2 (right). Note that channel 0 (master) does not seem to be supported (the set command returns 'who?')

AudioObjectPropertyAddress volumePropertyAddress = {
  kAudioDevicePropertyVolumeScalar,
  kAudioDevicePropertyScopeOutput,
  1 /*LEFT_CHANNEL*/
};

Float32 volume;
volumedataSize = sizeof(volume);

result = AudioObjectSetPropertyData(defaultOutputDeviceID,
                                    &volumePropertyAddress,
                                    0, NULL,
                                    sizeof(volume), &volume);
if (result != kAudioHardwareNoError) {
  // ... handle error ...
}

Hope this answers your question!