Why doesn't this simple CoreMIDI program produce MIDI output?

You're calling MIDISourceCreate to create a virtual MIDI source.

This means that your source will appear in other apps' MIDI setup UI, and that those apps can choose whether or not to listen to your source. Your MIDI will not get sent to any physical MIDI ports, unless some other app happens to channel it there. It also means that your app has no choice as to where the MIDI it's sending goes. I'm assuming that's what you want.

The documentation for MIDISourceCreate says:

After creating a virtual source, use MIDIReceived to transmit MIDI messages from your virtual source to any clients connected to the virtual source.

So, do two things:

• Remove the code that creates the output port. You don't need it.
• change MIDISend(outPort, midiOut, pktList) to: MIDIReceived(midiOut, pktlist).

That should solve your problem.

So what are output ports good for? If you wanted to direct your MIDI data to a specific destination -- maybe a physical MIDI port -- you would NOT create a virtual MIDI source. Instead:

1. Call MIDIOutputPortCreate() to make an output port
2. Use MIDIGetNumberOfDestinations() and MIDIGetDestination() to get the list of destinations and find the one you're interested in.
3. To send MIDI to one destination, call MIDISend(outputPort, destination, packetList).

I'm just leaving this here for my own reference. It's a full example based 100% on yours, but including the other side (receiving), my bad C code and the accepted answer's corrections (of course).

#import "AppDelegate.h"

@implementation AppDelegate

@synthesize window = _window;

#define NSLogError(c,str) do{if (c) NSLog(@"Error (%@): %u:%@", str, (unsigned int)c,[NSError errorWithDomain:NSMachErrorDomain code:c userInfo:nil]); }while(false)

static void spit(Byte* values, int length, BOOL useHex) {
    NSMutableString *thing = [@"" mutableCopy];
    for (int i=0; i<length; i++) {
        if (useHex)
            [thing appendFormat:@"0x%X ", values[i]];
        else
            [thing appendFormat:@"%d ", values[i]];
    }
    NSLog(@"Length=%d %@", length, thing);
}

- (void) startSending {
    MIDIEndpointRef midiOut;
    char pktBuffer[1024];
    MIDIPacketList* pktList = (MIDIPacketList*) pktBuffer;
    MIDIPacket     *pkt;
    Byte            midiDataToSend[] = {0x91, 0x3c, 0x40};
    int             i;

    MIDISourceCreate(theMidiClient, CFSTR("Magical MIDI Source"),
                     &midiOut);
    pkt = MIDIPacketListInit(pktList);
    pkt = MIDIPacketListAdd(pktList, 1024, pkt, 0, 3, midiDataToSend);

    for (i = 0; i < 100; i++) {
        if (pkt == NULL || MIDIReceived(midiOut, pktList)) {
            printf("failed to send the midi.\n");
        } else {
            printf("sent!\n");
        }
        sleep(1);
    }
}

void ReadProc(const MIDIPacketList *packetList, void *readProcRefCon, void *srcConnRefCon)
{
    const MIDIPacket *packet = &packetList->packet[0];

    for (int i = 0; i < packetList->numPackets; i++)
    {

        NSData *data = [NSData dataWithBytes:packet->data length:packet->length];
        spit((Byte*)data.bytes, data.length, YES);

        packet = MIDIPacketNext(packet);
    }
}

- (void) setupReceiver {
    OSStatus s;
    MIDIEndpointRef virtualInTemp;
    NSString *inName = [NSString stringWithFormat:@"Magical MIDI Destination"];
    s = MIDIDestinationCreate(theMidiClient, (__bridge CFStringRef)inName, ReadProc,  (__bridge void *)self, &virtualInTemp);
    NSLogError(s, @"Create virtual MIDI in");
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    MIDIClientCreate(CFSTR("Magical MIDI"), NULL, NULL,
                     &theMidiClient);
    [self setupReceiver];
    [self startSending];

}

@end