Will dispatch_async inside autoreleasepool cause issues? - objective-c

Question

I'm parsing midi messages inside an autoreleasepool. After determining the message that's being sent, I make calls to the main thread using dispatch_async(dispatch_get_main_queue(). This seems to work well most of the time but occasionally I'll get a crash where it stops on the line
while (semaphore_wait(midiReceivedSemaphore) == KERN_SUCCESS ) {
The method looks something like this:
#autoreleasepool {
unsigned int maxPacketLength = 0x100;
unsigned char* data = malloc(maxPacketLength);
UInt16 length;
while (semaphore_wait(midiReceivedSemaphore) == KERN_SUCCESS ) {
midi_packet_buffer_next_packet_length(midiPacketBuffer, &length);
if( length > 0) {
length = midi_packet_buffer_read(midiPacketBuffer, data, length);
for (unsigned int packetContentsIndex = 0; packetContentsIndex < length; packetContentsIndex++) {
while (packetContentsIndex < length) {
Byte command = data[packetContentsIndex];
switch (command) {
case 0xA0: //aftertouch
commandLength += 2;
int meterChannel = data[packetContentsIndex +1];
int meterValue = data[packetContentsIndex +2];
if (meterValue < 0x10)
{
dispatch_async(dispatch_get_main_queue(), ^{
[delegate sendLevelMeterPT:meterChannel withValue:meterValue ];
});
}
break;
default:
break;
}
}
}
}
}
free(data);
}
Is this a very bad way of handling the MIDI messages? The reason I used dispatch_async is so that it wouldn't slow down the midi processing thread, before using dispatch_async the method was missing some messages if they were coming in fast. Now it catches all the messages but I get the occasional crash. I'm thinking I might have to rewrite the whole midi processing method but want to know the proper way to go about it. Any advice would be great.