I have an application, running on MacOs, that communicates with a Bluetooth LE peripheral. I’m able to establish a L2CAP channel and after the channel is established, the peripheral sends a first (quite small) SDU on that channel to the application. The PSM is 0x80 and was chosen by the peripherals BLE stack. The application receives the PSM via GATT.

I can see the SDU being send in a single LL PDU with wireshark:

Frame 569: 44 bytes on wire (352 bits), 44 bytes captured (352 bits) on interface /dev/cu.usbmodem0006832334881-4.2, id 0
nRF Sniffer for Bluetooth LE
Bluetooth Low Energy Link Layer
    Access Address: 0x50657518
    [Master Address: Apple_7a:a5:d6 (f4:d4:88:7a:a5:d6)]
    [Slave Address: c4:ed:78:ac:5b:31 (c4:ed:78:ac:5b:31)]
    Data Header
        .... ..10 = LLID: Start of an L2CAP message or a complete L2CAP message with no fragmentation (0x2)
        .... .0.. = Next Expected Sequence Number: 0 [ACK]
        .... 1... = Sequence Number: 1 [OK]
        ...1 .... = More Data: True
        ..0. .... = CTE Info: Not Present
        00.. .... = RFU: 0
        Length: 18
    [L2CAP Index: 30]
    [Connection Parameters in: 473]
    CRC: 0x9850ef
Bluetooth L2CAP Protocol
    Length: 14
    CID: Dynamically Allocated Channel (0x004e)
    [Connect in frame: 562]
    [PSM: Unknown (0x0080)]
    SDU Length: 12
    Payload: 000509040401053c4c6f7600

I can also see the SDU being received in Apples PacketLogger. But I miss the corresponding call to a stream event handler.

The same BLE code ran in a different project without problems on MacOS and iOS. Now, I moved the code to an other project. Discovery and GATT communication works without any problem, yet the reception of L2CAP messages causes a problem.

The code creates a dispatch queue and passes it to the CBCentralManager:

dispatch_queue  = dispatch_queue_create("de.torrox.ble_event_queue", NULL);
manager         = [[CBCentralManager alloc] initWithDelegate:self queue:dispatch_queue options:nil];

When the L2CAP channel is established, the didOpenL2CAPChannel callback gets called from a thread within the dispatch_queue (has been verified with lldb):

- (void)peripheral:(CBPeripheral *)peripheral
didOpenL2CAPChannel:(CBL2CAPChannel *)channel
             error:(NSError *)error
{
    [channel inputStream].delegate = self;
    [channel outputStream].delegate = self;
    [[channel inputStream] scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
    [[channel outputStream] scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
    [[channel inputStream] open];
    [[channel outputStream] open];

    ...
    // a reference to the channel is stored in the outside channel object
    [channel retain];
    ...
}

Yet, not a single stream event is generated:

- (void)stream:(NSStream *)stream
   handleEvent:(NSStreamEvent)event_code
{
    Log( @"stream:handleEvent %@, %lu", stream, event_code );
    ...
}

What could be the problem here? How could I get further informations to track this down? Is there a chance, that received data is not stored until it is picked up by a call to read?