HI *, I have a multiprocessing program that shall host a JACK server and two JACK clients. The server is named "AVB_Processing", the clients are name "AVB_Talker" and "AVB_Listener". The JACK server is created and started before forking the other processes and looks fine: JACK server starting in realtime mode with priority 8 self-connect-mode is "Don't restrict self connect requests" Jack: JackPosixThread::StartImp : create non RT thread Jack: JackPosixThread::ThreadHandler : start Jack: playback device hw:1,0 Jack: capture device hw:1,0 Jack: capture device hw:1,0 Jack: playback device hw:1,0 Jack: apparent rate = 48000 Jack: frames per period = 128 Jack: JackDriver::Open capture_driver_name = hw:1,0 Jack: JackDriver::Open playback_driver_name = hw:1,0 Jack: Check protocol client = 8 server = 8 Jack: JackEngine::ClientInternalOpen: name = system Jack: JackEngine::AllocateRefNum ref = 0 Jack: JackLinuxFutex::Allocate name = jack_sem.0_AVB_Processing_system val = 0 Jack: JackEngine::NotifyAddClient: name = system Jack: JackGraphManager::SetBufferSize size = 128 Jack: JackConnectionManager::DirectConnect first: ref1 = 0 ref2 = 0 Jack: JackGraphManager::ConnectRefNum cur_index = 0 ref1 = 0 ref2 = 0 Jack: JackDriver::SetupDriverSync driver sem in flush mode creating alsa driver ... hw:1,0|hw:1,0|128|2|48000|2|2|nomon|swmeter|-|32bit configuring for 48000Hz, period = 128 frames (2.7 ms), buffer = 2 periods ALSA: final selected sample format for capture: 32bit integer little-endian ALSA: use 2 periods for capture ALSA: final selected sample format for playback: 32bit integer little-endian ALSA: use 2 periods for playback Jack: JackSocketServerChannel::Open Jack: JackServerSocket::Bind : addr.sun_path /dev/shm/jack_AVB_Processing_0_0 Jack: JackSocketServerChannel::BuildPoolTable size = 1 Jack: JackEngine::Open Jack: JackClientSocket::Connect : addr.sun_path /dev/shm/jack_AVB_Processing_0_0 Jack: JackEngine::ClientInternalOpen: name = freewheel Jack: JackEngine::AllocateRefNum ref = 1 Jack: JackLinuxFutex::Allocate name = jack_sem.0_AVB_Processing_freewheel val = 0 Jack: JackEngine::NotifyAddClient: name = freewheel Jack: JackDriver::ClientNotify ref = 1 driver = system name = freewheel notify = 0 Jack: JackDriver::ClientNotify ref = 0 driver = freewheel name = system notify = 0 Jack: JackConnectionManager::DirectConnect first: ref1 = 1 ref2 = 1 Jack: JackGraphManager::ConnectRefNum cur_index = 0 ref1 = 1 ref2 = 1 Jack: JackDriver::SetupDriverSync driver sem in flush mode Jack: JackGraphManager::SetBufferSize size = 128 Jack: JackAlsaDriver::Attach fBufferSize 128 fSampleRate 48000 Jack: JackEngine::PortRegister ref = 0 name = system:capture_1 type = 32 bit float mono audio flags = 22 buffer_size = 128 Jack: JackGraphManager::AllocatePortAux port_index = 1 name = system:capture_1 type = 32 bit float mono audio Jack: JackConnectionManager::AddOutputPort ref = 0 port = 1 Jack: JackEngine::ClientNotify: no callback for notification = 9 Jack: JackEngine::ClientNotify: no callback for notification = 9 Jack: JackAlsaDriver::Attach fCapturePortList[i] 1 Jack: JackEngine::PortRegister ref = 0 name = system:capture_2 type = 32 bit float mono audio flags = 22 buffer_size = 128 Jack: JackGraphManager::AllocatePortAux port_index = 2 name = system:capture_2 type = 32 bit float mono audio Jack: JackConnectionManager::AddOutputPort ref = 0 port = 2 Jack: JackEngine::ClientNotify: no callback for notification = 9 Jack: JackEngine::ClientNotify: no callback for notification = 9 Jack: JackAlsaDriver::Attach fCapturePortList[i] 2 Jack: JackEngine::PortRegister ref = 0 name = system:playback_1 type = 32 bit float mono audio flags = 21 buffer_size = 128 Jack: JackGraphManager::AllocatePortAux port_index = 3 name = system:playback_1 type = 32 bit float mono audio Jack: JackConnectionManager::AddInputPort ref = 0 port = 3 Jack: JackEngine::ClientNotify: no callback for notification = 9 Jack: JackEngine::ClientNotify: no callback for notification = 9 Jack: JackAlsaDriver::Attach fPlaybackPortList[i] 3 Jack: JackEngine::PortRegister ref = 0 name = system:playback_2 type = 32 bit float mono audio flags = 21 buffer_size = 128 Jack: JackGraphManager::AllocatePortAux port_index = 4 name = system:playback_2 type = 32 bit float mono audio Jack: JackConnectionManager::AddInputPort ref = 0 port = 4 Jack: JackEngine::ClientNotify: no callback for notification = 9 Jack: JackEngine::ClientNotify: no callback for notification = 9 Jack: JackAlsaDriver::Attach fPlaybackPortList[i] 4 Jack: Clock source : hpet Jack: JackServer::Start Jack: JackThreadedDriver::Start Jack: JackPosixThread::StartImp : create non RT thread Jack: JackPosixThread::ThreadHandler : start Jack: JackThreadedDriver::Init real-time Jack: JackPosixThread::AcquireRealTimeImp priority = 8 Jack: JackPosixThread::StartImp : create non RT thread Jack: JackPosixThread::ThreadHandler : start Jack: JackSocketServerChannel::ClientCreate socket Jack: JackSocketServerChannel::BuildPoolTable size = 2 Jack: JackSocketServerChannel::BuildPoolTable fSocketTable i = 1 fd = 56 Jack: JackSocketServerChannel::Execute : fPollTable i = 1 fd = 56 Jack: JackRequest::Notification Jack: JackDriver::ClientNotify ref = 1 driver = freewheel name = freewheel notify = 18 Jack: JackDriver::ClientNotify ref = 1 driver = freewheel name = freewheel notify = 18 Jack: JackDriver::ClientNotify ref = 1 driver = freewheel name = freewheel notify = 18 Jack: JackDriver::ClientNotify ref = 1 driver = freewheel name = freewheel notify = 18 Jack: JackEngine::ClientNotify: no callback for notification = 4 Jack: JackEngine::ClientNotify: no callback for notification = 4 Jack: **** alsa_pcm: xrun of at least -5118090.752 msecs Jack: ALSA XRun wait_status = 0 Jack: JackSocketServerChannel::Execute : fPollTable i = 1 fd = 56 Jack: JackRequest::Notification Jack: JackEngine::ClientNotify: no callback for notification = 3 Jack: JackEngine::ClientNotify: no callback for notification = 3 Jack: **** alsa_pcm: xrun of at least -5118163.456 msecs Jack: ALSA XRun wait_status = 0 Jack: JackSocketServerChannel::Execute : fPollTable i = 1 fd = 56 Jack: JackRequest::Notification Jack: JackEngine::ClientNotify: no callback for notification = 3 Jack: JackEngine::ClientNotify: no callback for notification = 3 After the JACK server is running, I fork four processes. Two of which try to open a JACK client without success: Jack: jack_client_open AVB_Talker Jack: JackInternalClient::Open name = AVB_Talker Jack: Check protocol client = 8 server = 8 Jack: JackEngine::ClientInternalOpen: name = AVB_Talker Jack: JackEngine::AllocateRefNum ref = 2 Jack: JackLinuxFutex::Allocate name = jack_sem.0_AVB_Processing_AVB_Talker val = 0 Jack: JackPosixProcessSync::TimedWait time out = 5000000 Jack: jack_client_open AVB_Listener Jack: JackInternalClient::Open name = AVB_Listener Jack: Check protocol client = 8 server = 8 Jack: JackEngine::ClientInternalOpen: name = AVB_Listener Jack: JackEngine::AllocateRefNum ref = 2 Jack: JackLinuxFutex::Allocate name = jack_sem.0_AVB_Processing_AVB_Listener val = 0 Jack: JackPosixProcessSync::TimedWait time out = 5000000 JackPosixProcessSync::LockedTimedWait error usec = 5000000 err = Connection timed out Jack: JackPosixProcessSync::TimedWait finished delta = 5000078.0 Driver is not running Cannot open client name = AVB_Talker JackPosixProcessSync::LockedTimedWait error usec = 5000000 err = Connection timed out Jack: JackPosixProcessSync::TimedWait finished delta = 5000069.0 Driver is not running Cannot open client name = AVB_Listener Jack: JackSocketServerChannel::Execute : fPollTable i = 1 fd = 56 The client probably fails with "Driver is not running" due to the ALSA xruns of the server: Jack: **** alsa_pcm: xrun of at least -5118090.752 msecs Jack: ALSA XRun wait_status = 0 Jack: JackSocketServerChannel::Execute : fPollTable i = 1 fd = 56 Jack: JackRequest::Notification Jack: JackEngine::ClientNotify: no callback for notification = 3 Jack: JackEngine::ClientNotify: no callback for notification = 3 Jack: **** alsa_pcm: xrun of at least -5118163.456 msecs Jack: ALSA XRun wait_status = 0 Here is the server code: static void set_jack_server_parameters( FILE* filepointer, const JSList * node_ptr) {     while (node_ptr != NULL) {         jackctl_parameter_t * parameter = (jackctl_parameter_t *)node_ptr->data;         if( ( strcmp( jackctl_parameter_get_name(parameter), "rate" ) == 0 ) ||             ( strcmp( jackctl_parameter_get_name(parameter), "period" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "nperiods" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "device" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "realtime-priority" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "realtime" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "inchannels" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "outchannels" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "playback" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "capture" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "verbose" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "clock-source" ) == 0 ) ||                 ( strcmp( jackctl_parameter_get_name(parameter), "name" ) == 0 ) ){             union jackctl_parameter_value value;             if( ( strcmp( jackctl_parameter_get_name(parameter), "name" ) == 0 ) ){                 sprintf( value.str, "AVB_Processing");             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "verbose" ) == 0 ) ){                 value.b = true;             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "clock-source" ) == 0 ) ){                 value.ui = 1;             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "rate" ) == 0 ) ){                 value.ui = 48000;             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "period" ) == 0 ) ){                 value.ui = 128;             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "nperiods" ) == 0 ) ){                 value.ui = 2;             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "device" ) == 0 ) ){                 sprintf( value.str, "hw:1,0");             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "realtime-priority" ) == 0 ) ){                 value.i = 8;             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "realtime" ) == 0 ) ){                 value.b = true;             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "inchannels" ) == 0 ) ){                 value.ui = 2;             } else if( ( strcmp( jackctl_parameter_get_name(parameter), "outchannels" ) == 0 ) ){                 value.ui = 2;             }             jackctl_parameter_set_value( parameter, &value );         }         node_ptr = jack_slist_next(node_ptr);     } } if( (server = jackctl_server_create(NULL, NULL) )); parameters = jackctl_server_get_parameters(server); set_jack_server_parameters(filepointer, parameters); drivers = jackctl_server_get_drivers_list(server); node_ptr = drivers; while (node_ptr != NULL) {        if( ( strcmp( jackctl_driver_get_name((jackctl_driver_t *)node_ptr->data), "alsa" ) == 0 ) ){                     set_jack_server_parameters( filepointer, jackctl_driver_get_parameters( ( jackctl_driver_t *) node_ptr->data) );          }          node_ptr = jack_slist_next(node_ptr); } if( (jackctl_server_open(server, jackctl_server_get_driver(server, "alsa")) )); if( (jackctl_server_start(server) )); Jack is working fine outside my program. I cloned the git repo of jack2 today ( jackdmp 1.9.12 ). What might be going wrong here? Thanks, Ck