Reported by valgrind using the command below:
$ valgrind jackd -p128 -t2000 -dalsa -dhw:0 -r44100 -p2048 -n2 -S
==8930== Memcheck, a memory error detector
==8930== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==8930== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==8930== Command: jackd -p128 -t2000 -dalsa -dhw:0 -r44100 -p2048 -n2 -S
==8930==
jackdmp 1.9.11
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2016 Grame.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
JACK server starting in realtime mode with priority 10
self-connect-mode is "Don't restrict self connect requests"
creating alsa driver ... hw:0|hw:0|2048|2|44100|0|0|nomon|swmeter|-|16bit
configuring for 44100Hz, period = 2048 frames (46.4 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 16bit little-endian
ALSA: use 2 periods for capture
ALSA: final selected sample format for playback: 16bit little-endian
ALSA: use 2 periods for playback
==8930== Thread 4:
==8930== Syscall param write(buf) points to uninitialised byte(s)
==8930==    at 0x571388D: ??? (syscall-template.S:81)
==8930==    by 0x4EA70B3: Jack::JackClientSocket::Write(void*, int) (JackSocket.cpp:233)
==8930==    by 0x4E94C62: Jack::JackClientCheckResult::Write(Jack::detail::JackChannelTransactionInterface*) (JackRequest.h:227)
==8930==    by 0x4ECDF3A: Jack::JackRequestDecoder::HandleRequest(Jack::detail::JackChannelTransactionInterface*, int) (JackRequestDecoder.cpp:58)
==8930==    by 0x4ED4219: Jack::JackSocketServerChannel::Execute() (JackSocketServerChannel.cpp:247)
==8930==    by 0x4EA4B63: Jack::JackPosixThread::ThreadHandler(void*) (JackPosixThread.cpp:59)
==8930==    by 0x59F5181: start_thread (pthread_create.c:312)
==8930==    by 0x572247C: clone (clone.S:111)
==8930==  Address 0x41da85e is on thread 4's stack
==8930==  in frame #3, created by Jack::JackRequestDecoder::HandleRequest(Jack::detail::JackChannelTransactionInterface*, int) (JackRequestDecoder.cpp:46)
==8930==
==8930== Syscall param write(buf) points to uninitialised byte(s)
==8930==    at 0x571388D: ??? (syscall-template.S:81)
==8930==    by 0x4EA70B3: Jack::JackClientSocket::Write(void*, int) (JackSocket.cpp:233)
==8930==    by 0x4ED619B: Jack::JackClientNotification::Write(Jack::detail::JackChannelTransactionInterface*) (JackRequest.h:1606)
==8930==    by 0x4ED5F2E: Jack::JackSocketNotifyChannel::ClientNotify(int, char const*, int, int, char const*, int, int, int*) (JackSocketNotifyChannel.cpp:56)
==8930==    by 0x4EB51DF: Jack::JackExternalClient::ClientNotify(int, char const*, int, int, char const*, int, int) (JackExternalClient.cpp:40)
==8930==    by 0x4EAFA9A: Jack::JackEngine::ClientNotify(Jack::JackClientInterface*, int, char const*, int, int, char const*, int, int) (JackEngine.cpp:274)
==8930==    by 0x4EAFD34: Jack::JackEngine::NotifyAddClient(Jack::JackClientInterface*, char const*, int) (JackEngine.cpp:318)
==8930==    by 0x4EB0C99: Jack::JackEngine::ClientExternalOpen(char const*, int, int, int*, int*, int*, int*) (JackEngine.cpp:636)
==8930==    by 0x4ED4486: Jack::JackLockedEngine::ClientExternalOpen(char const*, int, int, int*, int*, int*, int*) (JackLockedEngine.h:123)
==8930==    by 0x4ED3AD5: Jack::JackSocketServerChannel::ClientAdd(Jack::detail::JackChannelTransactionInterface*, Jack::JackClientOpenRequest*, Jack::JackClientOpenResult*) (JackSocketServerChannel.cpp:132)
==8930==    by 0x4ECE0B8: Jack::JackRequestDecoder::HandleRequest(Jack::detail::JackChannelTransactionInterface*, int) (JackRequestDecoder.cpp:73)
==8930==    by 0x4ECDFBA: Jack::JackRequestDecoder::HandleRequest(Jack::detail::JackChannelTransactionInterface*, int) (JackRequestDecoder.cpp:63)
==8930==  Address 0x41d9ccb is on thread 4's stack
==8930==  in frame #3, created by Jack::JackSocketNotifyChannel::ClientNotify(int, char const*, int, int, char const*, int, int, int*) (JackSocketNotifyChannel.cpp:51)
==8930==
==8930== Syscall param write(buf) points to uninitialised byte(s)
==8930==    at 0x571388D: ??? (syscall-template.S:81)
==8930==    by 0x4EA70B3: Jack::JackClientSocket::Write(void*, int) (JackSocket.cpp:233)
==8930==    by 0x4ED633C: Jack::JackClientNotification::Write(Jack::detail::JackChannelTransactionInterface*) (JackRequest.h:1612)
==8930==    by 0x4ED5F2E: Jack::JackSocketNotifyChannel::ClientNotify(int, char const*, int, int, char const*, int, int, int*) (JackSocketNotifyChannel.cpp:56)
==8930==    by 0x4EB51DF: Jack::JackExternalClient::ClientNotify(int, char const*, int, int, char const*, int, int) (JackExternalClient.cpp:40)
==8930==    by 0x4EAFA9A: Jack::JackEngine::ClientNotify(Jack::JackClientInterface*, int, char const*, int, int, char const*, int, int) (JackEngine.cpp:274)
==8930==    by 0x4EAFD34: Jack::JackEngine::NotifyAddClient(Jack::JackClientInterface*, char const*, int) (JackEngine.cpp:318)
==8930==    by 0x4EB0C99: Jack::JackEngine::ClientExternalOpen(char const*, int, int, int*, int*, int*, int*) (JackEngine.cpp:636)
==8930==    by 0x4ED4486: Jack::JackLockedEngine::ClientExternalOpen(char const*, int, int, int*, int*, int*, int*) (JackLockedEngine.h:123)
==8930==    by 0x4ED3AD5: Jack::JackSocketServerChannel::ClientAdd(Jack::detail::JackChannelTransactionInterface*, Jack::JackClientOpenRequest*, Jack::JackClientOpenResult*) (JackSocketServerChannel.cpp:132)
==8930==    by 0x4ECE0B8: Jack::JackRequestDecoder::HandleRequest(Jack::detail::JackChannelTransactionInterface*, int) (JackRequestDecoder.cpp:73)
==8930==    by 0x4ECDFBA: Jack::JackRequestDecoder::HandleRequest(Jack::detail::JackChannelTransactionInterface*, int) (JackRequestDecoder.cpp:63)
==8930==  Address 0x41d9d1d is on thread 4's stack
==8930==  in frame #3, created by Jack::JackSocketNotifyChannel::ClientNotify(int, char const*, int, int, char const*, int, int, int*) (JackSocketNotifyChannel.cpp:51)
==8930==