Valgrind errors in c++ threads

I am new to Valgrind. It throws 2 errors during memory check for the following code (http://ift.tt/1tWLqLO). Not sure how to interpret those errors.

#include <thread>
#include <mutex>
#include <iostream>

int g_i = 0;
std::mutex g_i_mutex;  // protects g_i

void safe_increment()
{
    std::lock_guard<std::mutex> lock(g_i_mutex);
    ++g_i;
    std::cout << std::this_thread::get_id() << ": " << g_i << '\n';
}

int main()
{
    std::cout << __func__ << ": " << g_i << '\n';
    std::thread t1(safe_increment);
    std::thread t2(safe_increment);
    t1.join();
    t2.join();
   std::cout << __func__ << ": " << g_i << '\n';
}

Ran this code on Mac using:

clang++ simplethread.cpp -o simplethread -lpthread -std=c++11

Code runs fine as expected but I get the following output from Valgrind.

    $ valgrind --tool=memcheck ./simplethread
    ==34831== Memcheck, a memory error detector
    ==34831== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
    ==34831== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
    ==34831== Command: ./simplethread
    ==34831== 
    ==34831== Syscall param msg->desc.port.name points to uninitialised byte(s)
    ==34831==    at 0x10045F34A: mach_msg_trap (in         /usr/lib/system/libsystem_kernel.dylib)
    ==34831==    by 0x10045E796: mach_msg (in /usr/lib/system/libsystem_kernel.dylib)
    ==34831==    by 0x100458485: task_set_special_port (in  /usr/lib/system/libsystem_kernel.dylib)
    ==34831==    by 0x1005F410E: _os_trace_create_debug_control_port (in /usr/lib/system/libsystem_trace.dylib)
    ==34831==    by 0x1005F4458: _libtrace_init (in /usr/lib/system/libsystem_trace.dylib)
    ==34831==    by 0x1000A99DF: libSystem_initializer (in /usr/lib/libSystem.B.dylib)
    ==34831==    by 0x10001BA1A:    ImageLoaderMachO::doModInitFunctions(ImageLoader::LinkContext const&) (in  /usr/lib/dyld)
==34831==    by 0x10001BC1D:   ImageLoaderMachO::doInitialization(ImageLoader::LinkContext const&) (in  /usr/lib/dyld)
    ==34831==    by 0x1000174A9:   ImageLoader::recursiveInitialization(ImageLoader::LinkContext const&, unsigned int, char const*, ImageLoader::InitializerTimingList&,    ImageLoader::UninitedUpwards&) (in /usr/lib/dyld)
    ==34831==    by 0x100017440: ImageLoader::recursiveInitialization(ImageLoader::LinkContext const&, unsigned int, char const*, ImageLoader::InitializerTimingList&, ImageLoader::UninitedUpwards&) (in /usr/lib/dyld)
    ==34831==    by 0x100016523: ImageLoader::processInitializers(ImageLoader::LinkContext const&, unsigned int, ImageLoader::InitializerTimingList&, ImageLoader::UninitedUpwards&) (in /usr/lib/dyld)
    ==34831==    by 0x1000165B8: ImageLoader::runInitializers(ImageLoader::LinkContext const&, ImageLoader::InitializerTimingList&) (in /usr/lib/dyld)
    ==34831==  Address 0x10488ed4c is on thread 1's stack
    ==34831==  in frame #2, created by task_set_special_port (???:)
    ==34831== 
    main: 0
    ==34831== Thread 2:
    ==34831== Invalid read of size 4
    ==34831==    at 0x1005BC899: _pthread_body (in /usr/lib/system/libsystem_pthread.dylib)
    ==34831==    by 0x1005BC886: _pthread_start (in /usr/lib/system/libsystem_pthread.dylib)
    ==34831==    by 0x1005BC08C: thread_start (in /usr/lib/system/libsystem_pthread.dylib)
    ==34831==  Address 0x18 is not stack'd, malloc'd or (recently) free'd
    ==34831== 
    ==34831== 
    ==34831== Process terminating with default action of signal 11 (SIGSEGV)
    ==34831==  Access not within mapped region at address 0x18
    ==34831==    at 0x1005BC899: _pthread_body (in /usr/lib/system/libsystem_pthread.dylib)
    ==34831==    by 0x1005BC886: _pthread_start (in /usr/lib/system/libsystem_pthread.dylib)
    ==34831==    by 0x1005BC08C: thread_start (in /usr/lib/system/libsystem_pthread.dylib)
    ==34831==  If you believe this happened as a result of a stack
    ==34831==  overflow in your program's main thread (unlikely but
    ==34831==  possible), you can try to increase the size of the
    ==34831==  main thread stack using the --main-stacksize= flag.
    ==34831==  The main thread stack size used in this run was 8388608.
    --34831:0:schedule VG_(sema_down): read returned -4
    ==34831== 
    ==34831== HEAP SUMMARY:
    ==34831==     in use at exit: 22,494 bytes in 168 blocks
    ==34831==   total heap usage: 184 allocs, 16 frees, 28,638 bytes allocated
    ==34831== 
    ==34831== LEAK SUMMARY:
    ==34831==    definitely lost: 16 bytes in 1 blocks
    ==34831==    indirectly lost: 56 bytes in 2 blocks
    ==34831==      possibly lost: 72 bytes in 3 blocks
    ==34831==    still reachable: 4,368 bytes in 10 blocks
    ==34831==         suppressed: 17,982 bytes in 152 blocks
    ==34831== Rerun with --leak-check=full to see details of leaked memory
    ==34831== 
    ==34831== For counts of detected and suppressed errors, rerun with: -v
    ==34831== Use --track-origins=yes to see where uninitialised values come from
    ==34831== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 1 from 1)
    Segmentation fault: 11

What does these errors mean ?