title: A very simple thread pool using pthreads in C++
body: <p>I'm trying to understand some of the basics of using POSIX pthreads.  The kind of thing I need to do (eventually) is parallelize some computations, using a thread pool model.  At present I want to ensure I have a very basic sense for how the POSIX pthread model works.   So I'm trying to create the simplest thread pool that's general enough to do the kinds of things I want to do.  There will be some shared memory, an input queue, and an output queue, and there will be mutexes protecting each.  I've written some code that does just that but valgrind's helgrind tool does not like what I've done.  I suspect I'm missing something basic.   Do you have insights into my code? </p>
  
  <pre><code>#include &lt;stdlib.h&gt;
  #include &lt;string&gt;
  #include &lt;sstream&gt;
  #include &lt;list&gt;
  #include &lt;iostream&gt;
  
  #include &lt;pthread.h&gt;
  #include &lt;signal.h&gt;
  #include &lt;sys/select.h&gt;
  
  // the muticies, protectors of the shared resources
  pthread_mutex_t coutLock;
  pthread_mutex_t inQueueLock;
  pthread_mutex_t outQueueLock;
  // the shared data
  std::list&lt; std::string &gt; inQueue;
  std::list&lt; std::string &gt; outQueue;
  
  struct thread_detail { // information to pass to worker threads
   unsigned long num;
  };
  
  extern "C" {
      void *workerThread(void *threadarg);
  }
  
  void *workerThread(void *threadarg) 
  {
     struct thread_detail *my_data;
     my_data = (thread_detail *) threadarg;
     int taskid = my_data-&gt;num;
     std::stringstream ss; ss&lt;&lt;taskid; std::string taskString(ss.str());
  
     bool somethingTodo=true;
     while (somethingTodo) // keep on working until inQueue is empty 
      {
        pthread_mutex_lock( &amp;inQueueLock );
        std::string workOnMe;
        if (inQueue.size()==0) { somethingTodo=false; }
        else
          {
           workOnMe = inQueue.front();
           inQueue.pop_front();
          }
        pthread_mutex_unlock( &amp;inQueueLock );
  
        if (!somethingTodo) break;
        workOnMe = "thread " + taskString + " worked on " + workOnMe;
        // let's pretend this takes some time, add a delay to the computation
        struct timeval timeout;
        timeout.tv_sec = 0;  timeout.tv_usec = 100000; // 0.1 second delay 
        select( 0, NULL, NULL, NULL, &amp; timeout );
  
        pthread_mutex_lock( &amp;outQueueLock );
        outQueue.push_back( workOnMe );
        pthread_mutex_unlock( &amp;outQueueLock );
      }
  
     pthread_exit(NULL);
  }
  
  
  int main (int argc, char *argv[])
  {
    unsigned long comp_DONE=0; 
    unsigned long comp_START=0;
    // set-up the mutexes
    pthread_mutex_init( &amp;coutLock, NULL );
    pthread_mutex_init( &amp;inQueueLock, NULL );
    pthread_mutex_init( &amp;outQueueLock, NULL );
  
    if (argc != 3) { std::cout&lt;&lt;"Program requires two arguments: (1) number of threads to use,"
                             " and (2) tasks to accomplish.\n"; exit(1); }
    unsigned long NUM_THREADS(atoi( argv[1] ));
    unsigned long comp_TODO(atoi(argv[2]));
    std::cout&lt;&lt;"Program will have "&lt;&lt;NUM_THREADS&lt;&lt;" threads, working on "&lt;&lt;comp_TODO&lt;&lt;" things \n";
    for (unsigned long i=0; i&lt;comp_TODO; i++) // fill inQueue will rubbish data since this isn't an actual computation...
     {
      std::stringstream ss; 
      ss&lt;&lt;"task "&lt;&lt;i; 
      inQueue.push_back(ss.str());
     }
  
    // start the worker threads
    std::list&lt; pthread_t* &gt; threadIdList; // just the thread ids
    std::list&lt; thread_detail &gt; thread_table; // for keeping track of information on the various threads we'll create
    for (unsigned long i=0; i&lt;NUM_THREADS; i++) // start the threads
     {
      pthread_t *tId( new pthread_t );   threadIdList.push_back(tId);
      thread_detail Y; Y.num=i; thread_table.push_back(Y);
      int rc( pthread_create( tId, NULL, workerThread, (void *)(&amp;(thread_table.back() )) ) );
      if (rc) { std::cout&lt;&lt;"ERROR; return code from pthread_create() "&lt;&lt;comp_START&lt;&lt;"\n"; std::cout.flush();
                exit(-1); }
     }
    // now we wait for the threads to terminate, perhaps updating the screen with info as we go. 
    std::string stringOut;
    while (comp_DONE != comp_TODO)
     {
           // poll the queue to get a status update on computation
           pthread_mutex_lock(&amp;inQueueLock);
           comp_START = comp_TODO -  inQueue.size();
           pthread_mutex_unlock(&amp;inQueueLock);
           pthread_mutex_lock(&amp;outQueueLock);
           comp_DONE = outQueue.size();
           pthread_mutex_unlock(&amp;outQueueLock);
  
           // update for users
           pthread_mutex_lock(&amp;coutLock);
           for (unsigned long i=0; i&lt;stringOut.length(); i++) std::cout&lt;&lt;"\b";
           std::stringstream ss; ss&lt;&lt;"started "&lt;&lt;comp_START&lt;&lt;" completed "&lt;&lt;comp_DONE&lt;&lt;" of "&lt;&lt;comp_TODO;  
           stringOut = ss.str();  std::cout&lt;&lt;stringOut;  std::cout.flush();
           pthread_mutex_unlock(&amp;coutLock);
  
           // wait one second per update
           struct timeval timeout;
           timeout.tv_sec = 1;  timeout.tv_usec = 0;  
           select( 0, NULL, NULL, NULL, &amp; timeout );
          } // big while loop
  
     // call join to kill all worker threads
     std::list&lt; pthread_t* &gt;::iterator i(threadIdList.begin());
    while (i!=threadIdList.end())
     {
      if (pthread_join( *(*i), NULL)!=0) { std::cout&lt;&lt;"Thread join error!\n"; exit(1); }
      delete (*i);
      threadIdList.erase(i++);  
     }
    std::cout&lt;&lt;"\n";
  
     // let the user know what happened
     for (std::list&lt; std::string &gt;::iterator i=outQueue.begin(); i!=outQueue.end(); i++)
      {
       std::cout&lt;&lt;(*i)&lt;&lt;"\n";
      }
      // clean-up
      pthread_mutex_destroy(&amp;coutLock);
      pthread_mutex_destroy(&amp;inQueueLock);  
      pthread_mutex_destroy(&amp;outQueueLock);  
      // pthread_exit(NULL);
  }
  </code></pre>
  
  <p>Here is the helgrind output when passing the arguments 2 40 to the compiled program. </p>
  
  <pre><code>
  valgrind -v --tool=helgrind ./thread1 2 40
  ==12394== Helgrind, a thread error detector
  ==12394== Copyright (C) 2007-2009, and GNU GPL'd, by OpenWorks LLP et al.
  ==12394== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
  ==12394== Command: ./thread1 2 40
  ==12394== 
  --12394-- Valgrind options:
  --12394--    --suppressions=/usr/lib/valgrind/debian-libc6-dbg.supp
  --12394--    -v
  --12394--    --tool=helgrind
  --12394-- Contents of /proc/version:
  --12394--   Linux version 2.6.32-24-generic (buildd@yellow) (gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) ) #38-Ubuntu SMP Mon Jul 5 09:20:59 UTC 2010
  --12394-- Arch and hwcaps: AMD64, amd64-sse3-cx16
  --12394-- Page sizes: currently 4096, max supported 4096
  --12394-- Valgrind library directory: /usr/lib/valgrind
  --12394-- Reading syms from /home/rybu/prog/regina/exercise/thread1 (0x400000)
  --12394-- Reading syms from /lib/ld-2.11.1.so (0x4000000)
  --12394-- Reading debug info from /lib/ld-2.11.1.so ..
  --12394-- .. CRC mismatch (computed 99d13f6f wanted 0962e544)
  --12394-- Reading debug info from /usr/lib/debug/lib/ld-2.11.1.so ..
  --12394-- Reading syms from /usr/lib/valgrind/helgrind-amd64-linux (0x38000000)
  --12394--    object doesn't have a dynamic symbol table
  --12394-- Reading suppressions file: /usr/lib/valgrind/debian-libc6-dbg.supp
  --12394-- Reading suppressions file: /usr/lib/valgrind/default.supp
  --12394-- Reading syms from /usr/lib/valgrind/vgpreload_core-amd64-linux.so (0x4a23000)
  --12394-- Reading syms from /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so (0x4c25000)
  --12394-- REDIR: 0x4018310 (index) redirected to 0x4c2be59 (index)
  --12394-- REDIR: 0x4018390 (strcmp) redirected to 0x4c2bf4b (strcmp)
  --12394-- REDIR: 0x40184a0 (strlen) redirected to 0x4c2bec5 (strlen)
  --12394-- Reading syms from /usr/lib/libregina-engine-4.6.1.so (0x4e31000)
  --12394-- Reading syms from /usr/lib/libgmp.so.3.5.2 (0x52f7000)
  --12394-- Reading debug info from /usr/lib/libgmp.so.3.5.2 ..
  --12394-- .. CRC mismatch (computed d65050b9 wanted 1e40f6c0)
  --12394--    object doesn't have a symbol table
  --12394-- Reading syms from /lib/libpthread-2.11.1.so (0x5557000)
  --12394-- Reading debug info from /lib/libpthread-2.11.1.so ..
  --12394-- .. CRC mismatch (computed 9da7e2f6 wanted 8161fac5)
  --12394-- Reading debug info from /usr/lib/debug/lib/libpthread-2.11.1.so ..
  --12394-- Reading syms from /lib/librt-2.11.1.so (0x5774000)
  --12394-- Reading debug info from /lib/librt-2.11.1.so ..
  --12394-- .. CRC mismatch (computed 0e4f4ece wanted 920c9bed)
  --12394-- Reading debug info from /usr/lib/debug/lib/librt-2.11.1.so ..
  --12394-- Reading syms from /lib/libz.so.1.2.3.3 (0x597c000)
  --12394-- Reading debug info from /lib/libz.so.1.2.3.3 ..
  --12394-- .. CRC mismatch (computed 86f1fa27 wanted 5f1ca823)
  --12394--    object doesn't have a symbol table
  --12394-- Reading syms from /usr/lib/libstdc++.so.6.0.13 (0x5b93000)
  --12394-- Reading debug info from /usr/lib/libstdc++.so.6.0.13 ..
  --12394-- .. CRC mismatch (computed 7b5bd5a5 wanted e2f63673)
  --12394--    object doesn't have a symbol table
  --12394-- Reading syms from /lib/libm-2.11.1.so (0x5ea7000)
  --12394-- Reading debug info from /lib/libm-2.11.1.so ..
  --12394-- .. CRC mismatch (computed 043548c3 wanted a081b93d)
  --12394-- Reading debug info from /usr/lib/debug/lib/libm-2.11.1.so ..
  --12394-- Reading syms from /lib/libgcc_s.so.1 (0x612a000)
  --12394-- Reading debug info from /lib/libgcc_s.so.1 ..
  --12394-- .. CRC mismatch (computed 7c01dfc9 wanted 9d78e511)
  --12394--    object doesn't have a symbol table
  --12394-- Reading syms from /lib/libc-2.11.1.so (0x6341000)
  --12394-- Reading debug info from /lib/libc-2.11.1.so ..
  --12394-- .. CRC mismatch (computed c73d5a83 wanted 02758e3e)
  --12394-- Reading debug info from /usr/lib/debug/lib/libc-2.11.1.so ..
  --12394-- Reading syms from /usr/lib/libxml2.so.2.7.6 (0x66c4000)
  --12394-- Reading debug info from /usr/lib/libxml2.so.2.7.6 ..
  --12394-- .. CRC mismatch (computed c2590bed wanted 7aaa27a0)
  --12394-- Reading debug info from /usr/lib/debug/usr/lib/libxml2.so.2.7.6 ..
  --12394-- Reading syms from /lib/libdl-2.11.1.so (0x6a14000)
  --12394-- Reading debug info from /lib/libdl-2.11.1.so ..
  --12394-- .. CRC mismatch (computed 4a29f474 wanted e0b8d72c)
  --12394-- Reading debug info from /usr/lib/debug/lib/libdl-2.11.1.so ..
  --12394-- REDIR: 0x55603c0 (pthread_mutex_lock) redirected to 0x4c299fb (pthread_mutex_lock)
  --12394-- REDIR: 0x5561a00 (pthread_mutex_unlock) redirected to 0x4c29e8c (pthread_mutex_unlock)
  --12394-- REDIR: 0x63bd520 (malloc) redirected to 0x4c28a06 (malloc)
  --12394-- REDIR: 0x63bf360 (calloc) redirected to 0x4c27cc9 (calloc)
  --12394-- REDIR: 0x5c5e380 (operator new[](unsigned long)) redirected to 0x4c28e97 (operator new[](unsigned long))
  --12394-- REDIR: 0x5c5e250 (operator new(unsigned long)) redirected to 0x4c2921f (operator new(unsigned long))
  --12394-- REDIR: 0x5c5c380 (operator delete(void*)) redirected to 0x4c28328 (operator delete(void*))
  --12394-- REDIR: 0x5c5c3c0 (operator delete[](void*)) redirected to 0x4c27fa4 (operator delete[](void*))
  --12394-- REDIR: 0x63c3fe0 (strlen) redirected to 0x4a235dc (_vgnU_ifunc_wrapper)
  --12394-- REDIR: 0x63c4010 (__GI_strlen) redirected to 0x4c2be91 (strlen)
  --12394-- REDIR: 0x63c7c60 (memcpy) redirected to 0x4c2bfdb (memcpy)
  Program will have 2 threads, working on 40 things 
  --12394-- REDIR: 0x555dd60 (pthread_create@@GLIBC_2.2.5) redirected to 0x4c2d4c7 (pthread_create@*)
  ==12394== Thread #2 was created
  ==12394==    at 0x64276BE: clone (clone.S:77)
  ==12394==    by 0x555E172: pthread_create@@GLIBC_2.2.5 (createthread.c:75)
  ==12394==    by 0x4C2D42C: pthread_create_WRK (hg_intercepts.c:230)
  ==12394==    by 0x4C2D4CF: pthread_create@* (hg_intercepts.c:257)
  ==12394==    by 0x401C22: main (in /home/rybu/prog/regina/exercise/thread1)
  ==12394== 
  ==12394== Thread #1 is the program's root thread
  ==12394== 
  ==12394== Possible data race during write of size 8 at 0x7fefffcf0 by thread #2
  ==12394==    at 0x4C2D54C: mythread_wrapper (hg_intercepts.c:200)
  ==12394==  This conflicts with a previous read of size 8 by thread #1
  ==12394==    at 0x4C2D440: pthread_create_WRK (hg_intercepts.c:235)
  ==12394==    by 0x4C2D4CF: pthread_create@* (hg_intercepts.c:257)
  ==12394==    by 0x401C22: main (in /home/rybu/prog/regina/exercise/thread1)
  ==12394== 
  started 21 completed 19 of 40==12394== Thread #3 was created
  ==12394==    at 0x64276BE: clone (clone.S:77)
  ==12394==    by 0x555E172: pthread_create@@GLIBC_2.2.5 (createthread.c:75)
  ==12394==    by 0x4C2D42C: pthread_create_WRK (hg_intercepts.c:230)
  ==12394==    by 0x4C2D4CF: pthread_create@* (hg_intercepts.c:257)
  ==12394==    by 0x401C22: main (in /home/rybu/prog/regina/exercise/thread1)
  ==12394== 
  ==12394== Possible data race during read of size 1 at 0x63401a7 by thread #3
  ==12394==    at 0x613A4D7: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A947: _Unwind_Resume (in /lib/libgcc_s.so.1)
  ==12394==    by 0x4019C9: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394==    by 0x555D9C9: start_thread (pthread_create.c:300)
  ==12394==    by 0x64276FC: clone (clone.S:112)
  ==12394==  This conflicts with a previous write of size 1 by thread #2
  ==12394==    at 0x6138331: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x5563F42: pthread_once (pthread_once.S:104)
  ==12394==    by 0x613A4C9: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A7B6: _Unwind_ForcedUnwind (in /lib/libgcc_s.so.1)
  ==12394==    by 0x556508F: __pthread_unwind (unwind.c:130)
  ==12394==    by 0x555EEB4: pthread_exit (pthreadP.h:265)
  ==12394==    by 0x40198E: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394== 
  ==12394== Possible data race during read of size 1 at 0x63401a6 by thread #3
  ==12394==    at 0x6139A12: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x6139AA8: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A724: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A982: _Unwind_Resume (in /lib/libgcc_s.so.1)
  ==12394==    by 0x4019C9: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394==    by 0x555D9C9: start_thread (pthread_create.c:300)
  ==12394==    by 0x64276FC: clone (clone.S:112)
  ==12394==  This conflicts with a previous write of size 1 by thread #2
  ==12394==    at 0x613832A: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x5563F42: pthread_once (pthread_once.S:104)
  ==12394==    by 0x613A4C9: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A7B6: _Unwind_ForcedUnwind (in /lib/libgcc_s.so.1)
  ==12394==    by 0x556508F: __pthread_unwind (unwind.c:130)
  ==12394==    by 0x555EEB4: pthread_exit (pthreadP.h:265)
  ==12394==    by 0x40198E: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394== 
  ==12394== Possible data race during read of size 1 at 0x63401b0 by thread #3
  ==12394==    at 0x6139AD7: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A724: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A982: _Unwind_Resume (in /lib/libgcc_s.so.1)
  ==12394==    by 0x4019C9: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394==    by 0x555D9C9: start_thread (pthread_create.c:300)
  ==12394==    by 0x64276FC: clone (clone.S:112)
  ==12394==  This conflicts with a previous write of size 1 by thread #2
  ==12394==    at 0x6138370: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x5563F42: pthread_once (pthread_once.S:104)
  ==12394==    by 0x613A4C9: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A7B6: _Unwind_ForcedUnwind (in /lib/libgcc_s.so.1)
  ==12394==    by 0x556508F: __pthread_unwind (unwind.c:130)
  ==12394==    by 0x555EEB4: pthread_exit (pthreadP.h:265)
  ==12394==    by 0x40198E: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394== 
  --12394-- REDIR: 0x63bede0 (free) redirected to 0x4c28616 (free)
  started 40 completed 40 of 40--12394-- REDIR: 0x555ef30 (pthread_join) redirected to 0x4c29796 (pthread_join)
  
  thread 0 worked on task 0
  thread 1 worked on task 1
  thread 0 worked on task 2
  thread 1 worked on task 3
  thread 0 worked on task 4
  thread 1 worked on task 5
  thread 0 worked on task 6
  thread 1 worked on task 7
  thread 0 worked on task 8
  thread 1 worked on task 9
  thread 0 worked on task 10
  thread 1 worked on task 11
  thread 0 worked on task 12
  thread 1 worked on task 13
  thread 0 worked on task 14
  thread 1 worked on task 15
  thread 0 worked on task 16
  thread 1 worked on task 17
  thread 0 worked on task 18
  thread 1 worked on task 19
  thread 0 worked on task 20
  thread 1 worked on task 21
  thread 0 worked on task 22
  thread 1 worked on task 23
  thread 0 worked on task 24
  thread 1 worked on task 25
  thread 0 worked on task 26
  thread 1 worked on task 27
  thread 0 worked on task 28
  thread 1 worked on task 29
  thread 0 worked on task 30
  thread 1 worked on task 31
  thread 0 worked on task 32
  thread 1 worked on task 33
  thread 0 worked on task 34
  thread 1 worked on task 35
  thread 0 worked on task 36
  thread 1 worked on task 37
  thread 0 worked on task 38
  thread 1 worked on task 39
  ==12394== 
  ==12394== ERROR SUMMARY: 7 errors from 4 contexts (suppressed: 804 from 64)
  ==12394== 
  ==12394== 1 errors in context 1 of 4:
  ==12394== Possible data race during read of size 1 at 0x63401a7 by thread #3
  ==12394==    at 0x613A4D7: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A947: _Unwind_Resume (in /lib/libgcc_s.so.1)
  ==12394==    by 0x4019C9: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394==    by 0x555D9C9: start_thread (pthread_create.c:300)
  ==12394==    by 0x64276FC: clone (clone.S:112)
  ==12394==  This conflicts with a previous write of size 1 by thread #2
  ==12394==    at 0x6138331: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x5563F42: pthread_once (pthread_once.S:104)
  ==12394==    by 0x613A4C9: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A7B6: _Unwind_ForcedUnwind (in /lib/libgcc_s.so.1)
  ==12394==    by 0x556508F: __pthread_unwind (unwind.c:130)
  ==12394==    by 0x555EEB4: pthread_exit (pthreadP.h:265)
  ==12394==    by 0x40198E: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394== 
  ==12394== 
  ==12394== 2 errors in context 2 of 4:
  ==12394== Possible data race during read of size 1 at 0x63401b0 by thread #3
  ==12394==    at 0x6139AD7: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A724: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A982: _Unwind_Resume (in /lib/libgcc_s.so.1)
  ==12394==    by 0x4019C9: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394==    by 0x555D9C9: start_thread (pthread_create.c:300)
  ==12394==    by 0x64276FC: clone (clone.S:112)
  ==12394==  This conflicts with a previous write of size 1 by thread #2
  ==12394==    at 0x6138370: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x5563F42: pthread_once (pthread_once.S:104)
  ==12394==    by 0x613A4C9: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A7B6: _Unwind_ForcedUnwind (in /lib/libgcc_s.so.1)
  ==12394==    by 0x556508F: __pthread_unwind (unwind.c:130)
  ==12394==    by 0x555EEB4: pthread_exit (pthreadP.h:265)
  ==12394==    by 0x40198E: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394== 
  ==12394== 
  ==12394== 2 errors in context 3 of 4:
  ==12394== Possible data race during read of size 1 at 0x63401a6 by thread #3
  ==12394==    at 0x6139A12: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x6139AA8: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A724: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A982: _Unwind_Resume (in /lib/libgcc_s.so.1)
  ==12394==    by 0x4019C9: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394==    by 0x555D9C9: start_thread (pthread_create.c:300)
  ==12394==    by 0x64276FC: clone (clone.S:112)
  ==12394==  This conflicts with a previous write of size 1 by thread #2
  ==12394==    at 0x613832A: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x5563F42: pthread_once (pthread_once.S:104)
  ==12394==    by 0x613A4C9: ??? (in /lib/libgcc_s.so.1)
  ==12394==    by 0x613A7B6: _Unwind_ForcedUnwind (in /lib/libgcc_s.so.1)
  ==12394==    by 0x556508F: __pthread_unwind (unwind.c:130)
  ==12394==    by 0x555EEB4: pthread_exit (pthreadP.h:265)
  ==12394==    by 0x40198E: workerThread(void*) (in /home/rybu/prog/regina/exercise/thread1)
  ==12394==    by 0x4C2D558: mythread_wrapper (hg_intercepts.c:202)
  ==12394== 
  ==12394== 
  ==12394== 2 errors in context 4 of 4:
  ==12394== Possible data race during write of size 8 at 0x7fefffcf0 by thread #2
  ==12394==    at 0x4C2D54C: mythread_wrapper (hg_intercepts.c:200)
  ==12394==  This conflicts with a previous read of size 8 by thread #1
  ==12394==    at 0x4C2D440: pthread_create_WRK (hg_intercepts.c:235)
  ==12394==    by 0x4C2D4CF: pthread_create@* (hg_intercepts.c:257)
  ==12394==    by 0x401C22: main (in /home/rybu/prog/regina/exercise/thread1)
  ==12394== 
  --12394-- 
  --12394-- used_suppression:    610 helgrind-glibc2X-101
  --12394-- used_suppression:    192 helgrind---...-*Unwind*-*pthread_unwind*
  --12394-- used_suppression:      2 helgrind-glibc2X-112
  ==12394== 
  ==12394== ERROR SUMMARY: 7 errors from 4 contexts (suppressed: 804 from 64)
  </code></pre>
  
  <p>I'm not so confident when it comes to interpreting helgrind output.  </p>
  
  <p>Thanks for any insights. </p>