Multithreading in MEX function using pthreads

I am building a multi-threaded MEX function which uses pthreads. I followed the example from http://robertoostenveld.nl/?p=33, and everything is going fine, except that I get a segmentation fault every time pthread_join() is called (works fine if I comment out pthread_join, but I need this functionality). Here is the output from gdb when this happens:

    Program received signal SIGSEGV, Segmentation fault.
    [Switching to Thread 0x7fffe35ea700 (LWP 20687)]
    0x00007ffff4c5ef53 in pthread_join () from /lib/libpthread.so.0

Here is a simplified version of the code in my mexFunction():

void mexFunction( int nlhs, mxArray *plhs[],
                  int nrhs, const mxArray *prhs[] )
{
    struct stableParams params;

    /* Assign pointers to each input */
    params.remCodePhase = *mxGetPr(prhs[0]);
    params.remCarrPhase = *mxGetPr(prhs[1]);
    /* etc */

    /*struct funcArgs args[params.n_threads]; */

    /* Create matrix for the return argument. */
    plhs[0] = mxCreateDoubleScalar(0);
    plhs[1] = mxCreateDoubleScalar(0);
    /* etc */

    /* Assign pointers to the output */
    params.remCodePhaseOut = mxGetPr(plhs[6]);
    params.remCarrPhaseOut = mxGetPr(plhs[7]);
    /* etc */

    struct Correlators correlators[params.n_threads];
    params.correlators[params.n_threads] = correlators;

    pthread_t threads[params.n_threads];
    int rc, t;

    for(t=1; t <= params.n_threads ; t++){
      struct funcArgs *args = mxMalloc(sizeof(*args));

      if (args){
          args->params = params;
          args->curr_thread = t;

          mexPrintf("In main: creating thread %d\n", t);

          rc = pthread_create(&threads[t], NULL, do_thread, (void *)args);

          if (rc){
              mexErrMsgTxt("problem with return code from pthread_create()");
          }
      }

    }
    int *ptr;

    for(t=0; t< params.n_threads; t++){

         int rc2=pthread_join(threads[t],(void**)&(ptr));

}

It's very possible I'm doing something wrong because I'm new to both C and pthreads. But has anyone managed to get pthreads working in a MEX function? I'm using, by the way, Ubuntu 10.04 and Matlab 2013a.

Answer 1

[Jan]

Do the threads reply a value by pthread_exit() or not? You call pthread_join twice, one time without catching the output and one time with it. You have to decide for one of the methods.

[EDITED] do_thread creates the variable used as output dynamically, such that the memory is released when the function returns. Then catching the pointer in the caller fails. Please try this:

int *ret2 = malloc(sizeof(int));
*ret2 = 200;
pthread_exit((void *) ret2)

[Robert]

Sorry, first call of pthread_join was a mistake. I meant to remove that. To answer your question, yes, I call pthread_exit with a return value, e.g.:

    int ret2  = 200;
    pthread_exit(&ret2)

[Jan]

See [EDITED]