mex file compiles fine but crashes when run (c code run externally runs no problem)

As the title states I'm able to run a piece of code outside mex without any problem but matlab crashes when run as mex.

the twiddle code is from here

CODE:

#include "math.h"
#include "mex.h"

#define PI M_PI

/*twiddle.c - generate all combinations of M elements drawn without replacement
  from a set of N elements.  
*/

mwSize twiddle(mwSize *x, mwSize *y, mwSize *z, mwSize *p)
{
    mwSize i, j, k;
    j = 1;

      while(p[j] <= 0)
          j++;

      if(p[j-1] == 0)
      {
          for(i = j-1; i != 1; i--)
              p[i] = -1;
          p[j] = 0;
          *x = *z = 0;
          p[1] = 1;
          *y = j-1;
      }
      else
      {
          if(j > 1)
              p[j-1] = 0;
          do
              j++;
          while(p[j] > 0);
          k = j-1;
          i = j;
          while(p[i] == 0)
              p[i++] = -1;
          if(p[i] == -1)
          {
              p[i] = p[k];
              *z = p[k]-1;
              *x = i-1;
              *y = k-1;
              p[k] = -1;
          }
          else
          {
              if(i == p[0])
                  return(1);
              else
              {
                  p[j] = p[i];
                  *z = p[i]-1;
                  p[i] = 0;
                  *x = j-1;
                  *y = i-1;
              }
          }
      }
      return(0);
  }

void inittwiddle(mwSize m, mwSize n, mwSize *p)
{
    mwSize i;
    p[0] = n+1;
    for(i = 1; i < n-m+1; i++)
        p[i] = 0;
    while(i < n+1)
    {
        p[i] = i+m-n;
        i++;
    }
    p[n+1] = -2;
    if(m == 0)
        p[1] = 1;
}

/*   */
void createConstMbyNaSM(mwSize Nt, mwSize Nr, mwSize M_full, double alpha, mwSize numsym,
                        double *all_symr, double *all_symi)
{
    mwSize constSize = 0, iNa;
    mwSize M, temp, i, iM, iMgray;
    mwSize x, y, z, *p, *b;
    double *temp_symr, *temp_symi;

      temp_symr = all_symr;
      temp_symi = all_symi;

      p = mxMalloc(Nt+2 * sizeof(*p));
      b = mxMalloc(Nt * sizeof(*b));

      for (iNa = 1; iNa < (Nt-1); iNa++)
      {
          temp = iNa;
          M = M_full;

          /*while(temp >>= 1)
              M = M/2;*/

          inittwiddle(iNa, Nt, p);
          /*for(i = 0; i != Nt-iNa; i++)
              b[i] = 0;
          while(i != Nt)
              b[i++] = 1;
          for (i = 0; i < Nt; i++)
          {
              *(temp_symr+constSize) = *(apm_symr+iM) * b[i];
              *(temp_symi+cobstSize) = *(apm_symi+iM) * b[i];
              constSize++;
          }
          while(!twiddle(&x, &y, &z, p))
          {
              b[x] = 1;
              b[y] = 0;
              for (iM = 0; iM < M; iM++)
              {
                  for (i = 0; i < Nt; i++)
                  {
                      /*iMgray = iM ^ (iM >> 1); 

                      /**(temp_symr+constSize) = cos(iMgray * 2*PI/M) * b[i];
                      *(temp_symi+constSize) = sin(iMgray * 2*PI/M) * b[i];
                      constSize++;
                  }
                  constSize++;
              }
          }*/

          constSize++;

          /* Free memory Allocation */

          /*all_symr[iNa] = constSize;*/
      }

      mxFree(b);
      mxFree(p);
  }

/* The gateway routine. */
void mexFunction( int nlhs, mxArray *plhs[],
                  int nrhs, const mxArray *prhs[] )
{
    double *all_symr, *all_symi, alpha;
    mwSize Nt, Nr, M, num_sym;

      /* check for the proper number of arguments */
      if(nrhs != 5)
        mexErrMsgIdAndTxt( "MATLAB:recvSM:invalidNumInputs",
                "Invalid number of inputs.");
      if(nlhs > 1)
        mexErrMsgIdAndTxt( "MATLAB:recvSM:maxlhs",
                "Too many output arguments.");

      Nt = mxGetScalar(prhs[0]);
      Nr = mxGetScalar(prhs[1]);
      M = mxGetScalar(prhs[2]);
      alpha = mxGetScalar(prhs[3]);
      num_sym = mxGetScalar(prhs[4]);

      plhs[0] = mxCreateDoubleMatrix(Nt, (mwSize) num_sym, mxCOMPLEX);

      all_symr = mxGetPr(plhs[0]);
      all_symi = mxGetPi(plhs[0]);

      createConstMbyNaSM(Nt, Nr, M, alpha, num_sym, all_symr, all_symi);
  }

[Pranav Koundinya]

I have commented out everything after call to inittwiddle in the for loop to check if that function is the cause (and it seems to be). But this compiles and runs fine outside matlab

On a side note I am on Ubuntu so debugging is hard with gdb