C++ openmp parallel computation calculates wrong results

Question

I have an algorithm that I want to execute in parallel using openmp. I could validate the results for single-threaded execution ( OMP_NUM_THREADS=1 ), but the results are slightly different as soon as I set the number of threads to 2 or higher. I also tried parallelizing the inner for -loops, but that doesn't yield correct results, either.

I'm fairly new to openmp and multi-threading in general. I suspect that my implementation shares variables between threads improperly somehow, but I can't figure it out.

extern "C" double *lomb_scargle(double *x, double *y, double *f, int NX, int NF) {
    double *result = (double*) malloc(2 * NF * sizeof(double));
    double w, tau, SS, SC, SST1, SST2, SCT1, SCT2, Ai, Bi;
    int i, j;

    #pragma omp parallel for
    for (i=0; i<NF; i++) {
        w = 2 * M_PI * f[i];
        SS = 0.;
        SC = 0.;
        for (j=0; j<NX; j++) {
            SS += sin(2*w*x[j]);
            SC += cos(2*w*x[j]);
        }
        tau = atan2(SS, SC) / (2 * w);
        SCT1 = 0.;
        SCT2 = 0.;
        SST1 = 0.;
        SST2 = 0.;
        for (j=0; j<NX; j++) {
           SCT1 += y[j] * cos(w * (x[j] - tau)); 
           SCT2 +=    pow(cos(w * (x[j] - tau)),2); 
           SST1 += y[j] * sin(w * (x[j] - tau)); 
           SST2 +=    pow(sin(w * (x[j] - tau)),2); 
        }
        Ai = SCT1 / SCT2;
        Bi = SST1 / SST2;
        // result contains the amplitude first, and then the phase 
        result[i] = sqrt(Ai*Ai + Bi*Bi);
        result[i+NF] = - (atan2(Bi, Ai) + w * tau);
    }
    return result;
}

EDIT: typo

Answer 1

By default, OpenMP shares all variables declared in an outer scope between all workers. You need to move the temporary variables into the inner block (or declare them private).