简单的OpenMP代码挂起3个线程

Question

我在使用openMP计算Pi这个简单的程序时遇到了麻烦。它适用于1,2或4个线程，但它挂起3个线程，我不知道为什么。以下是使用-fopenmp

在GCC中编译的完整代码

#include<stdio.h>
#include<math.h>
#include<omp.h>
#include<time.h>

#define NUM_THREADS 3

void main(){
  long num_steps = 100000;
  double c1,c2;
  int i,nthreads,id;
  double p=0.,sum[NUM_THREADS-1],pi=0.;
  double x,local_sum;
  double step = 1.0/((double)num_steps);

  c1=omp_get_wtime();   

    #pragma omp parallel num_threads(NUM_THREADS) shared(sum) private(i,id,local_sum,x)
  { 
        id = omp_get_thread_num();

                local_sum = 0.;

        for(i=id;i<=num_steps;i=i+NUM_THREADS)
    {
            x=(i+0.5)*step;
            local_sum=local_sum+4.0/(1.0+x*x);
    }
    sum[id] = local_sum;
  }

  for(i=0;i<=NUM_THREADS-1;i++) {pi += step*sum[i]; printf("sum[%d] = %lf\n",i,sum[i]);}

  printf("pi = %lf\n",pi);

  c2=omp_get_wtime();

    printf("tempo = %f s\n",(c2-c1));
}