// 
// atomic
//
#include <omp.h>
#include <stdio.h> 

const int n=100000000;
const int nthreads=8;
 
int main()
{
	long a=0, mysum;
	int i;
	double t0, t1;

// incorrect version
	t0=omp_get_wtime(); // 获取墙上时间，以秒为单位 
	#pragma omp parallel for num_threads(nthreads) private(i)
		for(i=1; i<n; i++)
		{
			a = a + 1;
		}
		
	t1=omp_get_wtime(); 	
	printf("a=%d, elapsed time=%.2e\n", a, t1-t0);	
	
// correct but not efficient	
	a = 0;
	t0=omp_get_wtime(); // 获取墙上时间，以秒为单位 
	#pragma omp parallel for num_threads(nthreads) private(i)
		for(i=1; i<n; i++)
		{
			#pragma omp atomic
				a = a + 1;
		}
		
	t1=omp_get_wtime(); 	
	printf("a=%d, elapsed time=%.2e\n", a, t1-t0);
	
// correct and efficient			
	a = 0;	
	t0=omp_get_wtime(); // 获取墙上时间，以秒为单位 
	#pragma omp parallel num_threads(nthreads) private(i,mysum)
	{
		mysum = 0; // 注意：私有变量缺省没有值
		#pragma omp for
			for(i=1; i<n; i++)
			{
				mysum = mysum + 1;
			}
		#pragma omp critical
			a = a + mysum;
	}

	t1=omp_get_wtime(); 	
	printf("a=%d, elapsed time=%.2e\n", a, t1-t0);
	
// correct and more efficient			
	a = 0;	
	t0=omp_get_wtime(); // 获取墙上时间，以秒为单位 
	#pragma omp parallel num_threads(nthreads) private(i,mysum)
	{
		mysum = 0; // 注意：私有变量缺省没有值
		#pragma omp for
			for(i=1; i<n; i++)
			{
				mysum = mysum + 1;
			}
		#pragma omp atomic
			a = a + mysum;
	}
		
	t1=omp_get_wtime(); 
	printf("a=%d, elapsed time=%.2e\n", a, t1-t0);

	return 0;
}