MPI计算pi值

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π的求解方式：

//由高数里的知识可知，根据上述公式可以求得pi/4。
#include<stdio.h>
#include<mpi.h>
int main(int argc,char *argv[]){
	int my_rank,num_procs;
	int i,n = 0;
	double sum,width,local,mypi,pi;
	double start =0.0,stop = 0.0;
	int proc_len;
	//MPI_MAX_PROCESSOR_NAME是MPI预定义的宏，，即MPI所允许的机器名字的最大长度。
	char processor_name[MPI_MAX_PROCESSOR_NAME];
	MPI_Init(&argc,&argv);
	MPI_Comm_size(MPI_COMM_WORLD,&num_procs);
	//determines the rank of the calling process in the communicator
	MPI_Comm_rank(MPI_COMM_WORLD,&my_rank);
	//mpi接口获取进程名 int MPI_Get_processor_name(char *name,int *resultlen)
	MPI_Get_processor_name(processor_name,&proc_len);
	printf("Process %d of %d\n",my_rank,num_procs);
	if(my_rank == 0){
		scanf("%d",&n);
		printf("\n");
		//MPI_Wtime返回一个用浮点数表示的秒数
		start = MPI_Wtime();
	}
	//MPI_Bcast(void *buffer,int count,MPI_Datatype datatype ,int root,MPI_Comm comm)
	MPI_Bcast(&n,1,MPI_INT,0,MPI_COMM_WORLD);
	sum =0.0;
	width = 1.0/n;
	//每个进程my_rank，计算4.0/(1.0+local*local)放入sum
	for(i = my_rank;i<n;i+=num_procs){
		local =width*((double)i+0.5);
		sum += 4.0/(1.0+local*local);
	}
	mypi = width*sum;
	MPI_Reduce(&mypi,&pi,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);
	//打印结果
	if(my_rank == 0){
		printf("PI is %.20f\n",pi);
		stop = MPI_Wtime();
		printf("Time:%f on %s\n",stop-start,processor_name);
		fflush(stdout);
	}
	MPI_Finalize();
	return 0;
}