open MP基础

open MP基础语法

代码展示了基础的语法，没有涉及同步的问题：

#include <iostream>
#include"omp.h"
#include<stdio.h>

int main()
{
	//parallel是让所有线程重复执行同样的任务，这里为了演示后面加omp_get_thread_num()来区分
	printf("以下是parallel：\n");
#pragma omp parallel
	{
		printf( "Hello World! \t thread:%d \n",omp_get_thread_num());
		//这里没有用cout是因为不同线程的执行速度有差异，对屏幕多次访问造成输出不连贯，显示不美观
	}
	int i = 0;
	
	//for不经常用，以下示例也表明循环只有一个线程在执行，并且for后直接加循环，而非大括号
	printf("以下是for：\n");
#pragma omp for
	for (i=0;i<6;i++)
		{ 
			printf("Hello World! \t thread:%d \t time:%d \n", omp_get_thread_num(),i);
		}

	//parallel for是比较常用的，相当于parallel+for
	//子句有多种schedule可以选择
	printf("以下是parallel for：\n");
#pragma omp parallel for if(i>4)
	for (i = 0; i < 10; i++)
	{
		printf("Hello World! \t thread:%d \t time:%d \n", omp_get_thread_num(), i);
	}

	//sections分区编程，每个独立分区用section表示
	//分区编程更具有普适性，不局限于循环并且比较灵活
	printf("以下是sections：\n");
	int sum1=0, sum2=0;
#pragma omp parallel sections
	{
#pragma omp section
		for (int j = 1; j <= 10; j++)
			sum1 = sum1 + j;
#pragma omp section
		for (int k = 11; k <= 20; k++)
			sum2 = sum2 + k;
	}
	printf("sum：%d \n",sum1+sum2);
}

程序执行结果如下：

以下是parallel：
Hello World!     thread:0
Hello World!     thread:1
Hello World!     thread:2
Hello World!     thread:3
以下是for：
Hello World!     thread:0        time:0
Hello World!     thread:0        time:1
Hello World!     thread:0        time:2
Hello World!     thread:0        time:3
Hello World!     thread:0        time:4
Hello World!     thread:0        time:5
以下是parallel for：
Hello World!     thread:0        time:0
Hello World!     thread:0        time:1
Hello World!     thread:0        time:2
Hello World!     thread:1        time:3
Hello World!     thread:1        time:4
Hello World!     thread:1        time:5
Hello World!     thread:3        time:8
Hello World!     thread:3        time:9
Hello World!     thread:2        time:6
Hello World!     thread:2        time:7
以下是sections：
sum：210