如何从不同的线程,OpenMP,C++写入文件

Question

我使用openMP来并行我的C++程序.我的并行代码具有非常简单的形式

#pragma omp parallel for shared(a, b, c) private(i, result)
        for (i = 0; i < N; i++){
         result= F(a,b,c,i)//do some calculation
         cout<<i<<" "<<result<<endl;
         }

如果两个线程同时尝试写入文件,则数据会混淆.我怎么能解决这个问题？

Answer 1

OpenMP提供了编译指示以帮助进行同步. #pragma omp critical只允许一个线程在任何时间执行附加语句(互斥关键区域).该#pragma omp orderedpragma确保循环迭代线程按顺序进入该区域.

// g++ -std=c++11 -Wall -Wextra -pedantic -fopenmp critical.cpp
#include <iostream>

int main()
{
  #pragma omp parallel for
  for (int i = 0; i < 20; ++i)
    std::cout << "unsynchronized(" << i << ") ";
  std::cout << std::endl;
  #pragma omp parallel for
  for (int i = 0; i < 20; ++i)
    #pragma omp critical
    std::cout << "critical(" << i << ") ";
  std::cout << std::endl;
  #pragma omp parallel for ordered
  for (int i = 0; i < 20; ++i)
    #pragma omp ordered
    std::cout << "ordered(" << i << ") ";
  std::cout << std::endl;
  return 0;
}

示例输出(一般每次不同):

unsynchronized(unsynchronized(unsynchronized(05) unsynchronized() 6unsynchronized() 1unsynchronized(7) ) unsynchronized(unsynchronized(28) ) unsynchronized(unsynchronized(93) ) unsynchronized(4) 10) unsynchronized(11) unsynchronized(12) unsynchronized(15) unsynchronized(16unsynchronized() 13unsynchronized() 17) unsynchronized(unsynchronized(18) 14unsynchronized() 19) 
critical(5) critical(0) critical(6) critical(15) critical(1) critical(10) critical(7) critical(16) critical(2) critical(8) critical(17) critical(3) critical(9) critical(18) critical(11) critical(4) critical(19) critical(12) critical(13) critical(14) 
ordered(0) ordered(1) ordered(2) ordered(3) ordered(4) ordered(5) ordered(6) ordered(7) ordered(8) ordered(9) ordered(10) ordered(11) ordered(12) ordered(13) ordered(14) ordered(15) ordered(16) ordered(17) ordered(18) ordered(19)