Question

#include "/usr/lib/gcc/i686-linux-gnu/4.6/include/omp.h"
#include <iostream>
#include<list>
using namespace std;

int main()
{
    list<int> lst;
    for(int i=0;i<5;i++)
        lst.push_back(i);

#pragma omp parallel for
    for(int i=0;i<5;i++)
    {
        cout<<i<<" "<<omp_get_thread_num()<<endl;
    }   
}

suppose that I can get this:

0  0
1  0
2  0
3  1
4  1

However, sometimes I can get this result:

30  0
1  0
2  0
  1
4  1

or even this kind of result:

30 1 0
4 1

1 0
2 0

I know this is because the output code:

cout<<i<<" "<<omp_get_thread_num()<<endl;

has been spliced into small segments and has no order when doing output.
But who can tell me how to prevent this from happening?
Thanks.

Answer 1

Your code

#pragma omp parallel for
    for(int i = 0; i < 5; i++)
    {
        cout << i << " " << omp_get_thread_num() << endl;
    }

is equivalent to

#pragma omp parallel for
    for(int i = 0; i < 5; i++)
    {
        cout << i;
        cout << " ";
        cout << omp_get_thread_num();
        cout << endl;
    }

Calls to << in the different threads may be executed in any order. For instance cout << i; in thread 3 may be followed by cout << i; in tread 0 which may be followed by cout << " "; in thread 3 etc. resulting in the garbled output 30 ....

The correct way is to rewrite the code so that each thread calls cout << only once in the loop:

#pragma omp parallel for
    for(int i = 0; i < 5; i++)
    {
        stringstream ss;
        ss << i << " " << omp_get_thread_num() << '\n';
        cout << ss.str();
    }