Something strange with shared variable in openMP parallel sections

Question

I got a strange phenomenon in openMP with shared monery and print function.

I tested this problem both in C++ and Fortran .

In C++:

#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#include <windows.h> 
int main (int argc, char *argv[]) 
{
int i=1;

#pragma omp parallel sections shared(i)
  {
    #pragma omp section
    {while(true){
        i = 1;
        printf("thread 1: %i\n", i);
    }}
    #pragma omp section
    {while(true){
        i = i - 1000;
        printf("thread 2: %i\n", i);
    }}
  }

}

This code is quite simple and the expected result is something like this:

thread 1: 1
thread 2: -999
thread 1: 1
thread 2: -999
thread 2: -1999
thread 1: 1

However, I could get this result:

thread 1: 1
thread 2: -1726999
thread 2: -1727999
thread 2: -1728999
thread 2: -1729999
thread 2: -1730999
thread 2: -1731999
thread 2: -1732999

It is confusing and looks like i is not shared! I tried to commented this line:

printf("thread 1: %i\n", i);

and got:

thread 2: 1
thread 2: -999
thread 2: 1
thread 2: 1
thread 2: -999
thread 2: 1

It looks fine now.

In Fortan:

OpenMP performances a little different in Fortran.

PROGRAM test
implicit none
integer*8 i
i = 1
 !$OMP parallel sections shared(i)
    !$OMP section
        do 
            i = 1 
            print *, "thread 1" ,i
            !call sleep(1)
        end do
    !$OMP section
        do 
            i = i-1000
            print *, "thread 2" ,i
            !call sleep(1)
        end do
 !$OMP end parallel sections
END PROGRAM

This code lead to the same problem as above. But if I comment the thread 1's print, the problem is still there.

I have to add sleep subroutine as the commented lines to get the expected result.

Anyone know the reason?

Another question, can a variable being modified in one thread as the same time as be reading in another thread?

Answer 1

You are modifying a shared variable from more than one thread without synchronization. This is known as a data race. The result of your program is unspecified - anything can happen. The same applies if you are writing to a variable in one thread and reading from another without synchronization.

See section 1.4.1 of the OpenMP 4.0 standard for more information.