Critical section of OpenMP thread synchronization

Synchronous mutexes are definitely used in multi-core/multithreaded programming. In addition to the mutex variable, it is the critical section.

A critical section is a section of code that is surrounded by {...}, which is used to allow only one thread to execute at one time.

The critical Zone declaration method in OpenMP is as follows:

#pragma omp critical [(name)]//[] indicates an optional name

{

Code that requires only one thread to access at the same time

}

As in the following code:

#include <stdio.h>
#include <omp.h>
    
int main ()
{
    int sum = 0;
#pragma omp parallel
    printf ("Hello World!!! \ n ");
    
#pragma omp parallel for    
    (int i = 0; i < 10000; ++i)
    {
#pragma omp critical
        {
          sum = sum + i% 7;
        }
    }
    printf ("Sum:%d\n", sum);
    return 0;
}

Execution results:

As you can see, the machine has 8 cores, and 8 threads are started. if the 13th line in the code is commented out, the sum in the execution result is basically indeterminate .