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

main(){
    int x,i,id;
    x=1;
    #pragma omp parallel sections  firstprivate(x)
    {   
        #pragma omp  section 
        {
           id = omp_get_thread_num();
           std::cout << "Thread id = " << id << std::endl;
           std::cout << "start x = " << x << std::endl;
           x=x+id;
           std::cout << "comput. x = x + id = " << x << std::endl;
        }

        #pragma omp  section 
        {
           id = omp_get_thread_num();
           std::cout << "Thread id = " << id << std::endl;
           std::cout << "start x = " << x << std::endl;
           x=x+id;
           std::cout << "comput. x = x + id = " << x << std::endl;
        }
    }
}

$ c++ -fopenmp firstpr.cc -o firstpr && ./firstpr
Thread id = 2
start x = 1
comput. x = x + id = 3
Thread id = 2
start x = 3
comput. x = x + id = 5

$ c++ -fopenmp firstpr.cc -o firstpr && ./firstpr
Thread id = 1
start x = 1
comput. x = x + id = 2
Thread id = 1
start x = 1
comput. x = x + id = 2

The FIRSTPRIVATE clause combines the behavior of the PRIVATE clause with automatic initialization of the variables in its list

Listed variables are initialized according to the value of their original objects prior to entry into the parallel or work-sharing construct.

BY>$ c++ -fopenmp firstpr.cc -o firstpr && ./firstpr
BY>Thread id = 2
BY>start x = 1
BY>comput. x = x + id = 3
BY>Thread id = 2
BY>start x = 3
BY>comput. x = x + id = 5

BY>$ c++ -fopenmp firstpr.cc -o firstpr && ./firstpr
BY>Thread id = 1
BY>start x = 1
BY>comput. x = x + id = 2
BY>Thread id = 1
BY>start x = 1
BY>comput. x = x + id = 2
BY>

$ c++ -fopenmp firstpr.cc -o firstpr && ./firstpr
Thread id = Thread id = 32
start x = 1
comput. x = x + id = 3

start x = 1
comput. x = x + id = 4



$ c++ -fopenmp firstpr.cc -o firstpr && ./firstpr
Thread id = 3
Thread id = 1start x = 1
start x = 1
comput. x = x + id = 2

comput. x = x + id = 4