private

Definition

private is the clause that contains the variables that each thread in the OpenMP parallel region will have a copy of. These copies are not initialised, specifically, they do not have the value that the original variable had at the time it was passed in the private clause. By contrast, firstprivate variables are initialised with the value of the original variable. The other possible data-sharing policies are shared, lastprivate and linear.

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private(list)

Parameters

list: The variables to pass as private, separated by commas.

Example

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#include <stdio.h>
#include <stdlib.h>
#include <omp.h>

/**
 * @brief Illustrates the OpenMP private policy.
 * @details This example shows that when a private variable is passed to a
 * parallel region, threads work on uninitialised copies and that whatever
 * modification is made to their copies is not reflected onto the original
 * variable. 
 **/
int main(int argc, char* argv[])
{
    // Use 4 OpenMP threads
    omp_set_num_threads(4);

    // Variable that will be private
    int val = 123456789;

    printf("Value of \"val\" before the OpenMP parallel region: %d.\n", val);

    #pragma omp parallel private(val)
    {
        printf("Thread %d sees \"val\" = %d, and updates it to be %d.\n", omp_get_thread_num(), val, omp_get_thread_num());
        val = omp_get_thread_num();
    }

    // Value after the parallel region; unchanged.
    printf("Value of \"val\" after the OpenMP parallel region: %d.\n", val);

    return EXIT_SUCCESS;
}