Data sharing

C | Fortran-90

default

Definition

default is the OpenMP clause that sets the data-sharing policy to apply to variables that have not been explicitly assigned one. If this clause is not specified, variables are considered as having been passed implicity to the shared clause.

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default(policy)

Parameters

policy

The policy to apply to all variables not declared in a data-sharing policy clause.
Possible values:

  • none: no policy will be applied by default; the programmer is required to explicitly declare the data-sharing policy of every variable.
  • shared: variables not explicitly assigned a policy will be considered as having been passed implicitly to a shared clause.

Example

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#include <stdio.h>
#include <stdlib.h>
#include <omp.h>
 
/**
 * @brief Illustrates the OpenMP default clause.
 * @details An int is passed to a parallel region. Then:
 *     - 1st step: thread 0 writes "123" in the int
 *     - 2nd step: thread 1 prints the value of the int
 * The default policy, set to shared, becomes visible when the value read by thread 1 is the one written by thread 0.
 **/
int main(int argc, char* argv[])
{
    // Use 2 OpenMP threads
    omp_set_num_threads(2);
 
    // The int that will be shared among threads
    int val = 0;
 
     // Variables not part of a data-sharing clause will be "shared" by default.
    #pragma omp parallel default(shared)
    {
        // Step 1: thread 0 writes the value
        if(omp_get_thread_num() == 0)
        {
            printf("Thread 0 sets the value of \"val\" to 123.\n");
            val = 123;
        }
 
        // Threads wait each other before progressing to step 2
        #pragma omp barrier
        
        // Step 2: thread 1 reads the value
        if(omp_get_thread_num() == 1)
        {
            printf("Thread 1 reads the value of \"val\": %d.\n", val);
        }
    }
 
    return EXIT_SUCCESS;
}