B9 - single, master and masked directives

single directive

#pragma omp parallel
{
    #pragma omp single
    {
        printf("Hello from thread %d\n",
               omp_get_thread_num());
    }
}

• the single directive in OpenMP tells the runtime that only one thread from the team should execute a particular block of code, while the other threads skip that block
• particularly useful for initialization, printing results, or performing an operation that should happen exactly once
• there is an implicit barrier at the end of single
• nowait can be used

master and masked directives

• the master directive makes only the master, thread 0, run the block
• there is no implicit barrier at the end
• it guarantees the correct serial order

#pragma omp parallel
{
    #pragma omp single
    {
        printf("Any one thread executes this\n");
    }

    #pragma omp masked
    {
        printf("Thread 0 executes this\n");
    }
}

• modern OpenMP standard has deprecated the master construct in favour of the masked construct
• masked defaults to the same behaviour (thread 0), but allows a filter clause to be specified

#pragma omp parallel num_threads(4)
{
    #pragma omp masked filter(2)
    {
        printf("Thread %d\n", omp_get_thread_num());
    }
}

>>> Thread 2