Task construct

Check following code:

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

int main(void)
{    
    #pragma omp parallel num_threads(4)
    {
        printf("Thread %d is running\n", omp_get_thread_num());
    }

    return 0;
}

What "#pragma omp parallel num_threads(4)" region actually does is creating 4 threads, and every thread is assigned an implicit task which prints the log.

Besides implicit task, using task construct can create explicit task. Check following code:

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

int main(void)
{    
    #pragma omp parallel num_threads(4)
    {
        if (1 == omp_get_thread_num())
        {
            #pragma omp task
            printf("Thread %d is running an explicit task\n", omp_get_thread_num());
        }
        printf("Thread %d is running\n", omp_get_thread_num());
    }

    return 0;
}

Build and run it:

# gcc -fopenmp parallel.c
# ./a.out
Thread 0 is running
Thread 3 is running
Thread 1 is running
Thread 0 is running an explicit task
Thread 2 is running

We can see thread whose ID is 1 created the task, but it was actually master thread who executed the task. The explicit task can run immediately or deferred to the future (like the above example).