Parallel loop construct

Parallel loop construct is a very common idiom in OpenMP programming. The following is the syntax of loop construct:

#pragma omp for [clause[ [,] clause] ... ] new-line
    for-loops

What loop construct does is distributing every iteration into threads in team, and makes iterations run concurrently. Check the following example:

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

int main(void)
{        
    #pragma omp for
    for (int index = 0; index < 10; index++)
    {
        printf("Thread id is %d\n", omp_get_thread_num());
    }

    return 0;
}

Build and run it on my 24-core machine:

$ gcc -fopenmp parallel.c
$ ./a.out
Thread id is 0
Thread id is 0
Thread id is 0
Thread id is 0
Thread id is 0
Thread id is 0
Thread id is 0
Thread id is 0
Thread id is 0
Thread id is 0

Since there is only main thread, the loop executed in sequential in fact. Add "#pragma omp parallel" to spawn other threads:

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

int main(void)
{    
    #pragma omp parallel
    {
        #pragma omp for
        for (int index = 0; index < 10; index++)
        {
            printf("Thread id is %d\n", omp_get_thread_num());
        }
    }

    return 0;
}

Build and run it again:

$ gcc -fopenmp parallel.c
$ ./a.out
Thread id is 1
Thread id is 0
Thread id is 9
Thread id is 6
Thread id is 3
Thread id is 7
Thread id is 2
Thread id is 5
Thread id is 4
Thread id is 8

This time every iteration is dispatched to different thread to run. Actually, we always use the following format:

#pragma omp parallel for
for (int index = 0; index < 10; index++)
{
    ......
}

which is the shortcut of:

#pragma omp parallel
{
    #pragma omp for
    {
        ......
    }
}

P.S. Please do not mix up the following code:

#pragma omp parallel
{
    for (...)
    {
        ......
    }
}

Which means executing for-loops in every thread.