C – Parallel for loop in OpenMP

Last Updated : 19 Dec, 2022

Prerequisites:

We have the for loops in the programs and we can parallelize them easily with the help of OpenMP Library. Add #include<omp.h>, as all the prototypes for these parallelization techniques of OpenMP lie in that header file.

Syntax:

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

Explanation:

The Threads split the range of iterations evenly and execute in parallel.
For Example,
- There are 4 threads
- The for loop goes from 1 to 1000. Total 1000 iterations.
- So, thread 0 executes 1 to 250
- Thread 1 executes 251 to 500
- Thread 2 executes 501 to 750
- Thread 3 executes 751 to 1000
So in this way, the time taken would reduce as the work is now not done in sequential order. Rather it is executed parallelly.
This is just an example for visualization, the actual range may vary a bit, but at the most, it divides evenly.

OpenMP For Loop| How to parallelize the for loop in C Program

Algorithm:

Start the program

There are many for loops in the program.

Add the for loop construct before all the for loops.

num_threads( n ) needs to be mentioned to get n threads

If not mentioned, by default, the no. of processor’s scores threads are formed.

So therefore parallelized.

Example:

C

#include <omp.h>
#include <stdio.h>
 
int main()
{
 
    #pragma omp parallel for num_threads(4)
    for (int i = 1; i <= 10; i++) {
        int tid = omp_get_thread_num();
        printf("The thread %d  executes i = %d\n", tid, i);
    }
 
    return 0;
}

Output:

Output

Here are 10 iterations, and 4 threads. Each thread has 2/3 iterations with it.
Another proof that parallelization has happened is the order of iteration:
- Here in the sequential or serial program, we have iterations i=1 to i=10 in the order one after another.
- But here you can see that the i=4 comes first and i=1 at 3rd last place.
- As in the above example, thread 1 finished before thread 0

Time Complexity: O(N)

Space Complexity: O(1)

do...while Loop in C

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