A short OpenMP example showing how you can make your code parallel with minimal effort.

Makefile

CC=gcc
# Without the fopenmp flag the program will normally still work and be
# completely serial. In this case however it won't because I used omp.h
CFLAGS=-fopenmp -Wall
LDFLAGS=$(CFLAGS)

all: omp_example

omp_example: omp_example.o

clean:
	rm -rf *.o omp_example

omp_example.c

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

int main(void){
    int th_id, nthreads, i;
    srand(time(NULL));

    // launch team of threads and keep th_id private
    #pragma omp parallel private(th_id)
    {
        th_id = omp_get_thread_num();
        printf("Hello thread %d\n",th_id);
        // all threads wait at this barrier so that total prints last
        #pragma omp barrier
        if (th_id == 0) {
            nthreads = omp_get_num_threads();
            printf("Total: %d threads\n",nthreads);
        }
    }

    // specify how many threads to launch explicitly and parallelize loop
    #pragma omp parallel for num_threads(3) private(i)
    for (i=0;i<6;++i) 
        printf("Boo %d\n",i);

    // parallel reduce
    int a[1000],b[1000];
    for (i=0;i<1000;i++) {
        a[i] = rand();
        b[i] = rand();
    }
    long sum = 0;
    #pragma omp parallel for reduction (+:sum) private(i)
    for (i=0;i<1000;i++)
        sum += a[i]*b[i];
    printf("Parallel scalar product result: %ld \n",sum);
    
    return 0;
}

You can control the number of threads spawned (where not specified in code) at runtime using

export OMP_NUM_THREADS=10

It is not clear to me whether it is necessary to define the loop variable as private in OMP when using C. This is default behaviour in C++.