multithreading - How does OpenMP works internally -

i tried write small c program figure out how openmp works. example supposed compute sum of 1 1000; however, printed out 0 in terminal. can desired result after commenting #pragma stuff out. can possibly tell me reason?

this guide says #pragma omp for divides work of for-loop among threads of current team. not create threads, divides work amongst threads of executing team. should have 1 main thread throughout execution, correct?

#include <stdio.h>  int main() {    int n, sum = 0;   #pragma omp     (n = 0; n <1000; n++) {       sum += n;     }    printf("%d\n");   return 0; } 

you have several issues such simple example...

1) not starting parallel region. this, use omp parallel for instead of omp for.

2) variables not being made private each thread working on different loops. each thread over-writing each other thread's version of variables. n needs made private.

3) attempting sum 1 shared variable across multiple threads. must done reduction clause.

4) you're not printing out. printf() syntax as-is not print proper result ever.

so example should like:

int n, sum = 0; #pragma omp parallel private(n) reduction(+:sum) (n = 0; n < 1000; n++) {   sum += n; }  printf("%d\n", sum); 

i'd suggest basic openmp tutorial (either online or in book). first 3 problems have been obvious little bit of research.