CODEWITHSUNDEEP
c++openmpsrand
user3159019
user3159019

Reputation: 33

openMP generate different random numbers with the same seed

I am new to openMP, in my program complex simulations are needed, to repeat the result, the seed is set for each simulation, however, when implementing openMP, different results are produced for each time I run it. so I write a simple example to check the problem as follows, I also generated different result each time:

#include <iostream>
#include <omp.h>
using namespace std;


int main () {

double A[10];

#pragma omp parallel for
for( int i=0;i<10;i++){
    srand(i+1);
    int m = rand()%100;
    A[i] = m;
}

cout<<"A= \n";

for(int i=0;i<10;i++){
    cout<<i<<" "<<A[i]<<" \n";
}
   return 0;
}

I run it twice, the results are: A= 0 86 1 25 2 78 3 1 4 46 5 95 6 77 7 83 8 15 9 8

and A= 0 15 1 41 2 65 3 1 4 75 5 85 6 95 7 83 8 74 9 8

Thank you very much!

Upvotes: 1

Views: 3410

Answers (2)

pburka
pburka

Reputation: 1474

rand() uses static state and is not threadsafe. You'll need to use a different, thread-safe, PRNG. See Thread-safe random number generation for Monte-Carlo integration or Do PRNG need to be thread safe?

Upvotes: 2

Ben Voigt
Ben Voigt

Reputation: 283733

This is a bug

A[i] += m;

You're reading the prior value of A[i] which has never been assigned. That's undefined behavior. try

A[i] = m;

Then, note that the random number state might not be threadlocal. Get a better RNG, where you have an explicit state variable instead of accessing shared global state.

Upvotes: 1

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