Why is this C++11 code containing rand() slower with multiple threads than with one?

Question

I\'m trying around on the new C++11 threads, but my simple test has abysmal multicore performance. As a simple example, this program adds up some squared random numbers.

Answer 1

On my system the behavior is same, but as Maxim mentioned, rand is not thread safe. When I change rand to rand_r, then the multi threaded code is faster as expected.

void add_multi(int N, double& result) {
double sum=0;
unsigned int seed = time(NULL);
for (int i = 0; i < N; ++i){
    sum+= sqrt(1.0*rand_r(&seed)/RAND_MAX);
}
result = sum/N;
}