Introduction to Concurrency in Programming Languages

284 Introduction to Concurrency in Programming Languages

Listing A.4: Demonstrating a reduction.

# inclu de < stdlib .h>

# inclu de < omp .h >

int main ( int argc , char ** argv ) {

int i , j, id , num ;

double mean = 0.0 , * x ;

int N = 20 000000;

x = ( double *) malloc ( sizeof ( double )*N );

/* populate x on main thread ... */

for ( i =0; i <N ; i ++)

x[i] = ( double ) rand () / ( double ) RAND_MAX ;

/* compute mean in pa rallel */

# pragma omp p arallel priva te ( id , i, num) \

reduc tion (+: mean )

{

id = omp_get_thread_num ();

num = omp_get_num_threads ();

for ( i = id; i <N; i += num ) {

mean = mean + x[i] / ( double ) N;

}

}

}

reduction(op: variable)

where ...