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/* File: omp_pi.c * Purpose: Estimate pi using OpenMP and the formula * * pi = 4*[1 - 1/3 + 1/5 - 1/7 + 1/9 - . . . ] * * Compile: gcc -g -Wall -fopenmp -o omp_pi omp_pi.c -lm * Run: omp_pi * thread_count is the number of threads * n is the number of terms of the series to use * * Input: none * Output: The estimate of pi and the value of pi computed by the * arctan function in the math library * * Notes: * 1. The radius of convergence is only 1. So the series converges * *very* slowly. * * IPP: Section 5.5.4 (pp. 229 and ff.) */ #include #include #include #include # define DEBUG 1 void Usage(char* prog_name); int main(int argc, char* argv[]) { long long n, i; int thread_count; double factor; double sum = 0.0; if (argc != 3) Usage(argv[0]); thread_count = strtol(argv[1], NULL, 10); n = strtoll(argv[2], NULL, 10); if (thread_count < 1 || n < 1) Usage(argv[0]); printf("Before parallel for, factor = %f.\n", factor); # pragma omp parallel for num_threads(thread_count) \ reduction(+: sum) private(factor) for (i = 0; i < n; i++) { factor = (i % 2 == 0) ? 1.0 : -1.0; sum += factor/(2*i+1); # ifdef DEBUG int my_rank = omp_get_thread_num(); printf("Thread %d > i = %lld\n", my_rank, i); # endif } printf("After parallel for, factor = %f.\n", factor); sum = 4.0*sum; printf("With n = %lld terms and %d threads,\n", n, thread_count); printf(" Our estimate of pi = %.14f\n", sum); printf(" pi = %.14f\n", 4.0*atan(1.0)); return 0; } /* main */ /*------------------------------------------------------------------ * Function: Usage * Purpose: Print a message explaining how to run the program * In arg: prog_name */ void Usage(char* prog_name) { fprintf(stderr, "usage: %s \n", prog_name); /* Change */ fprintf(stderr, " thread_count is the number of threads >= 1\n"); /* Change */ fprintf(stderr, " n is the number of terms and should be >= 1\n"); exit(0); } /* Usage */