Miscellany

#include <stdio.h>#include <unistd.h>#include <sys/time.h>#include <stdlib.h>#include <math.h>#include <assert.h>#include <gmp.h>long n_iters;unsigned long gcd(unsigned long a, unsigned long b){  unsigned long temp;  while(b) {    temp = b;    b = a % b;    a = temp;  }  return a;}long time_in_usecs(){  struct timeval tv;  gettimeofday(&tv, NULL);  return tv.tv_sec * 1000000L + tv.tv_usec;}void child(int result_pipe){  long s = 0;  mpz_t a, b, c;  gmp_randstate_t state;  mpz_init(a);  mpz_init(b);  mpz_init(c);  gmp_randinit_default(state);  gmp_randseed_ui(state, time_in_usecs() * result_pipe);  while(n_iters--) {    mpz_urandomb(a, state, 256);    mpz_urandomb(b, state, 256);    mpz_gcd(c, a, b);    s += mpz_cmp_ui(c, 1) == 0;  }  write(result_pipe, &s, sizeof(s));}double parent(int n_children){  int *pipes;  long results = 0;  long temp;  int pipefds[2];  int i;  int pid;  double ratio;      pipes = malloc(sizeof(int) * n_children);  assert(pipes);      for(i = 0; i < n_children; i++) {    if(pipe(pipefds)) {      perror("couldn't pipe");      exit(1);    }    pipes[i] = pipefds[0];    pid = fork();     if(pid < 0) {      perror("couldn't fork");      exit(1);    } else if(pid == 0) {      child(pipefds[1]);      exit(0);    }  }  for(i = 0; i < n_children; i++) {    read(pipes[i], &temp, sizeof(long));    results += temp;  }  ratio = (double)results/(double)(n_children * n_iters);  return sqrt(6.0/ratio);}int main(int argc, char **argv){  assert(argc == 3);  n_iters = atol(argv[1])/atol(argv[2]);  printf("%.20f\n", parent(atoi(argv[2])));  return 0;}