write random numbers

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
static inline double r(double beg, double end) {
  return (rand() + 1.) / (RAND_MAX + 2.) * (end - beg) + beg;
}
static inline void write_random(FILE *out, unsigned int upper) {
  int n = rand() % upper;
  for (int i = 0; i < n; ++i) { fprintf(out, "%f\n", r(-10., 10.)); }
}
int main(void) {
  srand(time(NULL));
  FILE *out = fopen("random.txt", "w");
  write_random(out, 100);
  fclose(out);
}