from_to.c

#include <stdbool.h>
#include <stdio.h>

typedef bool (*gen_f)(size_t *i);

gen_f from_to(size_t from, size_t to) {
  size_t i = from;

  bool g(size_t * j) {
    if (i <= to) {
      *j = i++;
      return true;
    } else {
      return false;
    }
  }

  return g;
}

int main(int argc, char **argv) {
  gen_f gen = from_to(5, 7);
  size_t i;

  while (gen(&i)) {
    printf("%ld\n", i);
  }

  return 0;
}