#include <unistd.h>

int main(void) {
  // duplicate stdout
  int duplicate = dup(1);
  unsigned counter = 0;
  for (; counter < 10; ++counter) {
    if (counter % 2 == 0) {
      // redirect IO on FD duplicate to stderr
      dup2(2, duplicate);
    } else {
      // redirect it back to stdout
      dup2(1, duplicate);
    }
    write (duplicate, "A ", 2);
    write (1, "B\n", 2);
  }
  close (duplicate);
  return 0;
}