c puzzle #4

The following program doesn't "seem" to print "hello-out". (Try executing it)

#include <stdio.h>
  #include <unistd.h>
  int main()
  {
          while(1)
          {
                  fprintf(stdout,"hello-out");
                  fprintf(stderr,"hello-err");
                  sleep(1);
          }
          return 0;
  }

What could be the reason?