The various BSD flavors were able to continue using a.out binaries long after Linux was forced to switch to ELF, due to the somewhat more flexible nature of the BSD a.out format compared to that of Linux.
一些BSD变体在Linux被迫转向ELF之后仍然用了很久的a.out格式，因为相比于Linux，BSD的a.out略微更灵活一些。