It could be emulated in userspace with F_DUPFD. But that's costly because the kernel table is optimized for dense data, not sparse. The rust standard library a

It could be emulated in userspace with F_DUPFD. But that's costly because the kernel table is optimized for dense data, not sparse. The rust standard library aborts the program in debug builds when it detects a double-close; at that point corruption may already have occurred but better than nothing.