But you're sharing file descriptors, which might be for devices, or for SOCK_SEQ connections, etc, and you can't just have the parent and child step all over ea

But you're sharing file descriptors, which might be for devices, or for SOCK_SEQ connections, etc, and you can't just have the parent and child step all over each other writing to them. Now, you wouldn't do that, but you might use a library that lets you end up doing that without noticing. Fork-safety is not trivial.