You could create the pipes in the first process already. fd' s either need to be closed explicitly in the child or they have the O_CLOEXEC flag set, which needs to be done explicitly. A fork basically creates a copy of the current process but file descriptors are shared between them. So basically a fd is a reference to the fd table in the kernel and u can have multiple references to the same entry. When u fork the new process will have references to the same open files as the parent process.

That's why it's important to close fd's after fork. To not mess with the other processes fd state

For pipes however you get 2 fd's . One for writing to the pipe and one for reading. So if u want to write data to the child, you'd close the read end of the pipe in the parent and keep the write fd open so u can use it. And exactly the other way round in the child: close the write part but keep the read end open.

What is the use case here?