That depends on what you're doing. Sendfile, splice and copy_file_range are the fastest if you want to shovel things back and forth between file descriptors. Th

That depends on what you're doing. Sendfile, splice and copy_file_range are the fastest if you want to shovel things back and forth between file descriptors. The latter supports reflink copy if the the underlying filesystem does. And I think there's some special casing in vmsplice that lets you transfer memory pages from process to process over a pipe. At least I've seen some patches for that, not sure if they made it into the kernel. Alternatively there's memfd. But if you want to coordinate processes you'll use a shared memory and futexes. If you just want to hand over an entire file to a different process you can send the file descriptor itself over a unix socket.