io_uring_register_ring_fd - Man Page
register a ring file descriptor
#include <liburing.h> int io_uring_register_ring_fd(struct io_uring *ring);
io_uring_register_ring_fd(3) registers the file descriptor of the ring.
Whenever io_uring_enter(2) is called to submit request or wait for completions, the kernel must grab a reference to the file descriptor. If the application using io_uring is threaded, the file table is marked as shared, and the reference grab and put of the file descriptor count is more expensive than it is for a non-threaded application.
Similarly to how io_uring allows registration of files, this allow registration of the ring file descriptor itself. This reduces the overhead of the io_uring_enter(2) system call.
If an application using liburing is threaded, then an application should call this function to register the ring descriptor when a ring is set up. See Notes for restrictions when a ring is shared.
When the ring descriptor is registered, it is stored internally in the struct io_uring structure. For applications that share a ring between threads, for example having one thread do submits and another reap events, then this optimization cannot be used as each thread may have a different index for the registered ring fd.
Returns 1 on success, indicating that one file descriptor was registered, or -errno on error.