io_uring_prep_msg_ring(3) liburing Manual io_uring_prep_msg_ring(3)

io_uring_prep_msg_ring_fd - send a direct descriptor to another ring

#include <liburing.h>
void io_uring_prep_msg_ring_fd(struct io_uring_sqe *sqe,
                        int fd,
                        int source_fd,
                        int target_fd,
                        __u64 data,
                        unsigned int flags);
void io_uring_prep_msg_ring_fd_alloc(struct io_uring_sqe *sqe,
                        int fd,
                        int source_fd,
                        __u64 data,
                        unsigned int flags);

io_uring_prep_msg_ring_fd(3) prepares an SQE to send a direct file descriptor to another ring. The submission queue entry sqe is setup to use the file descriptor fd, which must identify a target io_uring context, to send the locally registered file descriptor with value source_fd to the destination ring into index target_fd and passing data as the user data in the target CQE with the request modifier flags set by flags. Currently there are no valid flag modifiers, this field must contain 0.

io_uring_prep_msg_ring_fd_alloc(3) is similar to io_uring_prep_msg_ring_fd(3), but doesn't specify a target index for the direct descriptor. Instead, this index is allocated in the target ring and returned in the CQE res field.

None

These are the errors that are reported in the CQE res field.

The kernel was unable to allocate memory for the request.
One of the fields set in the SQE was invalid.
Target ring is identical to the source ring.
The descriptor passed in fd does not refer to an io_uring file descriptor, or the ring is in a disabled state.
The kernel was unable to fill a CQE on the target ring. This can happen if the target CQ ring is in an overflow state and the kernel wasn't able to allocate memory for a new CQE entry.
The direct descriptor table in the target ring was full, no new descriptors could be successfully allocated.
March 16, 2023 liburing-2.4