io_uring_register_sync_msg(3)

NAME
io_uring_register_sync_msg - send a synchronous message to another ring

SYNOPSIS
#include <liburing.h>

int io_uring_register_sync_msg(struct io_uring_sqe *sqe);

DESCRIPTION

io_uring_register_sync_msg(3) issues a synchronous MSG_RING request. The sqe parameter must have been cleared and initialized with io_uring_prep_msg_ring(3).

Normally message requests are sent from one ring to another ring. But there are also cases where a source ring is not available, yet it would be convenient to send a message to a destination ring. io_uring_register_sync_msg(3) exists for that purpose. A source ring is not required to send a message to another ring, instead the sqe parameter can be placed on the stack and filled in using the normal message helpers, and then io_uring_register_sync_msg(3) can be called. Since a source ring does not exist, the results of the operation is returned directly rather than via a CQE. On the destination/receiving end, a CQE is posted, as it would have been with a non-sync request.

Only data request are supported, sending files such as setup by io_uring_prep_msg_ring_fd(3) is not supported. The given SQE should be initialized by io_uring_prep_msg_ring(3) or io_uring_prep_msg_ring_cqe_flags(3), or any other helper that sets up a non-fd message request.

The targeted ring may be any ring that the user has access to, even the ring itself. This request can be used for simple message passing to another ring, allowing 32+64 bits of data to be transferred through the len and data fields. The use case may be anything from simply waking up someone waiting on the targeted ring, or it can be used to pass messages between the two rings.

io_uring_prep_msg_ring_cqe_flags(3) is similar to io_uring_prep_msg_ring(3). But has an addition cqe_flags parameter, which is used to set flags field on CQE side. That way, you can set the CQE flags field cqe->flags when sending a message. Be aware that io_uring could potentially set additional bits into this field.

Available since kernel 6.13.

RETURN VALUE
Returns 0 on success, or -errno on error.

SEE ALSO
io_uring_prep_msg_ring_cqe_flags(3), io_uring_prep_msg_ring(3)