.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_msg_ring 3 "March 10, 2022" "liburing-2.2" "liburing Manual"
.SH NAME
io_uring_prep_msg_ring   - send a message to another ring

.SH SYNOPSIS
.nf
.BR "#include <liburing.h>"
.PP
.BI "void io_uring_msg_ring(struct io_uring_sqe *sqe,"
.BI "                        int fd,"
.BI "                        unsigned int len,"
.BI "                        __u64 data,"
.BI "                        unsigned int flags)"

.SH DESCRIPTION
.PP
io_uring_prep_msg_ring() prepares a to send a CQE to an io_uring file
descriptor. The submission queue entry
.I sqe
is setup to use the file descriptor
.I fd
, which must identify a io_uring context, to post a CQE on that ring where
the target CQE
.BR res
field will contain the content of
.I len
and the
.BR user_data
of
.I data
with the request modifier flags set by
.I flags.
Currently there are no valid flag modifiers, this field must contain
.B 0.

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
.I len
and
.I 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.

The resulting CQE posted on the target ring will have
.B IORING_CQE_F_MSG
set in its
.I flags
member, indicating to the target ring that this CQE is posted without having
a relationship to an SQE issued on this ring.

.SH RETURN VALUE
None