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.\" Copyright (C) 2025 Jens Axboe <axboe@kernel.dk>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_sendmsg_zc_fixed 3 "January 18, 2025" "liburing-2.10" "liburing Manual"
.SH NAME
io_uring_prep_sendmsg_zc_fixed \- prepare a zero-copy sendmsg using fixed buffers
.SH SYNOPSIS
.nf
.B #include <liburing.h>
.PP
.BI "void io_uring_prep_sendmsg_zc_fixed(struct io_uring_sqe *" sqe ","
.BI " int " fd ","
.BI " const struct msghdr *" msg ","
.BI " unsigned " flags ","
.BI " unsigned " buf_index ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_prep_sendmsg_zc_fixed (3)
function prepares a zero-copy sendmsg request using fixed (registered)
buffers. The submission queue entry
.I sqe
is setup to send data on the socket indicated by the file descriptor
.I fd
using the message structure
.IR msg .
The
.I flags
argument contains flags for the sendmsg operation, as described in
.BR sendmsg (2).
The
.I buf_index
specifies the index of the registered buffer set to use. The buffers in
.I msg
must be part of the registered buffer set previously registered with
.BR io_uring_register_buffers (3).
Zero-copy sends avoid copying data from user to kernel space, improving
performance for large transfers. Using fixed buffers additionally avoids
the overhead of mapping buffers for each I/O operation.
Note that zero-copy sends require the application to wait for a notification
before reusing the buffer. See
.BR io_uring_prep_send_zc (3)
for more details on zero-copy semantics.
.SH RETURN VALUE
None
.SH ERRORS
The CQE
.I res
field will contain the result of the operation, the number of bytes sent
on success. On error, a negative errno value is returned.
.SH SEE ALSO
.BR io_uring_get_sqe (3),
.BR io_uring_submit (3),
.BR io_uring_prep_sendmsg_zc (3),
.BR io_uring_prep_sendmsg (3),
.BR io_uring_prep_send_zc_fixed (3),
.BR io_uring_register_buffers (3),
.BR sendmsg (2)
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