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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | SEE ALSO | COLOPHON |
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io_uring_prep_send(3) liburing Manual io_uring_prep_send(3)
io_uring_prep_send - prepare a send request
#include <liburing.h>
void io_uring_prep_send(struct io_uring_sqe *sqe,
int sockfd,
const void *buf,
size_t len,
int flags);
void io_uring_prep_sendto(struct io_uring_sqe *sqe,
int sockfd,
const void *buf,
size_t len,
int flags,
const struct sockaddr *addr,
socklen_t addrlen);
void io_uring_prep_send_bundle(struct io_uring_sqe *sqe,
int sockfd,
size_t len,
int flags);
The io_uring_prep_send(3) function prepares a send request. The
submission queue entry sqe is setup to use the file descriptor
sockfd to start sending the data from buf of size len bytes and
with modifier flags flags.
After calling this function, additional io_uring internal modifier
flags may be set in the SQE ioprio field. The following flags are
supported:
IORING_RECVSEND_POLL_FIRST
If set, io_uring will assume the socket is currently full
and attempting to send data will be unsuccessful. For this
case, io_uring will arm internal poll and trigger a send of
the data when the socket has space available. If poll does
indicate that space is available in the socket, the
operation will proceed immediately.
IORING_RECVSEND_BUNDLE
If set, the send operation will attempt to fill multiple
buffers with rather than just pick a single buffer to fill.
To send multiple buffers in a single send, the buffer group
ID set in the SQE must be of the ring provided type. If
set, the CQE res field indicates the total number of bytes
sent, and the buffer ID returned in the CQE flags field
indicates the first buffer in the send operation. The
application must process the indicated initial buffer ID
and until all res bytes have been seen to know which is the
last buffer in the send operation. The buffers consumed
will be contiguous from the initial buffer, in the order in
which they appear in the buffer ring. The CQE struct does
not contain the position of the buffer in the buffer ring,
therefore in order to identify buffers contained by the
bundle, it is advised to maintain the cached head index per
buffer ring. This uint16_t index represents the position of
the next buffer to be consumed within the ring. Upon
completion of a bundle send operation, the cached head
index should be incremented accordingly. Sending in
bundles can improve performance when more than one chunk of
data is available by eliminating redundant round trips
through the networking stack.
IORING_SEND_VECTORIZED
If set, addr must point to an array of struct iovec and len
must be the number of vectors in that array. This enables
use of vectorized IO for a normal send operation, rather
than needing a sendmsg variant to accomplish that.
Note that using IOSQE_IO_LINK with this request type requires the
setting of MSG_WAITALL in the flags argument, as a short send
isn't a considered an error condition without that being set.
This function prepares an async send(2) request. See that man page
for details.
The io_uring_prep_sendto(3) function prepares a sendto request.
The submission queue entry sqe is setup to use the file descriptor
sockfd to start sending the data from buf of size len bytes and
with modifier flags flags. The destination address is specified
by addr and addrlen and must be a valid address for the socket
type.
This function prepares an async sendto(2) request. See that man
page for details.
Both of the above send variants may be used with provided buffers,
where rather than pass a buffer in directly with the request,
IOSQE_BUFFER_SELECT is set in the SQE flags field, and
additionally a buffer group ID is set in the SQE buf_group field.
By using provided buffers with send requests, the application can
prevent any kind of reordering of the outgoing data which can
otherwise occur if the application has more than one send request
inflight for a single socket. This provides better pipelining of
data, where previously the app needed to manually serialize sends.
The bundle version allows the application to issue a single send
request, with a buffer group ID given in the SQE buf_group field,
which keeps sending from that buffer group until it runs out of
buffers. As with any other request using provided buffers,
IOSQE_BUFFER_SELECT must also be set in the SQE flags before
submission. Currently len must be given as 0 otherwise the request
will be errored with -EINVAL as the result code. Future versions
may allow setting to limit the transfer size. A single CQE is
posted for the send, with the result being how many bytes were
sent, on success. When used with provided buffers, send or send
bundle will contain the starting buffer group ID in the CQE flags
field. The number of bytes sent starts from there, and will be in
contiguous buffer IDs after that. Send bundle, and send with
provided buffers in general, are available since kernel 6.10, and
can be further identified by checking for the
IORING_FEAT_SEND_BUF_SELECT flag returned in when using
io_uring_init_queue_params(3) to setup the ring.
None
The CQE res field will contain the result of the operation. See
the related man page for details on possible values. Note that
where synchronous system calls will return -1 on failure and set
errno to the actual error value, io_uring never uses errno.
Instead it returns the negated errno directly in the CQE res
field.
io_uring_get_sqe(3), io_uring_submit(3),
io_uring_buf_ring_init(3), io_uring_buf_ring_add(3), send(2)
sendto(2)
This page is part of the liburing (A library for io_uring)
project. Information about the project can be found at
⟨https://github.com/axboe/liburing⟩. If you have a bug report for
this manual page, send it to io-uring@vger.kernel.org. This page
was obtained from the project's upstream Git repository
⟨https://github.com/axboe/liburing⟩ on 2025-08-11. (At that time,
the date of the most recent commit that was found in the
repository was 2025-08-02.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
liburing-2.2 March 12, 2022 io_uring_prep_send(3)
Pages that refer to this page: io_uring_prep_send(3), io_uring_prep_send_bundle(3), io_uring_prep_sendto(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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