io_uring_prep_sendto(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | SEE ALSO | COLOPHON

io_uring_prep_send(3)        liburing Manual        io_uring_prep_send(3)

NAME         top

       io_uring_prep_send - prepare a send request

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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.

RETURN VALUE         top

       None

ERRORS         top

       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.

SEE ALSO         top

       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)

COLOPHON         top

       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)