HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. |  |
NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |SEE ALSO |COLOPHON | |
io_uring_prep_send(3) liburing Manualio_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,intsockfd,const void *buf,size_tlen,intflags);void io_uring_prep_sendto(struct io_uring_sqe *sqe,intsockfd,const void *buf,size_tlen,intflags,const struct sockaddr *addr,socklen_taddrlen);void io_uring_prep_send_bundle(struct io_uring_sqe *sqe,intsockfd,size_tlen,intflags);
Theio_uring_prep_send(3) function prepares a send request. The submission queue entrysqe is setup to use the file descriptorsockfd to start sending the data frombuf of sizelen bytes and with modifier flagsflags. After calling this function, additional io_uring internal modifier flags may be set in the SQEioprio 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 CQEres field indicates the total number of bytes sent, and the buffer ID returned in the CQEflags field indicates the first buffer in the send operation. The application must process the indicated initial buffer ID and until allres 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 andlen 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 usingIOSQE_IO_LINKwith this request type requires the setting ofMSG_WAITALLin theflags argument, as a short send isn't a considered an error condition without that being set. This function prepares an asyncsend(2) request. See that man page for details. Theio_uring_prep_sendto(3) function prepares a sendto request. The submission queue entrysqe is setup to use the file descriptorsockfd to start sending the data frombuf of sizelen bytes and with modifier flagsflags. The destination address is specified byaddr andaddrlen and must be a valid address for the socket type. This function prepares an asyncsendto(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_SELECTis set in the SQEflags field, and additionally a buffer group ID is set in the SQEbuf_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 SQEbuf_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_SELECTmust also be set in the SQEflags before submission. Currentlylen must be given as0otherwise the request will be errored with-EINVALas the result code. Future versions may allow settingto limit the transfer size. A single CQE isposted 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 CQEflags 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 theIORING_FEAT_SEND_BUF_SELECTflag returned in when usingio_uring_init_queue_params(3) to setup the ring.
None
The CQEres 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-1on failure and seterrno to the actual error value, io_uring never useserrno. Instead it returns the negatederrno directly in the CQEres 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 theliburing (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 isnot part of the original manual page), send a mail to man-pages@man7.orgliburing-2.2 March 12, 2022io_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)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |