io_uring_prep_send(3)

NAME
io_uring_prep_send - prepare a send request

SYNOPSIS
#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

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_queue_init_params(3) to setup the ring.

RETURN VALUE
None

ERRORS
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.

NOTES
Despite accepting a size_t number of bytes, these functions can transfer at most INT_MAX bytes per call (the maximum for the underlying syscall interface).

SEE ALSO
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)