--- name: "io_uring_prep_sendto" section: "3" platform: "linux" package: "liburing" version: "2.15-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/io_uring_prep_sendto" --- # io\_uring\_prep\_sendto(3) prepare a send request ## NAME io\_uring\_prep\_send - prepare a send request ## SYNOPSIS ``` #include ``` **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)](/io_uring_prep_send/3.md) 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)](/send/2.md) request. See that man page for details. The [**io\_uring\_prep\_sendto**(3)](/io_uring_prep_sendto/3.md) 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)](/sendto/2.md) 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)](/io_uring_queue_init_params/3.md) 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_get_sqe/3.md), [**io\_uring\_submit**(3)](/io_uring_submit/3.md), [**io\_uring\_buf\_ring\_init**(3)](/io_uring_buf_ring_init/3.md), [**io\_uring\_buf\_ring\_add**(3)](/io_uring_buf_ring_add/3.md), [**send**(2)](/send/2.md) [**sendto**(2)](/sendto/2.md)