# io_uring_prep_send

## 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_init_queue_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.
    


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