.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_recv 3 "March 12, 2022" "liburing-2.2" "liburing Manual"
.SH NAME
io_uring_prep_recv \- prepare a recv request
.SH SYNOPSIS
.nf
.B #include <liburing.h>
.PP
.BI "void io_uring_prep_recv(struct io_uring_sqe *" sqe ","
.BI "                        int " sockfd ","
.BI "                        void *" buf ","
.BI "                        size_t " len ","
.BI "                        int " flags ");"
.PP
.BI "void io_uring_prep_recv_multishot(struct io_uring_sqe *" sqe ","
.BI "                                  int " sockfd ","
.BI "                                  void *" buf ","
.BI "                                  size_t " len ","
.BI "                                  int " flags ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_prep_recv (3)
function prepares a recv request. The submission
queue entry
.I sqe
is setup to use the file descriptor
.I sockfd
to start receiving the data into the destination buffer
.I buf
of size
.I len
and with modifier flags
.IR flags .

This function prepares an async
.BR recv (2)
request. See that man page for details on the arguments specified to this
prep helper.

The multishot version allows the application to issue a single receive request,
which repeatedly posts a CQE when data is available. It requires length to
be 0, the
.B IOSQE_BUFFER_SELECT
flag to be set and no
.B MSG_WAITALL
flag to be set.
Therefore each CQE will take a buffer out of a provided buffer pool for receiving.
The application should check the flags of each CQE, regardless of its result.
If a posted CQE does not have the
.B IORING_CQE_F_MORE
flag set then the multishot receive will be done and the application should issue a
new request.
Multishot variants are available since kernel 6.0.


After calling this function, additional io_uring internal modifier flags
may be set in the SQE
.I ioprio
field. The following flags are supported:
.TP
.B IORING_RECVSEND_POLL_FIRST
If set, io_uring will assume the socket is currently empty and attempting to
receive data will be unsuccessful. For this case, io_uring will arm internal
poll and trigger a receive of the data when the socket has data to be read.
This initial receive attempt can be wasteful for the case where the socket
is expected to be empty, setting this flag will bypass the initial receive
attempt and go straight to arming poll. If poll does indicate that data is
ready to be received, the operation will proceed.

Can be used with the CQE
.B IORING_CQE_F_SOCK_NONEMPTY
flag, which io_uring will set on CQEs after a
.BR recv (2)
or
.BR recvmsg (2)
operation. If set, the socket still had data to be read after the operation
completed. Both these flags are available since 5.19.

.TP
.B IORING_RECVSEND_BUNDLE
If set and provided buffers are used with
.B IOSQE_BUFFER_SELECT ,
the receive operation will attempt to fill multiple buffers with rather than
just pick a single buffer to fill. To receive multiple buffers in a single
receive, the buffer group ID set in the SQE must be of the ring provided type.
If set, the CQE
.I res
field indicates the total number of bytes received, and the buffer ID returned
in the CQE
.I flags
field indicates the first buffer in the receive operation. The application must
iterate from the indicated initial buffer ID and until all
.I res
bytes have been seen to know which is the last buffer in the receive operation.
The buffer IDs consumed will be contiguous from the starting ID, in the order
in which they were added to the buffer ring used. Receiving in bundles can
improve performance when more than one chunk of data is available to receive,
by eliminating redundant round trips through the networking stack. Receive
bundles may be used by both single shot and multishot receive operations. Note
that, internally, bundles rely on the networking stack passing back how much
data is left in the socket after the initial receive. This means that the
initial receive may contain less buffers than what is available, with the
followup receive(s) containing more buffers. Available since 6.10.
.P

.SH RETURN VALUE
None
.SH ERRORS
The CQE
.I 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
.B -1
on failure and set
.I errno
to the actual error value, io_uring never uses
.IR errno .
Instead it returns the negated
.I errno
directly in the CQE
.I res
field.
.SH SEE ALSO
.BR io_uring_get_sqe (3),
.BR io_uring_submit (3),
.BR io_uring_buf_ring_init (3),
.BR io_uring_buf_ring_add (3),
.BR recv (2)