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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | SEE ALSO | COLOPHON |
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io_uring_prep_recv(3) liburing Manual io_uring_prep_recv(3)
io_uring_prep_recv - prepare a recv request
#include <liburing.h>
void io_uring_prep_recv(struct io_uring_sqe *sqe,
int sockfd,
void *buf,
size_t len,
int flags);
void io_uring_prep_recv_multishot(struct io_uring_sqe *sqe,
int sockfd,
void *buf,
size_t len,
int flags);
The io_uring_prep_recv(3) function prepares a recv request. The
submission queue entry sqe is setup to use the file descriptor
sockfd to start receiving the data into the destination buffer buf
of size len and with modifier flags flags.
This function prepares an async 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 IOSQE_BUFFER_SELECT
flag to be set and no 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
IORING_CQE_F_MORE flag set, then the multishot receive is done and
the application must issue a new request if it still wishes to
receive data from the socket. Multishot variants are available
since kernel 6.0.
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 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 IORING_CQE_F_SOCK_NONEMPTY flag,
which io_uring will set on CQEs after a recv(2) or
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.
IORING_RECVSEND_BUNDLE
If set and provided buffers are used with
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 res field indicates
the total number of bytes received, and the buffer ID
returned in the CQE flags field indicates the first buffer
in the receive 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 receive
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
receive operation, the cached head index should be
incremented accordingly. 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.
None
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.
io_uring_get_sqe(3), io_uring_submit(3),
io_uring_buf_ring_init(3), io_uring_buf_ring_add(3), recv(2)
This page is part of the liburing (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 [email protected]. 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 is not part of the original manual page), send a mail to
[email protected]
liburing-2.2 March 12, 2022 io_uring_prep_recv(3)
Pages that refer to this page: io_uring_prep_recv(3), io_uring_prep_recv_multishot(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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