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 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 ioprio field. The following
flags are supported:
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.
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
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 firstname.lastname@example.org. This page
was obtained from the project's upstream Git repository
⟨https://github.com/axboe/liburing⟩ on 2023-12-22. (At that
time, the date of the most recent commit that was found in the
repository was 2023-12-19.) If you discover any rendering
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liburing-2.2 March 12, 2022 io_uring_prep_recv(3)