95 lines
2.8 KiB
Groff
95 lines
2.8 KiB
Groff
.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
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.\"
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.\" SPDX-License-Identifier: LGPL-2.0-or-later
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.\"
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.TH io_uring_prep_recvmsg 3 "March 12, 2022" "liburing-2.2" "liburing Manual"
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.SH NAME
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io_uring_prep_recvmsg \- prepare a recvmsg request
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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.B #include <sys/socket.h>
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.B #include <liburing.h>
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.PP
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.BI "void io_uring_prep_recvmsg(struct io_uring_sqe *" sqe ","
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.BI " int " fd ","
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.BI " struct msghdr *" msg ","
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.BI " unsigned " flags ");"
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.fi
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.SH DESCRIPTION
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.PP
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The
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.BR io_uring_prep_recvmsg (3)
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function prepares a recvmsg request. The submission queue entry
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.I sqe
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is setup to use the file descriptor
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.I fd
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to start receiving the data indicated by
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.I msg
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with the
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.BR recvmsg (2)
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defined flags in the
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.I flags
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argument.
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This function prepares an async
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.BR recvmsg (2)
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request. See that man page for details on the arguments specified to this
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prep helper.
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After calling this function, additional io_uring internal modifier flags
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may be set in the SQE
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.I off
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field. The following flags are supported:
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.TP
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.B IORING_RECVSEND_POLL_FIRST
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If set, io_uring will assume the socket is currently empty and attempting to
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receive data will be unsuccessful. For this case, io_uring will arm internal
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poll and trigger a receive of the data when the socket has data to be read.
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This initial receive attempt can be wasteful for the case where the socket
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is expected to be empty, setting this flag will bypass the initial receive
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attempt and go straight to arming poll. If poll does indicate that data is
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ready to be received, the operation will proceed.
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Can be used with the CQE
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.B IORING_CQE_F_SOCK_NONEMPTY
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flag, which io_uring will set on CQEs after a
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.BR recv (2)
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or
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.BR recvmsg (2)
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operation. If set, the socket still had data to be read after the operation
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completed. Both these flags are available since 5.19.
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.P
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.SH RETURN VALUE
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None
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.SH ERRORS
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The CQE
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.I res
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field will contain the result of the operation. See the related man page for
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details on possible values. Note that where synchronous system calls will return
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.B -1
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on failure and set
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.I errno
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to the actual error value, io_uring never uses
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.IR errno .
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Instead it returns the negated
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.I errno
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directly in the CQE
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.I res
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field.
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.SH NOTES
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As with any request that passes in data in a struct, that data must remain
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valid until the request has been successfully submitted. It need not remain
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valid until completion. Once a request has been submitted, the in-kernel
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state is stable. Very early kernels (5.4 and earlier) required state to be
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stable until the completion occurred. Applications can test for this
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behavior by inspecting the
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.B IORING_FEAT_SUBMIT_STABLE
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flag passed back from
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.BR io_uring_queue_init_params (3).
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.SH SEE ALSO
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.BR io_uring_get_sqe (3),
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.BR io_uring_submit (3),
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.BR recvmsg (2)
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