86 lines
2.6 KiB
Groff
86 lines
2.6 KiB
Groff
.\" Copyright (C) 2021 Stefan Roesch <shr@fb.com>
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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_writev 3 "November 15, 2021" "liburing-2.1" "liburing Manual"
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.SH NAME
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io_uring_prep_writev \- prepare vector I/O write request
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.SH SYNOPSIS
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.nf
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.B #include <sys/uio.h>
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.B #include <liburing.h>
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.PP
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.BI "void io_uring_prep_writev(struct io_uring_sqe *" sqe ","
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.BI " int " fd ","
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.BI " const struct iovec *" iovecs ","
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.BI " unsigned " nr_vecs ","
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.BI " __u64 " offset ");"
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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_writev (3)
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prepares a vectored IO write 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 writing
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.I nr_vecs
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from the
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.I iovecs
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array at the specified
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.IR offset .
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On files that support seeking, if the offset is set to
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.BR -1 ,
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the write operation commences at the file offset, and the file offset is
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incremented by the number of bytes written. See
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.BR write (2)
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for more details. Note that for an async API, reading and updating the
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current file offset may result in unpredictable behavior, unless access
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to the file is serialized. It is not encouraged to use this feature if it's
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possible to provide the desired IO offset from the application or library.
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On files that are not capable of seeking, the offset is ignored.
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After the write has been prepared it can be submitted with one of the submit
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functions.
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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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Unless an application explicitly needs to pass in more than iovec, it is more
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efficient to use
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.BR io_uring_prep_write (3)
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rather than this function, as no state has to be maintained for a
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non-vectored IO request.
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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_prep_write (3),
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.BR io_uring_prep_writev2 (3),
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.BR io_uring_submit (3)
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