118 lines
3.9 KiB
Groff
118 lines
3.9 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_openat2 3 "March 13, 2022" "liburing-2.2" "liburing Manual"
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.SH NAME
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io_uring_prep_openat2 \- prepare an openat2 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/stat.h>
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.B #include <fcntl.h>
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.B #include <linux/openat2.h>
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.B #include <liburing.h>
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.PP
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.BI "void io_uring_prep_openat2(struct io_uring_sqe *" sqe ","
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.BI " int " dfd ","
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.BI " const char *" path ","
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.BI " int " flags ","
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.BI " struct open_how *" how ");"
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.PP
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.BI "void io_uring_prep_openat2_direct(struct io_uring_sqe *" sqe ","
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.BI " int " dfd ","
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.BI " const char *" path ","
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.BI " int " flags ","
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.BI " struct open_how *" how ","
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.BI " unsigned " file_index ");"
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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_openat2 (3)
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function prepares an openat2 request. The submission queue entry
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.I sqe
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is setup to use the directory file descriptor
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.I dfd
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to start opening a file described by
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.I path
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and using the open flags in
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.I flags
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and using the instructions on how to open the file given in
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.IR how .
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For a direct descriptor open request, the offset is specified by the
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.I file_index
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argument. Direct descriptors are io_uring private file descriptors. They
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avoid some of the overhead associated with thread shared file tables, and
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can be used in any io_uring request that takes a file descriptor. To do so,
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.B IOSQE_FIXED_FILE
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must be set in the SQE
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.I flags
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member, and the SQE
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.I fd
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field should use the direct descriptor value rather than the regular file
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descriptor. Direct descriptors are managed like registered files.
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If the direct variant is used, the application must first have registered
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a file table using
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.BR io_uring_register_files (3)
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of the appropriate size. Once registered, a direct accept request may use any
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entry in that table, as long as it is within the size of the registered table.
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If a specified entry already contains a file, the file will first be removed
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from the table and closed. It's consistent with the behavior of updating an
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existing file with
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.BR io_uring_register_files_update (3).
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Note that old kernels don't check the SQE
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.I file_index
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field, which is not a problem for liburing helpers, but users of the raw
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io_uring interface need to zero SQEs to avoid unexpected behavior.
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If
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.B IORING_FILE_INDEX_ALLOC
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is used as the
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.I file_index
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for a direct open, then io_uring will allocate a free direct descriptor in
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the existing table. The allocated descriptor is returned in the CQE
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.I res
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field just like it would be for a non-direct open request. If no more entries
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are available in the direct descriptor table,
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.B -ENFILE
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is returned instead.
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These functions prepare an async
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.BR openat2 (2)
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request. See that man page for details.
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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 io_uring_register (2),
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.BR openat2 (2)
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