mirror of https://github.com/mkerrisk/man-pages
561 lines
14 KiB
Groff
561 lines
14 KiB
Groff
.\" Copyright (c) 2016 Julia Computing Inc, Keno Fischer
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.\" Description based on include/uapi/fuse.h and code in fs/fuse
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH FUSE 4 2018-02-02 "Linux" "Linux Programmer's Manual"
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.SH NAME
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fuse \- Filesystem in Userspace (FUSE) device
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.SH SYNOPSIS
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.nf
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.B #include <linux/fuse.h>
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.fi
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.SH DESCRIPTION
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.PP
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This device is the primary interface between the FUSE filesystem driver
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and a user-space process wishing to provide the filesystem (referred to
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in the rest of this manual page as the
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.IR "filesystem daemon" ).
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This manual page is intended for those
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interested in understanding the kernel interface itself.
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Those implementing a FUSE filesystem may wish to make use of
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a user-space library such as
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.I libfuse
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that abstracts away the low-level interface.
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.PP
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At its core, FUSE is a simple client-server protocol, in which the Linux
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kernel is the client and the daemon is the server.
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After obtaining a file descriptor for this device, the daemon may
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.BR read (2)
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requests from that file descriptor and is expected to
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.BR write (2)
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back its replies.
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It is important to note that a file descriptor is
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associated with a unique FUSE filesystem.
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In particular, opening a second copy of this device,
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will not allow access to resources created
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through the first file descriptor (and vice versa).
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.\"
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.SS The basic protocol
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Every message that is read by the daemon begins with a header described by
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the following structure:
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.PP
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.in +4n
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.EX
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struct fuse_in_header {
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uint32_t len; /* Total length of the data,
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including this header */
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uint32_t opcode; /* The kind of operation (see below) */
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uint64_t unique; /* A unique identifier for this request */
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uint64_t nodeid; /* ID of the filesystem object
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being operated on */
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uint32_t uid; /* UID of the requesting process */
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uint32_t gid; /* GID of the requesting process */
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uint32_t pid; /* PID of the requesting process */
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uint32_t padding;
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};
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.EE
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.in
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.PP
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The header is followed by a variable-length data portion
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(which may be empty) specific to the requested operation
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(the requested operation is indicated by
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.IR opcode ).
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.PP
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The daemon should then process the request and if applicable send
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a reply (almost all operations require a reply; if they do not,
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this is documented below), by performing a
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.BR write (2)
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to the file descriptor.
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All replies must start with the following header:
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.PP
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.in +4n
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.EX
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struct fuse_out_header {
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uint32_t len; /* Total length of data written to
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the file descriptor */
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int32_t error; /* Any error that occurred (0 if none) */
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uint64_t unique; /* The value from the
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corresponding request */
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};
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.EE
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.in
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.PP
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This header is also followed by (potentially empty) variable-sized
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data depending on the executed request.
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However, if the reply is an error reply (i.e.,
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.I error
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is set),
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then no further payload data should be sent, independent of the request.
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.\"
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.SS Exchanged messages
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This section should contain documentation for each of the messages
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in the protocol.
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This manual page is currently incomplete,
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so not all messages are documented.
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For each message, first the struct sent by the kernel is given,
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followed by a description of the semantics of the message.
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.TP
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.BR FUSE_INIT
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.IP
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.in +4n
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.EX
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struct fuse_init_in {
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uint32_t major;
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uint32_t minor;
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uint32_t max_readahead; /* Since protocol v7.6 */
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uint32_t flags; /* Since protocol v7.6 */
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};
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.EE
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.in
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.IP
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This is the first request sent by the kernel to the daemon.
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It is used to negotiate the protocol version and other filesystem parameters.
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Note that the protocol version may affect the layout of any structure
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in the protocol (including this structure).
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The daemon must thus remember the negotiated version
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and flags for each session.
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As of the writing of this man page,
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the highest supported kernel protocol version is
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.IR 7.26 .
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.IP
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Users should be aware that the descriptions in this manual page
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may be incomplete or incorrect for older or more recent protocol versions.
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.IP
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The reply for this request has the following format:
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.IP
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.in +4n
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.EX
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struct fuse_init_out {
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uint32_t major;
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uint32_t minor;
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uint32_t max_readahead; /* Since v7.6 */
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uint32_t flags; /* Since v7.6; some flags bits
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were introduced later */
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uint16_t max_background; /* Since v7.13 */
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uint16_t congestion_threshold; /* Since v7.13 */
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uint32_t max_write; /* Since v7.5 */
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uint32_t time_gran; /* Since v7.6 */
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uint32_t unused[9];
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};
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.EE
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.in
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.IP
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If the major version supported by the kernel is larger than that supported
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by the daemon, the reply shall consist of only
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.I uint32_t major
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(following the usual header),
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indicating the largest major version supported by the daemon.
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The kernel will then issue a new
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.B FUSE_INIT
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request conforming to the older version.
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In the reverse case, the daemon should
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quietly fall back to the kernel's major version.
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.IP
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The negotiated minor version is considered to be the minimum
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of the minor versions provided by the daemon and the kernel and
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both parties should use the protocol corresponding to said minor version.
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.TP
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.BR FUSE_GETATTR
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.IP
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.in +4n
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.EX
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struct fuse_getattr_in {
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uint32_t getattr_flags;
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uint32_t dummy;
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uint64_t fh; /* Set only if
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(getattr_flags & FUSE_GETATTR_FH)
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};
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.EE
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.in
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.IP
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The requested operation is to compute the attributes to be returned
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by
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.BR stat (2)
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and similar operations for the given file system object.
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The object for which the attributes should be computed is indicated
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either by
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.IR header\->nodeid
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or, if the
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.IR FUSE_GETATTR_FH
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flag is set, by the file handle
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.IR fh.
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The latter case of operation is analogous to
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.BR fstat (2).
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.IP
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For performance reasons, these attributes may be cached in the kernel for
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a specified duration of time.
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While the cache timeout has not been exceeded,
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the attributes will be served from the cache and will not cause additional
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.B FUSE_GETATTR
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requests.
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.IP
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The computed attributes and the requested
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cache timeout should then be returned in the following structure:
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.IP
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.in +4n
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.EX
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struct fuse_attr_out {
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/* Attribute cache duration (seconds + nanoseconds) */
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uint64_t attr_valid;
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uint32_t attr_valid_nsec;
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uint32_t dummy;
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struct fuse_attr {
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uint64_t ino;
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uint64_t size;
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uint64_t blocks;
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uint64_t atime;
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uint64_t mtime;
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uint64_t ctime;
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uint32_t atimensec;
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uint32_t mtimensec;
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uint32_t ctimensec;
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uint32_t mode;
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uint32_t nlink;
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uint32_t uid;
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uint32_t gid;
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uint32_t rdev;
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uint32_t blksize;
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uint32_t padding;
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} attr;
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};
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.EE
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.in
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.IP
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.TP
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.BR FUSE_ACCESS
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.IP
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.in +4n
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.EX
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struct fuse_access_in {
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uint32_t mask;
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uint32_t padding;
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};
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.EE
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.in
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.IP
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If the
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.I default_permissions
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mount options is not used, this request may be used for permissions checking.
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No reply data is expected, but errors may be indicated
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as usual by setting the
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.I error
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field in the reply header (in particular, access denied errors
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may be indicated by returning
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.BR \-EACCES ).
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.TP
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.BR FUSE_OPEN " and " FUSE_OPENDIR
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.in +4n
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.EX
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struct fuse_open_in {
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uint32_t flags; /* The flags that were passed
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to the open(2) */
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uint32_t unused;
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};
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.EE
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.in
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.IP
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The requested operation is to open the node indicated by
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.IR header\->nodeid .
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The exact semantics of what this means will depend on the
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filesystem being implemented.
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However, at the very least the
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filesystem should validate that the requested
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.I flags
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are valid for the indicated resource and then send a reply with the
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following format:
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.IP
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.IP
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.in +4n
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.EX
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struct fuse_open_out {
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uint64_t fh;
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uint32_t open_flags;
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uint32_t padding;
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};
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.EE
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.in
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.IP
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.IP
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The
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.I fh
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field is an opaque identifier that the kernel will use to refer
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to this resource
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The
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.I open_flags
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field is a bit mask of any number of the flags
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that indicate properties of this file handle to the kernel:
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.RS 7
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.TP 18
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.BR FOPEN_DIRECT_IO
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Bypass page cache for this open file.
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.TP
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.BR FOPEN_KEEP_CACHE
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Don't invalidate the data cache on open.
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.TP
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.BR FOPEN_NONSEEKABLE
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The file is not seekable.
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.RE
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.TP
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.BR FUSE_READ " and " FUSE_READDIR
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.IP
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.in +4n
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.EX
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struct fuse_read_in {
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uint64_t fh;
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uint64_t offset;
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uint32_t size;
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uint32_t read_flags;
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uint64_t lock_owner;
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uint32_t flags;
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uint32_t padding;
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};
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.EE
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.in
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.IP
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.IP
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The requested action is to read up to
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.I size
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bytes of the file or directory, starting at
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.IR offset .
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The bytes should be returned directly following the usual reply header.
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.TP
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.BR FUSE_INTERRUPT
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.in +4n
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.EX
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struct fuse_interrupt_in {
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uint64_t unique;
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};
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.EE
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.in
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.IP
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The requested action is to cancel the pending operation indicated by
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.IR unique .
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This request requires no response.
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However, receipt of this message does
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not by itself cancel the indicated operation.
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The kernel will still expect a reply to said operation (e.g., an
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.I EINTR
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error or a short read).
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At most one
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.B FUSE_INTERRUPT
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request will be issued for a given operation.
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After issuing said operation,
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the kernel will wait uninterruptibly for completion of the indicated request.
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.TP
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.BR FUSE_LOOKUP
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Directly following the header is a filename to be looked up in the directory
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indicated by
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.IR header\->nodeid .
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The expected reply is of the form:
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.IP
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.in +4n
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.EX
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struct fuse_entry_out {
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uint64_t nodeid; /* Inode ID */
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uint64_t generation; /* Inode generation */
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uint64_t entry_valid;
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uint64_t attr_valid;
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uint32_t entry_valid_nsec;
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uint32_t attr_valid_nsec;
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struct fuse_attr attr;
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};
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.EE
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.in
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.IP
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The combination of
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.I nodeid
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and
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.I generation
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must be unique for the filesystem's lifetime.
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.IP
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The interpretation of timeouts and
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.I attr
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is as for
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.BR FUSE_GETATTR .
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.TP
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.BR FUSE_FLUSH
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.in +4n
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.EX
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struct fuse_flush_in {
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uint64_t fh;
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uint32_t unused;
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uint32_t padding;
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uint64_t lock_owner;
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};
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.EE
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.in
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.IP
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The requested action is to flush any pending changes to the indicated
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file handle.
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No reply data is expected.
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However, an empty reply message
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still needs to be issued once the flush operation is complete.
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.TP
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.BR FUSE_RELEASE " and " FUSE_RELEASEDIR
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.in +4n
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.EX
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struct fuse_release_in {
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uint64_t fh;
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uint32_t flags;
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uint32_t release_flags;
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uint64_t lock_owner;
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};
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.EE
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.in
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.IP
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These are the converse of
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.BR FUSE_OPEN
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and
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.BR FUSE_OPENDIR
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respectively.
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The daemon may now free any resources associated with the
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file handle
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.I fh
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as the kernel will no longer refer to it.
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There is no reply data associated with this request,
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but a reply still needs to be issued once the request has
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been completely processed.
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.TP
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.BR FUSE_STATFS
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This operation implements
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.BR statfs (2)
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for this filesystem.
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There is no input data associated with this request.
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The expected reply data has the following structure:
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.IP
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.in +4n
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.EX
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struct fuse_kstatfs {
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uint64_t blocks;
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uint64_t bfree;
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uint64_t bavail;
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uint64_t files;
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uint64_t ffree;
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uint32_t bsize;
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uint32_t namelen;
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uint32_t frsize;
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uint32_t padding;
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uint32_t spare[6];
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};
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struct fuse_statfs_out {
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struct fuse_kstatfs st;
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};
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.EE
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.in
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.IP
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For the interpretation of these fields, see
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.BR statfs (2).
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.SH ERRORS
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.TP
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.B E2BIG
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Returned from
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.BR read (2)
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operations when the kernel's request is too large for the provided buffer
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and the request was
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.BR FUSE_SETXATTR .
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.TP
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.B EINVAL
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Returned from
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.BR write (2)
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if validation of the reply failed.
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Not all mistakes in replies will be caught by this validation.
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However, basic mistakes, such as short replies or an incorrect
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.I unique
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value, are detected.
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.TP
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.B EIO
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Returned from
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.BR read (2)
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operations when the kernel's request is too large for the provided buffer.
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.IP
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.IR Note :
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There are various ways in which incorrect use of these interfaces can cause
|
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operations on the provided filesystem's files and directories to fail with
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.BR EIO .
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Among the possible incorrect uses are:
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.RS
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.IP * 3
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changing
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.I mode & S_IFMT
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for an inode that has previously been reported to the kernel; or
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.IP *
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giving replies to the kernel that are shorter than what the kernel expected.
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.RE
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.TP
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.B ENODEV
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Returned from
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.BR read (2)
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and
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.BR write (2)
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if the FUSE filesystem was unmounted.
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.TP
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.B EPERM
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Returned from operations on a
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.I /dev/fuse
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file descriptor that has not been mounted.
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.SH CONFORMING TO
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The FUSE filesystem is Linux-specific.
|
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.SH NOTES
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|
The following messages are not yet documented in this manual page:
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.PP
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.\" FIXME: Document the following.
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.in +4n
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.EX
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.BR FUSE_BATCH_FORGET
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.BR FUSE_BMAP
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.BR FUSE_CREATE
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.BR FUSE_DESTROY
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.BR FUSE_FALLOCATE
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.BR FUSE_FORGET
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.BR FUSE_FSYNC
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.BR FUSE_FSYNCDIR
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.BR FUSE_GETLK
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.BR FUSE_GETXATTR
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.BR FUSE_IOCTL
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.BR FUSE_LINK
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.BR FUSE_LISTXATTR
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.BR FUSE_LSEEK
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.BR FUSE_MKDIR
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.BR FUSE_MKNOD
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.BR FUSE_NOTIFY_REPLY
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.BR FUSE_POLL
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.BR FUSE_READDIRPLUS
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.BR FUSE_READLINK
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.BR FUSE_REMOVEXATTR
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.BR FUSE_RENAME
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.BR FUSE_RENAME2
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.BR FUSE_RMDIR
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.BR FUSE_SETATTR
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.BR FUSE_SETLK
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.BR FUSE_SETLKW
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.BR FUSE_SYMLINK
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.BR FUSE_UNLINK
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.BR FUSE_WRITE
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.EE
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.in
|
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.SH SEE ALSO
|
|
.BR fusermount (1),
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.BR mount.fuse (8)
|