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NAME | SYNOPSIS | DESCRIPTION | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
FUSE(4) Linux Programmer's Manual FUSE(4)
fuse - Filesystem in Userspace (FUSE) device
#include <linux/fuse.h>
This device is the primary interface between the FUSE filesystem
driver and a user-space process wishing to provide the filesystem
(referred to in the rest of this manual page as the filesystem
daemon). This manual page is intended for those interested in
understanding the kernel interface itself. Those implementing a FUSE
filesystem may wish to make use of a user-space library such as
libfuse that abstracts away the low-level interface.
At its core, FUSE is a simple client-server protocol, in which the
Linux kernel is the client and the daemon is the server. After
obtaining a file descriptor for this device, the daemon may read(2)
requests from that file descriptor and is expected to write(2) back
its replies. It is important to note that a file descriptor is
associated with a unique FUSE filesystem. In particular, opening a
second copy of this device, will not allow access to resources
created through the first file descriptor (and vice versa).
The basic protocol
Every message that is read by the daemon begins with a header
described by the following structure:
struct fuse_in_header {
uint32_t len; /* Total length of the data,
including this header */
uint32_t opcode; /* The kind of operation (see below) */
uint64_t unique; /* A unique identifier for this request */
uint64_t nodeid; /* ID of the filesystem object
being operated on */
uint32_t uid; /* UID of the requesting process */
uint32_t gid; /* GID of the requesting process */
uint32_t pid; /* PID of the requesting process */
uint32_t padding;
};
The header is followed by a variable-length data portion (which may
be empty) specific to the requested operation (the requested
operation is indicated by opcode).
The daemon should then process the request and if applicable send a
reply (almost all operations require a reply; if they do not, this is
documented below), by performing a write(2) to the file descriptor.
All replies must start with the following header:
struct fuse_out_header {
uint32_t len; /* Total length of data written to
the file descriptor */
int32_t error; /* Any error that occurred (0 if none) */
uint64_t unique; /* The value from the
corresponding request */
};
This header is also followed by (potentially empty) variable-sized
data depending on the executed request. However, if the reply is an
error reply (i.e., error is set), then no further payload data should
be sent, independent of the request.
Exchanged messages
This section should contain documentation for each of the messages in
the protocol. This manual page is currently incomplete, so not all
messages are documented. For each message, first the struct sent by
the kernel is given, followed by a description of the semantics of
the message.
FUSE_INIT
struct fuse_init_in {
uint32_t major;
uint32_t minor;
uint32_t max_readahead; /* Since protocol v7.6 */
uint32_t flags; /* Since protocol v7.6 */
};
This is the first request sent by the kernel to the daemon.
It is used to negotiate the protocol version and other
filesystem parameters. Note that the protocol version may
affect the layout of any structure in the protocol (including
this structure). The daemon must thus remember the negotiated
version and flags for each session. As of the writing of this
man page, the highest supported kernel protocol version is
7.26.
Users should be aware that the descriptions in this manual
page may be incomplete or incorrect for older or more recent
protocol versions.
The reply for this request has the following format:
struct fuse_init_out {
uint32_t major;
uint32_t minor;
uint32_t max_readahead; /* Since v7.6 */
uint32_t flags; /* Since v7.6; some flags bits
were introduced later */
uint16_t max_background; /* Since v7.13 */
uint16_t congestion_threshold; /* Since v7.13 */
uint32_t max_write; /* Since v7.5 */
uint32_t time_gran; /* Since v7.6 */
uint32_t unused[9];
};
If the major version supported by the kernel is larger than
that supported by the daemon, the reply shall consist of only
uint32_t major (following the usual header), indicating the
largest major version supported by the daemon. The kernel
will then issue a new FUSE_INIT request conforming to the
older version. In the reverse case, the daemon should quietly
fall back to the kernel's major version.
The negotiated minor version is considered to be the minimum
of the minor versions provided by the daemon and the kernel
and both parties should use the protocol corresponding to said
minor version.
FUSE_GETATTR
struct fuse_getattr_in {
uint32_t getattr_flags;
uint32_t dummy;
uint64_t fh; /* Set only if
(getattr_flags & FUSE_GETATTR_FH)
};
The requested operation is to compute the attributes to be
returned by stat(2) and similar operations for the given file
system object. The object for which the attributes should be
computed is indicated either by header->nodeid or, if the
FUSE_GETATTR_FH flag is set, by the file handle fh. The
latter case of operation is analogous to fstat(2).
For performance reasons, these attributes may be cached in the
kernel for a specified duration of time. While the cache
timeout has not been exceeded, the attributes will be served
from the cache and will not cause additional FUSE_GETATTR
requests.
The computed attributes and the requested cache timeout should
then be returned in the following structure:
struct fuse_attr_out {
/* Attribute cache duration (seconds + nanoseconds) */
uint64_t attr_valid;
uint32_t attr_valid_nsec;
uint32_t dummy;
struct fuse_attr {
uint64_t ino;
uint64_t size;
uint64_t blocks;
uint64_t atime;
uint64_t mtime;
uint64_t ctime;
uint32_t atimensec;
uint32_t mtimensec;
uint32_t ctimensec;
uint32_t mode;
uint32_t nlink;
uint32_t uid;
uint32_t gid;
uint32_t rdev;
uint32_t blksize;
uint32_t padding;
} attr;
};
FUSE_ACCESS
struct fuse_access_in {
uint32_t mask;
uint32_t padding;
};
If the default_permissions mount options is not used, this
request may be used for permissions checking. No reply data
is expected, but errors may be indicated as usual by setting
the error field in the reply header (in particular, access
denied errors may be indicated by returning -EACCES).
FUSE_OPEN and FUSE_OPENDIR
struct fuse_open_in {
uint32_t flags; /* The flags that were passed
to the open(2) */
uint32_t unused;
};
The requested operation is to open the node indicated by
header->nodeid. The exact semantics of what this means will
depend on the filesystem being implemented. However, at the
very least the filesystem should validate that the requested
flags are valid for the indicated resource and then send a
reply with the following format:
struct fuse_open_out {
uint64_t fh;
uint32_t open_flags;
uint32_t padding;
};
The fh field is an opaque identifier that the kernel will use
to refer to this resource The open_flags field is a bit mask
of any number of the flags that indicate properties of this
file handle to the kernel:
FOPEN_DIRECT_IO Bypass page cache for this open file.
FOPEN_KEEP_CACHE Don't invalidate the data cache on open.
FOPEN_NONSEEKABLE The file is not seekable.
FUSE_READ and FUSE_READDIR
struct fuse_read_in {
uint64_t fh;
uint64_t offset;
uint32_t size;
uint32_t read_flags;
uint64_t lock_owner;
uint32_t flags;
uint32_t padding;
};
The requested action is to read up to size bytes of the file
or directory, starting at offset. The bytes should be
returned directly following the usual reply header.
FUSE_INTERRUPT
struct fuse_interrupt_in {
uint64_t unique;
};
The requested action is to cancel the pending operation
indicated by unique. This request requires no response.
However, receipt of this message does not by itself cancel the
indicated operation. The kernel will still expect a reply to
said operation (e.g., an EINTR error or a short read). At
most one FUSE_INTERRUPT request will be issued for a given
operation. After issuing said operation, the kernel will wait
uninterruptibly for completion of the indicated request.
FUSE_LOOKUP
Directly following the header is a filename to be looked up in
the directory indicated by header->nodeid. The expected reply
is of the form:
struct fuse_entry_out {
uint64_t nodeid; /* Inode ID */
uint64_t generation; /* Inode generation */
uint64_t entry_valid;
uint64_t attr_valid;
uint32_t entry_valid_nsec;
uint32_t attr_valid_nsec;
struct fuse_attr attr;
};
The combination of nodeid and generation must be unique for
the filesystem's lifetime.
The interpretation of timeouts and attr is as for
FUSE_GETATTR.
FUSE_FLUSH
struct fuse_flush_in {
uint64_t fh;
uint32_t unused;
uint32_t padding;
uint64_t lock_owner;
};
The requested action is to flush any pending changes to the
indicated file handle. No reply data is expected. However,
an empty reply message still needs to be issued once the flush
operation is complete.
FUSE_RELEASE and FUSE_RELEASEDIR
struct fuse_release_in {
uint64_t fh;
uint32_t flags;
uint32_t release_flags;
uint64_t lock_owner;
};
These are the converse of FUSE_OPEN and FUSE_OPENDIR
respectively. The daemon may now free any resources
associated with the file handle fh as the kernel will no
longer refer to it. There is no reply data associated with
this request, but a reply still needs to be issued once the
request has been completely processed.
FUSE_STATFS
This operation implements statfs(2) for this filesystem.
There is no input data associated with this request. The
expected reply data has the following structure:
struct fuse_kstatfs {
uint64_t blocks;
uint64_t bfree;
uint64_t bavail;
uint64_t files;
uint64_t ffree;
uint32_t bsize;
uint32_t namelen;
uint32_t frsize;
uint32_t padding;
uint32_t spare[6];
};
struct fuse_statfs_out {
struct fuse_kstatfs st;
};
For the interpretation of these fields, see statfs(2).
EPERM Returned from operations on a /dev/fuse file descriptor that
has not been mounted.
EIO Returned from read(2) operations when the kernel's request is
too large for the provided buffer.
Note: There are various ways in which incorrect use of these
interfaces can cause operations on the provided filesystem's
files and directories to fail with EIO. Among the possible
incorrect uses are:
* changing mode & S_IFMT for an inode that has previously
been reported to the kernel; or
* giving replies to the kernel that are shorter than what the
kernel expected.
EINVAL Returned from write(2) if validation of the reply failed. Not
all mistakes in replies will be caught by this validation.
However, basic mistakes, such as short replies or an incorrect
unique value, are detected.
E2BIG Returned from read(2) operations when the kernel's request is
too large for the provided buffer and the request was
FUSE_SETXATTR.
ENODEV Returned from read(2) and write(2) if the FUSE filesystem was
unmounted.
The FUSE filesystem is Linux-specific.
The following messages are not yet documented in this manual page:
FUSE_BATCH_FORGET
FUSE_BMAP
FUSE_CREATE
FUSE_DESTROY
FUSE_FALLOCATE
FUSE_FORGET
FUSE_FSYNC
FUSE_FSYNCDIR
FUSE_GETLK
FUSE_GETXATTR
FUSE_IOCTL
FUSE_LINK
FUSE_LISTXATTR
FUSE_LSEEK
FUSE_MKDIR
FUSE_MKNOD
FUSE_NOTIFY_REPLY
FUSE_POLL
FUSE_READDIRPLUS
FUSE_READLINK
FUSE_REMOVEXATTR
FUSE_RENAME
FUSE_RENAME2
FUSE_RMDIR
FUSE_SETATTR
FUSE_SETLK
FUSE_SETLKW
FUSE_SYMLINK
FUSE_UNLINK
FUSE_WRITE
fusermount(1), mount.fuse(8)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-12-10 FUSE(4)
Pages that refer to this page: filesystems(5)