fuse(4) Kernel Interfaces Manual fuse(4)
NAME
fuse - Filesystem in Userspace (FUSE) device
SYNOPSIS
#include <linux/fuse.h>
DESCRIPTION
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 inter-
face 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 de-
scriptor (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 re-
ply (almost all operations require a reply; if they do not, this is doc-
umented 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 mes-
sages are documented. For each message, first the struct sent by the
kernel is given, followed by a description of the semantics of the mes-
sage.
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 ver-
sion.
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 re-
turned by stat(2) and similar operations for the given filesystem
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 re-
quest may be used for permissions checking. No reply data is ex-
pected, 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 de-
pend 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 di-
rectly 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 opera-
tion. The kernel will still expect a reply to said operation
(e.g., an EINTR error or a short read). At most one FUSE_INTER-
RUPT request will be issued for a given operation. After issuing
said operation, the kernel will wait uninterruptibly for comple-
tion 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 indi-
cated 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 respec-
tively. 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 re-
ply 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).
ERRORS
E2BIG Returned from read(2) operations when the kernel's request is too
large for the provided buffer and the request was FUSE_SETXATTR.
EINVAL Returned from write(2) if validation of the reply failed. Not
all mistakes in replies will be caught by this validation. How-
ever, basic mistakes, such as short replies or an incorrect
unique value, are detected.
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 in-
terfaces 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.
ENODEV Returned from read(2) and write(2) if the FUSE filesystem was un-
mounted.
EPERM Returned from operations on a /dev/fuse file descriptor that has
not been mounted.
STANDARDS
Linux.
NOTES
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
SEE ALSO
fusermount(1), mount.fuse(8)
Linux man-pages 6.9.1 2024-05-02 fuse(4)
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