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
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
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).
