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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 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).
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. However, 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 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.
ENODEV Returned from read(2) and write(2) if the FUSE filesystem was unmounted.
EPERM Returned from operations on a /dev/fuse file descriptor that has not been mounted.
STANDARDS
The FUSE filesystem is Linux-specific.
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)