FUSE(4)                   Linux Programmer's Manual                  FUSE(4)

NAME         top

       fuse - Filesystem in Userspace (FUSE) device

SYNOPSIS         top

       #include <linux/fuse.h>

DESCRIPTION         top

       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 opera‐
       tion 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.


                  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

              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.


                  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 lat‐
              ter 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

              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;


                  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).

                  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.


                  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.

                  struct fuse_interrupt_in {
                      uint64_t unique;

              The requested action is to cancel the pending operation indi‐
              cated by unique.  This request requires no response.  However,
              receipt of this message does not by itself cancel the indi‐
              cated 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 uninter‐
              ruptibly for completion of the indicated request.

              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

                  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.

                  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 com‐
              pletely processed.

              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         top

       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

       ENODEV Returned from read(2) and write(2) if the FUSE filesystem was

CONFORMING TO         top

       The FUSE filesystem is Linux-specific.

NOTES         top

       The following messages are not yet documented in this manual page:

SEE ALSO         top

       fusermount(1), mount.fuse(8)

COLOPHON         top

       This page is part of release 4.13 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

Linux                            2017-09-15                          FUSE(4)

Pages that refer to this page: filesystems(5)