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 obtain-
       ing  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


                  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 lay-
              out 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 high-
              est 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

              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 ver-
              sion.  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


                  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  time-
              out  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;


                  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 indicated
              operation.  The kernel will still expect a reply to said  opera-
              tion  (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 uninterrupt-
              ibly 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 FUSE_GETATTR.

                  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  completely

              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).

       E2BIG  Returned  from  read(2)  operations when the kernel's request is
              too large for the provided buffer and the request was FUSE_SETX-

       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
              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

       EPERM  Returned from operations on a /dev/fuse file descriptor that has
              not been mounted.

       The FUSE filesystem is Linux-specific.

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


       fusermount(1), mount.fuse(8)

       This page is part of release 4.15 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                             2018-02-02                           FUSE(4)
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