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          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 pro-
          vide 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 impor-
          tant 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 cre-
          ated 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)

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          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.  How-
          ever, 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, fol-
          lowed 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 proto-
               col 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 */

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                       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 con-
               sist 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 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 addi-
               tional 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;

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                       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 vali-
               date that the requested flags are valid for the indi-
               cated resource and then send a reply with the following

                   struct fuse_open_out {

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                       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

                 FOPEN_KEEP_CACHE  Don't invalidate the data cache on

                 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
               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.

               Directly following the header is a filename to be
               looked up in the directory indicated by header->nodeid.

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               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
               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 associ-
               ated with this request, but a reply still needs to be
               issued once the request has been completely processed.

               This operation implements statfs(2) for this filesys-
               tem.  There is no input data associated with this
               request.  The expected reply data has the following

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                   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).

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

               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

          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 previ-
                  ously been reported to the kernel; or

               *  giving replies to the kernel that are shorter than
                  what the kernel expected.

               Returned from read(2) and write(2) if the FUSE filesys-
               tem was unmounted.

               Returned from operations on a /dev/fuse file descriptor

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               that has not been mounted.

          The FUSE filesystem is Linux-specific.

          The following messages are not yet documented in this manual


          fusermount(1), mount.fuse(8)

          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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