Merge remote-tracking branch 'remotes/dgilbert-gitlab/tags/pull-virtiofs-20200123b' into staging

virtiofsd first pull v2

Import our virtiofsd.
This pulls in the daemon to drive a file system connected to the
existing qemu virtiofsd device.
It's derived from upstream libfuse with lots of changes (and a lot
trimmed out).
The daemon lives in the newly created qemu/tools/virtiofsd

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

v2
  drop the docs while we discuss where they should live
  and we need to redo the manpage in anything but texi

# gpg: Signature made Thu 23 Jan 2020 16:45:18 GMT
# gpg:                using RSA key 45F5C71B4A0CB7FB977A9FA90516331EBC5BFDE7
# gpg: Good signature from "Dr. David Alan Gilbert (RH2) <dgilbert@redhat.com>" [full]
# Primary key fingerprint: 45F5 C71B 4A0C B7FB 977A  9FA9 0516 331E BC5B FDE7

* remotes/dgilbert-gitlab/tags/pull-virtiofs-20200123b: (108 commits)
  virtiofsd: add some options to the help message
  virtiofsd: stop all queue threads on exit in virtio_loop()
  virtiofsd/passthrough_ll: Pass errno to fuse_reply_err()
  virtiofsd: Convert lo_destroy to take the lo->mutex lock itself
  virtiofsd: add --thread-pool-size=NUM option
  virtiofsd: fix lo_destroy() resource leaks
  virtiofsd: prevent FUSE_INIT/FUSE_DESTROY races
  virtiofsd: process requests in a thread pool
  virtiofsd: use fuse_buf_writev to replace fuse_buf_write for better performance
  virtiofsd: add definition of fuse_buf_writev()
  virtiofsd: passthrough_ll: Use cache_readdir for directory open
  virtiofsd: Fix data corruption with O_APPEND write in writeback mode
  virtiofsd: Reset O_DIRECT flag during file open
  virtiofsd: convert more fprintf and perror to use fuse log infra
  virtiofsd: do not always set FUSE_FLOCK_LOCKS
  virtiofsd: introduce inode refcount to prevent use-after-free
  virtiofsd: passthrough_ll: fix refcounting on remove/rename
  libvhost-user: Fix some memtable remap cases
  virtiofsd: rename inode->refcount to inode->nlookup
  virtiofsd: prevent races with lo_dirp_put()
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 1991 insertions, 0 deletions

diff --git a/tools/virtiofsd/fuse_lowlevel.h b/tools/virtiofsd/fuse_lowlevel.h
new file mode 100644
index 0000000000..138041e5f1
--- /dev/null
+++ b/tools/virtiofsd/fuse_lowlevel.h
@@ -0,0 +1,1991 @@
+/*
+ * FUSE: Filesystem in Userspace
+ * Copyright (C) 2001-2007  Miklos Szeredi <miklos@szeredi.hu>
+ *
+ * This program can be distributed under the terms of the GNU LGPLv2.
+ * See the file COPYING.LIB.
+ */
+
+#ifndef FUSE_LOWLEVEL_H_
+#define FUSE_LOWLEVEL_H_
+
+/**
+ * @file
+ *
+ * Low level API
+ *
+ * IMPORTANT: you should define FUSE_USE_VERSION before including this
+ * header.  To use the newest API define it to 31 (recommended for any
+ * new application).
+ */
+
+#ifndef FUSE_USE_VERSION
+#error FUSE_USE_VERSION not defined
+#endif
+
+#include "fuse_common.h"
+
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/statvfs.h>
+#include <sys/types.h>
+#include <sys/uio.h>
+#include <utime.h>
+
+/*
+ * Miscellaneous definitions
+ */
+
+/** The node ID of the root inode */
+#define FUSE_ROOT_ID 1
+
+/** Inode number type */
+typedef uint64_t fuse_ino_t;
+
+/** Request pointer type */
+typedef struct fuse_req *fuse_req_t;
+
+/**
+ * Session
+ *
+ * This provides hooks for processing requests, and exiting
+ */
+struct fuse_session;
+
+/** Directory entry parameters supplied to fuse_reply_entry() */
+struct fuse_entry_param {
+    /**
+     * Unique inode number
+     *
+     * In lookup, zero means negative entry (from version 2.5)
+     * Returning ENOENT also means negative entry, but by setting zero
+     * ino the kernel may cache negative entries for entry_timeout
+     * seconds.
+     */
+    fuse_ino_t ino;
+
+    /**
+     * Generation number for this entry.
+     *
+     * If the file system will be exported over NFS, the
+     * ino/generation pairs need to be unique over the file
+     * system's lifetime (rather than just the mount time). So if
+     * the file system reuses an inode after it has been deleted,
+     * it must assign a new, previously unused generation number
+     * to the inode at the same time.
+     *
+     */
+    uint64_t generation;
+
+    /**
+     * Inode attributes.
+     *
+     * Even if attr_timeout == 0, attr must be correct. For example,
+     * for open(), FUSE uses attr.st_size from lookup() to determine
+     * how many bytes to request. If this value is not correct,
+     * incorrect data will be returned.
+     */
+    struct stat attr;
+
+    /**
+     * Validity timeout (in seconds) for inode attributes. If
+     *  attributes only change as a result of requests that come
+     *  through the kernel, this should be set to a very large
+     *  value.
+     */
+    double attr_timeout;
+
+    /**
+     * Validity timeout (in seconds) for the name. If directory
+     *  entries are changed/deleted only as a result of requests
+     *  that come through the kernel, this should be set to a very
+     *  large value.
+     */
+    double entry_timeout;
+};
+
+/**
+ * Additional context associated with requests.
+ *
+ * Note that the reported client uid, gid and pid may be zero in some
+ * situations. For example, if the FUSE file system is running in a
+ * PID or user namespace but then accessed from outside the namespace,
+ * there is no valid uid/pid/gid that could be reported.
+ */
+struct fuse_ctx {
+    /** User ID of the calling process */
+    uid_t uid;
+
+    /** Group ID of the calling process */
+    gid_t gid;
+
+    /** Thread ID of the calling process */
+    pid_t pid;
+
+    /** Umask of the calling process */
+    mode_t umask;
+};
+
+struct fuse_forget_data {
+    fuse_ino_t ino;
+    uint64_t nlookup;
+};
+
+/* 'to_set' flags in setattr */
+#define FUSE_SET_ATTR_MODE (1 << 0)
+#define FUSE_SET_ATTR_UID (1 << 1)
+#define FUSE_SET_ATTR_GID (1 << 2)
+#define FUSE_SET_ATTR_SIZE (1 << 3)
+#define FUSE_SET_ATTR_ATIME (1 << 4)
+#define FUSE_SET_ATTR_MTIME (1 << 5)
+#define FUSE_SET_ATTR_ATIME_NOW (1 << 7)
+#define FUSE_SET_ATTR_MTIME_NOW (1 << 8)
+#define FUSE_SET_ATTR_CTIME (1 << 10)
+
+/*
+ * Request methods and replies
+ */
+
+/**
+ * Low level filesystem operations
+ *
+ * Most of the methods (with the exception of init and destroy)
+ * receive a request handle (fuse_req_t) as their first argument.
+ * This handle must be passed to one of the specified reply functions.
+ *
+ * This may be done inside the method invocation, or after the call
+ * has returned.  The request handle is valid until one of the reply
+ * functions is called.
+ *
+ * Other pointer arguments (name, fuse_file_info, etc) are not valid
+ * after the call has returned, so if they are needed later, their
+ * contents have to be copied.
+ *
+ * In general, all methods are expected to perform any necessary
+ * permission checking. However, a filesystem may delegate this task
+ * to the kernel by passing the `default_permissions` mount option to
+ * `fuse_session_new()`. In this case, methods will only be called if
+ * the kernel's permission check has succeeded.
+ *
+ * The filesystem sometimes needs to handle a return value of -ENOENT
+ * from the reply function, which means, that the request was
+ * interrupted, and the reply discarded.  For example if
+ * fuse_reply_open() return -ENOENT means, that the release method for
+ * this file will not be called.
+ */
+struct fuse_lowlevel_ops {
+    /**
+     * Initialize filesystem
+     *
+     * This function is called when libfuse establishes
+     * communication with the FUSE kernel module. The file system
+     * should use this module to inspect and/or modify the
+     * connection parameters provided in the `conn` structure.
+     *
+     * Note that some parameters may be overwritten by options
+     * passed to fuse_session_new() which take precedence over the
+     * values set in this handler.
+     *
+     * There's no reply to this function
+     *
+     * @param userdata the user data passed to fuse_session_new()
+     */
+    void (*init)(void *userdata, struct fuse_conn_info *conn);
+
+    /**
+     * Clean up filesystem.
+     *
+     * Called on filesystem exit. When this method is called, the
+     * connection to the kernel may be gone already, so that eg. calls
+     * to fuse_lowlevel_notify_* will fail.
+     *
+     * There's no reply to this function
+     *
+     * @param userdata the user data passed to fuse_session_new()
+     */
+    void (*destroy)(void *userdata);
+
+    /**
+     * Look up a directory entry by name and get its attributes.
+     *
+     * Valid replies:
+     *   fuse_reply_entry
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param parent inode number of the parent directory
+     * @param name the name to look up
+     */
+    void (*lookup)(fuse_req_t req, fuse_ino_t parent, const char *name);
+
+    /**
+     * Forget about an inode
+     *
+     * This function is called when the kernel removes an inode
+     * from its internal caches.
+     *
+     * The inode's lookup count increases by one for every call to
+     * fuse_reply_entry and fuse_reply_create. The nlookup parameter
+     * indicates by how much the lookup count should be decreased.
+     *
+     * Inodes with a non-zero lookup count may receive request from
+     * the kernel even after calls to unlink, rmdir or (when
+     * overwriting an existing file) rename. Filesystems must handle
+     * such requests properly and it is recommended to defer removal
+     * of the inode until the lookup count reaches zero. Calls to
+     * unlink, rmdir or rename will be followed closely by forget
+     * unless the file or directory is open, in which case the
+     * kernel issues forget only after the release or releasedir
+     * calls.
+     *
+     * Note that if a file system will be exported over NFS the
+     * inodes lifetime must extend even beyond forget. See the
+     * generation field in struct fuse_entry_param above.
+     *
+     * On unmount the lookup count for all inodes implicitly drops
+     * to zero. It is not guaranteed that the file system will
+     * receive corresponding forget messages for the affected
+     * inodes.
+     *
+     * Valid replies:
+     *   fuse_reply_none
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param nlookup the number of lookups to forget
+     */
+    void (*forget)(fuse_req_t req, fuse_ino_t ino, uint64_t nlookup);
+
+    /**
+     * Get file attributes.
+     *
+     * If writeback caching is enabled, the kernel may have a
+     * better idea of a file's length than the FUSE file system
+     * (eg if there has been a write that extended the file size,
+     * but that has not yet been passed to the filesystem.n
+     *
+     * In this case, the st_size value provided by the file system
+     * will be ignored.
+     *
+     * Valid replies:
+     *   fuse_reply_attr
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi for future use, currently always NULL
+     */
+    void (*getattr)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi);
+
+    /**
+     * Set file attributes
+     *
+     * In the 'attr' argument only members indicated by the 'to_set'
+     * bitmask contain valid values.  Other members contain undefined
+     * values.
+     *
+     * Unless FUSE_CAP_HANDLE_KILLPRIV is disabled, this method is
+     * expected to reset the setuid and setgid bits if the file
+     * size or owner is being changed.
+     *
+     * If the setattr was invoked from the ftruncate() system call
+     * under Linux kernel versions 2.6.15 or later, the fi->fh will
+     * contain the value set by the open method or will be undefined
+     * if the open method didn't set any value.  Otherwise (not
+     * ftruncate call, or kernel version earlier than 2.6.15) the fi
+     * parameter will be NULL.
+     *
+     * Valid replies:
+     *   fuse_reply_attr
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param attr the attributes
+     * @param to_set bit mask of attributes which should be set
+     * @param fi file information, or NULL
+     */
+    void (*setattr)(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
+                    int to_set, struct fuse_file_info *fi);
+
+    /**
+     * Read symbolic link
+     *
+     * Valid replies:
+     *   fuse_reply_readlink
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     */
+    void (*readlink)(fuse_req_t req, fuse_ino_t ino);
+
+    /**
+     * Create file node
+     *
+     * Create a regular file, character device, block device, fifo or
+     * socket node.
+     *
+     * Valid replies:
+     *   fuse_reply_entry
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param parent inode number of the parent directory
+     * @param name to create
+     * @param mode file type and mode with which to create the new file
+     * @param rdev the device number (only valid if created file is a device)
+     */
+    void (*mknod)(fuse_req_t req, fuse_ino_t parent, const char *name,
+                  mode_t mode, dev_t rdev);
+
+    /**
+     * Create a directory
+     *
+     * Valid replies:
+     *   fuse_reply_entry
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param parent inode number of the parent directory
+     * @param name to create
+     * @param mode with which to create the new file
+     */
+    void (*mkdir)(fuse_req_t req, fuse_ino_t parent, const char *name,
+                  mode_t mode);
+
+    /**
+     * Remove a file
+     *
+     * If the file's inode's lookup count is non-zero, the file
+     * system is expected to postpone any removal of the inode
+     * until the lookup count reaches zero (see description of the
+     * forget function).
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param parent inode number of the parent directory
+     * @param name to remove
+     */
+    void (*unlink)(fuse_req_t req, fuse_ino_t parent, const char *name);
+
+    /**
+     * Remove a directory
+     *
+     * If the directory's inode's lookup count is non-zero, the
+     * file system is expected to postpone any removal of the
+     * inode until the lookup count reaches zero (see description
+     * of the forget function).
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param parent inode number of the parent directory
+     * @param name to remove
+     */
+    void (*rmdir)(fuse_req_t req, fuse_ino_t parent, const char *name);
+
+    /**
+     * Create a symbolic link
+     *
+     * Valid replies:
+     *   fuse_reply_entry
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param link the contents of the symbolic link
+     * @param parent inode number of the parent directory
+     * @param name to create
+     */
+    void (*symlink)(fuse_req_t req, const char *link, fuse_ino_t parent,
+                    const char *name);
+
+    /**
+     * Rename a file
+     *
+     * If the target exists it should be atomically replaced. If
+     * the target's inode's lookup count is non-zero, the file
+     * system is expected to postpone any removal of the inode
+     * until the lookup count reaches zero (see description of the
+     * forget function).
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure with error code EINVAL, i.e. all
+     * future bmap requests will fail with EINVAL without being
+     * send to the filesystem process.
+     *
+     * *flags* may be `RENAME_EXCHANGE` or `RENAME_NOREPLACE`. If
+     * RENAME_NOREPLACE is specified, the filesystem must not
+     * overwrite *newname* if it exists and return an error
+     * instead. If `RENAME_EXCHANGE` is specified, the filesystem
+     * must atomically exchange the two files, i.e. both must
+     * exist and neither may be deleted.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param parent inode number of the old parent directory
+     * @param name old name
+     * @param newparent inode number of the new parent directory
+     * @param newname new name
+     */
+    void (*rename)(fuse_req_t req, fuse_ino_t parent, const char *name,
+                   fuse_ino_t newparent, const char *newname,
+                   unsigned int flags);
+
+    /**
+     * Create a hard link
+     *
+     * Valid replies:
+     *   fuse_reply_entry
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the old inode number
+     * @param newparent inode number of the new parent directory
+     * @param newname new name to create
+     */
+    void (*link)(fuse_req_t req, fuse_ino_t ino, fuse_ino_t newparent,
+                 const char *newname);
+
+    /**
+     * Open a file
+     *
+     * Open flags are available in fi->flags. The following rules
+     * apply.
+     *
+     *  - Creation (O_CREAT, O_EXCL, O_NOCTTY) flags will be
+     *    filtered out / handled by the kernel.
+     *
+     *  - Access modes (O_RDONLY, O_WRONLY, O_RDWR) should be used
+     *    by the filesystem to check if the operation is
+     *    permitted.  If the ``-o default_permissions`` mount
+     *    option is given, this check is already done by the
+     *    kernel before calling open() and may thus be omitted by
+     *    the filesystem.
+     *
+     *  - When writeback caching is enabled, the kernel may send
+     *    read requests even for files opened with O_WRONLY. The
+     *    filesystem should be prepared to handle this.
+     *
+     *  - When writeback caching is disabled, the filesystem is
+     *    expected to properly handle the O_APPEND flag and ensure
+     *    that each write is appending to the end of the file.
+     *
+     *  - When writeback caching is enabled, the kernel will
+     *    handle O_APPEND. However, unless all changes to the file
+     *    come through the kernel this will not work reliably. The
+     *    filesystem should thus either ignore the O_APPEND flag
+     *    (and let the kernel handle it), or return an error
+     *    (indicating that reliably O_APPEND is not available).
+     *
+     * Filesystem may store an arbitrary file handle (pointer,
+     * index, etc) in fi->fh, and use this in other all other file
+     * operations (read, write, flush, release, fsync).
+     *
+     * Filesystem may also implement stateless file I/O and not store
+     * anything in fi->fh.
+     *
+     * There are also some flags (direct_io, keep_cache) which the
+     * filesystem may set in fi, to change the way the file is opened.
+     * See fuse_file_info structure in <fuse_common.h> for more details.
+     *
+     * If this request is answered with an error code of ENOSYS
+     * and FUSE_CAP_NO_OPEN_SUPPORT is set in
+     * `fuse_conn_info.capable`, this is treated as success and
+     * future calls to open and release will also succeed without being
+     * sent to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_open
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     */
+    void (*open)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi);
+
+    /**
+     * Read data
+     *
+     * Read should send exactly the number of bytes requested except
+     * on EOF or error, otherwise the rest of the data will be
+     * substituted with zeroes.  An exception to this is when the file
+     * has been opened in 'direct_io' mode, in which case the return
+     * value of the read system call will reflect the return value of
+     * this operation.
+     *
+     * fi->fh will contain the value set by the open method, or will
+     * be undefined if the open method didn't set any value.
+     *
+     * Valid replies:
+     *   fuse_reply_buf
+     *   fuse_reply_iov
+     *   fuse_reply_data
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param size number of bytes to read
+     * @param off offset to read from
+     * @param fi file information
+     */
+    void (*read)(fuse_req_t req, fuse_ino_t ino, size_t size, off_t off,
+                 struct fuse_file_info *fi);
+
+    /**
+     * Write data
+     *
+     * Write should return exactly the number of bytes requested
+     * except on error.  An exception to this is when the file has
+     * been opened in 'direct_io' mode, in which case the return value
+     * of the write system call will reflect the return value of this
+     * operation.
+     *
+     * Unless FUSE_CAP_HANDLE_KILLPRIV is disabled, this method is
+     * expected to reset the setuid and setgid bits.
+     *
+     * fi->fh will contain the value set by the open method, or will
+     * be undefined if the open method didn't set any value.
+     *
+     * Valid replies:
+     *   fuse_reply_write
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param buf data to write
+     * @param size number of bytes to write
+     * @param off offset to write to
+     * @param fi file information
+     */
+    void (*write)(fuse_req_t req, fuse_ino_t ino, const char *buf, size_t size,
+                  off_t off, struct fuse_file_info *fi);
+
+    /**
+     * Flush method
+     *
+     * This is called on each close() of the opened file.
+     *
+     * Since file descriptors can be duplicated (dup, dup2, fork), for
+     * one open call there may be many flush calls.
+     *
+     * Filesystems shouldn't assume that flush will always be called
+     * after some writes, or that if will be called at all.
+     *
+     * fi->fh will contain the value set by the open method, or will
+     * be undefined if the open method didn't set any value.
+     *
+     * NOTE: the name of the method is misleading, since (unlike
+     * fsync) the filesystem is not forced to flush pending writes.
+     * One reason to flush data is if the filesystem wants to return
+     * write errors during close.  However, such use is non-portable
+     * because POSIX does not require [close] to wait for delayed I/O to
+     * complete.
+     *
+     * If the filesystem supports file locking operations (setlk,
+     * getlk) it should remove all locks belonging to 'fi->owner'.
+     *
+     * If this request is answered with an error code of ENOSYS,
+     * this is treated as success and future calls to flush() will
+     * succeed automatically without being send to the filesystem
+     * process.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     *
+     * [close]:
+     * http://pubs.opengroup.org/onlinepubs/9699919799/functions/close.html
+     */
+    void (*flush)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi);
+
+    /**
+     * Release an open file
+     *
+     * Release is called when there are no more references to an open
+     * file: all file descriptors are closed and all memory mappings
+     * are unmapped.
+     *
+     * For every open call there will be exactly one release call (unless
+     * the filesystem is force-unmounted).
+     *
+     * The filesystem may reply with an error, but error values are
+     * not returned to close() or munmap() which triggered the
+     * release.
+     *
+     * fi->fh will contain the value set by the open method, or will
+     * be undefined if the open method didn't set any value.
+     * fi->flags will contain the same flags as for open.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     */
+    void (*release)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi);
+
+    /**
+     * Synchronize file contents
+     *
+     * If the datasync parameter is non-zero, then only the user data
+     * should be flushed, not the meta data.
+     *
+     * If this request is answered with an error code of ENOSYS,
+     * this is treated as success and future calls to fsync() will
+     * succeed automatically without being send to the filesystem
+     * process.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param datasync flag indicating if only data should be flushed
+     * @param fi file information
+     */
+    void (*fsync)(fuse_req_t req, fuse_ino_t ino, int datasync,
+                  struct fuse_file_info *fi);
+
+    /**
+     * Open a directory
+     *
+     * Filesystem may store an arbitrary file handle (pointer, index,
+     * etc) in fi->fh, and use this in other all other directory
+     * stream operations (readdir, releasedir, fsyncdir).
+     *
+     * If this request is answered with an error code of ENOSYS and
+     * FUSE_CAP_NO_OPENDIR_SUPPORT is set in `fuse_conn_info.capable`,
+     * this is treated as success and future calls to opendir and
+     * releasedir will also succeed without being sent to the filesystem
+     * process. In addition, the kernel will cache readdir results
+     * as if opendir returned FOPEN_KEEP_CACHE | FOPEN_CACHE_DIR.
+     *
+     * Valid replies:
+     *   fuse_reply_open
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     */
+    void (*opendir)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi);
+
+    /**
+     * Read directory
+     *
+     * Send a buffer filled using fuse_add_direntry(), with size not
+     * exceeding the requested size.  Send an empty buffer on end of
+     * stream.
+     *
+     * fi->fh will contain the value set by the opendir method, or
+     * will be undefined if the opendir method didn't set any value.
+     *
+     * Returning a directory entry from readdir() does not affect
+     * its lookup count.
+     *
+     * If off_t is non-zero, then it will correspond to one of the off_t
+     * values that was previously returned by readdir() for the same
+     * directory handle. In this case, readdir() should skip over entries
+     * coming before the position defined by the off_t value. If entries
+     * are added or removed while the directory handle is open, they filesystem
+     * may still include the entries that have been removed, and may not
+     * report the entries that have been created. However, addition or
+     * removal of entries must never cause readdir() to skip over unrelated
+     * entries or to report them more than once. This means
+     * that off_t can not be a simple index that enumerates the entries
+     * that have been returned but must contain sufficient information to
+     * uniquely determine the next directory entry to return even when the
+     * set of entries is changing.
+     *
+     * The function does not have to report the '.' and '..'
+     * entries, but is allowed to do so. Note that, if readdir does
+     * not return '.' or '..', they will not be implicitly returned,
+     * and this behavior is observable by the caller.
+     *
+     * Valid replies:
+     *   fuse_reply_buf
+     *   fuse_reply_data
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param size maximum number of bytes to send
+     * @param off offset to continue reading the directory stream
+     * @param fi file information
+     */
+    void (*readdir)(fuse_req_t req, fuse_ino_t ino, size_t size, off_t off,
+                    struct fuse_file_info *fi);
+
+    /**
+     * Release an open directory
+     *
+     * For every opendir call there will be exactly one releasedir
+     * call (unless the filesystem is force-unmounted).
+     *
+     * fi->fh will contain the value set by the opendir method, or
+     * will be undefined if the opendir method didn't set any value.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     */
+    void (*releasedir)(fuse_req_t req, fuse_ino_t ino,
+                       struct fuse_file_info *fi);
+
+    /**
+     * Synchronize directory contents
+     *
+     * If the datasync parameter is non-zero, then only the directory
+     * contents should be flushed, not the meta data.
+     *
+     * fi->fh will contain the value set by the opendir method, or
+     * will be undefined if the opendir method didn't set any value.
+     *
+     * If this request is answered with an error code of ENOSYS,
+     * this is treated as success and future calls to fsyncdir() will
+     * succeed automatically without being send to the filesystem
+     * process.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param datasync flag indicating if only data should be flushed
+     * @param fi file information
+     */
+    void (*fsyncdir)(fuse_req_t req, fuse_ino_t ino, int datasync,
+                     struct fuse_file_info *fi);
+
+    /**
+     * Get file system statistics
+     *
+     * Valid replies:
+     *   fuse_reply_statfs
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number, zero means "undefined"
+     */
+    void (*statfs)(fuse_req_t req, fuse_ino_t ino);
+
+    /**
+     * Set an extended attribute
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure with error code EOPNOTSUPP, i.e. all
+     * future setxattr() requests will fail with EOPNOTSUPP without being
+     * send to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     */
+    void (*setxattr)(fuse_req_t req, fuse_ino_t ino, const char *name,
+                     const char *value, size_t size, int flags);
+
+    /**
+     * Get an extended attribute
+     *
+     * If size is zero, the size of the value should be sent with
+     * fuse_reply_xattr.
+     *
+     * If the size is non-zero, and the value fits in the buffer, the
+     * value should be sent with fuse_reply_buf.
+     *
+     * If the size is too small for the value, the ERANGE error should
+     * be sent.
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure with error code EOPNOTSUPP, i.e. all
+     * future getxattr() requests will fail with EOPNOTSUPP without being
+     * send to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_buf
+     *   fuse_reply_data
+     *   fuse_reply_xattr
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param name of the extended attribute
+     * @param size maximum size of the value to send
+     */
+    void (*getxattr)(fuse_req_t req, fuse_ino_t ino, const char *name,
+                     size_t size);
+
+    /**
+     * List extended attribute names
+     *
+     * If size is zero, the total size of the attribute list should be
+     * sent with fuse_reply_xattr.
+     *
+     * If the size is non-zero, and the null character separated
+     * attribute list fits in the buffer, the list should be sent with
+     * fuse_reply_buf.
+     *
+     * If the size is too small for the list, the ERANGE error should
+     * be sent.
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure with error code EOPNOTSUPP, i.e. all
+     * future listxattr() requests will fail with EOPNOTSUPP without being
+     * send to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_buf
+     *   fuse_reply_data
+     *   fuse_reply_xattr
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param size maximum size of the list to send
+     */
+    void (*listxattr)(fuse_req_t req, fuse_ino_t ino, size_t size);
+
+    /**
+     * Remove an extended attribute
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure with error code EOPNOTSUPP, i.e. all
+     * future removexattr() requests will fail with EOPNOTSUPP without being
+     * send to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param name of the extended attribute
+     */
+    void (*removexattr)(fuse_req_t req, fuse_ino_t ino, const char *name);
+
+    /**
+     * Check file access permissions
+     *
+     * This will be called for the access() and chdir() system
+     * calls.  If the 'default_permissions' mount option is given,
+     * this method is not called.
+     *
+     * This method is not called under Linux kernel versions 2.4.x
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent success, i.e. this and all future access()
+     * requests will succeed without being send to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param mask requested access mode
+     */
+    void (*access)(fuse_req_t req, fuse_ino_t ino, int mask);
+
+    /**
+     * Create and open a file
+     *
+     * If the file does not exist, first create it with the specified
+     * mode, and then open it.
+     *
+     * See the description of the open handler for more
+     * information.
+     *
+     * If this method is not implemented or under Linux kernel
+     * versions earlier than 2.6.15, the mknod() and open() methods
+     * will be called instead.
+     *
+     * If this request is answered with an error code of ENOSYS, the handler
+     * is treated as not implemented (i.e., for this and future requests the
+     * mknod() and open() handlers will be called instead).
+     *
+     * Valid replies:
+     *   fuse_reply_create
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param parent inode number of the parent directory
+     * @param name to create
+     * @param mode file type and mode with which to create the new file
+     * @param fi file information
+     */
+    void (*create)(fuse_req_t req, fuse_ino_t parent, const char *name,
+                   mode_t mode, struct fuse_file_info *fi);
+
+    /**
+     * Test for a POSIX file lock
+     *
+     * Valid replies:
+     *   fuse_reply_lock
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     * @param lock the region/type to test
+     */
+    void (*getlk)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi,
+                  struct flock *lock);
+
+    /**
+     * Acquire, modify or release a POSIX file lock
+     *
+     * For POSIX threads (NPTL) there's a 1-1 relation between pid and
+     * owner, but otherwise this is not always the case.  For checking
+     * lock ownership, 'fi->owner' must be used.  The l_pid field in
+     * 'struct flock' should only be used to fill in this field in
+     * getlk().
+     *
+     * Note: if the locking methods are not implemented, the kernel
+     * will still allow file locking to work locally.  Hence these are
+     * only interesting for network filesystems and similar.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     * @param lock the region/type to set
+     * @param sleep locking operation may sleep
+     */
+    void (*setlk)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi,
+                  struct flock *lock, int sleep);
+
+    /**
+     * Map block index within file to block index within device
+     *
+     * Note: This makes sense only for block device backed filesystems
+     * mounted with the 'blkdev' option
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure, i.e. all future bmap() requests will
+     * fail with the same error code without being send to the filesystem
+     * process.
+     *
+     * Valid replies:
+     *   fuse_reply_bmap
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param blocksize unit of block index
+     * @param idx block index within file
+     */
+    void (*bmap)(fuse_req_t req, fuse_ino_t ino, size_t blocksize,
+                 uint64_t idx);
+
+    /**
+     * Ioctl
+     *
+     * Note: For unrestricted ioctls (not allowed for FUSE
+     * servers), data in and out areas can be discovered by giving
+     * iovs and setting FUSE_IOCTL_RETRY in *flags*.  For
+     * restricted ioctls, kernel prepares in/out data area
+     * according to the information encoded in cmd.
+     *
+     * Valid replies:
+     *   fuse_reply_ioctl_retry
+     *   fuse_reply_ioctl
+     *   fuse_reply_ioctl_iov
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param cmd ioctl command
+     * @param arg ioctl argument
+     * @param fi file information
+     * @param flags for FUSE_IOCTL_* flags
+     * @param in_buf data fetched from the caller
+     * @param in_bufsz number of fetched bytes
+     * @param out_bufsz maximum size of output data
+     *
+     * Note : the unsigned long request submitted by the application
+     * is truncated to 32 bits.
+     */
+    void (*ioctl)(fuse_req_t req, fuse_ino_t ino, unsigned int cmd, void *arg,
+                  struct fuse_file_info *fi, unsigned flags, const void *in_buf,
+                  size_t in_bufsz, size_t out_bufsz);
+
+    /**
+     * Poll for IO readiness
+     *
+     * Note: If ph is non-NULL, the client should notify
+     * when IO readiness events occur by calling
+     * fuse_lowlevel_notify_poll() with the specified ph.
+     *
+     * Regardless of the number of times poll with a non-NULL ph
+     * is received, single notification is enough to clear all.
+     * Notifying more times incurs overhead but doesn't harm
+     * correctness.
+     *
+     * The callee is responsible for destroying ph with
+     * fuse_pollhandle_destroy() when no longer in use.
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as success (with a kernel-defined default poll-mask) and
+     * future calls to pull() will succeed the same way without being send
+     * to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_poll
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     * @param ph poll handle to be used for notification
+     */
+    void (*poll)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi,
+                 struct fuse_pollhandle *ph);
+
+    /**
+     * Write data made available in a buffer
+     *
+     * This is a more generic version of the ->write() method.  If
+     * FUSE_CAP_SPLICE_READ is set in fuse_conn_info.want and the
+     * kernel supports splicing from the fuse device, then the
+     * data will be made available in pipe for supporting zero
+     * copy data transfer.
+     *
+     * buf->count is guaranteed to be one (and thus buf->idx is
+     * always zero). The write_buf handler must ensure that
+     * bufv->off is correctly updated (reflecting the number of
+     * bytes read from bufv->buf[0]).
+     *
+     * Unless FUSE_CAP_HANDLE_KILLPRIV is disabled, this method is
+     * expected to reset the setuid and setgid bits.
+     *
+     * Valid replies:
+     *   fuse_reply_write
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param bufv buffer containing the data
+     * @param off offset to write to
+     * @param fi file information
+     */
+    void (*write_buf)(fuse_req_t req, fuse_ino_t ino, struct fuse_bufvec *bufv,
+                      off_t off, struct fuse_file_info *fi);
+
+    /**
+     * Forget about multiple inodes
+     *
+     * See description of the forget function for more
+     * information.
+     *
+     * Valid replies:
+     *   fuse_reply_none
+     *
+     * @param req request handle
+     */
+    void (*forget_multi)(fuse_req_t req, size_t count,
+                         struct fuse_forget_data *forgets);
+
+    /**
+     * Acquire, modify or release a BSD file lock
+     *
+     * Note: if the locking methods are not implemented, the kernel
+     * will still allow file locking to work locally.  Hence these are
+     * only interesting for network filesystems and similar.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param fi file information
+     * @param op the locking operation, see flock(2)
+     */
+    void (*flock)(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi,
+                  int op);
+
+    /**
+     * Allocate requested space. If this function returns success then
+     * subsequent writes to the specified range shall not fail due to the lack
+     * of free space on the file system storage media.
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure with error code EOPNOTSUPP, i.e. all
+     * future fallocate() requests will fail with EOPNOTSUPP without being
+     * send to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param offset starting point for allocated region
+     * @param length size of allocated region
+     * @param mode determines the operation to be performed on the given range,
+     *             see fallocate(2)
+     */
+    void (*fallocate)(fuse_req_t req, fuse_ino_t ino, int mode, off_t offset,
+                      off_t length, struct fuse_file_info *fi);
+
+    /**
+     * Read directory with attributes
+     *
+     * Send a buffer filled using fuse_add_direntry_plus(), with size not
+     * exceeding the requested size.  Send an empty buffer on end of
+     * stream.
+     *
+     * fi->fh will contain the value set by the opendir method, or
+     * will be undefined if the opendir method didn't set any value.
+     *
+     * In contrast to readdir() (which does not affect the lookup counts),
+     * the lookup count of every entry returned by readdirplus(), except "."
+     * and "..", is incremented by one.
+     *
+     * Valid replies:
+     *   fuse_reply_buf
+     *   fuse_reply_data
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param size maximum number of bytes to send
+     * @param off offset to continue reading the directory stream
+     * @param fi file information
+     */
+    void (*readdirplus)(fuse_req_t req, fuse_ino_t ino, size_t size, off_t off,
+                        struct fuse_file_info *fi);
+
+    /**
+     * Copy a range of data from one file to another
+     *
+     * Performs an optimized copy between two file descriptors without the
+     * additional cost of transferring data through the FUSE kernel module
+     * to user space (glibc) and then back into the FUSE filesystem again.
+     *
+     * In case this method is not implemented, glibc falls back to reading
+     * data from the source and writing to the destination. Effectively
+     * doing an inefficient copy of the data.
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure with error code EOPNOTSUPP, i.e. all
+     * future copy_file_range() requests will fail with EOPNOTSUPP without
+     * being send to the filesystem process.
+     *
+     * Valid replies:
+     *   fuse_reply_write
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino_in the inode number or the source file
+     * @param off_in starting point from were the data should be read
+     * @param fi_in file information of the source file
+     * @param ino_out the inode number or the destination file
+     * @param off_out starting point where the data should be written
+     * @param fi_out file information of the destination file
+     * @param len maximum size of the data to copy
+     * @param flags passed along with the copy_file_range() syscall
+     */
+    void (*copy_file_range)(fuse_req_t req, fuse_ino_t ino_in, off_t off_in,
+                            struct fuse_file_info *fi_in, fuse_ino_t ino_out,
+                            off_t off_out, struct fuse_file_info *fi_out,
+                            size_t len, int flags);
+
+    /**
+     * Find next data or hole after the specified offset
+     *
+     * If this request is answered with an error code of ENOSYS, this is
+     * treated as a permanent failure, i.e. all future lseek() requests will
+     * fail with the same error code without being send to the filesystem
+     * process.
+     *
+     * Valid replies:
+     *   fuse_reply_lseek
+     *   fuse_reply_err
+     *
+     * @param req request handle
+     * @param ino the inode number
+     * @param off offset to start search from
+     * @param whence either SEEK_DATA or SEEK_HOLE
+     * @param fi file information
+     */
+    void (*lseek)(fuse_req_t req, fuse_ino_t ino, off_t off, int whence,
+                  struct fuse_file_info *fi);
+};
+
+/**
+ * Reply with an error code or success.
+ *
+ * Possible requests:
+ *   all except forget
+ *
+ * Whereever possible, error codes should be chosen from the list of
+ * documented error conditions in the corresponding system calls
+ * manpage.
+ *
+ * An error code of ENOSYS is sometimes treated specially. This is
+ * indicated in the documentation of the affected handler functions.
+ *
+ * The following requests may be answered with a zero error code:
+ * unlink, rmdir, rename, flush, release, fsync, fsyncdir, setxattr,
+ * removexattr, setlk.
+ *
+ * @param req request handle
+ * @param err the positive error value, or zero for success
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_err(fuse_req_t req, int err);
+
+/**
+ * Don't send reply
+ *
+ * Possible requests:
+ *   forget
+ *   forget_multi
+ *   retrieve_reply
+ *
+ * @param req request handle
+ */
+void fuse_reply_none(fuse_req_t req);
+
+/**
+ * Reply with a directory entry
+ *
+ * Possible requests:
+ *   lookup, mknod, mkdir, symlink, link
+ *
+ * Side effects:
+ *   increments the lookup count on success
+ *
+ * @param req request handle
+ * @param e the entry parameters
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_entry(fuse_req_t req, const struct fuse_entry_param *e);
+
+/**
+ * Reply with a directory entry and open parameters
+ *
+ * currently the following members of 'fi' are used:
+ *   fh, direct_io, keep_cache
+ *
+ * Possible requests:
+ *   create
+ *
+ * Side effects:
+ *   increments the lookup count on success
+ *
+ * @param req request handle
+ * @param e the entry parameters
+ * @param fi file information
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_create(fuse_req_t req, const struct fuse_entry_param *e,
+                      const struct fuse_file_info *fi);
+
+/**
+ * Reply with attributes
+ *
+ * Possible requests:
+ *   getattr, setattr
+ *
+ * @param req request handle
+ * @param attr the attributes
+ * @param attr_timeout validity timeout (in seconds) for the attributes
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_attr(fuse_req_t req, const struct stat *attr,
+                    double attr_timeout);
+
+/**
+ * Reply with the contents of a symbolic link
+ *
+ * Possible requests:
+ *   readlink
+ *
+ * @param req request handle
+ * @param link symbolic link contents
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_readlink(fuse_req_t req, const char *link);
+
+/**
+ * Reply with open parameters
+ *
+ * currently the following members of 'fi' are used:
+ *   fh, direct_io, keep_cache
+ *
+ * Possible requests:
+ *   open, opendir
+ *
+ * @param req request handle
+ * @param fi file information
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_open(fuse_req_t req, const struct fuse_file_info *fi);
+
+/**
+ * Reply with number of bytes written
+ *
+ * Possible requests:
+ *   write
+ *
+ * @param req request handle
+ * @param count the number of bytes written
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_write(fuse_req_t req, size_t count);
+
+/**
+ * Reply with data
+ *
+ * Possible requests:
+ *   read, readdir, getxattr, listxattr
+ *
+ * @param req request handle
+ * @param buf buffer containing data
+ * @param size the size of data in bytes
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_buf(fuse_req_t req, const char *buf, size_t size);
+
+/**
+ * Reply with data copied/moved from buffer(s)
+ *
+ * Possible requests:
+ *   read, readdir, getxattr, listxattr
+ *
+ * Side effects:
+ *   when used to return data from a readdirplus() (but not readdir())
+ *   call, increments the lookup count of each returned entry by one
+ *   on success.
+ *
+ * @param req request handle
+ * @param bufv buffer vector
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_data(fuse_req_t req, struct fuse_bufvec *bufv);
+
+/**
+ * Reply with data vector
+ *
+ * Possible requests:
+ *   read, readdir, getxattr, listxattr
+ *
+ * @param req request handle
+ * @param iov the vector containing the data
+ * @param count the size of vector
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_iov(fuse_req_t req, const struct iovec *iov, int count);
+
+/**
+ * Reply with filesystem statistics
+ *
+ * Possible requests:
+ *   statfs
+ *
+ * @param req request handle
+ * @param stbuf filesystem statistics
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_statfs(fuse_req_t req, const struct statvfs *stbuf);
+
+/**
+ * Reply with needed buffer size
+ *
+ * Possible requests:
+ *   getxattr, listxattr
+ *
+ * @param req request handle
+ * @param count the buffer size needed in bytes
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_xattr(fuse_req_t req, size_t count);
+
+/**
+ * Reply with file lock information
+ *
+ * Possible requests:
+ *   getlk
+ *
+ * @param req request handle
+ * @param lock the lock information
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_lock(fuse_req_t req, const struct flock *lock);
+
+/**
+ * Reply with block index
+ *
+ * Possible requests:
+ *   bmap
+ *
+ * @param req request handle
+ * @param idx block index within device
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_bmap(fuse_req_t req, uint64_t idx);
+
+/*
+ * Filling a buffer in readdir
+ */
+
+/**
+ * Add a directory entry to the buffer
+ *
+ * Buffer needs to be large enough to hold the entry.  If it's not,
+ * then the entry is not filled in but the size of the entry is still
+ * returned.  The caller can check this by comparing the bufsize
+ * parameter with the returned entry size.  If the entry size is
+ * larger than the buffer size, the operation failed.
+ *
+ * From the 'stbuf' argument the st_ino field and bits 12-15 of the
+ * st_mode field are used.  The other fields are ignored.
+ *
+ * *off* should be any non-zero value that the filesystem can use to
+ * identify the current point in the directory stream. It does not
+ * need to be the actual physical position. A value of zero is
+ * reserved to mean "from the beginning", and should therefore never
+ * be used (the first call to fuse_add_direntry should be passed the
+ * offset of the second directory entry).
+ *
+ * @param req request handle
+ * @param buf the point where the new entry will be added to the buffer
+ * @param bufsize remaining size of the buffer
+ * @param name the name of the entry
+ * @param stbuf the file attributes
+ * @param off the offset of the next entry
+ * @return the space needed for the entry
+ */
+size_t fuse_add_direntry(fuse_req_t req, char *buf, size_t bufsize,
+                         const char *name, const struct stat *stbuf, off_t off);
+
+/**
+ * Add a directory entry to the buffer with the attributes
+ *
+ * See documentation of `fuse_add_direntry()` for more details.
+ *
+ * @param req request handle
+ * @param buf the point where the new entry will be added to the buffer
+ * @param bufsize remaining size of the buffer
+ * @param name the name of the entry
+ * @param e the directory entry
+ * @param off the offset of the next entry
+ * @return the space needed for the entry
+ */
+size_t fuse_add_direntry_plus(fuse_req_t req, char *buf, size_t bufsize,
+                              const char *name,
+                              const struct fuse_entry_param *e, off_t off);
+
+/**
+ * Reply to ask for data fetch and output buffer preparation.  ioctl
+ * will be retried with the specified input data fetched and output
+ * buffer prepared.
+ *
+ * Possible requests:
+ *   ioctl
+ *
+ * @param req request handle
+ * @param in_iov iovec specifying data to fetch from the caller
+ * @param in_count number of entries in in_iov
+ * @param out_iov iovec specifying addresses to write output to
+ * @param out_count number of entries in out_iov
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_ioctl_retry(fuse_req_t req, const struct iovec *in_iov,
+                           size_t in_count, const struct iovec *out_iov,
+                           size_t out_count);
+
+/**
+ * Reply to finish ioctl
+ *
+ * Possible requests:
+ *   ioctl
+ *
+ * @param req request handle
+ * @param result result to be passed to the caller
+ * @param buf buffer containing output data
+ * @param size length of output data
+ */
+int fuse_reply_ioctl(fuse_req_t req, int result, const void *buf, size_t size);
+
+/**
+ * Reply to finish ioctl with iov buffer
+ *
+ * Possible requests:
+ *   ioctl
+ *
+ * @param req request handle
+ * @param result result to be passed to the caller
+ * @param iov the vector containing the data
+ * @param count the size of vector
+ */
+int fuse_reply_ioctl_iov(fuse_req_t req, int result, const struct iovec *iov,
+                         int count);
+
+/**
+ * Reply with poll result event mask
+ *
+ * @param req request handle
+ * @param revents poll result event mask
+ */
+int fuse_reply_poll(fuse_req_t req, unsigned revents);
+
+/**
+ * Reply with offset
+ *
+ * Possible requests:
+ *   lseek
+ *
+ * @param req request handle
+ * @param off offset of next data or hole
+ * @return zero for success, -errno for failure to send reply
+ */
+int fuse_reply_lseek(fuse_req_t req, off_t off);
+
+/*
+ * Notification
+ */
+
+/**
+ * Notify IO readiness event
+ *
+ * For more information, please read comment for poll operation.
+ *
+ * @param ph poll handle to notify IO readiness event for
+ */
+int fuse_lowlevel_notify_poll(struct fuse_pollhandle *ph);
+
+/**
+ * Notify to invalidate cache for an inode.
+ *
+ * Added in FUSE protocol version 7.12. If the kernel does not support
+ * this (or a newer) version, the function will return -ENOSYS and do
+ * nothing.
+ *
+ * If the filesystem has writeback caching enabled, invalidating an
+ * inode will first trigger a writeback of all dirty pages. The call
+ * will block until all writeback requests have completed and the
+ * inode has been invalidated. It will, however, not wait for
+ * completion of pending writeback requests that have been issued
+ * before.
+ *
+ * If there are no dirty pages, this function will never block.
+ *
+ * @param se the session object
+ * @param ino the inode number
+ * @param off the offset in the inode where to start invalidating
+ *            or negative to invalidate attributes only
+ * @param len the amount of cache to invalidate or 0 for all
+ * @return zero for success, -errno for failure
+ */
+int fuse_lowlevel_notify_inval_inode(struct fuse_session *se, fuse_ino_t ino,
+                                     off_t off, off_t len);
+
+/**
+ * Notify to invalidate parent attributes and the dentry matching
+ * parent/name
+ *
+ * To avoid a deadlock this function must not be called in the
+ * execution path of a related filesytem operation or within any code
+ * that could hold a lock that could be needed to execute such an
+ * operation. As of kernel 4.18, a "related operation" is a lookup(),
+ * symlink(), mknod(), mkdir(), unlink(), rename(), link() or create()
+ * request for the parent, and a setattr(), unlink(), rmdir(),
+ * rename(), setxattr(), removexattr(), readdir() or readdirplus()
+ * request for the inode itself.
+ *
+ * When called correctly, this function will never block.
+ *
+ * Added in FUSE protocol version 7.12. If the kernel does not support
+ * this (or a newer) version, the function will return -ENOSYS and do
+ * nothing.
+ *
+ * @param se the session object
+ * @param parent inode number
+ * @param name file name
+ * @param namelen strlen() of file name
+ * @return zero for success, -errno for failure
+ */
+int fuse_lowlevel_notify_inval_entry(struct fuse_session *se, fuse_ino_t parent,
+                                     const char *name, size_t namelen);
+
+/**
+ * This function behaves like fuse_lowlevel_notify_inval_entry() with
+ * the following additional effect (at least as of Linux kernel 4.8):
+ *
+ * If the provided *child* inode matches the inode that is currently
+ * associated with the cached dentry, and if there are any inotify
+ * watches registered for the dentry, then the watchers are informed
+ * that the dentry has been deleted.
+ *
+ * To avoid a deadlock this function must not be called while
+ * executing a related filesytem operation or while holding a lock
+ * that could be needed to execute such an operation (see the
+ * description of fuse_lowlevel_notify_inval_entry() for more
+ * details).
+ *
+ * When called correctly, this function will never block.
+ *
+ * Added in FUSE protocol version 7.18. If the kernel does not support
+ * this (or a newer) version, the function will return -ENOSYS and do
+ * nothing.
+ *
+ * @param se the session object
+ * @param parent inode number
+ * @param child inode number
+ * @param name file name
+ * @param namelen strlen() of file name
+ * @return zero for success, -errno for failure
+ */
+int fuse_lowlevel_notify_delete(struct fuse_session *se, fuse_ino_t parent,
+                                fuse_ino_t child, const char *name,
+                                size_t namelen);
+
+/**
+ * Store data to the kernel buffers
+ *
+ * Synchronously store data in the kernel buffers belonging to the
+ * given inode.  The stored data is marked up-to-date (no read will be
+ * performed against it, unless it's invalidated or evicted from the
+ * cache).
+ *
+ * If the stored data overflows the current file size, then the size
+ * is extended, similarly to a write(2) on the filesystem.
+ *
+ * If this function returns an error, then the store wasn't fully
+ * completed, but it may have been partially completed.
+ *
+ * Added in FUSE protocol version 7.15. If the kernel does not support
+ * this (or a newer) version, the function will return -ENOSYS and do
+ * nothing.
+ *
+ * @param se the session object
+ * @param ino the inode number
+ * @param offset the starting offset into the file to store to
+ * @param bufv buffer vector
+ * @return zero for success, -errno for failure
+ */
+int fuse_lowlevel_notify_store(struct fuse_session *se, fuse_ino_t ino,
+                               off_t offset, struct fuse_bufvec *bufv);
+
+/*
+ * Utility functions
+ */
+
+/**
+ * Get the userdata from the request
+ *
+ * @param req request handle
+ * @return the user data passed to fuse_session_new()
+ */
+void *fuse_req_userdata(fuse_req_t req);
+
+/**
+ * Get the context from the request
+ *
+ * The pointer returned by this function will only be valid for the
+ * request's lifetime
+ *
+ * @param req request handle
+ * @return the context structure
+ */
+const struct fuse_ctx *fuse_req_ctx(fuse_req_t req);
+
+/**
+ * Get the current supplementary group IDs for the specified request
+ *
+ * Similar to the getgroups(2) system call, except the return value is
+ * always the total number of group IDs, even if it is larger than the
+ * specified size.
+ *
+ * The current fuse kernel module in linux (as of 2.6.30) doesn't pass
+ * the group list to userspace, hence this function needs to parse
+ * "/proc/$TID/task/$TID/status" to get the group IDs.
+ *
+ * This feature may not be supported on all operating systems.  In
+ * such a case this function will return -ENOSYS.
+ *
+ * @param req request handle
+ * @param size size of given array
+ * @param list array of group IDs to be filled in
+ * @return the total number of supplementary group IDs or -errno on failure
+ */
+int fuse_req_getgroups(fuse_req_t req, int size, gid_t list[]);
+
+/**
+ * Callback function for an interrupt
+ *
+ * @param req interrupted request
+ * @param data user data
+ */
+typedef void (*fuse_interrupt_func_t)(fuse_req_t req, void *data);
+
+/**
+ * Register/unregister callback for an interrupt
+ *
+ * If an interrupt has already happened, then the callback function is
+ * called from within this function, hence it's not possible for
+ * interrupts to be lost.
+ *
+ * @param req request handle
+ * @param func the callback function or NULL for unregister
+ * @param data user data passed to the callback function
+ */
+void fuse_req_interrupt_func(fuse_req_t req, fuse_interrupt_func_t func,
+                             void *data);
+
+/**
+ * Check if a request has already been interrupted
+ *
+ * @param req request handle
+ * @return 1 if the request has been interrupted, 0 otherwise
+ */
+int fuse_req_interrupted(fuse_req_t req);
+
+/**
+ * Check if the session is connected via virtio
+ *
+ * @param se session object
+ * @return 1 if the session is a virtio session
+ */
+int fuse_lowlevel_is_virtio(struct fuse_session *se);
+
+/*
+ * Inquiry functions
+ */
+
+/**
+ * Print low-level version information to stdout.
+ */
+void fuse_lowlevel_version(void);
+
+/**
+ * Print available low-level options to stdout. This is not an
+ * exhaustive list, but includes only those options that may be of
+ * interest to an end-user of a file system.
+ */
+void fuse_lowlevel_help(void);
+
+/**
+ * Print available options for `fuse_parse_cmdline()`.
+ */
+void fuse_cmdline_help(void);
+
+/*
+ * Filesystem setup & teardown
+ */
+
+struct fuse_cmdline_opts {
+    int foreground;
+    int debug;
+    int nodefault_subtype;
+    int show_version;
+    int show_help;
+    int print_capabilities;
+    int syslog;
+    int log_level;
+    unsigned int max_idle_threads;
+};
+
+/**
+ * Utility function to parse common options for simple file systems
+ * using the low-level API. A help text that describes the available
+ * options can be printed with `fuse_cmdline_help`. A single
+ * non-option argument is treated as the mountpoint. Multiple
+ * non-option arguments will result in an error.
+ *
+ * If neither -o subtype= or -o fsname= options are given, a new
+ * subtype option will be added and set to the basename of the program
+ * (the fsname will remain unset, and then defaults to "fuse").
+ *
+ * Known options will be removed from *args*, unknown options will
+ * remain.
+ *
+ * @param args argument vector (input+output)
+ * @param opts output argument for parsed options
+ * @return 0 on success, -1 on failure
+ */
+int fuse_parse_cmdline(struct fuse_args *args, struct fuse_cmdline_opts *opts);
+
+/**
+ * Create a low level session.
+ *
+ * Returns a session structure suitable for passing to
+ * fuse_session_mount() and fuse_session_loop().
+ *
+ * This function accepts most file-system independent mount options
+ * (like context, nodev, ro - see mount(8)), as well as the general
+ * fuse mount options listed in mount.fuse(8) (e.g. -o allow_root and
+ * -o default_permissions, but not ``-o use_ino``).  Instead of `-o
+ * debug`, debugging may also enabled with `-d` or `--debug`.
+ *
+ * If not all options are known, an error message is written to stderr
+ * and the function returns NULL.
+ *
+ * Option parsing skips argv[0], which is assumed to contain the
+ * program name. To prevent accidentally passing an option in
+ * argv[0], this element must always be present (even if no options
+ * are specified). It may be set to the empty string ('\0') if no
+ * reasonable value can be provided.
+ *
+ * @param args argument vector
+ * @param op the (low-level) filesystem operations
+ * @param op_size sizeof(struct fuse_lowlevel_ops)
+ * @param userdata user data
+ *
+ * @return the fuse session on success, NULL on failure
+ **/
+struct fuse_session *fuse_session_new(struct fuse_args *args,
+                                      const struct fuse_lowlevel_ops *op,
+                                      size_t op_size, void *userdata);
+
+/**
+ * Mount a FUSE file system.
+ *
+ * @param se session object
+ *
+ * @return 0 on success, -1 on failure.
+ **/
+int fuse_session_mount(struct fuse_session *se);
+
+/**
+ * Enter a single threaded, blocking event loop.
+ *
+ * When the event loop terminates because the connection to the FUSE
+ * kernel module has been closed, this function returns zero. This
+ * happens when the filesystem is unmounted regularly (by the
+ * filesystem owner or root running the umount(8) or fusermount(1)
+ * command), or if connection is explicitly severed by writing ``1``
+ * to the``abort`` file in ``/sys/fs/fuse/connections/NNN``. The only
+ * way to distinguish between these two conditions is to check if the
+ * filesystem is still mounted after the session loop returns.
+ *
+ * When some error occurs during request processing, the function
+ * returns a negated errno(3) value.
+ *
+ * If the loop has been terminated because of a signal handler
+ * installed by fuse_set_signal_handlers(), this function returns the
+ * (positive) signal value that triggered the exit.
+ *
+ * @param se the session
+ * @return 0, -errno, or a signal value
+ */
+int fuse_session_loop(struct fuse_session *se);
+
+/**
+ * Flag a session as terminated.
+ *
+ * This function is invoked by the POSIX signal handlers, when
+ * registered using fuse_set_signal_handlers(). It will cause any
+ * running event loops to terminate on the next opportunity.
+ *
+ * @param se the session
+ */
+void fuse_session_exit(struct fuse_session *se);
+
+/**
+ * Reset the terminated flag of a session
+ *
+ * @param se the session
+ */
+void fuse_session_reset(struct fuse_session *se);
+
+/**
+ * Query the terminated flag of a session
+ *
+ * @param se the session
+ * @return 1 if exited, 0 if not exited
+ */
+int fuse_session_exited(struct fuse_session *se);
+
+/**
+ * Ensure that file system is unmounted.
+ *
+ * In regular operation, the file system is typically unmounted by the
+ * user calling umount(8) or fusermount(1), which then terminates the
+ * FUSE session loop. However, the session loop may also terminate as
+ * a result of an explicit call to fuse_session_exit() (e.g. by a
+ * signal handler installed by fuse_set_signal_handler()). In this
+ * case the filesystem remains mounted, but any attempt to access it
+ * will block (while the filesystem process is still running) or give
+ * an ESHUTDOWN error (after the filesystem process has terminated).
+ *
+ * If the communication channel with the FUSE kernel module is still
+ * open (i.e., if the session loop was terminated by an explicit call
+ * to fuse_session_exit()), this function will close it and unmount
+ * the filesystem. If the communication channel has been closed by the
+ * kernel, this method will do (almost) nothing.
+ *
+ * NOTE: The above semantics mean that if the connection to the kernel
+ * is terminated via the ``/sys/fs/fuse/connections/NNN/abort`` file,
+ * this method will *not* unmount the filesystem.
+ *
+ * @param se the session
+ */
+void fuse_session_unmount(struct fuse_session *se);
+
+/**
+ * Destroy a session
+ *
+ * @param se the session
+ */
+void fuse_session_destroy(struct fuse_session *se);
+
+/*
+ * Custom event loop support
+ */
+
+/**
+ * Return file descriptor for communication with kernel.
+ *
+ * The file selector can be used to integrate FUSE with a custom event
+ * loop. Whenever data is available for reading on the provided fd,
+ * the event loop should call `fuse_session_receive_buf` followed by
+ * `fuse_session_process_buf` to process the request.
+ *
+ * The returned file descriptor is valid until `fuse_session_unmount`
+ * is called.
+ *
+ * @param se the session
+ * @return a file descriptor
+ */
+int fuse_session_fd(struct fuse_session *se);
+
+/**
+ * Process a raw request supplied in a generic buffer
+ *
+ * The fuse_buf may contain a memory buffer or a pipe file descriptor.
+ *
+ * @param se the session
+ * @param buf the fuse_buf containing the request
+ */
+void fuse_session_process_buf(struct fuse_session *se,
+                              const struct fuse_buf *buf);
+
+/**
+ * Read a raw request from the kernel into the supplied buffer.
+ *
+ * Depending on file system options, system capabilities, and request
+ * size the request is either read into a memory buffer or spliced
+ * into a temporary pipe.
+ *
+ * @param se the session
+ * @param buf the fuse_buf to store the request in
+ * @return the actual size of the raw request, or -errno on error
+ */
+int fuse_session_receive_buf(struct fuse_session *se, struct fuse_buf *buf);
+
+#endif /* FUSE_LOWLEVEL_H_ */