man4/fuse.4

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  25 .\"
  26 .TH FUSE 4 2016-12-10 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 /dev/fuse \- Filesystem in Userspace (FUSE) device
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <linux/fuse.h>
  32 .nf
  33 .SH DESCRIPTION
  34
  35 This device is the primary interface between the FUSE filesystem driver
  36 and a user-space process wishing to provide the filesystem (referred to
  37 in the rest of this manual page as the
  38 .IR "filesystem daemon" ).
  39 This manual page is intended for those
  40 interested in understanding the kernel interface itself.
  41 Those implementing a FUSE filesystem may wish to make use of
  42 a user-space library such as
  43 .I libfuse
  44 that abstracts away the low-level interface.
  45
  46 At its core, FUSE is a simple client-server protocol, in which the Linux
  47 kernel is the client and the daemon is the server.
  48 After obtaining a file descriptor for this device, the daemon may
  49 .BR read (2)
  50 requests from that file descriptor and is expected to
  51 .BR write (2)
  52 back its replies.
  53 It is important to note that a file descriptor is
  54 associated with a unique FUSE filesystem.
  55 In particular, opening a second copy of this device,
  56 will not allow access to resources created
  57 through the first file descriptor (and vice versa).
  58 .\"
  59 .SS The basic protocol
  60 Every message that is read by the daemon begins with a header described by
  61 the following structure:
  62
  63 .in +4n
  64 .nf
  65 struct fuse_in_header {
  66     uint32_t len;       /* Total length of the data,
  67                            including this header */
  68     uint32_t opcode;    /* The kind of operation (see below) */
  69     uint64_t unique;    /* A unique identifier for this request */
  70     uint64_t nodeid;    /* ID of the filesystem object
  71                            being operated on */
  72     uint32_t uid;       /* UID of the requesting process */
  73     uint32_t gid;       /* GID of the requesting process */
  74     uint32_t pid;       /* PID of the requesting process */
  75     uint32_t padding;
  76 };
  77 .fi
  78 .in
  79
  80 The header is followed by a variable-length data portion
  81 (which may be empty) specific to the requested operation
  82 (the requested operation is indicated by
  83 .IR opcode ).
  84
  85 The daemon should then process the request and if applicable send
  86 a reply (almost all operations require a reply; if they do not,
  87 this is documented below), by performing a
  88 .BR write (2)
  89 to the file descriptor.
  90 All replies must start with the following header:
  91
  92 .in +4n
  93 .nf
  94 struct fuse_out_header {
  95     uint32_t len;       /* Total length of data written to
  96                            the file descriptor */
  97     int32_t  error;     /* Any error that occurred (0 if none) */
  98     uint64_t unique;    /* The value from the
  99                            corresponding request */
 100 };
 101 .fi
 102 .in
 103
 104 This header is also followed by (potentially empty) variable-sized
 105 data depending on the executed request.
 106 However, if the reply is an error reply (i.e.,
 107 .I error
 108 is set),
 109 then no further payload data should be sent, independent of the request.
 110 .\"
 111 .SS Exchanged messages
 112 This section should contain documentation for each of the messages
 113 in the protocol.
 114 This manual page is currently incomplete,
 115 so not all messages are documented.
 116 For each message, first the struct sent by the kernel is given,
 117 followed by a description of the semantics of the message.
 118 .TP
 119 .BR FUSE_INIT " ( 25 )"
 120
 121 .in +4n
 122 .nf
 123 struct fuse_init_in {
 124     uint32_t major;
 125     uint32_t minor;
 126     uint32_t max_readahead; /* Since protocol v7.6 */
 127     uint32_t flags;         /* Since protocol v7.6 */
 128 };
 129 .fi
 130 .in
 131
 132 This is the first request sent by the kernel to the daemon.
 133 It is used to negotiate the protocol version and other filesystem parameters.
 134 Note that the protocol version may affect the layout of any structure
 135 in the protocol (including this one).
 136 The daemon must thus remember the negotiated version
 137 and flags for each session.
 138 As of the writing of this man page,
 139 the highest supported kernel protocol version is
 140 .IR 7.26 .
 141
 142 Users should be aware that the descriptions in this manual page
 143 may be incomplete or incorrect for older or more recent protocol versions.
 144
 145 The reply for this request has the following format:
 146
 147 .in +4n
 148 .nf
 149 struct fuse_init_out {
 150     uint32_t major;
 151     uint32_t minor;
 152     uint32_t max_readahead;   /* Since v7.6 */
 153     uint32_t flags;           /* Since v7.6; some flags bits
 154                                  were introduced later */
 155     uint16_t max_background;  /* Since v7.13 */
 156     uint16_t congestion_threshold;  /* Since v7.13 */
 157     uint32_t max_write;       /* Since v7.5 */
 158     uint32_t time_gran;       /* Since v7.6 */
 159     uint32_t unused[9];
 160 };
 161 .fi
 162 .in
 163
 164 If the major version supported by the kernel is larger than that supported
 165 by the daemon, the reply shall consist of only
 166 .I uint32_t major
 167 (following the usual header),
 168 indicating the largest major version supported by the daemon.
 169 The kernel will then issue a new
 170 .B FUSE_INIT
 171 request conforming to the older version.
 172 In the reverse case, the daemon should
 173 quietly fall back to the kernel's major version.
 174
 175 The negotiated minor version is considered to be the minimum
 176 of the minor versions provided by the daemon and the kernel and
 177 both parties should use the protocol corresponding to said minor version.
 178 .TP
 179 .BR FUSE_GETATTR " ( 3 )"
 180 .in +4n
 181 .nf
 182 struct fuse_getattr_in {
 183     uint32_t getattr_flags;
 184     uint32_t dummy;
 185     uint64_t fh;      /* Set only if
 186                          (getattr_flags & FUSE_GETATTR_FH)
 187 };
 188 .fi
 189 .in
 190
 191 As usual, the filesystem object operated on is indicated by
 192 .IR header\->nodeid .
 193 The daemon should compute the attributes
 194 of this object and reply with the following message:
 195 .in +4n
 196
 197 .nf
 198 struct fuse_attr {
 199     uint64_t ino;
 200     uint64_t size;
 201     uint64_t blocks;
 202     uint64_t atime;
 203     uint64_t mtime;
 204     uint64_t ctime;
 205     uint32_t atimensec;
 206     uint32_t mtimensec;
 207     uint32_t ctimensec;
 208     uint32_t mode;
 209     uint32_t nlink;
 210     uint32_t uid;
 211     uint32_t gid;
 212     uint32_t rdev;
 213     uint32_t blksize;
 214     uint32_t padding;
 215 };
 216
 217 struct fuse_attr_out {
 218     /* Attribute cache duration (seconds + nanoseconds) */
 219     uint64_t attr_valid;
 220     uint32_t attr_valid_nsec;
 221     uint32_t dummy;
 222     struct fuse_attr attr;
 223 };
 224 .fi
 225 .in
 226
 227 The fields of
 228 .I struct fuse_attr
 229 describe the attributes of the required file.
 230 For the interpretation of these fields, see
 231 .BR stat (2).
 232 .TP
 233 .BR FUSE_ACCESS " ( 34 )"
 234
 235 .in +4n
 236 .nf
 237 struct fuse_access_in {
 238     uint32_t mask;
 239     uint32_t padding;
 240 };
 241 .fi
 242 .in
 243
 244 If the
 245 .I default_permissions
 246 mount options is not used, this request may be used for permissions checking.
 247 No reply data is expected, but errors may be indicated
 248 as usual in the reply header (in particular, access denied errors
 249 may be indicated, by setting such field to
 250 .BR \-EACCES ).
 251 .TP
 252 .BR FUSE_OPEN " ( 14 ) and " FUSE_OPENDIR " ( 34 )"
 253 .in +4n
 254 .nf
 255 struct fuse_open_in {
 256     uint32_t flags;     /* The flags that were passed
 257                            to the open(2) */
 258     uint32_t unused;
 259 };
 260 .fi
 261 .in
 262
 263 The requested operation is to open the node indicated by
 264 .IR header\->nodeid .
 265 The exact semantics of what this means will depend on the
 266 filesystem being implemented.
 267 However, at the very least the
 268 filesystem should validate that the requested
 269 .I flags
 270 are valid for the indicated resource and then send a reply with the
 271 following format:
 272
 273 .in +4n
 274 .nf
 275
 276 struct fuse_open_out {
 277     uint64_t fh;
 278     uint32_t open_flags;
 279     uint32_t padding;
 280 };
 281
 282 .fi
 283 .in
 284
 285 The
 286 .I fh
 287 field is an opaque identifier that the kernel will use to refer
 288 to this resource
 289 The
 290 .I open_flags
 291 field is a bit mask of any number of
 292 .B FOPEN_*
 293 flags, which indicate properties of this file handle to the kernel.
 294 .TP
 295 .BR FUSE_READ " ( 15 ) and " FUSE_READDIR " ( 28 )"
 296 .in +4n
 297 .nf
 298
 299 struct fuse_read_in {
 300     uint64_t fh;
 301     uint64_t offset;
 302     uint32_t size;
 303     uint32_t read_flags;
 304     uint64_t lock_owner;
 305     uint32_t flags;
 306     uint32_t padding;
 307 };
 308
 309 .fi
 310 .in
 311
 312 The requested action is to read up to
 313 .I size
 314 bytes of the file or directory, starting at
 315 .IR offset .
 316 The bytes should be returned directly following the out header,
 317 with no further special out structure.
 318 .TP
 319 .BR FUSE_INTERRUPT " ( 36 )"
 320 .in +4n
 321 .nf
 322 struct fuse_interrupt_in {
 323     uint64_t unique;
 324 };
 325 .fi
 326 .in
 327
 328 The requested action is to cancel the pending operation indicated by
 329 .IR unique .
 330 This request requires no response.
 331 However, receipt of this message does
 332 not by itself cancel the indicated operation.
 333 The kernel will still expect a reply to said operation (e.g., an
 334 .I EINTR
 335 error or a short read).
 336 At most one
 337 .B FUSE_INTERRUPT
 338 request will be issued for a given operation.
 339 After issuing said operation,
 340 the kernel will wait uninterruptibly for completion of the indicated request.
 341 .TP
 342 .BR FUSE_LOOKUP " ( 1 )"
 343 Directly following the header is a filename to be looked up in the directory
 344 indicated by
 345 .IR header\->nodeid .
 346 The expected reply is of the form:
 347
 348 .in +4n
 349 .nf
 350 struct fuse_entry_out {
 351     uint64_t nodeid;            /* Inode ID */
 352     uint64_t generation;        /* Inode generation */
 353     uint64_t entry_valid;
 354     uint64_t attr_valid;
 355     uint32_t entry_valid_nsec;
 356     uint32_t attr_valid_nsec;
 357     struct fuse_attr attr;
 358 };
 359 .fi
 360 .in
 361
 362 The combination of
 363 .I nodeid
 364 and
 365 .I generation
 366 must be unique for the filesystem's lifetime.
 367
 368 The interpretation of timeouts and
 369 .I attr
 370 is as for
 371 .BR FUSE_GETATTR .
 372 .TP
 373 .BR FUSE_FLUSH " ( 36 )"
 374 .in +4n
 375 .nf
 376 struct fuse_flush_in {
 377     uint64_t fh;
 378     uint32_t unused;
 379     uint32_t padding;
 380     uint64_t lock_owner;
 381 };
 382 .fi
 383 .in
 384
 385 The requested action is to flush any pending changes to the indicated
 386 file handle.
 387 No reply data is expected.
 388 However, an empty reply message
 389 still needs to be issued once the flush operation is complete.
 390 .TP
 391 .BR FUSE_RELEASE " ( 18 ) and " FUSE_RELEASEDIR " ( 29 )"
 392 .in +4n
 393 .nf
 394 struct fuse_release_in {
 395     uint64_t fh;
 396     uint32_t flags;
 397     uint32_t release_flags;
 398     uint64_t lock_owner;
 399 };
 400 .fi
 401 .in
 402
 403 These are the converse of
 404 .BR FUSE_OPEN
 405 and
 406 .BR FUSE_OPENDIR
 407 respectively.
 408 The daemon may now free any resources associated with the
 409 file handle
 410 .I fh
 411 as the kernel will no longer refer to it.
 412 There is no reply data associated with this request,
 413 but a reply still needs to be issued once the request has
 414 been completely processed.
 415 .TP
 416 .BR FUSE_STATFS " ( 17 )"
 417 This operation implements
 418 .BR statfs (2)
 419 for this filesystem.
 420 There is no input data associated with this request.
 421 The expected reply data has the following structure:
 422
 423 .in +4n
 424 .nf
 425 struct fuse_kstatfs {
 426     uint64_t blocks;
 427     uint64_t bfree;
 428     uint64_t bavail;
 429     uint64_t files;
 430     uint64_t ffree;
 431     uint32_t bsize;
 432     uint32_t namelen;
 433     uint32_t frsize;
 434     uint32_t padding;
 435     uint32_t spare[6];
 436 };
 437
 438 struct fuse_statfs_out {
 439     struct fuse_kstatfs st;
 440 };
 441 .fi
 442 .in
 443
 444 For the interpretation of these fields, see
 445 .BR statfs (2).
 446 .SH ERRORS
 447 .TP
 448 .B EPERM
 449 Returned from operations on a
 450 .I /dev/fuse
 451 file descriptor that has not been mounted.
 452 .TP
 453 .B EIO
 454 Returned from
 455 .BR read (2)
 456 operations when the kernel's request is too large for the provided buffer.
 457
 458 .IR Note :
 459 There are various ways in which incorrect use of these interfaces can cause
 460 operations on the provided filesystem's files and directories to fail with
 461 .BR EIO .
 462 Among the possible incorrect uses are
 463 .IP * 3
 464 changing
 465 .I mode & S_IFMT
 466 for an inode that has previously been reported to the kernel; or
 467 .IP *
 468 giving replies to the kernel that are shorter than what the kernel expected.
 469 .TP
 470 .B EINVAL
 471 Returned from
 472 .BR write (2)
 473 if validation of the reply failed.
 474 Not all mistakes in replies will be caught by this validation.
 475 However, basic mistakes, such as short replies or an incorrect
 476 .I unique
 477 value, are detected.
 478 .TP
 479 .B E2BIG
 480 Returned from
 481 .BR read (2)
 482 operations when the kernel's request is too large for the provided buffer
 483 and the request was
 484 .BR FUSE_SETXATTR .
 485 .TP
 486 .B ENODEV
 487 Returned from either operation if the FUSE filesystem was unmounted.