0intro.9p (14577B)
1 .TH INTRO 9P 2 .SH NAME 3 intro \- introduction to the Plan 9 File Protocol, 9P 4 .SH SYNOPSIS 5 .B #include <fcall.h> 6 .SH DESCRIPTION 7 A Plan 9 8 .I server 9 is an agent that provides one or more hierarchical file systems 10 \(em file trees \(em 11 that may be accessed by Plan 9 processes. 12 A server responds to requests by 13 .I clients 14 to navigate the hierarchy, 15 and to create, remove, read, and write files. 16 The prototypical server is a separate machine that stores 17 large numbers of user files on permanent media; 18 such a machine is called, somewhat confusingly, a 19 .I file 20 .IR server . 21 Another possibility for a server is to synthesize 22 files on demand, perhaps based on information on data structures 23 maintained in memory; the 24 .MR plumber (4) 25 server is an example of such a server. 26 .PP 27 A 28 .I connection 29 to a server is a bidirectional communication path from the client to the server. 30 There may be a single client or 31 multiple clients sharing the same connection. 32 .PP 33 The 34 .IR "Plan 9 File Protocol" , 35 9P, is used for messages between 36 .I clients 37 and 38 .IR servers . 39 A client transmits 40 .I requests 41 .RI ( T-messages ) 42 to a server, which 43 subsequently returns 44 .I replies 45 .RI ( R-messages ) 46 to the client. 47 The combined acts of transmitting (receiving) a request of a particular type, 48 and receiving (transmitting) 49 its reply is called a 50 .I transaction 51 of that type. 52 .PP 53 Each message consists of a sequence of bytes. 54 Two-, four-, and eight-byte fields hold unsigned 55 integers represented in little-endian order 56 (least significant byte first). 57 Data items of larger or variable lengths are represented 58 by a two-byte field specifying a count, 59 .IR n , 60 followed by 61 .I n 62 bytes of data. 63 Text strings are represented this way, 64 with the text itself stored as a UTF-8 65 encoded sequence of Unicode characters (see 66 .MR utf (7) ). 67 Text strings in 9P messages are not 68 .SM NUL\c 69 -terminated: 70 .I n 71 counts the bytes of UTF-8 data, which include no final zero byte. 72 The 73 .SM NUL 74 character is illegal in all text strings in 9P, and is therefore 75 excluded from file names, user names, and so on. 76 .PP 77 Each 9P message begins with a four-byte size field 78 specifying the length in bytes of the complete message including 79 the four bytes of the size field itself. 80 The next byte is the message type, one of the constants 81 in the enumeration in the include file 82 .BR <fcall.h> . 83 The next two bytes are an identifying 84 .IR tag , 85 described below. 86 The remaining bytes are parameters of different sizes. 87 In the message descriptions, the number of bytes in a field 88 is given in brackets after the field name. 89 The notation 90 .IR parameter [ n ] 91 where 92 .I n 93 is not a constant represents a variable-length parameter: 94 .IR n [2] 95 followed by 96 .I n 97 bytes of data forming the 98 .IR parameter . 99 The notation 100 .IR string [ s ] 101 (using a literal 102 .I s 103 character) 104 is shorthand for 105 .IR s [2] 106 followed by 107 .I s 108 bytes of UTF-8 text. 109 (Systems may choose to reduce the set of legal characters 110 to reduce syntactic problems, 111 for example to remove slashes from name components, 112 but the protocol has no such restriction. 113 Plan 9 names may contain any printable character (that is, any character 114 outside hexadecimal 00-1F and 80-9F) 115 except slash.) 116 Messages are transported in byte form to allow for machine independence; 117 .MR fcall (3) 118 describes routines that convert to and from this form into a machine-dependent 119 C structure. 120 .SH MESSAGES 121 .ta \w'\fLTsession 'u 122 .IP 123 .ne 2v 124 .IR size [4] 125 .B Tversion 126 .IR tag [2] 127 .IR msize [4] 128 .IR version [ s ] 129 .br 130 .IR size [4] 131 .B Rversion 132 .IR tag [2] 133 .IR msize [4] 134 .IR version [ s ] 135 .IP 136 .ne 2v 137 .IR size [4] 138 .B Tauth 139 .IR tag [2] 140 .IR afid [4] 141 .IR uname [ s ] 142 .IR aname [ s ] 143 .br 144 .br 145 .IR size [4] 146 .B Rauth 147 .IR tag [2] 148 .IR aqid [13] 149 .IP 150 .ne 2v 151 .IR size [4] 152 .B Rerror 153 .IR tag [2] 154 .IR ename [ s ] 155 .IP 156 .ne 2v 157 .IR size [4] 158 .B Tflush 159 .IR tag [2] 160 .IR oldtag [2] 161 .br 162 .IR size [4] 163 .B Rflush 164 .IR tag [2] 165 .IP 166 .ne 2v 167 .IR size [4] 168 .B Tattach 169 .IR tag [2] 170 .IR fid [4] 171 .IR afid [4] 172 .IR uname [ s ] 173 .IR aname [ s ] 174 .br 175 .IR size [4] 176 .B Rattach 177 .IR tag [2] 178 .IR qid [13] 179 .IP 180 .ne 2v 181 .IR size [4] 182 .B Twalk 183 .IR tag [2] 184 .IR fid [4] 185 .IR newfid [4] 186 .IR nwname [2] 187 .IR nwname *( wname [ s ]) 188 .br 189 .IR size [4] 190 .B Rwalk 191 .IR tag [2] 192 .IR nwqid [2] 193 .IR nwqid *( wqid [13]) 194 .IP 195 .ne 2v 196 .IR size [4] 197 .B Topen 198 .IR tag [2] 199 .IR fid [4] 200 .IR mode [1] 201 .br 202 .IR size [4] 203 .B Ropen 204 .IR tag [2] 205 .IR qid [13] 206 .IR iounit [4] 207 .IP 208 .ne 2v 209 .IR size [4] 210 .B Topenfd 211 .IR tag [2] 212 .IR fid [4] 213 .IR mode [1] 214 .br 215 .IR size [4] 216 .B Ropenfd 217 .IR tag [2] 218 .IR qid [13] 219 .IR iounit [4] 220 .IR unixfd [4] 221 .IP 222 .ne 2v 223 .IR size [4] 224 .B Tcreate 225 .IR tag [2] 226 .IR fid [4] 227 .IR name [ s ] 228 .IR perm [4] 229 .IR mode [1] 230 .br 231 .IR size [4] 232 .B Rcreate 233 .IR tag [2] 234 .IR qid [13] 235 .IR iounit [4] 236 .IP 237 .ne 2v 238 .IR size [4] 239 .B Tread 240 .IR tag [2] 241 .IR fid [4] 242 .IR offset [8] 243 .IR count [4] 244 .br 245 .IR size [4] 246 .B Rread 247 .IR tag [2] 248 .IR count [4] 249 .IR data [ count ] 250 .IP 251 .ne 2v 252 .IR size [4] 253 .B Twrite 254 .IR tag [2] 255 .IR fid [4] 256 .IR offset [8] 257 .IR count [4] 258 .IR data [ count ] 259 .br 260 .IR size [4] 261 .B Rwrite 262 .IR tag [2] 263 .IR count [4] 264 .IP 265 .ne 2v 266 .IR size [4] 267 .B Tclunk 268 .IR tag [2] 269 .IR fid [4] 270 .br 271 .IR size [4] 272 .B Rclunk 273 .IR tag [2] 274 .IP 275 .ne 2v 276 .IR size [4] 277 .B Tremove 278 .IR tag [2] 279 .IR fid [4] 280 .br 281 .IR size [4] 282 .B Rremove 283 .IR tag [2] 284 .IP 285 .ne 2v 286 .IR size [4] 287 .B Tstat 288 .IR tag [2] 289 .IR fid [4] 290 .br 291 .IR size [4] 292 .B Rstat 293 .IR tag [2] 294 .IR stat [ n ] 295 .IP 296 .ne 2v 297 .IR size [4] 298 .B Twstat 299 .IR tag [2] 300 .IR fid [4] 301 .IR stat [ n ] 302 .br 303 .IR size [4] 304 .B Rwstat 305 .IR tag [2] 306 .PP 307 Each T-message has a 308 .I tag 309 field, chosen and used by the client to identify the message. 310 The reply to the message will have the same tag. 311 Clients must arrange that no two outstanding messages 312 on the same connection have the same tag. 313 An exception is the tag 314 .BR NOTAG , 315 defined as 316 .B (ushort)~0 317 in 318 .BR <fcall.h> : 319 the client can use it, when establishing a connection, 320 to 321 override tag matching in 322 .B version 323 messages. 324 .PP 325 The type of an R-message will either be one greater than the type 326 of the corresponding T-message or 327 .BR Rerror , 328 indicating that the request failed. 329 In the latter case, the 330 .I ename 331 field contains a string describing the reason for failure. 332 .PP 333 The 334 .B version 335 message identifies the version of the protocol and indicates 336 the maximum message size the system is prepared to handle. 337 It also initializes the connection and 338 aborts all outstanding I/O on the connection. 339 The set of messages between 340 .B version 341 requests is called a 342 .IR session . 343 .PP 344 Most T-messages contain a 345 .IR fid , 346 a 32-bit unsigned integer that the client uses to identify 347 a ``current file'' on the server. 348 Fids are somewhat like file descriptors in a user process, 349 but they are not restricted to files open for I/O: 350 directories being examined, files being accessed by 351 .MR stat (3) 352 calls, and so on \(em all files being manipulated by the operating 353 system \(em are identified by fids. 354 Fids are chosen by the client. 355 All requests on a connection share the same fid space; 356 when several clients share a connection, 357 the agent managing the sharing must arrange 358 that no two clients choose the same fid. 359 .PP 360 The fid supplied in an 361 .B attach 362 message 363 will be taken by the server to refer to the root of the served file tree. 364 The 365 .B attach 366 identifies the user 367 to the server and may specify a particular file tree served 368 by the server (for those that supply more than one). 369 .PP 370 Permission to attach to the service is proven by providing a special fid, called 371 .BR afid , 372 in the 373 .B attach 374 message. This 375 .B afid 376 is established by exchanging 377 .B auth 378 messages and subsequently manipulated using 379 .B read 380 and 381 .B write 382 messages to exchange authentication information not defined explicitly by 9P. 383 Once the authentication protocol is complete, the 384 .B afid 385 is presented in the 386 .B attach 387 to permit the user to access the service. 388 .PP 389 A 390 .B walk 391 message causes the server to change the current file associated 392 with a fid to be a file in the directory that is the old current file, or one of 393 its subdirectories. 394 .B Walk 395 returns a new fid that refers to the resulting file. 396 Usually, a client maintains a fid for the root, 397 and navigates by 398 .B walks 399 from the root fid. 400 .PP 401 A client can send multiple T-messages without waiting for the corresponding 402 R-messages, but all outstanding T-messages must specify different tags. 403 The server may delay the response to a request 404 and respond to later ones; 405 this is sometimes necessary, for example when the client reads 406 from a file that the server synthesizes from external events 407 such as keyboard characters. 408 .PP 409 Replies (R-messages) to 410 .BR auth , 411 .BR attach , 412 .BR walk , 413 .BR open , 414 and 415 .B create 416 requests convey a 417 .I qid 418 field back to the client. 419 The qid represents the server's unique identification for the 420 file being accessed: 421 two files on the same server hierarchy are the same if and only if their qids 422 are the same. 423 (The client may have multiple fids pointing to a single file on a server 424 and hence having a single qid.) 425 The thirteen-byte qid fields hold a one-byte type, 426 specifying whether the file is a directory, append-only file, etc., 427 and two unsigned integers: 428 first the four-byte qid 429 .IR version , 430 then the eight-byte qid 431 .IR path . 432 The path is an integer unique among all files in the hierarchy. 433 If a file is deleted and recreated with the 434 same name in the same directory, the old and new path components of the qids 435 should be different. 436 The version is a version number for a file; 437 typically, it is incremented every time the file is modified. 438 .PP 439 An existing file can be 440 .BR opened , 441 or a new file may be 442 .B created 443 in the current (directory) file. 444 I/O of a given number of bytes 445 at a given offset 446 on an open file is done by 447 .B read 448 and 449 .BR write . 450 .PP 451 A client should 452 .B clunk 453 any fid that is no longer needed. 454 The 455 .B remove 456 transaction deletes files. 457 .PP 458 .B Openfd 459 is an extension used by Unix utilities to allow traditional Unix programs 460 to have their input or output attached to fids on 9P servers. 461 See 462 .IR openfd (9p) 463 and 464 .MR 9pclient (3) 465 for details. 466 .PP 467 The 468 .B stat 469 transaction retrieves information about the file. 470 The 471 .I stat 472 field in the reply includes the file's 473 name, 474 access permissions (read, write and execute for owner, group and public), 475 access and modification times, and 476 owner and group identifications 477 (see 478 .MR stat (3) ). 479 The owner and group identifications are textual names. 480 The 481 .B wstat 482 transaction allows some of a file's properties 483 to be changed. 484 .PP 485 A request can be aborted with a 486 flush 487 request. 488 When a server receives a 489 .BR Tflush , 490 it should not reply to the message with tag 491 .I oldtag 492 (unless it has already replied), 493 and it should immediately send an 494 .BR Rflush . 495 The client must wait 496 until it gets the 497 .B Rflush 498 (even if the reply to the original message arrives in the interim), 499 at which point 500 .I oldtag 501 may be reused. 502 .PP 503 Because the message size is negotiable and some elements of the 504 protocol are variable length, it is possible (although unlikely) to have 505 a situation where a valid message is too large to fit within the negotiated size. 506 For example, a very long file name may cause a 507 .B Rstat 508 of the file or 509 .B Rread 510 of its directory entry to be too large to send. 511 In most such cases, the server should generate an error rather than 512 modify the data to fit, such as by truncating the file name. 513 The exception is that a long error string in an 514 .B Rerror 515 message should be truncated if necessary, since the string is only 516 advisory and in some sense arbitrary. 517 .PP 518 Most programs do not see the 9P protocol directly; 519 on Plan 9, calls to library 520 routines that access files are 521 translated by the kernel's mount driver 522 into 9P messages. 523 .SS Unix 524 On Unix, 9P services are posted as Unix domain sockets in a 525 well-known directory (see 526 .MR getns (3) 527 and 528 .MR 9pserve (4) ). 529 Clients connect to these servers using a 9P client library 530 (see 531 .MR 9pclient (3) ). 532 .SH DIRECTORIES 533 Directories are created by 534 .B create 535 with 536 .B DMDIR 537 set in the permissions argument (see 538 .IR stat (9P)). 539 The members of a directory can be found with 540 .IR read (9P). 541 All directories must support 542 .B walks 543 to the directory 544 .B .. 545 (dot-dot) 546 meaning parent directory, although by convention directories 547 contain no explicit entry for 548 .B .. 549 or 550 .B . 551 (dot). 552 The parent of the root directory of a server's tree is itself. 553 .SH "ACCESS PERMISSIONS" 554 This section describes the access permission conventions 555 implemented by most Plan 9 file servers. These conventions 556 are not enforced by the protocol and may differ between servers, 557 especially servers built on top of foreign operating systems. 558 .PP 559 Each file server maintains a set of user and group names. 560 Each user can be a member of any number of groups. 561 Each group has a 562 .I group leader 563 who has special privileges (see 564 .IR stat (9P) 565 and 566 Plan 9's \fIusers\fR(6)). 567 Every file request has an implicit user id (copied from the original 568 .BR attach ) 569 and an implicit set of groups (every group of which the user is a member). 570 .PP 571 Each file has an associated 572 .I owner 573 and 574 .I group 575 id and 576 three sets of permissions: 577 those of the owner, those of the group, and those of ``other'' users. 578 When the owner attempts to do something to a file, the owner, group, 579 and other permissions are consulted, and if any of them grant 580 the requested permission, the operation is allowed. 581 For someone who is not the owner, but is a member of the file's group, 582 the group and other permissions are consulted. 583 For everyone else, the other permissions are used. 584 Each set of permissions says whether reading is allowed, 585 whether writing is allowed, and whether executing is allowed. 586 A 587 .B walk 588 in a directory is regarded as executing the directory, 589 not reading it. 590 Permissions are kept in the low-order bits of the file 591 .IR mode : 592 owner read/write/execute permission represented as 1 in bits 8, 7, and 6 593 respectively (using 0 to number the low order). 594 The group permissions are in bits 5, 4, and 3, 595 and the other permissions are in bits 2, 1, and 0. 596 .PP 597 The file 598 .I mode 599 contains some additional attributes besides the permissions. 600 If bit 31 601 .RB ( DMDIR ) 602 is set, the file is a directory; 603 if bit 30 604 .RB ( DMAPPEND ) 605 is set, the file is append-only (offset is ignored in writes); 606 if bit 29 607 .RB ( DMEXCL ) 608 is set, the file is exclusive-use (only one client may have it 609 open at a time); 610 if bit 27 611 .RB ( DMAUTH ) 612 is set, the file is an authentication file established by 613 .B auth 614 messages; 615 if bit 26 616 .RB ( DMTMP ) 617 is set, the contents of the file (or directory) are not included in nightly archives. 618 (Bit 28 is skipped for historical reasons.) 619 These bits are reproduced, from the top bit down, in the type byte of the Qid: 620 .BR QTDIR , 621 .BR QTAPPEND , 622 .BR QTEXCL , 623 (skipping one bit) 624 .BR QTAUTH , 625 and 626 .BR QTTMP . 627 The name 628 .BR QTFILE , 629 defined to be zero, 630 identifies the value of the type for a plain file.